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Equal channel angular pressing followed by combustion synthesis of titanium aluminides
Journal of Alloys and Compounds 429 (2007) L1–L4
Letter
Equal channel angular pressing followed by combustion synthesis of titanium aluminides K. Morsi ∗ , S. Goyal Department of Mechanical Engineering, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, USA Received 29 December 2005; received in revised form 7 April 2006; accepted 11 April 2006 Available online 15 May 2006
Abstract Titanium aluminides are one of the most promising intermetallics today. They have previously been processed using a variety of routes to generate different and unique microstructures. Recently, TiAl have been processed using severe plastic deformation by equal channel angular pressing (ECAP). However the temperature and energy requirements for this approach are high. Alternative lower energy approaches are therefore needed. The work presented in this paper investigates a new low energy approach consisting of ECAP of elemental titanium and aluminum powders (up to three passes) followed by combustion synthesis. The effect of number of passes during ECAP on the reaction characteristics, product homogeneity and microstructure are presented. © 2006 Elsevier B.V. All rights reserved. Keywords: Intermetallics; Powder metallurgy
1. Introduction The gamma TiAl phase is one that has attracted a considerable amount of attention due to its relatively high melting point and low density [1]. A number of processing approaches have been adopted in the fabrication of these materials including forging, extrusion and casting [2–4]. Recently low-energy processes such as combustion synthesis (CS) have also been used to process these materials in short processing times [5]. Severe plastic deformation was recently applied to elemental powders by ball milling prior to combustion synthesis. The process is termed, mechanically activated self-propagating high-temperature synthesis (MASHS). Here elemental powders with particle sizes in 10s of micrometer are milled together in a high-energy planetary ball mill for shorter times than are necessary to initiate any reaction, resulting in large agglomerates (100s of micrometers in diameter) of nanostructured elemental powders. The powders are then compacted and reacted via SHS leading to the production of nanostructured intermetallics [6–10]. This technique has a number of advantages including the production of nanostructured product phase; however the resulting product often
∗
Corresponding author. Tel.: +1 619 594 2903; fax: +1 619 594 3599. E-mail address: [email protected] (K. Morsi).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.04.015
contains high levels of porosity 40–50% [6]. On the other hand equal channel angular extrusion (ECAE) or pressing (ECAP) is a bulk severe plastic deformation process that has been effectively used to produce nano/ultrafine structured bulk materials starting from either bulk [11] or powder-based [12,13] compacts. For TiAl however ECAE often needs to be conducted at high temperatures, equal to and in excess of 1000 ◦ C [14,15], which raises cost and energy concerns. Recently one of the authors investigated the simultaneous combustion synthesis and forging/direct extrusion of iron and nickel aluminides [16–18]. The current work is a preliminary investigation into of the room temperature consolidation of elemental titanium and aluminum powders by ECAP, followed by combustion synthesis. To the best of the authors knowledge this approach has never been carried out for TiAl intermetallics, and, in essence it may be a viable route that avoids the high temperature processing requirements of ECAE for TiAl product. The effect of processing on the reaction characteristics, product homogeneity and microstructure is presented. 2. Experimental procedures Titanium (Ti) and aluminum (Al) powders both-325 mesh (Atlantic Equipment Engineers, NJ, USA) were first rotator mixed for one and a half hours in the composition of Ti–52 at.%Al (TiAl). Following this, powders were degassed at 150 ◦ C for 10 h in a vacuum oven, and then compacted. For uniaxial pressing and ECAP zinc stearate was used as the lubricant, and applied to the inner walls
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Fig. 3. Schematic of combustion synthesis setup.
Fig. 1. ECAE/ECAP H13 steel die.
Fig. 4. Effect of number of ECAP passes on ignition and combustion temperatures. The heating cycle used was 15 ◦ C/min to 680 ◦ C and dwell for 15 min. The time-temperature profile throughout the process was collected by a computer via an OMEGA OMB-DAQ-55 data acquisition system. At least three specimens were examined for each processing condition and an average calculated and reported. Samples were then cut, ground and polished to 1 m diamond finish. Light, electron microscopy and image analysis were used to observe the microstructures and determine porosity content and homogeneity of both green and reacted specimens.
