Effect of thermomechanical treatment on the sequence of phase transformations in TiNi base alloys

Abstract

1. The use of diagrams of phase transformations, plotted from the dependences of the deformation of specimens on the temperature in heat cycling under constant stress, is preferable to diagrams obtained by other methods because the former make it possible to choose more accurately an alloy for the required operating conditions. 2. The combination of deformation by reduction + annealing makes it possible to change the nature of the phase transformation and to initiate the martensitic transformation B2→R→B19′ in a large group of TiNi base alloys. 3. When alloys are treated by the optimal regime of deformation by reduction + annealing (tann=380–530°C), they have the maximal degree of strain of transformation and good operational properties. 4. When alloys in the process of heat treatment are annealed at temperatures higher than 580°C, their properties are impaired: the degree of strain of transformation is lowered, dislocational yield strength becomes lower, there is increased proneness to cumulation of residual strain in heat cycling over the range of martensitic transformations under load, as a result of which the degree of shape recovery is lowered. 5. The processes accompanying the annealing of deformed material differ substantially from the processes accompanying the annealing of homogenized of hardened metal; this provides new possibilities of controlling the properties of titanium nickelide base alloys.