Orientation dependence of fracture in copper bicrystals with symmetric tilt boundaries

Experimental results (Wang and Anderson (1991), Acta Metall. 39, 779–792) show that the fracture behavior of Σ9 copper bicrystals depends on the cracking direction. Near-interface transgranular fracture surfaces were observed in the case of the crack growing in the [ 14] direction, with an essentially ductile failure mode, while the case of the crack growing in the [1 ] direction showed far less toughness and had an intergranular fracture surface with cleavage tongues. Asymptotic and finite element models for stationary cracks in ideally plastic and strain hardening materials have been used to examine this cracking direction dependency from a small strain continuum mechanics point of view. The tensile stress ahead of the crack tip was found to be essentially identical for the two growth directions, with the brittle orientation resulting in only slightly higher stress values at small distances from the crack tip. However, the strain field was found to be different for the two orientations, with the overall plastic zone size being much larger in the ductile case. Also, the orientations of the zones of concentrated shearing ahead of the crack, observed in the ideally plastic model, suggest two different dislocation shearing mechanisms. In the ductile case, this zone is parallel to the slip plane, resulting in a regular shearing mechanism in which dislocations can be nucleated at the crack tip and glide on the (111) slip plane. In contrast, this zone is perpendicular to the (111) slip plane in the brittle case, resulting in a kinking shear mode in which dislocations from other external sources expand in a dipole mode to produce macroscopically concentrated shearing. Thus, apart from the dislocation nucleation considerations, continuum mechanics does not seem to be able to fully explain this difference in directional dependency of fracture in the Σ9 copper bicrystals.     

FATIGUE LIFE BEHAVIOR OF MONOCRYSTALLINE COPPER IN 0.1 M PERCHLORIC ACID

We have studied the fatigue lives of single crystals of copper in 0.1 M HClO₄under different polarization potentials. Perchloric acid was chosen for the aqueous environment because it allows us to control the corrosion reactions rigorously. Persistent slip band (PSB) behaviour and crack nucleation were studied during life, and fracture surfaces after failure. Different behavior was observed depending on strain amplitude. At 2 × 10^-3 plastic shear strain amplitude, anodic potential was observed to decrease life, whereas cathodic potenlial was found to be less damaging than laboratory air. Crack nucleation and propagation occurred along the primary slip plane for both conditions. The reduction of fatigue life under anodic potential is explained by enhanced localized strain at the PSB's and preferential dissolution within them. However, for a strain amplitude of 4 × 10^-3, cracks nucleated and propagated along the secondary slip system. We observed crack nucleation to be associated with deformation-induced stress concentrations, and the aqueous solution showed no aggressive effect under either anodic or cathodic potential.     

High-temperature mechanical properties of hot-pressed TiN with fine grain size

Mechanical properties of fine-grained TiN were studied by compression tests at temperatures ranging from 1073–1823 K and strain rates from 2×10-5 to 5×10-3 s-1. The temperature dependence of maximum (fracture or peak) stress of TiN reveals three regions with different activation energy and strain-rate sensitivity. At lower temperatures, brittle fracture takes place without plastic deformation. Fracture stress is independent of temperature, and greater than 1 Gpa. In the intermediate temperature region, specimens fracture in a quasi-brittle manner after a few per cent plastic deformation. Fracture stress decreases above 1300 K (in the intermediate temperature region), due to the deformation-assisted fracture. TiN becomes fully ductile at further higher temperatures, at which five independent slip systems are available. This ductile to brittle transition characteristic of TiN is similar to MgO (ionic) but different from TiC (covalent), though all three materials take the same NaCl type of lattice structure. In the high-temperature region, the activation energy for plastic deformation is close to that for diffusion of nitrogen in TiN. Strain-rate sensitivity in this region is typical of superplasticity, suggesting the possibility of superplastic forming. © 1998 Chapman and Hall     

A finite element analysis of fracture initiation in ductile/brittle periodically layered composites

