双相高强钢FeNiAlC的动态剪切行为及微结构机理

绝热剪切带是金属材料在高应变率载荷下常见的一种失效模式。利用霍普金森压杆装置,对双相钢Fe-24.86Ni-5.8Al-0.38C不同微结构的帽形样品施加冲击载荷,研究它的动态剪切变形行为及微结构机理。先通过对固熔处理得到的粗晶态样品进行大应变冷轧获得冷轧态样品,再使用透射电子显微镜和扫描电子显微镜表征两种样品冲击前后微结构的变化差异。结果表明,双相钢FeNiAlC拥有较优异的动态剪切性能,剪切强度达1.3 GPa,均匀剪切应变达1.5。变形前,材料由奥氏体相和马氏体相构成,马氏体体积分数约为20%。变形过程由位错滑移和孪生变形主导,但因应变速率较高致使马氏体相变被抑制。不同微结构样品内均形成绝热剪切带,带内发生动态再结晶,形成超细晶粒,平均晶粒尺寸约300 nm,且剪切带内不发生相变;冷轧态剪切带宽度的实验值（14.6 μm）与理论计算值(12.3 μm)较好吻合,而粗晶态剪切带宽度的实验值（14.6 μm）与理论计算值（30 μm）相差甚远,初步分析可能是因为粗晶态样品应变较大基本不满足完全绝热的理论条件。在变形过程中,粗晶态因塑性变形做功产生的绝热温升高达720 K,而冷轧态的只有190 K。通过实验结果与热塑模型分析,得出绝热温升不是形成绝热剪切带的唯一因素,而应考虑材料的微观结构和局部化变形等的共同影响。     

平头弹冲塞靶板的绝热剪切数值模拟

针对绝热剪切形成时由于变形高度局域化,塑性功产生的热导致局部高温,有时会伴随动态再结晶(DRX)的现象,采用一种考虑动态再结晶过程的绝热剪切破坏准则,利用有限元方法模拟了Arne工具钢平头弹冲塞Weldox 460 E钢靶板的实验.数值模拟揭示了剪切带产生、传播的过程,温度分布情况表明在绝热剪切带中具备动态再结晶形成的...     

再结晶组织对TA2纯钛绝热剪切行为的影响

使用二辊轧机对TA2工业纯钛进行多道次大应变冷轧处理,制备了冷轧总变形量为70%的TA2纯钛板。通过对冷轧TA2纯钛板进行500℃加热、不同保温时间的退火处理,获得了具有不同再结晶组织的钛板。基于帽形试样和限位环变形控制技术,在分离式霍普金森压杆装置上对不同再结晶组织的试样进行动态冲击冻结实验,结合光学显微镜和扫描电子显微镜表征试样冲击前后微观组织的变化,研究了再结晶组织对TA2纯钛绝热剪切行为的影响。结果表明,随着退火保温时间的延长,试样再结晶晶粒占比逐渐增大,晶粒分布由分散向局部聚集转变;在相同应变和应变率下,在所有试样中都观察到了绝热剪切带,再结晶晶粒占比高的试样更易诱发绝热剪切带中裂纹形核扩展。对比变形前后试样再结晶组织和几何必需位错变化,结合剪切区整体温升分析发现,再结晶晶粒作为材料软化点能够诱发剪切带的形成,而剪切带发展后期产生的绝热温升会促进剪切带内材料发生二次再结晶,提高剪切带内材料的韧性,延缓剪切裂纹的形成。     

高速切削锯齿形切屑内绝热剪切带微观特征研究

使用光学显微镜、显微硬度计、X射线衍射、SEM、TEM等方法对高速切削30CrNi3MoV高强度钢锯齿形切屑中第一和第二变形区内形成绝热剪切带和白层进行了观察和研究。结果表明,形成了两种形式的绝热剪切带,即低速下形成的形变带和高速下形成的转变带。转变带内的硬度高于形变带和切屑基体。X射线衍射表明白层内发生了马氏体相变。TEM观察发现,形变带内为经历了大塑性变形的回火马氏体组织。转变带是由尺寸为50～100 nm的等轴晶组成,绝热剪切带形成过程中发生了动态再结晶。     

