Severe Plastic Deformation on Magnesium and its Alloys...Severe Plastic Deformation on Magnesium and...
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Severe Plastic Deformation on Magnesium and its Alloys...Severe Plastic Deformation on Magnesium and its Alloys Academic Advisor: Shin, Kwang Seon Submitting a Master’s thesis of
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Severe Plastic Deformation on Magnesium and its
Alloys
Materials Science and Engineering
Severe Plastic Deformation on Magnesium and its
Alloys
Submitting a Master’s thesis of Materials Science &
Engineering
July 2018
Syed Hasnain Abbas
July 2018
Vice Chair Prof. Shin, Kwang Seon (Seal)
Examiner Prof. Choi, In-suk (Seal)
iv
Abstract
This study deals with the effect of extrusion temperature on ZX10,
a high strength
low alloy (HSLA) biodegradable material, from 180oC to 300oC at
40oC interval to
study the impact on corrosion and mechanical properties in 3.5%
NaCl saturated with
Mg(OH)2. The effect of addition of neodymium (Nd) and calcium (Ca)
in magnesium
has also been investigated at fixed extrusion temperature of 300oC.
All the results
were then compared with commercially pure magnesium processed under
similar
conditions.
characterized by optical microscopy (OM), x-ray diffraction (XRD),
scanning
electron microscope (SEM) and electron backscattered diffraction
(EBSD),
respectively. The electrochemical corrosion behavior was
investigated by
potentiodynamic polarization and electrochemical impedance
spectroscopy (EIS)
while immersion study was conducted using fishing line specimen
arrangement to
simultaneously measure corrosion rates from weight loss (Pw) and
hydrogen evolution
(PH) after 168 hours. The mechanical properties including tensile
and compression
properties were measured using a 2-ton Unitech R&B machine at
fixed strain rate of
2 x 10-4.
As the extrusion temperature decreases, fine dynamic recrystallized
(DRX) grains
(~1.67μm) and elongated coarse un-dynamic recrystallized (unDRX)
grains turned
out to be more dominant. The binary neodymium alloy precipitates
metastable
Mg12Nd and thermodynamically stable Mg41Nd5 phases while ternary
Mg-Nd-Ca
alloys precipitated Mg2Ca and Mg41Nd5 phases. The Mg12Nd phase is
known to act
as weak cathode phase and is better for corrosion resistance.
Plastic deformation is a convenient method to change the
microstructure and improve
corrosion behavior of materials. Since magnesium and ZX10 extruded
at 180oC has
the best mechanical and corrosion properties in as-extruded
condition, they were
v
subjected to isothermal severe plastic deformation processes, such
as screw rolling
(SR) and multi-directional forging (MDF) at 300oC.
In case of ZX10, by observing screw rolled sample at 300oC, it is
found that the
corrosion rate was reduced appreciably with increase of yield
strength. However,
MDF increased the grain size along with increased porosity and
Mg-Zn-Ca phase
leading localized attack. When ternary Mg-Nd-Ca alloys were
subjected to screw
rolling, the corrosion resistance of EX33 was increased by
impressively 78% at the
expense of strength when compared with as-extruded condition. This
elevated
temperature decline in yield strength and increment of corrosion
resistance could be
associated with proportions of unDRX grains and reduction of second
phases
(galvanic cells).
Finally, the corrosion rates of studied materials were plotted on a
corrosion model
recently proposed in the literature. The calculated and predicted
data values were
found to be in good agreement for most materials.
Key words: magnesium alloys, mechanical properties, corrosion
behavior,
isothermal severe plastic deformation, multi-directional forging,
screw rolling, grain
size.
vi
Contents
1.1.1. Alloying
Elements..............................................................................
2
1.1.3. Grain Size
...........................................................................................
9
1.1.4. Current Density
................................................................................
10
1.1.5. Texture Effect
..................................................................................
13
1.3 Research Objectives
.................................................................................
17
2.1 Extrusion
..................................................................................................
18
2.4 Mechanical properties
..............................................................................
20
2.5 Corrosion Measurements
.........................................................................
20
2.6 Microstructure & Phase Analysis
............................................................
21
Chapter 3: Results & Discussion
.............................................................................
22
3.1 Effect of Extrusion Temperature and ISPD on Properties of Mg
............ 22
3.1.1 Microstructures
................................................................................
22
3.1.4 Summary
..........................................................................................
32
3.2 Effect of Extrusion Temperature and ISPD on Properties of ZX10
........ 33
3.2.1 Microstructures
................................................................................
33
3.2.3 Mechanical Properties
......................................................................
38
3.2.4 Corrosion Measurements
.................................................................
40
3.3.1 Microstructures
................................................................................
45
3.3.3 Mechanical Properties
......................................................................
51
3.3.4 Corrosion Measurements
.................................................................
53
4.2 UTS vs Corrosion vs Extrusion Temperature
.......................................... 58
4.3 Yield Stress vs Yield Asymmetry vs Extrusion Temperature
................. 61
4.4 Corrosion Analysis
...................................................................................
63
Chapter 5: Conclusion
..............................................................................................
66
Chapter 6: References
..............................................................................................
68
7.1 Classification of Different Medium Based on Ionic Concentration
......... 75
7.2 Alloys Properties in Hanks
......................................................................
76
7.3 Alloys Properties in
SBF..........................................................................
77
7.5 Extrusion Conditions
...............................................................................
79
7.7 Oxide Morphologies
................................................................................
81
ix
Figures
Fig. 1: Cross-section images of corroded surfaces of HT-AZ31 Sheet
samples after
6 days in 5wt.% NaCl solution [21]
.........................................................................
8
Fig. 2: Dependence of Corrosion Rate on grain size: (a) hot-rolled
AZ31 sheet
samples with and without heat treatments immersed in 5 wt.% NaCl
solution [22]
...............................................................................................................................
11
Fig. 3: Volume percent Intermetallic vs Corrosion Rate for HPDC
Mg-RE alloys
where RE: Ce/La/Nd/Y [22]
..................................................................................
11
Fig. 4: Schematic representation of the MDF process [34]
.................................. 19
Fig. 5: Temperature distribution (°) of screw rolled sample after
one pass [35]
...............................................................................................................................
19
Fig. 6: The OM images of magnesium extruded at (a) 180oC, (b)
220oC, (c) 260oC,
(d) 300oC; (e) screw rolled at 300oC and; (f) MDFed at 300oC
............................. 24
Fig. 7: XRD Analysis of CP magnesium in as-extruded and as-SPD
condition .... 25
Fig. 8: (a) Tensile and (b) Compressive tests of magnesium extruded
from 180oC to
300oC and as-SPD at 300oC
...................................................................................
27
Fig. 9: Polarization and EIS data in terms of (a) Nyquist, (b)
Bode, (c) Tafel plots
for CP magnesium in as-extruded and as-ISPD condition after 1.5
hours immersion
in saturated 3.5 wt. % NaCl solution
.....................................................................
29
Fig. 10: (a) Hydrogen evolution and (b) corrosion rate of
as-extruded and as-SPD
magnesium after 168 hours of immersion in saturated 3.5 wt. % NaCl
solution.
...............................................................................................................................
30
Fig. 11: SEM images showing surface morphologies of extruded and
screw rolled
Mg samples after immersion in saturated 3.5 wt. % NaCl solution for
168 hrs. ... 31
Fig. 12: The OM images of ZX10 extruded at (a) 180oC, (b) 220oC,
(c) 260oC, (d)
300oC; screw rolled at (e) 300oC and; MDFed at (f) 300oC
................................... 35
Fig. 13: Solidification behavior calculated by JMatPro 7.0 [41]
........................... 36
Fig. 14: XRD analysis of ZX10 in as-extruded and as-SPD condition
.................. 37
Fig. 15: (a) Tensile and (b) Compressive tests of ZX10 extruded
from 180oC to
300oC and as-SPD at 300oC
...................................................................................
39
x
Fig. 16: Polarization and EIS data in terms of (a) Nyquist, (b)
Bode, (c) Tafel plots
for ZX10 alloys in as-extruded and as-ISPD condition after 1.5
hours immersion in
saturated 3.5 wt. % NaCl
solution..........................................................................
41
Fig. 17: (a) Hydrogen evolution and (b) corrosion rate of
as-extruded and as-SPD
ZX10 after 168 hours of immersion in saturated 3.5 wt. % NaCl
solution. ........... 42
Fig. 18: SEM images showing surface morphologies of extruded and
screw rolled
ZX10 alloy after immersion in saturated 3.5 wt. % NaCl solution for
168 hrs. .... 43
Fig. 19: Microstructure of EX series alloy after extrusion and
isothermal screw
rolling. (a-d) shows OM before etching, (e-h) shows OM after
etching, (i-l) after
EBSD
.....................................................................................................................
