Abstract
A method has been developed to produce fine grain sizes in Ti-50.7 at%Ni alloy by means of thermo-mechanical treatment (TMT). The TMT process involved sequential stages of intermediate annealing and cold rolling followed by recrystallization. The intermediate annealing was carried out for precipitating fine Ni-rich phase particles as well as for recovering internal strains. The relations of the cold rolling, recovery and recrystallization to transition behavior and the grain size dependence of transition temperatures have been studied.
In order to obtain uniform and equi-axed fine-grained recrystalllized structure, the recrystallizing heat treatment should be done at somewhat high temperatures at which all precipitates can be re-occluded in the matrix crystal. It was found that the martensitic transformation temperature (Ms) increased with a decrease in grain size. This may be caused by the facts that the grain boundaries can act as nucleation sites for the martensitic transformation and the insufficient recovery of Ni concentration in the matrix caused by undissolved precipitates. The TMT process used in the present study would not lower the properties of the alloy.
