镍基高温合金GH4738的凝固偏析和碳化物析出行为

高温合金铸锭凝固过程内部各区域散热条件不同,冷却速率存在明显差异。采用差示扫描量热分析(DSC)、高温共聚焦显微镜(HT-CLSM)原位观察和定向凝固(DS)的方法,研究了宽冷速范围下GH4738合金的凝固偏析和碳化物析出行为。结果表明, GH4738合金的凝固顺序为L→γ+L,L→γ+L+MC,L→γ+MC+η+(γ+γ′),其中MC型碳化物、η相和(γ+γ′)共晶相为合金凝固过程中的主要偏析产物;Ti、Mo元素是合金的主要枝晶间偏析元素;提高冷却速率能有效降低凝固前沿残余液相中的溶质富集程度;铸态组织中的的碳化物主要为富Ti的MC型碳化物(TiC、Ti(N)C)和以TiN或Al₂O₃为核心的MC型复合碳化物(Al₂O₃-TiC、TiN-TiC);随着冷却速率降低,碳化物平均尺寸增大,体积分数减小,形貌由小块状向长条状、汉字状和大块状演变。

Solidification segregation and carbide precipitation behavior of nickel-based superalloy GH4738

Internal cooling rate of superalloy ingot is obviously different due to the variation of heat dissipation conditions. The solidification segregation and carbide precipitation behavior of nickel-based superalloy GH4738 at a wide range of cooling rate were investigated by differential scanning calorimeter (DSC), high temperature confocal laser scanning microscopy (HT-CLSM) and directional solidification (DS) experiments. The results indicated that the solidification sequence of GH4738 superalloy was L→γ+L, L→γ+L+MC, L→γ+MC+η+(γ+γ′), among which MC, η and (γ+γ′) eutectic phase were the main segregation products during the solidification process. Ti and Mo were the main solute elements segregated to the interdendritic regions, and increasing the cooling rate could effectively reduce the concentration of solute in the residual liquid phase at solidification front. The carbides in the as-cast alloy were mainly divided into titanium-rich carbide, like TiC, Ti(N)C, and MC type composite carbides with TiN or Al₂O₃ as the core (Al₂O₃-TiC, TiN-TiC). With the decrease of cooling rate, the average size of carbide increased, but the volume fraction decreased, and the morphology evolved from small block to long strip, Chinese character and large block.