Mitsuaki Furui

Room-Temperature Superplasticity in an Ultrafine-Grained Magnesium Alloy

Superplasticity, a phenomenon of high tensile elongation in polycrystalline materials, is highly effective in fabrication of complex parts by metal forming without any machining. Superplasticity typically occurs only at elevated homologous temperatures, where thermally-activated deformation mechanisms dominate. Here, we report the first observation of room-temperature superplasticity in a magnesium alloy, which challenges the commonly-held view of the poor room-temperature plasticity of magnesium alloys. An ultrafine-grained magnesium-lithium (Mg-8 wt.%Li) alloy produced by severe plastic deformation demonstrated 440% elongation at room temperature (0.35 Tm) with a strain-rate sensitivity of 0.37. These unique properties were associated with enhanced grain-boundary sliding, which was approximately 60% of the total elongation. This enhancement originates from fast grain-boundary diffusion caused by the Li segregation along the grain boundaries and the formation of Li-rich interphases. This discovery introduces a new approach for controlling the room-temperature superplasticity by engineering grain-boundary composition and diffusion, which is of importance in metal forming technology without heating.

Multiple deformation effect on the ductility of chromium

The purpose of this paper is to present a remarkable effect of the deformation procedure on the ductility of chromium in air at the temperature just below the ductile-brittle transition temperature (DBTT). The experiments were carried out with not only chromium, but also molybdenum and tungsten, all of which are bcc metals of the 6A group in the periodic table. Furthermore, the relationship between the deformation procedure and test environments was investigated experimentally for chromium. All these experiments were performed using a small punch (SP) test.

Microstructure Observation of AM60 Magnesium Alloy Solidified by Rapidly Quench

In solidification theory, with a slow cooling rate such as sand mold casting, it is easy to segregate the solute aluminum near the grain boundary of primary α-Mg phase under the solidification in Mg-Al system alloys. Thus, volume fraction of none-equilibrium crystallized β-Mg17Al12 phase showed the higher value compared with metal mold casting with faster cooling rate. However, in our microstructure observation results, the volume fraction of β phase in permanent mold castings was larger than that of sand mold castings. In the present study, these contradictory behavior was investigated by observation of as-solidified microstructure obtained from rapid cooling castings at the just below the solidus temperature of 723, 773 and 823K.

Microstructure Evolution in AZ31 Magnesium Alloy Worked by Torsion at Warm Temperature

It had been already reported that the resistance of compression at warm condition can be decreased by the preliminary torsion working at AZ31B magnesium alloy. In the present study, it was found that the dynamic recrystallization occured during warm working by torsion. Dynamic recrystallization was slightly seen in the fractured edge of the bar at a rotation speed of 1rpm at temperature 573K and 623K. The amount of torsion to fracture was increased with increasing of deformation temperature. Remarkable dynamic recrystallization could be seen in the center of bar at the rotation speed of 1rpm at temperature of 673K.

Screw Form Rolling of Beta Type Titanium Alloy Preliminary Worked by Torsion

Beta type titanium alloys in a cold processability are light, have high strength, excellent corrosion resistance and the same level as Youngs modulus of human bone. Therefore, beta type titanium alloys are used for plant facilities such as nuclear plants, architectural materials, aircraft, car, biomaterial, medical equipment, glasses and golf club head, etc. Microstructure and mechanical properties of beta type titanium alloys processed by rolling and heat treatment have been reported [1]. Additionally, screw form rolling using beta type titanium alloys has also been reported [2]. However, the development in those characteristics after the preliminary working by torsion has been unknown.

Microstructure and Aging Behavior in AM60 Magnesium Alloy Cast into Sand and Permanent Molds

AM60 magnesium alloy castings gave the solution treatment at 688K for 86.4ks. After that, aging treatment was carried out at three temperatures of 473, 498 and 523K. The age hardening curve obtained, hardness of all the specimens in the condition of peak aging was increased by decreasing the aging temperature. In the condition of long aging time, a cellular precipitation grows up from grain boundary to crystal grain. Fine cellular precipitation and intergranular precipitation obviously occurs at the lower aging temperature.

Superplastic Deformation of a Mg-8% Li Alloy Processed at Room Temperature by ECAP

Significant phase refinement and dispersion were achieved in a cast Mg-8% Li alloy through processing by ECAP using a die having an internal channel angle of 135° and a pressing temperature of 293 K. Following extrusion and subsequent ECAP through 4 passes, the alloy exhibited superplasticity at a testing temperature of 473 K. A maximum fracture elongation of 1610% was attained with an initial strain rate of 1.5 x 10 -4 s -1 . The strain rate sensitivities under the optimum superplastic conditions were measured as about 0.6. The maximum elongation achieved in this investigation is very high by comparison with most Mg alloys tested in tension at similar temperatures and strain rates.

The Effect of Grain Size and Amount of β Phase on the Properties of Back-Torsion Working in 60/40 Brass

It was limited to 60/40 brass and the adjustment of grain size was performed, thereafter, it was processed by one way torsion working and back-torsion working. The effect of both workings on the work hardening and the surface roughness of 60/40 brass were investigated. The quenched material has excellent surface roughness in comparison with the furnace-cooled material both after and before workings. It seemed that a little difference in their grain size (15-55μm) affects the restoration ratio of surface roughness. For the back-torsion worked material, as the grain size is coarse, the amount of hardening increases. If the β phase ratio is more, it is easy to become hardened. Inversely, if the β phase ratio is less, it obtains the maximum amount of hardening.

Aging Property of AZ91D Magnesium Alloy Screw Thread-Rolled at Room Temperature Using Extrusion-Torsion Simultaneous Processing

We have proposed a new extrusion process functionally combined with torsion. Extrusion-torsion simultaneous processing is a very attractive technique for fabricating a rod-shape material with high strength and excellent workability. To improve the hardness, the aging treatment was performed with AZ91D magnesium alloy screw thread-rolled at room temperature using extrusion-torsion simultaneous processing. The distribution of hardness from the tip to center in as thread-rolled screw was modified to uniform distribution by the isothermal aging treatment at 423 K for 460.8 ks. The peak hardness was not depended on the working temperature and rotation speed during extrusion-torsion simultaneous processing. β-Mg17Al12 precipitates are obviously grown in as peak-aged condition comparing with as thread-rolled condition.

Room Temperature Screw Form Rolling of AZ91D Magnesium Alloy through Processing by Extrusion-Torsion Simultaneous Working

Extrusion-torsion simultaneous processing is a very attractive technique for fabricating a rod-shape material with fine grain and random texture. We have proposed a new screw form rolling process combined with preliminary extrusion-torsion simultaneous working. Microstructure evolution and mechanical property change of AZ91D magnesium alloy during extrusion-torsion simultaneous processing was examined through microstructure observation, X-ray diffraction analysis and micro-Vickers hardness measurement. By the addition of torsion, the crystal orientation of AZ91D magnesium alloy workpiece was drastically changed from basal crystalline orientation to the random orientation. Crystal grain occurred through the dynamic recrystallization and tended to coarsen with an increase of extrusion-torsion temperature. Grain refinement under 2 um was achieved at the lowest extrusion-torsion temperature of 523 K. M8 gauge AZ91D magnesium alloy screw was successfully formed at room temperature using the extrusion-twisted workpiece preliminary solution treating at 678 K for 345.6 ks. It was found that the extrusion-torsion temperature of 678 K must be selected to fabricate the good screw without any defects.