Plastic deformation of single crystals of the heusler alloy Cu 2 MnAl
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ATOMIC
o r d e r i n g generally increases the work hardening rate of an alloy, whereas the yield strength may i n c r e a s e or d e c r e a s e depending upon the d e g r e e of o r d e r , type of superlattice, and the size of the antiphase domains. Much information on the mechanical properties of binary o r d e r e d alloys is already available, ~ but information on ternary alloys is r a t h e r sparse. Marcinkowski and Brown (MB)2 speculated on the deformation mode and dislocation configuration in CueMnA1, but made no experimental observations. T o our knowledge, a high temperature c r e e p study of another Heusler alloy, Ni2A1Ti, is the only published experimental work on the mechanical behavior of Heusler alloys.3 As part of a g e n e r a l study conc e r n i n g the effects of plastic deformation on the magnetic behavior of Cu2MnA1, it was decided to e x a m i n e the mechanical properties of this alloy in some detail, The work on the mechanical properties of the polycrystalline m a t e r i a l has been reported4 and some magnetic results have appeared in the literatureJ In this paper, the s t r e s s strain behavior of single crystals def o r m e d in free and plane-strain compression is examined. The results a r e analyzed according t o crystal plasticity theory and discussed in t e r m s of the expected a c t i v e slip systems and of the dislocation structure r e v e a l e d by transmission electron m i c r o s c o p y . B a s i c t o this analysis was the determination of the slip mode as this knowledge was unavailable at the time of s t u d y . 2. EXPERIMENTAL PROCEDURE Single crystals of Cu2MnA1 were grown in h i g h p u r i t y alumina crucibles under purified a r g o n a t m o s phere, u s i n g a modified Bridgman technique. The cooling rate below 600°C was accelerated to avoid MARTIN L. GREEN and J. B. VANDER SANDEare GraduateStudent and Associate Professor, respectively, Department of Materials Science and Engineering, Massachusetts Instituteof Technology, Cambridge, MA. G.Y. CHIN is Head, Physical Metallurgy and Crystal Growth Research Department, Bell Laboratories, Murray Hill,NJ 07974. Manuscript submittedJune 29, 1976. METALLURGICAL TRANSACTIONS A
low-temperature decomposition. Samples from t h r e e runs were analyzed as Cu2Mn~99A1, CuzMr~Alo.~9 and Cu2Mn~Alo.88. T h e r e appeared t o be no significant differences in mechanical behavior a m o n g t h e m . The as-grown crystals were etched in dilute (50 pct) HNO3 t o r e m o v e oxides, oriented u s i n g the Laue back r e flection technique, and shaped by s p a r k cutting and planing. The shaped samples were annealed at 725°C for 1 h in quartz capsules evacuated t o 10-6 Torr and backfilled with purified a r g o n t o a p r e s s u r e of 0.2 T o r r . They were then electropotished at 100°C in an H~PO4 solution saturated w i t h CrOs. Magnetic moment measurements ~ and electron diffraction results on thin foils confirmed full L21 (Heusler structure) order. Compression testing was performed at room temperature on an Instron testing ma
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