Microscopic description of plasticity in computer generated metallic nanophase samples: a comparison between Cu and Ni

Simulations are reported on the plastic behavior of two model f.c.c. metals, Ni and Cu, with different stacking fault energies, and average grain sizes in the range of 3–12 nm. A change in deformation mechanism is observed: at the smallest grain sizes all deformation is accommodated in the grain boundaries. At higher grain sizes intragrain deformation is observed. Analysis of the atomic configurations shows that intrinsic stacking faults are produced by motion of Shockley partial dislocations generated and absorbed in opposite grain boundaries. In Cu the stacking faults are observed at smaller grain sizes than in Ni (8 nm in Cu, 12 nm in Ni) which is attributed to the lower stacking fault energy. Shockley partial dislocations appear on slip systems that are not necessarily those favored by the Schmid factor. Atomic displacement analysis shows that deformation starts at triple points, with grain boundary sliding followed by the creation of intragrain partial dislocations.     

金属退火硬化现象及机理的研究进展

通常金属进行退火时都会发生软化现象,而对于一些特殊的金属或者合金,将出现退火硬化的反常现象。对纯金属、铜合金、镍钨合金、锌铝合金和铝合金等体系中退火硬化现象及机理进行了总结与分析。铝钪、铝镱和铝锆系合金中存在铸态直接退火硬化现象,而其它合金体系需要进行冷变形才会出现退火硬化现象。退火硬化的机理主要包括:晶界溶质偏析、晶界弛豫、第二相颗粒的晶界钉扎、位错源限制硬化、溶质偏析对孪晶边界迁移或位错滑动的钉扎效应、退火孪晶、第二相纳米粒子强化等。     

Phase field crystal model of solute drag

The atomic scale interaction of solute with a migrating grain boundary has been studied using a binary phase field crystal (PFC) model. This model bridges between atomistic and continuum simulation techniques as it operates on diffusive timescales but at atomistic length scales. For this study, a two-dimensional channel containing two grains separated by a flat grain boundary has been constructed that allows for a channel length on the order of one micrometer. A new formalism has been developed to allow for the application of an external driving pressure for the growth of one grain. These simulations account for solute/grain boundary interactions, resulting in a solute drag effect on the grain boundary motion. The PFC simulations show good agreement with classical solute drag theory, though deviations due to the atomic scale nature of the interface exist.     

Revisiting the role of peritectics in grain refinement of Al alloys

The grain refining effect of four peritectic-forming solutes (Ti, V, Zr and Nb) as well as three eutectic-forming solutes (Cu, Mg and Si) on pure Al was investigated. Significant grain refinement is observed by the addition of peritectic-forming solutes, whereas the addition of eutectic-forming solutes only slightly decreases the grain size. The mechanisms underlying the grain refinement of these alloys were then studied by a new analytical methodology for assessing grain refinement that incorporates the effects of both alloy chemistry and nucleant potency. It is found that the low degree of grain refinement by the addition of eutectic-forming solutes is mainly attributed to the segregating power of solutes, i.e. the constitutional undercooling contribution. However, peritectic-forming solutes do not only cause grain refinement by their segregation power but, more importantly, they introduce copious potent nuclei into the melt and promote significant grain refinement via heterogeneous nucleation.     

Quasicontinuum study of incipient plasticity under nanoscale contact in nanocrystalline aluminum

Atomistic simulations using the quasicontinuum method are performed to examine the mechanical behavior and underlying mechanisms of surface plasticity in nanocrystalline aluminum with a grain diameter of 7 nm deformed under wedge-like cylindrical contact. Two embedded-atom method potentials for Al, which mostly differ in their prediction of the generalized stacking and planar fault energies, and grain boundary (GB) energies, are used and characterized. The simulations are conducted on a randomly oriented microstructure with 〈1 1 0〉-tilt GBs. The contact pressure–displacement curves are found to display significant flow serration. We show that this effect is associated with highly localized shear deformation resulting from one of three possible mechanisms: (1) the emission of partial dislocations and twins emanating from the contact interface and GBs, along with their propagation and intersection through intragranular slip, (2) GB sliding and grain rotation and (3) stress-driven GB migration coupled to shear deformation. Marked differences in mechanical behavior are observed, however, as a function of the interatomic potential. We find that the propensity to localize the plastic deformation at GBs via interface sliding and coupled GB migration is greater in the Al material presenting the lowest predicted stacking fault energy and GB energy. This finding is qualitatively interpreted on the basis of impurity effects on plastic flow and GB-mediated deformation processes in Al.     

