B.J. Diak

Bio: B.J. Diak is an academic researcher from Queen's University. The author has contributed to research in topics: Slip (materials science) & Strain rate. The author has an hindex of 12, co-authored 47 publications receiving 600 citations.

Effects of heat treatment on microstructure and tensile deformation of Mg AZ80 alloy at room temperature

Abstract: The plasticity of coarse and grain-refined Mg AZ80 alloys in the as-cast, γ-dissolved and homogenized states was investigated by specialized tensile testing at room temperature. Results indicate that microstructural parameters such as the activation volume and mean free path are important descriptors for these materials and capture the nature of the solute and second phase effect on strength and ductility. The as-cast alloys contain a microstructure consisting of α-Mg matrix, and divorced eutectic α-Mg/γ-Mg17Al12 phase with non-uniform Al solute content in the α-Mg. Dissolution of the majority of γ-phase occurs after annealing 5 h at 420 °C, and an almost uniform solid solution is obtained after 20 h at 420 °C. The yield strength is dependent upon the volume fraction of γ-phase and grain size. All alloys yield initially by basal slip and they exhibit different work hardening behaviour. The as-cast alloys show the fastest initial hardening and earliest saturation, and ultimately the lowest ductility. In contrast the solutionized alloys show a lower initial work hardening rate that is sustained, and enhanced ductility. The flow stress dependence of the strain rate sensitivity indicates that dynamical recovery processes associated with the dislocation–dislocation interactions, which develop in the as-cast alloys after small amount of deformation, lead to strain localizations and early failure. Results reveal that reducing the grain size and dissolving the γ-phase will enhance the ductility of AZ80 at room temperature.

Mechanistic origin and prediction of enhanced ductility in magnesium alloys.

Abstract: Pure magnesium exhibits poor ductility owing to pyramidal [Formula: see text] dislocation transformations to immobile structures, making this lowest-density structural metal unusable for many applications where it could enhance energy efficiency. We show why magnesium can be made ductile by specific dilute solute additions, which increase the [Formula: see text] cross-slip and multiplication rates to levels much faster than the deleterious [Formula: see text] transformation, enabling both favorable texture during processing and continued plastic straining during deformation. A quantitative theory establishes the conditions for ductility as a function of alloy composition in very good agreement with experiments on many existing magnesium alloys, and the solute-enhanced cross-slip mechanism is confirmed by transmission electron microscopy observations in magnesium-yttrium. The mechanistic theory can quickly screen for alloy compositions favoring conditions for high ductility and may help in the development of high-formability magnesium alloys.

Quantitative prediction of solute strengthening in aluminium alloys

Abstract: Despite significant advances in computational materials science, a quantitative, parameter-free prediction of the mechanical properties of alloys has been difficult to achieve from first principles. Here, we present a new analytic theory that, with input from first-principles calculations, is able to predict the strengthening of aluminium by substitutional solute atoms. Solute-dislocation interaction energies in and around the dislocation core are first calculated using density functional theory and a flexible-boundary-condition method. An analytic model for the strength, or stress to move a dislocation, owing to the random field of solutes, is then presented. The theory, which has no adjustable parameters and is extendable to other metallic alloys, predicts both the energy barriers to dislocation motion and the zero-temperature flow stress, allowing for predictions of finite-temperature flow stresses. Quantitative comparisons with experimental flow stresses at temperature T=78 K are made for Al-X alloys (X=Mg, Si, Cu, Cr) and good agreement is obtained.

Slip planes in bcc transition metals

Abstract: Slip in face centred cubic (fcc) metals is well documented to occur on {111} planes in 〈110〉 directions. In body centred cubic (bcc) metals, the slip direction is also well established to be 〈111〉, but it is much less clear as to the slip planes on which dislocations move. Since plasticity in metals is governed by the collective motion and interaction of dislocations, the nature of the relevant slip planes is of critical importance in understanding and modelling plasticity in bcc metals. This review attempts to address two fundamental questions regarding the slip planes in bcc metals. First, on what planes can slip, and thus crystallographic rotation, be observed to occur, i.e. what are the effective slip planes? Second, on what planes do kinks form along the dislocation lines, i.e. what are the fundamental slip planes? We review the available literature on direct and indirect characterisation of slip planes from experiments, and simulations using atomistic models. Given the technological importan...