孪生诱发塑性对V-5Cr-5Ti合金应变硬化行为的影响

钒合金(V-Cr-Ti)作为潜在重要的聚变反应堆用结构材料, 近年来受到广泛的关注. 为了研究 V-5Cr-5Ti 合金不同应变率压缩下的应变硬化行为, 特别是孪生对塑性变形的影响, 以位错密度和孪晶演化为基础, 建立了该合金的应变硬化模型. 模型中考虑了孪晶中的位错滑移对材料塑性应变的贡献. 模拟结果表明, 由于孪生诱发塑性, 从而使动态压缩时的位错密度小于准静态加载时的, 这使得 V-5Cr-5Ti 合金在动态压缩时的应变硬化率比准静态加载时的小. 当孪晶形成后, 位错滑移引起的塑性应变率随应变增大而增大, 并逐渐接近加载应变率, 而孪生引起的塑性应变率则随应变增大而减小.      

SiCp/Al复合材料增强颗粒尺寸效应的细观分析

运用Voronoi方法建立了反映金属基颗粒增强复合材料(MMCp)微结构的多晶集合体代表性单元(RVE);采用Taylor关系推导了包含颗粒结构尺寸和体积分数参数的位错滑移硬化函数;建立了由300个平均粒度约为20μm的晶粒组成的多晶集合体代表性单元,并对MMCp3.5-5、MMCp3.5-10、MMCp10-5、MMCp10-10四种具有不同粒径和体积分数的铝基SiC颗粒增强复合材料在宏观均匀变形条件下的应力应变响应进行了数值模拟。计算结果表明:复合材料的应力应变模拟曲线与试验曲线吻合得较好,说明所推导的模型和硬化模式能够合理地描述颗粒增强尺度效应的变化趋势;多晶体模型也能够合理地表现复合材料内部应力应变在空间分布上的细观不均匀性。数值模拟结果反映了颗粒增强区承载着较大的载荷份额,而非颗粒存在区(基体)则承受着高达18%的应变,在两个区域的交界处出现了高达310MPa的应力集中,与已有文献试验观测的结果比较吻合。     

铝单晶体潜在硬化的指向性行为研究

应用高纯铝单晶体，采用偏离弹性线法定义其在潜在滑移系统在屈服应力，研究了潜在滑移系在正负两个滑移方向上的屈服及应变硬化行为。结果表明，潜在滑移系负行为的差异要远远小于动滑移系的Ｂａｕｓｃｈｉｎｇｅｒ效应，其正负方向的屈服应力一般均等于或略小于预应变时的最大分切应力，大大高于动滑移系的负向屈服应力，潜在滑移系和原始滑移系的相对取向及预应变的大小对单晶体潜在移系在潜在硬化的影响不大，但对其起始过渡区应     

高应变率下TiC_P/Ti复合材料的动态拉伸性能

采用旋转盘式杆-杆型动态拉伸试验机对TiCP颗粒增强钛基复合材料及其基体钛合金的动态拉伸性能进行了研究.同时为了比较,在MTS810试验机上做了两种材料的准静态试验.试验结果表明,复合材料及基体材料屈服后至材料的迅速失效,几乎没有应变硬化效应;复合材料的抗拉强度和屈服强度较基体明显提高,但延性明显下降;钛合金基体和复合材料均有明显的应变率强化效应,但复合材料的应变率强化效应明显高于基体;建立了复合材料率相关的本构关系.最后从位错等微观角度分析了复合材料的强化机理、复合材料的应变率敏感性以及复合材料应变率敏感性高于基体的原因.     

SiC和石墨混杂增强铜基复合材料的高温摩擦磨损特性研究

采用MMU-5G型端面摩擦磨损试验机研究了SiC和石墨颗粒混杂增强铜基复合材料在250～400 ℃与GCr15钢对摩时的高温摩擦磨损特性,并与SiC/Cu复合材料进行对比分析.结果表明:加入石墨颗粒可以降低复合材料和偶件GCr15钢的磨损率,获得低而稳定的摩擦系数;同时,有效防止高温条件下严重粘着转移现象的发生,使得在400 ℃下混杂复合材料仍具有较低的磨损率.这是由于在混杂增强铜基复合材料的高温磨损表面上通过"磨屑机械混合→热压"的机制形成了连续的富石墨机械混合层,而对磨损表面起到良好的固体润滑作用,使得SiC和石墨混杂增强铜基复合材料具有良好的高温摩擦磨损特性.     

