Microstructure and mechanical properties of Ti-Al-Nb-based alloys were investigated as they are important in the development of new generation high temperature Ti alloys due to good oxidation resistance. The alloy compositions Ti-15Al-2Nb (to obtain α 2 precipitates) and Ti-15Al-2Nb-0.5Si (at%) (to obtain solid-solution hardening by Si) were selected. Heat treatment at 1000 C after forging and rolling at 900 C transformed the β phase to α phase and α martensite during cooling. By heat treatment at 900 C, a single α phase was obtained in both alloys. The α 2 phase was formed by aging treatment at 600 and 700 C of Ti-15Al-2Nb and at 600, 700, and 800 C of Ti-15Al-2Nb-0.5Si. Compressive strength was investigated for samples heat treated at 900 C with a single α phase and those aged at 600, 700, 800 C with α 2 precipitates. The solid solution hardening effect of Si was found at all test temperatures. Precipitation hardening effect by α 2 precipitates was also observed in both alloys and the effect improved with Si addition. Peaks of precipitation hardening were obtained at 300 C in both alloys; the precipitation hardening effect decreased with increase of test temperature above 300 C. It is suggested that a shearing mechanism occurs up to 300 C, which changes to a bypass mechanism above 450 C.
A microstructure evolution based on the processing and heat-treatment conditions was investigated for Ti13Al2Nb2Zr (at%) alloy, which has a promising oxidation resistance. Three processing temperatures, 900°C and 1000°C in the ¡+¢ phase field, and 1080°C in the ¢ 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 ¡+¢ phase field, an almost fully equiaxed structure, i.e., the equiaxed or ellipsoid ¡ phase surrounded by the ¢ phase, was formed through furnace cooling, and a bi-modal structure was formed using air cooling. The morphology of the ¡ phase in the near fully equiaxed and lamellar structure depends on the rolling reduction ratio; in other words, the equiaxed and ellipsoid ¡ phases are formed at rolling reduction ratios of 93% and 67%, respectively. The volume fraction of the equiaxed ¡ 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 ¢ phase field and heat-treated in either the ¡+¢ or ¢ 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 ¡ phase in the sample processed at 1000°C than that of the sample processed at 900°C.
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