Internal Structure of B19 Martensite in AuTi Shape Memory Alloy
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0980-II04-12
Internal Structure of B19 Martensite in AuTi Shape Memory Alloy Tomonari Inamura1, Ryosuke Tachi2, Kenji Wakashima1, and Hideki Hosoda1 1 Precision and Intelligence Labortory, Tokyo Institute of Technology, 4259Nagatsuta, Midori, Yokohama, 226-8503, Japan 2 Graduate student, Tokyo Institute of Technology, 4259Nagatsuta, Midori, Yokohama, 2268503, Japan
ABSTRACT Internal twin of B19 martensite in equiatomic AuTi binary alloy was examined by conventional transmission electron microscopy observation and the phenomenological theory of martensite crystallography (PTMC). The crystal structure of martensite was B19 (orthorhombic) with the lattice parameters of 0.2944nm, 0.4900nm and 0.4633nm. Most of martensite plates were internally twinned by {111}typeI twin. typeII twin was occasionally observed and {011}compound-twin relationship was observed at boundaries between adjacent martensite plates. However, no martensite plate entirely twinned by the typeII twinning or the {011}compound twinning was observed. PTMC analysis showed that the invariant plane is formed only by the introduction of the internal twin of {111}typeI or typeII twin in the present geometry of the transformation. The geometry of a typical martensite plate with internal twin of {111}typeI twin was in good agreement with that required for the formation of the habit plane with the invariant plane character. The observed {111}typeI twin is, therefore, considered to be the lattice invariant shear to minimize the elastic strain energy due to the transformation.
INTRODUCTION Martensitic transformation temperature (Ms) of NiTi binary alloy is around 400K at most [1]. The shape memory effect of NiTi alloy cannot be, therefore, used at the high-temperatures above 400K. The development of the shape memory alloy (SMA) with Ms higher than 400K has been strongly required to expand the applications related with SMAs. On the other hand, NiTi SMAs are recently used in human body as stents, bone plates and intramedullary nails. There is a possibility of the onset of Ni-hypersensitivity due to the implantation of NiTi. A new SMA containing no toxic element is strongly required in addition to the high temperature SMA. It has been known that PdTi, PtTi and AuTi alloys have a martensitic transformation from B2 to B19 (orthorhombic) structure with Ms much higher than 400K and exhibit the shape memory effect [2-6]. It has been demonstrated that Ms of PdTi and AuTi can be lowered and controlled by the ternary addition of various elements [7-8]. Titanium and platinum group elements are known as non-toxic elements for human body. These alloys are, therefore, attractive on the view point of the development of the high temperature SMA and/or the new biomedical SMAs with Ms around room temperature (RT). Shape memory properties such as recovery strain, hysteresis, stress to induce martensite and their orientation dependence are essentially characterized by the crystallography of the
martensitic transformation. It is necessary to know the detail of the crystallographic
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