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).

The effect of solute additions on the deformation and fracture behavior of KCl bicrystals was investigated. This material exhibited a definite transition from brittle to ductile fracture, the brittle type being of an intergranular nature and the ductile a necked matrix failure. This transition temperature was about 175 C for the high purity specimens and 400 C for tilt misoriented calcium doped (100 ppm) bicrystals. Calcium doped twist boundary bicrystals showed intergranular brittle fracture at temperatures as high as 600 C, which was the highest test temperature examined. It was found that wavy glide rather than the planar type was a prerequisite for ductile behavior and that the role of the solute was to suppress the onset of wavy glide, thus elevating the transition temperature. The data also showed a greater propensity for grain boundary sliding in the doped material in the tests which were conducted above the transition temperature. (Author).

Alpha-beta interfaces play an important role in the mechanical properties of alpha/beta alloys. These interfaces are the sites of hydrogen segregation and hydride precipitation. In addition there have been a number of investigations which have reported the presence of what has been designated interface phase, and the role of the interface phase on transmitting deformation across the interface has been considered in two separate investigations. Numerous observations have been made that voids formed at alpha/beta interfaces. Fatigue cracks have been observed to form and propagate along alpha/beta interfaces. Catastrophic fracture has also been found to occur at these interfaces. All of these observations attest to the importance of alpha/beta interfaces in controlling properties of alpha/beta Ti alloys. Although plastic deformation studies of single crystal alpha-Ti have been carried out and single crystal studies of Beta-Ti have been made, no studies, up to this investigation, have been carried out on alpha/beta bimetallic bicrystals.

The object of this program has been to evaluate the role of grain boundaries in the deformation and fracture behavior of beryllium. Utilizing bi-crystals grown by a parallel seed-floating zone technique, selected orientation configurations believed significant in polycrystalline deformation have been investigated. Features of flow and fracture in bi-crystals deformed in direct tests to failure and companion interrupted tests have been examined by optical microscopy as well as surface replication electron microscopy. Dislocation interactions at the boundary have been evaluated by electron transmission microscopy of thin foils. Brittle behavior was noted in the asymmetric and modified symmetric configurations investigated. Inability to accommodate mixed dislocation interactions, and complex grain rotations are believed responsible. In the absence of these adverse circumstances in a symmetric configuration, ductile behavior was observed. Other potentially ductile symmetric configurations should be evaluated to further clarify features of polycrystalline deformation. (Author).