Deformation twinning in nanocrystalline metals

Deformation twins have been oberved in nanocrystalline Al processed by cryogenic ball-milling and in nanocrystalline Cu processed by high-pressure torsion at a very low strain rate. They were formed by partial dislocations emitted from grain boundaries. This paper first reviews experimental evidences and atomistic simulation results on deformation twinning and partial dislocation emissions from grain boundaries and then discusses recent analytical models on the nucleation and growth of deformation twins. These models are compared with experimental results to establish their validity and limitations. This paper was presented at the International Symposium of Manufacturing, Properties, and Applications of Nanocrystalline Materials sponsored by the ASM International Nanotechnology Task Force and TMS Powder Materials Committee on October 18–20, 2004 in Columbus, OH.     

Deformation twinning in nanocrystalline Al by molecular-dynamics simulation

We use a recently developed, massively parallel molecular-dynamics code for the simulation of polycrystal plasticity to elucidate the intricate interplay between dislocation and GB processes during room-temperature plastic deformation of model nanocrystalline-Al microstructures. Our simulations reveal that under relatively high stresses (of 2.5 GPa) and large plastic strains (of ˜12%), extensive deformation twinning takes place, in addition to deformation by the conventional dislocation-slip mechanism. Both heterogeneous and homogeneous nucleation of deformation twins is observed. The heterogeneous mechanism involves the successive emission of Shockley partials from the grain boundaries onto neighboring slip planes. By contrast, the homogeneous process takes place in the grain interiors, by a nucleation mechanism involving the dynamical overlap of the stacking faults of intrinsically and/or extrinsically dissociated dislocations. Our simulations also reveal the mechanism for the formation of a new grain, via an intricate interplay between deformation twinning and dislocation nucleation from the grain boundaries during the deformation. The propensity for deformation twinning observed in our simulations is surprising, given that the process has never been observed in coarse-grained Al and that the well-known pole mechanism cannot operated for such a small grain size. It therefore appears that the basic models for deformation twinning should be extended with particular emphasis on the role of grain-boundary sources in nanocrystalline materials.     

Deformation twinning in Ni2MnGa

Deformation twinning is investigated in the martensitic phase of a Ni_46.75Mn₃₄Ga_19.25 (at.%) alloy. X-ray and electron diffraction are used to establish the crystallography of the non-modulated tetragonal martensite, and transmission electron microscopy is employed to deduce the twinning parameters. It is convenient to define the twinning parameters with respect to a “monoclinic” unit cell, designated 2M: then K₁, η₁, K₂, and η₂ are (0 0 1), [1 0 0], (1 0 0), and [0 0 1] respectively. The Burgers vector of the active twinning disconnections is close to 1/6[1 0 0] and the disconnections are associated with steps of height d₍₀₀₂₎. These defects are expected to be highly mobile since their motion does not require atomic shuffling. It is shown that periodic arrangements of two layer twins produce modulated crystal structures, such as 14M.     

Deformation twinning in TiAl: Effects of defect clustering

Possible roles of point defect clustering in the formation of deformation twins in γ-TiAl are critically assessed by reviewing the available models of dislocation-assisted twin nucleation and experimental data on deformation twinning in Ti-56 at.% Al single crystals and two-phase Ti-47 at.% Al alloys. According to the pole mechanism for twinning in the Ll₀ structure, a reasonable combination of the stress concentration (n≈r27) and the vacancy supersaturation (c/c₀≈13) is needed to overcome the critical stages of twin formation. The so-called radiation-induced ductility reported in Ti-47 at.% Al alloys is attributed to the effective formation of twin embryos in the presence of interstitial-type Frank loops and the subsequent nucleation and growth of twins during plastic deformation.     

Deformation twinning at aluminum crack tips

Recent experimental evidence has shown that even fcc materials that are not normally associated with deformation twinning, such as aluminum, will twin given a sufficiently high stress concentration such as at a crack tip. In this paper we present a computational study of the atomic structures that form at the tips of atomically sharp cracks in aluminum single crystals under loading. The simulations were carried out using the quasicontinuum method—a mixed continuum and atomistic approach. A variety of loading modes and orientations were examined. It was found that for certain combinations of loading mode and orientation, deformation twinning does occur at aluminum crack tips in agreement with experimental observation. For other configurations, either dislocation emission or in one case the formation of an intrinsic–extrinsic fault pair was observed. It was also found that the response at the crack tip can depend on the crack-tip morphology in addition to the applied loading and crystallographic orientation.     