A near-tip plane strain finite element analysis of a crack terminating at and normal to the interface in a laminate consisting of alternate brittle and ductile layers is conducted under mode-I loading. The studies are carried out for a system representing steel/alumina composite laminate. The Gurson constitutive model, which accounts for the ductile failure mechanisms of microvoid nucleation, growth and coalescence, is employed within the framework of small deformation plasticity theory. Evolution of plastic zone and damage in the ductile layer is monitored with increasing load. High plastic strain localization and microvoid damage accumulation are found to occur along the brittle/ductile interface at the crack-tip. Fracture initiation in the ductile phase is predicted and the conditions for crack renucleation in the brittle layer ahead of the crack are established for the system under consideration. Ductile fracture initiation has been found to occur before plasticity spreads in multiple ductile layers. Effects of material mismatch and yield strength on the plastic zone evolution are briefly discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Deformation and fracture of Al-8Fe rapidly solidified alloys

The effects of the rapid solidification on the deformation and fracture of Al-8Fe alloys, from TEM fracture specimens, have been studied. The most general conclusion which can be drawn in this study is clearly in agreement with a plastic deformation mechanism. Crack propagation occurs by localized plastic rupture mechanisms which result from enhanced slip along {111} planes. Crack propagation occurs within the deformed zone either by the nucleation, growth and coalescence of holes ahead of the crack-tip, or through the emission of dislocation from the crack-tip. The resulting fracture is along the active {111} slip planes. The principal effect of secondary phases (Al₁₃Fe₄) on the fracture propagation in Al-8Fe alloys was that the secondary phases increased the stress level at which plastic deformation occurs at the crack-tip and increased the stress level at which the crack propagates. This work clearly shows that in order to obtain coarse intermetallic precipitates in the specimens after ageing heat treatments the crack propagation and deformation processes occur at lower stresses compared to as-received rapidly solidified samples.     

A novel damage variable to characterize evolution of microstructure with plastic deformation for ductile metal materials under tensile loading

Damage and fracture of ductile metal materials are greatly associated with evolution of their microstructure under loading, which meso-dimensionally corresponds to grain deformation as well as nucleation, growth, and coalescence of microvoids in later local deformation. The evolution behavior of microstructure is important to realize ductile damage and fracture mechanism of materials. In the present work, a novel damage variable of shape factor of grains was put forward to quantitatively describe the character of microstructure; its evolution with the plastic deformation was built-up by microanalytical and mechanical experiments for Armco iron and mild steel tensile bars. The evolution rule based on the damage variable of shape factor is possible to be extended into all of ductile metal materials.     

X65管线钢焊接接头在模拟浅表海水环境中的应力腐蚀试验研究

在海水环境中,由于海水从海底管道外管焊缝浸入,导致外管焊接接头断裂。为了研究可能导致X65外管焊接接头断裂的因素,应用慢应变速率拉伸试验(SSRT),通过应力-应变曲线、扫描电镜(SEM)等手段分析了3个X65管线钢焊接接头在空气及模拟浅表海水环境中的应力腐蚀性能。结果表明:在空气中X65钢焊接接头试样的延长量最大,达5.6 mm,在模拟海水中试样的延长量均减小,其中2号试样延长量最小,仅3.6 mm,表明试样在浅表海水中塑性变形能力降低;模拟海水中3个试样的应力腐蚀敏感性指数均处于有应力腐蚀倾向的范围;在空气中试样的断裂为韧性断裂;在浅表海水环境中试样的断裂为韧性断裂与脆性断裂的混合断裂,有应力腐蚀开裂的趋势;海水中含有的大量Cl~-导致焊接接头的应力腐蚀敏感性升高,失效风险增加。     

温度对V-5Cr-5Ti合金拉伸性能及组织结构的影响

对真空自耗重熔制备的V-5Cr-5Ti合金进行了室温到1150℃温度范围的拉伸性能测试,获得了不同温度下的拉伸应力应变曲线,用SEM和光学显微镜对断口形貌和金相组织进行了观察,分析了温度对断口形貌和组织的影响。结果表明:V-5Cr-5Ti合金的屈服强度和极限强度总体上随温度升高而降低,但在300℃到700℃之间出现应变失效效应,断裂伸长率随温度升高而降低,断面收缩率随温度升高先增大再而降低,在400℃时断面收缩率最大;温度较低时塑性变形以滑移为主,温度较高时以晶界开裂为主,并伴随有晶界熔化的现象,高温断口表现为韧性断裂为主,具有韧性与脆性共存的现象。     

