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.
Ti6Al4Nb4Zr (mass%) was prepared by selective laser melting (SLM) under various conditions, and the microstructure evolution resulting from SLM processing and subsequent heat treatments was investigated. The effects of the unique SLM-induced microstructure on the high-temperature compressive strength and creep properties of the samples were then elucidated. Under rapid cooling conditions, the martensitic structure formed in a scale-like pattern, with a 100 µm in size, consistent with the laser scanning pattern. By contrast, under slow cooling conditions, the ¡/¢ lamellar structure formed in ¢ grains with a 300 µm grain size instead of in a scale-like pattern. The martensitic structure drastically changed to a Widmanstätten structure during heat treatment. The equiaxed ¡ phase also formed at the interface of the scale-like patterns. By contrast, the ¡/¢ lamellar structure did not exhibit a change in response to heat treatment. The compressive strength of the SLM samples was governed by the martensite ¡ size and the grain size, both of which depended on the cooling rate. The dominant creep deformation mechanism at 600°C and under a loading stress of 137 MPa was grain boundary sliding. The creep life depended on the grain size. The HIP treatment improved the creep life because it eliminated pores introduced by the SLM process.
Heat-resistant Ti-Al-Nb-Zr alloys, which don’t contain Sn, have been designed to obtain good oxidation resistance above 600 °C. In addition, to design Ti alloys with best balance of creep and fatigue properties, prior β grain size which affects fatigue properties and lamellar microstructure which affects creep properties were controlled by heat treatment. In the present study, the effect of microstructure on creep properties of one of the alloys, i.e., Ti-7.5Al-4Nb-4Zr alloy, with the bimodal (B), the lamellar structures in small prior β grains (LS), and the lamellar in large prior β grains (LL) were investigated at 600 °C. The creep deformation mechanism for each microstructure was a power-law creep. However, the creep life varied depending on the microstructures. The longest creep life was obtained in LS with prior β grain size of 90 μm and interlamellar spacing of approximately 10 μm, while the shortest creep life was obtained in LL with prior β grain size of 550 μm and fine interlamellar spacing of less than 2~3 μm. This suggests that creep life is more affected by interlamellar spacing than by prior β grain size.
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