Papers by Keyword: Ultrafine Grained

Abstract: The effect of working temperature on microstructure and mechanical properties of ultrafine grained monolithic Al and Al-5vol.%SiCp composite processed by accumulative roll bonding (ARB) was studied. The ARB was performed up to eight cycles (an equivalent strain of ~6.4) without lubricant. The working temperature was varied from ambient temperature to 200
C. The samples processed at temperatures below 100
C exhibited an ultrafine grained structure over almost all regions. However, the samples processed at 200
C showed an inhomogeneous structure in which a few coarse grains due to an occurrence of conventional recrystallization is partially seen. The tensile strength of both the monolithic Al and the composite decreased with increasing the ARB working temperature. The variation of microstructure and mechanical properties of the composite with the working temperature was compared to that of the monolithic aluminum.

Abstract: Accumulative roll-bonding (ARB) process was applied to an oxygen free copper for improvement of the mechanical properties via ultra grain refinement to nanometer order level. Two copper sheets 1mm thick, 30mm wide and 300mm long are degreased and wire-brushed for sound bonding. The sheets are then stacked to each other, and cold-roll-bonded by 50% reduction rolling. The sheet is then cut to the two pieces of same length and the same procedure was repeated to the sheets. The ARB process up to eight cycles (an equivalent thickness strain of 6.4) is successfully performed at ambient temperature. TEM observation reveals that ultrafine grains, hardly containing the dislocation interior, begin to develop at the third cycle, and after the sixth cycle they cover most of regions of samples. The morphology of ultrafine grains formed is different from that of aluminum alloys. Tensile strength of the ARB-processed copper increases with the equivalent strain up to a strain of ~3.2, in which it reached 390 MPa, ~2.1 times higher than the initial value. However, the strength hardly changed at the strain above ~3.2.

Abstract: An ultra-fine grained microstructure was obtained in high purity nickel by a combination of (a) equal-channel angular pressing (ECAP) and (b) hydrostatic extrusion (HE) with a cumulative true strain of ~11.2. The resulting microstructure was examined by light and TEM microscopy. Mechanical properties have been measured by tensile and hardness tests. It was found that HE of ECAP-ed samples leads to a significant grain size refinement (from 330 to 160nm) and to an increase in microstructural homogeneity. SPD nickel, made by a combination of the ECAP and hydrostatic extrusion methods, has high strength and ductility (i.e.: YS=1120MPa and εf = 11%). The microstructure transformation was accompanied by a strength increase of 78% compared to ECAP alone. The results obtained fit well with the Hall-Petch relationship. A combination of ECAP and HE has achieved much better properties than either single process and show it to be a promising procedure for manufacturing bulk UFG nickel.

Abstract: Recently the method for obtaining ultra-fine grained metallic materials has developed using severe plastic deformation (SPD), such as equal channel angular pressing (ECAP), accumulative roll bonding (ARB), torsion straining, and warm multiple deformation (WMD) etc. In order to enhance thermal stability of ultra-fine grained aluminum alloys manufactured by SPD process, the addition of Sc and Zr elements has been considered to devise fine Al3Sc, Al3Zr and Al3(Scx Zr1-x) precipitates for inhibiting the grain growth. In this study, the microstructure evolution has been investigated in Al-Mg alloys with and without Sc and Zr addition during the warm multiple deformation process. In addition Al-Mg alloys were compressed at a strain rate of 10-1 sec-1 by two different routes, that is, route A and route B. Route A is to rotate the specimen throughout 90o around the vertical axis of loading direction at every pass. Route B is to rotate the specimen throughout 90o around the parallel axis of loading direction and then rotate it again as route A. The specimen deformed by route B had finer grain size and more uniform distribution of grains than those deformed by route A. When the warm multiple deformation process repeated up to 8 passes at 673 K, the specimen consisted of ultra-fine grained structure with the average grain size less than 3 μm. The superplastic behavior can also be observed at the high strain rate and low temperature regime.

