高温合金单晶铸件中的杂乱共晶缺陷

Randomly Orientated Eutectic Grains in Single Crystal Castings of Superalloys

对镍基高温合金铸态组织中的γ/γ′共晶体及其形成机制进行了研究。发现含有碳元素的高温合金如CM247LC的γ/γ′共晶析出分为三种模式,分别为在γ相上形核生成γ基共晶、在MC碳化物上形核生成MC基共晶和从液体中形核形成自由共晶。其中后两种模式形成的γ/γ′共晶体的晶向是杂乱随机的,从而在单晶基体组织中形成大角度的晶界。对不含碳元素的高温合金如CMSX-4,其γ/γ′共晶的析出只有两种模式,即在γ相上形核和从液体中自由形核结晶,分别形成与γ相一致或完全无关的晶体取向。不管哪种合金,其铸态组织都含有随机杂乱取向的共晶组织,使得宏观上为单晶的铸件在微观上却是多晶体。

In the present work, the γ/γ’ eutectics in the as-cast structure of Ni-based superalloys and their formation mechanisms were investigated. For the superalloys with carbon addition such as CM247 LC, three different nucleation modes of γ/γ’ eutectics at the final stage of solidification are found: nucleation on the existing γ dendrites and on the MC carbides respectively, as well as nucleation directly in the interdendritic liquid. The γ/γ’ eutectics nucleated trough the latter two modes have different crystallographic orientation from the γ dendrites. For the superalloys without carbon addition such as CMSX-4, the γ/γ’ eutectics nucleate either on the existing γ dendrites or from the interdendritic liquid, revealing the same crystal orientation with γ matrix or the independent random orientations, respectively. As a result, there always exist the randomly orientated eutectic grains in the solidification structure of Ni-based superalloys. It indicates that the macroscopically defined SC castings are, in reality, polycrystalline at a microscopic scale.