奥氏体不锈钢中纳米孪晶的相变约束机制及尺度效应研究

Ultrahigh-strength austenite stainless steel (SS) is one of the most important structural materials in the development of advanced iron and steels. However, the commonly used methods to strengthen austenite stainless steels often lead to irreconcilable compensations such as low ductility and degrading corrosion resistance, which may originate from deformation-induced martensite transformation from metastable austenite phase. In our previous work, this martensite transformation in the austenite stainless steel is remarkably suppressed with thickness of austenitic twins decreasing. Inspired by this thickness-dependent martensite suppression, this project aims at 1) a systematic investigation of a constrained martensite transformation (CMT) effect, viz. martensite transformation shall not occur on the condition of twin thickness lower than a critical size in austenite stainless steel(This thickness effect has not been observed in other nanotwinned materials such as Cu or Ag thin film since no martensite transformation takes place in these matrix materials when deformed). Due to the CMT effect, when the twin thickness is smaller than the critical size, some other mechanisms including multiple twinning, detwinning, or partial dislocation slip are expected to be the governing factor(s) in deformation; 2) exploring effects of austenite stability, strain rate and low temperature on evolution of microstructures and formation mechanism(s) of nanotwins in austenite stainless steel. Deformation behavior of austenite twins with thickness from submicrometer to nanometer will be investigated to unveil the influence of CMT mechanism on strength and plasticity of SS; and 3) synthesis of bulk austenite SS with a high density of nanotwinned structure, which may exhibit improved ductility and intergranular corrosion resistance aside from high strength,by local severe plastic deformation with high strain rates. The proposed research results are expected not only to comprehensively understand the relationship of twin size with phase transformation, but to have important implications in optimizing the performance of austenite SS.

超高强度奥氏体不锈钢是先进钢铁材料重点发展的结构材料之一。现有的强化方法存在高强低塑性和耐蚀性下降等问题，其中亚稳态奥氏体的马氏体相变是导致性能下降的直接原因。本项目在我们前期工作发现随着奥氏体不锈钢中孪晶尺度减小，马氏体相变受到抑制的基础上，探讨当奥氏体孪晶小于临界尺度时可能不发生马氏体相变的约束效应（不发生相变如纳米孪晶铜薄膜则并无此效应），小于临界尺度的孪晶则表现出其他变形尺度效应。拟利用高应变速率局域变形方法在体材料中制备出高密度纳米孪晶结构，系统研究奥氏体稳定性、应变速率和温度对微观结构演化和孪晶形成机制的影响。通过全面分析从亚微米到纳米尺度奥氏体孪晶的变形过程，探讨奥氏体孪晶独特的相变约束机制和变形尺度效应对材料强塑性机制的影响，以期全面揭示孪晶尺寸与相变的关系并为应用纳米孪晶机制优化奥氏体不锈钢性能奠定坚实的科学基础。

超高强度奥氏体不锈钢是先进钢铁材料重点发展的结构材料之一。现有的强化方法存在高强低塑性和耐蚀性下降等问题。纳米孪晶（NT）结构金属具有非常独特的力学性能，可同时实现材料的超高强度及高塑性，有望突破传统强化技术所致的塑性降低难题。本项目采用纳米孪晶结构强化奥氏体不锈钢，利用高应变速率局域塑性变形方法在体材料中制备出高密度纳米孪晶结构。研究奥氏体稳定性、应变速率和温度对材料微观结构演化和孪晶形成机制的影响。所制备的纳米孪晶结构奥氏体不锈钢应达到超高强度水平同时具有良好的延展性及耐晶间腐蚀性能，解决其它强化方法中高强低塑性、耐蚀性降低问题。所取得的成果如下：.采用局域塑性变形方法在奥氏体不锈钢中制备出具有高密度纳米孪晶结构。研究发现低冲击速率有利于细化，但高应变速率诱发了孪晶变化，同时抑制了马氏体相变，导致晶粒尺寸（250 nm）不能细化到纳米尺度。理论预测和实验观察表明，局域变形条件下诱发孪晶变形的临界应力为587 MPa，且应力水平越高，孪晶密度越高，孪晶片层间距越小。.在不同冲击速率下处理304不锈钢，发现随着冲击速率的提高，材料的屈服强度（540-1000 MPa）和抗拉强度（810-1100 MPa）均表现出明显的改善。同时，断裂延伸率仍保持在较高水平（65-35%）。SMAT处理301不锈钢，其屈服强度最高可达1280 MPa和抗拉强度为1600 MPa，同时仍保持良好的延伸率（23%）。在原位和断裂结构的透射电子显微镜显示，纳米和亚微量孪晶帮助抑制并限制了马氏体相变形在孪晶片层之间。In -situ TEM拉伸下，小于5 纳米的孪晶发生孪晶化，交叉孪晶化以及退孪晶化变形。在5 -150 纳米件的孪晶发生二次孪晶化以及相变机制，并约束马氏体生长方向。这些变形机制不仅对材料的综合性能具有十分重要的影响，同时也对研究孪晶尺寸与相变的关系具有普遍的科学意义。