Effect of cooling rate on phase transformation in Ti2AlNb alloy

Phase transformation and microstructure evolution of a Ti2AlNb alloy upon cooling from B2/β phase at rates of 0.02–100 °C/s were investigated by using X-ray diffractometer, scanning electron microscope, and thermal dilatometer. At the lowest cooling rate of 0.02 °C/s, both α2 and O phases precipitate from the B2/β phase. The precipitation is prior to occur at the grain boundary of the parent phase, which even makes α2 phase almost disappear from the interior of the grain. Cooling the alloy at a higher rate results in a decreasing amount of the precipitates. The critical rate for the complete depression of α2 and O phase precipitation is 0.5 °C/s and 4 °C/s, respectively. According to the established continuous cooling transformation (CCT) diagram, the temperature interval for B2/β→O phase transformation decreases with increasing cooling rate. The alloy cooled at the rate of 0.1 °C/s has the highest microhardness for the appropriate size and amount of the precipitated phases.