Papers by Author: Zen Ji Horita

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 investigated to achieve. SPD techniques make use of plastic deformation process where no change in the cross-sectional dimension of a work piece occurs during straining. In this work, the effect of HPT on the aging behavior and microstructure in excess Mg-type Al-Mg-Si alloys including Cu are investigated. These alloys were investigated by hardness test and transmission electron microscopy (TEM) observation.The results show that processing by HPT leads to significant grain refinement with a grain size of 200-250nm.An age-hardening phenomenon is observed at 343K and 373K for the Al-Mg-Si alloys with HPT. Some precipitates were observed on grain boundaries.

Abstract: An Al 7075 alloy (5.63mass%Zn-2.56mass%Mg-1.68mass%Cu-0.21mass%Fe-0.19mass%Cr-0.14mass%Si-0.02mass%Ti with balance of Al) was processed by high-pressure torsion (HPT) under an applied pressure of 6 GPa for 1, 3 and 5 revolutions with a rotation speed of 1 rpm at room temperature. Vickers microhardness saturated to a level of 220 Hv after the HPT processing and the grain size was refined to 120 nm at the state of the hardness saturation. Tensile tests were conducted with initial strain rates from 2.0 × 10^-4 to 2.0 × 10^-2 s^-1 at temperatures as 200 °C and 250 °C (equivalent to 0.52T_m and 0.57T_m, respectively, where T_m is the melting point of the alloy). The HPT-processed samples for 3 revolutions exhibited superplastic elongations of 640% and 510% at 250 °C with initial strain rates of 2.0 × 10^-3 s^-1 and 2.0 × 10^-2 s^-1, respectively.

Abstract: The aging behavior of a cast Al-2 wt.% Fe alloy processed by High-Pressure Torsion (HPT) at room temperature was studied by subsequent aging treatments at 200 °C. Observations by Transmission Electron Microscopy (TEM) revealed that the microstructure after HPT processing reached an ultrafine-grained level with an average grain size in the Al matrix of ~120 nm. The initial eutectic structures were fragmented into particles with sizes of less than 400 nm and partially dissolved in the matrix up to a supersaturated Fe content of ~1% as confirmed by X-Ray Diffraction (XRD) analysis. The peak-age condition was achieved within 0.25 h of aging, which provides the maximum hardness of ~200 HV. Analyses by high-resolution S/TEM show that round particles of Al₆Fe with sizes of ~5-10 nm and semi-coherent with the matrix are the dominant precipitates in the peak-aged condition. The hardness increases by aging for 12 h above the as-HPT-processed level of 185 HV. The dominant precipitate phase transforms to Al₃Fe in the over-aged condition with a loss of coherency during growth. Enhanced precipitation kinetics was observed because of high density of lattice defects induced by the HPT processing, which were also confirmed by significant recovery in the electrical conductivity of the samples after aging.

Abstract: Dynamic interactions between plastic deformation and precipitation processes in Aluminium alloys can lead to large changes in precipitation kinetics as compared to conventional heat treatments. This can be due to a number of different mechanisms such as changes in vacancy concentrations or movement of structural defects. Understanding the underlying mechanisms requires a quantitative measurement of the precipitate evolution as a function of deformation conditions, temperature and time. The present contribution will review experimental results obtained by Small-Angle X-ray Scattering in three different configurations, all concerning Al-Zn-Mg-Cu alloys: first, dynamic precipitate evolution during plastic deformation at ageing temperature; second, dynamic precipitation during low cycle fatigue at room temperature; and third, precipitation during ageing of a material processed by room temperature high pressure torsion. The experimental evidence will be used to discuss the role of the different mechanisms of interaction with plasticity on the precipitate microstructure evolution.

Abstract: The effect of high-pressure torsion (HPT) processing on the microstructure and Vickers hardness of Co-Cr-Mo (CCM) alloys were investigated in this study. The microstructure of initial CCM alloy contains equiaxed grains with a grain diameter of approximately 50 μm and twins. The clear grain boundaries of equiaxed grains and twins disappear after HPT processing at a rotation number, N, of 10. The phase maps of initial CCM alloy and CCM alloy subjected to HPT processing at N = 5 measured by electron backscatter diffraction exhibit that the ratio of γ phase decreases from 93.5% to 34.1% and the ratio of ε phase increases from 6.5% to 65.9% by applying HPT processing. These results indicate that the ε phase is formed by high-strain, which is induced by the HPT processing. The Vickers hardness values on the surfaces of the CCM alloys subjected to HPT processing at N = 1, 5, and 10 increase with increasing the equivalent strain, ε_eq. These results suggest that an increase of Vickers hardness is correlated to an increase of the ratio of ε phase and the dislocation density, and grain refinement, which are caused by the high-strain induced by HPT processing.