| Popis: |
The effects of ball-milling on powders of covalent and interstitially bonded materials have been studied by x-ray diffraction. The materials included elements of Group IV, and carbides, nitrides and borides of transition metals. All the materials examined were found to be affected by ball-milling. As milling times were increased, the amounts of strain induced increased, while the crystal size decreased. In several materials there was evidence of strain saturation with an accompanying small crystal size. The values were different in different ball mills. Electron microscopy failed to reveal particles as small as the crystal sizes determined by x-ray line broadening, except with germanium. The amount of strain induced was a function of the bond strength and was comparable to that developed in filed metals. Non-stoichiometry and the presence of impurities were found to be important factors in the strain characteristics of the interstitial compounds. In the carbides of the most reactive metals, absorbed impurity atoms prevented slip on preferred planes, causing deformation isotropy. Some carbides were purified by the novel technique of precipitation from a liquid metal bath under argon. By this means perfect crystal lattices were obtained, and the materials were close to the stoichiometric MX value. Strain relief characteristics were determined for silicon and germanium, several carbides, and titanium nitride. Special techniques were developed to prevent oxidation of the carbides during the annealing process. In all materials, crystal growth and recovery and recrystallization reactions similar to those in metals were observed. The structures of tungsten carbide tool tips and block specimens were examined by x-ray diffraction using film methods, and it was found that strain was present in the surface of the tool after machining operations. There is a relationship between plastic deformation of the tool tip and its wear properties. |