On the twinning occurrence in bulk semiconductor crystal growth

Based on known theories of twinning in semiconductor crystal growth, a new model is proposed to study the occurrence of twins during the solidification of photovoltaic multicrystalline silicon ingots. It is expected that twins will appear on facets existing at the grain boundary–solid–liquid triple line. Necessary conditions for the existence of facets are derived and it is shown that twinning remains a function of the probability of nucleation of twinned nuclei. It is demonstrated that this probability is in qualitative agreement with the experimental observation for cases where the grain orientation is such that an angle of 132° occurs between a facet and a grain boundary. However, full validation of the model requires accurate values of interfacial energies at the melting point, which are currently lacking.     

Deformation kinetics of nanocrystalline nickel

The work hardening and the strain rate sensitivity of flow stress were studied in the temperature range from 298 to 473 K for nanocrystalline (NC) nickel with average grain sizes of about 25 and 80 nm produced by pulsed electrodeposition. The rate of work hardening, the maximum flow stress and the strain rate sensitivity of flow stress increase with decreasing grain size. The data are compared to published data for NC Ni and found to be consistent. The common analysis of strain rate sensitivity in terms of thermal activation is critically discussed. It is proposed that the activation analysis gives information about thermally activated processes at grain boundaries which may be related with recovery of dislocations.     

Deformation behaviour of nanocrystalline magnesium

The deformation behaviour of nanocrystalline magnesium was studied. Nanocrystalline magnesium powder was prepared by high energy ball milling in a dry inert gas atmosphere and vacuum cold pressed to form fully dense cylindrical specimens. Compression tests were carded out at room temperature. The samples exhibited remarkably high values of ductility, and a significant improvement in yield strength in sintered specimens. Large yield drops were observed on the stress-strain curve of sintered specimens. The strain rate sensitivity of the flow stress was also measured and compared to the values for commercially available pure magnesium bars. The values of activation volume as well as the stress exponent were found to be significantly lower in the nanocrystalline samples. It is speculated that grain boundary sliding controls the deformation mechanism in the nanocrystalline magnesium samples.     

Entropic effect on creep in nanocrystalline metals

We report a significant entropic effect on creep of nanocrystalline metal using molecular dynamics. Our simulations reveal that the activation entropy may contribute a multiplicative factor of many orders of magnitude to the steady-state creep rate. The relationship between activation entropy and enthalpy obeys an empirical Meyer–Neldel compensation rule. The activation volume is found to decrease with increasing temperature for dislocation nucleation creep, which agrees well with experimental results. The study opens up an avenue for quantitatively discussing the entropic effects on various thermally activated deformations in nanocrystals.     

Determining the stress required for deformation twinning in nanocrystalline and ultrafine-grained copper

Deformation twinning in nanocrystalline and ultrafine-grained materials has attracted much attention in recent years due to the ability of a high density of twin boundaries to dramatically improve mechanical properties such as yield strength and ductility. Various processing conditions such as ball milling, cryomilling, electrodeposition, and equi-channel angular extrusion have been used to form deformation twins in metals. Most techniques for estimating the shear stress needed to form deformation twins are based indirectly on the processing conditions. Here, a new method to directly measure the shear stress needed to form twin boundaries through in-situ transmission electron microscopy nanocompression testing will be described.     

Predicting twinning stress in fcc metals: Linking twin-energy pathways to twin nucleation

Deformation twinning is observed in numerous engineering and naturally occurring materials. However, a fundamental law for critical twinning stress has not yet emerged. We resolve this long-standing issue by integrating twin-energy pathways obtained via ab initio density functional theory with heterogeneous, dislocation-based twin nucleation models. Through a hierarchical theory, we establish an analytical expression that quantitatively predicts the critical twinning stress in face-centered cubic metals without any empiricism at any length scale. Our theory predicts a monotonic relation between the unstable twin stacking fault energy and twin nucleation stress revealing the physics of twinning.     

块体纳米晶Al-Zn-Mg-Cu合金的热处理

利用液氮球磨、真空热压和挤压工艺制备块体纳米晶Al-Zn-Mg-Cu合金,并对其固溶和时效处理进行研究,得到时效硬度曲线。利用X射线衍射仪和透射电镜对该合金热处理前后的微观组织进行分析,结果表明:块体制备过程中析出的MgZn2可以通过固溶处理使其回溶并在时效后沉淀析出;热压后晶粒尺寸为50~100 nm,热处理后晶粒长大到100 nm,部分晶粒达到200 nm。