Knowledge Management System of Institue of Mechanics, CAS
|Alternative Title||Study of strength- and ductility- enhancing mechanisms in low stacking fault energy heterostructure|
|Thesis Advisor||武晓雷 ; 谢季佳|
|Place of Conferral||北京|
|Keyword||异构 低层错能合金 非均匀变形诱导硬化 纳米孪晶|
强度和塑性是表征金属材料承载能力和变形能力的两大重要力学性能。在传统均质结构金属中存在强度和拉伸塑性此消彼长的关系，即当强度提高时，拉伸塑性会急剧下降。而异构金属在拉伸过程中产生非均匀变形诱导硬化(Hetero-deformation induced hardening, HDI hardening)提供额外应变硬化使金属具有优异的强塑性匹配，也在一定程度下打破强度和拉伸塑性此消彼长的关系。纳米孪晶结构由于孪晶界可以阻碍位错运动和提供位错储存空间来提高应变硬化，具有优异的力学性能。所以当异构含有纳米孪晶时，其强度和拉伸塑性得到进一步提升，而这类异构的界面（孪晶界和软硬相界面）如何提高结构材料的强度和应变硬化仍不够明确，需要进一步研究。本文将低层错能合金（黄铜(Cu-30Zn, wt.%)和CoCrNi中熵合金）通过一定工艺处理后得到具有优异力学性能的纳米孪晶异构，结合力学性能测试和微结构表征研究这类异构在拉伸过程中微结构演化和应变硬化机制。
针对黄铜(Cu-30Zn, wt.%)合金，本文通过冷轧和部分再结晶得到多级晶粒尺寸分布的异构。该结构表现出优异的强塑性匹配，其中在室温下屈服强度达到~580 MPa的同时仍然能保持~23%的均匀延伸率。同时在液氮温度下，强度和塑性同时提高。异构黄铜拉伸过程中在微米级晶粒和亚微米级晶粒的晶界处和三叉晶界处孪生出纳米级晶粒，从而实现动态晶粒细化增强的效果。并且异构中纳米级晶粒，亚微米级晶粒和微米级晶粒的强度差异很大，使其在变形过程中亚微米级晶粒和纳米级晶粒及微米级晶粒和纳米级晶粒之间的晶界处变形不协调产生很大应变梯度。为了协调应变梯度，晶界处堆积高密度几何必需位错进而产生HDI应力和HDI硬化。由于拉伸过程中不断产生晶粒细化使异构的结构非均匀性增加，从而持续产生HDI硬化提供额外加工硬化能力，使异构在高强度下保持了良好的均匀延伸率，即晶粒细化诱导塑性(Grain refinement induced plasicity, GRIP)效应。由于黄铜的低层错能，在液氮温度下更易发生孪生变形，所以GRIP效应更加显著。
对于CoCrNi-0.44%N (wt.%)中熵合金，通过冷轧和退火处理得到间隙氮原子强化的多级晶粒尺寸分布结构。相较于均质粗晶结构，其屈服强度均大于1 GPa，且仍然具有良好的均匀延伸率达到14%-28%之间。根据Hall-Petch关系，均质粗晶CoCrNi-0.44%N中熵合金的晶格阻力和Hall-Petch系数相对于均质CoCrNi中熵合金分别提高70%和116%。其强化机理主要是通过Cr2N析出相的析出强化和溶于基体的氮原子增强的固溶强化（包含化学短程有序和点阵畸变强化）。而异构CoCrNi-0.44%N中熵合金相对于粗晶结构，Hall-Petch系数进一步提高了63%。因为异构样品中各级晶粒的强度差异很大，在屈服阶段即是弹塑性共变形阶段，微米级晶粒和亚微米级晶粒均屈服发生塑性变形而纳米级晶粒没有屈服。所以在此阶段晶界处出现应变梯度，并产生几何必需位错协调应变梯度，进而产生HDI应力增强其屈服强度。间隙氮原子的析出强化和固溶强化再结合HDI强化协同作用提高异构CoCrNi-0.44%N中熵合金的屈服强度。同时异构样品在拉伸变形过程中产生GRIP效应和持续的HDI硬化，使其具有良好的均匀延伸率。并且在拉伸前后异构样品中均直接观察到化学短程有序，其平均尺寸和体积分数在变形前后几乎不变。说明在拉伸过程中化学短程有序和位错发生交互作用而不被破坏，这对材料的强化和应变硬化也有相应提高。
最后本文通过表面机械研磨处理得到的梯度结构CoCrNi中熵合金，其表层没有发生晶粒细化。梯度层是纳米孪晶层片间距和数密度呈梯度分布。梯度结构的屈服强度为~450 MPa，均匀延伸率达到~52%。相对粗晶芯部，屈服强度和均匀延伸率同时提高。同时随着距表层深度不断减少，剥层样品的屈服强度不断增加，而均匀延伸率不断减低。梯度结构表层的屈服强度达到~950 MPa，由于没有加工硬化能力，几乎没有均匀延伸率。在拉伸过程中，梯度结构由于梯度层和粗晶芯部和各层之间存在强度差异，所以在其界面之间变形不协调产生很大应变梯度。在粗晶芯部的界面和各层界面处堆积高密度几何必需位错产生HDI应力和HDI硬化。且梯度结构拉伸过程中孪生变形产生纳米孪晶，孪晶界附近堆积几何必需位错，孪晶界提供HDI应力和HDI硬化。梯度结构表层在拉伸过程中产生变形孪晶，同时高密度位错交互作用产生大量位错束和位错胞，从而提供应变硬化来抑制应变局域化。
Strength and ductility are the two major mechanical properties of materials, but there is a trade-off relationship between strength and ductility in traditional homogeneous structures. And hetero-deformation induced (HDI) hardening in the heterostructured metal provides extra strain hardening during the tensile test to have an excellent combination of strength and ductility. And it also breaks the trade-off relationship between strength and ductility. The nanotwin structure has excellent mechanical properties due to the effective block of the twin boundary to dislocation slip and the provision of space for dislocation storage. When the heterostructure contains nanotwins, its strength and ductility are further improved. It is not clear how interfaces (twin boundaries and interfaces of the soft and hard domain) can improve the strength and strain hardening in heterostructure including nanotwins. In this paper, low-stacking fault energy alloys (brass and CoCrNi medium-entropy alloy) are processed to obtain different nanotwin-containing heterostructures with excellent mechanical properties. Combining mechanical test and microstructure characterization to study the microstructure evolution and strain hardening mechanism of heterostructure with nanotwin during the tensile testing.
