Instability of the B2 Ordered Structure in TiNi Shape Memory Alloys due to Atom Displacements on (110) Planes

Abstract

Lattice instability due to the formation of lattice displacement waves (LDW) was investigated in the B2 ordered structure of Ti-Ni shape memory alloys using first principle calculations. The modified B2 structure with this LDW of the 1/2(011)[011] phonon mode have structural similarity with the B19′ monoclinic structure, which is martensitic structure known to appear in Ti-Ni alloys at low temperatures. The shift to optimal lattice positions for Ti and Ni atoms is determined by the force acting on each nucleus. Ni and Ti atoms are displaced from their original positions in the B2 structure without any potential barrier. The atomic displacements without any potential barrier can lead to precursor phenomenon in advance of martensitic transformations.