Influence of Alloying Element Segregation on the Growth of Ferrite/Austenite Interfaces

采用真空熔炼的方法，在1250ﾟC下均质化Fe-0.3质量%C-3质量%Mo合金50h，然后进行奥氏体化处理，在400~700ﾟC温度范围内等温反应，测定了各样品的相变分数和相变起始温度曲线。相变动力学在530ﾟC以下很快，在此温度下形成“简并铁素体”。在5 5 0~5 70ﾟC温度范围内，动力学变化缓慢，并随温度升高而迅速增大。测得区间温度为-560ﾟC。在透射电子显微镜下观察到，简并铁素体为等轴细小铁素体的集合体。这些颗粒被认为是一个巨大的铁氧体晶体的一部分，其形状类似于一棵树或珊瑚。还认为这些晶体的形成是在某些晶体方向上强烈抑制生长的结果，合金元素向铁素体/奥氏体界面处的偏析可能是阻碍生长的原因之一。用FE-TEM分析了界面的化学成分，在许多界面上观察到Mo含量是合金平均含量的2倍以上。随着等温保温时间的延长，Mo含量有增加的趋势，部分相变驱动力可能被界面区中偏析相的扩散所耗散。再加上Mo与碳的强烈相互作用，这可能是阻碍铁素体生长的原因，并解释了该合金系统中不规则的相变行为。

An Fe-0.3mass% C-3mass% Mo alloy was vacuum-melted and was homogenized at 1250ﾟC for 50 h. The specimens were then austenitized and reacted isothermally at temperatures ranging from 400 to 700ﾟC.The fraction transformed was measured in each specimen and TTT curves for the initiation of transforma-tion, determined.The transformation kinetics were fast below 530ﾟC,where "degenerate ferrite" was formed. The kinetics were slow between 550 and 570ﾟC and increased rapidly with increasing temperature. The bay temperature was deter-mined to be -560ﾟC.Under transmission electron microscope the degenerate ferrite was observed to be the aggregate of equiaxd fine grains of ferite. These grains were consider ed to be a section of one large ferrite crystal, whose shape resembled a tree or a coral. It was also considered that these crystals were formed as a result of strong inhibition of growth in some crystallographic directions.The segregation of alloying elements to ferrite/austenite interfaces may be one reason for the retardation of growth. The chemical composition of interfaces was analyzed by FE-TEM.The Mo concentration was observed to be more than 2 times the average concentration in the alloy at many interfaces. The Mo concentration tended to increase with the isothermal holding time.A part of driving force for transformation may be dissipated by diffusion of segregants in the interfacial region. Coupled with strong interaction of Mo with carbon this may cause the retardation of ferrite growth and account for the irregular transformation behavior in this alloy system.