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Effect of microalloying with rare-earth elements on the texture of extruded magnesium-based alloys
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Scripta Materialia 59 (2008) 772–775 www.elsevier.com/locate/scriptamat
Effect of microalloying with rare-earth elements on the texture of extruded magnesium-based alloys N. Stanford,a,* D. Atwell,b A. Beer,b C. Daviesc and M.R. Barnetta a
ARC Centre of Excellence for Design in Light Metals, Deakin University, Geelong, Vic. 3217, Australia b CRC for CAST Metals Manufacturing, Deakin University, Vic. 3217, Australia c ARC Centre of Excellence for Design in Light Metals, Monash University, Vic., 3168, Australia Received 23 May 2008; accepted 10 June 2008 Available online 18 June 2008
A series of alloys have been produced with microalloying additions of rare-earth (RE) elements in the range of 0.1–0.4 wt.%. The alloys have been extruded to produce grain sizes of 23 ± 5 lm. The texture of the extruded alloys was measured, and it was found that the extrusion texture was weakened by the addition of RE elements. The samples with weakened extrusion textures exhibited an increase in the tensile elongation. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium alloy; Texture; Microalloying; Ductility; Rare earth
The limited room temperature formability of Mg alloys is a hindrance to the wider uptake of this metal in applications that would benefit considerably from its low density. The sharp texture typically seen in wrought products plays a large role in this effect [1]. Amongst the various approaches that have been tried to weaken the texture, the addition of rare-earth (RE) alloying elements is quite promising [1,2]. Indeed, the benefits of rare-earth ‘‘microalloying” additions for cold formability (rolling in particular) have been long known [3–5]. Recent studies have shown that the texture weakening effect is most likely a consequence of the influence of RE addition on recrystallization [1] but the mechanism is poorly understood. It is also evident that the benefit is seen in both extrusions and in rolled material [1,6,7]. However, the relations between RE level, texture and ductility have yet to be fully elucidated. The present communication quantifies the impact of varying RE levels (Ce and/or La) on the texture sharpness and mechanical response of extruded rod. The alloys were based on the commercial alloy M1 (nominally Mg–1 wt.% Mn) to which RE elements were added in the form of a cerium-rich misch metal, containing approximately 60% Ce, with the balance predominantly lanthanum and minor weight percentages of
* Corresponding author. E-mail: [email protected]
other RE elements. The total RE content was varied from 0.1 to 0.4 wt.%. AZ31 (nominally Mg—3 wt.% Al—1 wt.% Zn—0.3 wt.% Mn) was used as a benchmark. Alloys were cast in 2 kg batches under a shielding gas, and the billets were homogenized and extruded. The speed and temperature of extrusion was such that each alloy exhibited a fully recrystallized grain size of 23 ± 5 lm (see Table 1 for further details). Three tensile specimens of 3 mm diameter and 12 mm gauge length were machined from each alloy extrudate. These tensile specimens were then tested to failure at room temperature using an MTS 100 kN tensile tester. A section of the extrudate from each alloy was then examined by X-ray diffraction to measure the extrusion texture. The texture was measured with a GBC-Mini Materials Analyser texture goniometer equipped with Cu Ka radiation and a polycapillary beam enhancer. The texture data was examined in the form of inverse pole figures, and three typical examples are shown in Figure 1. The Mn-containing alloys and AZ31 had the form shown in Figure 1a, in which the extrusion direction was centred around ½1010. This is a typical extrusion texture for Mg-based alloys. Figure 1b shows the extrusion texture for alloy MEZ1.2.5. This texture is typical of those alloys containing RE elements in which the texture is weakened and spread towards ½11 20. At the higher alloying level of 0.4% RE, a second texture peak at approximately ½1121 parallel to the extrusion direction is evident in the inverse pole figure shown in
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.06.008
N. Stanford et al. / Scripta Materialia 59 (2008) 772–775
773
Table 1. Compositions and extrusion conditions for the alloys examined in this study Alloy
Al
Zn
Mn
RE
Billet temperature (°C)
Vr (mm s1)
Grain size (lm)
Strain rate
Z
Ductility
AZ31 M1 M1 ME1.3 MEZ1.1.5 ME1.2 ME1.4 MEZ1.2.5 ME1.2
2.87 0 0 0 0 0 0.01 0.01 0.01
0.83 0.02 0.02 0 0.5 0 0 0.55 0
0.45 1.62 1.62 1.0 1.0 1.0 0.98 0.95 0.95
0 0 0 0.3 0.1 0.21 0.4 0.19 0.19
530 375 499 450 450 450 450 450 450
2 20 20 15 15 15 15 15 15
26.9 23.3 27.7 18.7 24.6 26.9 23.3 23.5 22.3
5.0 50.1 48.9 37.6 37.6 37.6 37.6 37.6 37.6
6.41E + 09 9.67E + 12 1.45E + 11 4.90E + 11 4.90E + 11 4.90E + 11 4.90E + 11 4.90E + 11 4.90E + 11
11.3 6.0 6.0 14.0 18.6 17.4 17.9 16.6 15.7
Vr, ram speed during extrusion. Z, Zener Hollomon parameter, assuming 140 kJ mol1 activation energy. All compositions shown are in weight percentage. RE, rare-earth elements (a mixture of Ce and La, see text for further details).
