Michael Callahan

Funding: UCL
Supervisor(s): Pascal Jacques

My research focuses on two topics. The first concerns the relationship between the processing parameters and the resulting microstructure in a CoCrFeMnNi multiple principal element alloy (MPEA). MPEAs represent a class of materials for which development has only just begun in the last decade or so. As such, the body of work regarding the influence of processing parameters on the obtained microstructure is quite lacking, with only a few studies employing any form of heat treatment after casting. Moreover, only one of these studies employed more than one annealing temperature.
It is of interest, then, to investigate different thermo-mechanical processing routes (cooling rates, plastic deformation, annealing temperatures/times) and observe their effects on the resulting microstructure of the alloy. Additionally, studies on non-equimolar compositions will be performed to explore phase equilibria and attempt to optimize the microstructure of these alloys to further improve mechanical performance.

The second aspect of the research is of a more fundamental nature, namely, the effect of lattice distortion on microstructure and plastic deformation. In MPEAs, 4+ elements are combined in near-equimolar compositions, which is hypothesized to result in significant distortion of the crystal lattice due to differences in atomic radii. This distortion should have an effect on microstructure formation and on dislocation mobility and plastic deformation. The lattice distortion effect will be quantified using a combination of methods such as HRTEM, EELS, and XRD. Its effect on dislocation behavior and dislocation core energy will be investigated in hopes of understanding the relationship between lattice distortion, chemical composition, and more macroscopic material properties.

IMMC main research direction(s):
Processing and characterisation of materials
Solid mechanics

metallic alloys
microstructure-processing relationships

Research group(s): IMAP