Functional properties of nanocrystalline, submicrocrystalline and polygonized Ti–Ni alloys processed by cold rolling and post-deformation annealing

Abstract

Thermomechanical processing consisting of cold rolling ( e = 0.3–2.0) and post-deformation annealing (300–450 °C, 1 h) was applied to binary Ti–Ni alloys to produce nanocrystalline structures (NS) or polygonized dislocation substructures (PDS), or their mixture. The evolution of the material structure and properties was studied using TEM, X-ray, microhardness, calorimetry and tensile testing techniques. Recovery stress and strain of the Ti–50.26 at.%Ni alloy and superelastic strain of the Ti–50.6 at.%Ni alloy were measured under static and fatigue conditions. It was found that higher true yield stress of NS alloys not only increases the recovery stress potential, but, since it is combined with a relatively low transformation yield stress; it increases the completely recoverable strain. NS alloys generate recovery stresses that are twice as high as those of PDS alloys (1200 MPa), completely recoverable strains that are 10% greater (up to 6% in tension), and they demonstrate a higher cyclic stability of shape memory and superelastic properties. This improvement comes with the cost of a lower NS alloy fatigue damage tolerance, aggravated by the presence of microcracks caused by cold working. Binary Ti–Ni alloys, processed by annealing of an intermediately cold-worked ( e = 0.75…1) alloy and containing mixed nanocrystalline structure and polygonized dislocation substructure, allow a high fatigue life combined with relatively high and cyclically stable functional properties.