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Microstructural evolution and grain growth kinetics of hot rolled GZ31magnesium alloy
M. Roostaei, M. Shirdel, M.H. Parsa, R. Mahmudi, H. Mirzadeh
PII: S1044-5803(15)30038-3DOI: doi: 10.1016/j.matchar.2015.10.035Reference: MTL 8080
To appear in: Materials Characterization
Received date: 10 June 2015Revised date: 7 August 2015Accepted date: 28 October 2015
Please cite this article as: Roostaei M, Shirdel M, Parsa MH, Mahmudi R, Mirzadeh H,Microstructural evolution and grain growth kinetics of hot rolled GZ31 magnesium alloy,Materials Characterization (2015), doi: 10.1016/j.matchar.2015.10.035
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Microstructural evolution and grain growth kinetics of hot rolled
GZ31 magnesium alloy
M. Roostaeia, M. Shirdel
a*, M. H. Parsa
a,b,c, , R. Mahmudi
a,b, and H. Mirzadeh
a
a School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box 11155–
4563, Tehran, Iran
b Center of Excellence for High Performance Materials, School of Metallurgy and Materials Engineering,
University of Tehran, Tehran, Iran
c Advanced Metalforming and Thermomechanical Processing Laboratory, School of Metallurgy and Materials
Engineering, University of Tehran, Tehran, Iran
Abstract
Grain growth behavior of Mg–3Gd–1Zn (GZ31) magnesium alloy was studied in a wide range of
annealing times and temperatures to clarify the kinetics of grain growth, microstructural
evolution and related metallurgical phenomena. This material exhibited typical normal grain
growth (NGG) behavior below annealing temperatures of 300 °C and soaking times of up to 240
min. However, the abnormality in grain growth was evident at annealing temperatures of 400 °C
and 500 °C. The dependence of abnormal grain growth (AGG) at these annealing temperatures
upon microstructural features such as dispersed precipitates, which were rich in Zn and Gd, was
investigated by optical micrographs, X-ray diffraction patterns, scanning electron microscopy
images, and energy dispersive X-ray analysis spectra. The bimodality in the grain-size
distribution histograms signified the occurrence of AGG. Finally, based on the experimental
grain growth data, the grain growth exponent and the activation energy were figured out.
Keywords: Grain growth; magnesium alloy, Rare earth elements; activation energy.
* Corresponding author
E-mail addresses: [email protected] (M.shirdel), [email protected] (M. Roustaei),
[email protected] (H. Mirzadeh), [email protected] (M.H. Parsa), [email protected] (R. Mahmudi).
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1. Introduction
Nowadays, the application of light magnesium alloys has received a remarkable attention
in automotive and aerospace industries [1–2]. However, their poor formability due to the
hexagonal close-packed (hcp) crystal structure limits their processing and applications [3].
Additionally, the strong anisotropy induced by metal forming adversely influences their
formability [4–5]. The experiments have demonstrated that the addition of rare-earth metals such
as Gd, Y, Ce, Nd to magnesium alloys weakens the basal texture and the corresponding
anisotropy [6–7]. It has been suggested that the particle-stimulated nucleation during
recrystallization process contributes to the development of a weaker and more random texture in
Mg alloys [8–9]. Moreover, the typical thermomechanical processing involving recrystallization
is shown to be effective in ductility enhancement of Mg alloys [10]. Therefore, a refined
microstructure of Mg alloys containing rare-earth metals could be an excellent means to meet the
ductility requirements, for which the control of the grain growth after completion of
recrystallization is a crucial step.
Grain growth is affected by microstructural features such as morphology of grains,
presence of second-phase particles, and texture [11]. In this regard, two grain growth modes have
been recognized: (I) normal grain growth (NGG) and (II) abnormal grain growth (AGG) [12].