中应变率拉伸载荷下DZ2车轴钢的动态力学响应与微观组织演变

高速列车运行过程中车轴可能遭受不同程度的冲击载荷作用，导致车轴的结构损伤与破坏，从而影响列车运营安全和服役寿命.因此，明晰冲击载荷下车轴材料的力学响应和变形损伤行为，对高速动车组车轴的运维与设计具有重要意义.论文研究了DZ2车轴钢在中应变率(0.1～100 s^-1)拉伸条件下的力学性能和微观结构演变，揭示了DZ2车轴钢的变形与失效机理，构建了可准确描述DZ2车轴钢力学响应行为的Zerilli-Armstrong模型.结果表明，位错滑移和韧性断裂是DZ2车轴钢塑性变形和失效的主要机制，但由于位错运动状态的改变，其强度的应变率依赖性在不同应变率范围内存在较大差异.当应变率低于10 s^-1时，DZ2车轴钢内的位错密度低，位错运动阻碍作用小，其强度不会随应变率增加而显著变化，具有低的应变率敏感性；而在应变率超过10 s^-1后，DZ2车轴钢内的位错密度大幅度增加，位错运动速率加快，位错短程作用增强，从而增大了材料的变形抗力，材料的强度随应变率增加而增大，表现出显著的应变率强化效应，应变率敏感性也明显提高.与实验数据相一致，Zeri...     

两种不同形状压头与单晶铜基体间接触力和摩擦力的纳观分析

在考虑单晶铜基体弹塑性形变和晶体各向异性情况下,基于原子尺度,采用混合势函数(EAM和Morse)和Verlet算法动态模拟了半球形和圆锥形两种不同形状压头与单晶铜基体的黏着接触和滑动摩擦过程,分析了接触力和摩擦力对单晶铜基体内失效原子变化情况.研究表明:当压头下压位移为0.9 nm时,由于半球形压头比圆锥形压头底部表面积大,导致半球形压头与基体之间的引力更大而更易产生黏着接触现象.在下压接触过程中,与半球形压头相接触的基体内出现位错原子长大成位错环,而与圆锥形压头相接触的基体未出现此位错环现象,但位错原子数均随压深的增加而增多;在滑动过程中,因半球形压头对基体的摩擦力和法向力比圆锥形压头对基体的摩擦力和法向力大,使得半球形压头比圆锥形压头正前方堆积的位错原子数多,但均随滑动距离的增加而增多.     

bcc-Fe刃型位错偶极子的动力学过程模拟

用分子动力学方法模拟了bcc单晶铁中滑移面为(011)晶面、柏氏矢量b-=±[100]的刃型位错偶极子吸引至湮灭的动力学过程.模拟结果显示:沿[100]晶向滑移的正刃型位错穿越Peierls势垒的方式为滑移面上方芯原子沿[111]晶向滑移,滑移面下方芯原子沿[111]晶向滑移;芯原子滑移方向分别偏转为[111]和[111]晶向,同时芯能量以格波形式释放.统计了位错运动速度和Peierls势垒随时间或两位错距离的变化.     

