Structural changes at heating of the β-phase in NiTi alloys

A note. Electron transmission microscopy and selected area diffraction studies of structural changes occurring on annealing between 300 and 700 deg C are described. Annealing in this temp. range leads to the formation of new phase domains with a f.c.c. structure (lattice constant a = 15.8 A), in grains of the parent phase. This structure cannot be referred to any well-known structures in the Ni--Ti system. The domain size of the phase depends on temp. and duration of annealing. Increasing the temp. causes domain growth; a max. size is reached at 600 deg C. Initially the domains are round and < 100 A in size with two or three orientations in each grain. As the domains grow, larger ones absorb smaller ones, and from 500 deg C the domains acquire a plate form with their long axis along < 110 > . The ordered domains disappear at 700 deg C.--R.E.