There are several criteria to distinguish between these grain growth modes. For instance, it has
been shown by Rios [13] and confirmed by Shirdel et. al [11] that when the relative size of the
candidate grain for abnormal growth (DAbnorm) increases, in such a way
that 0/)/( dtDDd averageAbnorm , the abnormal grain growth occurs. Recent studies on the thermal
stability and grain growth of Mg–Gd alloys have shown that they can offer a high level of
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resistance to grain growth, caused by the presence of different precipitates such as Mg24Gd5,
Mg5Gd, and Mg3Gd [14–15]. It has also been reported that additions of only 0.3–0.7 Gd to an
Mg–2Zn alloy suppressed grain growth during the annealing process [16]. More recently, a
comprehensive study of the grain growth kinetics in an extruded Mg–Gd–Y–Zr alloy has been
carried out by static annealing in the temperature range of 673–773 K [17]. It has been stated that
although the Mg3(Gd,Y) precipitates have sufficient thermal stability to restrict grain growth,
significant grain growth occurs at the highest temperatures where increased mobility of the grain
boundaries seems to be dominant. However, there is restricted growth at lower temperatures,
where the grain growth is controlled by lattice self-diffusion.
This work examines the microstructural evolution of the newly developed Mg–3Gd–1Zn
magnesium alloy [18–19] along with its grain growth kinetics during annealing in a wide range
of soaking times and temperatures.
2. Experimental materials and procedures
The GZ31 (Mg–3Gd–1Zn) alloy was prepared from high purity Mg, Zn, and an Mg–
30Gd master alloy, which were melted in an electric furnace under the Foseco MAGREX 36
covering flux to protect molten magnesium from oxidation. The rolling process started with an
initial reduction of 5% and the final reduction was 15% resulting in a total reduction of 85%.
After each pass, the rolling specimens were reheated to 460◦C and held for 150 min to maintain a
consistent rolling temperature.
The homogenization and hot rolling operations were used to refine the cast
microstructure. Rectangular strips, 51010 tWL mm3, were cut off and submitted to heat
treatments at temperatures of 200 °C, 300 °C, 400 °C, and 500 °C for 15, 30, 60, 120, 240 min to
systematically examine the microstructural evolution. The XRD analyses were performed using a
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Philips PW-3710 diffractometer with Cu Kα radiation. The microstructural observations were
carried out by optical microscopy (OM) and scanning electron microscopey (SEM) after etching
the quenched samples in a solution containing 100 ml ethanol, 2.5g picric acid, 25 ml acetic acid
and 25 ml water. Grain size measurements were conducted according to ASTM E-112–82, where
about 500 grains were considered for each grain size measurement. It is worth noting that at the
annealing temperature of 500 °C and soaking times of 120 and 240 min, because of the large size
of the grains, calculations were limited to about 200 grains. The Vickers microhardness
measurements were carried out by applying a load of 0.2 kg with dwell time of 15 s.
3. Results and discussion
3.1. The as-received state
The optical micrograph of the studied alloy in the as-cast condition is shown in Fig. 1a,
which represents a typical dendritic microstructure. Fig. 1b depicts the microstructure of the
investigated GZ31 alloy after homogenization at 500 °C for 10 h and the subsequent hot rolling.
As can be seen, the microstructure consists of equiaxed grains with an average grain size ( D ) of
14 μm. Also, the XRD pattern of this microstructure is shown in Fig. 1c, in which most peaks
correspond to the -Mg matrix and only one weak peak is indicative of the GdMg5 phase.
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3.2. Annealing treatment at 200 °C
Fig. 2 shows the microstructural evolution during annealing at 200 °C for soaking times
of 15, 30, 60, 120, 240 min. The corresponding grain size distribution histograms have been
shown below each micrograph. It is evident that the normal grain growth behavior is dominant
for all annealing conditions. Some twins can also be observed after annealing at 200 °C. In Mg
alloys, mechanical twinning may result during cutting, grinding, or handling if pressures are
excessive. For a straightforward comparison between grain growth of the investigated alloy and
other data of Mg alloys presented in the literature, the grain growth parameter (PG) defined as
00 /)( DDDPG , where D and 0D are, respectively, the grain sizes after each specific
annealing condition and the initial grain size, were used. The maximum value of GP for
annealing temperature of 200 °C and equals 2.29 and the histograms clearly tend to coarser grain
size ranges by continued annealing.
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3.3. Annealing treatment at 300 °C
Fig. 3 illustrates the evolution of grain growth at the annealing temperature of 300 °C for
soaking times of 15, 30, 60, 120, 240 min. The corresponding grain size distribution histogram of
each annealing condition can be seen below the corresponding optical micrographs. The
occurrence of NGG is evident for all annealing times. The GP parameter for the studied alloy and
other data present in the literature is shown in Table 1 [20–21]. As can be seen, the GP values of
GZ31 are smaller than those of AZ31. This can be attributed to the smaller initial grain size of
the AZ31 alloy and the low diffusion coefficient of Gd in the Mg matrix.