Deformation mechanism at impact test of Al-11% Si alloy processed by equal-channel angular pressing with rotary die

Al-11%Si(mass fraction)alloy was transformed into a ductile material by equal-channel angular pressing(ECAP)with a rotary die.Two mechanisms at impact test,slip deformation by dislocation motion and grain boundary sliding,were discussed.The ultrafine grains with modified grain boundaries and the high content of fine particles(<1μm)were necessary for attaining high absorbed energy.The results contradict the condition of slip deformation by dislocation motion and coincide with that of grain boundary sliding.Many fine zigzag lines like a mosaic were observed on the side surface of the tested specimens.These observed lines may show grain boundaries appeared by the sliding of grains.     

Plastic deformation of nanocrystalline aluminum at high temperatures and strain rate

The deformation of nanocrystalline aluminum was studied using molecular dynamics simulation at homologous temperatures up to 0.97. The microstructures and stress–strain response were examined in a polycrystalline and bicrystal configuration. The activation energies for dislocation-based deformation as well as grain boundary sliding and migration were quantified by fitting simulation data to temperature using an Arrhenius relation. The activation energy for the flow stress response suggests that deformation is largely accommodated by sliding and migration of grain boundaries. This is in agreement with simulated microstructures, indicating a negligible degree of dislocation interaction within each grain, and microstructural observations from high strain rate processes are also consistent with this result. A steady-state grain size is maintained in the recrystallized structure following yielding due to boundary migration and grain rotation mechanisms, rather than by diffusion-based dislocation climb.     

Thermomechanical characterization of β-stabilized Ti–45Al–7Nb–0.4W–0.15B alloy

The correlation between hot deformation parameters and the workability of β-stabilized Ti–45Al–7Nb–0.4W–0.15B (at. %) alloy was studied in the temperature range 1000–1200 °C and the strain rate range 0.001–1 s^?1. Deformation mechanisms were characterized by detailed analyses of the deformation behavior and microstructural observations. The results indicate that the deformation and recrystallization occurred preferentially in the grain boundary β phases because its good high temperature deformability enhances grain boundary sliding and migration, and thus improves the workability. Decomposition of the β phase to α₂ and γ phases partly accommodates the stress concentration and is thus beneficial in hot deformation. Appropriate deformation processing parameters were suggested based on the processing map, and were successfully applied in the quasi-isothermal canned forging of industrial-scale billets.     

Enhanced strain-rate sensitivity in fcc nanocrystals due to grain-boundary diffusion and sliding

Recent experiments on face-centered cubic (fcc) and hexagonal close packed (hcp) nanocrystalline metals reported an increase of more than 10-fold in strain-rate sensitivity in contrast to their conventional coarse-grained counterparts. To improve our understanding of this issue, we consider a mesoscopic continuum model of a two-dimensional polycrystal with deformation mechanisms including grain interior plasticity, grain-boundary diffusion and grain-boundary sliding. The model captures the transition from sliding- and diffusion-dominated creep in nanocrystals with relatively small grain sizes at low strain rates to plasticity-dominated flow in nanocrystals with larger grain sizes at higher strain rates. The strain-rate sensitivity obtained from our calculations matches well with the experimental data for nanocrystalline Cu. Based on this analysis, an analytical model incorporating the competition between grain interior plasticity and grain-boundary deformation mechanisms is proposed to provide an intuitive understanding of the transition in strain-rate sensitivity in nanostructured metals.     

Phase field modeling of grain boundary migration with solute drag

The grain boundary (GB) motion in the presence of GB segregation is investigated by means of phase field simulations. It is found that the solute concentration at the moving GB may increase with increasing velocity and becomes larger than the equilibrium value, which is unexpected according to the solute drag theory proposed by Cahn, but has been observed in some experiments. A non-linear relation between the driving force (curvature) and the GB velocity is found in two cases: (1) the GB motion undergoes a transition from the low-velocity extreme to the high-velocity extreme; (2) the GB migrates slowly in a strongly segregating system. The first case is consistent with the solute drag theory of Cahn. As for the second case, which is unexpected according to solute drag theory, the non-linear relation between the GB velocity and curvature comes from two sources: the non-linear relation of the solute drag force with GB velocity, and the variation in GB energy with curvature. It is also found that, when the diffusivity is spatially inhomogeneous, the kinetics of GB motion is different from that with a constant diffusivity.     

