Electron microscopy studies of martensitic transformations in Ti 50Ni 50− xCu x alloys. Part I. Compositional dependence of one-third reflections from the matrix phase

Abstract

A transmission electron microscopy study has been made of the high temperature phase of Ti 50Ni 50− x Cu x alloys, where x was varied from 1 to 19, the Ti content being essentially constant. The high temperature phase was confirmed to have B2-type ordered structure over the composition range studied. Contrary to previous results, however, the M s temperature of the alloys was found to vary considerably, ranging from room temperature to 150°K. On cooling alloys with lower Cu content ( x ≤ 3), the one-third reflections, considered to be associated with a rhombohedral distortion of the B2 structure, appeared prior to the subsequent martensitic transformation, as in binary TiNi alloys. Their appearance was accompanied by microstructural changes, i.e., the formation of needles and domains. The domains were observed to persist in the martensite. In the alloy x = 19, however, the one-third reflections were not observed, even close to the M s temperature. This suggests that the B2 → R (rhombohedral) transformation may be suppressed when the substitution of Cu for Ni exceeds a certain value. The absence of the B2 → R transformation is expected to affect significantly the crystallography of the martensites subsequently produced.