Dong Hyuk Shin

Microstructural Evolution and Mechanical Properties of Ultrafine Grained Commercially Pure 1100 Aluminum Alloy Processed by Accumulative Roll-Bonding (ARB)

The ARB process has been carried out up to seven cycles on a commercial purity 1100 aluminum alloy to obtain ultra-fine grains with the average grain size of 500 nm. Microstructural evolution of the ARB processed aluminum alloy was examined by a transmission electron microscopy as a function of accumulated total strain. Mechanical properties including hardness, tensile property, and sliding wear characteristics of the severely deformed Al alloy were also investigated. Grain boundaries of the ARB processed alloy were diffusive and poorly defined after the initial ARB cycles, however they changed to well-defined high angle boundaries with the increase of the accumulated strain. Though hardness and strength of the ARB processed alloy were enhanced significantly, wear resistance of the processed alloy hardly increased. The mechanical properties were discussed in connection with the microstructure.

Sliding Wear Characteristics of Ultrafine-Grained Non-Strain-Hardening Aluminum-Magnesium Alloys

Coarse grains of commercial 5052 Al and 5083 Al alloys were refined by the accumulative roll bonding (ARB) process. Average grain size of the refined microstructure was 200 nm. The 5083 Al alloy that has higher Mg content required more deformation for the refinement. Dry sliding wear behavior of the ultra-fine grained (UFG) Al alloys was investigated using a pin-on-disk wear tester at room temperature. The UFG microstructure of the processed alloys hardly increased the wear resistance of the Al alloys in spite of the increased strength and hardness. Wear rate of the UFG Al alloys was higher than that of the non processed coarse-grained starting alloys. The SEM observation of worn surfaces revealed that surface deformation controlled the wear. The low wear resistance of the UFG Al alloys was attributed to non-equilibrium and unstable grain boundaries and low strain hardening capability of the alloys.

Effect of Rolling Direction on the Microstructure and Mechanical Properties of Accumulative Roll Bonding (ARB) Processed Commercially Pure 1050 Aluminum Alloy

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.

Effect of deformation temperature on the formation of ultrafine grains in the 5052 Al alloy

Effects of the annealing temperature on microstructures and mechanical properties of 5052 Al alloy that have received 88% reduction at cryogenic temperature were investigated for an annealing temperature range of 150–300°C, in comparison with those at room temperature. Equiaxed grains, approximately 200nm in diameter, were observed in 5052 Al alloy deformed 88% and annealed at 200°C for 1 h. When compared with the deformation at room temperature, the deformation at cryogenic temperature showed higher strengths and equivalent elongation after annealing at temperatures below 200°C. However, for annealing above 250°C, materials deformed at cryogenic temperature showed lower strength than those deformed at room temperature. This behavior might be attributable to the higher rate of recrystallization and growth in materials deformed at cryogenic temperature during annealing, due to the lager density of dislocations accumulated during the deformation.

Ultrafine Grained Dual Phase Steels Fabricated by Equal Channel Angular Pressing

Ultrafine grained (UFG) ferrite-martensite dual phase steels were fabricated by equal channel angular pressing and subsequent intercritical annealing. Their room temperature tensile properties were examined and compared to those of coarse grained counterpart. The formation of UFG martensite islands of ~ 1 μm was not confined to the former pearlite colonies but they were uniformly distributed throughout UFG matrix. The strength of UFG dual phase steels was much higher than that of coarse grained counterpart but uniform and total elongation were not degraded. More importantly, unlike most UFG metals showing negligible strain hardening, the present UFG dual phase steels exhibited extensive rapid strain hardening.

Superplastic Deformation of Ultrafine Grained Al Alloy Processed by ECAP and Post-Rolling

Superplastic behavior of an ultrafine grained (UFG) 5154 Al alloy processed by ECAP and cold rolling (ECAP+CR sample) was investigated and compared with that of the alloy processed by only ECAP without rolling (ECAP sample) in the strain rate range of 10-4~5×10-1 s-1 at 723 K. Processing of the ECAP+CR sample consisted of ECAP of 4 passes, which was less than that showing the optimum microstructure for high strain rate superplasticity of UFG Al alloys (i.e. 8 passes), with route Bc and subsequent cold rolling (70% thickness reduction). The superplastic elongation was remarkably enhanced by post-rolling. An analysis of the mechanical data revealed that deformation of the ECAP+CR sample was dominated by grain boundary sliding, but dislocation viscous glide was the main deformation mechanism for the ECAP sample. In addition, cavitation in the ECAP+CR sample was insignificant up to ∼300% elongation.

Monte-carlo modeling of grain growth in Zr equal channel angular pressed and recrystallized

Grain boundary character distribution in equal-channel-angular pressed Zr was studied. Using a die design of 90°/20° and an operation temperature of 350°C. The initial grain size of 20 μm was reduced to about 270 nm with 4 passes via route Bc. The grain growth kinetics of the recrystallized state was obtained by experiment and Monte-Carlo computer simulation, respectively, which showed good agreement. Based on kinetics and morphological characteristics, it was concluded that the grain coarsening mechanism was governed by normal grain growth. No sign of abnormal grain growth was detected either in the experiment or in simulation despite taking into consideration anisotropy in grain boundary energy as well as its mobility. This indicates that grain boundaries produced by severely deformed Zr are stable against explosive coarsening. The evolution characteristics of the microstructure in the present ECA pressed and recrystallized Zr differed from those of cold rolled Ti in that the grain boundary misorientation distribution and texture were rather stable during grain growth.

Tribological characteristics of coarse and ultra-fine grained ferrite-martensite dual phase steel fabricated by equal channel angular pressing

The dual phase steel, which consists of hard martensite islands embedded in a ductile ferrite matrix, is known to possess high strength, toughness, and superior wear resistance. However, the detailed wear mechanism of the steel has not yet been understood thoroughly. In the present study, dry sliding friction and wear characteristics of an ultra-fine grained ferrite-martensite dual phase steel has been investigated at room temperature. Wear tests of the steel were carried out using a pin-on-disk wear tester against an AISI 52100 bearing steel ball at loads ranging from 1N to 10N. Normalizing heat treatment was also performed on the steel to produce a ferrite-pearlite microstructure, and the wear characteristics of the normalized specimen were compared with that of the dual phase steel. The dual phase steel exhibited lower wear rates than the normalized steel, but the steady-state friction coefficients of the two steels were similar. The wear of the dual phase steel proceeded with a tribochemical reaction on the wearing surface accompanied with subsurface strain hardening, which explained the lower wear rate of the steel.

Mechanical Behaviors of Ultrafine Grained Metallic Materials

Ultrafine grained materials fabricated by severe plastic deformation exhibit both superior and inferior mechanical properties, as the prominent structural materials, compared to coarse grained counterparts. The superior mechanical properties are ultrahigh strength and exceptional ductility at high temperatures (i.e., superplasticity). The inferior mechanical properties are lack of strain hardenability and room temperature ductility. In this study, the relationship between microstructure and mechanical properties of ultrafine grained materials fabricated by severe plastic deformation is investigated in order to provide insight broadening their future applicability.