NUCLEATION AND SELF-ACCOMMODATION OF THE R-PHASE IN TI-NI ALLOYS

Transformation and self-accommodation mechanisms of the R-phase in Ti-Ni-Fe and Ti-Ni-Al shape memory alloys were studied by means of optical and electron microscope observations. Sequences of transformation events were recorded successfully with video tapes using a TV camera. Through the observation we found the following. The R-phase transformation takes place by nucleation and growth in a heterogeneous manner. Nucleations take place at stress concentrated places, e.g., matrix-inclusion interfaces and dislocation. The R-phase can nucleate from the stress field of a single dislocation. The R-phase self-accommodates during growth. The fundamental unit of the self-accommodation is composed of four variants. These variants are twin related one another and there are two types of twin relations. One is (1121BAR)(112BAR]H type compound twin and the other is (1122BAR)[111]H type compound twin. The four variants self-accommodate around one of <001>P axes, and there are six groups of self-accommodation units (two for each one of <001>P axes) for the single parent.