Optimization of the cold rolling processing for continuous manufacturing of nanostructured Ti–Ni shape memory alloys

Abstract

A Ø 1.0 mm Ti–50.26 at.% Ni wire is cold rolled by varying its thickness reduction (true thickness reduction from e = 0.25 to 2.0), the back and forward pulling tensions applied to the strip (from 0.1 to 0.5 of the material yield stress, σ y) under dry and lubricated conditions, and then annealed to obtain a nanoscale grain structure or polygonized dislocation substructure, or their mixture. For the as-cold worked material, calorimetry, microhardness and optical microscopy are used to evaluate the quantity of the amorphized and ultrafine-grained phases, their mechanical properties, and the surface quality of the material. For the annealed material, tensile testing and recovery stress measurement are used to evaluate the mechanical and shape memory properties. It is found that for the same level of cold work, increasing the pulling tension leads to a decrease in the roll force and to a closer correspondence between the rolled product thickness and the set-up thickness. The higher the cold work reduction, the higher the quantity of the amorphized and ultrafine-grained phases and the higher the recovery stresses obtained after post-deformation annealing. On the other hand, the higher the thickness reduction, the larger the number of microcracks appearing on the samples’ edges. The average and the maximum lengths of the microcracks increase as the pulling stress applied to the strip increases. Lubrication decreases the average length of the microcracks, but does not affect their maximum length. From a functional properties perspective, lubricated cold rolling with a thickness reduction of e = 1.5 using back and forward pulling tensions of 0.1 σ y constitute optimal processing conditions, which allow a better trade-off between higher values of recovery stresses and the structural integrity of the sample.