Thermally-induced α→β phase transformation interweaving with abnormal α grain growth in hot extruded TNM alloy

The microstructural and crystallographic features of the β phase in TNM alloy during the hot extrusion at 1290 °C and the subsequent cooling process were investigated. Except the original grain boundary β phase, two categories of β particles, along grain boundaries of primarily recrystallized α grains (intergranular β) and in grain interiors of abnormally grown α grains (intragranular β), were found in the extruded microstructure. The former obeyed the Burgers orientation relationship with, at least, one of the neighboring α grains, whereas the latter did not show any specific orientation relationship with the α host. Analysis revealed that the thermally-induced α→β phase transformation interweaving with the abnormal α grain growth happened during the transition period from end-extrusion to water-quenching. The intergranular β particles were derived from the thermally-induced α→β phase transformation, and the intragranular β particles were resulted from the intergranular β precipitates after being swallowed by the abnormally grown α grains. The present work provides new information of thermally-induced phase transformation by adiabatic heating and the annexation of the second phase by abnormal grain growth during high-speed hot deformation.