Change of Ms Temperatures and its Correlation to Atomic Configurations of Offstoichiometric NiTi-Cr and NiTi-Co Alloys

Abstract

ABSTRACTEffects of compositional deviation from the stoichiometry and Cr and Co additions on martensitic-transformation-slart and auslenite-start temperatures (Ms and As) of offstoichiometric NiTi alloys are investigated. Ms and As 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 Ms and As in single phase regions are reduced with increasing amount of constituent element, Ni, and ternary elements, Cr and Co. On the other hand, Ms and As do not depend on Ti concentration when Ti concentration is more than 52mol.%. NiTi alloys are in two phase region in the case. Ms 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 Ms are evaluated to be -65K / mol.% in Ni poor side and -45K / mol.% in Ni rich side. The former is similar to the Ms 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 Ms 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.