Hydrogen-induced Modification in the Deformation and Fracture of a Precipitation-hardened Fe-Ni Based Austenitic Alloy

Hydrogen-induced modification in the deformation and fracture of a precipitation-hardened Fe-Ni based austenitic alloy has been investigated in the present study by means of thermal hydrogen charging experiment, tensile tests, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. It is found that the γ＇ particles are subjected to the multiple shearing by dislocations during plastic deformation, which promotes the occurrence of the dislocation planar slip. Moreover, the alloy will be enhanced by hydrogen resulting in the formation of strain localization at macroscale. So, the mechanisms of deformation and fracture in the alloy have been proposed in terms of serious hydrogen-induced planar slip at microscale which can lead to macroscopic strain localization.     

Numerical simulations of hole growth and ductile fracture initiation under mixed-mode loading

Numerical simulations of ductile fracture initiation caused by the interaction between a notch tip and a nearby hole under mixed-mode loading involving modes I and II are performed. Attention is restricted to plane strain, small-scale yielding conditions. The Gurson constitutive model that accounts for the ductile failure mechanisms of micro-void nucleation, growth and coalescence is employed within the framework of a finite deformation plasticity theory. The failure of the ligament connecting the notch tip and the hole by either microvoid coalescence or by intense plastic strain localization is modelled. The effect of mode-mixity on the notch tip deformation, hole growth and the critical value of J at fracture initiation is examined. The dominant failure mechanism which is operative near the notch tip for various extents of mixity of modes I and II is identified.     

基于微孔贯通细观损伤模型的金属韧性断裂分析

韧性材料断裂过程通常可看作是材料内部微孔洞的形核、扩展及相互贯通的积累。经典的Gurson- Tvergaard (GT)模型能够很好地模拟具有变形均匀、各向同性的孔洞的萌生及扩展过程;但无法模拟由孔洞贯通而引起的局部变形过程,因此需要对其修正,引入相应的孔洞贯通准则。该文采用两种贯通准则对经典GT模型进行修正,即Thomason的塑性极限载荷准则和临界等效塑性应变准则。借助用户自定义程序UMAT将采用这两种贯通准则修正的GT本构关系嵌入至商用有限元软件ABAQUS中,从而可通过对金属材料应力状态和断裂机理的分析控制孔洞的贯通。以一组含有不同缺口根半径的圆棒拉伸试验件为例,分析了该类金属构件自孔洞萌生至最终断裂的整个损伤演化过程,并与试验数据进行了对比,验证了该模型的有效性。该文还讨论了金属断裂过程中应力三轴度对微裂纹萌生与扩展的影响。     

沉淀强化奥氏体合金的氢致断裂行为

研究了未充氢和热充氢沉淀强化奥氏体合金的拉伸断裂行为,分析了其氢脆敏感性与拉伸断裂行为间的联系,研究了氢对合金局部塑性变形及微裂纹形核的影响。结果表明:氢使沉淀强化合金由单一的韧窝断裂转变为韧窝断裂、沿晶断裂和滑移带开裂的混合断裂方式。其原因是:一方面,氢促进位错平面化滑移趋势、加剧局部塑性变形;另一方面,滑移带被晶界、孪晶界以及不同取向的滑移带所阻碍,引起了位错塞积和氢聚集。     

Cyclic stress response and deformation behaviour of precipitation-hardened aluminium-lithium alloys

