NUMERICAL SIMULATION FOR DEFORMATION OF NANO-GRAINED METALS

Electro-deposition technique is capable of producing nano-grained bulk copper specimens that exhibit superplastic extensibility at room temperature. Metals of such small grain sizes deform by grains sliding,with little distortion occur- ring in the grain cores.Accommodation mechanisms such as grain boundary diffusion, sliding and grain rotation control the kinetics of the process.Actual deformation min- imizes the plastic dissipation and stored strain energy for representative steps of grain neighbor switching.Numerical simulations based on these principles are discussed in this paper.

Deformation Behavior and Formability of Gradient Nano-grained AISI 304 Stainless Steel Processed by Ultrasonic Impact Treatment

The deformation behavior and formability of gradient nano-grained（GNG） AISI 304 stainless steel in uniaxial and biaxial states were investigated by means of tensile test and small punch test（SPT）. The GNG top layer was fabricated on coarse grains（CG） AISI 304 by ultrasonic impact treatment. The results showed that the CG substrate could effectively suppress the strain localization of NC in GNG layer, and an approximate linear relationship existed between the thickness of substrate（h） and uniform true strain before necking（ε_（unif））. Grain growth of NC was observed at the stress state with high Stress triaxiality T, which led to better ductility of GNG/CG 304 in SPT, as well as similar true strain after the onset of necking（ε_（neck）） compared with coarse 304 in tensile test. Ei-values of GNG/CG 304 with different structures were nearly the same at different punch speeds, and good formability of GNG/CG 304 was demonstrated. However, punch speed and microstructure needed to be optimized to avoid much lost of membrane strain region in biaxial stress state.

固体力学跨尺度计算若干问题研究

本文展示了固体力学领域跨尺度计算的若干问题和研究概况。(1)建立位错动力学与有限元耦合DDD-FEM的计算模型,实现了能够基于纳米尺度离散位错运动机制计算分析连续介质有限变形晶体塑性问题,提出微纳尺度(200 nm~10μm)晶体塑性流动应力解析公式,结合试验数据揭示了在无应变梯度下强度和变形的尺寸效应;(2)建立具有微相分离结构的纳米尺度粗粒化分子动力学模型CG-MD,计算获得聚脲材料在时域和频域下的存储模量和损耗模量,通过动态加载分析的DMA试验和超声波试验的数据验证,解决了连续介质尺度下微相分离高分子共聚物的设计难题;(3)通过数据驱动关联高分辨率的微米尺度CT影像和临床低分辨率的毫米尺度CT影像的特征值,建立了围关节松质骨小梁的等效模量和结构张量,为骨组织增材制造点阵结构设计和实现个性化骨缺损重建奠定了基础。

轧制压下率、路径对NiCoCr合金微观结构和力学性能的影响

通过开展不同轧制压下率和路径的轧制实验,制备了具有不同比例和尺寸的纳米孪晶和纳米晶结构NiCoCr合金。透射电镜表征结果表明,单向轧制压下率50%样品中微观结构特征以纳米孪晶和高密度位错为主。当压下率达到90%,由于剪切带在纳米孪晶片层结构中的开动,孪晶体积分数下降,同时剪切带内形成大量尺寸为35 nm的纳米晶结构。多方向轧制样品中微观结构以纳米晶结构为主,平均尺寸为27 nm,略小于单向轧制压下率90%样品中的纳米晶尺寸。拉伸实验结果表明:相比于轧制前固溶态粗晶合金,单向轧制压下率50%纳米孪晶镍基合金强度大幅提高,合金屈服强度和抗拉强度分别达到1312 MPa和1396 MPa。压下率90%轧制样品屈服强度高达1599 MPa,变方向轧制样品屈服强度更是高达1705 MPa。同时拉伸试验数据表明,晶粒细化可以有效地提高材料强度,但是材料的延伸率显著下降。

Friction and wear behaviors of a gradient nano-grained AISI 316L stainless steel under dry and oil-lubricated conditions

