The morphology of martensite in sputter-deposited thin films of Ti-48.5 at.%Ni was studied. After annealing at 873 K for 1 h, fine Ti2Ni precipitates formed in the matrix of a TiNi phase. Electron diffraction verified that (001) compound twins are dominantly observed as internal defects, which lie in three orientations. Morphologically it was found that there are three kinds of boundary among martensite domains of (001) twinning, namely {113} type, (001)//{111} type and {111} type. In fact, the wave-like {113}-type boundary is also the (001)//{111} type from the atomic viewpoint. These matching planes beside the boundaries derive from the same close packed planes in a bcc structure, and produce low energy boundaries. The formation of (001) twinning is attributed to the impingement of growing martensite variants with the Ti2Ni precipitates during martensitic transformation. In some local regions, (11 (1) over bar) type I or [011) type II twinning can be found owing to the relatively inhomogeneous distribution of these precipitates. [References: 32]
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