Occupancy Sites of Constituent Atoms and their Effects on the Martensitic Transformations in some Cu-Based and Ti-Ni-Based Ternary Alloys

Abstract
Atomic configurations in β phase Cu-Al-Ni, Cu-Au-Zn and Cu-Zn-Al ternary alloys before and after martensitic transformations, and occupancy sites of third elements X in the B2 parent lattice of Ti-Ni-X ternary alloys, where X = 3d elements, Pd and Au, have been investigated by means of an electron channeling enhanced microanalysis method, called ALCHEMI. Large increases in M_s and A_s temperatures by several tens of K or more observed in the β phase alloys upon aging at ambient temperatures are attributed to ordering in the parent phase and to disordering in the martensite phase, respectively, both taking place between certain atoms occupying specific sites. The occupancy sites of third elements X in Ti-Ni-X alloys have been systematically examined, changing compositions in the following formulae : Ti_0.5-xNi_0.5X_x, Ti_0.5-x/2Ni_0.5-x/2X_x and Ti_0.5Ni_0.5-xX_x, where x≤0.05. While Fe, Co and Pd atoms occupy the Ni site preferentially, Sc atoms occupy the Ti site irrespective of composition, and the others the Ni and/or Ti site, depending on composition. These results for the Ti-Ni-X alloys are reasonably explained by a parameter D, which is defined by D = (V_TiX-V_NiX)/V_TiNi, where V_ij is pairwise interaction between i and j atoms. Decreases in M_s temperature for the B2→B19' martensitic transformation are discussed in terms of an average number of valence electrons at the specific sites in the parent lattice.