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单相合金定向凝固时,固液界面处液相溶质浓度会受抽拉速率的跃迁变化而发生较大的改变.理论分析和数值计算结果表明:当抽拉速率从V0跃迁变化到V时,定向凝固系统并不能立即达到抽拉速率V,而是经过一个非稳态凝固过程才能达到V.跃迁加速中,界面处液相溶质浓度呈现小大小的不对称分布,且刚开始时,界面处液相的溶质浓度随抽拉速率跃迁比V/V0,原始抽拉速率V0和温度梯度GL增加而增大,相应的溶质扩散长度随V/V0和GL增加而减小.对于跃迁减速过程,变化情形刚好相反.另外在跃迁加速中,即使合金跃迁变化的抽拉速率仍处于平界面临界失稳速率范围之内,如果固液界面处液相溶质浓度过大,也会使合金凝固的平界面发生失稳现象.上述分析结果在Al-2%Cu合金的数值计算中得到了验证.
When the single-phase alloy is directionally solidified, the concentration of liquid solute at the solid-liquid interface will be greatly changed by the transition of the pull rate.Theoretical analysis and numerical results show that when the pull rate changes from V0 transition to V, The directional solidification system does not reach the drawing rate V immediately, but only after a non-steady-state solidification process V. In the transitional acceleration, the liquid solute concentration at the interface shows a small size asymmetric distribution, and at the beginning, The solute concentration of the liquid phase increases with the jump ratio of the pull rate V / V0, the original pull rate V0 and the temperature gradient GL, and the corresponding solute diffusion length decreases with the increase of V / V0 and GL.For the transition deceleration process, In addition, during the transition acceleration, even though the pull rate of the transition of the alloy transition is still within the critical interface instability rate, if the concentration of the liquid solute at the solid-liquid interface is too large, the alloy will solidify The interface instability occurs.The above analysis results have been verified in the numerical calculation of Al-2% Cu alloy.