47
Fig. 20: SEM/EDX and mapping of EX series alloys (a) EX33, (b)
EX36, (c)
EX33SR and, (d) EX36SR, ‘+’ indicates edx results in table,
rectangle indicates
mapping and SR denotes isothermal screw rolling at 300oC
................................. 48
Fig. 21: Solidification behavior calculated by JMatPro 7. [41]
............................ 49
Fig. 22: XRD Analysis of ‘E-series’ in as-extruded and as-screw
rolled conditions50
Fig. 23: (a) Tensile and (b) Compressive tests of ‘E series’
extruded at 300oC and
as-SPD at 300oC
.....................................................................................................
52
Fig. 24: Polarization and EIS data in terms of (a) Nyquist, (b)
Bode, (c) Tafel plots
for E series alloys in as-extruded and as-ISPD condition after 1.5
hours of
immersion in saturated 3.5 wt. % NaCl solution.
.................................................. 54
Fig. 25: (a) Hydrogen evolution and (b) corrosion rate of
as-extruded and as-SPD
E-series alloys after 168 hours of immersion in saturated 3.5 wt. %
NaCl solution
...............................................................................................................................
55
Fig. 26: SEM images showing surface morphologies of extruded and
screw rolled
E-series alloys after immersion in saturated 3.5 wt. % NaCl
solution for 168 hrs .
...............................................................................................................................
56
Fig. 27: Effect of alloying on corrosion and mechanical properties
of magnesium
extruded at 300oC
...................................................................................................
59
Fig. 28: The corrosion and mechanical properties of magnesium and
its alloys (a)
extruded between 180oC and 300oC, (b) ISPD at 300oC
........................................ 60
xi
Fig. 29: Relationship between yield strength and yield asymmetry
(a) at extrusion
temperature between 180oC and 300oC, (b) ISPD at
300oC................................... 62
Fig. 30: The relationship between calculated and predicted
corrosion rates [44] .. 65
Fig. 31: Ionic concentration of Hanks, Simulated body fluid (SBF)
and Plasma in
terms of ionic concertation in millimoles per Litre (mmol/L)
............................... 75
Fig. 32: Effect of alloying elements in as-cast and as-extruded
condition on
instantaneous corrosion rate (mm/y) in Hanks solution and
respective mechanical
properties (yield strength (YS, MPa), ultimate tensile strength
(UTS, MPa),
elongation (%))
......................................................................................................
76
Fig. 33: Effect of alloying elements in as-cast and as-extruded
condition on
instantaneous corrosion rate (mm/y) in SBF and respective
mechanical properties
(yield strength (YS, MPa), ultimate tensile strength (UTS, MPa),
elongation (%))
...............................................................................................................................
77
Fig. 34: Effect of alloying elements in as-cast and as-extruded
condition on
instantaneous corrosion rate (mm/y) in NaCl and respective
mechanical properties
(yield strength (YS, MPa), ultimate tensile strength (UTS, MPa),
elongation (%))
...............................................................................................................................
78
Fig. 35: Most commonly studied extrusion ratios and temperature,
(b) extrusion
temperature effect on corrosion rate [38, 42, 55]
................................................... 79
Fig. 36: Inverted Pole Figures (IPZ) of selected alloys. Note the
twinning in screw
rolled (SR) samples
................................................................................................
80
xii
Table
Table 1. Corrosion Rates of Nd-Zn-Zr, Nd-Zn alloys after immersion
in 5 wt.% NaCl
for 3 days [17]
...........................................................................................................
6
Table 2. Electrochemical Parameters of Mg-RE Cathode Phase [1]
....................... 12
Table 3. Physical and Chemical Properties of Mg & RE Elements
[1] ................... 12
Table 4. Corrosion Potentials of Mg-Zn-xNd(Gd) [1]
............................................. 15
Table 5. Composition and Processing Conditions of Pure Magnesium
................... 23
Table 6. Grain size and mechanical properties of magnesium in
as-extruded and as-
ISPD condition
.........................................................................................................
27
Table 7. Corrosion rates of magnesium in terms of hydrogen evolved
and weight loss
in as-extruded and as-ISPD condition
......................................................................
30
Table 8. Composition and Processing Conditions of Mg-1Zn-0.3Ca
(ZX10) ......... 34
Table 9. Solidification and Phase Calculations for Mg-1Zn-0.3Ca
(ZX10) alloys .. 36
Table 10. Grain size and mechanical properties of ZX10 in
as-extruded and as-ISPD
condition
..................................................................................................................
39
Table 11. Corrosion rates of ZX10 in terms of hydrogen evolved and
weight loss in
as-extruded and as-ISPD condition
..........................................................................
42
Table 12. Composition and Processing Conditions of ‘E-series’
alloys .................. 46
Table 13. Average sizes of DRXed and unDRXed grains (μm) from Fig.
19 ......... 46
Table 14. Solidification and Phase Calculations for ‘E Series’
alloys ..................... 49
Table 15. Grain sizes and mechanical properties of EX alloys in
as-extruded and as-
ISPD condition
.........................................................................................................
52
Table 16. Corrosion rates of E series alloys in terms of hydrogen
evolved and weight
loss in as-extruded and as-ISPD condition
..............................................................
55
Table 17. Total fraction of second phases
................................................................
64
Table 18. Volta potentials of second phases in this study [43, 44,
54] .................... 64
1
Chapter 1: Introduction
In biomedical industry where implants, such as plates, screws and
pins are used to
repair bone fractures, a subsequent surgical procedure to remove
these implants from
the human body after the tissues have healed, is not always
necessary. This is because
repeated surgery generally increases health risks to the patient
[1]. The widely used
bio-inert metals (Ti alloys, stainless steels etc.) may induce a
‘stress-shielding’ effect
owing to their much higher elastic modulus, compared to that of
bones, leading to a
risk of secondary fracture of bones. Stress shielding occurs when a
bone is shielded
by an implant from carrying a load. As a result, the bone tends to
weaken over time,
resulting in more damage. To minimize the effect of stress
shielding on bones while
still retaining enough strength, a soft lightweight and degradable
(in physiological
environment) metal (implant) is desirable.
Magnesium (Mg) is the most abundant element, second only to sodium
(Na) in the
hydrosphere and the fourth most abundant cation in living organisms
(human body:
Ca > K > Na > Mg) [2]. It exerts a large variety of
biological functions. Mg is
abundant in the human body, and bone tissue stores approximately
half of the total
physiological Mg [3]. This element is also vital to metabolism
processes in almost all
enzymatic systems involved in DNA processing as an essential
cofactor, and a key
component of the ribosomal machinery that translates the genetic
information
encoded by mRNA into polypeptide [4]. Moreover, researches show
that Mg has
stimulatory effects on the formation of new bone [5]. It was also
demonstrated that
biodegradable Mg screws have had a low rate of complications,
including avascular
necrosis and nonunion. In addition, Mg and its alloys possess
considerable potential
as fixation implants for orthopedic applications. The
reconsideration of Mg alloys as
biomaterials is mainly to improve corrosion resistance.
2
1.1 Factors Controlling Corrosion Performance
To improve corrosion, it is important to know the factors that
control corrosion.
1.1.1. Alloying Elements
It is well known fact that the addition of alloying elements plays
crucial role in
controlling corrosion and mechanical properties by inducing defects
in the matrix.
The choice of elements in magnesium for invitro environment to
improve corrosion
performance is limited due to potential electrochemical corrosion
reactions between
matrix and reinforcements. This part describes the alloying
elements and their effects
while the next part will describe the effect of defects and
intermetallic in terms of
potential difference.
1.1.1.1 Calcium
Calcium (Ca) has lower density 1.55 g/cc than magnesium (1.74 g/cc)
and it is very
cost effective. In terms of biocompatibility, Ca is one of the
major elements in bones
causes healing and growth and second largest cations in human body
contributing to
osteosynthesis phenomenon [6].
Li Z [7] studied the effect of binary alloy Mg-xCa (x=1,2,3 wt.%)
in as-cast, as-rolled
and as-extruded condition. In as-cast state, the mechanical
properties (YS, UTS,
Elong) deteriorates with increasing Ca content with Mg-2Ca being
optimum. There
are generally two phases α-Mg and Mg2Ca [8]. The latter phase has
higher corrosion
rate due to increased cathode-to-anode ratio. After hot processing,
the grain size
further reduced contributing positively towards mechanical and
corrosion properties.
