Abstract
The melt with a nominal composition of Nb-16Si-22Ti-2Hf-2Cr-2Al was poured in a ceramic shell mould with a temperature gradient of about 4℃/mm, and the ingot with the dimension of 60 mm×170 mm was obtained. The relationship between the microstructure and mechanical properties was measured, and the effects of the silicide on fracture toughness at ambient temperature and compression strength at high temperature were analyzed. It is revealed that the microstructure of the alloy consists of Nb solid solution and silicides, and the cooling rate can obviously change primary phase and constituent phases. The volume fraction of Nb_3Si formed at rapid cooling rate is significantly increased, and Nb_5Si_3 phase is formed with decreasing in cooling rate, whereas the volume fraction of the eutectic colonies and Nbss dendrites increase evidently, especially the second dendrite arm. Fine and uniform eutectic colonies are contributed to the ambient tensile strength, while the coarse primary Nb_3Si decreases the tensile strength but improves the compression strength of the alloy. When the microstructure is mainly composed of fine (Nb_(SS)+Nb_5Si_3) eutectic colonies, the tensile strength and elongation of the alloy reach 449 MPa and 0.3%, respectively. When lath-like Nb_3Si phase has 80μm in width and 50% in volume fraction as well as its long axis is parallel to compression direction, the compression strength of the alloy at 1250℃is about 650 MPa.
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