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.