Between as-rolled and as-extruded samples, hot extruded sample’s
ultimate tensile
strength (UTS: 240 MPa) and elongation (El: 10.70 %) of Mg-1Ca is
superior than
as-rolled.
When 0.3wt.% Ca is added to extruded Mg-1Zn alloy (ZX10, wt.%),
very fine
microstructure (grain size < 2μm) with outstanding mechanical
properties, high
strength-ductility, as well as low mechanical anisotropy had been
reported. Hofstetter
J [9] referred it as high-strength-low-alloyed (HSLA) Mg. It also
possesses superior
3
implant applications.
Wang L [10] studied the effect of Ca (0, 0.3, 0.6, 0.9 wt.%)
addition on mechanical
and biocorrosion of as-cast ZK30 (Zn: 3, Zr: 0.3 wt.%). Beyond 0.6
wt.% of Ca, there
is no obvious grain refinement and hydrogen precipitation at
cathode is not further
inhibited. It is important to balance grain size effect and
negative effect of Ca2Mg6Zn3
for optimum mechanical (UTS: 200 MPa, El: 19%) and corrosion
properties (0.26
mm/y, mass loss method) of ZK30 alloys.
1.1.1.2 Strontium
Calcium and Strontium (Sr) belongs to group II with Sr being
slightly soluble (0.108
wt.%) in magnesium. Sr is also a component of human bone and has
been known to
promote growth of osteoblasts and prevent bone resorption [11]. The
maximum daily
average intake of Sr is 4 mg/day.
Binary alloys of Mg-xSr (x: 0.3-2.5 wt.%) has been studied for
cardiovascular and
other orthopedic applications and to understand how these alloys
biodegrade in
physiological conditions [12]. The average corrosion rate of
Mg-0.3Sr and Mg-0.5Sr
is ~2.5 and 2 mg/day/cm2 respectively. Consequently, Mg-0.5Sr has
lowest
degradation rate (0.01 mg/day < 4 mg/day) in SBF due to less
micro-galvanic cells
between Mg-matrix and Mg17Sr2.
Due to biocompatibility of Ca and Sr, Bornapour [13] added 0.3 wt.%
Ca in Mg-0.3Sr
and found the corrosion rate decrease by 90% because of (1) third
element effect; (2)
Globular Ca/Sr-rich phases. Additionally, Mg-0.3Sr-0.3Ca has better
as-cast
mechanical properties (YS: 52MPa, UTS:107 MPa, El: 8.8%) than
Mg-0.3Sr (YS:
44MPa, UTS: 98 MPa, El: 4.0%). However, Mg-0.5Sr-0.6Ca increases
the corrosion
rate due to significant micro galvanic effect. Moreover,
Sr-substituted hydroxyapatite
is also beneficial leading to slower biocorrosion with enhancing
cell growth, their
proliferation and healing around bone implants.
4
1.1.1.3 Zirconium
Human body contains, on average 250mg of zirconium (Zr) with intake
requirements
of 4.15mg/day. As such there is no adverse biological effect of
Zr.
The improvement of corrosion resistance by Zr addition is probably
due to: (1) high
Zr content at grain center with respect to grain boundaries; (2)
reduction of cathodic
activity of Zr-phases; (3) grain refinement ability of Zr provides
more continuous
layers of corrosion resistant intermetallic RE phases around grain
boundaries,
decelerating corrosion between grain [14].
Zr, being effective grain refiner, its addition lead to equiaxed
single-phase (α-Mg)
solid solution alloy (grain size: 34.7μm) improving mechanical and
corrosion
properties [15]. Comparatively, mechanical properties of
Mg-1.5Zn-0.6Zr (wt.%) and
AZ91D are: YS: 83 & 97 MPa; UTS: 168 & 151 MPa; and El: 9.1
& 2.7 %,
respectively. The lower yield strength of Mg-1.5Zn-0.6Zr is due to
the absence of
second phase. On the other hand, corrosion rate by mass loss
technique (0.304 mm/y)
of as-cast alloy is almost half of AZ91D (0.726 mm/y) when tested
in Hank’s solution
for 7 days.
1.1.1.4 Rare Earth Elements
Rare earth elements are very reactive because of their low
electronegativity, become
positive ions (+3 valency) while remaining stable in alkali. It is
due to this reason, RE
exist as stable compounds in magnesium alloys and in nature.
RE-compounds formed
have very low electrode potentials which help lower the overall
potentials of cathode
phase, consequently the corrosion current when combined with high
potential
elements. The AlRE phase has a lower potential than β-AlMg,
therefore the addition
of RE elements can improve the anticorrosion capability of AZ
series alloys.
The RE advantages include high melting point and can be alloyed by
solid solution
strengthening. However, the anticorrosion capability of Mg-RE
alloys varies
substantially depending on the alloying components and their
structures.
5
1.1.1.4.1 Magnesium-Neodymium Alloys
The maximum solubility of Nd in Mg is 3.6 wt,% but decreases with
temperature
from 0.8 ~1.0wt.% at room temperature. Therefore, the optimum
content is between
3 and 4 wt.%. In Mg-Nd alloys, Nd forms Mg12Nd phase depending upon
processing
condition, which is similar to Mg24Y5 and Mg-Y-Nd phases. It is a
high temperature
reinforced phase since the melting point of Mg12Nd is between those
of the Mg24Y5
and Mg-Y-Nd phases at 545oC. The Mg12Nd solidifies after the α-Mg
matrix and
therefore distributes at the crystal grain boundaries. After solid
solution treatment,
almost all of them can be solubilized in the matrix. Moreover,
Mg12Nd is a weak
cathode and its potential is close to that of Mg24Y5.
Zr is known to be effective grain refiner whereas Zn, when added in
small amounts,
contribute towards castability. Zhang X [16] compared the corrosion
behavior of
AZ31, WE43 and JDBM (Mg-3.09Nd-0.22Zn-0.44Zr) in SBF for 10 days.
JDBM has
almost 57% more and twice corrosion resistance than of AZ31 and
WE43,
respectively. It is due to low cathode/anode surface ratio and
lower potential
difference and therefore low degree of local corrosion.
1.1.1.4.2 Magnesium-Neodymium-Zirconium Based Alloys
Zr forms insoluble precipitates with impurities (especially Fe
& Ni), so enrichment
of Zr in grain center leads to a higher resistance in this zone. On
the other hand, Mg-
RE alloys can also result in higher purity and corrosion
resistance. In different heat
treatment conditions, corrosion resistance is T4>T6>F (Table
1.) [17]. The reason is
galvanic corrosion rate relates strongly to morphology of cathode
phase like Mg12Nd
such as in Mg-Gd alloy.
6
Table 1. Corrosion Rates of Nd-Zn-Zr, Nd-Zn alloys after immersion
in 5 wt.%
NaCl for 3 days [17]
7
1.1.1.5 Detrimental Alloying in Magnesium
Aluminum (Al) is known to slow down the biodegradation rate of
magnesium. In
solutions, Al releases ions which could easily combine with
inorganic phosphates,
leading to a lack of phosphate in the human body, and an increased
concentration of
Al ions in the brain. This may lead to Alzheimer’s disease. Severe
hepatotoxicity has
also been detected after the administration of RE elements such as
cerium,
praseodymium and yttrium [18].
Although, zinc (Zn) in trace amount (15mg/day) is essential to
human body, the main
drawback is Zn’s biocompatibility is it releases Zn2+ in chlorides
(especially HCl)
forming ZnCl2, known to damage stomach parietal cells [19] . Wan
studied the
surface nanomechanical performance and corrosion behavior Mg–Ca
alloys ion-
implanted with zinc. They concluded zinc ion implantation
(dose:
0.9 × 1017 ions/cm2) is not a favorable for biomedical Mg-Ca as it
deteriorates
corrosion resistance [20].
The crystallographic defects, such as grain boundaries,
dislocations, and twins, are
usually corroded preferentially because of their higher chemical
activity. If the
intermetallic is distributed randomly and discontinuously, the
corrosion of the matrix
is accelerated through galvanic effects. For example, after 6 days
of immersion in
5wt.% NaCl, the exposed grain boundaries and twins of heat-treated
AZ31 sheet
sample appear to be dissolved slightly deeper as shown in Fig. 1
[21]. Therefore, it is
important to understand how these crystallographic defects and
those intermetallic
precipitates influence corrosion performance.
