An attempt was made to determine the mechanisms of deformation and fracture in polycrystalline Cd, to correlate these mechanisms with pertinent aspects of single crystal behavior, and to determine whether the slip systems and mode of fracture of polycrystalline Cd could be altered by reduction of the axial ratio (1.886) byALLOYING. Tests on polycrystalline Cd and Cd-Mg alloys indicated that the yield stress, flow stresses for arbitrary small strains, and the average coefficient of work hardening were relatively insensitive to temperature from room temperature to 4.2 K. The tensile strength of Cd was strongly temperature dependent but relatively independent of grain size between room temperature and -175C; below-175C it was dependent on grain size, but not on temperature. Cd had a ductile to quasi-brittle transition at about -155C which did not depend on grain size or purity. The minimum ductility, about 15% reduction in area, was reached near -196 and persisted at -269C.

Particulate erosion of brittle materials is a complex process of fracture nucleation, growth, and interaction which leads to removal of material from the exposed surface. The temporal and spatial development of the localized pressure loading imparted to a material surface by a water-drop or soft-body collision is not clearly defined. For the impact conditions investigated, the duration of the applied pressure pulse is generally less than 100 ns. Correspondingly, the exact form of the transient stress states generated within the impacted body is not easily determined. However, insights into the nature of the damage produced by these relatively short stress cycles are being obtained from microscope examinations of the residual deformation and fracture patterns in transparent materials. The response of single crystals of magnesium oxide to water-drop collisions is described. It was found that for the high strain rates imposed slip and fracture occur in the same systems and planes as are associated with quasi-static deformations. However, water-drop impacts produce slip only on the four {110}45-deg slip systems, whereas rounded solid particles produce dominant slip on two {110} 90-deg slip systems, as well as on the {110} 45-deg slip systems. The fractures associated with each condition are also distinctly different. It was also found that fracture initiation could be suppressed with the introduction of numerous active slip sources on the impact face of the crystal.

Several basic aspects concerning the effects of grain boundaries in niobium bicrystals were explored. The investigation led to the following findings: (1) A new method of growing oriented niobium bicrystals (the Y shaped seed technique) was developed. Large cylindrical bicrystals, 0.63 cm in diameter and over 10 cm in length, have been grown from the melt using this method. (2) Niobium bicrystals exhibit excess hardening at the grain boundary, as shown by microhardness measurements. The degree of boundary hardening increased as the angle of misorientation increases. In addition, hardening is greater in bicrystals with tilt boundaries than with twist boundaries. (3) In tensile deformation of niobium bicrystals, Stage I hardening is absent. In Stage II, extra slip traces are activated from the boundary. In some cases, the primary slip system of the bicrystal differs from that of the single crystal of similar orientation. (4) In conjunction with the experimental program, a refined analysis was made to examine the dislocation-boundary interaction. It was found that in non-symmetrical bicrystals an edge dislocation wall has a long-range stress field which contrasts with the classical result in a single crystal. (Author).