冷却速率对Ni-Nb合金非平衡凝固组织的影响

采用阶梯铜模喷铸方式开展了Ni-10%Nb(质量分数)合金非平衡凝固实验,并利用Procast软件对不同铜模内径所对应的冷却速率进行了计算。综合采用光学显微镜、扫描电镜及能谱分析仪对不同冷速条件下合金二次枝晶间距、溶质含量等组织参量进行表征。实验结果表明,随铜模内径减小,合金冷却速率增加,二次枝晶间距减小,枝晶主干中溶质含量得到明显提高。在此基础上,利用枝晶生长模型对不同过冷度条件下的枝晶生长速率进行了计算。分析结果表明,随冷却速率增加,凝固发生时过冷度增大,枝晶生长速度提高,溶质截留现象趋于严重,导致溶质过饱和现象发生。此外,冷却速率的提高还有利于减小液/固界面前沿溶质扩散区长度,导致二次枝晶间距减小。

Effect of Cooling Rate on Non-equilibrium Solidified Microstructure of Ni-Nb Alloy

Non-equilibrium solidification of Ni-10wt%Nb alloy was carried out by a step copper-mould casting method and the cooling rate for different inner diameters was calculated using Procast software. The microstructure features including secondary dendrite spacing and solute content were characterized by optical microscope, scanning electron microscope and energy dispersive spectrum equipment. Results show that the cooling rate increases with decreasing the inner diameter of copper-mould, which causes the reduction of secondary dendrite spacing and the enhancement of solute content in dendrite trunk. On this basis, the migration rate of solid/liquid interface was calculated by a dendrite growth theory. The analysis shows that both the undercooling prior to solidification and dendrite growth rate increase with increasing cooling rate. As a consequence, the solute trapping phenomenon becomes serious, resulting in solute supersaturated in dendrite trunk. Moreover, the increased cooling rate is beneficial for reducing the length of solute-rich region ahead of solid/liquid interface, which contributes to the decrease of secondary dendrite spacing