Achieving high strength and high ductility in nanostructured metals: Experiment and modelling

Engineering nanostructures in metallic materials such as nanograins and nanotwins can promote plastic performance significantly. Nano/ultrafine-grained metals embedded in coarse grains called bimodal metals and nanotwinned polycrystalline metals have been proved to possess extensively improved yield strength whilst keeping good ductility. This paper will present an experimental study on nanostructured stainless steel prepared by surface mechanical attrition treatment (SMAT) with surface impacts of lower strain rate (10 s^?1–10³ s^?1) and higher strain rate (10⁴ s^?1–10⁵ s^?1). Microstructure transition has been observed from the original γ-austenite coarse grains to α′-martensite nanograins with bimodal grain size distribution for lower strain rates to nanotwins in the ultrafine/coarse grained austenite phase for higher strain rates. Meanwhile, we will further address the mechanism-based plastic models to describe the yield strength, strain hardening and ductility in nanostructured metals with bimodal grain size distribution and nanotwinned polycrystalline metals. The proposed theoretical models can comprehensively describe the plastic deformation in these two kinds of nanostructured metals and excellent agreement is achieved between the numerical and experimental results. These models can be utilized to optimize the strength and ductility in nanostructured metals by controlling the size and distribution of nanostructures.     

Grain-boundary sliding and separation in polycrystalline metals: application to nanocrystalline fcc metals

In order to model the effects of grain boundaries in polycrystalline materials we have coupled a crystal-plasticity model for the grain interiors with a new elastic-plastic grain-boundary interface model which accounts for both reversible elastic, as well irreversible inelastic sliding-separation deformations at the grain boundaries prior to failure. We have used this new computational capability to study the deformation and fracture response of nanocrystalline nickel. The results from the simulations reflect the macroscopic experimentally observed tensile stress-strain curves, and the dominant microstructural fracture mechanisms in this material. The macroscopically observed nonlinearity in the stress-strain response is mainly due to the inelastic response of the grain boundaries. Plastic deformation in the interior of the grains prior to the formation of grain-boundary cracks was rarely observed. The stress concentrations at the tips of the distributed grain-boundary cracks, and at grain-boundary triple junctions, cause a limited amount of plastic deformation in the high-strength grain interiors. The competition of grain-boundary deformation with that in the grain interiors determines the observed macroscopic stress-strain response, and the overall ductility. In nanocrystalline nickel, the high-yield strength of the grain interiors and relatively weaker grain-boundary interfaces account for the low ductility of this material in tension.     

Molecular dynamics simulation of polycrystalline copper

An approach for molecular dynamics simulation of the formation of polycrystalline materials from a melt during its cooling is proposed. Atomic configurations of copper corresponding to polycrystals with the mean grain size from 2 to 16 nm are obtained. Isothermal uniaxial tension and compression of these polycrystals is studied by the molecular dynamics method. For the mean grain size of polycrystalline copper being smaller than 10 nm, it is shown that Young’s modulus and yield stress decrease as the grain size decreases. Shock adiabats for polycrystalline copper are constructed. For a material with the grain size approximately equal to 2 nm, the temperature behind the shock wave front is demonstrated to be 10% higher than that in a polycrystal with the grain size greater than 10 nm. Molecular dynamics calculations predict the presence of copper with a body-centered cubic lattice behind the shock wave front at pressures ranging from 100 to 200 GPa.     

多晶冰抗压强度的粒径效应

对平均粒径分别为０５ｍｍ、１ｍｍ、２ｍｍ、３ｍｍ、４ｍｍ和６５ｍｍ的人造多晶冰在－５℃和６μｓ－１应变率下进行了单轴抗压强度试验．试验结果表明，多晶冰的极限抗压强度对粒径变化十分敏感，且与粒径平方根的倒数呈正比例关系，破坏应变随粒径的增大而呈线性减小．     

3-D simulation of spatial stress distribution in an AZ31 Mg alloy sheet under in-plane compression

A complete 3-D crystal plasticity finite element method (CPFEM) that considered both crystallographic slip and deformation twinning was applied to simulate the spatial distribution of the relative amount of slip and twin activities in a polycrystalline AZ31 Mg alloy during in-plane compression. A microstructure mapping technique that considered the grain size distribution and microtexture measured by electron backscatter diffraction (EBSD) technique was used to create a statistically representative 3-D microstructure for the initial configuration. Using a 3-D Monte Carlo method, a 3-D digital microstructure that matched the experimentally measured grain size distribution was constructed. Crystallographic orientations obtained from the EBSD data were assigned on the 3-D digital microstructure to match the experimentally measured misorientation distribution. CPFEM captured the heterogeneity of the stress concentration as well as the slip and twin activities of a polycrystalline AZ31 Mg alloy during in-plane compression.     