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Alloys
Values
GZ31 (present study) AZ31[20] AZ31B[21]
0D 14 μm 5 μm 8.2 μm
GP (15 min) 1 2 -
GP (30 min) 1.47 2.6 -
GP (60 min) 2.35 3.4 -
GP (120 min) 3.42 4 -
GP (240 min) 4.5 - 0.13
It should also be noted that the shown micrograph of specimen annealed for 240 min is
composed of two micrographs to illustrate the grain boundaries more clearly. An interesting
point regarding annealing at 300 °C is appearing of some darker grains. The experiments at this
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temperature were repeated two times and the same results were observed. As a result, the
available characterization tools were deployed to reveal the probable metallurgical reasons
behind it. Fig. 4 shows the XRD patterns of the samples annealed at 300 °C for above-mentioned
soaking times. As can be seen, except for the change in the peak intensities, no other major
differences could be observed. However, for sample annealed for 240 min, three peaks of
precipitate Mg3Gd2Zn3 were detected. By careful inspection of the microstructure of the sample
annealed for 240 min presented in Fig. 3, a few numbers of large grains (about 1.8 times larger
than the average grain size) are observable. The presence of these large grains may be ascribed to
the preferential pinning of some grain boundaries by detected precipitates. To our knowledge
[11], this could be related to the onset of abnormal grain growth. In other words, by continued
annealing at 300 °C, the abnormal grain growth would occur.
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The SEM micrograph of the sample annealed at 300 °C for 240 min along with the
corresponding EDAX spectra are presented in Fig. 5. As can be observed in Fig. 5a, a relatively
random distribution of precipitates exists in the matrix. However, some arrays of very fine
precipitates can be readily seen along the grain boundaries. Previous investigations reported that
the large difference in atomic size between Gd (2.54 Å) and Mg (1.72 Å) atoms results in a high
tendency for Gd atoms to segregate toward grain boundaries [22-25]. Recently, Bugnet et al. [26]
have reported a considerable segregation of Gd solute atoms at high-angle GBs in addition to the
quasi-random distribution of the same element in the Mg matrix of the Mg–0.28 at.% Gd binary
alloy. Moreover, a negligible and limited amount of Gd-rich precipitates was reported in their
investigation. However, in the case of GZ31 alloy, a considerable amount of Gd-rich precipitates
are formed during annealing at 300 °C for 240 min. An important observation about precipitation
in the studied alloy at 300 °C for 240 min is the formation of parallel precipitation rows in
specific orientation within each grain. These rows are shown by yellow arrows. Returning to the
discussion about appearance of some darker grains, the line EDAX analysis was also performed
across various positions. A typical example of this analysis is shown in Fig. 5b. As can be seen,
no considerable changes in chemical composition can be observed. However, it may be ascribed
to the employed etchant, which results in different contrast for differently oriented grains.
Therefore, the existence of such darker grains may be related to another metallurgical
phenomenon and more rigorous characterization methods would be helpful to elucidate this
interesting finding.
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3.4. Annealing treatment at 400 °C
The optical micrographs of samples annealed at 400 °C for 15, 30, 60, 120, and 240 min
with associated grain size distribution histograms are shown in Fig. 6. The PG values of the
studied alloy, AZ31 alloy [20], and AZ31B alloy [21] obtained from annealing at 400 °C are
summarized in Table 2. As seen, a significant difference exists in grain growth behavior of AZ31
alloy and AZ31B alloy. For instance, the required soaking time for achievement of average grain
size of 40 μm were reported to be 60 min and 10000 min for AZ31 alloy and AZ31B alloy,
respectively. However, the grain growth data obtained in the current study are more close to that
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reported for AZ31 alloy. Based on the results of Fig. 6, annealing at 400 °C for 240 min has led
to occurrence of AGG. In this case, the ratio of Dmax/Daverage was determined to be ~ 4.8.