Figure 2. Strength of the ½1010 texture peak in each of the alloys investigated. The square data point represents AZ31.
Figure 1. Inverse pole figures of the extrusion texture measured using X-ray diffraction. RE, rare-earth content. Inverse pole figures refer to the extrusion direction.
Figure 1c. It appears that the strength of this second peak, compared to the peak at ½10 10 increases with increasing RE alloy content. However, in all cases this second peak was less intense than the main extrusion texture component at ½10 10. If we examine the strength of the texture peak at ½10 10, it is apparent that the RE-containing alloys have a weaker texture than those without. This becomes clear when the data is plotted as a function of RE content (Fig. 2). This clearly demonstrates that very small microalloying additions to Mg-based alloys can significantly alter the strength of the extrusion texture. The effect of texture strength on the measured room temperature ductility of the alloys investigated is shown in Figure 3. It can be seen from Figure 3 that there is a trend of increasing ductility with decreasing texture
strength. The ductility of Mg alloys is well known to depend on the grain size, with the ductility increasing with decreasing grain size [8]. This has been attributed in the past to decreased twinning activity in smaller grain sized samples [9]. The quantitative effect of a grain size change on the ductility will depend on the texture of the alloy and the direction in which the deformation is applied with respect to this texture. Therefore, although an approximate number can be determined from literature (e.g. [8]), a closer approximation was determined for the present alloys (Fig. 3b). In this figure, data is shown for a number of samples with different grain sizes that were produced by extruding under different conditions. From the line of best fit shown, it was determined that the ductility decreases somewhere in the order of 0.2% per micron increase in grain size, although this relationship only holds for the grain size range examined here. This estimate was used to normalize the ductility data with respect to the grain size variation from the median of 23 lm (Fig. 3c). The reasons for the improved ductility with a weaker texture have already been discussed in a previous paper by two of the authors [6], but will briefly be described here. The principle deformation mechanism in Mg is basal slip. The extrusion texture developed in Mg alloys is such that the basal poles are aligned perpendicular to the extrusion direction. This texture results in a majority of the grains being poorly aligned for basal slip during
774
N. Stanford et al. / Scripta Materialia 59 (2008) 772–775
Figure 3. Effect of the texture strength on ductility. See text for further details on the normalized ductility values. The square data points in (a) and (c) represent AZ31.
tensile deformation in the extrusion direction. However, in the case of those alloys with RE addition, the strength of the texture is weaker. The broader spread in orientations results in there being more grains better aligned for basal slip in that case. This is evident in the yield point data for the alloys as shown in Figure 4, where it can be seen that decreasing the strength of the texture also reduces the yield point. Polycrystal plasticity simulations were used to examine if the changes in the mechanical properties resulting from RE addition could be attributed to the different textures that are developed. The calculations were made using a viscoplastic self-consisted model that has been
described in Ref. [10]. The values used for parameters such as the critical resolved shear stress for slip and the Voce hardening law parameters are given in Table 2. Two idealized textures were created for the simulation, one with a conventional extrusion texture of 6.5 times random intensity. The other texture had an overall strength of 1.5 times random intensity and comprised two texture components, one at ½1010 and another at ½1121 parallel to the extrusion direction. These two textures are shown as inverse pole figures in Figure 4. The stress–strain curves that result from simulation of tensile deformation of the two textures are also shown in Figure 4. It can be seen that the weaker texture exhibits a
Figure 4. (a) Effect of texture strength on the measured tensile yield point of the alloys examined. (b) Model predictions for the stress–strain behaviour of two idealized textures as shown. Inverse pole figures refer to the extrusion and tensile directions.