Effect of microstructure on the nucleation of deformation twins in polycrystalline high-purity magnesium: A multi-scale modeling study

A multi-scale, theoretical study of twin nucleation from grain boundaries in polycrystalline hexagonal close packed (hcp) metals is presented. A key element in the model is a probability theory for the nucleation of deformation twins based on the idea that twins originate from a statistical distribution of defects in the grain boundaries and are activated by local stresses at the grain boundaries. In this work, this theory is integrated into a crystal plasticity constitutive model in order to study the influence of these statistical effects on the microstructural evolution of the polycrystal, such as texture and twin volume fraction. Recently, a statistical analysis of exceptionally large data sets of {101?2} deformation twins was conducted for high-purity Mg (Beyerlein et al., 2010a). To demonstrate the significantly enhanced accuracy of the present model over those employing more conventional, deterministic approaches to twin activation, the model is applied to the case of {101?2} twinning in Mg to quantitatively interpret the many statistical features reported for these twins (e.g., variant selection, thickness, numbers per grain) and their relationship to crystallographic grain orientation, grain size, and grain boundary misorientation angle. Notably the model explains the weak relationship observed between crystal orientation and twin variant selection and the strong correlation found between grain size and the number of twins formed per grain. The predictions suggest that stress fluctuations generated at grain boundaries are responsible for experimentally observed dispersions in twin variant selection.     

Fatigue of polycrystalline copper with different grain sizes and texture

Fatigue experiments of polycrystalline copper with different grain sizes and texture were conducted under tension–compression, torsion, and non-proportional loading. The grain sizes ranged from 10 μm to 2 mm. The stress–strain response was found to be a strong function of the grain size and texture. A plasticity-based critical plane multiaxial fatigue criterion was used to predict the fatigue lives of the polycrystalline copper. It was found that the criterion was able to correlate all the experimental results with one single set of material constants. This indicates that the fatigue failure of the material under consideration is dominated by the fatigue resistance of the grains with an insignificant influence of the grain boundaries on the fatigue of the polycrystalline material. It was found that the fatigue model with the material constants obtained from fatigue experiments can be applied to predict failure under monotonic torsion.     

考虑尺寸效应的多晶金属循环塑性分析

本文采用多晶塑性分析方法,设材料点包含一定数量的各向异性单晶晶粒并考虑晶粒尺寸的影响,计算材料点的应力和应变时利用了Taylor假设。模型引入考虑尺寸效应的晶体滑移硬化函数,同时针对晶体滑移引入背应力及其方向性硬化的描述,以反映不同晶粒尺寸材料在循环加载条件下的力学行为。利用该模型,本文第一作者采用显式格式编制了与ABAQUS商用有限元软件接口的用户材料子程序（VUMAT）,实例计算证实该模型可以反映和描述多晶金属材料在材料反复加载条件下的循环塑性行为与尺寸效应。     

微纳米晶金属的应变率敏感性及应变硬化行为分析

基于亚微米、纳米晶粒组织塑性变形过程中多种变形机制(位错机制、扩散机制及晶界滑动机制)共存,建立了理论模型,用于定量研究亚微米、纳米晶粒组织的塑性变形行为.以铜为模型材料,计算分析了晶粒尺度、应变率以及温度对亚微米、纳米晶粒组织塑性变形行为的影响.结果表明:相比粗晶铜,亚微米晶铜表现出明显的应变率敏感性,并且应变率敏感系数随晶粒尺度及变形速率的减小而增大;同时,增大变形速率或降低变形温度都能提高材料的应变硬化能力,延缓颈缩发生,进而提高材料的延性.计算分析结果与实验报道吻合.     

Characterization of macroscopic tensile strength of polycrystalline metals with two-scale finite element analysis

The objective of this contribution is to develop an elastic-plastic-damage constitutive model for crystal grain and to incorporate it with two-scale finite element analyses based on mathematical homogenization method, in order to characterize the macroscopic tensile strength of polycrystalline metals. More specifically, the constitutive model for single crystal is obtained by combining hyperelasticity, a rate-independent single crystal plasticity and a continuum damage model. The evolution equations, stress update algorithm and consistent tangent are derived within the framework of standard elastoplasticity at finite strain. By employing two-scale finite element analysis, the ductile behaviour of polycrystalline metals and corresponding tensile strength are evaluated. The importance of finite element formulation is examined by comparing performance of several finite elements and their convergence behaviour is assessed with mesh refinement. Finally, the grain size effect on yield and tensile strength is analysed in order to illustrate the versatility of the proposed two-scale model.     

