The strain-aging behavior of annealed Zircaloy-2 in vacuum was studied as a function of deformation temperature, integrated fast-neutron dose, and degree of cold work. Strain aging occurred in the temperature range 200 to 500 C. Transmission electron microscopy showed that in the temperature range 300 to 450 C, dislocations were arranged in a cell structure, and that at 500 C two-dimensional hexagonal dislocation nets were present. Between 200 and 400 C, the strain aging was found to be due to the presence of oxygen; however, the origin of the strain aging between 400 and 500 C was not identified. At 300 C the strain aging decreased with increasing neutron dose and decreasing amounts of prior cold work. Post-irradiation annealing experiments between 400 and 600 C increased the yield stress of the material at 300 C after irradiation at 325 C.

The effect of Hertzian stresses resulting from loads acting at the points of contact of beryllia spheres was determined. As-drawn glass and glass ground with various grades of silicon carbide were indented with beryllia spheres, 1 in. in diameter. Also, pairs of these beryllia spheres were pressed together. Circular cracks due to Hertzian stresses were produced; an optical technique was developed for detecting them in a beryllia surface. The outer crack radius was proportional to the cube root of the maximum applied load. Average values obtained for the respective inner crack radii were used to determine average values of the minimum load to fracture these materials. The average minimum load to fracture 1 in. diameter beryllia spheres in air at room temperature under dynamic loading was 235 lb. The crack radius in glass and beryllia was about 20% greater than the radius of the contact surface. This was not significantly affected (in glass) by flaw density. It is considered that the Hertz analysis does not give the correct location and value of the maximum tensile stress when finite displacements of material occur.

No abstract

No abstract

Beryllia beams were loaded at temperature in four-point bending to a deflection of 0.001 in. or 0.002 in. and the decrease in load necessary to maintain the deflection constant was measured as a function of time. The beryllia was fine-grained, in the range ~-~ 1 to 7 Fm, and the porosity varied between 3 and 21 ~. The test conditions covered a temperature range of 850 to 1240~ experiments took from 10 min to 8 h: the loads used produced initial maximum outer fibre stresses of the order 3000 to 6000 psi.A method was developed for calculating the stress distribution in a beam at any time and this was used to analyse the results. The stress relaxation process was found to be stress-activated, and the conversion of elastic to plastic strain could be expressed as a creep law having the formThe activation energy was 100 • 2 kcal mole-' and the activation volume was large, probably of the order of 103 atoms. The rate constant ko was approximately proportional to the fraction of intergranular porosity and inversely proportional to the cube of the grain diameter. It is suggested that the mechanism of grain boundary sliding is consistent with the observations.