Effect of {0 0 1}〈1 1 0〉 texture on superelastic strain of Ti–Nb–Al biomedical shape memory alloys

Abstract

Anisotropy in recovery strains was examined in a Ti–Nb–Al superelastic β-Ti alloy with a well-developed {0 0 1} β〈1 1 0〉 β texture. An ingot of Ti–24 mol% Nb–3 mol% Al alloy was fabricated by arc melting, homogenized at 1273 K for 7.2 ks, cold-rolled with 99% reduction in thickness and then solution-treated at 873 K for 3.6 ks. X-ray diffraction pole figure and electron backscatter pattern analysis revealed that {0 0 1} β〈1 1 0〉 β texture with grain size of 2 μm was developed in the alloy under the condition of the above thermo-mechanical treatment. Cyclic loading-unloading tensile test was made to evaluate superelastic properties. Tensile specimens were made with various orientation between the rolling direction (RD) and the longitudinal direction. Superelastic strain ( ɛ SE) was measured as the sum of the transformation strain ( ɛ TR) of β–α″ thermoelastic martensitic transformation and pure elastic strain ( ɛ E). It was found that ɛ TR at ϕ = 0° and 90° was almost the same and decreased to a minimum at around ϕ = 45°. On the other hand, it was found that ɛ E increased to a maximum at around ϕ = 45°. These orientation dependences of ɛ TR and ɛ E were in good agreement with the crystallography of the transformation and the anisotropy in Young's modulus in the alloy. ɛ SE was almost a constant regardless of ϕ. It was found that anisotropy in the tensile component of the superelastic strain available along the directions lying within the normal plane can be lowered by the development of the {0 0 1} β〈1 1 0〉 β texture.