N. Stanford et al. / Scripta Materialia 59 (2008) 772–775 Table 2. Voce hardening parameters used for the simulations shown in Figure 4 Deformation mode
s0
s1
h0
h1
Basal slip Prismatic slip hc + aislip 10 12 twin
10 55 60 30
30 80 90 0
100 500 1500 30
0 0 0 30
Values are taken directly from Ref. [11], in which further details regarding these parameters can be found.
lower yield point, and that the stress at higher strains is consistently predicted to be lower than for the conventional extrusion texture. Compared to the measured yield points, the simulation has underpredicted the yield point of both textures. However, the difference in yield point of about 50 MPa between the two textures is consistent with the line of best fit in Figure 4a, showing approximately the same gradient. It therefore appears that the decreased yield point and increased ductility of the RE-containing alloys can indeed be attributed to texture modification. Another interesting result from the simulations was the marked difference in the predicted twin fraction for the two textures. The sharp extrusion texture was predicted to have 4% twin fraction after a tensile strain of 0.15. After the same strain, the weaker texture is pre-
775
dicted to have a twin fraction of 14%. This is consistent with previous observations of RE-containing alloys that have shown significantly higher twin fractions compared to alloys that do not contain RE additions [6]. The work described in this paper was funded jointly by the Centre of Excellence for Design in Light Metals and the CAST CRC. The VPSC5 code used in this work was developed by R.A. Lebensohn and C.N. Tome, and we gratefully acknowledge their generous distribution of the programme to our research group. [1] J. Bohlen, M.R. Nurnberg, J.W. Senn, D. Letzig, S.R. Agnew, Acta Mater. 55 (2007) 2101. [2] E.A. Ball, B. Prangnell, Scripta Metall. Mater. 31 (1994) 111. [3] S.L. Couling, J.F. Pashak, L. Sturkey, Trans. ASM 51 (1959) 94. [4] J.C. McDonald, Trans. Metall. Soc. AIME (1958) 45. [5] J.C. McDonald, Trans. AIME 138 (1941) 179. [6] N. Stanford, M.R. Barnett, Scripta Mater. 58 (2008) 179. [7] M.R. Barnett, M.D. Nave, C.J. Bettles, Mater. Sci. Eng. A 386 (2004) 205. [8] J.A. Chapman, D.V. Wilson, J. Inst. Metals 91 (1962) 39. [9] N. Ecob, B. Ralph, J. Mater. Sci. 18 (1983) 2419. [10] R.A. Lebensohn, C.N. Tome´, Acta Metall. 41 (1993) 2611. [11] A. Jain, S.R. Agnew, Scripta Mater. 48 (2003) 1003.
Scripta Materialia 59 (2008) 772–775 www.elsevier.com/locate/scriptamat
Effect of microalloying with rare-earth elements on the texture of extruded magnesium-based alloys N. Stanford,a,* D. Atwell,b A. Beer,b C. Daviesc and M.R. Barnetta a
ARC Centre of Excellence for Design in Light Metals, Deakin University, Geelong, Vic. 3217, Australia b CRC for CAST Metals Manufacturing, Deakin University, Vic. 3217, Australia c ARC Centre of Excellence for Design in Light Metals, Monash University, Vic., 3168, Australia Received 23 May 2008; accepted 10 June 2008 Available online 18 June 2008
A series of alloys have been produced with microalloying additions of rare-earth (RE) elements in the range of 0.1–0.4 wt.%. The alloys have been extruded to produce grain sizes of 23 ± 5 lm. The texture of the extruded alloys was measured, and it was found that the extrusion texture was weakened by the addition of RE elements. The samples with weakened extrusion textures exhibited an increase in the tensile elongation. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium alloy; Texture; Microalloying; Ductility; Rare earth
The limited room temperature formability of Mg alloys is a hindrance to the wider uptake of this metal in applications that would benefit considerably from its low density. The sharp texture typically seen in wrought products plays a large role in this effect [1]. Amongst the various approaches that have been tried to weaken the texture, the addition of rare-earth (RE) alloying elements is quite promising [1,2]. Indeed, the benefits of rare-earth ‘‘microalloying” additions for cold formability (rolling in particular) have been long known [3–5]. Recent studies have shown that the texture weakening effect is most likely a consequence of the influence of RE addition on recrystallization [1] but the mechanism is poorly understood. It is also evident that the benefit is seen in both extrusions and in rolled material [1,6,7]. However, the relations between RE level, texture and ductility have yet to be fully elucidated. The present communication quantifies the impact of varying RE levels (Ce and/or La) on the texture sharpness and mechanical response of extruded rod. The alloys were based on the commercial alloy M1 (nominally Mg–1 wt.% Mn) to which RE elements were added in the form of a cerium-rich misch metal, containing approximately 60% Ce, with the balance predominantly lanthanum and minor weight percentages of
* Corresponding author. E-mail: [email protected]
