Cu content and annealing temperature dependence of martensitic transformation of Ti36Ni49−xHf15Cux melt spun ribbons

The martensitic transformation in Ti36Ni49−xHf15Cux (x = 1, 3, 5, 8) ribbons has been investigated. Only B2 to B19′ transformation was detected in all the present ribbons. The martensitic transformation temperatures do not change obviously with increase in the Cu content except that they decrease when the Cu content is 3 at.%. The lattice parameters of B19′ martensite, a and c increase, b almost remains constant, while the monoclinic angle β decreases with increase in the Cu content. For the ribbons with Cu content of 1 and 3 at.%, the martensitic transformation temperatures change slightly when the annealing temperature increases. For the ribbons with Cu content of 5 and 8 at.%, with increase in the annealing temperature, the martensitic transformation temperatures almost do not change and then decrease rapidly when the annealing temperature is higher than 873 K. TEM observation shows that the microstructure of the ribbons with Cu content of 1 and 3 at.% contains the martensite matrix and the (Ti,Hf)2Ni particles with the size of about 150 nm, which does not change obviously when the annealing temperature increases. This results in that the martensitic transformation temperatures are not sensitive to the annealing temperature in the ribbons with 1 and 3 at.% Cu content. However, nano-scale (Ti,Hf)2Ni particles precipitate in the ribbons with Cu content of 5 and 8 at.% when the annealing temperature is 773 and 873 K, and then the (Ti,Hf)2Ni particles grow and coarsen rapidly with further increase in the annealing temperature. The coarsening of the (Ti,Hf)2Ni particles should be responsible for the dramatic decrease of the martensitic transformation when the annealing temperature is higher than 873 K. For all the present ribbons, the substructure of B19′ martensite is (001) compound twins, and the inter-variant relationship is mainly (011) type I twinning.