12. - 14. 10. 2010, Olomouc, Czech Republic, EU

HIGH-TEMPERATURE DEFORMATION CHARACTERISTICS OF ECAP Al-BASED ALLOYS STABILIZED BY Sc+Zr

Přemysl MÁLEK, Kryštof TURBA, Miroslav CIESLAR

Charles University Prague, Faculty of Mathematics and Physics, Ke Karlovu 5, 12116 Prague 2, Czech Republic, malek@met.mff.cuni.cz

Abstract

Several Al-based alloys were prepared using equal-channel angular pressing. Their sub-microcrystalline

structure was stabilized by the addition of small amount of Sc + Zr. The paper compares high-temperature

deformation characteristics of these alloys, especially their potential to exhibit superplastic behaviour. The

strain rate sensitivity parameter m and ductility were determined as a function of deformation temperature

and strain rate. The deformation behaviour was correlated with the microstructure development. The atom

force microscopy was used to verify the operating deformation mechanism.

Keywords: Al-based alloys, sub-microcrystalline structure, ECAP, superplasticity

1. INTRODUCTION

Numerous Al-based alloys exhibit superplastic behaviour at elevated temperatures if prepared with the grain

size bellow 10 µm. The region of superplastic behaviour can be displaced either to much higher strain rates

or to lower temperatures if the grain size is further reduced, e.g. using the method of severe plastic

deformation. Equal-channel angular pressing (ECAP) is at present the most efficient method for the

processing of bulk materials with sub-microcrystalline structure.

Grain boundary sliding is considered as the main deformation process during superplastic forming and the

grain boundaries are the most important lattice defects in this case [1]. The high-temperature deformation

characteristics are influenced not only by the number of grain boundaries but also by their structure – only

high–angle boundaries can slide. Both the number and the structure of interfaces can be influenced by the

parameters of ECAP (number of ECAP passes, pressing temperature [2, 3]) and by the thermal treatment

after ECAP processing. In order to obtain the superplastic behaviour, the sub-microcrystalline structure with

a sufficiently high fraction of high-angle boundaries has to be retained at temperatures of superplastic

forming, i.e. the grain growth has to be suppressed or strongly limited. The addition of small amounts of Sc +

Zr is a very efficient method how to stabilize the sub-microcrystalline structure. Very fine coherent particles of

the Al3(ScxZr1-x) phase pin the grain boundaries and retard their migration [4]. These particles are very stable

and resisting against coarsening.

2. EXPERIMENTAL MATERIAL AND PROCEDURE

The chemical composition of the studied alloys is given in table 1. All materials were pressed through a die

consisting of 2 channels (cross section 14x14 mm at Al-Mg-based alloys and 10x10 mm at Al-7075- and Al-

Mn-based alloys) intersecting at an angle of 90o. The original length of the pressed samples was 140 and 70

mm, respectively. ECAP was performed at different temperatures (see table 1) with 6 to 8 passes using the

Bc rotation between subsequent passes. The rate of the crosshead displacement during ECAP was 5

mm/min.

12. - 14. 10. 2010, Olomouc, Czech Republic, EU

Composition in wt. % Mg Zn Mn Cu Sc Zr Fe+Si TECAP [oC]

Al-1.5 Mg-Sc-Zr 1.5 - - - 0.21 0.18 - 150

Al-4.5 Mg-Sc-Zr 4.5 - - - 0.2 0.2 - 250

Al-7075-Sc-Zr 2.5 5.9 0.17 1.3 0.2 0.11 0.7 120, 170

Al-Mn-Sc-Zr - - 1.35 - 0.27 0.23 0.1 20

Table 1: Chemical composition of investigated materials in wt. % and the corresponding ECAP temperatures

The microstructure was studied using transmission electron microscopy both in the as-pressed (after ECAP)

and in annealed states. The electron back-scatter diffraction (EBSD) method was used in selected materials

for the evaluation of the fraction of high-angle boundaries and for the control of the grain shape. The

deformation behaviour was studied using tensile tests in the temperature range between 300 and 525 oC.

Flat samples with the thickness of about 1 mm were cut from the pressed materials parallel to the ECAP

direction. The strain rate sensitivity parameter m defined as

. log / log m εσ ∂∂=

(σ represents the true stress and .ε the true strain rate) was evaluated by the strain rate change method.

The sample was pre-strained to 10 % of elongation at the strain rate of 10-3 s-1, afterwards the strain rate was

reduced to 10-5 s-1 and then gradually increased in small steps (the ratio of successive strain rates was from

1.4:1 to 1.7:1) up to the value of 10-1 s-1. For each temperature of straining, the strain rate dependence of the

parameter m over a broad strain rate range was obtained from the measurement on only one sample. The

optimum strain rate and temperature conditions for superplastic deformation were thus evaluated and used

for further tensile tests with a constant crosshead velocity in order to determine the values of ductility. Some

tensile samples were polished prior to high-temperature deformation and the deformation relief of samples

strained at superplastic conditions to relatively small elongations (20 to 40 %) was studied using atom force

microscopy (AFM) in order to find the main deformation mechanism and to verify the deformation

homogeneity.

