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异构金属强韧化及塑性变形行为研究
Alternative TitleStudy of strength- and ductility- enhancing mechanisms and plastic deformation behaviors in hetero-nanostructured metals
王宇驰
Thesis Advisor武晓雷
2019-05-28
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype硕士
Degree Discipline固体力学
Keyword塑性 背应力硬化 异构 中锰trip钢 铜铝合金
Abstract

  金属材料的高强度和高塑性一直是人们追求的目标,位错林硬化的不足是高强度下提高塑性所面临的挑战。提出利用强度差异较大的相结构或跨尺度的晶粒分布获得异构材料,塑性变形时在晶/相界面处引入应变梯度,实现背应力硬化,在高强度下获得满意的塑性。本文主要研究了两种金属材料中异构的拉伸变形行为与微结构演化,阐明了异构诱导背应力硬化的微结构机制。

  为此,选取了两类典型金属材料来获得异构。第一类是Fe-0.2C-7Mn (wt. %) 中锰钢,通过冷轧和临界区退火,得到双相异构。第二类是低层错能的Cu-15Al (at. %) 合金,通过冷轧和不同工艺退火,得到具有跨尺度晶粒结构的异构。

  针对以上两类异构,进行了系列力学性能测试及微观结构表征,揭示了变形物理和微结构演化机制,特别阐明了应变硬化机理。主要结论如下:

  针对双相中锰相变诱导塑性 (TRIP) 钢的异构,研究并获得了强度与塑性的良好匹配,达到了1.111.13 GPa的高屈服强度,同时仍保持31.7%29.3%的均匀拉伸伸长率,且具有显著的屈服平台现象,可以达到15%-24%的拉伸应变,在均匀延伸率中占有很大比例。在拉伸变形过程中,样品的标距段内形成应变局域化带,并沿加载方向扩展,在局域化带的前沿造成显著的应变梯度,导致强烈的背应力硬化。同时,在带的前沿产生显著的TRIP效应,使带内的马氏体体积分数增加、位错密度增大,背应力和TRIP耦合效应延迟了应变局域化,使异构中锰TRIP钢样品表现出良好的拉伸塑性变形能力。在应变局域化带扩展过程中,带内奥氏体在屈服平台结束时已全部转变为马氏体,因此在随后的应变硬化阶段不存在TRIP效应。在屈服平台和应变硬化阶段,几乎没有可动位错产生,因此林位错硬化也不足以带来显著的应变硬化。背应力在整个拉伸变形过程中始终存在,是造成后续应变硬化的主要原因。在双相异构中锰TRIP钢的微结构中,KAM值在小晶粒的晶界处达到峰值,且随着应变增大而增大,为几何必须位错在晶界处堆积导致背应力硬化提供了直接证据。

  针对铜铝合金的异构,研究并获得了高强度下的高塑性,异构铜铝合金样品的屈服强度与粗晶样品相比提高到3倍以上,达到500450 MPa,同时仍保持超过9.5%23.7%的均匀延伸率。在拉伸变形过程中,在屈服点前存在弹塑性变形阶段,由异构带来背应力强化,从而获得屈服强度的提高。同时在应变硬化过程中,由于存在背应力硬化作用,提供额外的应变硬化,从而保持塑性变形,抑制颈缩破坏。统计表明,在异构铜铝合金塑性屈服时,KAM值在异构界面处达到峰值,并明显高于粗晶晶界处的KAM值,给出由几何必须位错持续塞积导致背应力强化的直接实验证据。

Other Abstract

  High strength and ductility are always the targets of metallic materials, insufficient of forest dislocation is the challenge of enhancing ductility at high strength. By phase structure or cross-scale grain distribution with large difference in strength, heterostructure materials bring large strain gradient at grain/phase boundary during plastic deformation, and induce back stress hardening. So that obtaining satisfactory ductility at high strength. This paper mainly studies the tensile deformation behaviors and microstructure evolution in two metals with heterostructure, and illuminates microcosmic mechanisms of heterostructure induced back stress hardening.

  For this purpose, we choose two kind of typical metallic materials to obtain heterostructure. The first one is Fe-0.2C-7Mn (wt. %) medium Mn transformation induced plasticity (TRIP) steel, we obtain dual-phase heterostructure by cold rolling and intercritical annealing. The second one is Cu-15Al (at. %) alloy with low stacking fault energy, we use cold rolling and annealing process to obtain heterostructure with cross-scale grain structure.

  For the above two kind of heterostructure, a series of mechanical properties tests and microstructure observation have been done to reveal the deformation physics and mechanism of microstructure evolution, especially the mechanism of strain hardening. main conclusions are as follows:

  For the heterostructure in medium Mn TRIP steel, we study and obtain a good strength and ductility matching. It can reach a high yield strength about 1.11 and 1.13 GPa and maintain a high uniform elongation about 31.7% and 29.3%. At the same time, an ultra-long yield plateau elongation (YPE) extending to tensile strains of as large as 15%-24%. It takes up a large part of the apparent uniform elongation. During the tensile deformation, a strain localization band (SLB) appears and propagates along the axial direction, and shows a high strain gradient at the tip of the SLB, and induces a large back stress hardening. At the same time, TRIP effect happens at the tip of the band, which makes the martensite fraction and dislocation density increase in the SLB. So that, back stress and TRIP synergistic effect inhibit strain localization, and bring a good tensile ductility in dual-phase heterostructured medium Mn TRIP steel. During the propagation of SLB, austenite all change into martensite at the end of YPE. This indicate the absence of TRIP effect during the following uniform elongation. On the other hand, hardly production of mobile dislocations during the YPE and following uniform elongation. Thus the forest hardening alone cannot bring enough strain hardening. Back stress always exists during the tensile deformation, and be the primary cause of the following strain hardening. In the microstructure of dual-phase heterostructured medium Mn TRIP steel, the largest KAM values lies mainly at grain boundary of small grains, and increase with increasing tensile strain. The large KAM values directly support that geometrically necessary dislocations plie up near the grain boundary in the heterostructure and lead to back stress hardening.

  For the heterostructure in Cu-Al alloy, we study and obtain good ductility at high strength. The yield strength of heterostructured Cu-Al alloy is 3 times higher than coarse-grain specimen, reaching 500 and 450 MPa. At the same time, it maintains a uniform elongation about 9.5% and 23.7%, shows a better strength-ductility matching. During the tensile deformation of Cu-Al alloy, the heterostructure renders an elasto-plastic co-deformation during yielding, and the back stress strengthening improve the yield strength. During the uniform elongation, back stress hardening provide extra strain hardening, which contributes to improve ductility and inhibit necking. During yielding of Cu-Al alloy, KAM value reaches maximum near the grain boundary, and higher than coarse-grain specimen, which indicates that geometrically necessary dislocations plie up near the grain boundary in the heterostructure and lead to back stress strengthening.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/78940
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
王宇驰. 异构金属强韧化及塑性变形行为研究[D]. 北京. 中国科学院大学,2019.
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