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

Atomic configurations in P phase Cu-Al-Ni, Cu-Au-Zn and Cu-Zn-A1 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 chanueling enhanced microanalysis method, called ALCHEMI. Large increases in M, and A, temperatures by several tens of K or more observed in the P 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, cllanging co~npositions in the following formulae: Ti0.5.xNi0.SXx, Ti0.5x12Ni0.5.x12Xx and Ti0.5Ni0.5.xXx. where x s: 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 ~ ~ V ~ ~ ~ ) / v ~ i ~ i , where Vi, is pairwise interaction hetweeu i aud j atoms. Decreases in h& temperature for the B2-B19' martensitic transformation are discussed in tenlls of a11 averagc number of valence electrons'at the specific sites in the parent lattice.