Microstructure and microhardness of heat-treated Ti–6Al–2Sn–4Zr–6Mo alloy

Ti–6Al–2Sn–4Zr–6Mo is a α + β titanium alloy that combines the long-term strength at elevated temperature with the hardenability by heat-treatments. In this work the effect of conventional heat treatments on the microstructure is investigated, in relation with the hardness properties of the material, by Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersion X-ray Spectroscopy (EDXS) and X-ray diffraction (XRD). According to the Rietveld method the quantitative phase analysis by XRD has been carried out, showing the evolution of the phases with the different heat treatments. Solution-treated samples exhibit a microstructure of α-phase crystals embedded in a matrix of retained-β phase and/or α″ martensite, depending on solution treatment temperature; the α-phase amount depends on the solution-treatment temperature. The presence of the α″ martensite does not lead to a significant hardening of the material. The aging treatment induces the transformation of the retained-β phase and the martensite in a very fine α + β structure that hardens the alloy. The hardness is related to the amount of the fine (α + β)-phase and a linear dependence on the β-phase amount formed during the solution treatment can be identified.