A PROGRAM OF BASIC RESEARCH ON MECHANICAL PROPERTIES OF REACTOR MATERIALS. Quarterly Progress Report for the Period Ending June 30, 1962

The effect of modifying the dislocation structure by room-temperature prestraining on the subsequent yielding behavior at 77 deg K is being studied. Results on tantalum single crystals indicated that a considerable decrease in the early yield stress for a given strain is effected by prestraining at room temperature. Twinning was observed in the tantalum at 77 deg K and normal rates of strain. It is shown that the potential barrler to dislocation motion in crystalline solids can be measured in relatively pure bcc transition metals such as Nb, Ta, Mo, and W. Work is being carried out to extend the method of differential calorimetry to measurements of stored-energy-release spectra in deformed bcc metals. Measurements for copper are discussed. (M.C.G.