A gradient nano-grained （GNG） surface layer was fabricated on an AIS1316L stainless steel （SS） by using the surface mechanical rolling treatment （SMRT）. Reciprocating dry and oil-lubricated sliding tests of the GNG 316L SS in air at room temperature were conducted in comparison with the coarse-grained （CG） counterpart. Worn surface morphologies and subsurface microstructures were investigated for both 316L SS samples. 316L SS with a GNG surface layer shows a significantly improved wear resistance, especially under oil-lubricated condition. The notably wear resistance enhancement of the GNG 316L SS is attributed to the GNG surface layer with high strain accommodation ability and high hardness, which can reduce the wear volume in the running-in stage effectively.

Fatigue Property of Nano-grained Delaminated Low-carbon Steel Sheet

Tension-tension fatigue life tests on nano-grained delaminated low-carbon steel sheet under different fatigue loads are carried out to study the fatigue properties of the steel. The three-dimensional microstructures of the steel are observed by TEM. In addition, the morphology of the fatigue fracture of the specimen under different loads is observed by SEM. The results show that micro-cracks form on the weak interface of the nano-grained steel under low-stress conditions, which hinders the propagation of the main cracks and reduces the fatigue crack propagation rate, resulting in the extending fatigue life of the steel.

A low-to-high friction transition in gradient nano-grained Cu and Cu–Ag alloys

A unique low-to-high friction transition is observed during unlubricated sliding in metals with a gradient nano-grained(GNG)surface layer.After persisting in the low-friction state(0.2–0.4)for tens of thousands of cycles,the coefficients of friction in the GNG copper(Cu)and copper-silver(Cu–5Ag)alloy start to increase,eventually reaching a high level(0.6–0.8).By monitoring the worn surface morphology evolution,wear-induced damage accumulation,and worn subsurface structure evolution during sliding,we found that the low-to-high friction transition is strongly correlated with distinct microstructural instabilities induced by vertical plastic deformation and wear-off of the stable nanograins in the subsurface layer.A very low wear loss of the GNG samples was achieved compared with the coarse-grained sample,especially during the low friction stage.Our results suggest that it is possible to postpone the initiation of low-to-high friction transitions and enhance the wear resistance in GNG metals by increasing the GNG structural stability against grain coarsening under high loading.

Excellent strength-ductility synergy properties of gradient nano-grained structural CrCoNi medium-entropy alloy

Tailoring heterogeneities could bring out excellent strength-ductility synergy properties.A gradient nanograined(GNG)structure,i.e.grain size range from nanometer(~50 nm)at topest surface layer to micrometer(~1.3μm)at center layer,was successfully introduced into CrCoNi medium-entropy alloy(MEA)by means of high energy shot peening in this work.Experimental results demonstrated that this GNG CrCoNi MEA shows excellent strength and ductility combination properties,exhibiting high yield strength and ultimate tensile strength of~1215 MPa and~1524 MPa,respectively,while remaining a good ductility of~23.0%.The extraordinary hetero-deformation induced(HDI)hardening origins from heterogeneous structure,i.e.GNG structure,which contributes to the majority strength enhancement.Dynamical reinforced heterogeneous structure during tension process results in the enhanced HDI hardening effect,which facilitates excellent ductility and strain hardening capacity at high-level strength.Our work provide not only a feasible and effective way to strengthen the CrCoNi MEA,and other low stacking faults energy(SFE)materials,but also an useful insight to understanding HDI hardening in heterogeneous structure.

Orientation dependence of mechanically induced grain boundary migration in nano-grained copper

Tensile tests were carried out on gradient nanograined copper samples to investigate the grain orientation dependence of mechanically induced grain boundary migration(GBM) process. The relationship between GBM and the orientations of nanograins relative to loading direction was established by using electron backscatter diffraction. GBM is found to be more pronounced in the grains with higher Schmid factors where dislocations are easier to slip. As a result, the fraction of high angle grain boundaries decreases and that of low angle grain boundaries increases after GBM.