The cyclic stress response of two lithium-containing aluminium alloys aged to contain ordered precipitates was studied in different environments over a range of plastic strains. The specimens were cycled using tension-compression loading under total strain control. The peak-aged Al---Li---Mn alloy cyclically hardened to failure, whereas the peak-aged Al---Li---Cu alloy displayed softening for most of the fatigue life. The presence of shearable softening for most of the fatigue life. The presence of shearable precipitates in the two alloys results in a local decrease in resistance to dislocation movement, leading to a progressive loss of ordering contributions to hardening and slip concentration. This, coupled with the presence of precipitate free zones, promotes strain localization in intense slip bands and results in early crack nucleation. Transmission electron microscopy observations revealed homogeneous deformation in specimens cycled at high plastic strain amplitudes. However, at lower plastic strain amplitudes, deformation was inhomogeneous in the two alloy systems with the formation of intense planar slip bands. Results of this study reveal that the initial hardening observed is due to dislocation-dislocation and dislocation-precipitate interaction and that the softening observed in the Al---Li---Cu alloy is a mechanical and not an environmental effect.     

Cyclic deformation response and dislocation structure of Cu single crystals oriented for double slip

Cyclic deformation behavior of double-slip oriented Cu single crystals with a stress axis in the [034] direction was investigated under plastic strain control mode for a shear strain amplitude range of 1 × 10⁻⁴ to 5 × 10⁻³. Dislocation structures in the tested samples were observed using a transmission electronic microscope. It has been found that the effect of the operation of critical slip in these [034] crystals on cyclic responses and dislocation structures is nearly the same as that of increase in strain amplitude. The nucleation stress and number of cycles for PSB formation at each specific strain amplitude in the double-slip oriented crystals were found to be both considerably lower than those observed in single-slip oriented crystals. This observation is in a good agreement with the Kuhlmann-Wilsdorf and Laird analysis, in that the formation of PSBs is associated with glide behavior on the secondary slip system. A dislocation “cord” structure has also been observed and is believed to be caused by the operation of the cross-slip system during cyclic deformation. Labyrinth wall structures were found to form with increase in strain amplitude by the operation of critical slip and cross-slip systems. However, the formation of labyrinth structure was suppressed by the coplanar slip at high strain amplitudes.     

Novel In Situ Nanostructure-Dendrite Composites in Zr-Base Multicomponent Alloy System

The evolution of a nanostructure-dendrite composite microstructure of two Zr-base alloys solidified through different casting routes is presented. The alloys were designed by adding different amounts of Nb to the Zr-based multicomponent glass-forming alloy system. The refractory metal Nb promotes the formation of a primary phase having dendritic morphology, whereas the residual melt solidifies to a nanostructured/amorphous matrix. The volume fraction and the morphology of the dendritic phase varied with the Nb content and the adopted casting route. A correlation between the alloy composition and adopted casting method with evolved microstructures and mechanical properties is revealed. These composites exhibit a unique combination of high fracture strength up to 1922 Mpa, as well as plastic strain over 15.8% under uniaxial compression testing at room temperature. The high strength of these composites is imparted by the nanostructured matrix, whereas the large plastic strain is a consequence of the retardation of excessive localized shear banding in the matrix by ductile dendrites. The significant work hardening of the composites prior to fracture is attributed to dislocation multiplication in the solid solution-strengthened dendritic phase.     

Fracture criterion on the basis of uniformity of plastic work of polycrystalline ductile materials under various stress states

Studies on the fracture criterions of structural materials would be of great significance to the service security of engineering structures. This note proposes a novel criterion on the basis of the uniformity of plastic work under various stress states. In order to realize abundant stress states, modified Arcan fixtures were designed and integrated with a self-made in situ tensile device. Accordingly, by changing the stress ratio of tensile to shear components, true stress–strain relationships of a typical polycrystalline ductile material (Gr-4 titanium alloy) specimens on the basis of various stress states were obtained and piecewisely fitted. By, respectively, adopting linear and polynomial fitting in the elastic and plastic deformation stages, the plastic work, namely the envelope areas of true \({\sigma_{\rm t}}\)–\({\varepsilon_{\rm t}}\) curves, was quantitatively calculated. Approximate uniformity of plastic work was verified as no correlation between plastic work and stress state was observed. Moreover, the evolution behavior of microvoids inside a single grain and the equivalent average slip distance of polycrystalline ductile materials during trans-granular fracture process were also analyzed theoretically to explain the uniformity. The trans-granular slip and fracture behavior of Gr-4 titanium alloy specimen and orientations of crack propagation of extruded AZ61B magnesium alloy specimens were also examined experimentally.     