The primary driving force for magnesium alloys corrosion reaction
is the varied
electrode potentials on microscale alloy surface. The varied
potentials can be
attributed to the multi-phase alloy system as well as the crystal
defects. It is known
that the potential at the boundary of two phases is lower than that
in crystal matrix. A
deep and wide phase boundary makes inter crystalline corrosion
liable to occur.
8
Fig. 1: Cross-section images of corroded surfaces of HT-AZ31 Sheet
samples after
6 days in 5wt.% NaCl solution [21]
9
Depending upon the solubility of alloying elements, the second
phases form. When
this second phase(s) precipitate(s) after solidification, due to
the contact with matrix,
a potential difference is created. The current starts to flow from
higher Fermi energy
level to lower Fermi energy level until thermodynamic equilibrium
has been
maintained. Balancing of Fermi energy level does not indicate the
electric potentials
are also equal. The electric potential is controlled by its work
function. Elements
which have low work function values than pure Mg (3.66) are Ca
(2.8), Ce (3.1), Nd
(3.2) and Y (3.1). The resultant second phase forms become anode
with respect to
matrix due to lower potential than the matrix and hence,
preferentially corrode.
Inorder to measure the surface potential between the matrix and
intermetallic,
Scanning Kelvin Probe Force Microscope (SKPFM) is used which
measures the
relative potential through difference of work function. What is
implied from the
relative potential measured from phases in isolation from say,
SKPFM, is that a phase
more positive than the matrix will be polarized cathodically at the
open circuit
potential of an alloy in which it populates, and therefore will
likely act as a local
cathode. SKPFM also provides an indication of expected relative
driving force for
microgalvanic couples (ex-situ) or changes in the modified work
function in the
presence of corrosion or surface layers (in-situ).
1.1.3. Grain Size
As described in above section, the presence of intermetallic
particles and crystal
defects can accelerate the corrosion of Mg alloys. The
crystallographic defects and
intermetallic particles can evolve with grain size during a surface
deformation or heat
treatment process and finally influence the corrosion performance
of a Mg alloy.
Therefore, grain size is not an independent factor and almost
always evolve with
crystal defects and intermetallic particles.
When the grain size of AZ31 is large (e.g. >10 micron), the
corrosion rate is relatively
high; but if the grain size is small (e.g. <100 micron) the
corrosion rate increases with
decreasing size. This non-monotonous relationship between the grain
size and
corrosion rate of AZ31 suggests that not only the density of
crystal defects but also
10
the amount of precipitated particles are responsible for corrosion
behavior. The
detrimental effect of crystallographic defects (dislocations, grain
boundaries and
twins) on corrosion performance interprets the slightly worsened
corrosion resistance
when grain is small (<10 micron). After heat-treatment, or in
case of squeeze casting,
while grains grow significantly, the number of particles also
increases. The later
variation deteriorates the corrosion performance of AZ31, Fig. 2
[21].
1.1.4. Current Density
Corrosion of magnesium alloys leads to reduced mass
(macroscopically) and
corrosion current (microscopically). Cathode current determines the
corrosion current
and in effect controls the corrosion rate.
Besides the α-Mg matrix, binary alloys, that is Mg-Ce, Mg-La, Mg-Nd
and Mg-Y,
contain only the Mg12Ce, Mg12La, Mg3Nd and Mg12Y5 phases. Fig. 3
shows Mg-La
& Mg-Nd has gradual slope while Mg-Ce & Mg-Y has steeper
slope between the
intermetallic volume percentage and the corrosion current [22]. The
current density
on the surface of the cathode phase is an indicator for
hydrogen-evolution efficiency,
which is positively related with the corrosion sensitivity of the
magnesium alloy. The
polarization resistance, a barometer for the extent of cathode
polarization, is a
parameter for the catalytic activity in the hydrogen-evolution
reaction on the cathode
surface. Zero polarization resistance means non-polarized while ∞
resistance
represents complete polarization. Lower cathode current density
means higher
polarization resistance, which is more beneficial to the
anticorrosion properties.
The current density depends on the potential difference between the
cathode phase
and the matrix. The matrix potential also changes with the matrix
content. It can be
seen from Table 2 that the Mg12Y5 phase, which has the lowest
polarization resistance,
will generate a strong corrosion current even in the absence of a
high potential
difference between the RE phase and the matrix [22].
11
Fig. 2: Dependence of Corrosion Rate on grain size: (a) hot-rolled
AZ31 sheet
samples with and without heat treatments immersed in 5 wt.% NaCl
solution [22]
Fig. 3: Volume percent Intermetallic vs Corrosion Rate for HPDC
Mg-RE alloys
where RE: Ce/La/Nd/Y [22]
Table 2. Electrochemical Parameters of Mg-RE Cathode Phase
[1]
Table 3. Physical and Chemical Properties of Mg & RE Elements
[1]
13
Previous studies show that the corrosion rate strongly depends on
crystallographic
orientations [23]. In order to determine this dependency, single
crystal studies are
usually carried out to study effect of specific orientations.
However, this methodology
limits the corrosion studies to few crystal planes which does not
necessarily provide
understanding of the entire system. In corrosion studies, the grain
orientations vary
significantly only few degrees difference in crystallographic
orientations.
Heat treatment can effectively alter the microstructure without
changing the
composition of an alloy. A properly controlled heat treatment will
not significantly
change the grain orientation of a hot-rolled AZ31 sheet. However,
the heat treatment
changes the grain size also effecting corrosion. The factors to be
considered while
studying the effect of crystallographic orientations on corrosion
are as follows:
For one type of alloy,
1. Effects of crystal structure on
a. Atomic bonding between nearest neighbors,
b. Coordination, the number of dangling bonds
c. Resultant surface energy
Specifically, for magnesium and its alloys:
1. Mg is a system which corrodes in [Cl-] solutions far from
equilibrium. There has
been conflicting data on orientation dependence of Mg corrosion.
One study
claims near normal <0001> directions with respect to sample
surface are prone
to corrosion in NaCl environments containing CrO3. While, another
study
14
examined (0001) planes as more corrosion resistant for Mg in 0.1N
HCl. The
Mg (0001) surface exhibited pitting corrosion susceptibility at
open circuit
potential (OCP) while the (10-10) and (11-20) surfaces were passive
at open
circuit and pitting only initiated at potentials slightly anodic to
their OCP in
similar environment [24].
2. In AZ31, the corrosion occurs slowly on closely packed plane but
the presence
of secondary phases also effects [25].
3. Another study found in two Mg grains that the one grain near
(0001) (between
0001 and 10-10) had higher corrosion rate than the twin containing
grain close
to (11-20), when the oxide film was thin in NaCl and saturated
Mg(OH)2
solution [26].
16
1.2 Severe Plastic Deformation
Since, the strength coefficent values of magnesium is four times
than that of
aluminum, utilizing severe plastic deformation mechanisms is an
effective way for
grain refinement. Several SPD techniques, e.g. screw rolling (SR,
PSW), equal
channel angular pressing (ECAP), high-pressure torsion (HPT),
multidirectional
forging (MDF), accumulative roll bonding (ARB) etc., lead to the
development of
ultrafine-grained (UFGed) structures in a whole volume of the
products. However,
MDF and SR seems very attractive especially for commercialization
where relatively
large samples can be processed.
1.2.1 Multidirectional Forging (MDF)
In the context of grain refinement described above, multi
directional forging (MDF)
is a process in which simple plane strain condition is used to
deform FCC and HCP
materials without changing its dimensions. As a result, Zn-Al finds
their applications
in sleeve bearings, thrust washers, house couplings &
connectors, pressure tight
housings and many other structural applications. The zinc aluminum
alloys are known
for their excellent castability and wear resistance. When Zn-Al
sample is cast, it
shows dendritic structure leading to heterogenous mechanical
properties, negative
effect on ductility [27]. The addition of other alloying elements
produces several
metastable phases which have negative effect on dimensional
instability.
Miura H [28] studied the evolution of ultra-fine grains in AZ31 and
AZ61 by MDF
under decreasing temperature conditions from 350oC to 150oC at a
strain rate of 3 x
10-3 s-1. The strength level reported for average of 1-micron grain
structure of AZ31
and AZ61 are 850MPa and 1.2GPa, respectively, at cumulative strain
of 4.0 and 4.8.
