高Cr铸铁中M_(7)C_(3)碳化物与奥氏体共生长的元胞自动机模拟

Cellular automaton simulation on cooperative growth of M_(7)C_(3) carbide and austenite in high Cr cast irons

高铬铸铁中M_(7)C_(3)碳化物大小适中、弥散均匀分布,有利于提高合金的耐磨性.为分析凝固过程中M_(7)C_(3)碳化物晶粒在基体中的形貌及分布、M_(7)C_(3)碳化物与奥氏体晶粒生长的相互作用、引起的溶质偏聚对最终M_(7)C_(3)碳化物粒径分布的影响,本文开发了Fe-C-Cr三元合金小面晶M_(7)C_(3)碳化物与奥氏体晶粒共生长的二维微观元胞自动机模型,模型中加入潜热释放对凝固过程温度场的影响,由C,Cr两溶质界面扩散共同确定晶体生长速度,由凝固路径数据表插值获取液相元胞的溶质平衡浓度,设定M_(7)C_(3)碳化物邻胞结构并优化形状因子来保持M_(7)C_(3)碳化物小面晶形貌,模拟了Fe-4%C-17%Cr三元合金(C和Cr的质量分数分别为4%和17%)初生M_(7)C_(3)碳化物和共晶奥氏体晶粒的生长演变过程.研究表明,M_(7)C_(3)碳化物和奥氏体晶粒各自的生长速度随着界面液相中C,Cr溶质的超饱和度和贝克列数的增大而增大;随着奥氏体的析出和晶粒生长,M_(7)C_(3)碳化物晶粒的生长速度明显增快;当奥氏体晶粒逐渐接触并包围M_(7)C_(3)碳化物晶粒时,两相晶粒生长速度逐渐降低.凝固过程中,奥氏体晶粒生长向外排出C,Cr溶质,与吸收C,Cr溶质生长的M_(7)C_(3)碳化物晶粒互补,致使二者生长互相促进,最终奥氏体晶粒包围M_(7)C_(3)碳化物晶粒生长.预测的冷却曲线与实验冷却曲线变化趋势相符;最终凝固组织形貌和M_(7)C_(3)碳化物体积分数与实验相符;剩余液相、奥氏体中C,Cr溶质浓度演变也与Gulliver-Scheil,Partial Equilibrium,Lever Rule模型预测结果相符.

M_(7)C_(3)carbide’s amount,size,morphology and distribution in the microstructure contribute much to the wear resistance of high chromium cast irons.In the present paper,a two-dimensional microscopic cellular automaton model for the growth of the faceted M_(7)C_(3)carbide together with the austenitic dendrite grains in an Fe-4%C-17%Cr ternary alloy is developed to obtain the evolution of M_(7)C_(3)carbide grain morphology,the concentration redistribution and their interaction during the growth of M_(7)C_(3)carbide and austenite grains,and also the total influence on the final M_(7)C_(3)carbides’size.The model includes the effect of latent heat release on the temperature drop.The grain growth velocity is determined by both the diffusion of C solute and the diffusion of Cr solute at the S/L interface.The equilibrium concentration in liquid cells is interpolated from the tablulated solidification path which is prescribed by Gulliver-Scheil approximation coupling with the thermodynamic equilibrium calculation.The morphology of the faceted M_(7)C_(3)carbide is maintained through setting its neighborhood relations and optimizing its shape factor at grain growth.The results show that the individual grain growth velocity for M_(7)C_(3)carbide and austenite increases with the increase of the supersaturation and Peclet number of solute C and Cr.The austenite precipitation and grain growth obviously speed up the growth velocity of M_(7)C_(3)carbide grains.While with the austenite grains gradually touching and enveloping the M_(7)C_(3)carbide grain,the growth velocities for both kinds of grains decrease.The rejection of solute C and Cr during austenite grain growth complements the absorption of solute C and Cr during M_(7)C_(3)carbide grain growth,thus promoting their growth.The predicted cooling curve fits with the evolution tendency of the experimental one.The predicted final solidification microstructure and M_(7)C_(3)carbide amount in volume fraction are in agreement with the experimental ones.Furthermore,both C solute concentration distribution and Cr solute concentration distribution in both residual liquid and austenite are consistent with the predictions by the Gulliver-Scheil,partial equilibrium and lever rule model.