Fabrication of nano-grained negative temperature coefficient thermistors with high electrical stability

Dense nano-grained Ni_(0.7)Mn_(2.3O4) negative temperature coefficient(NTC) thermistors were fabricated by a novel two-step sintering approach that combines rapid sintering and principle of conventional two-step sintering technique.Samples were sintered at 1042℃ for 30 s in the first rapid step and then at 850-950℃ for 20 h in the second soaking step.Crystal phase,microstructure and electrical properties of sintered samples were studied by X-ray diffraction(XRD),scanning electron microscopy(SEM),resistance temperature relationship and aging performance.Sintered samples show a single-phase cubic spinel structure and indicate a high relative density ranging from 84% to 91% of the theoretical density.Moreover,average grain sizes of sintered samples under SEM are distributed between 254 and 570 nm.Meanwhile,the resistivity and the aging coefficient significantly decrease when soaking sintering temperature rises.In addition,the obtained material constant(B) ranges from 3931 to 3981 K.Ni_(0.7)Mn_(2.3)O_(4)-3(soaking at 900℃) and Ni_(0.7)Mn_(2.3O4)-4(soaking at 950℃) present little aging behavior,implying great electrical stability.

Grain boundary segregation and relaxation in nano-grained polycrystalline alloys

The present study carries out systematic thermodynamics analysis of Grain Boundary(GB)segregation and relaxation in NanoGrained(NG)polycrystalline alloys.GB segregation and relaxation is an internal process towards thermodynamic equilibrium,which occurs naturally in NG alloys without any applied loads,causes deformation and generates internal stresses.The analysis comprehensively investigates the multiple coupling effects among chemical concentrations and mechanical stresses in GBs and grains.A hybrid approach of eigenstress and eigenstrain is developed herein to solve the multiple coupling problem.The analysis results indicate that the GB stress and grain stress induced by GB segregation and relaxation can be extremely high in NG alloys,reaching the GPa level,which play an important role in the thermal stability of NG alloys,especially via the coupling terms between stress and concentration.The present theoretic analysis proposes a novel criterion of thermal stability for NG alloys,which is determined by the difference in molar free energy between a NG alloy and its reference single crystal with the same nominal chemical composition.If the difference at a temperature is negative or zero,the NG alloy is thermal stable at that temperature,otherwise unstable.

剧烈塑性变形铜锌合金的纳米压痕性能研究

采用室温扭转试验对Cu-Zn合金进行剧烈塑性变形,并利用纳米压痕技术对扭转不同圈数的Cu-Zn合金热处理后的力学性能开展研究。基于Oliver-Pharr模型和量纲分析获得Cu-Zn合金在不同扭转圈数下的杨氏模量、硬度、屈服应力与应变硬化指数。结果表明,随着扭转圈数增加,Cu-Zn合金平均晶粒尺寸逐渐减小;在相同的扭转圈数下,轴向平均晶粒尺寸小于径向晶粒尺寸。合金的显微硬度和杨氏模量随载荷的增加而减小。基于量纲分析获得的合金平均屈服强度为0.099 GPa,平均应变硬化指数为0.26,与试验测试结果基本符合。

机械表面处理铜合金研究现状

铜合金具有良好的导电性和导热性,进一步提升铜合金力学性能可扩展其应用领域。通过机械表面处理在铜合金内构筑梯度纳米晶结构能在无合金元素添加的条件下,大幅提升铜合金强度-塑性匹配、抗疲劳等性能,具有工程应用潜力。本文首先综述了国内外制备梯度纳米晶结构铜合金常用的机械表面处理技术;其次,分析了机械表面处理对铜合金强度-塑性匹配性、疲劳性能和耐腐蚀性能的影响,并系统阐述了梯度纳米晶结构铜合金组织稳定性的调控方法;最后,总结了机械表面处理铜合金研究领域的发展趋势及面临的挑战。