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Alloys
Values
GZ31 (present study) AZ31[20] AZ31B[21]
0D 14 μm 5 μm 11 μm
GP (15 min) 1.4 4.8 -
GP (30 min) 2.25 6 -
GP (60 min) 3.39 7 -
GP (120 min) 4.8 8.6 0.45
GP (240 min) 6.8 - 0.63
3.4. Annealing treatment at 500 °C
Fig. 7 illustrates the microstructural evolution of GZ31 alloy during soaking at annealing
temperature of 500 °C. As seen, the grain size distribution histograms have been changed
significantly by increasing soaking time, in which a transition of NGG to AGG can be easily
deduced for annealing time of 120 min by the appearance of a bimodal distribution. Furthermore,
a few large grains are observable in optical micrograph of sample annealed for 15 min. As seen,
these larger grains are not presented in microstructure of sample annealed for 30 min. This
observation shows that a temporary transition of AGG to NGG was occurred. This transient
behavior has been demonstrated in many experimental works as well as in simulation studies
[28-30].
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Many previous studies have shown that the impurity-based particles could restrict the
grain boundaries motion and induce AGG. Hereby, the sample annealed for 240 min has been
chosen for more detailed experiments. Fig. 8 shows the SEM micrograph of this sample along
with the EDAX spectra to characterize the type of the elements present in the precipitate which
pinned the shown boundary. Based on the results of Fig. 8b, the matrix has the relatively same
chemical composition in comparison with initial one. Also, the EDAX analysis of Fig. 8c reveals
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that the second phase particles pinned the grain boundaries are rich in Gd and Zn. Repetition of
such analysis on many second phase particles has yielded the similar results.
Therefore, by consideration of the lower initial amount of Zn in the studied alloy, it can
be concluded that the contribution of Zn element in precipitation of second phase particles is
more noticeable than that of Gd in GZ31 alloy. In this respect, the calculation of the impurity
diffusion coefficient ( diffD ) might be useful to elucidate the metallurgical phenomenon behind it.
Impurity diffusion coefficients for Gd in hcp Mg are compared with those of Zn in Fig. 9. For
better comparison, the self-diffusion coefficient of Mg is also plotted. Previous investigations
have shown the anisotropic diffusion behaviors in MG alloys. So, the D and D stand for
perpendicular to c-axis and along c-axis, respectively. The following relations are used for
calculation of anisotropic impurity diffusion coefficients [28–29]:
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)268/RT 79exp(1027.1 9
hcpMg
GdD
)/RT16878exp(1079.1 9 hcpMg
IIGdD
)/RT379791exp(1075.1 4
hcpMg
MgD
)/RT138943exp(1078.1 4 hcpMg
IIMgD
)/RT327251exp(1098.4 5
hcpMg
ZnD
)/RT135488exp(1033.7 5 hcpMg
IIZnD
Briefly, the diffusion coefficients of Zn is bigger than those of Gd. Low diffusion
coefficients for Gd can be ascribed to the large atomic size of Gd compared to those of Mg and
Zn. Such conclusions are in good agreement with those reported in the literature [31–32].
3.5. Grain growth kinetics
The Grain size variations against annealing time at different temperatures are depicted in
Fig. 10a. Obviously, increasing annealing temperature has more substantial influence on grain
growth behavior of GZ31 alloy. The empirical observations and the theoretical treatments
generally support a parabolic equation of the form )/exp(00 RTQtkDD GG
nn , where D is the
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average grain size, D0 is the average initial grain size, t is the annealing time, T is the absolute
annealing temperature, and R is the universal gas constant. Additionally, n, K0, and QGG are the
grain growth exponent, the proportionally constant, and the activation energy of grain growth,
respectively [30]. This law can be expressed as KtDD nn 0. Taking natural logarithm from
both sides of that equation results in tKDD nn lnln)ln( 0 . The linear solution of this
equation needs to neglect D0. To overcome this difficulty, a nonlinear regression was employed
by considering D0. The equation tKDD nn lnln)ln( 0 as a nonlinear function with respect to
D was used in plots of lnt vs. D to determine the ultimate values of n and K. The results are
shown in Fig. 10b. Now, drawing the plot of ln (k) against 1/T, as shown in Fig. 10c, enable us to
work out the value of GGQ for GZ31 alloy as ~ 101 kJ/mol. It should be noted that the activation
energy for grain boundary diffusion and bulk diffusion in Mg have been reported to be 92 kJ/mol
and 135 kJ/mol, respectively. According to Gottstein et al. [33], the general observations
demonstrated that the GGQ of metals falls between the activation energy for the bulk and the
grain boundary diffusion.
