The Influence of Point Defects on the Mechanical Behavior of Rutile Single Crystals

The temperature dependence of the stress-strain relation was determined for rutile over the range 400 to 1100c in atmospheres of air, helium, and vacuum. It was found that there are two deformation processes occurring, one in the temperature range 400 to 700 and the other in the range 900 to 1100c. The deformation mechanism in the low temperature range has an apparent activation energy of 28.5 kcal/mol and is thought to be dislocation glide on several slip systems. An apparent activation energy of 67.5 kcal/mol for the high temperature range suggests a deformation mechanism controlled by the self-diffusion of point defects. This high temperature activation energy is independent of defect concentration; i.e., there is good agreement between the values obtained for specimens deformed in air, helium and vacuum. In light of the above results, a mechanism is proposed to explain the diffusion controlled high temperature deformation. The mechanism is based on the formation of long prismatic loops resulting from the intersections of screw dislocations with voids formed by the condensation of vacancies. These prismatic loops are the active slip sources which initiate plastic deformation of the crystal. The yield stress is dependent on the length of the prismatic loops; and, therefore, on the concentration of vacancies. The dependence of yield stress on temperature and point defect concentration reported in this investigation qualitatively support this mechanism