Search

An Al-0.2%Sc alloy and an Al-3%Mg-0.2%Sc alloy were subjected to equal-channel angular pressing to a strain of - 8 to produce grain sizes of ∼ 0.7 and 0.2 μm, respectively. Because of the presence of some areas of subgrain boundaries in the Al-3%Mg-0.2%Sc alloy, additional samples of this alloy were subjected to ECA pressing to a strain of ∼12. In both alloys, the grain sizes were reasonably stable up to annealing temperatures above 700 K. Tensile specimens were machined from the as-pressed samples for testing at elevated temperatures. The Al-3%Mg-0.2%Sc alloy pressed to a strain of ∼ 12 exhibited exceptional ductility with elongations up to a maximum of ∼ 1560% at 673 K at the high strain rate of 3.3 x 10 -2 S -1 . By contrast, the Al-0.2%Sc alloy failed under all conditions at elongations

Diffusion experiments have been conducted using the Ni 3 Al/NiAl couples at annealing temperatures of 873 and 923 K. It is shown that the Ni 5 Al 3 phase forms between the Ni 3 Al and NiAl phases during the diffusion annealing. Optical microscopy, transmission electron microscopy and electron probe microanalysis (EPMA) are employed for the identification of the Ni 5 Al 3 phase. Concentration profiles of Al across the diffusion-couple interfaces are measured using EPMA to determine the region of the Ni 5 Al 3 single-phase in the Ni-Al binary phase diagram. Interdiffusion coefficients in the Ni 5 Al 3 phase are also determined from the concentration profiles.

The lattice misfits at interphase interfaces, NiAl(B 2 )-Ni 2 AlTi(L2 I ) and Ni 2 AlTi(L2 I )-NiTi(B2), in a pseudobinary NiAl(B2)-NiTi(B2) system have been estimated by transmission electron microscopy. When the size of Ni 2 AlTi precipitates is smaller than a critical value, the precipitates keep their lattice coherency with the NiAI and NiTi matrices. After longer periods of aging the Ni 2 AlTi precipitates lose their coherency by introducing interface dislocations surrounding them. The coherency loss occurs catastrophically and the shape of the precipitates changes to a globular one after the loss of coherency, while keeping the cubic-cubic orientation relationship. Contrast experiments have revealed that the Burgers vectors of these misfit dislocations are of a type with the pure edge component lying nearby on the surface of the Ni 2 AlTi phase. The magnitudes of the lattice misfits at NiAl-Ni 2 AlTi and NiTi-Ni 2 AlTi interfaces have been estimated from the spacings of the misfit dislocations and found to increase with decreasing aging temperature in both the systems. The present results are compared with the previously reported theoretical and experimental values. A NiAl-NiTi pseudobinary phase diagram is proposed based upon microstructure observation, lattice misfit measurements and analytical electron microscopy.

High tensile ductility may be achieved in metallic alloys over a limited range of strain rates at elevated temperatures when the grain size is very small (typically less than ∼ 10 μm). These superplastic materials are often suitable for industrial forming operations. This paper considers the possibility of attaining a superplastic forming capability at lower temperatures and/or faster strain rates by a reduction of the grain size into the submicrometer range. Procedures are available for the fabrication of ultrafine-grained materials by imposing a very high plastic strain. These procedures are examined with reference to results obtained using Al-Mg solid solution alloys.

Constant strain rate tests were conducted using the miniaturized double-shear specimens of pure Al and Al-5 mass% Mg. The stress-strain curves and the stress and temperature dependence of the strain rate are examined. The results are compared with other published results in order to evaluate the applicability of the miniaturized double-shear specimen to an examination of high temperature deformation behavior.

Phase equilibria between γ and γ' in Ni-base Ni-Al-Ta alloys have been investigated by analytical electron microscopy (AEM). The chemical compositions of the γ and γ' phases in the two-phase alloys were determined by an energy dispersive X-ray spectrometer (EDS) and then tie lines in the γ+γ' two-phase region and the partitioning of Ta in the γ and γ' phases were evaluated. The ALCHEMI (atom location by channeling enhanced microanalysis) technique revealed that Ta occupies most favorably the Al sites in the γ' phases irrespective of stoichiometry