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

将Fe-0.3mass% C-3mass% Mo合金真空熔化，在1250℃下均匀化50 h，然后在400 ~ 700℃范围内进行奥氏体化和等温反应。在每个试样中测量转化的分数，并确定转化开始的TTT曲线。在530℃以下，形成了“简并铁素体”。在550 ~ 570℃范围内，反应速度较慢，随着温度的升高，反应速度迅速加快。测得温度为-560ºC。透射电镜下观察到简并铁素体为铁素体等轴细晶粒的聚集体。这些颗粒被认为是一个大铁氧体晶体的一部分，其形状类似于树或珊瑚。也有人认为，这些晶体是由于在某些晶体学方向上的生长受到强烈抑制而形成的。合金元素在铁素体/奥氏体界面上的偏析可能是生长迟缓的原因之一。用FE-TEM分析了界面的化学成分。在许多界面处，Mo的浓度是合金中平均浓度的2倍以上。Mo浓度随等温保温时间的延长而增加。部分转化动力可能因界面区分离体的扩散而耗散。再加上Mo与碳的强相互作用，这可能导致铁素体生长迟缓，并解释了该合金体系中的不规则转变行为。

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.