二次硬化合金钢中多组元强化相M_2C碳化物的粗化动力学研究

COARSENING KINETICS OF MULTI-COMPONENT M_2C PRECIPITATES IN SECONDARY HARDENING ALLOY STEELS

高Co—Ni超高强度合金钢在482℃等温回火处理,首先析出渗碳体,回火30 min后,针状合金碳化物M_2C以共格方式从基体α-Fe中析出,回火时间延长到10 h以上,M_2C仍与基体间保持良好的共格关系。该共格关系会随过时效而失去,高分辨电镜观察表明,M_2C的尺寸r和回火时间t接近r^2∝t关系,明显偏离扩散控制的三次方关系的经典LSW粗化理论,对相近的合金钢材料有关M_2C析出动力学实验数据进行分析得到相似的结论。这不仅和第二相碳化物的形态有关,还与多组元扩散的体系相关,表明经典理论不完全适合多组分的碳化物粗化行为。从经典扩散理论出发,考虑针状沉淀相多元扩散问题,得到的针状相粒子粗化方程与实验结果吻合。

When high Co-Ni alloy steels are isothermally tempered at 482 degreeC, cementite is precipitated first, and then the needle-shaped carbides of M2C begin to precipitate in a coherent way with martensitic ferrite matrix after 30 min. This coherent:relationship can be hold as long as 10 h, and tends to be lost when the steels are overaged with increasing tempering time. High resolution electron microscopy investigation shows that the size of M2C precipitates increases with tempering time following the relationship r(2) proportional to t, and this relationship c an be confirmed by other studies in some similar alloy steels, which obviously deviates from the classical theory of LSW for diffusion-controlled coarsening of precipitate particles in binary solutions. This result might be related to the multicomponent diffusion system as well as the morphology of the precipitates. It indicates that the classical theory cannot be used properly to describe the coarsening behavior of multi-component carbides even the coarsening controlled by volume diffusion. With a view of the needle shaped precipitates and two or more solutes diffusion, a model for the coarsening rate of the precipitates with time has been derived, which is well consistent with the observed results.