[en] Plastic deformation in elemental BCC metals and dilute alloys is controlled by the slower of the kink pair nucleation and kink migration processes along screw dislocations. In alloys nucleation is facilitated and migration inhibited, leading to a concentration- and temperature-dependent transition from nucleation dominance to migration dominance. Here, an analytical statistical model for the stress-dependent kink migration barrier in dilute BCC alloys is developed and validated. The barrier depends only on a clearly-defined solute/screw dislocation interaction parameter, the kink width, and dislocation length between jogs. The analytic model is extensively validated via fully atomistic nudged-elastic band calculations and stochastic simulations in a model Fe-Si alloy. Combined with a recent validated double-kink nucleation theory, a fully-analytic model for the temperature- and concentration-dependent flow stress is obtained that includes the transition from nucleation to migration control. The overall model is applied to Fe-Si and W-Re using independently-determined material properties and good agreement is obtained with experiments over a range of concentrations and temperatures. Overall, the two theories represent a unified, fully-statistical, parameter-free understanding of screw dislocation strength in dilute BCC alloys.

[en] Cross-slip is a fundamental process of screw dislocation motion and plays an important role in the evolution of work hardening and dislocation structuring in metals. Cross-slip has been widely studied in pure FCC metals but rarely in FCC solid solutions. Here, the cross-slip transition path in solid solutions is calculated using atomistic methods for three representative systems of Ni-Al, Cu-Ni and Al-Mg over a range of solute concentrations. Studies using both true random alloys and their corresponding average-alloy counterparts allow for the independent assessment of the roles of (i) fluctuations in the spatial solute distribution in the true random alloy randomness and (ii) average alloy properties such as stacking fault energy. The results show that the solute fluctuations dominate the activation energy barrier, i.e. there are large sample-to-sample variations around the average activation barrier. The variations in activation barrier correlate linearly with the energy difference between the initial and final states. The distribution of this energy difference can be computed analytically in terms of the solute/dislocation interaction energies. Thus, the distribution of cross-slip activation energies can be accurately determined from a parameter-free analytic model. The implications of the statistical distribution of activation energies on the rate of cross-slip in real alloys are then identified.

[en] We discuss experimental and theoretical results which probe the identity, characteristics and thermodynamics of interstitial hydrogen storage sites in amorphous transition metal alloys. Utilizing experimental findings which describe both the local nature of hydrogen interstitial site properties and the thermodynamics of these sites, we emphasize the development of site statistical models to explain these observations. Recent models provide detailed methods for calculating hydrogen site binding energies and for predicting thermodynamic properties such as pressure-composition isotherms. (author) 36 refs., 9 figs

[en] The plastic response of a sheet metal is governed by the collective response of the underlying grains. Intragranular plasticity depends on intrinsic variables such as crystallographic orientation and on extrinsic variables such as grain interactions; however, the role of the latter is not well understood. A finite element crystal plasticity formulation is used to investigate the importance of grain interactions on intragranular plastic deformation in initially untextured polycrystalline aggregates. A statistical analysis reveals that grain interactions are of equal (or more) importance for determining the average intragranular deviations from the applied strain as compared to the orientation of the grain itself. Furthermore, the influence of the surrounding grains is found to extend past nearest neighbor interactions. It is concluded that the stochastic nature of the mesoscale environment must be considered for a proper understanding of the plastic response of sheet metals at the grain-scale

[en] The excellent mechanical properties of carbon nanotubes (CNTS) are driving research into the creation of new strong, tough nanocomposite systems. Here, the first evidence of toughening mechanisms operating in carbon-nanotube-reinforced ceramic composites is presented. A highly ordered array of parallel multiwall CNTs in an alumina matrix was fabricated. Nanoindentation introduced controlled cracks and the damage was examined by scanning electron microscopy. These nanocomposites exhibit the three hallmarks of toughening found in micron-scale fiber composites: crack deflection at the CNT/matrix interface; crack bridging by CNTs; and CNT pullout on the fracture surfaces. Interface debonding and sliding can thus occur in materials with microstructures approaching the atomic scale. Furthermore, for certain geometries a new mechanism of nanotube collapse in 'shear bands' occurs, rather than crack formation, suggesting that these materials can have multiaxial damage tolerance. The quantitative indentation data and computational models are used to determine the multiwall CNT axial Young's modulus as 200-570 GPa, depending on the nanotube geometry and quality. Three-dimensional FEM analysis indicates that matrix residual stresses on the order of 300 MPa are sustained in these materials without spontaneous cracking, suggesting that residual stress can be used to engineer enhanced performance. These nanoscale ceramic composites thus have potential for toughening and damage tolerance at submicron scales, and so are excellent candidates for wear-resistant coatings

