Ti3SiC2 and Ti3AlC2 single crystal elastic shear modulus: investigation via inelastic neutron scattering and computer simulation

This thesis explores the harmonic and isotropic nature of Ti3SiC2 and Ti3AlC2 by taking an experimental approach. This approach would have been greatly simplified if a sizeable single crystal of these materials was available, but as this is not the case currently, more complex experiments and analysis have been done in order to characterise the materials. The need to explore the harmonic and isotropic nature of MAX phases has arisen from the difference between the single crystal elastic tensor derived from simulations and preliminary experiments. This research measures the Phonon Density of States and compares it to simulations using Density Functional Theory and Molecular Dynamics methodologies. The result of this comparison and further analysis is that all atoms behave harmonically in the Z or c direction. The titanium and carbon within these materials undergoes anharmonic motion in the X-Y or a-b plane consistent with the atoms experiencing flat bottom potentials. The difference between Ti3SiC2 and Ti3AlC2 arises from the motion of the A element with aluminium behaving fairly harmonically, and, silicon behaving anharmonically consistent with experiencing a double potential well. The effect of this atomic motion on the elastic constants was observed by measuring C44 via coherent inelastic neutron scattering on an oriented polycrystal. Ti3SiC2 was found to be shear stiff similar to previous experimental evidence and DFT-MD simulation, but, contrary to DFT simulations. Ti3AlC2 was found to be quasi-isotropic matching DFT simulated values and providing the first known experimental measurement of a single crystal elastic constant for this material.