Ha-Guk Jeong

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.

Effects of addition of magnesium on interface structure and high-strain-rate superplasticity in Si3N4-reinforced Al-alloy composites

Abstract The mechanical properties and interface structure of fine-grained Al–Cu alloy and Al–Cu–Mg alloy composites, reinforced with Si 3 N 4 whiskers (Si 3 N 4 w) and particles (Si 3 N 4 p), are investigated to reveal the role of magnesium addition in high-strain-rate superplasticity. The Al–Cu alloy composites, which exhibit lower elongations ( 3 N 4 crystals without any interfacial reaction. On the other hand, the interfaces in the Al–Cu–Mg alloy composites, which exhibit high elongations (>280%), show strong reaction between the Al-matrix and Si 3 N 4 crystals. The result suggests that the addition of magnesium causes the reaction between the Al-matrix and Si 3 N 4 crystals, and consequently that partially melting of the reaction phases at the tensile-testing temperature results in the relaxation of stress concentration and suppresses the development of microcracks and cavities at the interfaces during superplastic deformation.

High strain rate superplasticity in powder metallurgy processed Al-16Si-5Fe alloy

In the present paper, new additional experimental results for these aluminum alloys with a very fine grain structure are demonstrated over a strain range of 10{sup {minus}4} {approximately} 10{sup {minus}1} s{sup {minus}1} in the temperature range of 673 {approximately} 793 K, to analyze the possible mechanisms of superplasticity at high strain rates in very fine grained aluminum alloys produced by powder metallurgy method.

The temperature dependence of superplastic characteristics and microstructures of PM Al–16 Si–5 Fe alloy

Abstract Temperature dependence of the superplasticity and microstructure of an Al-16 wt.% Si—5 wt.% Fe—0.9 wt.% Cu—0.4 wt.% Mg—0.9 wt.% Zr alloy fabricated by powder metallurgy processing was examined at elevated temperatures. Superplastic large elongations of >300% were obtained at a temperature range, in which high strain-rate sensitivities ( m value) were observed, and elongation rapidly decreased with increasing temperature beyond superplastic flow region. While evidence of grain sliding and rotation was observed in the superplastic flow region, a brittle-like fracture was observed in high temperature region beyond superplastic flow region. It has also been realized in significant grain growth during deformation leading to the decline of superplasticity in the present alloy.

Strain-rate dependence of microstructure and superplasticity of an Al-16 wt.% Si-5 wt.% Fe-0.9 wt.% Cu-0.4 wt.% mg-0.9 wt.% Zr alloy fabricated by powder metallurgy

Abstract Strain-rate dependence of flow stress and elongation of an Al–16 wt.% Si–5 wt.% Fe–0.9 wt.% Cu–0.4 wt.% Mg–0.9 wt.% Zr alloy fabricated by powder metallurgy processing was examined in a range of strain rate from 1.4×10 −4 to 7×10 −1 s −1 under a constant temperature of 783 K. A superplastic elongation of 350% is obtained at a strain rate of 1.4×10 −1 s −1 , and it rapidly decreases with an increasing strain rate. Transmission electron microscopic observations of the alloy showing the superplastic elongation reveal the same fine-grained morphology as that of an as-extruded alloy, whereas in the alloy tensile-tested at 1.4×10 −4 s −1 , which show a small elongation of 100%, grain growth occurs.

High-strain-rate Superplastic Flow in 6061 Al Composite Enhanced by Liquid Phase

High-strain-rate superplastic behavior of powder-metallurgy processed 0%, 10%, 20%, and 30% SiC particulate reinforced 6061 Al composites was studied over a range of temperatures from 430 to 610 °C. The strength of the 6061 Al composites was lower than that of the 6061 Al matrix alloy in the temperature range where grain boundary sliding is believed to control the plastic flow. The difference in their strength was also observed to be temperature dependent, increasing with increase in temperature. Abnormally high activation energy for superplastic flow was another important feature of the 6061 Al composites. These behaviors in particle weakening and activation energy have strong resemblance to those noted in the high-strain-rate superplastic 2124 Al composites studied previously. The observed particle weakening was attributed to liquid-enhanced superplastic flow and discussed by adopting the concept of effective diffusivity considering mass flow through liquid phase formed at the solute-segregated region near SiC/Al interfaces.

The effect of Ca addition on viscosity and electrochemical properties of Mg-alloys produced by casting

Summary form only given. The composition of different Mg alloys is known to affect their current capacity, potential, and anode efficiency. Many alloying elements have been used in attempts to improve the electrochemical properties of magnesium anodes. Significant improvements of electrochemical properties have been achieved by controlling the adverse effects of impurity elements such as Fe, Ni, Cu with alloying elements. Out of many elements, Ca is considered as a very effective element that can improve the electrochemical properties of Mg-alloys because of its relatively low potential in comparison with specified elements such as Mn, Al, Zn in high Mn alloys or AZ63 alloys with the effect of grain refining. Ca has recently been used as a common inhibitor for the ignition of molten Mg alloys. However, the viscosity of pure Mg is markedly increased with increasing Ca content. Ca is responsible for making the casting of Mg alloys from Mg melt difficult at desirable pouring temperatures. In the present study, the effect of Ca addition on the viscosity and electrochemical properties of Mg-Ca alloys is investigated. Viscosity as well as electrochemical data will be correlated with chemical composition of impurities, and the microstructural change before and after Ca is added.