Firstly, the hetero-structured brass (Cu-30Zn, wt.%) with multi-level grain size distribution has an excellent combination between strength and ductility, where the yield strength at 298 K reaches ~580 MPa, while still maintaining a uniform elongation of ~23%. At the same time, the strength and ductility are improved simultaneously at liquid nitrogen temperature. During the tensile testing, the hetero-structured samples form fine grains by twinning at the grain boundaries and triple junction of the micrometer grains and submicrometer grains, thereby realizing the dynamic grain refinement during deformation. In addition, the strength of micrometer grains, submicrometer grains, and fine grains in hetero-structure is very different, so that, the incompatible deformation at grain boundaries of submicrometer grains and fine grains, as well as the micro-scale grains and nano-scale grains, produces a large strain gradient. To coordinate the strain gradient, the high-density geometrically necessary dislocations (GNDs) near the grain boundary must be piled up to produce HDI stress and HDI hardening. The continuous grain refinement during the deformation increases the heterogeneity of the heterostructure so that the continuous HDI hardening provides extra strain hardening. And the heterostructure still has an excellent uniform elongation under high strength, that is, the grain refinement-induced plasticity (GRIP) effect. Due to the low stacking fault energy of brass, twin deformation is more likely to occur at liquid nitrogen temperature, so the GRIP effect is more significant.
Second is the hetero-structured CoCrNi-0.44%N (wt.%) medium-entropy alloy (MEA) with multi-level grain size distribution strengthened by interstitial nitrogen atoms. Compared with the homogeneous coarse-grained structure, the yield strength of hetero-structures is greater than 1 GPa and heterostructures still have excellent uniform elongation from 14% to 28%. According to the Hall-Petch relationship, the lattice friction and Hall-Petch coefficient of homogeneous coarse-grained CoCrNi-0.44%N MEA are increased to 70% and 116%, respectively, compared with that of homogeneous CoCrNi MEA. Mainly the precipitation strengthening of the Cr2N precipitation and solid solution strengthening (including chemical short-range order and lattice distortion strengthening) by the nitrogen atoms improve yield strength. Compared with the coarse-grained structure of the hetero-structured CoCrNi-0.44%N MEA, the Hall-Petch coefficient is further increased by 63%. Due to the strengths of different levels grains in hetero-structured samples are very different, the yield stage of heterostructure is the elastoplastic co-deformation stage. Both micrometer and submicrometer grains yield and undergo plastic deformation, while fine grains do not yield. Therefore, the strain gradient appears at the grain boundary resulting in high-density GNDs coordinating strain gradient, which generates HDI stress to enhance its yield strength. The interstitial nitrogen atom precipitation strengthening and solid solution strengthening combined with HDI strengthening synergistically increase the yield strength of the heterostructured CoCrNi-0.44%N MEA. At the same time, the heterostructured samples produce the GRIP effect and continuous HDI hardening during the tensile deformation, and HDI hardening dominates the strain hardening, making it have outstanding uniform elongation. Moreover, chemical short-range order (CSRO) was directly observed in the hetero-structured samples before and after tensile testing. And the average size and volume fraction of CSRO is almost unchanged before and after deformation. It shows the interaction of chemical short-range order and dislocations during the tensile deformation, which may also improve the strengthening and strain hardening in the metal.
The last is the gradient structure CoCrNi MEA, in the gradient layer of which the spacing and number density of nanotwin lamellae are in a gradient distribution and has no grain refinement. The yield strength of the gradient structure is ~450 MPa, and the uniform elongation reaches ~52%. Compared with the coarse grain structure of the core, the yield strength and uniform elongation are improved simultaneously. At the same time, as the depth from the surface decreases, the yield strengths of the delaminated samples continue to increase, while the uniform elongations continue to decrease. The yield strength of the surface layer reaches ~950 MPa, and there is almost no uniform elongation due to the lack of strain hardening. During the tensile testing, the gradient structure due to the strength difference among the gradient layer, core, and each layer, so the incoordination deformation between the interface produces a large strain gradient. And the interface of the core and the interface of each layer accumulate high-density GNDs to produce HDI stress and HDI hardening. In addition, during tensile deformation, the gradient structure produces deformation twins, and the GNDs must be piled up near the twin boundaries, which provides HDI stress and HDI hardening. The surface layer of gradient structure produces deformation twins, and dislocation bundles and dislocation cells are produced by high-density dislocations, which provides strain hardening to inhibit strain localization.
|薛琪琦. 低层错能异质结构强塑性机理研究[D]. 北京. 中国科学院大学,2021.|
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