低活化钢闭塞式变截面往复挤-扭-镦强化相回溶析出及其对高温和辐照性能的影响机理研究

High temperature creep and irradiation damage caused by the high temperature, high pressure and energetic particle flux in the reduced activation ferritic/martensitic (RAFM) steels restrict their applications in the field of nuclear fusion. The grain size and strengthening phases of RAFM steels have important influence on strength and toughness, high temperature creep and radiation resistance. Severe plastic deformation (SPD) can significantly refine grains and redistribute strength phases, which is an effective way to improve the comprehensive performance of RAFM steels and prolong service life. In this study, a new type of blocking varietal cross section cyclic twist-extrusion-upsetting process is developed, which is based on cyclic extrusion compression (CEC) and twist extrusion (TE).The process has advantages of good process plasticity and large deformation, which is benefit to optimize the deformation process and fabricate large size bulk RAFM steels with high performance. At the same time, the theoretical analysis and experimental research are combined to analyze the mechanism of grain refinement, phase transition, dissolution and precipitation of strengthening phase during the processing of severe plastic deformation and heat treatment, and the influence of main process factors on the properties of RAFM steels will be investigated. In addition, the influence of microstructures on the strength and toughness, high temperature creep and radiation resistance will be discussed through the grain boundary evolution and the interaction between strengthening phases and dislocations in the processed RAFM steels. It has important theoretical significance and scientific research value for improving the comprehensive performance of RAFM steels, prolong service life, and promote the application of RAFM steels in the field of nuclear fusion.

低活化钢在高温高压和高能粒子流的多重作用下产生的高温蠕变和辐照损伤等缺陷，限制了其在可控核聚变领域的应用。低活化钢晶粒尺度和强化相尺寸及分布对其强韧性、高温蠕变性和辐照性能有重要影响。大塑性变形能够大大细化晶粒、有效改善强化相尺寸及分布，是提高低活化钢综合性能进而延长其使用寿命的有效手段。本研究将传统的往复挤压和挤扭工艺有机结合，开发出一种工艺塑性好、变形量大的闭塞式变截面往复挤-扭-镦工艺，实现工艺的最优化以制备高性能的大尺寸块体低活化钢。将理论分析与实验研究相结合，深入分析在大变形和热处理过程中低活化钢的晶粒细化、相转变、强化相回溶与析出规律，揭示主要工艺因素对低活化钢性能的影响机理，并通过位错、晶界与强化相的相互作用阐明低活化钢微观组织对其强韧性、高温蠕变和辐照性能的影响机理。这对改善低活化钢的综合性能，延长其使用寿命，进而促进低活化钢在核聚变领域的应用具有重要的理论意义和科研价值。

低活化钢作为核聚变堆包层第一壁的首选结构材料，在聚变环境下会遭受离位损伤和嬗变反应形成的氦气泡的作用，这对其抗辐照损伤性能提出了苛刻要求。当低活化钢成分确定时，通过大塑性变形技术对其进行改性，并揭示微结构对其服役性能的影响规律及机理，对包层第一壁结构材料的设计和开发具有指导价值。600℃下对低活化钢进行往复挤扭镦变形结果表明，晶粒得到明显细化，位错密度得到了有效的提升。变形后低活化钢的高温屈服强度提高了约19.8%，往复挤扭镦变形对低活化钢的高温拉伸性能有积极影响。挤扭镦变形试样的蠕变应力指数n为28.3，蠕变损伤容限因子为6.3。变形试样中析出相的细化和位错密度的增大有利于提升阀值应力σth，从而降低蠕变速率，起到强化作用。.辐照后初始样品的室温平均纳米硬度相比辐照前增加了21.1%，而TEU试样变形区的室温平均纳米硬度仅比辐照前增加了9.2%。同样的，辐照后初始样品的抗拉应变比辐照前下降了27.1%，而TEU试样变形区的抗拉应变仅比辐照前下降了14.9%。这表明TEU变形能够有效抑制低活化钢的辐照硬化和辐照脆化。辐照后初始样品存在裂纹和烧蚀等粗糙损伤表面，裂纹宽度大于80 nm，最大烧蚀坑直径约为8 μm。辐照后TEU大变形区表面的烧蚀和裂纹缺陷消失，仅出现浮泡和沟道等较为光滑的辐照损伤形貌，表现出优异的抗辐照损伤性能。TEU温变形能使低活化钢的晶粒细小均匀，位错密度显著增加，从而有效抑制其辐照硬化和辐照脆化。