Grain boundary segregation,chemical ordering and stability of nanocrystalline alloys: Atomistic computer simulations in the Ni–W system

Atomistic computer simulations are used to investigate the equilibrium solute distribution and alloying energetics in nanocrystalline Ni–W. Composition and grain size-dependent trends in grain boundary segregation and chemical ordering behavior are evaluated and we find the equilibrium state to be significantly influenced by the nanostructure. The energetics of alloying are assessed through computation of the segregation, formation, and grain boundary energy, and these quantities are linked to previous thermodynamic models of nanostructure stability. With comparison to experiments, we conclude that nanocrystalline Ni–W alloys are synthesized in a metastable state. These findings have important consequences for theories of nanostructure control in general and particularly for the thermal stability of nanocrystalline Ni–W.     

Effect of minor Sc and Zr on superplasticity of Al-Mg-Mn alloys

The effect of Sc and Zr on the superplastic properties of Al-Mg-Mn alloy sheets was investigated by control experiment. The superplastic properties and the mechanism of superplastic deformation of the two alloys were studied by means of optical microscope, scanning electronic microscope and transmission electron microscope. The elongation to failure of Al-Mg-Mn-Sc-Zr alloy is larger than that of Al-Mg-Mn alloy at the same temperature and initial strain rate. The variation of strain rate sensitivity index is similar to that of elongation to failure. In addition, Al-Mg-Mn-Sc-Zr alloy exhibits higher strain rate superplastic property. The activation energies of the two alloys that are calculated by constitutive equation and linear regression method approach the energy of grain boundary diffusion. The addition of Sc and Zr decreases activation energy and improves the superplastic property of Al-Mg-Mn alloy. The addition of Sc and Zr refines the grain structure greatly. The main mechanism of superplastic deformation of the two alloys is grain boundary sliding accommodated by grain boundary diffusion. The fine grain structure and high density of grain boundary, benefit grain boundary sliding, and dynamic recrystallization brings new fine grain and high angle grain boundary which benefit grain boundary sliding too. Grain boundary diffusion, dislocation motion and dynamic recrystallization harmonize the grain boundary sliding during deformation.     

Grain boundary migration and grain rotation studied by molecular dynamics

We report on molecular dynamics simulations of an isolated cylindrical grain in copper shrinking under capillary forces. At low temperatures, the coupling between grain boundary (GB) migration and grain translation induces rotation of the grain towards higher or lower misorientation angles, depending on the initial misorientation. The dynamics of the GB motion and grain rotation are studied as functions of the initial misorientation angle and temperature. The effects of imposed constraints blocking the grain rotation or exerting a cyclic torque are examined. The simulation results verify several predictions of the model proposed by Cahn and Taylor [Acta Mater 52, 4887 (2004)]. They also indicate that the GB motion is never perfectly coupled but instead involves at least some amount of sliding. This, in turn, requires continual changes (annihilation or nucleation) in the GB dislocation content. Dislocation mechanisms that can be responsible for the motion of curved GBs and dislocation annihilation in them are proposed.     

TA15钛合金高温变形过程的介观模拟计算

以多晶体位错滑移及塑性流动机制为基础,探究了TA15钛合金在高温变形过程中介观层次上形变不均匀性和力学响应。基于率相关晶体塑性理论,建立了描述体心立方结构金属力学行为的本构模型,同时考虑了主滑移系和次滑移系的运动;确定了合理的材料本构参数,高温压缩实验与模拟得到的真应力-应变曲线基本一致。通过对TA15钛合金高温变形模拟结果进行分析,包括应力和应变分布、滑移系开动情况和晶界面积变化,得出:(1)由于晶粒几何及取向的随机性造成应力和应变分布非均匀性;(2)晶粒间相互作用的复杂性会导致各个滑移系开动的差异性;(3)形变程度越大,晶粒密度越大,晶界面积变化率越大。模拟结果为相变等显微组织演变及多尺度同步耦合提供了参考。     

Inter- and intragranular plasticity mechanisms in ultrafine-grained Al thin films: An in situ TEM study

The nature of the elementary deformation mechanisms in small-grained metals has been the subject of numerous recent studies. In the submicron range, mechanisms other than intragranular dislocation mechanisms, such as grain boundary (GB)-based mechanisms, are active and can explain the deviations from the Hall–Petch law. Here, we report observations performed during in situ transmission electron microscopy (TEM) tensile tests on initially dislocation-free Al thin films with a mean grain size around 250 nm prepared by microfabrication techniques. Intergranular plasticity is activated at the onset of plasticity. It consists of the motion of dislocations in the GB plane irrespective of the GB character. Surface imperfections, such as GB grooves, are supposed to trigger intergranular plasticity. At larger deformations, the motion of the intergranular dislocations leads to GB sliding and eventually cavitation. In the meantime, GB stress-assisted migration and dislocation emission inside the grain from GB sources have also been observed. The observation of these different mechanisms during the deformation provides an important insight into the understanding of the mechanical properties of metallic thin films.     