3. Results and discussion Fig. 2. Schematic of copper cap used and its dimensions.
of the hardened steel dies with the aid of acetone. For ECAP an H13 hardened steel die with a 90◦ angle and 14.08 mm inner diameter was used (Fig. 1). The uniaxially pressed specimens were weighed and dimensions measured using digital micrometers and calipers in order to determine the green density. Specimens were uniaxially pressed to 75% of theoretical green density (TGD). For the ECAP experiments, powders were first placed in a copper cap (Fig. 2) (to avoid cracking of the green compact if extruded without an outer cap) and then pressed (passed through to the die exit, as seen in Fig. 1). More passes were made by pressing the specimen back and forth without specimen rotation. For ECAP, pressing was conducted at a speed of 0.55 mm/s then after pressing the cap was removed. Specimens that have been uniaxially pressed and those that have undergone ECAP were both combustion synthesized in a tube furnace under argon atmosphere (Fig. 3).
The use of elemental powders in ECAE/ECAP provides an important benefit in reducing the extrusion/consolidation pressure requirements compared to extruding/consolidating the intermetallic phase, and can enable room temperature extrusion/pressing. Our materials were pressed at room temperature requiring pressures between 70 and 90 MPa. In combustion synthesis under the volumetric combustion mode of ignition, the green compact is heated uniformly to an ignition temperature at which the elemental powders react exothermically and convert to product(s), raising the compact to the maximum combustion temperature, before cooling down again due to heat losses. Fig. 4 shows that the ignition temperature is unaffected by the number of ECAP passes (pressings),
Fig. 5. Effect of number of ECAP passes on the microstructure of green compacts (from left to right; 75% TGD uniaxially pressed, 1, 2 and 3 pass ECAP specimens).
K. Morsi, S. Goyal / Journal of Alloys and Compounds 429 (2007) L1–L4
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Fig. 6. Effect of number of ECAP passes on the average porosity content of green and reacted specimens.
with ignition starting approximately around the melting point of aluminum (660 ◦ C). The melting of aluminum is an important part of the reaction mechanism for this system. According to previous work in the area [19], the presence of transient liquid aluminum and its subsequent spreading is an integral part of the process. The combustion temperature was also found to be little affected by the number of ECAP passes. Fig. 5 shows light micrographs of the microstructures for uniaxially pressed (∼75% of theoretical green density) and ECAP (1, 2 and 3 pass) green specimens. The figure shows some consolidation of the green compact with number of ECAP passes. Fig. 6 shows the effect of number of passes on the measured product porosity for green and reacted specimens, it can be seen that after three passes the green compact contains around 9% porosity. The figure also shows the generation of excessive porosity in the reacted specimens, compared to the green specimens. This was also the case for the uniaxially pressed specimens. Factors that may have contributed to the porosity in the final product include porosity initially present in the green compact, molar volume difference between the product and reactant phases (can contribute as much as 8.9% for TiAl [20]), the development of Kirkendall porosity during the heating stage of CS (due to imbalanced diffusivities of the elemental species) and the presence of agglomerated Al powder after mixing resulting in large prior particle sites following the melting and spreading of Al. The figure also shows that there maybe a slight decrease in CS average product porosity with an increase in number of passes possibly corresponding to the slight decrease in porosity of the green compact with number of passes prior to the reaction. Backscattered SEM micrographs reveal that after one and two passes in-homogeneities are present in the product microstructure, however after three passes the in-homogeneity is greatly reduced, as can be seen in (Fig. 7). It is believed that the improved homogeneity with increased number of passes may be a result of the corresponding increase in green density in addition to possible formation of defect structures that could facilitate improved diffusion characteristics and therefore better homogeneity. The 75% TGD reacted (uniaxially pressed) specimens contained 46.9 ± 5.8% porosity; this is close to a reported value of 52% porosity for TiAl that underwent thermal explosion at a heating rate of 10 ◦ C/min [20]. Alman [21] reported an even higher value of 64% product porosity in TiAl combustion synthesized in vacuum starting with an elemental compact of 80% TGD. A higher level of porosity would normally be expected in an argon atmosphere (such as in our case) compared to vac-
Fig. 7. Inhomogeneous combustion synthesized microstructures for (a) one- and (b) two-pass ECAP and improved homogeneity for (c) three-pass ECAP.
uum [22]. However it appears that in general, specimens that underwent ECAP exhibited slightly lower levels of porosity, than the previously reported values for uniaxially pressed specimens. Further work however is needed to confirm this. 4. Conclusions From the current investigation into the ECAP (up to three passes) of Ti and Al elemental powders, the following conclusions can be drawn: 1. Neither the ignition temperature nor the combustion temperatures were significantly affected by the number of passes.
