Wonbaek Kim

Texture development and its effect on mechanical properties of an AZ61 Mg alloy fabricated by equal channel angular pressing

Abstract Microstructure and texture development of an AZ61 Mg alloy during equal channel angular pressing (ECAP) was investigated and correlated with the mechanical properties. The microstructure was effectively refined by ECAP, and the original fiber texture of the extruded AZ61 alloy was disintegrated and a new texture was gradually developed by repetitive ECAP pressing. After 8 ECAP passes following route Bc, the yield stress is lower than for the as-extruded AZ61 alloy, indicating that the texture softening is dominant over the strengthening due to grain refinement. When route A was used, on the other hand, the yield stress slightly increased after 8 passes. This result is primarily due to a difference in texture. The dominant textures after 8 passes were ( 10 1 1 )[ 0 1 11 ] + ( 10 1 2 )[ 1 2 1 0 ] and ( 10 1 2 )[ 1 2 1 0 ] when processed by route Bc and route A, respectively. Tensile ductility increased after ECAP and the effect of ECAP on ductility is more remarkable when the initial grain size is large.

Mechanical properties and microstructures of an AZ61 Mg Alloy produced by equal channel angular pressing

The negative slope of yield stress versus d−1/2 in ECAPed AZ61 alloys can be explained by the gradual transition of the texture during repetitive equal channel angular pressing (ECAP) to control the grain size. Electron backscatter diffraction results of the present study show that texture modification occurs during ECAP.

Optimization of strength and ductility of 2024 Al by equal channel angular pressing (ECAP) and post-ECAP aging

Abstract Equal channel angular pressing (ECAP) in the solid solution state with post-ECAP low-temperature aging treatment was found to be quite effective in enhancing the mechanical properties of 2024 Al alloy. Yield stress (YS) as high as 630 MPa could be achieved. Moreover, the ductility of ECAPed alloy was improved after the aging treatment.

Effect of aging treatment on heavily deformed microstructure of a 6061 aluminum alloy after equal channel angular pressing

Abstract Pre-ECAP solid-solution treatment combined with post-ECAP aging treatment has been found to be more effective than pre-ECAP peak-aging treatment in enhancing the strength of a 6061 Al alloy. An increase of ∼40% in UTS and yield stress was obtained in the post-ECAP aged material compared to the T6 treated commercial 6061 Al alloy.

A new method for the identification and quantification of magnetite-maghemite mixture using conventional X-ray diffraction technique.

The electrical explosion of Fe wire in air produced nanoparticles containing the binary mixture of magnetite (Fe(3)O(4)) and maghemite (γ-Fe(2)O(3)). The phase identification of magnetite and maghemite by the conventional X-ray diffraction method is not a simple matter because both have the same cubic structure and their lattice parameters are almost identical. Here, we propose a convenient method to assess the presence of magnetite-maghemite mixture and to further quantify its phase composition using the conventional peak deconvolution technique. A careful step scan around the high-angle peaks as (511) and (440) revealed the clear doublets indicative of the mixture phases. The quantitative analysis of the mixture phase was carried out by constructing a calibration curve using the pure magnetite and maghemite powders commercially available. The correlation coefficients, R(2), for magnetite-maghemite mixture was 0.9941. According to the method, the iron oxide nanoparticles prepared by the wire explosion in this study was calculated to contain 55.8 wt.% maghemite and 44.2 wt.% magnetite. We believe that the proposed method would be a convenient tool for the study of the magnetite-maghemite mixture which otherwise requires highly sophisticated equipments and techniques.

Improvement of high-cycle fatigue life in a 6061 Al alloy produced by equal channel angular pressing

In the present study, mechanical properties and fatigue behavior of solid solution treated 6061 Al alloy fabricated by equal channel angular pressing (ECAP) process were evaluated. Yield stress and ultimate tensile stress significantly increased after ECAP. A remarkably large enhancement in fatigue life, by a factor of about 10, compared to a T6 treated commercial 6061 Al alloy was recognized to occur in low and high cycle regimes after a single pass. Further deformation by ECAP, however, virtually eliminated this improvement especially in the high cycle regime. Fine-grained microstructure with low-grain boundary misorientation angles was proposed to yield the best result in fatigue performance in a 6061 Al alloy. The current result suggests that one may pay attention to a single passed material rather than multi-passed material if improvement in fatigue life is a primary concern for successful engineering applications.