基于微结构动态演化机制的单晶镍基高温合金晶体塑性本构及其有限元模拟

因其优异的高温力学性能,镍基单晶高温合金在航空航天和能源等领域得到了广泛的应用.镍基单晶高温合金优异的高温性能来源于其特有的两相微结构.基于代表体胞模型及分块均匀化方法,以位错密度为主要内变量,发展了一个包含两相微结构和位错演化信息的单晶镍基高温合金塑性行为的本构模型.该本构模型充分考虑了镍基单晶合金中位错在基体相和沉淀增强相中的多种演化机制,例如,基体位错八面体滑移、立方滑移、位错攀移、交滑移、位错弓出、位错切过沉淀增强相以及位错Kear-Wilsdolf(K-W)锁形成与解锁等.在商用有限元软件ABAQUS的框架下,编制了UMAT用户材料子程序.利用该用户子程序,对单晶和多晶镍基高温合金在不同温度、不同加载方向下的单调塑性、循环塑性、蠕变等典型行为进行了计算模拟.结果表明:该晶体塑性本构模型能"统一地"刻画镍基高温合金在不同温度、不同方向下的多种变形行为,并与实验结果具有良好的一致性.     

45号钢的动态力学性能研究

对45号钢在不同环境温度(25 300℃)和不同应变率(10-4～103s-1)的 关系进行了研究。修正了Johnson Cook模型中的应变率强化系数C,确定了45号钢的本构关系。金相观察表明,与应变率强化相比,较高的环境温度使硬化速率降低,且占主要因素,其结果使材料的强度降低。透射电镜分析表明,高应变率在使位错运动的能量提高的同时,增加了位错在晶界处的阻力,而较高的环境温度则为位错提供了较多的滑移面和滑移方向,其结果是使材料更容易压缩。     

A phase-field theory of dislocation dynamics, strain hardening and hysteresis in ductile single crystals

A phase-field theory of dislocation dynamics, strain hardening and hysteresis in ductile single crystals is developed. The theory accounts for: an arbitrary number and arrangement of dislocation lines over a slip plane; the long-range elastic interactions between dislocation lines; the core structure of the dislocations resulting from a piecewise quadratic Peierls potential; the interaction between the dislocations and an applied resolved shear stress field; and the irreversible interactions with short-range obstacles and lattice friction, resulting in hardening, path dependency and hysteresis. A chief advantage of the present theory is that it is analytically tractable, in the sense that the complexity of the calculations may be reduced, with the aid of closed form analytical solutions, to the determination of the value of the phase field at point-obstacle sites. In particular, no numerical grid is required in calculations. The phase-field representation enables complex geometrical and topological transitions in the dislocation ensemble, including dislocation loop nucleation, bow-out, pinching, and the formation of Orowan loops. The theory also permits the consideration of obstacles of varying strengths and dislocation line-energy anisotropy. The theory predicts a range of behaviors which are in qualitative agreement with observation, including: hardening and dislocation multiplication in single slip under monotonic loading; the Bauschinger effect under reverse loading; the fading memory effect, whereby reverse yielding gradually eliminates the influence of previous loading; the evolution of the dislocation density under cycling loading, leading to characteristic ‘butterfly’ curves; and others.     

Plastic size effect analysis of lamellar composites using a discrete dislocation plasticity approach

Plastic size effect analysis of lamellar composites consisting of elastic and elastic-plastic layers is performed using a discrete dislocation plasticity approach, which is based on applying periodic homogenization to the superposition method for discrete dislocation plasticity. In this approach, the decomposition of displacements into macro and perturbed components circumvents the calculation of superposing displacement fields induced by dislocations in an infinitely homogeneous medium, resulting in two periodic boundary value problems specialized for the analysis of representative volume elements. The present approach is verified by analyzing a model lamellar composite that includes edge dislocations fixed at interfaces. The plastic size effects due to dislocation pile-ups at interfaces are also analyzed. The analysis shows that, strain hardening in elastic-plastic layers arises depending on two factors, namely the thickness and stiffness of elastic layers; and the gap between slip planes in adjacent elastic-plastic layers. In the case where the thickness of elastic layers is several dozen nm, strain hardening in elastic-plastic layers is restrained as the gap of the slip planes decreases. This particular effect is attributed to the long range stress due to pile-ups in adjacent elastic-plastic layers.     

