Effects of grain-boundary liquid particles on fracture of a copper bicrystal

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The main purposes of this project are to understand(1) the roles of liquid particles on grain boundaries in high- temperature fracture, and(2) the effects of dispersed particles on various mechanical properties of composites. Experimental work to study, deformation and fracture behavior of Cu-BィイD22ィエD2OィイD23ィエD2 and Cu-GeOィイD22ィエD2 alloys and theoretical work using micromechanics are made.Cu alloys containing small BィイD22ィエD2OィイD23ィエD2 or GeOィイD22ィエD2 glass particles have been made by internal oxidation treatments. Both the particles are dispersed in-grain and on-grain-boundaries of Cu matrix. Although the GeOィイD22ィエD2 particles in Cu are considered as plastically non-deformable solid, the BィイD22ィエD2OィイD23ィエD2 particles become liquid particles at high temperatures. From results of tensile tests using the Cu-BィイD22ィエD2OィイD23ィエD2 and Cu-GeOィイD22ィエD2 alloys at various temperatures, the roles of liquid particles on grain boundaries in high-temperature grain-boundary fracture have been discussed. The liquid BィイD22ィエD2OィイD23ィエD2 particles on grain boundaries cause brittle fracture of the Cu alloy. Void formationin the liquid particles is the reason of this fracture.Stress relaxation caused by the combination of interfacial sliding and diffusion around spherical inclusions has been discussed. Rate equations of the relaxation processes have been derived. Effects of shape and volume fraction of second phase on stress states in two-phase materials have been evaluated. The average field method has been applied for the prediction of stable shapes of coherent precipitates under uniaxial stresses. Effects of Poisson's ratio on the variation of the average Young's moduli of composites with the transitions of second-phase shape have been discussed.