THE FRACTURE-BEHAVIOR OF SINGLE-CRYSTAL Y3AL5O12

The fracture behavior of single-crystal yttrium aluminum garnet in ambient air has been studied as a function of temperature for two different crystallographic orientations. A Chevron notch specimen geometry was used so that stable crack propagation could be achieved. It was found that the room temperature fracture toughness, K-Ic = 1.8 MPa m(1/2), increased to K-Ic = 4.0 MPa m(1/2) at 1600 degrees C. At 1750 degrees C, samples did not fracture even when the load was brought to an equivalent stress intensity factor of 11.3 MPA m(1/2). The fracture toughness was found not to depend significantly on crystallographic orientation. The increase in toughness with temperature was attributed to the activation of slip systems, which were evaluated by trace analysis. It was found that the degree of plasticity is extremely sensitive to the strain rate. Finally, periodic striations were observed on the fracture surfaces which are believed to be related to periodic chemical or structural fluctuations which arise during the Czochralski growth process.