Abstract: Accumulative Roll Bonding (ARB) is a technique of grain refinement by severe plastic deformation, which involves multiple repetitions of surface treatment, stacking, rolling, and cutting. The rolling with 50% reduction in thickness bonds the sheets. After several cycles, ultrafine-grained (UFG) materials are produced. Since ARB enables the production of large amounts of UFG materials, its adoption into industrial practice is favoured. ARB has been successfully used for preparation of UFG sheets from different ingot cast aluminium alloys. Twin-roll casting (TRC) is a cost and energy effective method for manufacturing aluminium sheets. Fine particles and small grain size are intrinsic for TRC sheets making them good starting materials for ARB. The paper presents the results of a research aimed at investigating the feasibility of ARB processing of three TRC alloys, AA8006, AA8011 and AA5754, at ambient temperature. The microstructure and properties of the ARB were investigated by means of light and transmission electron microscopy and hardness measurements. AA8006 specimens were ARB processed without any problems. Sound sheets of AA8011 alloy were also obtained even after 8 cycles of ARB. The AA5754 alloy suffered from severe edge and notch cracking since the first cycle. The work hardening of AA8006 alloy saturated after the 3rd cycle, whereas the hardness of AA5754 alloy increased steadily up to the 5th cycle. Monotonous increase in strength up to 280 MPa was observed in the ARB processed AA8011 alloy.

Abstract: The reaction between the zinc plate (ZP) and the IF steel with near surface ultra fine grains (NSUFG) structure with grain size of about 89 nm was studied in temperature range of 473K to 623K in order to elucidate the temperature dependence of the reactions and its mechanism, by comparison with the reactions of ZP to coarse grains (CG) sheet, superficial cold rolled CG sheet (CG+R) and superficial cold rolled NSUFG sheet (NSUFG+R). It was found that this NSUFG structure considerably affected reactions between IF steel and ZP. There was almost no effect of superficial cold rolling on their reactions, but the NSUFG structure dramatically enhanced the reactions. The incubation times for appearance of the reaction layer and its layer width of ZP /(NSUFG or NSUFG+R) reactions are shorter and thicker than those of the ZP/(CG or CG+R) ones. The activation energy for reaction was 107kJ/mol, which indicates that the volume diffusion in zinc side and the grain boundary diffusion in the iron side play an important role in the reaction. The layer growth up to the layer thickness less than about 10µm was controlled mainly by the interface reaction and it over about 10µm mainly by the diffusion mass transfer.

Abstract: Internal stress field in a severely deformed aluminium with ultrafine grained microstructure has been studied by convergent-beam electron diffraction (CBED) technique in transmission electron microscopy (TEM). A commercial purity aluminium (99.1%Al) sheet was highly strained by the accumulative roll-bonding (ARB) process to evolve an ultrafine grained structure. Higher-order Laue zone (HOLZ) lines in the incidence disk of the ] 12 1 [ zone axis have been observed at various positions within an identical ultrafine grain. The key finding is that the HOLZ line pattern taken from the vicinity (~50nm) of the grain boundary (lamellar boundary) looses ) 1 1 0 ( mirror symmetry, whereas the pattern from the grain centre has the symmetry. The former and the latter represent the existence of a large non-hydrostatic stress field and a small internal stress field, respectively. The magnitude of the internal stress becomes larger with approaching to a grain boundary.

Abstract: Ultra-fine grained AA8011 alloy sheets manufactured by the accumulative roll-bonding (ARB) process exhibited unique tensile deformation behavior. Tensile strength of the ARB processed AA8011 sheets increased up to three cycles, but then showed nearly the same value after three cycles. Meanwhile, the total elongation grew significantly with an increasing nember of ARB cycles. It was found that the strain-rate sensitivities (m) of the AA8011 sheets increased up to 0.047 by the ARB process. A large number of high-angle boundaries were introduced by the ARB process and the fraction of high-angle boundaries reached 70% after eight ARB cycles. In this paper, we discusse the increase in total elongation on the basis of strain-rate sensitive deformation of the material, which is also correlated with dynamic recovery.

Abstract: The cross-ARB (C-ARB) process, which adopts cross rolling of the two stacked plates, has been performed up to seven cycles on a commercial purity 1050 aluminum alloy to obtain ultrafine grains with an average grain size of 0.7μm. Microstructural evolution of the C-ARB processed aluminum alloy was examined by a transmission electron microscopy as a function of process cycle number (accumulated plastic strain). Tensile property of the severely deformed Al alloy was also explored. Grain size of grains of the C-ARB processed alloy varied across thickness of the rolled plate. The size of grains at the top and bottom of the rolled plate converged to 0.65μm, while that of grains at the center of the plate increased with the number of ARB cycles. Tensile strength of the CARB processed 1050 Al alloy increased from 100MPa (as-received) to 160MPa. Tensile elongation varied with the number of cycles, but 15% of failure strain was measured from the 6-cycle C-ARB processed specimen. The variation of the elongation with the cycle number coincided exactly with the variation of grain size at the center of the processed plate.