Deformation of martensite in a polycrystalline Cu-Zn-AI alloy
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I.
INTRODUCTION
THEpurpose of this paper is to characterize and interpret the deformation mechanisms of martensitic microstructures in a polycrystalline shape memory alloy at various levels of strain. This topic is of fundamental interest as well as being relevant to the behavior of such alloys in typical applications. It will be useful to precede the discussion of the present results with a brief review of the deformation modes which are possible, and known to be prevalent, in this kind of microstructure. A fully martensitic thermoelastic alloy consists of an array of 24 martensite plate variants derived from each parent grain. These 24 variants are arranged in 6 plate groups with 4 plate variants per plate group. The plate variants within a group are designated as A, B, C, and D. When stress is applied to a fully martensitic thennoelastic alloy, several types of microscopic deformation can occur in order to produce the macroscopic shape change. These include four major deformation modes, as follows: (a) coalescence and rearrangement of preexisting martensite plate variants, ~-s (b) adjustment and development of internal twinnings in individual plates, 6'7 (c) structural (martensiteto-martensite) transformations, involving changes in the stacking sequence of close-packed planes, 4'8-~5and (d) ordinary slip. The coalescence-of-variants mechanism proceeds toward a single crystal corresponding to the variant in the original microstructure which gives the maximum strain in the direction of the applied stress. This generally coincides with the variant with the maximum Schmid factor9 and is carried out mainly through the motion of three basic types of interfaces, namely, the A:B, A:C, and A:D types, 2 which join the variants in a given plate group. In 2H martensite there are {121} transformation twins. Aside from this, a single crystal of the 2H lattice has six internal twin planes: four {121}'yi type and two {101}3,1 type. 3'6 In 18R martensite, there are no transformation twins. However, a single crystal of the 18R lattice also has six (experimentally confirmed) twin planes: two {]28}, two {1210}, as well as (1010) and (0018), 7 and possibly six othKENJI ADACHI is with Central Research Laboratory, Sumimoto Metal Mining Company, lchikawa, Chiba, Japan. JEFF PERKINS is Professor of Materials Science, Department of Mechanical Engineering, Naval Postgraduate School, Monterey, CA 93943. Manuscript submitted November 13, 1984. METALLURGICALTRANSACTIONS A
ers, namely, the conjugates of all these. The action of these twinnings comprises the "adjustment and development of internal twinnings" to which we refer. Most of these twin planes are derived from the mirror planes in the parent ordered bcc phase, i . e . , the {110} and {100}. Some of these twinnings are introduced to make a conversion from one plate group to another, according to Saburi and Nenno. 7 Martensite-to-martensite phase transformation under stress is well known in Cu-AI-Ni shape memory alloys, where one of the transformation sequences from the parent /31 ph
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