The Martensitic Transformation and Extra Reflections Appearing Prior to the Transformation in AuCd Alloy
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TRANSFORMATION AND EXTRA TO THE TRANSFORMATION IN
REFLECTIONS AuCd ALLOY
T. OHBA* and K. OTSUKA** *Department of Materials Science and Engineering, Teikyo University, Toyosatodai, Utsunomiya 320, Japan, [email protected] "**Instituteof Materials Science, University of Tsukuba, Tsukuba 305, Japan ABSTRACT AuCd alloy is a typical martensitic alloy. The crystal structural of the ý 2 '(trigonal) martensite was recently determined and the analysis indicated the possibility of phonon softening. Despite a large absorption coefficient of Cd phonon softening was actually observed by using an isotope 114Cd. In the present study, precise x-ray scattering studies were performed with a rotating anode x-ray source and synchrotron radiation at Photon Factory in KEK, using small crystals. The structure factors of the parent phase and the diffraction profiles along * direction were measured with four circle diffractometers. The structure factors decrease when approaching the transformation temperature. Superstructure reflections were observed prior to the onset of the transformation. They are very weak and rather sharp and appear at q = (1 / 3,1 / 3,0) within the experimental error, i. e. at the commensurate position. INTRODUCTION Shape memory alloys are one of the intelligent materials. They have great potential for industrial and medical applications. The shape memory effect is closely related to the martensitic transformation which was known and used for a long time such as steel hardening. The martensitic transformation occurring in the thermoelastic alloys shows the shape memory effect. The surface relief, shape strain and the existence of the habit plane attracted many researchers. Those characteristic features led to the phenomenological theory(1-3] of martensitic transformation. The phenomenological theory has been applied to several alloys and described well the morphology of the martensite. The microscopic understanding of the martensitic transformation is also important. The martensitic transformation is a typical first order transformation and is found in many materials these days. The transformation mechanism from the microscopic point of view becomes more important in these situations. Diffraction is the most powerful technique for the microscopic approach. Many efforts using electron microscopy, x-ray and neutron were performed and most of them were concentrated on finding the precursor phenomenon of the martensitic transformation. From a structural consideration, the phonon softening studies of CuAINi[4], AuCuZn 2 [5-6] and InTl[7] were performed. However no clear softening was observed in these alloys. Clear phonon softening was reported in the TiNi alloy.[8-10] Diffraction works on TiNi alloys were summarized well in the paper of Shapiro et al.[1 1] They observed the 'premartensitic state' of Ti-Ni alloy and reported that the extra peaks appear at the incommensurate positions and do not have the periodicity of the Brillioun zone. Yamada et al.[12-13] and Gooding and Krumhansl[14] proposed the transfo
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