Effect of Cooling Rate on α Variant Selection and Microstructure Evolution in TB17 Titanium Alloy.

The α variant selection and microstructure evolution in a new metastable β titanium alloy TB17 were studied in depth by DTA, microhardness, XRD, SEM, and EBSD characterization methods. Under the rapid cooling rate conditions (150 °C/min-400 °C/min), only a very small amount of granular α (α Widmanstatten precipitates within the grains) precipitated within the grains. The secondary α phase precipitated in the alloy changed from granular to fine needle-like at moderate cooling rates (15 °C/min-20 °C/min). When continuing to slow down the cooling rates (10 °C/min and 1 °C/min), the α (α precipitates along the β grain boundaries), α (α Widmanstatten precipitates that developed from β grain boundaries or α) and α grew rapidly. Moreover, the continuous cooling transformation (CCT) diagram illustrated the effect of cooling rate on the β/α phase transition. EBSD analysis revealed that the variants selection of α near the original β grain boundary is mainly divided into three categories. (i) The double-BOR (Burgers orientation relationship) α colonies within neighboring β grains grow in different directions but have the same crystallographic orientation. (ii) The double-BOR α colonies within neighboring β grains have different growth directions and different crystallographic orientations. (iii) The double-BOR α colonies within the same grain have the same growth direction, but different crystallographic directions. And these double-BOR α colonies correspond to two variants of the given {0001}α//{110}β.