3. EXPERIMENTAL RESULTS

Figure 1 documents the presence of the sub-microcrystalline structure after ECAP processing in all studied

materials. Both the individual recrystallized grains and strongly deformed regions can be found. Whereas the

structure of both Al-Mg-based alloys is nearly equiaxed, the Al-7075- and Al-Mn-based alloys exhibit

elongated bands of grains containing numerous low-angle boundaries oriented perpendicularly to the bands.

The finest grain size was observed in the Al-Mn-based alloy, the coarsest one in the Al-4.5 Mg based alloy.

This result does not probably reflect the influence of chemical or phase composition, but it results from the

different ECAP temperature (see table 1). Our previous experiments performed on the Al-7075-Sc-Zr alloy

revealed that an increase in the ECAP temperature resulted in a coarser grain size [3].

12. - 14. 10. 2010, Olomouc, Czech Republic, EU

Al-1.5 Mg-Sc-Zr, 6 passes at 150 oC Al-4.5 Mg-Sc-Zr 6 passes at 250 oC

Al-7075-Sc-Zr, 6 passes at 170 oC Al-Mn-Sc-Zr alloy, 8 passes at 20 oC

Fig. 1: Microstructure of the studied materials after ECAP processing

Figure 2 brings the strain rate dependences of the parameter m for all studied materials strained at different

temperatures. A common feature can be found in all materials – the maxima of the strain rate dependences

of the parameter m are located at strain rates of the order 10-2 s-1. This reflects the sub-microcrystalline

character of all materials. However, there are important differences between individual materials. The

maximum of m increases with increasing temperature at both Al-Mg-based alloys and reaches the values ≥

0.6. The position of this maximum remains at 10-2 s-1 even at 500 oC. This suggests a very good stability of

the fine-grained structure up to very high temperatures. The Al-7075-Sc-Zr exhibits the maximum m-values

slightly above 0.4 at 450 oC. Similar m-values were also observed at 500 oC, however, the curve is clearly

displaced to lower strain rates. Such displacement reflects the grain coarsening occurring above 450 oC.

Finally, the Al-Mn-Sc-Zr alloy exhibits the maximum values of m only slightly above 0.3. Such values

correspond to the bottom limit of superplastic behaviour. As the grain size remains fine (close to 1 µm at 400 oC and 2 µm at 500 oC) the lower fraction of high-angle boundaries might explain so bad superplastic

characteristics.

12. - 14. 10. 2010, Olomouc, Czech Republic, EU

523 K

573 K

673 K

723 K

.

Al-1.5 Mg-Sc-Zr

ε [s-1

]10-5 10-4 10-3 10-2

m

0,0

0,2

0,4

0,6

300 oC

400 oC

500 oC

Al-4.5 Mg-Sc-Zr

ε [s-1

]10-5 10-4 10-3 10-2

m

0,0

0,2

0,4

0,6

300 oC

400 oC

500 oC

Al-1.5 Mg-Sc-Zr, 6 passes at 150 oC Al-4.5 Mg-Sc-Zr 6 passes at 250 oC

523 K

573 K

673 K

723 K

.

Al-7075-Sc-Zr

ε [s-1

]10-5 10-4 10-3 10-2

m

0,0

0,2

0,4

0,6

400 oC

450 oC

500 oC

Al-Mn-Sc-Zr

ε [s-1

]10-5 10-4 10-3 10-2

m

0,0

0,2

0,4

0,6 400 oC

450 oC

500 oC

Al-7075-Sc-Zr, 6 passes at 170 oC Al-Mn-Sc-Zr alloy, 4 or 8 passes at 20 oC

Fig. 2: Strain rate dependence of the parameter m for all studied materials.

Alloy maximum ductility [%] temperature [oC] initial strain rate [s-1]

Al-1.5 Mg-Sc-Zr > 900 437 10-2

Al-4.5 Mg-Sc-Zr 2130 500 10-2

Al-7075-Sc-Zr 700 450 6.4x10-2

Al-Mn-Sc-Zr 350 500 2.10-2

Table 2: The maximum ductility values of ECAP Al-based alloys and corresponding straining conditions

It is well known from the literature on superplastic deformation (e.g. [1]) that the high values of the parameter

m usually correlate with high values of ductility. This tendency was also verified in our materials (see table 2

and fig. 3). The highest values of ductility were found in the Al-4.5 Mg-Sc-Zr where extremely high values

exceeding 2000 % of elongation were observed. The Al-7075-Sc-Zr alloy exhibits “medium” superplastic

12. - 14. 10. 2010, Olomouc, Czech Republic, EU

ductility values, however, these values are preserved up to very high strain rates of 10-1 s-1. The lowest

ductility of 350 % was observed in the Al-Mn-Sc-Zr alloy.

