The grain boundaries of MgO bicrystal specimens were subjected to creep-rupture tests in the temperature range of 1300-1500C and at shear stresses varying from 150 to 10,000 gm/mm2. The boundary was oriented at 45 degrees to the compression direction. The stress for grain boundary sliding and fracture varied markedly with crystal misorientation; high twist-low tilt boundaries being much weaker than other misorientations. For a given misorientation the was considerable scatter in the fracture and sliding stress data which was found to be a result of boundary irregularities (jog content) present in the as-received material or stress-induced during test. Most specimens did not exhibit controlled grain boundary sliding but rather, would slide uncontrollably to fracture after an incubation period of a few minutes at a critical stress.

The grain boundaries of MgO bicrystal specimens were subjected to creep-rupture tests in the temperature range of 1300-1500C and at shear stresses varying from 150 to 10,000 gm/mm2. The boundary was oriented at 45 degrees to the compression direction. The stress for grain boundary sliding and fracture varied markedly with crystal misorientation; high twist-low tilt boundaries being much weaker than other misorientations. For a given misorientation the was considerable scatter in the fracture and sliding stress data which was found to be a result of boundary irregularities (jog content) present in the as-received material or stress-induced during test. Most specimens did not exhibit controlled grain boundary sliding but rather, would slide uncontrollably to fracture after an incubation period of a few minutes at a critical stress.

The effect of solute additions on the deformation and fracture behavior of KCl bicrystals was investigated. This material exhibited a definite transition from brittle to ductile fracture, the brittle type being of an intergranular nature and the ductile a necked matrix failure. This transition temperature was about 175 C for the high purity specimens and 400 C for tilt misoriented calcium doped (100 ppm) bicrystals. Calcium doped twist boundary bicrystals showed intergranular brittle fracture at temperatures as high as 600 C, which was the highest test temperature examined. It was found that wavy glide rather than the planar type was a prerequisite for ductile behavior and that the role of the solute was to suppress the onset of wavy glide, thus elevating the transition temperature. The data also showed a greater propensity for grain boundary sliding in the doped material in the tests which were conducted above the transition temperature. (Author).

This volume constitutes the Proceedings of a Symposium on the Plastic Deformation of Ceramic Materials, held at The Pennsylvania State University, University Park, Pennsylvania, July 17, 18, and 19, 1974. The theme of this conference focused on single crystal and polycrystalline deformation processes in ceramic materials. The 31 contributed papers by 52 authors, present a current understand ing of the theory and application of deformation processes to the study and utilization of ceramic materials. The program chairmen gratefully acknowledge the financial assistance for the Symposium provided by the United States Atomic Energy Commission, The National Science Foundation, and The College of Earth and Mineral Sciences of The Pennsylvania State University. Special acknowledgment is extended to Drs. Louis C. Ianniello and Paul K. Predecki of the AEC and NSF, respectively. Of course, the proceedings would not have been possible without the excellent cooperation of the authors in preparing their manuscripts. Special appreciation is extended to the professional organi zation services provided by the J. Orvis Keller Conference Center of The Pennsylvania State University. In particular, Mrs. Patricia Ewing should be acknowledged for her excellent program organization and planning. Finally, we also wish to thank our secretaries for the patience and help in bringing these Proceedings to press.

This volume "Deformation of Ceramic Materials II" constitutes the proceedings of an international symposium held at The Pennsyl vania State University, University Park, PA on July 20, 21, and 22, 1983. It includes studies of semiconductors and minerals which are closely related to ceramic materials. The initial conference on this topic was held in 1974 at Penn State and the proceedings were published in the volume entitled "Deformation of Ceramic Materials." This conference emphasized the deformation behavior of crystals and po1ycrysta11ine and polyphase ceramics with internationally recognized authorities as keynote lecturers on the major subtopics. Several papers dealing with cavity nucleation and creep crack growth represent a major new research thrust in ceramics since the first conference. This collection of papers represents the state-of-the art of our understanding of the plastic deformation behavior of ceramics and the crystals of which they are composed. We are grateful for the suggestions of our International Advisory Committee .in recommending experts in their respective countries to participate. We are particularly grateful that the organizers of the previous Dislocation-Point Defect Interaction Workshops agreed to participate in the Penn State Symposium as an alternative at the suggestion of Prof. A. H. Heuer. We acknowledge the financial support of the National Science Foundation for this conference.