Orientation relationship between TiC carbides and B2 phase in as-cast and heat-treated NiTi shape memory alloys

In the present study, Ni50.7 at.%–Ti shape memory alloys were prepared by vacuum induction melting using graphite crucibles. During melting the carbon concentration of the alloy increased and TiC carbides formed during solidification. The microstructure of the as-cast NiTi alloys was examined using scanning and transmission electron microscopy (SEM, TEM). Only eutectic TiC fibers were found in a low carbon NiTi alloy. For higher carbon concentrations, primary TiC carbides were observed in addition to the fine eutectic TiC phase. TEM results suggest that there is no specific orientation relationship between primary TiC carbides and B2. For the eutectic TiC phase, however, the following orientation relationship could be identified for low and high carbon contents in the NiTi alloy: B2 [0 0 1]||TiC [0 0 1], B2 (2 0 0)||TiC (2 2 0), B2 ( 1   1 ¯   0 ) ||TiC (2 0 0), and B2 (1 1 0)||TiC (0 2 0). It was also found that the interface between TiC and B2 is semi-coherent. During a subsequent post-cast heat treatment new carbides formed with different morphology (sizes and shapes) but with the same orientation relationship. Experimental evidence including SEM and TEM micrographs and diffraction patterns is presented to document these findings.