镍基单晶高温合金DD11杂晶形核的热力学研究

Thermodynamics Study on the Stray Grain Nucleation in Ni-Based Single Crystal Superalloy DD11

以单晶叶片在研制过程中常出现杂晶而导致产品不合格为研究背景,以DD11合金单晶叶片缘板杂晶为研究对象,通过测试叶片缘板在定向凝固中的冷却速率,借助差热分析法(DSC)以相同冷却速率条件下测定DD11合金的形核临界过冷度及熔化潜热。利用扫描电镜(SEM)观察合金与型壳之间界面的化学反应情况,采用液滴法测量了合金熔体与型壳的润湿角,并完成了合金固液界面能及杂晶形核功的定量分析。结果表明,DD11合金与型壳基体属不润湿系统,润湿角为145.47°;DD11合金单晶叶片缘板杂晶形核临界半径为43.07 nm,临界形核功为0.938×10^(-15) J;较大的合金异质形核临界过冷度及合金熔体与型壳的润湿角有利于减少杂晶的产生。本研究对控制单晶叶片杂晶形核及提高产品合格率具有一定理论指导意义。

Having the disqualification caused by frequent stray grain in single crystal turbine blades development as a background, taking the stray grain on the margin plate ofDD11 single crystal turbine blades as object of study, the cooling rate of the blade margin plate during directional solidification was measured, and the critical undercooling rate of nucleation and enthalpy of DD11 alloy was calculated with differential thermal analysis（DSC） under the same cooling rate. The stability of solid-liquid interface was investigated with scanning electron microscopy（SEM）, the wetting angle between melting alloy and shell mould was measured with sessile drop method, and the quantitative analysis of the solid-liquid interface energy and stray grain nucleation energy of the alloy was accomplished. The results show that DD11 alloy and shell mould formed a nonwetting system, of which wetting angle was 145.47°; the critical radius of stray grain formation on the margin plate of DD 11 single crystal turbine blades was 43.07 nm and the critical nucleation energy was 0.938×10-15 J; high critical undercooling rate of heterogeneous medium nucleation and large wetting angle between the melting alloy and shell restrained the formation of stray grain. This research has theoretical and guiding significance on reducing stray grain formation in single crystal turbine blades and improving the product qualification rate.