Atomistic simulation of hillock growth

This paper explores the mechanisms of hillock and whisker growth in stressed polycrystalline films by molecular dynamics simulations. The initial geometry consists of three grains with a triple line aligned perpendicular to a free surface, plus a fourth pyramidal-shaped grain implanted between the triple line and the surface. This simulated grain geometry corresponds to that observed in experiments during hillock and whisker growth, with the fourth grain serving as a seed for hillock growth. The simulations, performed under an applied in-plane biaxial compression, reveal an upward motion and growth of the seed grain. The growth occurs by stress-driven grain boundary diffusion from below the seed grain onto some of its internal faces. Accretion of atoms to those faces pushes the seed grain upwards and sideways. The different diffusion and accretion rates at different boundaries also give rise to internal stresses, which can be partially accommodated by grain boundary motion coupled to shear deformation. The hillock growth is countered by surface diffusion, which can slow the growth or even suppress it completely. Other mechanisms involved in hillock growth are also discussed.     

Effects of solute Mg on grain boundary and dislocation dynamics during nanoindentation of Al–Mg thin films

Using in situ nanoindentation in a transmission electron microscope (TEM) the indentation-induced plasticity in ultrafine-grained Al and Al–Mg thin films has been studied, together with conventional quantitative ex situ nanoindentations. Extensive grain boundary motion has been observed in pure Al, whereas Mg solutes effectively pin high-angle grain boundaries in the Al–Mg alloy films. The proposed mechanism for this pinning is a change in the atomic structure of the boundaries, possibly aided by solute drag on extrinsic grain boundary dislocations. The mobility of low-angle boundaries is not affected by the presence of Mg. Based on the direct observations of incipient plasticity in Al and Al–Mg, it was concluded that solute drag accounts for the absence of discrete strain bursts in indentation of Al–Mg.     

Tensile deformation of an ultrafine-grained aluminium alloy: Micro shear banding and grain boundary sliding

This work investigates the relationship between the strain rate and the ductility and the underlying deformation mechanisms in an ultrafine-grained Al6082 alloy. At room temperature the uniform elongation of the material exhibits a marked increase with decreasing strain rate. This effect is related to the activation of micro shear banding, which is controlled by grain boundary sliding. The contribution of these mechanisms to uniform elongation is estimated. It is proposed that the grain boundary sliding suppresses the transformation of micro shear bands into macro shear bands. The activity of other deformation mechanisms during plastic deformation of the ultrafine-grained material is also discussed.     

Aluminum Σ3 grain boundary sliding enhanced by vacancy diffusion

Grain boundary sliding is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of vacancies in the grain boundary vicinity on the sliding of Al bi-crystals at 750 K. The threshold stress for grain boundary sliding was computed for a variety of grain boundaries with different structures and energies. These structures included one symmetrical tilt grain boundary and five asymmetrical tilt grain boundaries. Without vacancies, low energy Σ3 grain boundaries exhibited significantly less sliding than other high energy grain boundaries. The addition of vacancies to Σ3 grain boundaries decreased the threshold stress for grain boundary sliding by increasing the grain boundary diffusivity. A higher concentration of vacancies enhanced this effect. The influence of vacancies on grain boundary diffusivity and grain boundary sliding was negligible for high energy grain boundaries, due to the already high atom mobility in these boundaries.     

一个新的动态再结晶过程的分析模型

应用不可逆热力学方法，本文建立了一个新的动态再结晶过程的分析模型。在这个模型中根据动态再结晶重复形核有限长大机理，分别考虑原始晶粒和再结晶晶粒分布的变化。由于模型不考虑再结晶晶粒形成细节，所以本模型也适用于由亚晶直接转变成新晶粒的情况。这种情况中只有晶界能增加并无明显晶粒长大。由此模型导出了再结晶体积分数及有关晶粒尺寸的演化方程。将这些演化方程插人到作者给出的考虑不同变形机制的热塑性本构关系中，便得到了有动态再结晶过程伴随的热塑性本构关系。这一本构关系不仅适用于一般的热加工变形，还适用于动态再结晶引起晶粒细化所造成的变形机理改变的热变形过程。最后给出一些算例并对一些有关问题进行了讨论。