Papers by Author: Erik Johnson

Abstract: Alloys displaying positive enthalpy of mixing demix below a critical temperature. In Co-Cu and related ternaries the miscibility gap is metastable, i.e. it occurs at temperatures lower than the liquidus. In order to study the liquid phase separation high melt undercooling is necessary. This was obtained via rapid solidification techniques using melt spinning and casting in moulding devices, as well as high temperature DSC experiments with samples embedded in a flux. Results are given for Co-Cu, Co-Cu-Fe and Co-Cu-Ni systems. Phase diagrams were optimised using the DSC data. The mechanism of phase separation was investigated by comparing samples produced under different cooling conditions. The hierarchy of microstructures obtained was interpreted accounting for the processing technique and the phase diagram. They constitute a database useful for the interpretation of the thermal history of samples processed in microgravity.

Abstract: Thermal motion of nanoscale liquid Pb inclusions attached to fixed dislocations in thin Al foils is investigated using in-situ TEM. In contrast with 3D random motion of free inclusions, the attached inclusions demonstrate oscillations in the close proximity of the dislocations. This is due to the elasticity of the dislocations. It is found that inclusions captured by one dislocation repulse at small separations, and attract at large ones and this is also caused by the dislocation elasticity. Such behaviour of trapped inclusions can be considered as a motion in a potential well or in coupled potential wells in the case of motion of several trapped inclusions on one dislocation. The potential of interaction of an inclusion with a dislocation and the potential of mutual interaction between inclusions are determined.

Abstract: Diffusion of nano-sized liquid Pb inclusions in thin aluminum foils is investigated using in-situ transmission electron microscopy (TEM). Free diffusion of the inclusions in the bulk and diffusion constrained by dislocations trapping is studied. The motion of trapped Pb inclusions is spatially confined in close proximity to the dislocations. The diffusion coefficients of free motion of the inclusions are determined using Einstein's equation. The diffusion coefficients of trapped inclusions were obtained using an equation based on Smoluchowski's analysis of the Brownian motion of particle in a harmonic potential. The agreement of the diffusion coefficients of free and trapped inclusions indicates the same underlying microscopic mechanism, and no strong influence from dislocations. The microscopic mechanism controlling the mobility is discussed.