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2. Homogeneous reacted microstructures were generated after three ECAP passes. Inhomogenieties existed at lower number of passes. 3. There is a slight decrease in product average porosity with number of ECAP passes. 4. Future experiments will address the effect of a greater number of passes, extrusion temperature and simultaneous CS and ECAP (which should greatly help in generating high density products) on the developed microstructure (grain size) and properties. Acknowledgments The authors would like to thank Mr. Greg Morris and Mr. Satyajit Shinde for their appreciated help with this project. References [1] L.H. Yi, Y. Yu, M. Xia, Y. Sheng, Int. J. SHS 1 (3) (1992) 447–452. [2] T.W. Lee, C.H. Lee, J. Mater. Sci. Lett. 18 (10) (1999) 801–803. [3] T. Tetsui, K. Shindo, K. Satura, M. Takeyama, Intermetallics 11 (4) (2003) 299–306. [4] R. Yang, Y.Y. Cui, L.M. Dong, Q. Jia, J. Mater. Process. Tech. 135 (2/3) (2003) 179–188. [5] Y.-D. Hahn, Y.-T. Lee, Particul. Mater. Process. 9 (1992) 309–318.
[6] E. Gaffet, F. Bernard, Ann. Chim. Sci. Mater. 27 (6) (2002) 47–59. [7] V. Gauthier, F. Bernard, E. Gaffet, C. Josse, J.P. Larpin, Mater. Sci. Eng. A 272 (2) (1999) 334–341. [8] V. Gauther, F. Bernard, E. Gaffet, M. Gailhanou, J.P. Larpin, Intermetallics 10 (4) (2002) 377–389. [9] V. Gauthier, C. Josse, F. Bernard, E. Gaffet, J.P. Larpin, Mater. Sci. Eng. A 265 (1/2) (1999) 117–128. [10] U. Anselmi-Tamburini, F. Maglia, G. Spinolo, S. Doppiu, M. Monagheddu, G. Cocco, J. Mater. Synth. Process. 8 (5/6) (2000) 377–383. [11] F.D. Torre, R. Lapovok, J. Sandlin, P.F. Thomson, C.H.J. Davies, E.V. Pereloma, Acta Mater. 52 (2004) 4819–4832. [12] M. Haouaoui, I. Karaman, H.J. Maier, K.T. Hartwig, Metall. Mater. Trans. A 35 (9) (2004) 2935–2949. [13] A. Parasiris, K.T. Hartwig, Int. J. Refract. Met. Hard Mater. 18 (2000) 23–31. [14] M.L. Shankar, A. Sastry, N.R. Mahapatra, Mater. Sci. Eng. A 329–331 (2001) 872–877. [15] S.L. Semiatin, V.M. Segal, R.L. Goetz, Scripta Metall. Mater. 33 (4) (1995) 535–540. [16] K. Morsi, S.O. Moussa, J. Wall, J. Mater. Sci. 40 (2005) 1027–1030. [17] J. Rodriguez, S.O. Moussa, J. Wall, K. Morsi, Scripta Mater. 48 (2003) 707–712. [18] K. Morsi, J. Rodriguez, J. Mater. Sci. 39 (2004) 4849–4854. [19] R.M. German, Liquid Phase Sintering, Plenum Pub. Corp., New York, 1985, pp. 164–172. [20] H.C. Yi, A. Petric, J.J. Moore, J. Mater. Sci. 27 (1992) 6797–6806. [21] D.E. Alman, Intermetallics 13 (2005) 572–579. [22] W. Misiolek, R.M. German, Mater. Sci. Eng. A 144 (1/2) (1991) 1–10.
