Papers by Author: Won Kyu Bang

Abstract: The formability of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass has been investigated in the present study in relation to the heating rate. A series of extrusion tests after rapid heating has been performed in a laboratory scale together with sheet forming tests after slow heating for comparison purpose. The basic processing map based on dynamic materials model (DMM) and compression test data has been utilized to evaluate feasible forming conditions. The macroscopic formability, classified by fully formed, partially formed or a catastrophic fracture, is found to have a good correspondence with the iso-efficiency contour in the processing map. The region of high power dissipation efficiencies with η>0.8 is found to be broaden by avoiding crystallization events due to reduced exposure time in extrusion process with a faster heating rate.

Abstract: In this study, optimum processing conditions for strip-cast AZ31 Mg alloy was investigated on the basis of processing map and microstructural analysis. To obtain the processing map, isothermal compression tests were carried out to a strain of 0.5 at temperatures of 200 ∼ 400°C with the strain rates of 0.01 ∼ 10s-1. It was found that maximum efficiency indicating the optimum processing condition occurred at 300°C and 10s-1. The possible deformation mechanisms operating at high temperature was also discussed.

Abstract: The superplastic deformation behavior of a fine-grained 7075 Al has been investigated to clarify the issue of threshold stress. A series of mechanical tests has been conducted at various temperatures for the specimens with various grain sizes. The quantitative constitutive parameters have been determined from load relaxation test by applying the internal variable theory of structural superplaticity (SSP) proposed by Chang et al. The GBS flow could be formulated as a viscosity-type equation, characterized by the Newtonian exponent of 1.0. The unresolved issue of threshold stress is clarified and identified as a critical stress required for the GBS. The micro-mechanical roll of grain size refinement has also been manifested in terms of proposed constitutive parameters.