Shogo Komura

INFLUENCE OF STACKING FAULT ENERGY ON MICROSTRUCTURAL DEVELOPMENT IN EQUAL-CHANNEL ANGULAR PRESSING

Equal-channel angular (ECA) pressing is a procedure having the capability of introducing an ultrafine grain size into a material. Experiments were conducted to examine the effect of the low stacking fault energy in pure Cu on microstructural development during ECA pressing at room temperature. The results show that the low 0stacking fault energy and the consequent low rate of recovery lead to a very slow evolution of the microstructure during pressing. Ultimately, a stable grain size of −0.27 μm was established in pure Cu but the microstructure was not fully homogeneous even after pressing to a total strain of ∼10. It is shown by static annealing that the as-pressed grains are stable up to ∼400 K, but at higher temperatures there is grain growth. These results lead to the conclusion that a low stacking fault energy is especially favorable for the introduction of an exceptionally small grain size using the ECA pressing procedure.

Thermal stability of ultrafine-grained aluminum in the presence of Mg and Zr additions

Abstract Superplastic deformation occurs at high temperatures and requires the presence of a very small grain size. Experiments demonstrate that equal-channel angular (ECA) pressing is capable of introducing ultrafine grain sizes in pure Al, Al–Mg and Al–Zr alloys, with grain sizes lying in the sub-micrometer range for the alloys. It is shown by static annealing that these ultrafine grain sizes are not stable in pure Al and the Al–Mg alloys at elevated temperatures but in the Al–Zr alloys the grains remain small up to temperatures of ≈600 K.

Optimizing the procedure of equal-channel angular pressing for maximum superplasticity

Abstract The imposition of severe plastic straining on an Al–3% Mg–0.2% Sc alloy through equal-channel angular pressing (ECAP) reduces the grain size to ca. 0.2 μm and provides an opportunity to achieve superplastic elongations at high strain rates. Experiments were conducted to evaluate the procedure for optimizing the ECAP process in order to attain maximum superplastic ductilities. The results show that: (i) high elongations are achieved only after each sample has been subjected to a sufficiently high strain to establish a homogeneous array of grains separated by high angle boundaries and (ii) elongations are a maximum when using processing route B C in which the sample is rotated by 90° in the same sense between each consecutive pressing through the ECAP die.

An Evaluation of the Flow Behavior during High Strain Rate Superplasticity in an Al-Mg-Sc Alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 µm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10−2 s−1. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys.

Influence of scandium on superplastic ductilities in an Al–Mg–Sc alloy

Ultrafine grain sizes, of the order of approximately 0.2 ?m, may be introduced into Al-Mg-Sc alloys by subjecting the material to severe plastic deformation through the process of equal-channel angular pressing (ECAP). Experiments were conducted to evaluate the influence of the solution treatment temperature on the ductility of an Al-3% Mg-0.2% Sc alloy after ECAP. The results show the highest ductilities are achieved when the solution treatment temperature is within the narrow range of approximately 878 to about 883 K, immediately below the temperature associated with the onset of partial melting. These high temperatures serve to maximize the amount of scandium in solid solution and this leads, on subsequent heating, to an extensive precipitation of fine secondary Al3Sc particles which inhibit grain growth at the higher temperatures. Conversely, solution treatments at temperatures below approximately 878 K give less Sc in solid solution within the matrix and the precipitation of the Al3Sc particles is then insufficient to retain a uniform ultrafine microstructure.