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

真空熔炼Fe-0.3mass%C-3mass%Mo合金，并在1250 ℃下均匀化50小时。在400 ~ 700 ℃范围内等温反应，测定了各试样的相变分数和相变起始的TTT曲线，在530 ℃以下，相变动力学较快，在此温度下形成“退化铁素体”。动力学在550 ~ 570 ℃之间缓慢，随温度升高而迅速增加。实验室温度为-560 ℃，透射电镜下观察到退化铁素体是铁素体等轴细晶的集合体。这些颗粒被认为是一个大的铁氧体晶体的一部分，其形状类似于树或珊瑚。这些晶体的形成是由于某些晶向生长受到强烈抑制的结果，合金元素在铁素体/奥氏体界面上的偏聚可能是抑制生长的原因之一。用FE-TEM分析了界面的化学成分，在许多界面处，Mo的浓度超过合金中平均浓度的2倍。随着等温时间的延长，Mo含量有增加的趋势，部分相变驱动力可能通过偏析相在界面区的扩散而消散。再加上Mo与碳的强相互作用，这可能导致铁素体生长的延迟，并解释了该合金系统中的不规则相变行为。

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.