电弧增材制造不锈钢动态加载绝热剪切带内组织研究

绝热剪切失效是增材制造金属材料在高应变率载荷下的重要失效方式。使用电火花从冷金属过渡电弧增材技术制备的316L不锈钢单壁上沿着制造方向和扫描方向割出动态加载圆柱试样(尺寸为?4 mm×4 mm)。采用分离式霍普金森杆对增材制造316L试样在应变率4 000到6 000 s^-1下加载至绝热剪切状态，研究了其动态剪切变形行为特别是剪切带内微观组织特征结构。不同应变率动态加载下，电弧增材制造316L不锈钢的动态应力首先由于应变硬化而增大，随后绝热剪切热软化与应变硬化的平衡导致了动态变形最后阶段的应力平台效应。绝热剪切带中亚晶经历了动态再结晶过程，具有与基体完全不同的等轴晶形貌，晶粒尺寸大约在200～300 nm。动态剪切复杂热力过程导致剪切带内的亚晶形成了双重织构，既有与基体一致的沿着压缩方向的<110>丝织构，也有与宏观剪切方向相关的晶体学织构，即(111)沿着宏观剪切面，<112>沿着宏观剪切方向。不同剪切带的等轴亚晶都有大量残余Σ3 60°晶界，同时存在与基体相同的孪生织构，可以证明孪生再结晶是绝热剪切带内亚晶主要的动态再结晶机制。宏观绝热...     

块体金属玻璃及其复合材料的压缩剪切特性和侵彻/穿甲“自锐”行为

块体金属玻璃及其复合材料以其优异的力学、物理和化学性能, 正成为国内外科技和工程领域的研究热点. 特别是金属玻璃复合材料具有高剪切敏感性和剪切``自锐'特性, 使其有可能成为新型穿甲弹芯材料, 因而具有重要的军事意义. 本文综述了针对块体金属玻璃及其复合材料的压缩剪切变形和断裂特性及高速冲击、侵彻/穿甲过程中剪切``自锐'行为等方面的研究进展, 包括相关实验研究、理论分析及有限元模拟等, 最后给出未来相关工作的一些建议.      

冲击载荷下钨合金圆台试件绝热剪切变形局部化的数值模拟

采用有限元计算编码ABAQUS模拟了钨合金圆台试件在冲击载荷下的变形和剪切局部化行为.计算采用二维轴对称应变条件下的绝热模型.钨合金的本构方程采用热粘塑性形式的Johnson-Cook模型.为了得到不同尺度的变形信息,计算中用了两种网格;先用粗糙网格分析试件变形局部化的概貌;接着,用细密网格(在变形局部化区域,网格尺寸达到10μm)分析绝热剪切带的形成和发展.有限元模拟得到的绝热剪切带位置和方向与实验一致.计算结果表明,绝热剪切带的形成和发展与试件的应力状态密切相关.     

冲击载荷下钨合金圆台试件绝热剪切变形局部化的数值模

采用有限元计算编码ABAQUS模拟了钨合金圆台试件在冲击载荷下的变形和剪切局部化行为。计算采用二维轴对称应变条件下的绝热模型。钨合金的本构方程采用热粘塑性形式的Johnson Cook模型。为了得到不同尺度的变形信息 ,计算中用了两种网格 ;先用粗糙网格分析试件变形局部化的概貌 ;接着 ,用细密网格 (在变形局部化区域 ,网格尺寸达到 10 m)分析绝热剪切带的形成和发展。有限元模拟得到的绝热剪切带位置和方向与实验一致。计算结果表明 ,绝热剪切带的形成和发展与试件的应力状态密切相关。     

