Ta对低Cr高W铸造镍基高温合金显微组织的影响

Effect of Ta Addition on Microstructure of Cast Nickel Base Superalloys Containing Low Level of Cr and High Level of W

用金相显微镜、定量金相、扫描电镜(SEM)、能谱分析(EDX)及 X 射线衍射(XRD)研究了 0%,4%,6%Ta(质量分数) 3 种成分的低 Cr 高 W 铸造镍基高温合金铸态及热暴露后的显微组织,结果表明:Ta 是强 MC 碳化物形成元素,它形成 MC 碳化物的倾向低于 Nb、高于 Ti,当合金中 Ta 的原子百分含量超过 Nb 3 倍时,Ta 在 MC 碳化物中的原子百分含量超过 Nb。Ta 是强γ′形成元素,它促进共晶γ′的形成,在合金成分中以等量原子百分含量的 Ta 代替 Al 不会改变合金的共晶γ′数量。在高 W 铸造高温合金中 M6C 是比 MC 更加稳定的碳化物,Ta 的加入不能抑制 1 100℃,500 h 长时热暴露后 MC 完全转变为粒状或片状 M6C。MC 分解时释放的碳还促使α(W,Mo)+C→M6C 反应。对 Ni-1.5Cr-10Co-16W-2Mo-5Al-1Ti-1Nb 基础成分合金,其最佳的含 Ta 量为 4% (质量分数),而且含碳量应低于 0.07%(质量分数)。

Three heats of the low Cr and high W content superalloys with 0%, 4%, 6% Ta were prepared by vacuum induction melting (VIM). Microstructures of as-cast and thermally exposed at 1 100degreesC, 500 h specimens were investigated by optical metallography, quantitive metallography, SEM, EDS and XRD. The results indicate that Ta is a strong MC carbide former, and its tendency of forming MC carbide is stronger than that of element Ti, but weaker than that of Nb. When the atomic fraction of Ta in alloy is three times higher than that of Nb, the atomic percent of Ta in MC carbide exceeds Nb. Ta is a strong gamma'former and promotes the formation of eutectic gamma'.. Substituting equivalent atomic percent of Al with that of Ta will not change the quantity of eutectic gamma'. M6C is more stable carbide than MC in cast nickel base superalloys containing high level of W, and Ta addition does not inhibits the reaction, in which MC carbide has completely transformed to granular or plate M6C carbide after 1 100degreesC, 500 h thermal exposure. Carbon from the decomposed MC carbide even promotes the reaction a (W,Mo)+C-->M6C. Therefore, for the based composition alloy of Ni-1.5Cr-10Co-16W-2Mo-5Al-lTi-lNb, the optimum content of Ta is 4% (mass fraction) and carbon content should be controlled to be lower than 0.07% (mass fraction).