Al-4.5 Mg-Sc-Zr alloy Al-7075-Sc-Zr alloy

Fig. 3: The samples of selected ECAP Al-based alloys after superplastic deformation at various conditions

In order to explain the deformation characteristics of the studied materials the main deformation mechanism

has to be elucidated. The method of EBSD yields information on the size, shape, and especially

crystallographic orientation of grains. The structure of the Al-1.5 Mg-Sc-Zr alloy was studied on samples

strained under optimum superplastic conditions to 20 and 900 % of elongation. The comparison of pole

figures obtained from EBSD measurements revealed that the initial texture resulting from ECAP was

completely destroyed during superplastic straining to 900 % [5]. Additionally, the shape of grains remained

nearly equiaxed, i.e. the elongation of individual grains along the tensile axis was negligible in comparison

with the sample elongation. Such texture and grain shape development is typical for the process of grain

boundary sliding accompanied by grain rotations with no orientation relationship to the sample and with a

relatively small contribution of dislocation slip in the grain interiors.

The direct verification of the activity of grain

boundary sliding was obtained from AFM

experiments [5, 6]. The perfectly polished

samples of the Al-1.5 Mg-Sc-Zr alloy were

subjected to straining at optimum superplastic

conditions to various elongations. The

deformation relief revealed displacements of

neighboring grains along their common

boundaries and no slip lines in the grain

interiors. The size of individual grain

displacements was of the order of 100 nm

dependent on the elongation and straining

conditions. The maximum displacements were

close to the grain size.

elongation 20 % elongation 900 %

Fig. 4: EBSD image of the Al-1.5 Mg-Sc-Zr alloy

12. - 14. 10. 2010, Olomouc, Czech Republic, EU

It follows from the above mentioned experiments that

the grain boundary sliding is the main deformation

mechanism during high-temperature straining of ECAP

Al-based alloys. The differences in their deformation

characteristics should be thus explained on the basis

of different conditions for the development of grain

boundary sliding. The loss of superplastic

characteristics of the Al-7075-Sc-Zr alloy at high

temperatures results from the grain coarsening. The

temperature at which the grain coarsening starts

depends on the amount of deformation energy stored

during ECAP. Lower ECAP temperature leads to

larger stored deformation energy and, therefore, to

larger driving force for grain coarsening which then

occurs at lower temperatures.

Fig. 5: The Al-Mn-Sc-Zr alloy strained at 500 oC

Another important feature is the presence of coarse particles which can be located at grain boundaries and

retard their sliding. Simultaneously, these particles can serve as places where grain boundary sliding cannot

be accommodated with a sufficient rate and where cavities are nucleated. This is the case of the of the Al-

7075-Sc-Zr alloy. Similar situation occurs in the Al-Mn-Sc-Zr alloy (fig. 5) where numerous coarse particles

are located at grain boundaries.

4. CONCLUSIONS

• All ECAP Al-based alloys exhibit superplastic behaviour at elevated temperatures and strain rates of the

order of 10-2 s-1.

• The difference in the superplastic characteristics are connected with the conditions for grain boundary

sliding – number of grain boundaries and the presence of coarse second phase particles at boundaries.

ACKNOWLEDGEMENT

The work was supported by the grant of the GACR N. 106/07/0303.

LITERATURE

[1] EDINGTON, J. W., MELTON, K. N., CUTLER, C.P. Superplasticity. Prog. Mater. Sci., 1976, V. 21, p. 61-158.

[2] MÁLEK, P., CIESLAR, M., OČENÁŠEK, V. Deformation behaviour of the Al-Mn-Sc-Zr alloy produced using ECAP. In Metal

2010. 18. – 20.5.2010 Rožnov pod Radhoštěm, TANGER: May 2010.

[3] TURBA, K. et all. The optimization of ECAP conditions to achieve high strain-rate superplasticity in a Zr- and Sc-modified AA

7075 aluminum alloy. Int. J. Mat. Res. 2009, V. 100, p. 851-857.

[4] LEE, S. ET ALL. Influence of scandium and zirconium on grain size stability and superplastic ductilities in ultrafine-grained Al-

Mg alloys. Acta Mater. 2002, V. 50, p. 553-564.

[5] MÁLEK, P. et al. Structure development during superplastic deformation of an Al-Mg-Sc-Zr alloy. Mater. Sci. Eng. A, 2007, V.

462, p. 95-99.

[6] MÁLEK, P. et al. Superplastic behaviour of the Al-MgSc-Zr alloy processed by ECAP. Kovove Mater., 2005, V. 43, p. 245-257.