Toshiji Mukai

The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

Abstract Fine-grained alloys of Mg-3Al-1Zn-0.2Mn in wt.% (AZ31B) were obtained by an equal-channel angular extrusion technique and subsequent annealing at elevated temperatures. Tensile tests were performed at room temperature at a strain rate of 1x10-3 s-1. The alloys exhibited an apparent steady-state deformation region and a large tensile elongation of 47%. The deformed microstructure at an elongation of 2% indicated substantial cross-slip to non-basal planes induced by plastic compatibility stress associated with grain boundaries. The non-basal segment of dislocations was found to consist of 40% of the total dislocation density at a yield anisotropy factor of only 1.1 instead of an expected value of 100 obtained from single-crystal experiments. The deformed microstructure at an elongation of 16% indicated recovered regions within twins as well as untwinned matrices. These results indicate that dynamic recovery can occur in Mg alloys at room temperature.

Deformation mechanism in a coarse-grained Mg-Al-Zn alloy at elevated temperatures

Deformation behavior of a coarse-grained AZ31 magnesium alloy was investigated at elevated temperatures using commercial rolled sheet. The as-received material had equiaxed grains with an average grain size of 130 μm. The tensile tests revealed that the material exhibited high ductility of 196% at 648 K and 3×10−5 s−1. Stress exponent, grain size exponent and activation energy were characterized to clarify the deformation mechanism. It was suggested from the data analysis that the high ductility was attributed to the deformation mechanism of glide-controlled dislocation creep. In addition, constitutive equation was developed for the present alloy.

Superplastic deformation mechanism in powder metallurgy magnesium alloys and composites

Abstract The parametric dependencies for superplastic flow in powder metallurgy (PM) magnesium alloys and composites were characterized so as to elucidate the deformation mechanism. The mechanism was proposed to be slip accommodated grain boundary sliding. However, the PM alloys and composites were strengthened at low temperatures below ∼550K. This was different from the case in ingot metallurgy (IM) magnesium alloys, that behaved identically over a wide range of temperatures. The critical strain rate, below which the effect of intragranular particle is lost, was developed by considering the dislocation–particle interaction during slip accommodation process. It was suggested that the diffusional relaxation around the intragranular oxide particles was not completed during the slip accommodation process at low temperatures, and this caused the dislocation pile-up at the intragranular particles. It was expected that the dislocation pile-up at the intragranular particles would contribute to the strengthening at low temperatures in PM alloys and PM composites.

Experimental study for the improvement of crashworthiness in AZ91 magnesium foam controlling its microstructure

Abstract Metallic foams are expected to be used as the impact energy absorber material because of their unique deformation characteristics, which almost constant compressive stress appears in a wide range of strain. This phenomenon is well known as the regime of collapse plateau. It is very important to know strain rate dependence of the plateau stress, and the impact energy for suitable design of automotive components. Only limited amount of mechanical response data of metallic foams under dynamic loading are, however, available comparing with those of polymeric foams. In this study, the absorbed energy of an open-celled magnesium foams with a relative density of 0.03–0.06 is evaluated at a dynamic strain rate of ∼103 s−1 in compression by using the split Hopkinson pressure bar apparatus. In order to investigate the effect of microstructure in the solid material, solution treatment and aging are performed to all the specimens and then examined for the same strain rates. Peak stress and plateau stress per (relative density)3/2 for as-received and heat treated AZ91 foams showed the strain rate dependence, which decreased by the heat treatment. Therefore, it is possible to control the absorption energy of the AZ91 metallic foam by means of microstructural improvement, which controls the ductility in the solid material.

Compressive deformation behavior of Al2O3 foam

Deformation behavior of an Al2O3 foam with an open-cellular structure was investigated by compressive tests. The variation in flow stress with strain was significantly large and there was no densification region. Breakage of the columns is responsible for the large variation in flow stress with strain and no densification region. The relative stress of the Al2O3 foam was lower than the value predicted by Gibson and Ashby. This is probably because of the high degree of cracking in the columns and the presence of partial closed-faces.

Experimental prediction of deformation mechanism after continuous dynamic recrystallization in superplastic P/M7475

The deformation mechanism in high-strain-rate superplastic P/M7475 before and after continuous dynamic recrystallization (CDRX) was investigated. The recrystallization process in P/M7475 differed from that in conventional superplastic material, I/M7475. In I/M7475, the fine-grained microstructure was obtained by static recrystallization before deformation. On the other hand, the substructure in P/M7475 evolved into fine grains during deformation by CDRX. The percentage of high-angle and random boundaries was low at an initial stage of deformation. However, it increased with strain in P/M7475. The microstructural change in P/M7475 influenced a deformation mechanism and affected grain boundary sliding (GBS). The ratio of contribution of GBS to total elongation was low at an early stage of deformation in P/M7475. However, it increased with deformation progressed. It is suggested that the deformation behavior in P/M7475 changed from dislocation creep to superplasticity as the dominant deformation mechanism changed to GBS. The activation energy for superplastic flow in P/M7475 was close to that for lattice self-diffusion in pure aluminum. It is therefore concluded that the dominant deformation mechanism after CDRX in P/M7475 is GBS accommodated by dislocation movement controlled by lattice self-diffusion, similar to that in I/M7475.

Dynamic Deformation of Regularly Cell-Structured Materials

The open cell-structured materials are high-lighted as an effective shock-energy absorber for car crashworthiness. Without reliable, optimum cell-structured materials design, they are difficult to use in practice. Dynamic response of open cell-structured materials is investigated to understand the effect of geometric configuration, ductility and cell size on their uniaxial compression behavior. Copper cell-structured materials are employed to describe the effect of topological regularity on the static and dynamic deformation behavior. Essential difference is recognized between normally and regularly cell-structured materials on the dynamic/static stress ratio and the strain-rate sensitivity. Topological design of cells becomes important for open cell-structured materials.