J. R. Cahoon

Hydrogen diffusion and its relevance to intergranular cracking in nickel

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,Dg, the critical hydrogen concentration to cause intergranular fracture, Cg*, and the binding energy of grain boundaries with hydrogen,Eb. 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.

Creep regimes for directionally solidified Al-Al3Ni eutectic composite

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.

Macro-segregation in a hyper-eutectic Al-Cu alloy - a microgravity experiment

Both ground based as well as experiments in microgravity conditions aboard Space Shuttle STS-47 were performed on an Al-38% Cu alloy. Directional solidification was carried out in a GAS-Can. Segregation of Cu was studied and distinct differences were noted between the ground based and microgravity based specimens.