Structure and thermoelastic martensitic transformations in ternary Ni–Ti–Hf alloys with a high-temperature shape memory effect

The effect of alloying by 12–20 at % Hf on the structure, the phase composition, and the thermoelastic martensitic transformations in ternary alloys of the quasi-binary NiTi–NiHf section is studied by transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction. The electrical resistivity is measured at various temperatures to determine the critical transformation temperatures. The data on phase composition are used to plot a full diagram for the high-temperature thermoelastic $B$2 $\leftrightarrow$ $B$19' martensitic transformations, which occur in the temperature range 320–600 K when the hafnium content increases from 12 to 20 at %. The lattice parameters of the $B$2 and $B$19’ phases are measured, and the microstructure of the $B$19’ martensite is analyzed.