β-Phase Stability of Two Biomedical β-Titanium Alloys During Severe Plastic Deformation

Abstract

The β-phase stability and deformation behavior patterns of two β-type titanium bioalloys, viz. Ti-Nb-Ta-Zr (TNTZ) type and Ti-Nb-Ta (TNT) type, processed by a series of intense plastic deformations have been investigated theoretically and experimentally. Firstly, theoretical analysis was carried out, including an estimation of possible deformation mechanisms based on the electronic parameters of the studied alloys identified with the aid of the Bo–Md diagram. Secondly, phase composition and structural parameters determined by x-ray diffraction (XRD) analysis revealed that the application of severe plastic deformation (SPD) induces grain refinement (in particular for one of the two alloys), accompanied by residual stress generation and some partial phase transformation. Scanning electron microscopy (SEM)/transmission electron microscopy (TEM) imaging and some measurements of the texture completed the deformation behavior analysis. TNT alloy, with higher β stability (Moeq ~ 12.5 wt.%), presented an almost unmodified β-grain dimension from 29.4 nm to 24.4 nm (and thus poor β-grain refinement), coupled with a very fine dispersion of nanometric (~ 8.4 nm) crystallites of orthorhombic α″-stress-induced martensite. TNTZ alloy, also with high β stability (Moeq ~ 10.1 wt.%), showed accentuated β-grain refinement (from 27.8 nm to 9.9 nm), with a very small amount of orthorhombic α″-stress-induced martensite, but grain dimensions almost three times larger than that of the TNT alloy (~ 20.8 nm). The theoretical estimations concerning the possible deformation mechanisms are supported by the analysis of the experimental results.