Classification and Characterization of the Shape Memory Binary Alloys

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0980-II04-07

Classification and Characterization of the Shape Memory Binary Alloys Mitsuo Notomi1, Krystyn J.. Van Vliet2, and Sidney Yip2 1 Mechanical Engineering, Meiji University, Kanagawa, 214-8571, Japan 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139-4307

ABSTRACT All binary shape memory alloys (BSMA) that exhibit not only perfect shape recovery but also partial shape recovery were reviewed and classified into three groups, B2, A2 and A1 type, according to the parent phases. There are the thirteen, six and eleven alloys belonging to B2, A2 and A1 type, respectively. In the group of B2 type BSMA the alloys are divided into two categories due to the combination of the elements. Over A1 and A2 type BSMA the atomic composition of one element is larger than the other so the larger one is called a majority element. The majority elements, Ti, U, Fe, and Cu, of A2 type BSMA do not belong to the group 5 and 6 in which the elements have a typical BCC (A2) crystal structure. In the A1 type BSMA there are four majority elements, Mn, Fe, Co, and In and the BSMA except for In-based BSMA have ferromagnetic or antiferromagnetic natures. This magnetic properties might cause the shape memory effect (SME) on A1 (FCC) type BSMA might. INTRODUCTION The shape memory effect was firstly observed on the AuCd alloy in 1951 [1]. Afterwards, the same behavior was found on the TiNi alloy [2] that exhibits the most excellent performance in all shape memory alloys (SMA) so far. For more than forty years after the discovery of TiNi SMA, it was found that some kinds of alloys exhibit shape memory behaviors and a lot of effort has gone into revealing the mechanics of shape memory effect (SME). In some SMA, especially TiNi and its based alloys, most of the controversies have been solved according to the outstanding review for TiNi SMA and its based alloys written by Otsuka and Ren [3]. Nevertheless, they stated that the general problems still remain to be solved or clarified. One of them is if there is a common microscopic mechanics for all martensitic transformation (MT). Any kinds of alloys, that don’t belong to the category of SMA in the view of less thermoelasticity [4], exhibit the behavior similar to SME with faulted martensite [5]. Moreover, MT of In-Tl alloys, that is one of typical SMA, might be hardly explained on the basis of Zener’s scenario [6] for the instability of BCC/B2 structure. It will be considered that a review of all alloys that have partial and perfect SME is useful to solve and clarify the mechanism of SME and its related behaviors. We have complied just binary alloys that exhibited SME and/or superelasticity confirmed by published papers and proceedings so far. For escaping the complexity we focus on binary not ternary and more alloys. As the results the 30 kinds BSMA are found as shown in Table I and can be divided into three groups with respect to crystal structure at parent. Three groups are named as B2, A2 and A1 type according to Strukturbericht Designation, not e

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