1.2.2 Screw Rolling (SR)
The current economic crisis and trade wars, there is an increasing
pressure on leading
economies to reduce labor costs and minimizing the processing time
of products
between manufacturers and supplier. Hence, the need for lean
production is becoming
more evident. Screw rolling is one such processing technology
suitable for small
diameter but hard to deform materials such as high carbon steels,
high temperature
17
nickel-cobalt superalloys and white cast iron. The material is
deformed in helicoid
motion at a fixed distance of the rollers, creating bulk
macroshear. The final structure
consists of small individual isotropic particles with improved
physicomechanical
properties, most notably plastic and ductile properties [29].
Sheremetyev V processed shape memory alloy, Ti-18Zr-14Nb, by screw
rolling for
bone implants. The results show excellent fatigue properties in
conjunction with 35%
of ductility [30]. Akopyan TK recently investigated the effect of
screw rolling on Al-
Ni and Al-Ca alloys with up to 2.5 times improved strength than
as-cast or annealed
samples [31].
The properties of magnesium alloys can also be improved by
employing screw rolling
[32]. Diez M improved the yield strength of pure magnesium to 116
MPa under
decreasing temperature condition with 13% total elongation
[33].
1.3 Research Objectives
Due to increased demands in aerospace, automotive, biomaterials and
electronics
industry, it is vital to study effect of processing parameters on
properties of
magnesium and its alloys. Therefore, following are the main
objectives of the current
study:
a. Thorough review on corrosion medium on properties of magnesium
alloys:
effect of ionic concentration on corrosion and strength
properties
b. Prepare Al-free magnesium alloys with fine microstructure
(<2μm)
c. Systematic investigation on effect of extrusion temperature on
strength and
corrosion properties of ZX10 and Mg-Nd/Ca alloys
d. Analyze the effect of isothermal severe plastic deformation
temperature,
300oC
f. Propose optimum corrosion-strength-ductility
g. Fit corrosion data in the corrosion model recently proposed in
the literature
18
2.1 Extrusion
Pure magnesium (Mg, 99.8%) ingot was melted in electric resistance
furnace under
anti-oxidizing gas atmosphere (1 vol.% SF6 + 99 vol.% CO2) in a low
carbon steel
crucible with diameter 80mm at 750oC. Then, zinc (Zn), calcium (Ca)
and
neodymium (Nd) were added according to alloy design. After 30
minutes of stirring,
Mg and alloys were water quenched. Afterwards, the as-cast ingots
were machined
into cylindrical samples with dimensions Φ75 x 150 mm2 . The
machined billets were
then heat treated before carrying out indirect extrusion at fixed
extrusion ratio of 9
and constant ram speed.
2.2 Multidirectional Forging (MDF)
The MDF samples were prepared by machining the center of extruded
bar into
rectangular cubes with dimensions, 17 x 17 x 28 mm3. These samples
were heated at
300oC in the furnace to carry out isothermal forging. The principle
of MDF is
presented in Fig. 4. Accordingly, after each pass the sample was
rotated 90o and
abraded to recover deformed surface. Instron-type 5582 machine was
used to carry
out forging at an initial strain rate of 3 × 10-3 s-1 with
cumulative strain of ∑ = 2.7.
The applicable strain by MDF is theoretically unlimited when the
sample is not
fractured. The first forging axis was parallel to the extrusion
direction.
19
Fig. 4: Schematic representation of the MDF process [34]
Fig. 5: Temperature distribution (°) of screw rolled sample after
one pass [35]
20
2.3 Screw rolling (SR)
The as-extruded bar was cut 70 mm length for three-roll mill, Fig.
5. Prior to planetary
milling, rods were heated in an air furnace for 30 minutes to carry
out isothermal
rolling at 300oC. After six passes, the final diameter obtained was
15 mm at a mill
speed of 80 mm/s. The rolling direction was also similar to the
extrusion direction.
2.4 Mechanical properties
The mechanical properties were characterized by means of tensile
and compression
tests at a strain rate of 2 × 10-4 s-1. A 2-ton Unitech R&B
machine was utilized to test
tensile specimen with gauge length 12 mm and 1 mm thickness while
the dimensions
of compression samples were 3 x 3 x 4.5 mm3. An average of five
specimens for each
sample was taken at room temperature.
2.5 Corrosion Measurements
Specimen for corrosion studies were machined from the center of
extruded and
deformed samples by spark-erosion wire cutting with an exposed
surface parallel to
final processing direction.
The properties of as-extruded, as-MDF and as-screw rolled samples
were measured
by electrochemical, weight loss and hydrogen evolution measurements
in 3.5 wt.%
NaCl saturated with Mg(OH)2 at 25 + 1oC. The experimental
conditions were similar
as described by Arthanari S [36]. The dimensions of all corrosion
samples were kept
constant, 10 x 10 x 2 mm3 sliced from biscuit (ASTM E8). Prior to
corrosion
characterization, the samples were ground on abrasive silicon
carbide (SiC) with
granularity 2000. Finally, samples were cleaned in an ultrasonic
bath using acetone
and blow dried with hot air. The corrosion rate (mm/y) was
calculated by using
following equations [37]:
Hydrogen Evolution, PH = 2.088 VH /At (2)
Polarization, Pi = 22.85 icorr (3)
21
Where ‘ΔW’ is the difference of masses before and after immersion,
‘A’ is the area
of the specimen (cm2), ‘t’ is the time (d), icorr (mA/ cm2) is the
current density obtained
from tafel plots.
2.6 Microstructure & Phase Analysis
The microstructures evolved after extrusion, MDF and screw rolling
were analyzed
perpendicular to the final processing direction. The specimens were
ground, polished
and etched using picral. Finally, the average grain size was
measured by using image
analysis software.
The corroded morphology after electrochemical and immersion tests
was analyzed by
scanning electron microscopy (JSM-6360) equipped with an energy
dispersive X-ray
spectroscopy (EDS). The accelerating voltage range 15.0 kV and
working distance of
10 mm is kept for all samples.
X-Ray Diffraction (XRD) was carried in D8-Bruker machine with 2θ
scan range from
10o to 90o at a scan rate of 0.03o for investigating the formed
phases.
22
of Mg
The first goal of the present work is to study effect of extrusion
temperature on
commercially pure magnesium at a constant ratio. The properties of
pure magnesium
are very important especially in biomaterials and in magnesium
batteries due to
economic viability in latter when compared with doped-magnesium
batteries [38].
Due to absence of any second phase, the grain size plays the
primary role in deriving
the properties. Additionally, the isothermal severe plastic
deformation at 300oC was
carried out on low grain size samples to study its effect on
corrosion and mechanical
properties.
3.1.1 Microstructures
The commercially pure magnesium ingots were extruded from 180oC to
300oC at a
fixed extrusion ratio of 9. The chemical compositions determined by
spark emission
spectroscopy and extrusion conditions are listed in Table 5. Fig. 6
shows the optical
microstructure of magnesium in as-extruded and as-SPD state. As the
extrusion
temperature increases, the grain size increases due to the growth
of recrystallized
grains suggesting the linear relationship. Since, Mg180 possessed
the smallest grain
size, isothermal rolling and forging at 300oC were carried out to
see the effect on
microstructure. The microstructure of screw rolled samples showed
several
deformation twins whereas MDF sample shows a more homogenous
microstructure.
The grain sizes are listed in Table 6.
23
Material
Temp (oC)
HT (oC/h)
kg/cm3 - 340 516 548 - 402 oC
Mg
180
350/12
220 64 99.60 - 0.01 0.05 0.03 - 0.31
260 55 99.87 - 0.04 0.03 0.02 0.01 0.03
300 48 99.90 - 0.03 0.02 - - 0.05
24
Fig. 6: The OM images of magnesium extruded at (a) 180oC, (b)
220oC, (c) 260oC,
(d) 300oC; (e) screw rolled at 300oC and; (f) MDFed at 300oC
25
Fig. 7: XRD Analysis of CP magnesium in as-extruded and as-SPD
condition
26
3.1.2 Mechanical Properties
The tensile and compression tests were carried out at room
temperature on
magnesium samples extruded between 180oC and 300oC. The tensile
strength of
the as-extruded samples has no significant difference but both
tensile and
compressive elongation decreases with extrusion temperature.
To optimize the mechanical properties of pure magnesium, Mg180 was
subjected
to isothermal screw rolling and MDF at 300oC. The screw rolled
samples shows
the drastic increase in strength due to increased dislocation
density. However, the
tensile/compressive slope of MDF samples indicates the softening of
the matrix
even though the grains were reduced from 9.27 to 7.13 μm. This can
be explained
on the basis of the fact that the grain boundary diffusion
coefficient of Mg is large
enough to cause the deformation at higher temperature causing grain
boundary
sliding [39]. To overcome, Mabuchi concluded an increase strain
rate value can
decrease grain boundary sliding.
