反向梯度纳米结构316L不锈钢的强韧化机制研究

With outstanding corrosion resistance and ductility, 316L austenitic stainless steel (316L ss) has great potential in industry applications but limited by its poor strength. Although grain refinement could enhance the strength significantly, it usually sacrifices the ductility and corrosion resistance. It has been a challenge to optimum the synergy of strength, ductility and corrosion. .In this application, we propose an inverse gradient nanograined (IGNG) structure within which the grain size gradually decreases from micrometer scale in the surface to nanometer scale in the inner core. In IGNG, nanograins are attributed to the high strength, and gradient grain structure is responsible for the superior ductility while coarse grains in the surface could maintain the corrosion resistance of 316L ss..Through ultra-high frequency electromagnetic induction, laminated nanograined 316L ss fabricated by high strain cold rolling was heated with great speed. Nanograins in various depths were recrystallized to different extent, hence forming an IGNG. In this application, the recrystallization behavior of nanograins under electromagnetic heating will be systematically studied, from which a controllable method to fabricate IGNG would be derived. Based on the observations of the microstructure evolution, the deformation mechanisms of the nanograins in the inner core under the restraint of IGNG will be illustrated. The mechanical response of the recrystallized structure at various depths would be investigated and the underlying strengthening and toughening mechanism would be revealed. Our researches will shed a light on fabrication of high strength and high toughness 316L stainless steel with well reserved corrosion resistance. Also the results would be a fundamental for the performance optimization of gradient structured materials.

316L奥氏体不锈钢具有良好的耐蚀性和塑性，但强度偏低。细晶强化可以显著提高其强度，却易损害其塑性和耐蚀性。如何优化强度、塑性及耐蚀性三者的匹配是亟待解决的难题。本项目提出反向梯度纳米结构316L不锈钢，其晶粒尺寸由表及里从微米级逐渐减小到纳米级。该结构中，纳米晶提供强度，梯度结构保障塑性，同时表层的微米级粗晶保持耐蚀性不降低。本课题通过超高频电磁感应加热，对大应变量冷轧制备的层片状纳米晶316L不锈钢进行快速表面加热，使不同深度的纳米晶发生不同程度的再结晶，形成反向梯度纳米结构。系统研究层片状纳米晶在快速加热时的再结晶规律，掌握反向梯度纳米结构的可控制备方法；阐明层片状纳米晶在反向梯度纳米结构约束下的塑性变形机制；揭示不同深度组织结构的变形机制，及其与整体强塑性之间的关系，理解其强韧化机制，发展出高强高韧而耐蚀性不受损失的316L不锈钢的制备新方法，为类似结构材料的性能优化提供理论参考。

针对纳米金属材料强度，塑性和耐蚀性之间相互矛盾的问题展开了以下研究：.第一，提出并完善超高频电磁感应加热处理制备反向梯度纳米结构工艺，重点研究影响反向梯度纳米结构制备的主要因素和技术关键问题。分别制备出反向梯度纳米结构316L不锈钢、反向梯度纳米结构纯镍、反向梯度结构非晶以及反向梯度纳米结构铝合金。.第二，利用SEM、TEM、XPS、XRD等等手段分析测试表征了反向梯度纳米结构316L不锈钢纳米结构组态以及再结晶比例、分布、形貌，并研究了纳米晶在快速加热时的再结晶规律以及反向梯度纳米结构形态调节机制。.第三，系统研究了反向梯度纳米结构316L不锈钢的综合力学性能，探索力学性能与结构的关系。重点研究了不同纳米结构组态在反向梯度纳米结构约束下的变形机制，优化反向梯度结构性能；研究了反向梯度纳米结构316L不锈钢的低周疲劳性能，发现了影响纳米金属低周疲劳性能的新机制；探索利用超高频电磁感应加热处理修复疲劳损伤的原理；利用电化学测试分析了晶粒尺寸效应、晶粒分布效应、表面钝化膜状态对耐蚀性能的影响，兼顾强塑性的情况下改善反向梯度纳米结构电化学性能，获得高强高塑高耐蚀反向梯度纳米结构316L不锈钢。