Microstructural evolution and strengthening mechanisms in Ti–Nb–Zr–Ta, Ti–Mo–Zr–Fe and Ti–15Mo biocompatible alloys

The microstructural evolution and the strengthening mechanisms in the two quaternary alloys, TNZT (Ti–34Nb–9Zr–8Ta) and TMZF (Ti–13Mo–7Zr–3Fe), and one binary alloy, Ti–15Mo, have been investigated. In the homogenized condition both the quaternary alloys exhibited a microstructure consisting primarily of a β Ti matrix with grain boundary α precipitates and a low volume fraction of primary α precipitates while the binary alloy showed single-phase microstructure with large β grains. On ageing the homogenized alloys at 600 °C for 4 h, all the alloys exhibited the precipitation of refined scale secondary α precipitates distributed homogeneously in the β matrix. However, after ageing while the hardness of TMZF marginally increased, that of the TNZT and Ti–15Mo alloys decreased. Furthermore, the modulus of TNZT decreased while other two alloys showed opposite trends. TEM studies indicate that there is initially a B2 ordering in TNZT that is destroyed after ageing causing a reduction in both hardness and modulus of this alloy. Also in Ti–15Mo, dissolution of ω precipitates on ageing causes the hardness to reduce, while the precipitation of secondary α causes an increase in the modulus. Using these examples, the important influence of thermal processing on the property–microstructure relationships in orthopaedic alloys for implant applications will be highlighted.