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Abstract: This investigation examines the effect of hydrogen precharging upon the fracture mode of pure nickel at 77 K. Specific attention is given to the diffusion coefficient of hydrogen along grain boundaries,D g , the critical hydrogen concentration to cause intergranular fracture, C g* , and the binding energy of grain boundaries with hydrogen,E b . Both scanning electron microscope (SEM) observations and a newly developed grain boundary diffusion model indicate that the fracture mode changes from transgranular (TG) to intergranular (IG) when the hydrogen concentration in grain boundaries reaches a critical value, in the range of 6.5 to 9.8 at. pct. The experimental results further show that the observed increment of IG cracking depth with the precharging time could be accounted for by lattice diffusion alone, thus implying that hydrogen transport in this material is not enhanced by grain boundaries. Finally, the binding energy of grain boundaries with hydrogen is found to be 11.3 to 12.3 kJ/mol.

Abstract: The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ y, MPa) is represented as a function of the grain size,d (mm), the average concentration, Co (wt pct), and the local concentration, C (wt pct), by σy = [(15.7 + 22.6 Co) + (1.24 + 1.04 Co)d -1/2] + [15.7 △C], where △C = C - Co. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.

Abstract: Creep characteristics of Al-Al3Ni eutectic composites directionally solidified at 2.2 � 10-2 mm/s were determined over a wide range of stress and temperature. Four distinct regions of creep were observed. The rate controlling mechanisms for the four regions appear to be high-temperature dislocation climb in the Al matrix, low-temperature climb in the Al matrix, boundary sliding, and a mechanism involving deformation of the Al3Ni fibers. Creep rates of the Al-Al3Ni composite are several orders of magnitude smaller than for pure Al, and apparently, in the regions where deformation of the Al matrix is rate controlling, only a very small fraction of the matrix is deforming during creep of the composite.

Abstract: The silver decoration technique proposed by Sch�ber and Dieker[10] for revealing local hydrogen flux in terms of microstructural characteristics is reinterpreted. The results of decoration tests in polycrystalline nickel and palladium with and without hydrogen charging show that hydrogen flux is not solely responsible, nor even a prerequisite, for the deposition of silver crystals on metal surfaces. There is, consequently, no one-to-one correspondence between escaped hydrogen atoms and deposited silver atoms during the decoration process as reported previously. It is the oxidation film formed during electropolishing that prevents silver deposition. Any damage to the film and any factor which reduces its stability may result in spontaneous silver deposition in the absence of hydrogen. With this new interpretation, one can use the silver decoration technique to obtain qualitative information regarding hydrogen transport along grain boundaries in metallic materials.

Directionally solidified Al-4 wt pct Cu-0.2 wt pct Ti alloy ingots with equiaxed structures were analyzed for solute distribution. Inverse segregation was found to exist, and the extent of segregation in the first 50 pct of the ingot solidified agreed well with that calculated from inverse segregation theory based on the flowback of enriched liquid. However, at higher points in the ingot, the solute distribution deviated from the theoretical one. Also, more and bigger shrinkage pores existed in the equiaxed grained ingot than in a columnar grained ingot. It is suggested that the formation of randomly distributed, equiaxed grains restricts the flowback of enriched interdendritic liquid much more than the formation of columnar grains where interdendritic channels exist. In an equiaxed grained ingot solidified vertically from the bottom upward, the hydrostatic pressure becomes insufficient to cause complete flowback of enriched liquid in the latter stages of solidification. This effect causes porosity in the ingot and a solute distribution which deviates from that predicted by the theory of inverse segregation.

Abstract: A new experimental technique for the rapid determination of phase diagrams is described in this paper. The technique consists of producing a liquid diffusion couple of the Boltzmann-Matano type with the concentration of elements varying continuously across the specific portion of the phase diagram being investigated, heat treating the couple, and then performing microscopic and microchemical analysis to identify various phases. Since the couple contains all phase fields in the composition range, there is little ambiguity in the identification of phases. Some results on the use of this technique to resolve the confusion about the middle portion of the Al−Mg binary system are presented and discussed.