Letter
Equal channel angular pressing followed by combustion synthesis of titanium aluminides K. Morsi ∗ , S. Goyal Department of Mechanical Engineering, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, USA Received 29 December 2005; received in revised form 7 April 2006; accepted 11 April 2006 Available online 15 May 2006
Abstract Titanium aluminides are one of the most promising intermetallics today. They have previously been processed using a variety of routes to generate different and unique microstructures. Recently, TiAl have been processed using severe plastic deformation by equal channel angular pressing (ECAP). However the temperature and energy requirements for this approach are high. Alternative lower energy approaches are therefore needed. The work presented in this paper investigates a new low energy approach consisting of ECAP of elemental titanium and aluminum powders (up to three passes) followed by combustion synthesis. The effect of number of passes during ECAP on the reaction characteristics, product homogeneity and microstructure are presented. © 2006 Elsevier B.V. All rights reserved. Keywords: Intermetallics; Powder metallurgy
1. Introduction The gamma TiAl phase is one that has attracted a considerable amount of attention due to its relatively high melting point and low density [1]. A number of processing approaches have been adopted in the fabrication of these materials including forging, extrusion and casting [2–4]. Recently low-energy processes such as combustion synthesis (CS) have also been used to process these materials in short processing times [5]. Severe plastic deformation was recently applied to elemental powders by ball milling prior to combustion synthesis. The process is termed, mechanically activated self-propagating high-temperature synthesis (MASHS). Here elemental powders with particle sizes in 10s of micrometer are milled together in a high-energy planetary ball mill for shorter times than are necessary to initiate any reaction, resulting in large agglomerates (100s of micrometers in diameter) of nanostructured elemental powders. The powders are then compacted and reacted via SHS leading to the production of nanostructured intermetallics [6–10]. This technique has a number of advantages including the production of nanostructured product phase; however the resulting product often
∗
Corresponding author. Tel.: +1 619 594 2903; fax: +1 619 594 3599. E-mail address: [email protected] (K. Morsi).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.04.015
contains high levels of porosity 40–50% [6]. On the other hand equal channel angular extrusion (ECAE) or pressing (ECAP) is a bulk severe plastic deformation process that has been effectively used to produce nano/ultrafine structured bulk materials starting from either bulk [11] or powder-based [12,13] compacts. For TiAl however ECAE often needs to be conducted at high temperatures, equal to and in excess of 1000 ◦ C [14,15], which raises cost and energy concerns. Recently one of the authors investigated the simultaneous combustion synthesis and forging/direct extrusion of iron and nickel aluminides [16–18]. The current work is a preliminary investigation into of the room temperature consolidation of elemental titanium and aluminum powders by ECAP, followed by combustion synthesis. To the best of the authors knowledge this approach has never been carried out for TiAl intermetallics, and, in essence it may be a viable route that avoids the high temperature processing requirements of ECAE for TiAl product. The effect of processing on the reaction characteristics, product homogeneity and microstructure is presented. 2. Experimental procedures Titanium (Ti) and aluminum (Al) powders both-325 mesh (Atlantic Equipment Engineers, NJ, USA) were first rotator mixed for one and a half hours in the composition of Ti–52 at.%Al (TiAl). Following this, powders were degassed at 150 ◦ C for 10 h in a vacuum oven, and then compacted. For uniaxial pressing and ECAP zinc stearate was used as the lubricant, and applied to the inner walls
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K. Morsi, S. Goyal / Journal of Alloys and Compounds 429 (2007) L1–L4
Fig. 3. Schematic of combustion synthesis setup.
Fig. 1. ECAE/ECAP H13 steel die.
Fig. 4. Effect of number of ECAP passes on ignition and combustion temperatures. The heating cycle used was 15 ◦ C/min to 680 ◦ C and dwell for 15 min. The time-temperature profile throughout the process was collected by a computer via an OMEGA OMB-DAQ-55 data acquisition system. At least three specimens were examined for each processing condition and an average calculated and reported. Samples were then cut, ground and polished to 1 m diamond finish. Light, electron microscopy and image analysis were used to observe the microstructures and determine porosity content and homogeneity of both green and reacted specimens.
