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

Electron microscopy and differential Scanning calorimetry in combination with a thermodynamic analysis were used to investigate the structure and conditions for the realization of martensitic transformations in NiTi-Hf alloys with 9.5-20 at. % Hf rapidly quenched from the liquid state. The hafnium effect on the proportion of amorphous and crystalline components (with grains of various size) was studied. In the crystalline component, two types of martensite were observed, namely, the monoclinic martensite B19' and the orthorhombic martensite B19. It was found that the transformation of the high-temperature B2 phase with the formation of the B19' martensite was realized 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 inhibiting effect of amorphous layers located at grain boundaries on the martensite transformation and a relationship between the grain size and type of martensite formed in them.