金属合金等温相变的体激活能及相变机制 Ⅰ．钢的中温(贝氏体)等温相变

OVERALL ACTIVATION ENERGY OF ISOTHERMAL TRANSFORMATION IN METAL ALLOY AND ITS MECHANISM Ⅰ.Medium Temperature (Bainite) Isothermal Transformation in Steels

钢的中温(贝氏体)等温处理获得的上、下贝氏体和粒状组织,都具有各自独立的C曲线;在转变初期均能获得单一的组织,而在转变的中、后期通常可获得相邻两组织的复合体.必须用单一组织和Arrhenius关系求相变产物的体激活能.将体激活能、组织形貌和体自由能曲线相结合可诠释下述论点:下或上贝氏体是碳原子扩散控制下在奥氏体中的贫碳或极贫碳区,进行"军队型"(队列式)原子无扩散马氏体样切变相变;而粒状组织是碳原子扩散控制下在奥氏体中的最贫碳区,发生"平民型"(非队列式)原子无扩散界面控制相变.

Upper bainite, lower bainite and granular structure in isothermally treated steels have all themselves C curves. A single microstructure is always produced in the primary stage, whereas the nearby two microstructure complex is usually produced in the medium or final stage of the transformation. Only a single microstructure and Arrhenius equation must be used to deduce the overall activation energy for the transformation product. A combination of overall activation energy, morphology and free energy curve can explain the bainitic transformation mechanism and granular structure formation mechanism. The former follows military atom diffusionless martensite like shear, which occurred in carbon-depleted region controlled by carbon atom diffusion in austenite, and the latter results from civilian atom diffusionless interface control transformation, which occurred in the most carbon-depleted region controlled by carbon atom diffusion in austenite.