Microstructure development by thermomechanical processing in duplex stainless steel and β titanium alloy

In the case of duplex stainless steel, when the supersaturated alpha (bcc) specimen is aged after heavy cold rolling, subgrains are rapidly formed by recovery of alpha matrix and then gamma (fcc) phase precipitates at alpha subgrain boundaries. By this process, the microduplex structure which consists of alpha subgrain matrix and fine gamma precipitates is obtained. The alpha subgrains in the microduplex structure are stable and the recrystallization does not occur even after the prolonged annealing, because subgrain boundaries are pinned by fine gamma precipitates. By the cyclic treatment of slight cold rolling and short time annealing in the microduplex structure, the misorientation of alpha subgrain boundaries is continuously increased with an increase in the number of repetition and subgrains change to grains with high angle boundaries. For the Ti-15V-3Cr-3Sn-3Al alloy, alpha (hcp) phases precipitate on dislocations before the subgrain formation of heavily deformed beta (bcc) matrix. As the aging is prolonged, the rearrangement of dislocations in beta matrix proceeds, resulting in the formation of subgrain. In this case, the distribution of fine alpha precipitates is not so uniform because of the inhomogeneity of deformation structure. In order to obtain more uniform distribution of alpha precipitates, the recovery treatment (short time annealing at higher temperatures of slightly cold rolled specimen) prior to aging is effective.