Martensitic transformations in ribbons of rapidly quenched titanium nickelide alloyed with hafnium

Electron microscopy and differential Scanning calorimetry in combination wi th a thermodynamic analysis were used to investigate the structure and cond itions for the realization of martensitic transformations in NiTi-Hf alloys with 9.5-20 at. % Hf rapidly quenched from the liquid state. The hafnium e ffect on the proportion of amorphous and crystalline components (with grain s of various size) was studied. In the crystalline component, two types of martensite were observed, namely, the monoclinic martensite B19' and the or thorhombic martensite B19. It was found that the transformation of the high -temperature B2 phase with the formation of the B19' martensite was realize d mainly in fine grains with a size less than 2-4 nm. The B19 martensite is formed in the coarser grains. Data were obtained that indicate the inhibit ing effect of amorphous layers located at grain boundaries on the martensit e transformation and a relationship between the grain size and type of mart ensite formed in them.