岩土介质中局部化变形研究的一些新进展

岩土中的剪切带产生通常是断裂和失稳的前兆, 常引起滑坡等地 质灾害, 对油气或水合物等矿藏的开采容易导致压实带形成. 压实带的产生使岩土孔隙减小, 颗粒破碎, 从而导致沉陷、对油气的挤压等.本文重点对剪切带的宽度和压实带的研究现状作综述, 阐述在该问题的研究上从微观 观测到宏观表述的重要性. 通过分析, 认为还有如下问题值得研究: 怎样 准确得出局部化变形带的宽度和方向?局部化变形带形成后孔隙水和油气的渗流会发生什么 变化?颗粒层次的微结构特性是通过什么方式影响宏观表现的等等.     

影响金属玻璃中剪切带行为的微观机制

论文对金属玻璃发生剪切失稳形成剪切带的行为进行了分析,得到了其发生剪切失稳时的临界自由体积浓度,预测结果与实验观察和模拟结果吻合;利用两种方法对其剪切带厚度进行了预测,结果表明基于剪切失稳临界波长预测金属玻璃剪切带厚度的方法只在发生剪切失稳后极短的时间内有效,对成熟剪切带厚度的预测必须考虑自由体积的扩散效应;考察了金属玻璃的宏微观材料参数对其剪切带厚度的影响及其微观机制,发现金属玻璃剪切带厚度对其宏观材料参数(泊松比)不敏感,对与剪切相变区相关的微结构参数敏感.     

完整岩石拉压应力状态下的张裂破坏准则

一点的应力状态可由3个主应力$\sigma_{1}$, $\sigma_{2}$, $\sigma_{3}$来表示, 当规定主应力以压为正时, 沿最大主应力$\sigma_{1}$方向将产生收缩变形, 若中间主应力$\sigma_{2}$和最小主应力$\sigma_{3}$都远小于$\sigma_{1}$, 则沿$\sigma_{2}$和$\sigma_{3}$方向会产生横向扩张变形, 当横向扩张变形达到一定极限时, 将会在平行于$\sigma _{1}$的方向产生张裂破坏. 如何建立这种张裂破坏的强度准则目前尚缺乏研究, 最大拉应变理论(第二强度理论)有时被用来解释张裂破坏, 但最大拉应变理论难以应用于三向受力状态. 本文分别用$\varepsilon_{1}$, $\varepsilon_{2}$表示最大张应变和次大张应变, 则最大拉应变理论认为当$\varepsilon_{1}$达到单向拉伸屈服应变时, 材料将产生破坏. 而本文将根据$\varepsilon_{1}+\varepsilon_{2}$之和达到极限值$\varepsilon_u$来建立张裂破坏准则. 可以证明$\varepsilon_{1} +\varepsilon_{2}$所表示的是$\sigma_{1}$主平面的面积增长率. 当$\sigma_{3}<\sigma_{2} \ll \sigma_{1}$时, 大部分岩石都具有脆性破坏的特点, 所以可将破坏前的岩石视为满足广义胡克定律的线弹性材料, 这样用$\varepsilon_{1}$, $\varepsilon_{2}$表示的强度准则可通过$\sigma_{1}$, $\sigma_{2}$, $\sigma_{3}$来表示. 在这个过程中还可考虑岩石在拉伸和压缩时具有不同弹性参数和强度的特点, 并可通过单向拉伸和单向压缩的破坏状态来确定$\varepsilon_u$. 不管$\sigma_{1}$, $\sigma_{2}$, $\sigma_{3}$是压应力, 还是拉应力, 或者$\sigma_{1}$, $\sigma_{2}$, $\sigma_{3}$中有拉有压的情形, 基于$\varepsilon_{1} +\varepsilon_{2} =\varepsilon_u$都可建立相应的强度准则. 所建立的准则可以反映中间应力$\sigma_{2}$对强度的影响规律, 通过建立的强度准则还可以证明: 静水拉力能引起屈服, 而静水压力不能产生屈服; 压缩破坏能使塑性体积增大, 其结果比Mohr-Coulomb准则更能反映实际情形. 并通过拉压应力状态下的试验数据验证了所建立的强度准则, 所得理论计算结果和已有的试验数据吻合得很好. 通过提出的强度准则和圆盘劈裂的试验结果, 可获得更为可靠的岩石单轴抗拉强度.      

