| BT25y titanium alloy developed by Russian can be used at 550℃, but is not well studied in China. The effect of heat theatment on microstructure and tensile properties of BT25 y titanium alloy was studied with the optical microscope, scanning electron microscope, transmission electron microscope, X-ray diffractometer and tensile tester to provide the basis for the application of BT25 y alloy.The morphology of duplex microstructure can be observed at the arallel part of as-forged BT25 y alloy. The coarse barlike primary α phase nearly dissolves at 950℃. With the increase of solution temperature, the equiaxed primary α phase dissolves gradually and the primary β grains start to grow. The equiaxed primary α phase fully dissolves at 980℃.Various phase components will form at different cooling rate after the solid-solution treatment, where the microstructure of air-cooled specimen after solution treatment at 950℃ consists of α and β phases, and the microstructures of water-quenched and oil-quenched specimens consist of α, α’ and α’’ phases. The lattice parameters of α’’ martensite phase obtained by water-quenched are different from those obtained by oil-quenched, where the value of a is smaller and the value of b is larger in the water-quenched specimen than in oil-quenched one. Different cooling modes have no effect on the type of texture, the solid-solution treatment at low temperature in two phase region will not affect the type of texture but will affect the strength of texture, while the solution treatment at high temperature in two phase region changes the type of texture.With the same holding duration, the temperature of aging treatment only changes the size of aging α phase but does not change the morphology and size of primary α phase. Aging treatment at 400℃ for 6h has no obvious effect on the microsturcture of the alloy. With performing the aging treatment at the higher temperatures. the layer size of aging α phase increases. After aging at 550℃ for 6h, the microstructures of air-cooled, oil-quneched and water-qunenched specimens comsist of α phase and β phase. After performing the anneal treatment at temperatures lower than 600℃ for 2h, the microstructure does not change obviously compared with one of unannealed alloy. The layer size of aging α phase increases after annealing at 700℃ for 2h. The S2 type of silicide precipitates after aging treatment, and the amount of silicide is larger after aging treatment at 800℃.The tensile test results for the BT25 y alloy subjected to different heat treatmens show that with the increase of solid-solution temperature, the amount of primary equiaxed α phase obviously decreases, which will lead to a considerable decrease in the plasticity but has little influence on the strength. The cooling rate after solid-solution treatment has a large effect on the performances of the alloy. The sharp increase in cooling rate causes a dramatical increase of strength and a obvious decrease in plasticity of the alloy. The aging treatment can sharply increase the strength of the alloy, while the plasticity of the alloy decreases slightly. The performances of the alloy subjected to aging treatment at 400℃ for 6h are close to those of the unaged alloy which has the lower strength and the higher plasticity. When the aging temperature is above 500℃, the strength of the alloy decreases with increasing the aging temperature, which is associated with the growth of α phase. The maximum value of tensile strengths can be obtained for the alloy aged at 550℃, where the yeiled strength and ultimate tensile strength are 1066 MPa and 1234.5MPa respectively, and the elogation and area reduction are 16% and 53% respectively. The performances of the alloy annealed below 600℃ are close to those of the unannealed alloy, and the room-temperature plasticity of the alloy annealed at 700℃ decreases. |
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