The rate dependent response of a titanium alloy subjected to quasi-static loading in ambient environment

A dual phase Ti-6A1-4V alloy was tested in uniaxial tension over a large quasi-static loading range (10^–5–10^–1 s^–1) in ambient environment. As strain rate increases, strength of the alloy was found to increase at the expense of ductility. In the low strain-rate region, strain rate sensitivity of the material experienced a gradual decrease during plastic deformation. In the high strain-rate region, strain-rate sensitivity of the material was largely constant for most part of the plastic deformation. The different rate dependent behaviours are believed to be caused by a change of governing plastic deformation mechanism from dislocation slip at low strain rates to twinning at the highest strain rate. Strong fractographic and metallographic evidence was obtained to understand the micromechanisms of plastic deformation.     

Strain hardening in nanolayered thin films

Experimental results indicate that metal–ceramic multilayered thin films have unusual properties such as high strength, measurable plasticity and high strain hardening rate when both layers are nanoscale. Furthermore, the strength and strain hardening rate show a pronounced size effect, depending not only on the layer thickness but also on the layer thickness ratio. We analyze the strain hardening behavior of nanoscale multilayers using a three-dimensional crystal elastic–plastic model (3DCEPM) that describes plastic deformation based on the evolution of dislocation density in metal and ceramic layers according to confined layer slip mechanism. These glide dislocations nucleate at interfaces, glide inside layers and are deposited at interfaces that impede slip transmission. The high strain hardening rate is ascribed to the closely spaced dislocation arrays deposited at interfaces and the load transfer that is related to the layer thickness ratio of metal and ceramic layers. The measurable plasticity implies the plastically deformable ceramic layer in which the dislocation activity is facilitated by the interaction force among the deposited dislocations within interface and in turn is strongly related to the ceramic layer thickness.     

Plastic deformation of polyethylene and ethylene copolymers: Part I Homogeneous crystal slip and molecular mobility

The plastic behaviour of polyethylene and ethylene copolymers is studied under uniaxial tensile testing in parallel with the viscoelastic properties. Homogeneous plastic deformation is shown to take place at temperatures above the crystalline mechanical relaxation. The activation of homogeneous crystal slip is discussed in relation to the crystal lamella thickness and the molecular mobility of the crystalline chain stems. The thermally activated process of nucleation and propagation of screw dislocations that is proposed for the mechanism of the homogeneous crystal slip relies on the generation of 180° chain twists in the crystal stems of the sheared crystals. This kind of conformational chain defect is the basic link between the plastic and the viscoelastic properties of the materials. Homogeneous crystal slip can take place as long as the applied strain rate is consistent with the strain rate affordable by the screw dislocation propagation. The dependence on draw temperature of the crystal thickness in the fibre structure is ascribed to the stress-induced activation of 180° chain twists which allows an adjustment of the crystal thickness to the temperature of the experiment faster than an annealing treatment will. © 1998 Chapman & Hall     

Novel Ti-base nanostructure-dendrite composite with enhanced plasticity

Single-phase nanocrystalline materials undergo inhomogeneous plastic deformation under loading at room temperature, which results in a very limited plastic strain (smaller than 0-3%). The materials therefore display low ductility, leading to catastrophic failure, which severely restricts their application. Here, we present a new in situ-formed nanostructured matrix/ductile dendritic phase composite microstructure for Ti-base alloys, which exhibits up to 14.5% compressive plastic strain at room temperature. The new composite microstructure was synthesized on the basis of the appropriate choice of composition, and by using well-controlled solidification conditions. Deformation occurs partially through dislocation movement in dendrites, and partially through a shear-banding mechanism in the nanostructured matrix. The dendrites act as obstacles restricting the excessive deformation by isolating the highly localized shear bands in small, discrete interdendritic regions, and contribute to the plasticity. We suggest that microscale ductile crystalline phases might therefore be used to toughen nanostructured materials.