Strain gradient crystal plasticity effects on flow localization

In metal grains one of the most important failure mechanisms involves shear band localization. As the band width is small, the deformations are affected by material length scales. To study localization in single grains a rate-dependent crystal plasticity formulation for finite strains is presented for metals described by the reformulated Fleck–Hutchinson strain gradient plasticity theory. The theory is implemented numerically within a finite element framework using slip rate increments and displacement increments as state variables. The formulation reduces to the classical crystal plasticity theory in the absence of strain gradients. The model is used to study the effect of an internal material length scale on the localization of plastic flow in shear bands in a single crystal under plane strain tension. It is shown that the mesh sensitivity is removed when using the nonlocal material model considered. Furthermore, it is illustrated how different hardening functions affect the formation of shear bands.     

通过表面机械加工调控Zr52.5Cu17.9Ni14.6Al10Ti5非晶合金的结构和韧性

作为一种新型结构材料, 非晶态合金的韧性需要进一步提高. 提高非晶态合金韧性的方法有引入枝晶相、调整其成分改变其泊松比影响其剪切带衍生、裂纹扩展等.本文通过表面机械加工的方法来调控非晶态合金的微观结构及韧性. 我们采用真空电弧熔炼、亚稳态薄板离心浇铸系统制备了Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (原子百分比) (Vit105)非晶合金板,并用表面机械研磨处理方法(surface mechanical attrition treatment, SMAT), 在Vit105板上形成纳米尺度局域类晶体序结构. 基于差示扫描量热分析、纳米压痕实验, 我们发现SMAT处理后的Vit105合金板表面附近弛豫焓更低, 微观结构更加均匀、稳定. 通过显微维氏硬度计测试, 发现SMAT处理后样品的表面附近硬度增大,硬度值分布也更均匀. 通过三点弯断裂实验, 可得到SMAT处理后合金板缺口韧度值从70.7 ± 4.7 MPa·m1/2提高到112.8 ± 3.7 MPa·m1/2. SMAT处理后合金板断裂后, 缺口前端剪切带密度比未处理的更大. Vit105合金板韧性的提高源于SMAT处理对剪切带萌生的促进作用. 该研究表明,表面机械加工可以在非晶态合金中形成局域类晶体有序结构, 影响其结构均匀性, 增大其硬度, 促进剪切带萌生, 提高其韧性. 表面机械加工作为一种新型的改变材料性能的手段, 具有广阔的应用前景.      

16MnR钢缺口试样解理断裂行为研究

研究了低合金热轧钢16MnR缺口试样在$-196\,{^\circ}$C和$-130\,{^\circ}$C的解理断裂机 理. 拉伸试验、单、双缺口四点弯曲实验、断口形貌观察以及有限元分析结果表明, 缺口试 样发生解理断裂时均起裂于夹杂物粒子, 一种位于缺口根部前端(IC型), 另一种位于距缺口 根部较远的条形裂纹前端(SIC型); 且随温度升高, 起裂源的类型从$-196\,{^\circ}$C下的IC 型转变为$-130\,{^\circ}$C下的SIC型. 微裂纹均形核于夹杂物, 最终的断裂由铁素体晶粒尺 寸的微裂纹扩展控制. 缺口试样IC型解理断裂遵循裂纹形核条 件$\varepsilon_{\rm p} \ge \varepsilon_{\rm pc}$和裂纹扩展条件$\sigma_{yy} \ge \sigma_{f}$, 而SIC型解理断裂条件则演化为$\varepsilon_{\rm p}+\varepsilon_{\rm ps} \ge \varepsilon_{\rm pc}$和$\sigma_{yy} +\sigma_{yy{\rm s}} \ge \sigma_{f}$.     

