Change of M s Temperatures and its Correlation to Atomic Configurations of Offstoichiometric NiTi-Cr and NiTi-Co Alloys
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Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4249 Nagatsuta, Midori-ku, Yokohama 226, Japan, Now with Institute for Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-77, Japan. Now with NKK Cooperation, Minamiwatarida, Kawasaki-ku, Kawasaki 210, Japan, of Materials Science and Engineering, University of Washington, Seattle, WA 98195-2120, USA, " .Department Professor Emeritus, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan. ABSTRACT Effects of compositional deviation from the stoichiometry and Cr and Co additions on martensitictransformation-start and austenite-start temperatures (M. and A,) of offstoichiometric NiTi alloys are investigated. M. and A,are determined using conventional differential thermal analysis (DTA), where the temperature range investigated is between 77K and 423K. Alloys are widely chosen with both Ni- and Ti-rich compositions, to which ternary elements, Cr and Co, are added. It is clearly shown that MKand A, in single phase regions are reduced with increasing amount of constituent element, Ni, and ternary elements, Cr and Co. On the other hand, M, and A, do not depend on Ti concentration when Ti concentration is more than 52mo1.%. NiTi alloys are in two phase region in the case. M, changes by Co at offstoichiometry are evaluated to be -15K / mol.% in Ni poor side and -30K / mol.% in Ni rich side. These values correspond to -22K / mol.% for the stoichiometric NiTi alloys. Also, effects of Cr on M, are evaluated to be -65K / mol.% in Ni poor side and -45K / mol.% in Ni rich side. The former is similar to the M, change in stoichiometric alloys, and the latter is close to our prediction of -30K / mol.% in comparison with the reported value of -120K / mol.% for stoichiometric alloys. It is concluded for offstoichiometric NiTi alloys that effects of ternary additions on M, can be explained using electronic structures of ternary elements by taking atomic configurations into account, as well as the stoichiometric NiTi alloys. Effect of degree of order is also discussed. INTRODUCTION
For designing smart materials composed of shape memory alloys (SMA), it is important to control martensilic transformation temperatures and temperature ranges for superelasticity of SMA. Figure 1 shows a schematic deformation map for SMA describing phase stability related to stress and temperature [1]. This map is a modification of the map reported by Otsuka and Wayman [2]. Here, MKand Mf are martensitic transformation startand finish temperatures, A, and At are austenite transformation start and finish temperatures, and M,1 is a maximum temperature below which martensitic transformation can occur by either stress or temperature. Using the map it is easily understood for designing SMA that both transformation temperatures (M,, Mf, A,, Af and Md) and critical stress for plastic deformation must be controlled. Recently we have conducted alloy design of intermetallics through controlling atomic configurations, i. e. defect structures at offstoichiometry and site
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