C.W. Sinclair

Role of internal stresses in co-deformed two-phase materials

The mechanical response of two-phase materials during plastic co-deformation is examined with a particular emphasis on the role of the internal stresses developed during deformation. A framework is developed to describe the accumulation of these internal stresses during plastic co-deformation in terms of a small number of physical parameters. This simplified framework is used to rationalize the observed behaviour of copper–chromium and copper–niobium two-phase materials in terms of their monotonic strain hardening rates, the response to changes in path (tension/compression) and their dimensional stability on heating.

Dislocation glide through non-randomly distributed point obstacles

Abstract Classical meso-scale models for dislocation–obstacle interactions have, by and large, assumed a random distribution of obstacles on the glide plane. While a good approximation in many situations, this does not represent materials where obstacles are clustered on the glide plane. In this work, we have investigated the statistical problem of a dislocation sampling a set of clustered point obstacles in the glide plane using a modified areal-glide model. The results of these simulations show two clear regimes. For weak obstacles, the spatial distribution does not matter and the critically resolved shear stress is found to be independent of the degree of clustering. In contrast, above a critical obstacle strength determined by the degree of clustering, the critical resolved shear strength becomes constant. It is shown that this behaviour can be explained semi-analytically by considering the probability of interaction between the dislocation line and obstacles at a given level of stress. The consequences for alloys exhibiting solute clustering are discussed.

Biaxial Deformation of the Magnesium Alloy AZ80

The multiaxial deformation of magnesium alloys is important for developing reliable, robust models for both the forming of components and also analysis of in-service performance of structures, for example, in the case of crash worthiness. The current study presents a combination of unique biaxial experimental tests and biaxial crystal plasticity simulations using a visco-plastic self-consistent (VPSC) formulation conducted on a relatively weak AZ80 cast texture. The experiments were conducted on tubular samples which are loaded in axial tension or compression along the tube and with internal pressure to generate hoop stresses orthogonal to the axial direction. The results were analyzed in stress and strain space and also in terms of the evolution of crystallographic texture. In general, it was found that the VPSC simulations matched well with the experiments. However, some differences were observed for cases where basal 〈a〉 slip and

Carbon diffusion in supersaturated ferrite: a comparison of mean-field and atomistic predictions

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Factors contributing to plastic strain amplification in slip dominated deformation of magnesium alloys

extension twinning were in close competition such as in the biaxial tension quadrant of the plastic potential. The evolution of texture measured experimentally and predicted from the VPSC simulations was qualitatively in good agreement. Finally, experiments and VPSC simulations were conducted on a second AZ80 material which had a stronger initial texture and a higher level of mechanical anisotropy. In the previous case, the agreement between experiments and simulations was good, but a larger difference was observed in the biaxial tension quadrant of the plastic potential.

The Mechanisms of Transformation and Mechanical Behavior of Ferrous Martensite

Experiments have been conducted on a model Cu–Cr alloy to investigate the role of fine-scale embedded phases in co-deformation. The results of this work show that, when sufficiently fine, plasticity in embedded phases is accompanied by very large internal stresses. This has been attributed to the difficulty of maintaining compatibility between the phases. A simple model is proposed that is able to capture the essential features of the strengthening in this material under uniaxial tension at low temperatures.

The Effect of Obstacle Strength Distribution on the Critical Resolved Shear Stress of Engineering Alloys

Hillerts mean-field elastic prediction of the diffusivity of carbon in ferrite is regularly used to explain the experimental observation of slow diffusion of carbon in supersaturated ferrite. With increasing carbon supersaturation, the appropriateness of assuming that many-body carbon interactions can be ignored needs to be re-examined. In this work, we have sought to evaluate the limits of such mean-field predictions for activation barrier prediction by comparing such models with molecular dynamics simulations. The results of this analysis show that even at extremely high levels of supersaturation (up to 8 at% C), mean-field elasticity models can be used with confidence when the effects of carbon concentration on the energy of carbon at octahedral and tetrahedral sites are considered. The reasons for this finding and its consequences are discussed.

A model for the grain size dependent work hardening of copper

The kinetic interplay between helium (He) segregation, He cluster formation and curvature-driven grain boundary migration in bcc iron (-Fe) has been investigated using molecular dynamics simulations. He atoms that segregate to the migrating boundary are found to be trapped in vacant substitutional sites emitted by the migrating boundary. He atoms that form clusters in the bulk restrain the boundary migration via a pinning mechanism. The pinning pressure of He clusters is proportional to the density and the squared radius of each cluster. A cluster pinning model has been developed by taking into account the two-fold effect of clusters on the boundary migration: (1) reducing the boundary mobility and (2) acting as pinning objects that delay or even completely halt the boundary migration. The model is found to be in agreement with the simulation results.