Koichi Ishikawa

Low temperature superplasticity of a fine-grained ZK60 magnesium alloy processed by equal-channel-angular extrusion

Abstract Fine-grained ZK60 magnesium alloy with the grain size of 1.4 μm was processed by equal-channel-angular extrusion. The material exhibited low temperature superplasticity. The normalized plot suggested that the present material had equilibrium grain boundaries at the superplastic temperature in contrast to typical materials processed by severe plastic deformation.

Realization of high-strain-rate superplasticity at low temperatures in a Mg-Zn-Zr alloy

Abstract Possibility of combination of high-strain-rate superplasticity and low-temperature superplasticity was experimentally confirmed using extremely fine-grained magnesium alloy. In order to achieve such a superior superplastic behavior, the required grain size and the suitable processing was considered. It was suggested that the required grain size of ≤≈0.4 μm can be obtained through the processing from powder metallurgy route, using rapidly solidified powder. Therefore, superplastic behavior was examined using powder metallurgy processed Mg–Zn–Zr alloy (ZK61). Tensile tests revealed that high-strain-rate superplasticity was obtained even at low temperatures of ≈473 K, which is corresponding to half the absolute melting point of the material. This was attributed to the fine grain size of 0.65 μm.

Experimental study of a structural magnesium alloy with high absorption energy under dynamic loading

It has been demonstrated that pure Mg exhibits low ductility under dynamic loading at room temperature owing to its HCP structure. Very limited data are currently available for magnesium alloys under dynamic loading. In order to be used for structural components, it is necessary to improve the mechanical properties of magnesium alloys. Lahaise et al. reported the yield strength of the AZ91 magnesium alloy increased with refining its microstructure. Mohri et al. has already been reported the ductility enhancement of a Mg-Y-RE(Rare Earth) alloy by hot extrusion. They mentioned the enhancement of ductility is due to the refining microstructure of magnesium. Thus refining microstructure enables to raise the possibility for the development of a structural magnesium alloy with high ductility at dynamic strain rate. In this paper, the possibility of a fine-grained WE43 magnesium alloy is investigated to raise the high speed impact performance against the foreign object damage by the enhancement of ductility and absorption energy under dynamic loading.

Influence of strain rate on the mechanical properties in fine-grained aluminum alloys

Abstract Fine-grained IN905XL aluminum alloys with five grain sizes between 0.8 and 8.1 μm have been developed by a combination of mechanical alloying and conventional extrusion in order to investigate the influence of the strain rate on the mechanical properties. Negative strain rate sensitivity of flow stress is observed up to 10 s −1 for all samples. Above the strain rate of 1 × 10 3 s −1 , however, all samples show the positive strain rate sensitivity of strength. Total elongation at high strain rates is generally larger than that at low strain rates. Flow stresses increase with decreasing grain size for all strain rates. The measured values of strength of the coarse grained IN905XL with sizes above 4.3 μm agree with the values estimated from a cooperation of the strengthening by the grain size refinement, magnesium solute atoms and oxides and carbides dispersion. In the Hall-Petch relations at high strain rates, the gradient of the curve increases with increasing reciprocal square root of the grain size. For fine-grained samples, therefore, an additional strengthening mechanism should be considered such as the difference in the characteristics of the boundary.

Consolidation of machined magnesium alloy chips by hot extrusion utilizing superplastic flow

An examination of consolidation conditions by hot extrusion of AZ31 magnesium alloy machined chips was conducted to enhance the bonding of individual chips, in order to improve the mechanical properties. Hot extrusions were carried out in the superplastic and non-superplastic region. Microstructural observations revealed that grain refinement was attained by extruding machined chips, and the grain sizes of the chip-extruded materials were smaller than 5 μm. The interfaces of individual chips of extruded materials were not identified when the chips were extruded in the superplastic region. The ultimate tensile strength was about 300 MPa and elongation-to-failure was about 10% for chip-extruded materials that were extruded in the superplastic region. These materials were comparable with the as-received alloy with respect to the room temperature strength, although the ductility was reduced to half. It was confirmed that chip consolidation utilizing superplastic flow is useful to enhance the bonding of individual grains.

Strength and ductility under dynamic loading in fine- grained IN905XL aluminum alloy

Three IN905XL aluminum alloys with fine grain (1 μm), intermediate grain (3 μm), and coarse grain (5 μm) have been developed by a combination of mechanical alloying (MA) and conventional extrusion in order to investigate their mechanical properties at dynamic strain rates of 1 × 103 and 2 × 103 s−1 and a quasi-static strain rate of 10-3 s−1. Flow stresses are found to increase with decreasing grain size for all the strain rates tested. Negative strain-rate sensitivity of flow stress is observed up to 1 × 103 s−1 in both intermediate- and coarse-grained IN905XL. At the highest strain rate of 2 × 103 s−1 however, all samples showed a positive strain-rate sensitivity of strength. Total elongation at high strain rates is generally larger than that at low strain rates. Total elongation also decreases with grain size for all the strain rates. This decrease in elongation results from an initiation of microcracks at interfaces between the matrix and particles finely dispersed near grain boundary regions, introduced during MA processing; then, this initiation leads elongation of alloys to small limited values.