3. Results and discussion Fig. 2. Schematic of copper cap used and its dimensions.
of the hardened steel dies with the aid of acetone. For ECAP an H13 hardened steel die with a 90◦ angle and 14.08 mm inner diameter was used (Fig. 1). The uniaxially pressed specimens were weighed and dimensions measured using digital micrometers and calipers in order to determine the green density. Specimens were uniaxially pressed to 75% of theoretical green density (TGD). For the ECAP experiments, powders were first placed in a copper cap (Fig. 2) (to avoid cracking of the green compact if extruded without an outer cap) and then pressed (passed through to the die exit, as seen in Fig. 1). More passes were made by pressing the specimen back and forth without specimen rotation. For ECAP, pressing was conducted at a speed of 0.55 mm/s then after pressing the cap was removed. Specimens that have been uniaxially pressed and those that have undergone ECAP were both combustion synthesized in a tube furnace under argon atmosphere (Fig. 3).
The use of elemental powders in ECAE/ECAP provides an important benefit in reducing the extrusion/consolidation pressure requirements compared to extruding/consolidating the intermetallic phase, and can enable room temperature extrusion/pressing. Our materials were pressed at room temperature requiring pressures between 70 and 90 MPa. In combustion synthesis under the volumetric combustion mode of ignition, the green compact is heated uniformly to an ignition temperature at which the elemental powders react exothermically and convert to product(s), raising the compact to the maximum combustion temperature, before cooling down again due to heat losses. Fig. 4 shows that the ignition temperature is unaffected by the number of ECAP passes (pressings),
Fig. 5. Effect of number of ECAP passes on the microstructure of green compacts (from left to right; 75% TGD uniaxially pressed, 1, 2 and 3 pass ECAP specimens).
K. Morsi, S. Goyal / Journal of Alloys and Compounds 429 (2007) L1–L4
L3
Fig. 6. Effect of number of ECAP passes on the average porosity content of green and reacted specimens.
with ignition starting approximately around the melting point of aluminum (660 ◦ C). The melting of aluminum is an important part of the reaction mechanism for this system. According to previous work in the area [19], the presence of transient liquid aluminum and its subsequent spreading is an integral part of the process. The combustion temperature was also found to be little affected by the number of ECAP passes. Fig. 5 shows light micrographs of the microstructures for uniaxially pressed (∼75% of theoretical green density) and ECAP (1, 2 and 3 pass) green specimens. The figure shows some consolidation of the green compact with number of ECAP passes. Fig. 6 shows the effect of number of passes on the measured product porosity for green and reacted specimens, it can be seen that after three passes the green compact contains around 9% porosity. The figure also shows the generation of excessive porosity in the reacted specimens, compared to the green specimens. This was also the case for the uniaxially pressed specimens. Factors that may have contributed to the porosity in the final product include porosity initially present in the green compact, molar volume difference between the product and reactant phases (can contribute as much as 8.9% for TiAl [20]), the development of Kirkendall porosity during the heating stage of CS (due to imbalanced diffusivities of the elemental species) and the presence of agglomerated Al powder after mixing resulting in large prior particle sites following the melting and spreading of Al. The figure also shows that there maybe a slight decrease in CS average product porosity with an increase in number of passes possibly corresponding to the slight decrease in porosity of the green compact with number of passes prior to the reaction. Backscattered SEM micrographs reveal that after one and two passes in-homogeneities are present in the product microstructure, however after three passes the in-homogeneity is greatly reduced, as can be seen in (Fig. 7). It is believed that the improved homogeneity with increased number of passes may be a result of the corresponding increase in green density in addition to possible formation of defect structures that could facilitate improved diffusion characteristics and therefore better homogeneity. The 75% TGD reacted (uniaxially pressed) specimens contained 46.9 ± 5.8% porosity; this is close to a reported value of 52% porosity for TiAl that underwent thermal explosion at a heating rate of 10 ◦ C/min [20]. Alman [21] reported an even higher value of 64% product porosity in TiAl combustion synthesized in vacuum starting with an elemental compact of 80% TGD. A higher level of porosity would normally be expected in an argon atmosphere (such as in our case) compared to vac-
Fig. 7. Inhomogeneous combustion synthesized microstructures for (a) one- and (b) two-pass ECAP and improved homogeneity for (c) three-pass ECAP.
uum [22]. However it appears that in general, specimens that underwent ECAP exhibited slightly lower levels of porosity, than the previously reported values for uniaxially pressed specimens. Further work however is needed to confirm this. 4. Conclusions From the current investigation into the ECAP (up to three passes) of Ti and Al elemental powders, the following conclusions can be drawn: 1. Neither the ignition temperature nor the combustion temperatures were significantly affected by the number of passes.