The results are summarized in Fig. 8 and Table 6.
27
Fig. 8: (a) Tensile and (b) Compressive tests of magnesium extruded
from 180oC to
300oC and as-SPD at 300oC
Table 6. Grain size and mechanical properties of magnesium in
as-extruded and as-
ISPD condition
Designation Grain
Size (μm)
28
3.1.3 Corrosion Measurements
The instantaneous corrosion rate was measured by means of
electrochemical tests,
Fig. 9, while as-extruded magnesium was immersed over a period of
seven days in a
saturated NaCl solution to determine corrosion rate by hydrogen
evolution and weight
loss method, Fig. 10.
In as-extruded condition, Mg 180 has the lowest corrosion rate (PH)
of 2.96 mm/y
when grain size was 9.27 μm. To further enhance the properties,
Mg180 was
subjected to ISPD.
In Nyquist plot, the capacitive loop at the high frequency region
indicates the charge
transfer reaction on the sample surface and electrolyte, and the
dimension of
capacitance loop determines the charge transfer resistance. In Fig.
9a, the diameter of
high frequency capacitance loop of screw rolled sample is larger
than that of the
extruded and MDF, indicating that the charge transfer resistance
has remarkably
decreased after screw rolling. The bode plot, Fig. 9b, shows a drop
for all samples in
low frequencies except MDF which suggests resistance of the surface
film against
breakdown [3]. This improved resistance is attributed to grain
refinement being
principally confined to controlling the rate of anodic reactions
and having little role
in altering the rate at which cathodic reactions can be sustained
[4]. The homogeneity
of microstructure achieved by MDF forms an effective passivation
decreasing
corrosion rate of magnesium to 0.74 mm/y in terms of hydrogen
evolved, Fig. 10.
Also, the current density (μA/cm2) decrease from 93 to 32 from
tafel analysis, Fig.
9c, confirms the corrosion resistance trend of Mg180 MDF
sample.
Fig. 11 shows the corroded surfaces after immersion of screw rolled
and MDFed
samples in saturated NaCl. The surface of MDF sample was corroded
uniformly and
wrinkled with some pin holes and small shallow areas.
29
Fig. 9: Polarization and EIS data in terms of (a) Nyquist, (b)
Bode, (c) Tafel plots
for CP magnesium in as-extruded and as-ISPD condition after 1.5
hours immersion
in saturated 3.5 wt. % NaCl solution
30
Fig. 10: (a) Hydrogen evolution and (b) corrosion rate of
as-extruded and as-SPD
magnesium after 168 hours of immersion in saturated 3.5 wt. % NaCl
solution.
Table 7. Corrosion rates of magnesium in terms of hydrogen evolved
and weight
loss in as-extruded and as-ISPD condition
Sample Hydrogen
Volume (ml/cm2)
62
(a)
(b)
Fig. 29: Relationship between yield strength and yield asymmetry
(a) at extrusion
temperature between 180oC and 300oC, (b) ISPD at 300oC
63
slips dominates at higher temperatures. Yin (2009) demonstrated
that high frequency
of twinning under basal texture in Mg-3Al-1Zn leads to tension
compression yield
asymmetry [53].
Hence, we can say when the grain size reaches to a transition
threshold, the yield
asymmetry value can be minimized, ZX180 forged at 300oC, with the
dominant
deformation mechanisms change from twinning to slip.
4.4 Corrosion Analysis
Ahmad B [44] presented a corrosion model to predict corrosion rate
by estimating
phases fraction, grain sizes, volta potential difference between
matrix and second
phase. To corroborate the findings of current work, the area
fraction of all second
phases and their respective volta potentials are shown in Table 17
and Table 18,
respectively.
From the data plot, Fig. 30, of calculated and predicted corrosion
rates, the findings
of most materials were found to be in good agreement.
64
Sample C)o( Condition ∑f A,i
∑f A,i
ISR 0.0606 9.0975
Table 18. Volta potentials of second phases in this study [43, 44,
54]
Alloy (wt.%) Intermetallic Volta Potential (mV)
Mg-0.6Ca-0.5Mn Ca-Mg-Zn 350
Mg-2.6Nd Mg-Nd 150
Mg-4Y-2.2Nd Mg12Nd 25
Mg-3.5Zn-0.7Ca-0.5Mn Mg2Ca -130
65
Fig. 30: The relationship between calculated and predicted
corrosion rates [44]
66
To achieve optimum corrosion-mechanical properties, warm and hot
extrusion of
magnesium & its alloy and subsequent isothermal severe plastic
deformation have
been studied. The current industrial demand to produce alloys for
warm extrusion can
be achieved by low alloying and modification of processing
parameters to achieve
balance of desired properties, as achieved in the case of screw
rolled ZX10, for
instance.
The linear relationship between corrosion rates and extrusion
temperatures in
magnesium is a result of localized corrosion, breaking of
passivation film and
increase dissolution of black areas rich with MgO, Mg(OH)2 and,
impurities. The
more homogenous microstructure, as after forging of Mg180, can form
a thick
passivation of corrosion products over prolonged period which in
turn increase
corrosion resistance.
The ZX10 alloys shows a very fine microstructure (1.677μm) for a
technical wrought
alloy even in as-extruded condition. A good combination of
mechanical properties
such as high strength and high ductility after screw rolling at
300oC has been achieved.
The increased porosity and presence of Mg-Zn-Ca phase, which is
less noble than the
matrix, severed the corrosion rate of MDF samples.
For both Mg and ZX10, the decreased strength after MDF could be
associated with
softening of matrix because of lower strain rate employed and high
deformation
temperature causes grain boundary sliding (high diffusion
coefficient value of Mg)
resulting in decreased strength.
When Mg-3Nd was immersed in harsh chloride environment and
corrosion rate was
higher than previously reported in SBF. However, the corrosion
resistance role of
Mg12Nd phase was found to be consistent with the literature. The
volta potential of
Mg12Nd is only ~25mV than that of Mg-matrix making it a weak
cathode phase.
67
The addition of Ca in Mg-Nd is found to be suitable for mechanical
properties in as-
extruded condition. However, the corrosion properties need
optimization by
decreasing unDRX fraction and promoting formation of Mg12Nd phase.
When EX
series alloys were subjected to screw rolling, the corrosion
resistance of EX33 was
increased by impressively 78% than as-extruded state. This elevated
temperature
decline in yield strength and increment of corrosion resistance
could be associated
with increased grain size and reduction of second phases (galvanic
cells). The results
are consistent with previous reported data.
To design magnesium alloys for structural applications, the yield
asymmetry can be
lowered by decreasing extrusion temperature, for example ZX10
series. Further
decline was achieved by forging ZX180 at 300oC resulting in yield
asymmetry value
of 0.4 (minimum). The opposite trend of Mg and ZX10 yield asymmetry
values can
be explained on the basis of precipitates in latter, grain size
difference and texture
evolution with dominant deformation mechanisms changing from
twinning to slip,
mainly prismatic slip.
Finally, the corrosion rates of studied materials in this work were
plotted on a
corrosion model, recently proposed in the literature. For most
alloys, a good
agreement between calculated and predicted values was
obtained.
68
[1] G. Song, Corrosion Prevention of Magnesium Alloys, Woodhead
Publishing,
2013.
[2] F. Wolf and A. Cittadini, "Chemistry and Biochemistry of
Magnesium,"
Molecular Aspects of Medicine, Vols. 1-3, pp. 3-9, 2003.
[3] M. P. Staiger, A. M. Pietaka, H. Herawala and G. Dias,
"Magnesium and its
alloys as orthopedic biomaterials: A review," Biomaterials, vol.
27, no. 9, pp.
1728-1734, 2006.
[4] A. Hartwig, "Role of magnesium in genomic stability," Mutation
Researc,
vol. 475, pp. 113-121, 2001.
[5] H. Zreiqat, P. C. Howlett, A. Zannettino, P. Evans, G.
SchulzeTanzil, C.
Knabe and M. Shakibaei, "Mechanisms of magnesium-stimulated
adhesion
of osteoblastic cells to commonly used orthopaedic implants,"
Journal of
Biomedical Materials Research Banner, vol. 62, no. 2, pp. 175-184,
2002.
[6] B. Istrate, C. Munteanu, M. Matei, B. Oprisan, M. Moisei and K.
Earar,
"Microstructural analysis and mechanical properties of
biodegradable Mg-
1.3Ca-5.5Zr alloy," IOP Conference Series: Materials Science
and
Engineering, vol. 145, 2016.