other RE elements. The total RE content was varied from 0.1 to 0.4 wt.%. AZ31 (nominally Mg—3 wt.% Al—1 wt.% Zn—0.3 wt.% Mn) was used as a benchmark. Alloys were cast in 2 kg batches under a shielding gas, and the billets were homogenized and extruded. The speed and temperature of extrusion was such that each alloy exhibited a fully recrystallized grain size of 23 ± 5 lm (see Table 1 for further details). Three tensile specimens of 3 mm diameter and 12 mm gauge length were machined from each alloy extrudate. These tensile specimens were then tested to failure at room temperature using an MTS 100 kN tensile tester. A section of the extrudate from each alloy was then examined by X-ray diffraction to measure the extrusion texture. The texture was measured with a GBC-Mini Materials Analyser texture goniometer equipped with Cu Ka radiation and a polycapillary beam enhancer. The texture data was examined in the form of inverse pole figures, and three typical examples are shown in Figure 1. The Mn-containing alloys and AZ31 had the form shown in Figure 1a, in which the extrusion direction was centred around ½1010. This is a typical extrusion texture for Mg-based alloys. Figure 1b shows the extrusion texture for alloy MEZ1.2.5. This texture is typical of those alloys containing RE elements in which the texture is weakened and spread towards ½11 20. At the higher alloying level of 0.4% RE, a second texture peak at approximately ½1121 parallel to the extrusion direction is evident in the inverse pole figure shown in
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.06.008
N. Stanford et al. / Scripta Materialia 59 (2008) 772–775
773
Table 1. Compositions and extrusion conditions for the alloys examined in this study Alloy
Al
Zn
Mn
RE
Billet temperature (°C)
Vr (mm s1)
Grain size (lm)
Strain rate
Z
Ductility
AZ31 M1 M1 ME1.3 MEZ1.1.5 ME1.2 ME1.4 MEZ1.2.5 ME1.2
2.87 0 0 0 0 0 0.01 0.01 0.01
0.83 0.02 0.02 0 0.5 0 0 0.55 0
0.45 1.62 1.62 1.0 1.0 1.0 0.98 0.95 0.95
0 0 0 0.3 0.1 0.21 0.4 0.19 0.19
530 375 499 450 450 450 450 450 450
2 20 20 15 15 15 15 15 15
26.9 23.3 27.7 18.7 24.6 26.9 23.3 23.5 22.3
5.0 50.1 48.9 37.6 37.6 37.6 37.6 37.6 37.6
6.41E + 09 9.67E + 12 1.45E + 11 4.90E + 11 4.90E + 11 4.90E + 11 4.90E + 11 4.90E + 11 4.90E + 11
11.3 6.0 6.0 14.0 18.6 17.4 17.9 16.6 15.7
Vr, ram speed during extrusion. Z, Zener Hollomon parameter, assuming 140 kJ mol1 activation energy. All compositions shown are in weight percentage. RE, rare-earth elements (a mixture of Ce and La, see text for further details).
Figure 2. Strength of the ½1010 texture peak in each of the alloys investigated. The square data point represents AZ31.
Figure 1. Inverse pole figures of the extrusion texture measured using X-ray diffraction. RE, rare-earth content. Inverse pole figures refer to the extrusion direction.
Figure 1c. It appears that the strength of this second peak, compared to the peak at ½10 10 increases with increasing RE alloy content. However, in all cases this second peak was less intense than the main extrusion texture component at ½10 10. If we examine the strength of the texture peak at ½10 10, it is apparent that the RE-containing alloys have a weaker texture than those without. This becomes clear when the data is plotted as a function of RE content (Fig. 2). This clearly demonstrates that very small microalloying additions to Mg-based alloys can significantly alter the strength of the extrusion texture. The effect of texture strength on the measured room temperature ductility of the alloys investigated is shown in Figure 3. It can be seen from Figure 3 that there is a trend of increasing ductility with decreasing texture
strength. The ductility of Mg alloys is well known to depend on the grain size, with the ductility increasing with decreasing grain size [8]. This has been attributed in the past to decreased twinning activity in smaller grain sized samples [9]. The quantitative effect of a grain size change on the ductility will depend on the texture of the alloy and the direction in which the deformation is applied with respect to this texture. Therefore, although an approximate number can be determined from literature (e.g. [8]), a closer approximation was determined for the present alloys (Fig. 3b). In this figure, data is shown for a number of samples with different grain sizes that were produced by extruding under different conditions. From the line of best fit shown, it was determined that the ductility decreases somewhere in the order of 0.2% per micron increase in grain size, although this relationship only holds for the grain size range examined here. This estimate was used to normalize the ductility data with respect to the grain size variation from the median of 23 lm (Fig. 3c). The reasons for the improved ductility with a weaker texture have already been discussed in a previous paper by two of the authors [6], but will briefly be described here. The principle deformation mechanism in Mg is basal slip. The extrusion texture developed in Mg alloys is such that the basal poles are aligned perpendicular to the extrusion direction. This texture results in a majority of the grains being poorly aligned for basal slip during
774
N. Stanford et al. / Scripta Materialia 59 (2008) 772–775
Figure 3. Effect of the texture strength on ductility. See text for further details on the normalized ductility values. The square data points in (a) and (c) represent AZ31.
