Alloys Design of PdTi-Based Shape Memory Alloys Based on Defect Structures and Site Preference of Ternary Elements

Abstract

Defect structures and site preference of ternary elements in B2-type PdTi alloys are evaluated in order to design PdTi-based smart materials using the pseudo-ground state analysis (or analysis performed at near zero Kelvin). It has been found that this analysis can predict types of defect structures in these alloys which are of antistructure (substitution) type regardless of alloy compositions. Substitution behavior predicted is as follows: (1) most elements belonging to groups 1A through 5A occupy Ti sites only; (2) most elements belonging to groups 6A through 8A occupy Pd sites only; and (3) the others studied occupy either sites when these sites are unfilled with the corresponding constituent elements (Pd or Ti). Moreover, effects of both offstoichiometry and 3dtransition elemental additions on martensitic transformation start temperature (MS) in PdTi alloys are evaluated as a function of electron-atom ratio (e/a). It has been found that M, is strongly related to the e/a value when 3d electrons of ternary additions are taken into account, although the effect of e/a on M, differs if their substitution behavior is different. Changes in M, per e/a are estimated to be 900 K when Pd-site substitution elements, such as Cr, Mn, Fe, Co, and Ni are added, while changes in M, per e/a are 790 K when Ti-site substitution elements such as V are added. This suggests that M, is influenced not only by the e/a value but also by atomic configurations.