Numerical investigation of grain boundary effects on elevated-temperature deformation and fracture

A three-dimensional (3D) polycrystal intergranular model that accounts for grain boundary deformation and intergranular weakening at elevated temperatures is presented. The effects of grain boundaries on the accumulated slip deformation of grain interiors and lattice rotation have been investigated through a comparison between results from a model including grain boundary region (GBM) and a model representing only the grain interiors not the grain boundary region directly (NGBM). It is found that the presence of grain boundaries seems to suppress the grain interior slip deformation, and this suppressive role is reduced with increased relative thickness of the grain boundaries. In addition, grain boundaries promote the lattice rotation of individual grains in shear bands but suppress that of individual grains within non-shear bands. Mutual rotation of grains in both shear and non-shear bands is caused by the introduction of grain boundary regions. Rate-dependence of high-temperature plasticity could be more accurately captured by the GBM than by the NGBM. By considering creep damage of grain boundary, when the damage variable reaches a critical value, the corresponding grain boundary element is eliminated to describe dynamic intergranular fracture processes. The volume-averaged stress–strain curve by a model considering grain boundary damage (DGBM) showed better agreement with experimental results than that by a model not considering grain boundary damage (GBM).     

Detwinning of High-Purity Zirconium: <Emphasis Type="Italic">In-Situ</Emphasis> Neutron Diffraction Experiments

Twinning is an important deformation mode in hexagonal metals to accommodate deformation along the c-axis. It differs from slip in that it accommodates shear by means of crystallographic reorientation of domains within the grain. Such reorientation has been shown to be reversible (detwinning) in magnesium alloy aggregates. In this paper we perform in-situ neutron diffraction reversal experiments on high-purity Zr at room temperature and liquid nitrogen temperature, and follow the evolution of twin fraction. The experiments were motivated by previous studies done on clock-rolled Zr, subjected to deformation history changes (direction and temperature), in the quasi-static regime, for temperatures ranging from 76 K to 450 K. We demonstrate here for the first time that detwinning of { 10[`1] 2 } á 10[`1][`1] ñ\left\{ {10\overline 1 2} \right\}\left\langle {10\overline 1 \overline 1 } \right\rangle tensile twins is favored over the activation of a different twin variant in grains of high-purity polycrystalline Zr. A visco-plastic self-consistent (VPSC) model developed previously, which includes combined slip and twin deformation, was used here to simulate the reversal behavior of the material and to interpret the experimental results in terms of slip and twinning activities.     

基于等温吸附的页岩水分传输特征研究

研究页岩的水分传输特征至关重要,不仅有助于认识页岩的物理化学性质,而且也有助于评价页岩气的吸附扩散和流动能力.本文设计了页岩的水分传输实验装置,采用美国伍德福德和中国南方龙马溪组页岩为研究对象,开展了不同温度、不同湿度下页岩的水分传输实验,研究了页岩的水分传输特征和影响因素.结果表明,页岩的水分吸附属于II型曲线,包含着单分子层吸附、多分子层吸附和毛细凝聚的过程,GAB模型可用于描述页岩的水分吸附过程;水分吸附随着相对压力的增大而增强,有机碳含量和温度对页岩水分吸附起着增强作用,而方解石会抑制页岩的水分吸附;随着相对压力的增大,页岩的水分扩散系数呈现先增大后减小随后增加的趋势,其系数大约在8.73$\times$10$^{ - 9}\sim $5.95$\times $10$^{ - 8 }$m$^{2}$/s之间;伍德福德页岩的等量吸附热均大于龙马溪页岩的等量吸附热,这与其页岩的成熟度有关.研究结果为认识页岩的物理化学性质和力学性能以及评价页岩气的吸附流动能力提供参考依据.      

