Effect of cooling rate on α variant selection and microstructure evolution in a near β Ti–5Al–3Mo–3V–2Cr–2Zr–1Nb–1Fe alloy

The microstructure evolution and selection of α variants in a new near β titanium alloy Ti-5321 (Ti–5Al–3Mo–3V–2Cr–2Zr–1Nb–1Fe) were thoroughly investigated by dilatometry under continuous cooling conditions accompanied with OM, XRD, SEM, EBSD and TEM. Cooling rates from β region to room temperature significantly influenced the microstructure based on the competitive growth of α_GB, α_WGB and α_WI. The low (0.01 °C/s and 0.1 °C/s), intermediate (0.5 °C/s and 1 °C/s) and high (2 °C/s, 4 °C/s and 5 °C/s) cooling rates resulted in bi-lamellar, basket-weave and single metastable-β microstructures, respectively. Moreover, the transformed microtexture was also found in the cooling process at low cooling rate. In addition, a continuous cooling transformation (CCT) diagram obtained by the dilatometric analysis revealed the sensibility of the β→α phase transformation temperature range to the cooling rate. EBSD analysis revealed that microtexture formed by α colonies with the similar orientations, occurred in the adjacent β grains and maintained Burgers orientation relationship (OR) with at least one of the adjacent β grains. Two typical cases, i.e. the single-BOR α colonies (the colonies which maintain Burgers OR with β grain only on one side) and the double-BOR α colonies (α colonies which maintain Burgers OR with both the adjacent β grains) were studied. The single-BOR α colonies corresponded to the different α variants from the parent β grain. The double-BOR α colonies occurred in which the adjacent β grains shared nearly common {110}_β plane, moreover, the double-BOR α colonies could possess the identical orientation in the adjacent β grains which have a 60° rotation about a common <111>_β direction.