上贝氏体铁素体的形核及长大

 贝氏体铁素体的长大速度与转变机制密切相关。应用QUANTA 400型环扫电镜，观测了20CrMo钢和35CrMo钢的贝氏体铁素体形核及长大情况。结果表明，上贝氏体铁素体在原奥氏体晶界形核，可沿着晶界生长，也可平行地向晶内长大。测得贝氏体铁素体片条沿晶界延伸的平均速度为14 998 nm/s，而向晶内长大线速度为17 763 nm/s。应用计算和理论分析方法研究了贝氏体片条的长大机制，认为在晶界形成的上贝氏体铁素体晶核与两侧的奥氏体不同时具有共格界面，因此不能以共格切变长大。按照体扩散和界面扩散进行理论计算，计算结果表明：铁素体长大速度比实测值小3~4个数量级，因此扩散 台阶机制不能成立。另外，提出了上贝氏体铁素体晶核长大的原子热激活跃迁机制。

Nucleation and Growth of Upper Bainitic Ferrite

It is very important to investigate transformation dynamics of bainitic ferrite since the growth dynamics is highly related to the transformation mechanism. Nucleation and growth of bainitic ferrite of 20CrMo and 35CrMo steels were investigated by using SEM QUANTA 400.The results show that the upper bainitic ferrite nucleated at original austenite boundary. The outspread rate of bainitic ferrite slip along grain boundary was 14 998 nm/s, while the line rate inside crystal was 17 763 nm/s. The growth mechanism of bainitic slip was studied by means of calculation and theoretical method. The upper bainitic ferrite crystal nucleus and austenite have no coherent interphase at the same time, which can′t grow up by coherent shear. The theoretical calculation in term of body and interface diffusion indicated that the growth rate of ferrite was smaller than measured value, ie the difference is of the order of 3-4, that the diffusion step mechanism was not valid, and atom heat activation transition mechanism of upper bainitic ferrite crystal nucleus growth was given.