Applications of microdynamics and lattice mechanics to problems in plastic flow and fracture. Progress report, January 1, 1976--December 31, 1976

Research in progress has concentrated on experimental studies of the temperature dependence of crack propagation in molybdenum single crystals, on a theoretical investigation of screw dislocations via lattice dynamic methods and on the crystal growth of niobium--molybdenum alloys. Cleavage observations at 770K for [100] oriented molybdenum crystals revealed significant plastic relaxation at the crack-tip. Raising the temperature to 2980K causes ''slow'' crack growth to occur, but only when the macroscopic proportional limit is exceeded. The implications of these findings with respect to cleavage energetics is discussed. Lattice dynamics calculations on a model crystal have determined the local optical mode frequencies and inelastic neutron scattering cross section for screw dislocation arrays. A method of assessing the screw dislocation core energy from such data is indicated. Finally a series of Nb--Mo alloy crystals of homogeneous composition and controlled microstructure have been produced