纳米TiC复合铝空气电池阳极组织与电化学性能

针对铝空气电池金属阳极在碱性电解液中由于自腐蚀严重和钝化导致电化学性能无法提升的问题,通过引入TiC纳米粒子来细化铝阳极晶粒,提高晶界的数量,抑制自腐蚀反应,增加其电化学活性。实验利用Mg元素改善TiC与铝基体的润湿性,并通过真空感应熔炼炉制备了0.3%(质量分数)纳米TiC的铝阳极。结果表明:与纯Al和Al-Mg对比,Al-Mg-TiC晶粒平均尺寸从1.9664 mm降低到0.5326 mm,自腐蚀得到明显抑制,各项电化学性能包括工作电压和阳极利用率均有所提高,放电位点数量增加并且分布更加均匀。

添加纳米Ti(C,N)对粗晶WC-8Co硬质合金微观组织和力学性能的影响

将粒度为0.2μm的Ti(C,N)纳米粉末添加到粗晶WC-8Co硬质合金中,采用低压烧结技术制备了WC-xTi(C,N)-8Co系列硬质合金。利用XRD、SEM和EDS研究了添加纳米Ti(C,N)的硬质合金试样的物相组成和微观组织,并分析和讨论了合金的钴磁、矫顽磁力、洛氏硬度和抗弯强度等性能。结果表明,添加纳米Ti(C,N)烧结后的合金均出现了典型的Ti(C,N)芯-环结构,提高了硬质合金的硬度。然而,微量的Ti(C,N)添加会导致合金的力学性能下降,当添加量达到5%时,合金的综合力学性能最好,但当添加量达到15%以上时,基体出现脱碳相,导致合金的抗弯强度大幅下降。

立筒仓筒壁气密性修补技术探索

针对立筒仓气密性较差,绿色充氮气调储粮技术使用成本较高、整体效果不佳、虫害处理不彻底等问题,本文深入研究了纳米气密性涂料在立筒仓筒壁修补中的应用,结果表明该技术可有效提高示范仓的气密性、延长使用寿命,为提升绿色充氮气调储粮效果提供了新的研究方向。

CNT网络材料微观结构变化的数值模拟研究

为探究力学载荷作用对碳纳米管(carbon nano-tube,CNT)网络材料微观结构的影响,笔者建立了CNT网络材料的粗粒化模型,运用粗粒化分子动力学模拟方法对CNT网络在力学载荷作用下的微观结构变化行为进行仿真分析。结果表明:CNT网络材料的取向程度在拉伸应变增加初期增大速率较快,后续随着拉伸应变的增加取向程度趋于平稳,最终会趋近于稳定值0.95;在压缩载荷作用下,CNT网络材料的取向程度会随着应变的增大而减小,且最后减小至0.15;力学载荷速率不会影响CNT网络取向程度的变化规律。

用等径角挤压变形法制备纳米晶金属结构材料的组织演变

用实验方法研究了工业纯铁在等径角挤压变形 (路径 C)过程中晶粒细化过程。实验结果表明 ,经 4道次剪切变形后开始出现纳米级晶粒。晶粒细化过程为 :原始粗晶粒→晶粒被剪切变形带分割→位错线分割滑移带→位错线发展为位错墙 ,把变形带分割成细小的亚晶→亚晶界的位错密度增加→形成大角度晶界的纳米晶粒。测试了不同变形道次下材料的显微硬度值。

纳米Al_2O_3/Ni复合电刷镀层的表征与微动磨损机理

通过在镍基电刷镀液中添加纳米Al2O3颗粒,制备出了分散均匀、与基质金属Ni结合良好的Al2O3/Ni复合电刷镀层。复合镀层的组织致密,显微硬度高。微动磨损试验表明,复合镀层在常温及高温下抵抗微动磨损的能力均强于普通快镍镀层。这是因为,随着纳米颗粒的加入,Al2O3/Ni复合电刷镀层受到细晶强化、位错强化和弥散强化,抵抗塑性变形和粘着破坏的能力增强,从而具有良好的耐微动磨损性能。