Discrete dislocation plasticity analysis of the grain size dependence of the flow strength of polycrystals

The grain size dependence of the flow strength of polycrystals is analyzed using plane strain, discrete dislocation plasticity. Dislocations are modeled as line singularities in a linear elastic solid and plasticity occurs through the collective motion of large numbers of dislocations. Constitutive rules are used to model lattice resistance to dislocation motion, as well as dislocation nucleation, dislocation annihilation and the interaction with obstacles. The materials analyzed consist of micron scale grains having either one or three slip systems and two types of grain arrangements: either a checker-board pattern or randomly dispersed with a specified volume fraction. Calculations are carried out for materials with either a high density of dislocation sources or a low density of dislocation sources. In all cases, the grain boundaries are taken to be impenetrable to dislocations. A Hall–Petch type relation is predicted with Hall–Petch exponents ranging from ≈0.3 to ≈1.6 depending on the number of slip systems, the grain arrangement, the dislocation source density and the range of grain sizes to which a Hall–Petch expression is fit. The grain size dependence of the flow strength is obtained even when no slip incompatibility exists between grains suggesting that slip blocking/transmission governs the Hall–Petch effect in the simulations.     

Three dimensional structures of the geometrically necessary dislocations in matrix-inclusion systems under uniaxial tensile loading

Plastic slip deformation in matrix-inclusion systems, in which a cuboidal or spherical shaped inclusion is embedded in a softer matrix, are numerically analyzed by a finite element technique. Edge and screw components of the geometrically necessary dislocations on slip systems are evaluated for each finite element from the spatial gradient of plastic shear strain. The character of the dislocation segments in each element is deduced from the data for edge and screw components and the directions of dislocation segments are determined. The aggregate of the dislocation segments in the whole specimen shows typical structures of dislocations, such as the Orowan loops around the inclusion and tilt boundaries that develop perpendicular to the primary slip plane. Stress state and shape of dislocations in deformable inclusions are discussed.     

An experimental investigation on cyclic plastic deformation and substructures of polycrystalline copper

Cyclic deformation under proportional and nonproportional loading of a textured copper was experimentally studied, and the results were compared with those of texture-free copper with the same grain size. The texture had a great influence on the equivalent cyclic stress–strain (CSS) curves under proportional loading but insignificant influence on the CSS curves under nonproportional loading. By comparing the slip patterns on the specimen surface and dislocation substructures under proportional and nonproportional loading, the mechanism of nonproportional hardening was discussed. The slip multiplicity inherited from originally multiple-slip oriented grains plays a minor role. Nonproportional hardening is the result of enhanced activated slip systems and more uniform activation of slip systems due to the rotation of maximum shear stress under nonproportional loading. At high strain amplitudes, cells were the primary substructures for both proportional and nonproportional loading but the diameters of the cells under nonproportional loading were smaller for similar strain magnitude. A linear relationship existed between the saturation equivalent stress magnitude and the reciprocal of the diameter of the dislocation cells. Such a relationship was independent of the loading modes and texture. The saturation stress magnitude was related to the bowing stress of screw dislocations in the interior area of dislocation cells. The mechanical response was practically recoverable either when the loading magnitude was changed from a higher value to a lower value or when the loading was changed from a nonproportional loading path to a proportional loading path. However, the dislocation substructures cannot be completely recovered.     

A mesoscale investigation of strain rate effect on dynamic deformation of single-crystal copper

A combined finite element (FE) simulation and discrete dislocation dynamics (DD) approach has been developed in this paper to investigate the dynamic deformation of single-crystal copper at mesoscale. The DD code yields the plastic strain based on the slip of dislocations and serves as a substitute for the 3D constitutive form used in the usual FE computation, which is implemented into ABAQUS/Standard with a user-defined material subroutine. On the other hand, the FE code computes the displacement and stress field during the dynamic deformation. The loading rate effects on the yield stress and the deformation patterning of single-crystal copper are investigated. With the increasing of strain rate, the yield stress of single-crystal copper increases rapidly. A critical strain rate exists in each single-crystal copper block for the given size and dislocation sources, below which the yield stress is relatively insensitive to the strain rate. The dislocation patterning changes from non-uniform to uniform under high-strain-rate. The shear stresses in the bands are higher than that in the neighboring regions, which are formed shear bands in the crystal. The band width increases with the strain rate, which often take places where the damage occurs.     