Effects of Mg concentration on the quasi-superplasticity of coarse-grained Al-Mg alloys

Strain rate-stress relationship and tensile ductility behavior of coarse-grained Al-Mg binary alloys containing 5.3, 7 and 11wt.% of Mg, respectively, were investigated. For all the three alloys, solute drag creep is rate-controlling deformation mechanism. As Mg concentration and temperature is decreased, however, dislocation climb creep arises as the rate-controlling deformation mechanism. The Al-7Mg alloy shows the best tensile ductility. The maximum tensile elongation upto 350% could be obtained from the Al-7Mg alloy at 450 °C and 1 × 10−2s−1. The Al-7Mg and Al-5.3Mg alloys exhibited reasonably high tensile elongations of up to 250% even at a very high strain rate of 1 × 10−1s−1.

Fabrication of Nanostructured Ti from Ti and TiH2 by Rapid Sintering and Its Mechanical Properties

Titanium has good deformability, high hardness, high biocompatibility, excellent corrosion resistance and low density. Due to these attractive properties, it has been used in many industrial applications. Dense nanostructured Ti was sintered from mechanically activated Ti and TiH2 powders by high frequency induction heating under pressure of 80 MPa. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. TiH2 powder was decomposed to Ti during sintering. The hardness of Ti increased and the average grain size of Ti decreased with increasing milling time. The average grain sizes of Ti samples sintered from Ti and TiH2 powder milled for 5 hrs were about 26 nm, 44 nm, respectively. The hardness of Ti sintered from Ti and TiH2 powder milled for 5 h was 504 kg/mm and 567 kg/mm, respectively. (Received June 1, 2011)

Flame-resistant Ca-containing AZ31 magnesium alloy sheets with good mechanical properties fabricated by a combination of strip casting and high-ratio differential speed rolling methods

This study reported that a combination of strip casting and high-ratio differential speed rolling (HRDSR) can produce flame-resistant Mg alloy sheets (0.7 wt%Ca-AZ31: 0.7Ca-AZ31) with good room-temperature mechanical properties and high-temperature formability. HRDSR effectively refined the coarse microstructure of the strip-casting processed 0.7Ca-AZ31 alloy. As the result, the (true) grain size was reduced to as small as 2.7 μm and the (Mg, Al)2Ca phase was broken up to fine particles with an average sizes of 0.5 μm. Due to the advantage of having such a highly refined microstructure, the HRDSR-processed 0.7Ca-AZ31 alloy sheet exhibited a high yield stress over 300 MPa and good superplasticity at elevated temperatures. The deformation mechanism of the fine-grained 0.7Ca-AZ31 alloy in the superplastic regime was identified to be grainboundary-diffusion or lattice-diffusion controlled grain boundary sliding.

Catalytic properties of Ni-Zn alloy prepared by mechanical alloying for steam reforming from methanol

Amorphous Zn65Ni35 alloy, the composition of which lies between β1-NiZn and γ-NiZn phases, was prepared by mechanical alloying for 200 hours. The alloy was heat treated at various temperatures and leached in NaOH solution in an effort to enhance the catalytic properties for hydrogen production from methanol. X-ray diffraction study revealed that the amorphous phase crystallized during the heat treatment to the equilibrium β1-NiZn and γ-NiZn phases. It was found that Zn65Ni35 alloy leached after heat treatment at 928 K showed the highest catalytic activity for steam reforming of methanol. It is believed that the enhanced catalytic activity of the Zn65Ni35 alloy heat treated at 928 K is due to the dispersed Ni particles on β1-ZnNi matrix which was formed during leaching of the γ-Zn21Ni5 phase.