tensile deformation in the extrusion direction. However, in the case of those alloys with RE addition, the strength of the texture is weaker. The broader spread in orientations results in there being more grains better aligned for basal slip in that case. This is evident in the yield point data for the alloys as shown in Figure 4, where it can be seen that decreasing the strength of the texture also reduces the yield point. Polycrystal plasticity simulations were used to examine if the changes in the mechanical properties resulting from RE addition could be attributed to the different textures that are developed. The calculations were made using a viscoplastic self-consisted model that has been
described in Ref. [10]. The values used for parameters such as the critical resolved shear stress for slip and the Voce hardening law parameters are given in Table 2. Two idealized textures were created for the simulation, one with a conventional extrusion texture of 6.5 times random intensity. The other texture had an overall strength of 1.5 times random intensity and comprised two texture components, one at ½1010 and another at ½1121 parallel to the extrusion direction. These two textures are shown as inverse pole figures in Figure 4. The stress–strain curves that result from simulation of tensile deformation of the two textures are also shown in Figure 4. It can be seen that the weaker texture exhibits a
Figure 4. (a) Effect of texture strength on the measured tensile yield point of the alloys examined. (b) Model predictions for the stress–strain behaviour of two idealized textures as shown. Inverse pole figures refer to the extrusion and tensile directions.
N. Stanford et al. / Scripta Materialia 59 (2008) 772–775 Table 2. Voce hardening parameters used for the simulations shown in Figure 4 Deformation mode
s0
s1
h0
h1
Basal slip Prismatic slip hc + aislip 10 12 twin
10 55 60 30
30 80 90 0
100 500 1500 30
0 0 0 30
Values are taken directly from Ref. [11], in which further details regarding these parameters can be found.
lower yield point, and that the stress at higher strains is consistently predicted to be lower than for the conventional extrusion texture. Compared to the measured yield points, the simulation has underpredicted the yield point of both textures. However, the difference in yield point of about 50 MPa between the two textures is consistent with the line of best fit in Figure 4a, showing approximately the same gradient. It therefore appears that the decreased yield point and increased ductility of the RE-containing alloys can indeed be attributed to texture modification. Another interesting result from the simulations was the marked difference in the predicted twin fraction for the two textures. The sharp extrusion texture was predicted to have 4% twin fraction after a tensile strain of 0.15. After the same strain, the weaker texture is pre-
775
dicted to have a twin fraction of 14%. This is consistent with previous observations of RE-containing alloys that have shown significantly higher twin fractions compared to alloys that do not contain RE additions [6]. The work described in this paper was funded jointly by the Centre of Excellence for Design in Light Metals and the CAST CRC. The VPSC5 code used in this work was developed by R.A. Lebensohn and C.N. Tome, and we gratefully acknowledge their generous distribution of the programme to our research group. [1] J. Bohlen, M.R. Nurnberg, J.W. Senn, D. Letzig, S.R. Agnew, Acta Mater. 55 (2007) 2101. [2] E.A. Ball, B. Prangnell, Scripta Metall. Mater. 31 (1994) 111. [3] S.L. Couling, J.F. Pashak, L. Sturkey, Trans. ASM 51 (1959) 94. [4] J.C. McDonald, Trans. Metall. Soc. AIME (1958) 45. [5] J.C. McDonald, Trans. AIME 138 (1941) 179. [6] N. Stanford, M.R. Barnett, Scripta Mater. 58 (2008) 179. [7] M.R. Barnett, M.D. Nave, C.J. Bettles, Mater. Sci. Eng. A 386 (2004) 205. [8] J.A. Chapman, D.V. Wilson, J. Inst. Metals 91 (1962) 39. [9] N. Ecob, B. Ralph, J. Mater. Sci. 18 (1983) 2419. [10] R.A. Lebensohn, C.N. Tome´, Acta Metall. 41 (1993) 2611. [11] A. Jain, S.R. Agnew, Scripta Mater. 48 (2003) 1003.