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2. Homogeneous reacted microstructures were generated after three ECAP passes. Inhomogenieties existed at lower number of passes. 3. There is a slight decrease in product average porosity with number of ECAP passes. 4. Future experiments will address the effect of a greater number of passes, extrusion temperature and simultaneous CS and ECAP (which should greatly help in generating high density products) on the developed microstructure (grain size) and properties. Acknowledgments The authors would like to thank Mr. Greg Morris and Mr. Satyajit Shinde for their appreciated help with this project. References [1] L.H. Yi, Y. Yu, M. Xia, Y. Sheng, Int. J. SHS 1 (3) (1992) 447–452. [2] T.W. Lee, C.H. Lee, J. Mater. Sci. Lett. 18 (10) (1999) 801–803. [3] T. Tetsui, K. Shindo, K. Satura, M. Takeyama, Intermetallics 11 (4) (2003) 299–306. [4] R. Yang, Y.Y. Cui, L.M. Dong, Q. Jia, J. Mater. Process. Tech. 135 (2/3) (2003) 179–188. [5] Y.-D. Hahn, Y.-T. Lee, Particul. Mater. Process. 9 (1992) 309–318.
[6] E. Gaffet, F. Bernard, Ann. Chim. Sci. Mater. 27 (6) (2002) 47–59. [7] V. Gauthier, F. Bernard, E. Gaffet, C. Josse, J.P. Larpin, Mater. Sci. Eng. A 272 (2) (1999) 334–341. [8] V. Gauther, F. Bernard, E. Gaffet, M. Gailhanou, J.P. Larpin, Intermetallics 10 (4) (2002) 377–389. [9] V. Gauthier, C. Josse, F. Bernard, E. Gaffet, J.P. Larpin, Mater. Sci. Eng. A 265 (1/2) (1999) 117–128. [10] U. Anselmi-Tamburini, F. Maglia, G. Spinolo, S. Doppiu, M. Monagheddu, G. Cocco, J. Mater. Synth. Process. 8 (5/6) (2000) 377–383. [11] F.D. Torre, R. Lapovok, J. Sandlin, P.F. Thomson, C.H.J. Davies, E.V. Pereloma, Acta Mater. 52 (2004) 4819–4832. [12] M. Haouaoui, I. Karaman, H.J. Maier, K.T. Hartwig, Metall. Mater. Trans. A 35 (9) (2004) 2935–2949. [13] A. Parasiris, K.T. Hartwig, Int. J. Refract. Met. Hard Mater. 18 (2000) 23–31. [14] M.L. Shankar, A. Sastry, N.R. Mahapatra, Mater. Sci. Eng. A 329–331 (2001) 872–877. [15] S.L. Semiatin, V.M. Segal, R.L. Goetz, Scripta Metall. Mater. 33 (4) (1995) 535–540. [16] K. Morsi, S.O. Moussa, J. Wall, J. Mater. Sci. 40 (2005) 1027–1030. [17] J. Rodriguez, S.O. Moussa, J. Wall, K. Morsi, Scripta Mater. 48 (2003) 707–712. [18] K. Morsi, J. Rodriguez, J. Mater. Sci. 39 (2004) 4849–4854. [19] R.M. German, Liquid Phase Sintering, Plenum Pub. Corp., New York, 1985, pp. 164–172. [20] H.C. Yi, A. Petric, J.J. Moore, J. Mater. Sci. 27 (1992) 6797–6806. [21] D.E. Alman, Intermetallics 13 (2005) 572–579. [22] W. Misiolek, R.M. German, Mater. Sci. Eng. A 144 (1/2) (1991) 1–10.