Papers by Author: W.J. Clegg

Abstract: This paper examines the effect of anisotropy on the effective composite thermal conductivity, Kc, of metal matrix composites containing carbon-based inclusions. Added effects of thermal interface conductance, hbd, and size have also been considered. It has been found that at high hbd values, the effective thermal conductivity of the composite is limited by inclusion anisotropy. At lower hbd values and smaller inclusion sizes, this effect is greatly diminished due to the more dominant effect of limited heat flow across the interface.

Abstract: The tensile creep behaviour of a mullite-SiC nanocomposite containing 5 vol% of SiC particles deformed under stresses from 4 to 50 MPa at 1400 °C has been studied. After grain-size effects had been accounted for, the creep-rate of the nanocomposite was found to be approximately 30× less than that of the monolithic mullite. It is suggested that this reduction is caused not by a threshold stress but by the extra work required to drive diffusion in the low diffusivity SiC particles so that they can move with the grain boundaries during creep. A model is presented which predicts the rate of creep under these conditions and gives reasonable agreement with the experiments at low stresses.

Abstract: A method is presented for more than doubling the volume yield from an AlPO4 precursor, aluminium chlorophosphate ethanolate, from approximately 20% to 50% by replacing the ethanol ligands with water. It is shown that this increases the volume yield from about 20% to 50%. The precursor is used to make an Al2O3 powder bonded with an amorphous AlPO4 with a Young modulus of 80 GPa and a strength of 60 MPa.

Abstract: The aim of this paper is to study the effect of relaxing the assumption in the Peierls analysis that the dislocation must be wide compared to the atom spacing. To do this the use of the continuum description of the in-plane strains caused by the presence of an edge dislocation is replaced by an atomistic interaction taken to be linear elastic. It is found that in this case the inplane interactions give a contribution to the overall misfit energy changes that are not present in the Peierls analysis because of the use of a continuum approach. This contribution modifies these energy changes so that the total misfit energy is a minimum at the conventional low energy positions (whereas in the Peierls analysis it is a maximum) and gives values of the Peierls stress in reasonable agreement with those measured.

Abstract: The correlation between hardness, H, and yield strength, Y, for hard materials is reexamined using an analytical approach to provide a physical interpretation, which explains the trends observed. Existing analytical predictions using the analogy of the spherical cavity fail to reproduce experimental and finite element results because the accommodation of the intrusion of the indenter due to surface deformation is not taken into account, while experiments made here and elsewhere indicate that the latter can be substantial. A simple modification is proposed, which accounts for this, and allows for a reasonable prediction of the relation between hardness and yield strength for materials, which have a well determined yield point and do not strain harden. The treatment also allows explaining the effect of indenter geometry on measured hardness for such materials.

Abstract: Observations elsewhere have shown that multilayer structures with layers ~10 nm thick can be harder than monolithic ones. Here we see whether these effects can be observed at high temperatures and investigate the effect of temperature on the manner of deformation. The hardness of an AlN/CrN multilayer structure with a range of wavelengths from 6-200 nm has been measured at temperatures from room temperature to 400 oC. The changes in hardness have been related to the deformation behaviour observed by cross-sectional transmission electron microscopy and atomic force microscopy. These observations suggest that the mechanical properties of the coatings are dominated by the refinement in the columnar microstructure rather than directly by an effect of the layer interfaces on dislocation motion.