Eun-Yoo Yoon

Ultrafine Grained Cu-diamond Composites using High Pressure Torsion

【In this work, powder metallurgy and severe plastic deformation by high-pressure torsion (HPT) approaches were combined to achieve both full density and grain refinement at the same time. Pure Cu powders were mixed with 5 and 10 vol% diamonds and consolidated into disc-shaped samples at room temperature by HPT at 1.25 GPa and 1 turn, resulting in ultrafine grained metallic matrices embedded with diamonds. Neither heating nor additional sintering was required with the HPT process so that in situ consolidation was successfully achieved at ambient temperature. Significantly refined grain structures of Cu metallic matrices with increasing diamond volume fractions were observed by electron backscatter diffraction (EBSD), which enhanced the microhardness of the Cu-diamond composites.】

Densification and Nanocrystallization of Water-Atomized Pure Iron Powder Using High Pressure Torsion

In this study, powder metallurgy and severe plastic deformation by high-pressure torsion (HPT) approaches were combined to achieve both full density and grain refinement at the same time. Water-atomized pure iron powders were consolidated to disc-shaped samples at room temperature using HPT of 10 GPa up to 3 turns. The resulting microstructural size decreases with increasing strain and reaches a steady-state with nanocrystalline (down to ~250 nm in average grain size) structure. The water-atomized iron powders were deformed plastically as well as fully densified, as high as 99% of relative density by high pressure, resulting in effective grain size refinements and enhanced microhardness values.

Consolidation and Mechanical Behavior of Gas Atomized MgZn 4.3 Y 0.7 Alloy Powders using High Pressure Torsion

In this paper, rapid solidified Mg-4.3Zn-0.7Y (at.%) alloy powders were prepared using an inert gas atomizer, followed by a severe plastic deformation technique of high pressure torsion (HPT) for consolidation of the powders. The gas atomized powders were almost spherical in shape, and grain size was as fine as less than 5 μm due to rapid solidification. Plastic deformation responses during HPT were simulated using the finite element method, which shows in good agreement with the analytical solutions of a strain expression in torsion. Varying the HPT processing temperature from ambient to 473 K, the behavior of powder consolidation, matrix microstructural evolution and mechanical properties of the compacts was investigated. The gas atomized powders were deformed plastically as well as fully densified, resulting in effective grain size refinements and enhanced microhardness values.

Densification of Copper Powders using High-pressure Torsion Process

In this study, electrolytic copper powders were consolidated by high-pressure torsion process (HPT) which is the most effective process to produce bulk ultrafine grained and nanocrystalline metallic materials among various severe plastic deformation processes. The bulk samples were manufactured by the HPT process at 2.5 GPa and 1/2, 1 and 10 turns. After 10 turns, full densification was achieved by high pressure with shear deformation and ultrafine grained structure (average grain size of 677 nm) was observed by electron backscatter diffraction and a scanning transmission electron microscope.

Investigation of Microhardness and Microstructure of AZ31 Alloy after High Pressure Torsion

Cast commercial magnesium alloy AZ31 was processed by high pressure torsion (HPT) at room temperature for 1, 3, 5 and 15 rotations (strain ranged from 1 to 7). Micro structure evolution with strain imposed by HPT was observed by light and electron microscopy. HPT was shown to be a very effective method of grain refinement. The initial coarse grain structure was refined by a factor of almost 200 already after one HPT turn (e ≈ 4). Mechanical properties were investigated by detailed 2-D microhardness measurements. HPT straining was found to introduce a radial inhomogeneity in the material which is manifested by a pronounced drop in the center and the maximum near the specimen periphery. With increasing strain due to HPT this inhomogeneity is continuously smeared out tending to saturate with increasing strain. Integrated 3-D meshes across the total surface of disks revealed the undulating character of microhardness variations. The strain imposed by HPT was shown to saturate with increasing number of HPT turns.

Quantitative Analysis of Roughness of Powder Surface Using Three-Dimensional Laser Profiler and its Effect on Green Strength of Powder Compacts

Green strength is an important property of powders since high green strength guarantees easy and safe handling before sintering. The green strength of a powder compact is related to mainly mechanical and surface characters, governed by interlocking of the particles. In this study, the effect of powder surface roughness on the green strength of iron powders was investigated using a transverse rupture test. Three-dimensional laser profiler was employed for quantitative analyses of the surface roughness. Two different surface conditions, i.e. surface roughness, of powders were compared. The powders having rough surfaces show higher green strength than the round surface powders since higher roughness leads increasing interlocked area between the contacting powders.

Microstructure and Thermal Stability of Copper - Carbon Nanotube Composites Consolidated by High Pressure Torsion

Blends of Cu powders and 3 vol. % carbon nanotubes (CNTs), and an additional sample from pure Cu powder were consolidated by High Pressure Torsion (HPT) at room temperature (RT) and 373 K. The grain size, the lattice defect densities as well as the hardness of the pure and composite materials were determined. Due to the pinning effect of CNTs, the dislocation density is about three times larger, while the grain size is about half of that obtained in the sample consolidated from the pure Cu powder. The increase of the HPT-processing temperature from RT to 373 K resulted in only a slight increase of the grain size in the Cu-CNT composite while the dislocation density and the twin boundary frequency were reduced significantly. The flow stress obtained experimentally agrees well with the value calculated by the Taylor-formula indicating that the strength in both pure Cu and Cu-CNT composites is determined mainly by the interaction between dislocations. The addition of CNTs to Cu yields a significantly better thermal stability of the UFG matrix processed by HPT.