Papers by Author: Daichi Akama

Abstract: Severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP), accumulative roll bonding (ARB) and high pressure torsion (HPT), have been extensively researched to achieve. SPD process makes use of the plastic deformation where no change in the cross-sectional dimension of a work piece occurs during straining. In this work, the effect of HPTed or rolled on aging behavior and microstructure in Al-Mg-Si alloys were investigated. The results show that processing by HPTed leads to significant grain refinement with a grain size of ~400nm. Age-hardening phenomenon was confirmed at 343K and 373K but not at 423K for the HPTed Al-Mg-Si alloy. For the rolled alloys, there is little difference between the hardness of the alloys with different rolling rate. On the other hand, crystal grain with high dislocation density and grain boundary precipitates are observed with TEM analysis for HPTed samples aged at 473 K.

Abstract: This research investigates simultaneous strengthening by grain refinement and fine precipitation in age-hardenable Al-Mg-Si alloys containing an additional element of either Ag, Cu, Pt or Pd. The alloys were solution-treated and processed by high-pressure torsion (HPT) at room temperature under a pressure of 6 GPa. They were aged at a temperature of 373 K for up to a total period of 6.7 hours. Vickers microhardness was measured after selected periods of aging and the microstructures were observed by transmission electron microscopy. It was found that, in all alloys, the grain sizes after HPT were refined to 300-400 nm and there were significant increases in the hardness through the HPT processing. The hardness was further increased by the subsequent ageing treatment, confirming the simultaneous strengthening by grain refinement and fine precipitation. However, the aging behavior was different depending on the alloying compositions.

Abstract: This study presents an application of high-pressure torsion (HPT) to an Al-Li-Cu-Mg alloy (2091). The alloy was subjected to solid solution treatment at 505oC for 30 minutes and was processed by HPT under 6 GPa for 5 revolutions at room temperature. The hardness increased with straining and saturated to a constant level at 225 Hv. Aging was undertaken on the HPT-processed alloy at 100, 150 and 190oC for the total periods up to 9.3 days. The aging treatment led to a further increase in the hardness to ~275 Hv. It is shown that the simultaneous strengthening of the alloy due to grain refinement and age hardening was successfully achieved by application of HPT and subsequent aging treatment. The enhancement of the strength is prominent when compared with the application of a conventional rolling process.