https://html.mechse.illinois.edu/files/2014/08/Predicting-twinning-stress-in-fcc-metals-linking-twin-energy-pathways-to-twin-nucleation.pdf

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

We present atomistic simulations of the tensile and compressive loading of single crystal face-centered cubic (FCC) nanowires with 〈 1 0 0 〉 and 〈 1 1 0 〉 orientations to study the propensity of the nanowires to deform via twinning or slip. By studying the deformation characteristics of three FCC materials with disparate stacking fault energies (gold, copper and nickel), we find that the deformation mechanisms in the nanowires are a function of the intrinsic material properties, applied stress state, axial crystallographic orientation and exposed transverse surfaces. The key finding of this work is the first order effect that side surface orientation has on the operant mode of inelastic deformation in both 〈 1 0 0 〉 and 〈 1 1 0 〉 nanowires. Comparisons to expected deformation modes, as calculated using crystallographic Schmid factors for tension and compression, are provided to illustrate how transverse surface orientations can directly alter the deformation mechanisms in materials with nanometer scale dimensions.

https://imechanica.org/files/parkJMPS2006.pdf

Deformation of FCC nanowires by twinning and slip

Microstructural based models for bcc and fcc metals with temperature and strain rate dependency

Twinnability is the property describing the ease with which a metal plastically deforms by twinning relative to deforming by dislocation-mediated slip. In this paper a theoretical measure for twinnability in face-centered-cubic (fcc) metals is obtained through homogenization of a recently introduced criterion for deformation twinning (DT) at a crack tip in a single crystal. The DT criterion quantifies the competition between slip and twinning at the crack tip as a function of crack orientation and applied loading. The twinnability of bulk material is obtained by constructing a representative volume element of the material as a polycrystal containing a distribution of microcracks and integrating the DT criterion over all possible grain and microcrack orientations. The resulting integral expression depends weakly on Poisson's ratio and significantly on three interfacial energies: the stacking-fault energy, the unstable-stacking energy and the unstable-twinning energy. All these four quantities can be computed from first principles. The weak dependence on Poisson's ratio is exploited to derive a simple and accurate closed-form approximation for twinnability which clarifies its dependence on the remaining material parameters. To validate the new measure, the twinnability of eight pure fcc metals is computed using parameters obtained from quantum-mechanical tight-binding calculations. The ranking of these materials according to their theoretical twinnability agrees with the available experimental evidence, including the low incidence of DT in Al, and predicts that Pd should twin as easily as Cu.

A first-principles measure for the twinnability of FCC metals

Despite its fundamental importance for a broad range of applications, little is understood about the behaviour of metals during the initial phase of shock compression. Here, we present molecular dynamics (MD) simulations of shock-wave propagation through a metal allowing a detailed analysis of the dynamics of high strain-rate plasticity. Previous MD simulations have not seen the evolution of the strain from one- to three-dimensional compression that is observed in diffraction experiments. Our large-scale MD simulations of up to 352 million atoms resolve this important discrepancy through a detailed understanding of dislocation flow at high strain rates. The stress relaxes to an approximately hydrostatic state and the dislocation velocity drops to nearly zero. The dislocation velocity drop leads to a steady state with no further relaxation of the lattice, as revealed by simulated X-ray diffraction.

Shock deformation of face-centred-cubic metals on subnanosecond timescales

The response of metals to high-strain-rate deformation is successfully described by physically-based mechanisms which incorporate dislocation dynamics, twinning, displacive (martensitic) phase transformations, grain-size, stacking fault, and solution hardening effects. Several constitutive equations for slip have emerged, the most notable being the Zerilli–Armstrong and MTS. They are based on Becker’s and Seeger’s concepts of dislocations overcoming obstacles through thermal activation. This approach is illustrated for tantalum and it is shown that this highly ductile metal can exhibit shear localization under low temperature and high-strain-rate deformation, as predicted from the Zerilli–Armstrong equation. A constitutive equation is also developed for deformation twinning. The temperature and strain-rate sensitivity for twinning are lower than for slip; on the other hand, its Hall–Petch slope is higher. Thus, the strain rate affects the dominating deformation mechanisms in a significant manner, which can be quantitatively described. Through this constitutive equation it is possible to define a twinning domain in the Weertman– Ashby plot; this is illustrated for titanium. A constitutive description developed earlier and incorporating the grain-size dependence of yield stress is summarized and its extension to the nanocrystalline range is implemented. Computational simulations enable the prediction of work hardening as a function of grain size; the response of polycrystals is successfully modeled for the 50 nm–100 m range. The results of shock compression experiments at pulse durations of 3–10 ns (this is two–three orders less than gas-gun experiments) are presented. They prove that the defect structure is generated at the shock front; the substructures observed are similar to the ones at much larger durations. A mechanism for dislocation generation is presented, providing a constitutive description of plastic deformation. The dislocation densities are calculated which are in agreement with observations. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle–Grady relationship. © 2002 Elsevier Science B.V. All rights reserved.

http://www.meyersgroup.ucsd.edu/papers/journals/Meyers%20228.pdf?origin=publication_detail

H.-H. Fu

Papers

Constitutive description of dynamic deformation: physically-based mechanisms