材料期刊网

采用OM, XRD, SEM和TEM研究了由纯铁穿透渗氮所得高氮奥氏体经225 ℃中温转变后的显微组织, 确定了转变产物的种类和形态. 等温转变属于γ→α-Fe +γ’-Fe4N上贝氏体转变, 转变产物由α-Fe板条和γ’- Fe4N板条交替排列而成的贝氏体板条团以及离散分布在贝氏体团块间的残余奥氏体γr小团块组成. 该贝氏体相变在晶界和晶内位错线上优先形核、长大, 具有扩散型相变的特征, 其领先相是γ’-Fe4N. γ’-Fe4N与奥氏体晶体结构上的相似性及界面良好的共格性, 保证了γ’-Fe4N从奥氏体的顺利析出.

Abstract A microstructural study was carried out to clarify the nature and morphology of the microstructure produced in high-nitrogen austenite generated by nitriding thin pure-iron sheets and subjected to an intermediate-temperature isothermal transformation at 225C, using optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that an upper bainitic transformation, γ-Fe +γ′-Fe4N, occurred at either the grain boundaries or the dislocation lines in the interior of austenite grains, producing a microstructure which, although morphologically looking markedly different, consisted of bainitic colonies composed of parallel bainitic ferrite (-Fe ) laths with γ′-Fe4N laths entrapped between them, and was interspersed with discrete patches of retained austenite(γr). Transformations occurring at the grain boundaries initiated earlier, proceeded faster, and produced a coarser microstructure, than those occurring in the interior of austenite grains did, due to the difference between the two types of nucleation sites in thermodynamics (nucleation energy) and kinetics (diffusivity). The bainitic transformation was diffusional in nature, with γ′-Fe4N serving as the leading phase. The striking similarity of the γ′-Fe4N to the high-nitrogen austenite in crystal structure, and the good coherence of the γ/γ′-Fe4N interface greatly facilitated the precipitation of γ′-Fe4N from austenite.