Effect of equal channel angular extrusion on the shape memory property of Ti-Nb Alloys

Abstract

Bio-implants are materials that are used to replace the hard tissues in human body such as bone. Continual ageing of world population has brought an ever-increasing demand for bio-implants. Among the metallic bio-implant materials stainless steels and Co-Cr alloys have commonly been used. However, their high Young’s modulus led bone resorption resulted in the development and application of low modulus $\beta$ type titanium alloys in 1979. In that scenario, shape memory alloys (SMA) were recognized as bio-implant materials in late 1980 due to their functional shape memory and superelastic properties. Since then equi-atomic Nickel-titanium (Ni-Ti) SMA are widely being applied as cardiovascular stents, guide wires and orthodontic wires due to its excellent mechanical, corrosion resistance and superelastic properties. But recent studies have shown Ni induced hypersensitivity is harmful to human body. In order to solve this issue Ni-free SMA are under development. Titanium-niobium (Ti-Nb) binary alloys are potential candidates for the purpose. The useful amount of superelasticity for medical applications is 2%. Though the Ti-Nb binary alloys can exhibit superelasticity more than 2%, their poor mechanical properties hinder to realize it thus hampers its application. Efforts are being underway to improve the mechanical and functional properties by employing various thermo-mechanical treatments (TMT) and by adding alloying elements. Recently, ultrafine grain (UFG) Ni-Ti alloys processed via severe plastic deformation processes such high pressure torsion, equal channel angular extrusion (ECAE) shown to have excellent superelastic recovery strain along with ultrahigh strength and good elongation to fracture. Also UFG materials are significantly differ in the behavior from their coarse grain counterpart. However, the conventional cold rolling process was the only method used to process the Ti-Nb alloys till date. There has been no effort to produce UFG Ti-Nb to...