Abstract
We investigate the structure and substructure of the solution-treated martensitic Ni50.3Ti29.7Hf20 high-temperature shape memory alloy (SMA) by transmission electron microscopy and precession electron diffraction (PED). Most martensite plates contain high-density internal (0 0 1) compound twins. Four martensite variants (i.e., A, B, C, and D) were observed using PED. The orientation relationships were determined to be (1 1 0.64) Type II twins between A and B, and (1 1 1) Type I twins between C and D. B and C are divided by junction planes, in which (1 1 1) in C is parallel to (1 1 0.64) in B. These orientation relationships are rarely observed in other NiTi or NiTiHf SMAs, which offers new insight to understand the phase transformation behavior in Ni50.3Ti29.7Hf20 SMAs. Several low-angle grain boundaries were observed near the variant interfaces, which may explain the difficulty of martensite reorientation and detwinning in NiTiHf SMAs.
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