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3.6. Grain growth map
To extend the present grain growth results to a more exhaustive time and temperature
range, the contour lines of PG against the homologous temperature (T/Tm) and annealing time are
shown in Fig. 11. The synergic effect of raising annealing time and temperature on quick grain
growth can be readily seen in this map. The developed map can be used as a guide for prediction
of final grain size for a wide range of annealing conditions.
3.7. Grain size dependence of mechanical properties
For polycrystalline materials, the hardness (or the strength) is related to the average grain size
using the well-known Hall-Petch relationship [34]:
2/1
0
avgv kDHH
This relationship has been confirmed in both dislocation theory and experiment in many metallic
materials. Figure 12 illustrates the dependence of the Vickers hardness on the grain size for the
GZ31 alloy. As shown, the Hall–Petch type relationship is valid for a wide range of grain sizes.
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This also shows the magnificent effect of grain growth on the mechanical properties, which
signifies the importance of the developed grain growth map in controlling the properties of this
new rare-earth containing Mg alloy.
4. Conclusions
The following conclusions can be drawn from the study on the microstructural evolution
and grain growth kinetics of GZ31 magnesium alloy in a wide range of annealing temperatures
and times:
(1) The grain growth mode of GZ31 magnesium alloy at annealing temperatures of 200 °C and
300 °C was identified as normal for the annealing times considered in the current work.
(2) At annealing conditions of (400 °C/240min) and (500 °C/120min), a transition in grain
growth mode from normal to abnormal was detected to be caused by the impurity-based particles
that restrict the grain boundaries motion and induce AGG. This was demonstrated by the
appearance of a bimodal grain size distribution histogram.
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(3) It was shown that the contribution of Zn element in precipitation of second phase particles is
more noticeable than that of Gd in GZ31 magnesium alloy.
(4) The activation energy for grain growth of the GZ31 alloy was obtained to be 101 kJ/mol,
which falls between the activation energy for the bulk and the grain boundary diffusion.
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Figure Captions
Fig. 1: Optical micrograph of the alloy a) as cast and b) after homogenization at 500 °C for 10 h
and subsequent hot rolling, and c) its corresponding XRD patterns.
Fig. 2: Optical micrographs and the corresponding grain-size distribution histograms obtained
upon annealing at 200 °C.
Fig. 3: Optical micrographs and the corresponding grain-size distribution histograms obtained
upon annealing at 300 °C.
Fig. 4: XRD patterns of the samples annealed at 300 °C for a) 15 min b) 60 min c) 120 min d)
240 min.
Fig. 5: The analyses performed on the sample annealed at 300 °C for 240 min: (a) SEM image,
(b) corresponding line EDAX spectra.
Fig. 6: Optical micrographs and the corresponding grain-size distribution histograms obtained
upon annealing at 400 °C.
Fig. 7: Optical micrographs and the corresponding grain-size distribution histograms obtained
upon annealing at 500 °C.
Fig. 8: The analyses performed on the sample annealed at 500 °C for 240 min: (a) SEM image,
(b) EDAX spectra of matrix and (c) EDAX spectra of a typical precipitate.
Fig. 9: Impurity diffusion coefficients of Gd and Zn in hcp Mg besides self-diffusion coefficient
of Mg.
Fig. 10: The curves of a) intercept length versus annealing time, b) Ln (t) versus intercept length
and c) Ln (K) versus 1/T for GZ31 alloy.
Fig. 11: The grain growth map for GZ31 alloy represented by the contour lines of grain growth
parameter as a function of annealing temperature and time.
Fig. 12: The Hall–Petch plots representing the hardness versus grain size.
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Table Captions
Table 1: The magnitudes of GP parameter for the studied alloy, AZ31, and AZ31B alloys at 300
°C.
Table 2: The magnitudes of GP parameter for the studied alloy, AZ31, and AZ31B alloys at 400
°C.
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Graphical abstract
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Research Highlights
Abnormal grain growth (secondary recrystallization) in GZ31 magnesium alloy
A transition in grain growth mode from normal to abnormal at annealing conditions of
(400 °C/240min) and (500 °C/120min)
The more contribution of Zn element in precipitation of second phase particles than that
of Gd in GZ31 magnesium alloy
The activation energy for grain growth of the GZ31 alloy was obtained to be 101 kJ/mol
