Nanostructures Formation and Improvement of the Functional Properties of Ti-50.26 at.%Ni Alloy by Warm Rolling and Intermediate Annealing Included in the TMT Schedule

Warm rolling (~150°C) included in the schedule of thermomechanical processing of Ti-50.26at.%Ni shape memory alloy and its influence on the alloy’s functional properties were studied. The alloy was subjected to thermomechanical treatment (TMT) including cold (CR), warm rolling (WR) and intermediate annealing (IA) technological steps in various combinations. Identical post-deformation annealing (PDA) thermal treatment was performed after all the TMT routes.

TEM analysis reveals the formation of nanocrystalline (NC) and nanosubgrained (NS) structures in different combinations. The following results were obtained:  1) the greater the CR contribution, the greater the quantity of NC versus NS structure, and the lower the average dislocation density; 2) the greater the heat energy input during processing (WR±IA in contrast to CR), the greater the quantity of NS at the expense of NC, and the larger the size of grains, subgrains and polygonized areas.

It is shown that the fatigue life of Ti-50.26at.%Ni alloy is significantly improved in two cases: either when CR strain is decreased or when WR/IA are introduced in the TMT schedule, for the same level of a total plastic strain. This fatigue life improvement is a consequence of the reduction of the processing-induced damage combined with the development of a favorable B2-austenite texture. This imrpovement is reached however at a certain expense of the alloy’s functional stability. The main texture component of austenite after all the processing routes corresponds to {100}<110>B2, and after CR(e=1)+IA(400°C, 1h)+WR(e=0.2), the austenite texture sharpness reaches its maximum, while after CR(e=1.2), it reaches its minimum. The lattice parameters of martensite (measured by X-ray diffractometry) and the crystallographic resource of recovery strain ε_max are similar after all the TMT routes.