Microstructure Evolution and Creep Behavior of Near-α Ti Alloy Produced by Thermomechanical Processing

A microstructure evolution based on the processing and heat-treatment conditions was investigated for Ti–13Al–2Nb–2Zr (at%) alloy, which has a promising oxidation resistance. Three processing temperatures, 900°C and 1000°C in the α+β phase field, and 1080°C in the β phase field, and two rolling reduction ratios, 93% and 67%, were selected as the processing conditions. In the samples processed and heat-treated in the α+β phase field, an almost fully equiaxed structure, i.e., the equiaxed or ellipsoid α phase surrounded by the β phase, was formed through furnace cooling, and a bi-modal structure was formed using air cooling. The morphology of the α phase in the near fully equiaxed and lamellar structure depends on the rolling reduction ratio; in other words, the equiaxed and ellipsoid α phases are formed at rolling reduction ratios of 93% and 67%, respectively. The volume fraction of the equiaxed α phase in the bi-modal structure is processed at 900°C, which is higher than that of the bi-modal structure processed at 1000°C despite the same heat-treatment temperature applied. This is because the induced strain when processed at 1000°C is smaller than that when processed at 900°C. By contrast, in the samples processed in the β phase field and heat-treated in either the α+β or β phase field, a lamellar structure is formed. The creep behavior of the bi-modal structure obtained upon processing at 900°C and 1000°C for up to a 93% rolling reduction ratio was investigated. The creep life of the sample processed at 1000°C was two-times longer than the sample processed at 900°C. This is because a smaller volume fraction of the equiaxed α phase in the sample processed at 1000°C than that of the sample processed at 900°C.