A Study of the Rubber-Like Behavior of Mono-Domain Au-Cd Martensite
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ABSTRACT The origin of the rubber-like behavior in mono-domain Au-Cd martensite was explained in terms of a new model that focused attention on the change of long-range elastic interaction energy among vacancies during a domain reversion. Vacancies in martensite, the lower-symmetry phase, produce stress fields with lower symmetry. During martensite aging, vacancies tend to rearrange themselves to lower elastic interaction energy. The low-symmetry elastic field results in a lowsymmetry vacancy configuration. When a stabilized martensite domain reverts to a new domain (twin) under external stress, the original vacancy configuration is inherited to the new domain, but such a configuration becomes a high energy configuration because of the lower symmetry of elastic field, and thus it tends to restore the original configuration by reverse twinning. The above vacancy reconfiguration model is consistent with the fact that the rubber-like behavior is closely related to vacancies. INTRODUCTION The rubber-like behavior (RLB) of martensite refers to the curious pseudoelasticity in well-aged martensitic state, which was first found in 1932 by Olander in Au-47.5%Cd martensite[1]. The pseudoelasticity in martensite (i.e., the rubber-like behavior) is quite distinct from the pseudoelasticity in the parent phase. The latter is associated with the reversible movement of martensite/parent interface due to the balance of external stress and internal restoring force arising from the free energy difference between martensite and parent[2]; the former is associated with the reversible movement of martensite domain (twin) boundary [3] due to the balance of external stress and unknown internal restoringforce developed during aging. In the past decades there have been quite a few models trying to explain the origin of the unknown restoring force. However, the origin still remains obscure. These models can be classified into two categories: one is based on the domain boundary effect[4,5], the other is based on the volume effect or bulk effect. Recent experiment on mono-domain martensite [6,7] has showed that the RLB even occurs in martensites without domain boundary. This result suggests that domain boundary is not a necessary factor for the RLB, although it promotes the process. As volume effect is concerned, several recent models have been proposed based on the following possible processes during martensite aging: "* decrease of degree of long-range order (LRO model) [8,9] or increase of degree of short-range order (SRO model) [10,11] "• rearrangement of vacancy-pairs(strain-dipole model) [12] It should be pointed out that in essentially ordered alloys (most shape memory alloys belong to this category), LRO and SRO are closely related to each other, an increase of SRO will lead to a decrease of LRO. Thus the SRO model can be considered to be similar to LRO model in nature, and in the following we will indiscriminately refer them as reordering models. 305 Mat. Res. Soc. Symp. Proc. Vol. 459 @1997 Materials Research Society
The reordering
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