[7] Z. Li, X. Gu and Zheng Y, "The development of binary Mg-Ca
alloys for use
as biodegradable materials within bone," Biomaterials, vol. 29, no.
10, pp.
1329-1344, 2008.
[8] K. Kubok, L. Lityska-Dobrzyska, J. Wojewoda-Budka and A.
Góral,
"Structural investigation of Mg-3Ca, Mg-3Zn-1Ca and Mg-3Zn-3Ca as
cast
alloys," Inynieria Materiaowa, vol. 35, no. 2, pp. 162-165,
2014.
[9] J. Hofstetter, M. Becker, E. Martinelli, A. M. Weinberg, B.
Mingler, H.
Kilian, S. Pogatscher, P. J. Uggowitzer and J. F. Löffler,
"High-Strength
Low-Alloy (HSLA) Mg–Zn–Ca Alloys with Excellent
Biodegradation
69
Performance," The Journal of The Minerals, Metals & Materials
Society , vol.
66, no. 4, p. 566–572, 2014.
[10] L. Wang, J. B. Li, K. B. Nie, J. S. Zhang, C. W. Yang, P. W.
Yan, Y. P. Liu
and C. X. Xu, "Microstructure, mechanical and bio-corrosion
properties of
Mg–Zn–Zr alloys with minor Ca addition," Materials Science
and
Technology, vol. 33, no. 1, pp. 9-16, 2016.
[11] W. Zhang, Y. Shen, P. Haobo, K. Lin, X. Liu, B. W. Darwell, W.
W. L, J.
Chang, L. Deng, D. Wang and W. Huang , "Effects of strontium in
modified
biomaterials," Acta Biomaterialia, vol. 7, pp. 800-808, 2011.
[12] M. Bornapour, N. Muja, D. Shum-Tim, M. Cerruti and M.
Pekguleryuz,
"Biocompatibility and biodegradability of Mg–Sr alloys: The
formation of
Sr-substituted hydroxyapatite," Acta Biomaterialia, vol. 9, p.
5319–5330,
2013.
[13] M. Bornapour, M. Celikin, M. Cerruti and M. Pekguleryuz,
"Magnesium
implant alloy with low levels of strontium and calcium: The third
element
effect and phase selection improve bio-corrosion resistance and
mechanical
performance," Materials Science and Engineering C, vol. 35, pp.
267-282,
2014.
[14] M. Peron, J. Torgersen and F. Berto, "Mg and Its Alloys for
Biomedical
Applications: Exploring Corrosion and Its Interplay with Mechanical
failure,"
Metals, vol. 7, no. 252, pp. 1-41, 2017.
[15] T. Li, H. Yong , H. Zhang and X. Wang , "Microstructure ,
mechanical
property and in vitro biocorrosion behavior of single phase
biodegradable
Mg-1.5Zn-0.6Zr alloy," Journal of Magnesium and Alloys, vol. 2, pp.
181-
189, 2014.
[16] X. Zhang, G. Yuan, L. Mao, J. Niu and W. DIng, "Biocorrosion
properties of
as-extruded Mg–Nd–Zn–Zr alloy compared with commercial AZ31
and
WE43 alloys," Materials Letters, vol. 66, pp. 209-211, 2012.
70
[17] J. W. Chang, J. Duo, Y. Z. Xiang, H. Y. Yang, W. J. Ding and
Y. H. Peng,
"Influence of Nd and Y additions on the corrosion behaviour of
extruded Mg-
Zn-Zr alloys," International Journal of Minerals, Metallurgy and
Materials,
vol. 18, no. 2, pp. 203-209, 2011.
[18] N. Hort, Y. Huang, D. Fechner, M. Störmer , C. Blawert, F.
Witte, C. Vogt,
H. Drücker, R. Willumeit, K. U. Kainer and F. Feyerabend,
"Magnesium
alloys as implant materials--principles of property design for
Mg-RE alloys,"
Acta Biomaterialia, vol. 6, no. 5, pp. 1714-1725, 2010.
[19] S. Zhang, X. Zhang, C. Zhao, J. Li, Y. Song, C. Xie, H. Tao,
Y. Zhang, Y.
He, Y. Jiang and Y. Bian, "Research on an Mg-Zn alloy as a
degradable
biomaterial," Acta Biomaterialia, vol. 6, no. 2, pp. 626-640,
2010.
[20] Y. Z. Wan, G. Y. Xiong, H. L. Luo, F. He, Y. Huang and Y. L.
Wang,
"Influence of zinc ion implantation on surface nanomechanical
performance
and corrosion resistance of biomedical magnesium–calcium alloys,"
Applied
Surface Science, vol. 254, no. 17, pp. 5514-5516, 2008.
[21] G. L. Song, "The Effect of Texture on the Corrosion Behavior
of AZ31 Mg
Alloy," The Journal of The Minerals, Metals & Materials
Society, vol. 64, no.
6, pp. 671-679, 2012.
[22] A. D. Sudholz, K. Gusieva, X. B. Chen, B. C. Muddle, M. Gibson
and N.
Birbilis, "Electrochemical behaviour and corrosion of Mg–Y
alloys,"
Corrosion Science, vol. 53, no. 6, pp. 2277-2282, 2011.
[23] D. J. Horton, A. W. Zhu, J. R. Scully and M. Neurock ,
"Crystallographic
controlled dissolution and surface faceting in disordered
face-centered cubic
FePd," MRS Communications, vol. 4, no. 3, pp. 113-119, 2014.
[24] L. G. Bland, K. Gusieva and J. R. Scully, "Effect of
Crystallographic
Orientation on the Corrosion of Magnesium: Comparison of Film
Forming
and Bare Crystal Facets using Electrochemical Impedance and
Raman
Spectroscopy," Electrochimica Acta, vol. 227, p. 136 –151,
2017.
71
[25] G. L. Song and Z. Xu, "Effect of microstructure evolution on
corrosion of
different crystal surfaces of AZ31 Mg alloy in a chloride
containing solution,"
Corrosion Science, vol. 54, pp. 97-105, 2012.
[26] G. L. Song and Z. Xu, "Crystal orientation and electrochemical
corrosion of
polycrystalline Mg," Corrosion Science, vol. 63, pp. 100-112,
2012.
[27] P. C. Sharath, K. R. Udupa and G. P. Kumar, "Effect of Multi
Directional
Forging on the Microstructure and Mechanical Properties of Zn-24
wt% Al-
2 wt% Cu Alloy," Transactions of the Indian Institute of Metals,
vol. 70, no.
1, pp. 89-96, 2017.
[28] H. Miura, X. Yang and T. Sakai , "Evolution of Ultra-Fine
Grains in AZ31
and AZ61 Mg Alloys during Multidirectional Forging and Their
Properties,"
Materials Transaction, vol. 49, no. 5, pp. 1015-1020, 2008.
[29] S. P. Galkin, "Radial shear rolling as an optimal technology
for lean
production," Steel in Translation, vol. 44, no. 1, pp. 61-64,
2014.
[30] V. Sheremetyev, A. Kudryashova, S. Dubinskiy, S. Galkin , S.
Prokoshkin
and V. Brailovski, "Structure and functional properties of
metastable beta Ti-
18Zr-14Nb (at. %) alloy for biomedical applications subjected to
radial shear
rolling and thermomechanical treatment," Journal of Alloys and
Compounds,
vol. 737, pp. 678-683, 2018.
[31] T. K. Akopyan, A. S. Aleshchenko, N. A. Belov and S. P.
Galkin, "Effect of
Radial–Shear Rolling on the Formation of Structure and
Mechanical
Properties of Al–Ni and Al–Ca Aluminum–Matrix Composite Alloys
of
Eutectic Type," Physics of Metals and Metallography, vol. 119, no.
3, pp.
241-250, 2018.
[32] A. Bahmani and K. S. Shin, "Effects of Severe Plastic
Deformation on
Mechanical Properties and Corrosion Behavior of Magnesium Alloys,"
TMS
Annual Meeting & Exhibition, pp. 369-371, 2018.
72
[33] M. Diez, H. E. Kim, V. Serebryany, D. Dobatkin and Y. Estrin,
"Improving
the mechanical properties of pure magnesium by three-roll planetary
milling,"
Materials Science and Engineering: A, vol. 612, pp. 287-292,
2014.
[34] Y. Zheng, Y. Chao, Z. Luo, Y. Cai and R. Song, "Effect of
Multidirectional
Forging and Heat Treatment on Mechanical Properties of In
Situ
ZrB2p/6061Al Composites," High Temperature Materials and Processes,
pp.
