Stabilization and two-way shape memory effect in Cu-Al-Ni single crystals
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I.
INTRODUCTION
THE two-way shape memory effect (TWME) is generally obtained by a special training, during which the sample is repeatedly cycled in constrained condition through the martensitic transformation. Most commonly, and for technological applications, the cycling is done at or above room temperature, at which diffusional processes have also been observed.[1,2,3] Recent experimental results have shown, however, that in Cu-Zn-Al single crystals, it is not necessary to apply this training in order to obtain a stable TWME. Instead, it is sufficient to stabilize the martensite by diffusion already at room temperature, subsequently retransforming partially or completely to the austenite phase by heating.[4] Not only is it possible to obtain a full TWME in this way, but in addition, the martensite variant can be induced on cooling against an applied force. The work that it can do is of the same amount as when the TWME is obtained by stress cycling 104 times at 50 7C.[5] Apparently small martensite plates are retained, whose volume fraction is too small to be accessible by measurements of length changes. These plates, which will be called nuclei in the following, then grow on cooling and serve as obstacles for the growth of martensite plates of other variants. The TWME disappears when the sample is heated to sufficiently elevated temperatures. However, of more importance than the maximum reached temperature is the heating velocity.[6] A slow heating velocity of 0.05 7C/s implies the stabilization and retention of the nuclei to high temperatures, whereas during rapid heating (;5 7C/s), diffusional processes are not sufficiently efficient to stabilize the nuclei by diffusion after the transformation. Although the nuclei have not yet been studied by transmission electron microscopy,
E. CINGOLANI was formerly a graduate student with the Metals Division at the Instituto Balseiro, Centro Ato´mico Bariloche, Bariloche 8400, Argentina. M. AHLERS, Professor is with the Metals Division at the Instituto Balseiro, Centro Ato´mico Bariloche, 8400 Bariloche, Argentina. J. VAN HUMBEECK, Professor, is with the Materials and Applied Metallurgy Department at the Catholic University of Leuven, 3030 Heverlee, Belgium. Manuscript submitted January 5, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
their disappearance manifests itself strongly by acoustic emission far above the austenite finish temperature Af.[7] It is well known[8] that in Cu-Zn-Al single crystals, the diffusion in martensite can lead to a strong increase in retransformation temperature As of the order of 100 7C, and it is therefore not surprising that retained nuclei can increase their stability and their As if sufficient time for diffusion is available. It is therefore legitimate to ask what is ascribed to the introduction of dislocations during the training at and above room temperature if the TWME is also obtained without this training procedure, i.e., if only diffusional processes in the martensite are permitted to occur. This immediately leads to two questi
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