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

The beta-phase stability and deformation behavior patterns of two beta-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 beta stability (Mo-eq similar to 12.5 wt.%), presented an almost unmodified beta-grain dimension from 29.4 nm to 24.4 nm (and thus poor beta-grain refinement), coupled with a very fine dispersion of nanometric (similar to 8.4 nm) crystallites of orthorhombic alpha ''-stress-induced martensite. TNTZ alloy, also with high beta stability (Mo-eq similar to 10.1 wt.%), showed accentuated beta-grain refinement (from 27.8 nm to 9.9 nm), with a very small amount of orthorhombic alpha ''-stress-induced martensite, but grain dimensions almost three times larger than that of the TNT alloy (similar to 20.8 nm). The theoretical estimations concerning the possible deformation mechanisms are supported by the analysis of the experimental results.