层片纳米结构的力学行为及变形机理研究

发布时间：2018-03-14 02:50

  本文选题：层片结构　切入点：软硬微区　出处：《太原理工大学》2017年硕士论文　论文类型：学位论文

【摘要】：对于材料而言,不管均匀结构或不均匀结构,其最终目标都是为了得到优异的力学性能。根据霍尔-佩奇定律,纳米晶材料的屈服强度极高,可塑性极低,会迅速颈缩失稳。粗晶材料有很好塑性,但牺牲了强度。为了兼具高屈服强度和良好塑性,需要增加其应变硬化能力。为此,已有研究报道了多种增强增塑的策略。其中,梯度纳米结构和层片纳米结构使材料应变硬化能力提高很多,也获得了很好的强度塑性。梯度结构硬化的原因是存在应变梯度,导致塑性应变不相容,但缺乏细致的变形物理及微结构研究,不适合工业化生产。层片纳米结构材料的特点是层片状的纳米级微观结构,软硬相差值很大,甚至几个量级。对其变形机理虽提出了背应力概念及背应力测试方法,但研究者对其认识及计算方法存在较大差异,对层片纳米结构的应变硬化机理及微结构关系研究不深入,还没有系统性认识其演化规律,故有必要对层片纳米结构进行剖析。从微元法和微结构角度看,梯度纳米结构相当于是一层层纳米层片结构组合而成的,其微结构机理是相通的。因此,探究并揭示层片纳米结构的力学行为及其应变硬化机理便具有很高的学术意义和应用价值。本文选取10号钢和多层板作为模型材料。利用冷轧10号钢板得到超细晶的层片结构以及热轧多层板,然后退火处理调控微观结构,得到软硬不均匀分布,晶粒大小不同的组织。通过单轴准静态拉伸测试、显微硬度测试、加卸载测试、应力松弛测试及应变场测试等,研究并分析其力学行为、背应力硬化、林位错硬化及局部应变云图演化;借助光学显微镜(OM)、扫描电子显微镜(SEM)及透射电子显微镜(TEM)表征微观结构组织和观察断口形貌,最后分析并阐明了层片纳米结构的变形行为及其应变硬化机理。论文的主要结论如下:(1)不同退火处理的层片结构10号钢试样中存在软-硬微区结构,且硬度差值及硬度分布不均匀性越大,塑性应变不相容越明显,导致背应力越大,从而使背应力硬化显著提高加工硬化能力。拉伸时,软-硬微区层片结构试样表面的局部应变是不均匀的,存在局部应变场及局部应变速率场。不均匀的软硬微区结构能有效抑制塑性变形初期的应变局域化,避免过早颈缩失稳。(2)10号钢层片组织的瞬态应变硬化实质是变形时非均匀软硬微区的塑性应变不相容,产生背应力,使Kernel平均晶粒取向差变大,从而增加几何必需位错密度,导致背应力硬化。同时,可动位错密度增加,激活体积减小以及位错运动受阻,导致产生交滑移和多滑移,引起位错缠结或交割,从而形成位错墙或位错界面,导致位错交互作用增强,继而产生林位错硬化。背应力硬化和林位错硬化共同影响变形时的加工硬化过程。(3)大尺度不均匀复合板的力学行为与层片结构10号钢的相似,即硬度差值越大,结构越不均匀,应变硬化能力可能越强,从而得较好强度与塑性匹配。复合板界面处的晶粒尺度存在梯度分布,靠近界面处的晶粒较大,心部晶粒较小。多层板界面很稳定,可能是影响力学性能的一个关键因素。
[Abstract]:For the materials, regardless of uniform structure or uneven structure, its ultimate goal is to obtain excellent mechanical properties. According to Holzer Paige's law, nanocrystalline materials of high yield strength, plasticity is very low, will quickly necking instability. Coarse grained materials have good plasticity, but the expense of strength. In order to both high yield strength and good plasticity, the need to increase the strain hardening ability. Therefore, researchers have reported a variety of enhanced plasticizing strategies. Among them, gradient nano structure and nano structure layer material strain hardening ability to improve a lot, also won the strength of good plastic. Cause hardening of the gradient structure is there is strain gradient plastic strain, resulting in inconsistent, but the lack of detailed physical deformation and microstructure study, not suitable for industrialized production. The characteristics of lamellar nanostructured materials is nanometer lamellar microstructure, soft hard phase difference A large, even several orders of magnitude. The deformation mechanism is put forward the concept of back stress and back stress testing methods, but there is a big difference on the understanding and Research on the calculation method of depth of strain hardening, mechanism of lamellar nanostructure and microstructure studies, yet the evolution rules of the system knowledge. It is necessary to lamellar nanostructures were analyzed. From the micro element method and micro structure perspective, gradient nano structure is equivalent to a layer of nano lamellar structure formed by combining the micro structure mechanism is the same. Therefore, to explore and reveal the lamellar structure of the nanojunction mechanical behavior and strain hardening mechanism is very high the academic significance and application value. This paper selects 10 steel and plywood as model material. The use of cold rolled steel plate to be the No. 10 layer structure of ultra-fine grain hot rolling and multilayer board, then annealing microstructure control, get Soft and uneven distribution of grain size of different tissues. Through uniaxial quasi-static tensile test, microhardness test, loading test, stress relaxation test and strain field testing, research and analysis of its mechanical behavior, stress hardening, dislocation hardening and local Ying Bianyun forest graph evolution by means of optical microscope (OM); (SEM), scanning electron microscopy and transmission electron microscopy (TEM) microstructure and fractographic morphology characterization, finally analyzed and discussed the deformation behavior of lamellar nanostructure and strain hardening mechanism. Following the knot theory: (1) different annealing structure of 10 steel specimens are soft hard microstructure, hardness and hardness difference and uneven distribution of the larger plastic strain incompatibility is more obvious, leading to the back stress, so that the back stress hardening significantly improve work hardening ability. The tensile, soft hard micro layer The local strain surface sheet structure of the specimen is not uniform, the existence of local strain and local strain rate field. The uneven soft micro structure can effectively inhibit the plastic deformation of the initial strain localization, to avoid premature necking instability. (2) the transient strain No. 10 steel sheet of hard tissue is in essence uniform micro non deformation of soft and hard plastic strain incompatibility, produce back stress, the Kernel average grain misorientation increases, thereby increasing the density of geometrically necessary dislocations, resulting in hardening back stress. At the same time, the movable dislocation density increases and the activation volume decreases and the dislocation motion is blocked, resulting in cross slip and multiple slip. Delivery caused by entanglement or dislocation, thus forming dislocation walls or dislocation interface, resulting in enhanced dislocation interaction, and then generate the forest dislocation hardening back stress and hardening. Common dislocation hardening effect of hardening during deformation process. (3) a large scale The mechanical behavior of composite plate with uniform layer structure of 10 steel is similar to that of the hardness difference is bigger, the more uneven structure, strain hardening ability is stronger, and better strength and plasticity. The gradient distribution of grain size exists at the interface of composite plate, near the interface of grain is bigger, heart the grain is smaller. Multilayer interface is very stable, may be a key factor affecting the mechanical properties.

【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.1;TB383.1

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