Combined grain size, strain rate and loading condition effects on mechanical behavior of nanocrystalline Cu under high strain rates

Molecular dynamics simulations of nanocrys-talline Cu with average grain sizes of 3.1 nm, 6.2 nm, 12.4 nm and 18.6 nm under uniaxial strain and stress tension at strain rates of 10 8 s 1 , 10 9 s 1 and 10 10 s 1 are performed to study the combined grain size, strain rate and loading condition effects on mechanical properties. It is found that the strength of nanocrystalline Cu increases as grain size increases regardless of loading condition. Both the strength and ductility of nanocrystalline Cu increase with strain rate except that there is no monotonic relation between the strength and strain rate for specimens under uniaxial strain loading. Moreover, the strength and ductility of specimens under uniaxial strain loading are lower than those under uniaxial stress loading. The nucleation of voids at grain boundaries and their subsequent growth characterize the failure of specimens under uniaxial strain loading, while grain boundary sliding and necking dominate the failure of specimens under uniaxial stress loading. The rate dependent strength is mainly caused by the dynamic wave effect that limits dislocation motion, while combined twinning and slipping mechanism makes the material more ductile at higher strain rates.     

Homogenization scheme for brittle intergranular decohesion in polycrystalline aggregates

The overall fracture behaviour of polycrystalline aggregates is strongly conditioned by intergranular failure, as is the case in copper alloys subjected to dynamic embrittlement. The self-consistent scheme is extended to account for grain boundary decohesion using a nonlinear cohesive law. The effective tensile response up to failure is computed for a Cu–Ni–Si alloy based on the homogenization method. In particular, the proposed approach allows for identification of the grain boundary critical energy release rate from the macroscopic tensile curve.     

晶界结构及其对力学性质的影响(Ⅱ)

<正> 3 晶界结构对力学性质的影响 晶界是固体材料中的一种面缺陷.由于晶界和位错以及其他缺陷和杂质之间的相互作用      

Phase-field simulation of the coupled evolutions of grain and twin boundaries in nanotwinned polycrystals

Nanotwinned polycrystals exhibit an excellent strength-ductility combination due to nanoscale twins and grains. However, nanotwin-assisted grain coarsening under mechanical loading reported in recent experiments may result in strength drop based on the Hall-Petch law. In this paper, a phase-field model is developed to investigate the effect of coupled evolutions of twin and grain boundaries on nanotwin-assisted grain growth. The simulation result demonstrates that there are three pathways for coupled motions of twin and grain boundaries in a bicrystal under the applied loading, dependent on the amplitude of applied loading and misorientation of the bicrystal. It reveals that a large misorientation angle and a large applied stress promote the twinning-driven grain boundary migration. The resultant twin-assisted grain coarsening is confirmed in the simulations for the microstructural evolutions in twinned and un-twinned polycrystals under a high applied stress.     

铜双晶体的循环应力-应变响应

比较了铜单晶体和多晶体疲劳行为的异同,提出了研究双晶体疲劳行为的必要性．总结了具有不同晶体取向和晶界的铜双晶体的疲劳行为的最新进展．利用平行晶界铜双晶体的取向因子和晶界影响区,总结了在循环载荷作用下的晶界强化模型．分析了垂直晶界铜双晶体循环塑性变形行为的特点,讨论了组元晶体取向对垂直晶界铜双晶体循环应力－应变曲线的影响．提出了提高单晶体和双晶体疲劳强度的控制因素．     

单纤维微拉伸力学性能测试与分析

针对微尺度材料力学性能测试与尺度效应实验研究的需要,自行研制了一台FMT-I型高精度纤维材料微拉伸力学性能实验装置,并基于LabVIEW软件平台开发了相应的数据采集与控制系统,实现了测试过程的全自动化。该装置的测力传感器由非接触式激光位移传感器和两端固定的薄梁组成,可同时测量试样的拉伸力和上夹持端的位移量,帮助精确地获取试样的载荷-变形曲线。采用该装置对微米级直径的多晶铜丝、316L不锈钢纤维和T300碳纤维进行了拉伸测试。实验结果表明,直径为18～105μm多晶铜丝的拉伸力学行为并无明显的尺度效应;多晶铜丝和316L不锈钢纤维的弹性模量分别在43.9～60.0GPa和102.9～111.5GPa之间,均低于宏观尺度下材料的弹性模量;316L不锈钢纤维的抗拉强度和延伸率随着丝径的减小而降低;T300碳纤维的弹性模量为235.4±12.4GPa,抗拉强度为3238.2±280.8MPa,断裂应变约为1.5%。另外,相同的细铜丝材料的测试结果与Instron5848型商用拉伸试验机的测试结果进行了对比,吻合良好。通过系统的实验分析表明,该装置具有较高的精度和稳定性,适用于各种纤维材料的拉伸力学性能测试。