马氏体形态的平行多层截面法研究

来源期刊：金属学报2010年第8期

论文作者：柳永宁 张贵一 李伟

文章页码：930 - 934

关键词：马氏体相变； 高碳马氏体； 低碳马氏体； 透镜体； 椭球体

Key words：martensitic transformation； high carbon martensite； low carbon martensite； lens； ellipsoid

摘    要：设计了一种平行截面原位观察方法,并研究了一种C含量(质量分数)为1.37%的高碳钢的马氏体形态.采用激光打孔定位、超细抛光多层减薄方法原位追踪测量了目标马氏体的长度、宽度与减薄厚度的变化.以厚度累加变化为横坐标,测量的长度与宽度对称分布为纵坐标,还原出的马氏体空间形态为扁椭球体,而不是传统教科书中描述的透镜体,椭球的几何参数为a/b≈3,a/c≈20.热力学分析表明:形核能量除了与材料的物理特性有关外,还与新相的形状有关.在同等晶粒尺寸条件下,扁椭球状新相的形核功明显低于透镜状新相的形核功.含C量降低会导致相变驱动力降低,并使形核功增加,马氏体新相晶核为板条形态时可弥补因含C量下降导致的形核功增加.

Abstract: In metallography the shape of high carbon martensite is commonly considered to be lens-like in three dimensions,which is of needle or bamboo leaf on the OM micrographs of section planes.However,a round martensite on OM micrographs have never been seen so far.This question has not been answered over the years.It has not been theoretically explanined why is martensite lath-like for low carbon steels and lens-like for high carbon steels on OM micrographs.For answering these questions,the martensitic morphology of a steel with carbon content 1.37% was observed in the same view by OM using a method of multiple sectioning of one sample.The length,width of the aimed martensite and sectioned thickness of samples were measured before observing.It is found that the martensite is a flat ellipsoid rather than lens-like as described in traditional textbooks.The ratio of a/b is about 3 and a/c is about 20 for the martensite ellipsoid.The thermal dynamic analysis indicates that the nucleation energy for a martensite is closely related to its shape.In the austenite with the same grain size,the nucleation energy (ΔG^*) of martensite will be lower with ellipsoid shape than with lens shape.The driving force for martensitic transformation will reduce with the decrease of carbon content,resulting in the formation of lath-like martensite in steels with low carbon content.