Composition Dependence of the Atom Location of the Third Element in Ti–Ni–X Shape Memory Alloys

The composition dependence of the atom locations of the third elements in Ti–Ni–X shape memory alloys (X=Cr, Mn, Fe, Co or Cu) has been investigated by the electron channelling enhanced microanalysis. As a result, the followings were clarified: All the added third elements mainly occupied the Ni atom site for the Ti50Ni50−xXx alloys. However, different atom sites were occupied depending on the kind of the third elements for the Ti50−xNi50Xx alloys, that is, Mn and Cu mainly occupied the Ti atom site while Fe and Co did the Ni atom site, and Cr occupied both the atom sites with nearly the same probability. A thermodynamic consideration has been done on the above results, that is, the free energy of the alloy systems was calculated on the basis of the Bragg-Williams approximation, taking account of the configurational entropy, and the atom location of the third elements was determined so as to minimize the free energy. The atom location thus obtained was qualitatively in good agreement with the experimentally determined one. A slight disagreement was found between the calculation and experiment, and it appeared to be due to the size difference among the third element and the host atoms.