Influence of the ageing conditions and the initial microstructure on the precipitation of α phase in Ti-17 alloy

Abstract

The precipitation of α phase during ageing was investigated in the near-β titanium alloy Ti-17 considering either a fully βmetastable initial microstructure or a 35% αprimary + 65% βmetastable initial microstructure. In-situ electrical resistivity and high energy X-ray diffraction measurements revealed the influence of the initial microstructure, with different α morphologies (size and distribution of αprimary), as well as the heating rate on the precipitation sequences and kinetics following the decomposition of the β-metastable phase. Various amounts of metastable phases (ωisothermal and α″isothermal) precipitate in temperature ranges that increase with the heating rate. From temperatures about 500 °C, the orthorhombic α″isothermal structure evolved towards the hexagonal close-packed α as temperature increased. SEM microstructure characterisations showed that slow heating rates promoted a fine and dense α precipitate distribution through the formation of ωisothermal and/or α″isothermal, leading to higher hardness values. A higher heating rate restricted the precipitation of α″isothermal and shifted to the one of α at a higher temperature, leading to coarser precipitates. Furthermore, precipitation kinetics of α″isothermal/α were quicker considering an initial intragranular α precipitation as compared to α colonies.