Effects of texture on shear band formation in plane strain tension/compression and bending

In this study, effects of typical texture components observed in rolled aluminum alloy sheets on shear band formation in plane strain tension/compression and bending are systematically studied. The material response is described by a generalized Taylor-type polycrystal model, in which each grain is characterized in terms of an elastic–viscoplastic continuum slip constitutive relation. First, a simple model analysis in which the shear band is assumed to occur in a weaker thin slice of material is performed. From this simple model analysis, two important quantities regarding shear band formation are obtained: i.e. the critical strain at the onset of shear banding and the corresponding orientation of shear band. Second, the shear band development in plane strain tension/compression is analyzed by the finite element method. Predictability of the finite element analysis is compared to that of the simple model analysis. Third, shear band developments in plane strain pure bending of a sheet specimen with the typical textures are studied. Regions near the surfaces in a bent sheet specimen are approximately subjected to plane strain tension or compression. From this viewpoint, the bendability of a sheet specimen may be evaluated, using the knowledge regarding shear band formation in plane strain tension/compression. To confirm this and to encompass overall deformation of a bent sheet specimen, including shear bands, finite element analyses of plane strain pure bending are carried out, and the predicted shear band formation in bent specimens is compared to that in the tension/compression problem. Finally, the present results are compared to previous related studies, and the efficiency of the present method for materials design in future is discussed.     

Elongational rheology of NIPAM-based hydrogels

Hydrogels of different composition based on the copolymerization of N-isopropyl acrylamide and surfmers of different chemical structure were tested in elongation using Hencky/real definitions for stress, strain, and strain rate, offering a more scientific insight into the effect of deformation on the properties. In a range between $\dot {\varepsilon }=10$ and 0.01 s $^{-1}$ , the material properties are independent of strain rate and show a very clear strain hardening with a “brittle” sudden fracture. The addition of surfmer increases the strain at break $\varepsilon _{\mathrm {H}}^{\max }$ and at the same time leads to a failure of hyperelastic models. The samples can be stretched up to Hencky strains $\varepsilon _{\mathrm {H}}^{\max }$ between 0.6 and 2.5, depending on the molecular structure, yielding linear Young’s moduli E $_{0}$ between 2,700 and 39,000 Pa. The strain-rate independence indicates an ideal rubberlike behavior and fracture in a brittle-like fashion. The resulting stress at break $\sigma _{\textrm max}$ can be correlated with $\varepsilon _{\mathrm {H}}^{\max } $ and $E_{0}$ as well as with the solid molar mass between the cross-linking points $M_{\mathrm {c}}^{\textrm {solids}} $ , derived from $E_{0}$ .     

考虑平均应力的同频拉扭多轴高周疲劳寿命评价方法

目前基于临界平面理论的高周疲劳寿命预测模型, 大都充分考虑了法向平均应力对材料疲劳寿命的影响, 但是没有有效反映剪切平均应力对疲劳寿命的影响. 通过分析7075-T651铝合金的试验数据发现, 与法向拉平均应力类似, 剪切平均应力同样对材料的疲劳寿命产生不利影响. 因此, 如果寿命预测模型中忽略剪切平均应力的影响, 存在明显剪切平均应力加载工况下, 预测寿命可能偏于危险. 由此, 本文定义具有较大法向应力的最大剪应力范围平面为临界平面, 建立了一个能够同时反映法向和剪切平均应力影响的高周疲劳寿命预测模型, 并给出了模型中材料常数的确定方法. 新模型首先将基于应变的Fatemi-Socie准则, 推广到材料的高周疲劳寿命预测, 给出了Fatemi-Socie准则的应力表述形式. 然后, 引入剪切和法向Walker因子, 反映剪切和法向平均应力对材料疲劳寿命的影响. 剪切和法向Walker因子的取值都介于0和1之间, 不同取值反映了材料对剪切和法向平均应力敏感程度的不同. 新模型适用于范围内的金属塑性材料. 利用5种材料在12种存在平均应力加载工况下的试验数据, 对所建模型进行了试验验证, 结果表明预测结果与试验结果吻合良好, 绝大多数寿命预测结果分布在3倍误差带以内.