Papers by Author: Ki Duk Seong

Abstract: The Effects of Li content and annealing treatment on microstructure and damping capacity for Mg-X%Li alloys have been investigated, based on experimental results from X-ray diffractometry (XRD), optical microscopy (OM), hardness tests and vibration damping tests in a flexural mode. The Mg-X%Li alloys containing Li of 3%, 8% and 13% consist of α (HCP) single phase, (α + β (BCC)) dual phases and β single phase, respectively. In as-rolled state, the damping capacity for Mg-Li alloys shows a similar level regardless of Li content. The annealing treatments at 200oC and 400οC give rise to an enhancement of damping capacity only for the Mg-3%Li and Mg-8%Li alloys containing α phase, and at the same annealing temperature, the Mg-3%Li alloy with fully α structure exhibits higher damping capacity. This result indicates that the damping capacity of Mg-Li alloys depends principally on α phase, and that the annealing treatment is necessary to improve its damping capacity.

Abstract: Semi-Solid Al-5%Mg-3%Zn-1%Mn alloys with or without 0.2%Zr were prepared by the isothermal heating of either cast ingot or extruded billet. The microstructural and tensile properties variations during the isothermal heating, depending on alloy composition and slurry preparation method, were investigated. The grain (globule) size of semi-solid microstructure was not remarkably changed by the slurry preparation method, however it was significantly reduced by the Zr additions. The room temperature tensile elongation of the semi-solid formed alloys was generally higher in case of the extruded billet. T6 heat treatment was not found effective in improving the strength of the semi-solid alloys, unlike the casting alloys.

Abstract: Changes in tensile properties of A206 Al-Cu-Mg casting alloys containing trace amount of rare earth(RE) elements up to 0.2% were investigated and discussed in relation to the microstructural evolution. The A206 alloy with 0.1%RE shows higher values of ultimate tensile strength(UTS) and yield strength(YS) than A206 alloy at room and elevated temperatures up to 300oC, whereas no remarkable change is found in elongation with respect to RE addition. In view of X-ray diffractometry(XRD) and transmission electron microscopy(TEM), the improvement of tensile strength in the A206 alloy with 0.1%RE would be caused by higher number density of Al2Cu(q¢) strengthening precipitates.