1-10, 2017.
[35] Y. L. Wang, A. Molotnikov, M. Diez, R. Lapovok, H. E. Kim, J.
T. Wang
and Y. Estrin, "Graident structure produced by three roll planetary
milling:
Numerical simulation and microstructural observations," Materials
Science
& Engineering A, vol. 639, pp. 165-172, 2015.
[36] S. Arthanari, J. C. Jang and K. S. Shin, "Corrosion Behavior
of High Pressure
Die Cast Al-Ni and Al-Ni-Ca Alloys in 3.5% NaCl Solution,"
Corrosion
Science & Technology, vol. 16, no. 3, pp. 100-108, 2017.
[37] F. Cao, Z. Shi, G. L. Song, M. Liu and A. Atrens, "Corrosion
behaviour in
salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of
as-cast and
solution heat-treated binary Mg–X alloys: X = Mn, Sn, Ca, Zn, Al,
Zr, Si,
Sr," Corrosion Science, vol. 76, pp. 60-97, 2013.
[38] T. Zheng, Y. Hu and S. Yang, "Effect of grain size on the
electrochemical
behavior of pure magnesium anode," Journal of Magnesium and Alloys,
vol.
5, no. 4, pp. 404-411, 2017.
[39] M. Mabuchi, Y. Chino and H. Iwasaki, "Tensile Properties at
Room
Temperature to 823K of Mg-4Y-3RE Alloy," Materials Transactions,
vol. 43,
no. 8, pp. 2063-2068, 2002.
[40] A. C. Hänzi, A. S. Sologubenko, P. Gunde, M. Schinhammer and
P. J.
Uggowitzer, "Design considerations for achieving simultaneously
high-
strength and highly ductile magnesium alloys," Philosophical
Magazine
Letters , vol. 92, no. 9, pp. 417-427, 2012.
73
[41] N. Saunders, U. Z. Guo, X. Li, A. P. Miodownik and J. P.
Schillé, "Using
JMatPro to model materials properties and behavior," The Journal of
The
Minerals, Metals & Materials Society, vol. 55, no. 12, pp.
60-65, 2003.
[42] H. Yao, J. Wen, Y. Xiong, Y. Lu, F. Ren and Cao W, "Extrusion
temperature
impacts on biometallic Mg-2.0Zn-0.5Zr-3.0Gd (wt%) solid-solution
alloy,"
Journal of Alloys and Compounds, vol. 739, pp. 468-480, 2018.
[43] G. Williams, K. Gusieva and N. Birbilis, "Localized Corrosion
of Binary Mg-
Nd Alloys In Chloride Containing Electrolyte Using Scanning
Vibrating
Electrode Technique," Corrosion, vol. 68, no. 6, pp. 489-498,
2012.
[44] A. Bahmani , Development of High Strength and Corrosion
Resistant
Magnesium Alloys Using Severe Plastic Deformation Process, :
, 2018.
[45] J. W. Seong and W. J. Kim, "Mg-Ca binary alloy sheets with Ca
contents of
≤1 wt.% with high corrosion resistance and high toughness,"
Corrosion
Science, vol. 98, pp. 372-381, 2015.
[46] H. J. Fei, G. L. Xu, L. B. Liu, H. Bo, L. J. Zeng and C. P.
Chen, "Phase
equilibria in Mg-rich corner of Mg–Ca–RE (RE=Gd, Nd) systems at
400°C,"
Transactions of Nonferrous Metals Society of China, vol. 23, no. 4,
pp. 881-
888, 2013.
[47] P. Mao, B. Yu, Z. Liu, F. Wang and Y. Ju, "First-principles
calculations of
structural, elastic and electronic properties of AB2 type
intermetallics in Mg–
Zn–Ca–Cu alloy," Journal of Magnesium and Alloys , vol. 1, no. 3,
pp. 256-
262, 2013.
[48] N. Y. Yurchenko, N. D. Stepanov, G. A. Salishchev, L. L.
Rokhlin and S. V.
Dobatkin, "Effect of multiaxial forging on microstructure and
mechanical
properties of Mg-0.8Ca alloy," in IOP Conference Series: Materials
Science
and Engineering, 2014.
74
[49] G. K. Levy and E. Aghion, "Influence of Heat Treatment
Temperature on
Corrosion Characteristics of Biodegradable EW10X04 Mg Alloy Coated
with
Nd," Advanced Engineering Materials, vol. 18, no. 2, pp. 269-276,
2016.
[50] R. C. Zeng, W. C. Qi, H. Z. Cui, F. Zhang, S. Q. Li and E. H.
Han, "In vitro
corrosion of as-extruded Mg-Ca alloys: The influence of Ca
concentration,"
Corrosion Science, vol. 96, pp. 23-31, 2015.
[51] D. Li, V. Joshi, C. Lavender, M. Khaleel and S. Ahzi, "Yield
asymmetry
design of magnesium alloys by integrated computational
materials
engineering," Computational Materials Science, vol. 79, p. 448–455,
2013.
[52] H. Yu, Y. Xin, A. Chapius, X. Huang, R. Xin and Q. Liu, "The
different
effects of twin boundary and grain boundary on reducing
tension-
compression yield asymmetry of Mg alloys," Scientific Reports, vol.
6, pp.
1-8, 2016.
[53] D. L. Yin, J. T. Wang, J. Q. Liu and X. Zhao, "On
tension–compression yield
asymmetr y in an extrude d Mg–3Al–1Zn alloy," Journal of Alloys
and
Compounds, vol. 478, p. 789–795, 2009.
[54] A. E. Coy, F. Viejo, P. Skeldon and G. E. Thompson,
"Susceptibility of rare-
earth magnesium alloys to micro-galvanic corrosion," Corrosion
Science, vol.
52, pp. 3896-3906, 2010.
[55] X. Zhang, G. Yuan, L. Mao, J. Niu, P. Fu and W. Ding, "Effects
of extrusion
and heat treatment on the mechanical properties and biocorrosion
behaviors
of a Mg–Nd–Zn–Zr alloy," Journal of the Mechanical Behavior
of
Biomedical Materials, vol. 7, pp. 77-86, 2012.
75
Concentration
Fig. 31: Ionic concentration of Hanks, Simulated body fluid (SBF)
and Plasma in
terms of ionic concertation in millimoles per Litre (mmol/L)
76
7.2 Alloys Properties in Hanks
Fig. 32: Effect of alloying elements in as-cast and as-extruded
condition on
instantaneous corrosion rate (mm/y) in Hanks solution and
respective mechanical
properties (yield strength (YS, MPa), ultimate tensile strength
(UTS, MPa),
elongation (%))
77
7.3 Alloys Properties in SBF
Fig. 33: Effect of alloying elements in as-cast and as-extruded
condition on
instantaneous corrosion rate (mm/y) in SBF and respective
mechanical properties
(yield strength (YS, MPa), ultimate tensile strength (UTS, MPa),
elongation (%))
78
7.4 Alloys Properties in NaCl
Fig. 34: Effect of alloying elements in as-cast and as-extruded
condition on
instantaneous corrosion rate (mm/y) in NaCl and respective
mechanical properties
(yield strength (YS, MPa), ultimate tensile strength (UTS, MPa),
elongation (%))
79
(a)
(b)
Fig. 35: Most commonly studied extrusion ratios and temperature,
(b) extrusion
temperature effect on corrosion rate [38, 42, 55]
80
7.6 EBSD of Selected Alloys
Fig. 36: Inverted Pole Figures (IPZ) of selected alloys. Note the
twinning in screw
rolled (SR) samples
Chapter 1: Introduction
1.1.1. Alloying Elements
2.6 Microstructure & Phase Analysis
Chapter 3: Results & Discussion
3.1 Effect of Extrusion Temperature and ISPD on Properties of
Mg
3.1.1 Microstructures
3.1.4 Summary
3.2 Effect of Extrusion Temperature and ISPD on Properties of
ZX10
3.2.1 Microstructures
3.3.1 Microstructures
4.2 UTS vs Corrosion vs Extrusion Temperature
4.3 Yield Stress vs Yield Asymmetry vs Extrusion Temperature
4.4 Corrosion Analysis
Chapter 5: Conclusion
Chapter 6: References
7.2 Alloys Properties in Hanks
7.3 Alloys Properties in SBF
7.4 Alloys Properties in NaCl
7.5 Extrusion Conditions
7.7 Oxide Morphologies