Stability map for nanocrystalline and amorphous materials (original) (raw)

Rotational diffusion and grain size dependent shear instability in nanostructured materials

Acta Materialia, 2008

Previous experimental observations on some nanostructured metals have indicated a transition from homogeneous to non-homogeneous plastic deformation with a reduction in grain size [Jia D, Ramesh KT, Ma E. Acta Mater 2003;51:349]. We present a model that predicts the development of shear bands in such materials under quasi-static loading rates. Motivated by microscopic observations, a grain rotation based geometric softening mechanism is implemented as an internal variable within a viscoplastic constitutive setting. The model predicts the occurrence of the shear bands at small grain sizes, whereas at larger grain sizes it predicts homogeneous plastic flow. The model also predicts the phenomenon of shear band broadening with strain, which is experimentally observed, and attributes this to the ''rotational diffusion'' mechanism along with the restoration of material hardening within the band following the saturation of grain reorientation. Finally, we provide a localization index that can be used to classify nanostructured metals in terms of the susceptibility to this shear band mechanism.

Grain Rotation as a Mechanism of Grain Growth in Nanocrystalline Materials

ICASE/LaRC Interdisciplinary Series in Science and Engineering, 2003

Grain-boundary (GB) properties in a polycrystalline system are generally anisotropic; in particular, both the GB energy and mobility depend on the GB misorientation. Moreover, in nanocrystalline materials, in which the grain size is less than 100 nm, grain rotations leading to the coalescence of neighboring grains via elimination of the common GB between them may provide a new mechanism for grain growth. Here we investigate the combined effect of curvature-driven GB migration and grain-rotation grain-coalescence on the kinetics, topology and morphology of grain growth. A stochastic velocity-Monte-Carlo algorithm based on a variational formulation for the dissipated power is implemented. The presence of both growth mechanisms introduces a physical length scale R c into the system, enabling the growth process to be characterized by two regimes. If the average grain size is smaller than R c , grain growth is dominated by the grain-rotation-coalescence mechanism. By contrast, if the average grain size is greater than R c , growth is dominated by curvature-driven GB migration. The values of the growth exponents, different for the two growth regimes and different from a system with isotropic GB properties, are rationalized in terms of the detailed growth mechanism and the continuous change of the fraction of low-angle GBs in the system. An extended von Neumann-Mullins relation based on averaged GB properties is proposed and verified.

Decay of low-angle tilt boundaries in deformed nanocrystalline materials

Journal of Physics D: Applied Physics, 2004

A theoretical model is suggested, which describes a decay of low-angle grain boundaries in deformed nanocrystalline materials (NCMs). In the framework of the model, lattice dislocations that form a low-angle boundary are under the action of the forces owing to external (applied) and internal stresses. The balance of the forces causes the critical shear stress at which a low-angle boundary decays. Such decay processes result in the formation of high-density ensembles of mobile lattice dislocations that are capable of inducing plastic flow localization (shear banding) in mechanically loaded NCMs.

Grain-size dependent mechanical behavior of nanocrystalline metals

Grain size has a profound effect on the mechanical response of metals. Molecular dynamics continues to expand its range from a handful of atoms to grain sizes up to 50 nm, albeit commonly at strain rates generally upwards of 10 6 s À 1. In this review we examine the most important theories of grain size dependent mechanical behavior pertaining to the nanocrystalline regime. For the sake of clarity, grain sizes d are commonly divided into three regimes: d4 1 μm, 1 μm od o100 nm; and d o100 nm. These different regimes are dominated by different mechanisms of plastic flow initiation. We focus here in the region d o 100 nm, aptly named the nanocrystalline region. An interesting and representative phenomenon at this reduced spatial scale is the inverse Hall–Petch effect observed experimentally and in MD simulations in FCC, BCC, and HCP metals. Significantly, we compare the results of molecular dynamics simulations with analytical models and mechanisms based on the contributions of Conrad and Narayan and Argon and Yip, who attribute the inverse Hall–Petch relationship to the increased contribution of grain-boundary shear as the grain size is reduced. The occurrence of twinning, more prevalent at the high strain rates enabled by shock compression, is evaluated.

