淬火-配分钢配分过程中马氏体-奥氏体界面互迁移的研究

By controlling the retained austenite, Quenching & Partitioning (Q&P) heat treatment has become a highly promising technique for developing low-cost advanced steels with improved mechanical properties. A knowledge based model to describe the composition and microstructure change during the partitioning process is essential to proper design of a Q&P heat treatment. Due to the lack of in-depth understanding the mutual migration of martensite-austenite interfaces during partitioning, existing models have limited capability to provide a correct description about the microstructure evolution during partitioning. In this project, martensite-austenite interfaces before and after partitioning will be investigated both experimentally and theoretically. In the first step, martensite transformation crystallography, martensite-austenite interfacial structure in the quenched samples will be characterized using TEM, and a quantitative analysis will be made to gain a self-consistent interpretation of the observed results and to build a proper model for martensitic phase transformation. In the second step, a model combining crystallography, thermodynamics and kinetics aspects of the interface migration will be established to describe the mutual migration of martensite-austenite interfaces during partitioning in a Q&P steel. The applicant’s group has rich research experience in phase transformation crystallography and in-situ studying of interface migration. The outcome of this project will yield new insight into mutual migration of martensite-austenite interfaces for better understanding partitioning process. It will also shed light on the transformation theory of lath martensite.

通过控制残余奥氏体，淬火-配分（Q&P）工艺可有效提高许多低碳合金钢的力学性能，已成为极具潜力的发展低成本先进钢铁材料的技术路线。精确设计Q&P工艺的先决条件是科学理论指导下的定量建模。受限于对配分过程中马氏体-奥氏体（马-奥）界面互迁移的认识不够深入，目前的定量模型还难以准确描述配分时的组织演化过程。本项目拟对Q&P钢配分前后马-奥界面进行系统的实验和理论研究，从而弥补定量建模的短板。第一步要在对淬火组织精确表征的基础上，获得对马氏体相变晶体学、马-奥界面结构和界面迁移的自洽解释。第二步将所获得晶体学知识与热力学和动力学理论相结合，建立Q&P钢配分过程马-奥界面互迁移的定量模型 （简称二步三结合建模）。相变晶体学和界面迁移的原位研究是申请团队的特长，项目研究结果可望增进对Q&P钢配分过程中马-奥界面互迁移的理解，为优化Q&P钢配分工艺提供科学依据，同时对板条马氏体相变理论发展有所贡献。

许多固态相变过程中界面如何迁移的问题是长期未攻克之难点，本项目原计划对热点材料淬火-配分（Q&P）钢的配分过程中马氏体和奥氏体间界面迁移开展研究。早期研究开展了对模型Q&P钢的成分和工艺设计，获得了预期的马氏体和奥氏体两相组织，并采用常规透射电镜表征了相变晶体学。由于电镜样品薄膜几何的影响，原位实验中发现钢中既有相界面不能迁移。后期工作增加了其它实验合金，但是其它钢中的既有相界面同样不能迁移。所有钢在原位加热过程中均可以经形核长大形成奥氏体，由于新界面通常垂直于膜面，难以开展界面结构研究。观察发现Fe-20Ni-5Mn中马氏体里形成三类奥氏体，其中A型奥氏体具有板条状形貌，其相变晶体学服从三条择优规律。Fe-25Cr-7Ni-3Mo中铁素体内形成板条状奥氏体。定量分析结果表明，不同钢中形成的奥氏体板条长轴都沿薄膜含的不变线，说明利用膜面不变线引起的界面能和应变能降低可能对新相的相变晶体学起支配作用，而匹配各向同性和错配微小的G相则不受膜面影响。Ti-2.6Mo中观察到原回火马氏体中析出的板条状β在原位加热中收缩，发现侧面和惯习面通过台阶机制迁移，平行于板条长轴的位错或台阶只能改变长度但位置不变。运用晶体相场方法模拟板条状沉淀相三维非保守生长过程，在原子尺度揭示了不同部位界面迁移过程中界面位错和台阶形核生长的细节，生长模拟结果与β相板条收缩的实验结果相符。总之，原位观察揭示板条状沉淀相界面迁移的台阶机制，获得薄膜几何对相变晶体学影响规律的认识，提出针对板条状相的既有界面原位电镜研究应选择样品膜面平行于原有不变线的建议。此外本项目还进一步发展了电镜晶体学数据分析方法，有效提高界面取向测量精度，并贡献了透射电镜在线晶体学分析工具tompas。对相变晶体学理论方法也进行了开拓，包括提出了两段判据的普适分析法，发展了倒易空间列匹配模型及直接矢量匹配方法，促进解释或预测相变晶体学。