To improve the stress rupture and creep properties, the element W is added into a superalloy for serving at elevated temperatures. The microstructure at varied state, solidification, homogenization, hot-deformation and properties of the alloy are investigated in this study for the sake of application.
The as-cast microstructure of the alloy is made up of γ matrix, η and Laves phases precipitated in the interdendritic region, MC and γ′ phases precipitated at both the dendrite core and interdendritic region.
The solidification and segregation of the alloy are studied by DSC and by high-temperature soaking and quenching test. The solidification sequence is determined to be as followings: L→γ; L→MC; L→η+Laves, followed by solid-state reactions of γ→η(needle-like) and γ→γ′. All elements show some extent of segregation, among which the segregation of W, Nb and Ti are greatest. Nb and Ti exhibit normal segregation, while W shows negative segregation.
The dissolution of low melting point phases and the diffusion of the elements at high temperature are investigated, and homogenization regime of the alloy is determined to be as followings: 1150℃×10h+1220℃×40h. The low melting point phases are completely dissolved into the γ matrix by the soaking at 1150℃ for10h, while needle-like and massive μ phase particles precipitate during this stage annealing. The μ phase is dissolved and elemental distribution becomes uniform satisfactorily after soaking at 1220℃ for 40h, with the MC carbide dissolving noticeably.
The hot deformation of the alloy is difficult due to the high contents of refractory element W and other strengthening elements. Thus the processing abilities of samples with different original microstructures are examined by depressing the samples at varied temperatures with thermal simulation tests. The samples homogenized and homogenized followed by μ needles precipitation soaking have the best hot plasticity. The homogenized structure is selected for forging.
The as-cast alloy is homogenized, forged and heat treated. The μ particles with a diameter below 1 μm and globular γ′ particles with a diameter of 15nm are distributed uniformly in the grains of the alloy as heat treated. The plate-like particles are distributed densely along the grain boundary.
The tensile and stress rupture properties are tested. The ultimate and yield strength of the alloy at room and elevated temperatures are high, which can be attributed to the solution strengthening of higher W addition and the precipitating strengthening by γ′ phase. But the tensile plasticity decreases with the increasing temperature, because the grain boundaries are not strengthened properly. The alloy has good stress rupture properties at 700℃-750℃. The samples is ruptured with the intergranular fracture mode. The high temperature tensile plasticity and stress rupture property can be ameliorated by the improvement of grain boundary precipitation.
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