高强度超细晶金属材料塑性行为及增塑研究进展

强度和塑性是金属结构材料最重要的力学性能指标,金属高性能化的关键是在高强度水平下保证良好的塑性,然而两者往往不能兼顾。在众多强化方法中,晶粒细化长期以来被认为是强化金属最理想的手段,在传统晶粒尺寸范围,细化晶粒既可以显著提高材料的强度,又能改善材料的塑韧性。因此,近几十年来超细晶/纳米晶金属得到了广泛研究和发展,出现了以大塑性变形(SPD)、先进形变热处理(ATMP)技术为代表的超细晶制备方法,所得晶粒可以细化到亚微米或纳米尺度,金属性能大大提高。然而,大量研究证实当晶粒细化到亚微米或纳米尺度时金属强度提高但塑性显著下降,与传统的细晶强化规律不符。对此,国内外学者进行了很多研究,试图阐明其机理、揭示晶粒超细化导致塑性降低的物理本质。此外,由于细化晶粒方法受到塑性的限制,新的高强度水平下增强塑性的方法成为钢铁材料高性能化的研究热点。针对塑性下降的事实,为了进一步提高超细晶金属材料性能,研究者开展了许多增强塑性的工作,获得了较好的效果,但仍存在一些不足。关于金属晶粒超细化导致塑性降低的普遍共性现象,目前广泛认可的理论主要有晶界捕获(吸收)位错的动态回复理论、位错运动湮灭理论、高初始位错密度以及位错源缺失机制等。前三者都主要关注超细晶金属材料低(无)加工硬化能力,并将其归结为延伸率降低所致。主要是因为低(无)加工硬化使材料在变形早期发生塑性失稳或局部变形从而表现出低塑性。超细晶金属增塑研究主要体现在增塑方法和机理方面,目前,增塑方法主要有(1)形成纳米孪晶;(2)获得粗晶-细晶双峰组织;(3)利用相变诱发塑性/孪生诱发塑性(TRIP/TWIP)效应;(4)引入铁素体软相;(5)利用纳米第二相粒子等。这些增塑方法的主要机理是利用组织结构的改变提高超细晶金属的加工硬化能力以维持良好的均匀塑性变形以及利用组织相变提高塑性。本文归纳了常用的超细晶金属制备方法,综述了超细晶金属材料塑性降低的研究进展,总结了超细晶金属增塑的研究结果,分析了目前研究中存在的不足,探讨了超细晶金属增强增塑的发展趋势,以期为超细晶金属塑性降低理论及增强增塑研究提供参考。

A Review on the Plastic Behavior and Improvement of Plasticity in High Strength Ultrafine-grained Metallic Materials

Strength and plasticity are the most important mechanicalproperties of structural materials. The key to high performance steels is to ensure favorable ductilityat high strength level.However, a metallic material cannot gain high ductility and high strength at the same time.Among the strengthening methods, grain refining has long been considered as the most ideal means for strengthening metals. It has been widely recognized that metals and alloys with refined grains generally exhibit substantially higher strength and ductility than their crystalline counterparts with coarse grains. Therefore, extensive researches on ultrafine-grained/nanocrystalline metallic materialshave been conducted over the past decades. The preparation method of ultrafine grain, represented by severe plastic deformation (SPD) and advanced thermomechanical processing (ATMP) technology, has been developed, which can refine the grain size to submicron or nanometer scale and greatly improve the metal properties. However, a large number of studies have confirmed that the strength of the metal increases but the plasticity decreases drama-tically when the grain is refined to submicron or nanometer size, which is not consistent with the traditional law of grain refining strengthening. In regard to this phenomenon, on one hand, lots of works at home and abroad have been carried out to reveal or elucidate the mechanism of plasticity decrease caused by grain refining. Besides, because grain refining cannot guaranty good ductility of steels at highstrength level, it has been a hot topic to develop new means for strength and ductility improvement. In recent years, many techniques have been proposed for enhancing the property of ultrafine-grained materials and gratifying results have been achieved, but there are still some shortcomings. At present, theories including dynamic recovery, dislocation annihilation, high initial dislocation density and dislocation source deletion mechanism are widely adopted to explain the decrease of plasticity of ultrafine-grained materials. The former three theories mainly focus on the low or absence of work hardening and ascribe the low or absence of work hardening to the reason of poor ductility, because the plastic instability (necking) or strain localization of materials could occur at early stage of deformation due to the low or absence of work hardening, resulting in poor elongation plasticity. At present, the studies of improving plasticity of ultrafine-grained metallic materials primarily focus on plasticizing methods and mechanisms. Which include the formation of nanoscale twins in materials, obtaining coarse/fine bimodal grain structure, TRIP/TWIP effective, formation of soft ferrite phase in hard matrix phase and precipitation of nanoscale second dispersed precipitates, etc. The mechanism of these methods is to improve the work hardening ability of ultrafine-grained metal by changing the structure, so as to maintain good uniform plastic deformation or improve the plasticity by phase transformation. This review introduces the preparation methods of ultrafine-grained metallic materials, offers a retrospection of the research efforts to the plastic behavior and improvement of plasticity, analyze the insufficiency of previous works, and then discusses the trend and prospective of research on the ductility improvement of ultrafine-grained materials. It is expected to provide the inspiration and reference for the research on mechanism of plasticity decrease and ductility improvement.