On the influence of Ta/W ratio on the morphological evolution of γ+γ’ eutectics and solidification path in cast nickel-base superalloys

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ABSTRACTThe relation between Ta/W ratio and mechanism of γ/γ’ eutectics morphological changes and solidification path was investigated for experimental nickel-base superalloys. Thermal analysis and thermodynamic modelling was used to determine the solidification path. The results indicated that an increase in Ta content alters chemical composition of the dominant carbides from HfC to (Hf,Ta)C and then to TaC. Meanwhile, at low Ta/W ratios, uniformly distributed fine eutectic pools of γ+γ’ form and large phases of γ’ are rarely observed beside these pools, while at high Ta/W ratios, relatively coarse eutectic pools of γ+γ’ are formed with a rather non-uniform distribution grown epitaxially during the latest solidification step. According to the proposed heat treating guide map, the alloys with Ta/W ratio lower than 0.5 or greater than 2 with a rather narrow or smaller than 10°C heat treatment window are located in the region with a high risk of incipient melting.