High temperature titanium alloys have been used widely in aerospace because of the low density, specific strength and good performance at high temperature. Ti-60A alloy which is belong to Ti-Al-Zr-Mo-Si-Sn alloy system is a near-α high temperature titanium alloy. It can be used stably up to 600℃. Silicide and β grain size are two important factors in Ti-60A alloy, which are not studied sufficiently yet. Therefore, this thesis focused on silicide precipitation and solution temperature, precipitation position, shape, and size. Furthermore, the thesis studied the effect of silicide precipitation on high temperature creep and creep rupture behavior. At last the thesis explored how to control β grain size and distribution in Ti-60A alloy.
Microstructure analysis by TEM showed that silicide precipitation begins at 990℃, and solution completely at 1030℃, under 2 hours annealing. The most precipitation appears at about 850℃. The silicide size is about tens nanometers to hundreds nanometers. Silicide precipitation had a small quantity and less than 100 nanometers When the annealing temperature below 600℃.Annealing at 900℃, silicide is spindle, and has indistinct boundary with the matrix. At 900℃~800℃, silicide precipitated as polyhedral. Below 750℃, silicide looks like short stick. Silicide had orientation relationship with the α phase matrix: [0001] S2∥[123(__)1] α, (1(__)21(__)0) S2∥(01(__)12) α.
Silicide precipitation between α plates anchored α and β phase interface, when the creep stress is low, so it can improve the creep resistance at 600℃and this effect become more pronounced when α2 phase precipitated in the matrix. However, when the creep stress is high, the effect of silicide on the creep rupture property was negative. Because of the silicide precipitation, dislocation tangled near the silicide, then, cracks initiated from the tangled dislocation.
Precipitation of α2 phase was beneficial both to high temperature creep and creep rupture properties. Increase of silicide in solution by way of lowering the silicide precipitation was concluded to be more beneficial to creep rupture properties with higher applied stress than to creep properties with lower applied stress.
Different grain size and grain distribution of primary β grains could be achieved by controlling the deformation rate and temperature of precision forging and hot rolling. We found that the effects of the thermomechanical processing on grain size and distribution were minor when Ti-60A was heat treated near the α+β/β transformation temperature, while it was heat treated above the α+β/β transformation temperature, the differences of the grain size and distribution of all the samples were disappeared.
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