Abstract
It has been found that hall-milling produces lattice strains and a decrease of crystallite size in refractory and other related oxides, These strains have been measured by the X-ray line broadening methods frequently applied to metals. Strain values were different for different directions, and were related to the anisotropy in elastic properties. The mean induced strain values were related to the heat of formation, lattice energy, and hardness of the oxides. Strain was also found to effect the radiation colouring. The magnitudes of the induced strains were often greater than those induced in metals by filing. The maximum stored energy values, several calories per gram, were up to one hundred times greater than those obtained in cold-worked metals, and an order of magnitude greater than the surface energy of the crystallites. The strain relief characteristics for cadmium oxide, thoria, magnesia, and alumina were determined. In all cases the strain relief was uniform during the greater part of the strain relief process. Strained oxides produced a caking effect on annealing which was not observed with unstrained oxides. Use of the excess free energy of a strained crystalline material in producing activated reactions was demonstrated for the sintering of alumina. The crushing strengths of sintered alumina were found to be related to the degree of prior stored strain energy in this oxide. Variations of stored strain with crystallite size were observed, and the effect of the density of the grinding pestles on the stored strain values for alumina was investigated. Close correspondence was found between crystallite, particle and surface area size values. The deformation behaviour of sub-micron and macroscopic single crystals is shown to be different, in agreement with microscale deformation results for brittle materials obtained by other workers.