Mapping Shear Bands in Metallic Glasses: From Atomic Structure to Bulk Dynamics

Physical Review Letters

A deep understanding of the mechanisms controlling shear banding is of fundamental importance for improving the mechanical properties of metallic glasses. Atomistic simulations highlight the importance of nanoscale stresses and strains for shear banding, but corresponding experimental proofs are scarce due to limited characterization techniques. Here, by using precession nanodiffraction mapping in the transmission electron microscope, the atomic density and strain distribution of an individual shear band is quantitatively mapped at 2 nm resolution. We demonstrate that shear bands exhibit density alternation from the atomic scale to the submicron scale and complex strain fields exist, causing shear band segmentation and deflection. The atomic scale density alternation reveals the autocatalytic generation of shear transformation zones, while the density alternation at submicron scale results from the progressive propagation of shear band front and extends to the surrounding matrix, forming oval highly strained regions with density consistently higher (∼0.2%) than the encapsulated shear band segments. Through combination with molecular dynamic simulations, a complete picture for shear band formation and propagation is established.

Nucleation of shear bands in amorphous alloys

Proceedings of the National Academy of Sciences, 2014

The initiation and propagation of shear bands is an important mode of localized inhomogeneous deformation that occurs in a wide range of materials. In metallic glasses, shear band development is considered to center on a structural heterogeneity, a shear transformation zone that evolves into a rapidly propagating shear band under a shear stress above a threshold. Deformation by shear bands is a nucleation-controlled process, but the initiation process is unclear. Here we use nanoindentation to probe shear band nucleation during loading by measuring the first pop-in event in the load-depth curve which is demonstrated to be associated with shear band formation. We analyze a large number of independent measurements on four different bulk metallic glasses (BMGs) alloys and reveal the operation of a bimodal distribution of the first pop-in loads that are associated with different shear band nucleation sites that operate at different stress levels below the glass transition temperature, T g . The nucleation kinetics, the nucleation barriers, and the density for each site type have been determined. The discovery of multiple shear band nucleation sites challenges the current view of nucleation at a single type of site and offers opportunities for controlling the ductility of BMG alloys.

Role of atomic migration in nanocrystalline stability: Grain size and thin film stress states

Current Opinion in Solid State and Materials Science, 2015

As the length scale of materials decreases to the nanometer regime, grain boundaries occupy a relatively larger volume fraction. Consequently, they play an important role in stabilizing nanocrystalline systems. This review looks at the role of solute segregation to grain boundaries in stabilizing such systems. In recent years, grain size stabilization from solute segregation has led to new types of thermodynamic stability maps as a materials design tool. We propose to extend and adapt these concepts of grain boundary solute segregation as a stabilizing effect to thin film stress states. A recent study on Fe-Pt alloy films, where one species enriched the boundaries, was shown to manipulate the stress from tensile-to-compressive as a function of composition. This suggests that intrinsic segregation can be used as a tunable variable to manipulate stress states, analogous to changing film processing parameters, such as deposition rate, pressure, etc. The application of such solute segregation is at the precipice of new opportunities in materials design of thin films.

Grain rotation dependent non-homogeneous deformation behavior in nanocrystalline materials

Materials Science and Engineering: A, 2010

Many experimental observations have indicated a transition from homogeneous to non-homogeneous plastic deformation on nanocrystalline materials. Based on a grain rotation theory of diffusionaccommodated grain-boundary sliding, a new phase mixture model was developed to predict the softening of nanocrystalline materials considering non-homogeneous plastic deformation due to shear bands subjected to quasi-static rates of loading. In this phase mixture model, nanocrystalline materials were treated as composites consisting of grain interior and grain boundary phases. Grain interior phase was divided into soft-grain interior part with soft orientation and hard-grain interior part with hard orientation. The grain rotation rate driving force for grain rotation was derived from energy considerations, including the dissipation induced by mass diffusion and that by GB sliding viscosity. The model predicts the effect of softening mechanism for total stress-strain relation; the grain size and mean maximum Schmid factor effect was considered in the phase mixture model. Further discussion was presented for calculation results and relative experimented observations.

Mechanical anisotropy at the nanoscale in amorphous solids

Journal of Applied Physics, 2015

Amorphous solids are randomly disordered without any long-range periodic atomic arrangement and thus appear isotropic. Here, we show in metallic glasses that this view does not hold at small scales: Strong mechanical anisotropy emerges when the sample size decreases below about 15 nm as shown by the marked deviation in stress-strain relations as well as elastic modulus along different loading directions. The size induced mechanical anisotropy is naturally related to structural anisotropy that is absent before loading. The anisotropic stress and modulus versus the size yield different scaling exponents in different stages of deformation, hinting at different deformation mechanisms. The size effect discovered here points to the existence of intrinsic heterogeneity defined by the anisotropy, which may play an important role in structure-property relations in amorphous solids.