[en] A nonperturbative approach to the density-functional description of phase transitions in fluid composed of anistropic particles is presented. The theory is exact to second order in functional perturbation theory, and at higher orders satisfies all sum rules derived from density derivatives of the two-point direct correlations. The authors have applied this theory to orientational and translational freezing of hard ellipsoids, and find that the description of the structure of crystalline phases is improved in comparison to the commonly used second-order theory. For the case of orientational freezing, it is found that the higher-order contributions do not modify the predictions of the second-order theory, and that accurate liquid structure appears to be the key factor leading to improved description of the nematic phase

[en] Strengthening of basal slip could enhance formability of magnesium by decreasing the ratio of pyramidal to basal critical resolved shear stress (P/B CRSS ratio). Here solute strengthening of basal slip is predicted for a wide range of solutes as a function of temperature and solute concentration at experimental strain rates using a recently-developed parameter-free model. The model is simplified by approximating the solute/dislocation interaction energy as the sum of a misfit strain contribution and a stacking fault interaction term, with the dislocation stress field estimated using a Peierls-Nabarro-type model of the basal edge dislocation. The approach is validated against DFT results for both Al and Zn solutes, and the predicted strengths agree well with experiments. The model is then applied to predict basal strengthening of many other solutes versus temperature, with key parameters tabulated for general future use. Comparisons to experimental data on 0.5 at% Dy and 1.0 at% Y versus temperature show moderate agreement, and the predicted effects of deviatoric misfit strains are shown to be small. An analytic formula is developed to predict strengthening as a function of solute misfit volume and stacking fault energies, enabling rapid assessment for many solutes and their combinations because. the model naturally extends to multiple solutes. Predictions are made for a range of existing ternary and higher alloys. Overall, the analysis and models here provide an accurate and easy formulation for estimating basal solute strengthening in dilute multicomponent Mg alloys and thus estimating the P/B CRSS ratio.

[en] The ultimate tensile strength (UTS) of metal and intermetallic matrix unidirectional composites can be significantly lower than expected from the rule of mixtures prediction. One possible explanation is that the fibers in the as-processed state are in a residual state of stress and in some cases are broken because of the inhomogeneous nature of the densification during manufacture. Three main results emerge from the effort to include the effect of this processing damage on the composite UTS. First is the development of a simple but accurate analytical version of Curtin's model for predicting the stress-strain response and UTS of this class of composites. Second is the generalization of Curtin's model to include both process induced fiber bending and fracture. Third is that the reduction in strength is a sensitive function of the consolidation conditions; thus a link is established between the quality of the composite and the conditions of its manufacture

[en] By means of a simple experimental heat-treatment procedure, the anomalous far-infrared absorption of superconducting Sn particle composites is shown to be associated with clustering. The structural insights thus obtained lead to a new theory in which the composite electric dipole absorption is dominated by poorly conducting clusters and is much larger than that of isolated metal particles. For superconducting particles, the theory predicts the absorption at frequencies above the gap frequency to be larger than in normal state, in agreement with experiment

[en] Twinning in fcc High Entropy Alloys (HEAs) has been implicated as a possible mechanism for hardening that enables enhanced ductility. Here, a theory for the twinning stress is developed analogous to recent theories for yield stress. Specifically, the stress to move a twin dislocation, i.e an fcc partial dislocation moving along a pre-existing twin boundary, through a random multicomponent alloy is determined. A reduced elasticity theory is then introduced in which atoms interact with the twin dislocation pressure field and the twin boundary. The theory is applied to NiCoCr using results from both interatomic potentials and elasticity theory. Results are also used to predict the increased stress for the motion of (i) a single partial dislocation leaving a trailing stacking fault and (ii) adjacent partial dislocations involved in twin nucleation. Increased strength is predicted for all processes involved in the nucleation and growth of fcc twins. Comparison to single-crystal experiments at room temperature then suggests that twinning is controlled by twin nucleation, with reasonable quantitative agreement. When solute/fault interactions are neglected, the theory shows that twinning and lattice flow stresses are related. The theory also provides insight into how other dilute solute additions could suppress twinning, as found experimentally.