Plane strain compression of Cu2MnAl single crystals

Scripta METALLURGICA

Vol. 9, pp~ 1201-1203, 1975 Printed in the United States

Pergamon Press, Inc

PLANE STRAIN COMPRESSION OF Cu2MnAI SINGLE CRYSTALS by Martin L. Green and G.Y. Chin Bell Laboratories Murray Hill, New Jersey 07974 (Received July 30, 1975)

The plastic anisotropy of single crystals has been treated successfully in a number of cases (I-3) using the Taylor-Bishop-Hill (4,5) (TBH) analysis. In this analysis a theoretically derived Taylor factor is used to convert the applied stress strain curves for crystals deformed in various orientations to the resolved shear stresss resolved shear strain curves. If the analysis is valid, the latter curves fall into a single band. An important assumption in the analysis, however, is that latent hardening and active hardening be equal. This assumption is nearly correct in the metals and alloys studied to date. Ordered alloys, however, are expected to exhibit much greater latent hardening due to the presence of superlattice dislocations nd the associated antiphase boundaries. Large overshoots beyond the [I0~ 111J symmetry line have been noted, for example, in tensile deformation of Ni3Fe ordered single crystals (6). Thus it would be of interest to see if the TBH analysis is valid in ordered alloys.

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As part of a general study concerning the effects of plastic deformation on the magnetic behavior of the Heusler alloy, Cu2MnA1, we performed some plane strain compression experiments on single crystals of this alloy. The results indicate a deviation from the TBH analysis in a way consistent with large latent hardening. Large single crystals of Cu2MnA1 (25 mm diam. x 125 mm long) were grown using a modified Bridgman technique. Samples were shaped by electric spark machining following orientation using the Laue technique. They were then annealed at 725 °C for one hour in evacuated quartz capsules backfilled with purified argon and air cooled to room temperature. This resulted in samples with full L21 long range order as checked by magnetic moment measurements. The plane strain compression tests were made using a channel device (I). The notation (hkl)[uvw] denotes that (hkl) is the compression plane and [uvw] is the elongation directions widening in the width direction is suppressed by the channel. Typical dimensions of plane strain compression samples were 5 mm wide x 8 mm long x 2.5 mm thick. Teflon foils were used as lubricant. The nominal strain rate was 0.05 min -~. Samples were periodically unloaded for dimensional measurement and for renewal of lubricant. Stressstrain data were obtained from these measurements.

*Presently a graduate student at the Department of Materials Science and Engineering, M.I.T., Cambridge, Mass. 1201

1202

COMPRESSION OF Cu2MnAl CRYSTALS

Vol. 9, No. Ii

Optical microscopy revealed that slip traces a r e gener~lly straight and readily discernible. The slip plane was determined r o b e ~110~ in all cases. In this plane the low index slip direction may be (100>, <110> or (111~. Attempts using transmission electron microscopy to determine the direction of the Burgers vector b based on the g.bxu invisibility criterion turned out unsuccessful as strong residual images were observed for all three possible slip directions. However, the dislocation features are much like those of nearly isostructural Fe3AI, known to slip in the <111> direction (7). In addition, th~ dislocations were noted to occur predominantly in pairs spaced about 850 A apart and not in the form of dipoles. This means that they are superlattice dislocations and thus rules out <110> as the slip direction, as the latter dislocations are not expected to pair up into superlattice types (8). Finally, only the <111> slip direction is consistent with the observed shape changes and lattice rotations of all samples tested. Thus it is concluded that <111> is the slip direction. On this basis, t h e resolved shear stress, shear strain ~ - ~ ) curves were obtained from the compression stress, strain (~-£) dat~ using the expressions T = ~/M and ~ = MZ where M is the Taylor factor. Values of M or crystals of the present orientations have been given previously for 11~ slip (1). It was not necessary to correct for a change in M due to lattice rotation since the effect is small in the present range of strains. The r e s u l t i n g ~ - ~ curves are shown in Fig. I.

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ORENTATONS

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FIG. I Resolved shear stress, shear strain curves for Cu2MnAI crystals deformed in plane strain compression.

As s e e n from F i g . 1, t h e e x t r a p o l a t e d y i e l d s t r e s s i s a b o u t t h e same for all curves, but the difference in work hardening rate separates the curves into two ~ands. The slopes for the two bands differ by a factor of twO, at 314 MN/m and 615 MN/m respectively. Thus the TBH analysis is invalidated in the case of Cu2MnAI.

Vol. 9, No. II

COMPRESSION OF Cu2MnAI CRYSTALS

1203

Close examination of the curves showed that under plane strain compression and deformation by {110}
28, 131 (1968). 7. H.J. Leamy, F.X. Kayser and M.J. Marcinkowski, Phil. Mag. 20, 763, 779 (1969). 8. P.R. Strutt, G.M. Rowe, J.C. Ingram and Y.H. Choo, in Electron Microscopy and Structure of Materials, G. Thomas, et al., eds. (Univ. Berkeley Press, Calif. 1972), p. 722. 9. I.L. Dillamore, E. Butler and D. Green, Metal Sci. J. ~, 161 (1968).