Strain hardening in 2D discrete dislocation dynamics simulations: A new ‘2.5D’ algorithm

The two-dimensional discrete dislocation dynamics (2D DD) method, consisting of parallel straight edge dislocations gliding on independent slip systems in a plane strain model of a crystal, is often used to study complicated boundary value problems in crystal plasticity. However, the absence of truly three dimensional mechanisms such as junction formation means that forest hardening cannot be modeled, unless additional so-called ‘2.5D’ constitutive rules are prescribed for short-range dislocation interactions. Here, results from three dimensional dislocation dynamics (3D DD) simulations in an FCC material are used to define new constitutive rules for short-range interactions and junction formation between dislocations on intersecting slip systems in 2D. The mutual strengthening effect of junctions on preexisting obstacles, such as precipitates or grain boundaries, is also accounted for in the model. The new ‘2.5D’ DD model, with no arbitrary adjustable parameters beyond those obtained from lower scale simulation methods, is shown to predict athermal hardening rates, differences in flow behavior for single and multiple slip, and latent hardening ratios. All these phenomena are well-established in the plasticity of crystals and quantitative results predicted by the model are in good agreement with experimental observations.     

Nucleation of partial dislocations at a crack and its implication on deformation mechanisms of nanostructured metals

Nucleation of partial dislocations at a crack is analyzed based a multiscale model that incorporates atomic information into continuum-mechanics approach. The crack and the slip plane as the extension of the crack are modeled as a surface of displacement discontinuities embedded in an elastic medium. The atomic potential between the adjacent atomic layers along the slip plane is assumed to be the generalized stacking fault energy, which is obtained based on atomic calculations. The relative displacements along the slip plane, corresponding to the configurations of partial dislocations and stacking faults, are solved through the variational boundary integral method. The energetics of partial dislocation nucleation at the crack in FCC metals Al and Cu are comparatively studied for their distinctive difference in the intrinsic stacking fault energy. Compared with nucleation of perfect dislocations in previous studies, several new features have emerged. Among them, the critical stress and activation energy for nucleation of partial dislocations are markedly lowered. Depending on the value of stacking fault energy and crack configuration, the saddle-point configurations of partial dislocations can be vastly different in terms of the nucleation sequence and the size of the stacking fault. These findings have significant implication for identifying the fundamental dislocation and grain-boundary-mediated deformation mechanisms, which may account for the limiting strength and the high strain rate sensitivity of nanostructured metals.     

A crystal plasticity theory for latent hardening by glide twinning through dislocation transmutation and twin accommodation effects

Imagine a residual glide twin interface advancing in a grain under the action of a monotonic stress. Close to the grain boundary, the shape change caused by the twin is partly accommodated by kinks and partly by slip emissions in the parent; the process is known as accommodation effects. When reached by the twin interface, slip dislocations in the parent undergo twinning shear. The twinning shear extracts from the parent dislocation a twinning disconnection, and thereby releases a transmuted dislocation in the twin. Transmutation populates the twin with dislocations of diverse modes. If the twin deforms by double twinning, double-transmutation occurs even if the twin retwins by the same mode or detwins by a stress reversal. If the twin deforms only by slip, transmutation is single. Whether single or double, dislocation transmutation is irreversible. The multiplicity of dislocation modes increases upon strain, since the twin finds more dislocations to transmute upon further slip of the parent and further growth of the twin. Thus, the process induces an increasing latent hardening rate in the twin. Under profuse twinning conditions, typical of double-lattice structures, this rate-increasing latent hardening combined with crystal rotation to hard orientations by twinning is consistent with a regime of increasing hardening rate, known as Regime II or Regime B. In this paper, we formulate governing equation of the above transmutation and accommodation effects in a crystal plasticity framework. We use the dislocation density based model originally proposed by Beyerlein and Tomé (2008) to derive the effect of latent hardening in a transmuting twin. The theory is expected to contribute to surmounting the difficulty that current models have to simultaneously predict under profuse twinning, the stress-strain curves, intermediate deformation textures, and intermediate twin volume fractions.