Wide Range Observation on Crystal Zone of Single Variant of R-phase and Martensite in Ti-50.4 at%Ni Alloy

Wide range selected area electron diffraction (SAD) patterns on 〈110〉 crystal-zones (CZ) of a single variant of R-phase in Ti-50.4 at%Ni alloy quenched from 1223 K indicated that the 〈110〉 CZs can be classified into two types, CZ-A and CZ-B, with respect to the 1/3〈110〉* reflections. Especially the absence of 1/3〈110〉* on the CZ-A could not be explained by the models proposed up to now. Based on the experimental results, a map that expressed the SAD patterns of single R-phase variant on the (111) stereographic projection was presented. It was suggested that the true structure of R-phase should fulfill three conditions on characteristics of SAD patterns, that is, the reproduction of the unique (111) pattern, the absence of 1/3〈110〉* reflection at the specific planes on CZ-A and the retainment of three-fold symmetry with respect to the [111]. Bending the TEM sample composed of several R-phase variants caused the integration of the R-phase variants, and resulted in the formation of the B19″ martensite. However both the single variant and the B19″ martensite returned to the initial state of several R-phase variants after 120 hours at room temperature. It was proposed that the unification of the R-phase variants plays an important role on the R-M transformation.