Heung Nam Han

Fatigue-Free, Electrically Reliable Copper Electrode with Nanohole Array

Design and fabrication of reliable electrodes is one of the most important challenges in flexible devices, which undergo repeated deformation. In conventional approaches, mechanical and electrical properties of continuous metal films degrade gradually because of the fatigue damage. The designed incorporation of nanoholes into Cu electrodes can enhance the reliability. In this study, the electrode shows extremely low electrical resistance change during bending fatigue because the nanoholes suppress crack initiation by preventing protrusion formation and damage propagation by crack tip blunting. This concept provides a key guideline for developing fatigue-free flexible electrodes.

Diffusion-controlled transformation plasticity of steel under externally applied stress

The transformation plasticity of steel during phase transformation under external stress was modelled on a migrating interface diffusion mechanism. Atomic diffusion along the migrating phase interface is assumed to cause transformation plasticity by an accelerated Coble creep. A creep equation on transformation plasticity is derived as a function of transformation rate, temperature and externally applied stress. Predictions are compared with dilatometric measurements during the austenite-to-ferrite and ferrite-to-austenite transformation of steel under various levels of uniaxial compressive stress. Good agreement was found between the calculated and experimental transformation strain. The model proposed also successfully describes the thermally activated behaviour of the transformation strain. The evaluated effective diffusion coefficients on the migrating interface are three to four orders of magnitude larger than those reported for stationary boundaries.

Prediction of forming limit in hydro-mechanical deep drawing of steel sheets using ductile fracture criterion

It has been observed that the forming limit curve at fracture (FLCF) of steel sheets, with a relatively higher ductility limit have linear shapes, similar to those of a bulk forming process. In contrast, the FLCF of sheets with a relatively lower ductility limit have rather complex shapes approaching the forming limit curve at neck (FLCN) towards the equi-biaxial strain paths. In this study, the FLCFs of steel sheets were measured and compared with the fracture strains predicted from specific ductile fracture criteria, including a criterion suggested by the authors, which can accurately describe FLCFs with both linear and complex shapes. To predict the forming limit for hydro-mechanical deep drawing of steel sheets, the ductile fracture criteria were integrated into a finite element simulation. The simulation, results based on the criterion suggested by authors accurately predicted the experimetal, fracture limits of steel sheets for the hydro-mechanical deep drawing process.

Material properties of friction stir spot welded joints of dissimilar aluminum alloys

Abstract Mechanical properties and material mixing patterns of friction stir spot welded (FSSW) joints of dissimilar aluminum alloys were investigated. Two aluminum alloys typically used in automotive applications, 5052-H32 and 6061-T6, were selected. During the experiment, the process parameters including the z-axis force and torque histories were measured as a function of the tool displacement. The mechanical properties were investigated by microhardness measurements of the joint, and the material mixing in the stir zone was investigated by EPMA. The experimental results illustrate different process parameter histories, material mixing in the stir zone and material properties including microhardness distributions for FSSW joints of dissimilar aluminum alloys, likely due to different mechanical behaviors of the selected aluminum alloys in the FSSW process temperature range.

Improvement in Oxidation Resistance of Ferritic Stainless Steel by Carbon Ion Implantation

The oxidation characteristics of ferritic stainless steel (Crofer22APU), a potential candidate for interconnects in solid oxide fuel cells, were studied. Carbon was introduced on the surface by ion implantation, and both the carbon-implanted and unimplanted samples were subjected to oxidation at 800°C for 500 h. Ion implantation produced a carbon-rich amorphous region at the top surface. Fast diffusion induced by ion implantation promoted the formation of spinel oxides. The carbon-implanted samples showed significantly less mass gain than the unimplanted samples.

Improvement of Formability and Spring-Back of AA5052-H32 Sheets Based on Surface Friction Stir Method

A process to improve formability and spring-back was developed for AA5xxx-H temper sheets based on the surface friction stir (SFS) method. In the SFS method, a rotating probe stirs the sheet surface so that material flow and heat, which result from plastic deformation and friction, change the microstructure and macroscopic mechanical properties of the stirred zone and therefore, ultimately, the formability and spring-back performances of the whole sheet. When applied to AA5052-H32 sheets, the process improved formability and spring-back, as experimentally and numerically confirmed in the limit dome height and unconstrained bending tests.

Characterization of the crystallographic microstructure of the stress-induced void in Cu interconnects

Stepwise cross-sectional crystallographic measurement was performed on the stress-induced void (SIV) of copper interconnect and it was possible to investigate the three-dimensional crystallographic structures near the submicron scale void. The void mainly happened at a triple junction of grain boundaries and there were at least two grains with high biaxial elastic modulus near the triple junction. The preferred site for the SIV could well be understood by considering the elastic anisotropy of each grain and the grain boundary type.

Analysis of Transformation Plasticity in Steel Using a Finite Element Method Coupled with a Phase Field Model

An implicit finite element model was developed to analyze the deformation behavior of low carbon steel during phase transformation. The finite element model was coupled hierarchically with a phase field model that could simulate the kinetics and micro-structural evolution during the austenite-to-ferrite transformation of low carbon steel. Thermo-elastic-plastic constitutive equations for each phase were adopted to confirm the transformation plasticity due to the weaker phase yielding that was proposed by Greenwood and Johnson. From the simulations under various possible plastic properties of each phase, a more quantitative understanding of the origin of transformation plasticity was attempted by a comparison with the experimental observation.

Morphology transformation of patterned, uniform and faceted GaN microcrystals

We report on the growth and characterization of patterned and uniformly distributed GaN microcrystals with well-defined facets and epitaxy. The microcrystals were grown on a mask patterned by lithography. The GaN microcrystals were formed by selective-area epitaxy using metal-organic chemical-vapour deposition. The GaN microcrystals have similar sizes and shapes. Each microcrystal consists of an upper and a lower part, which are rotated by 30°. Transmission electron microscopy shows that there is a rather clear interface between the two parts of the crystal, suggesting a sudden change in the growth direction. We performed ab initio calculations for the surface energies of hexagonal GaN, and the growth morphology is explained based on surface energy considerations.

Boundary Migration Induced Plasticity during Recrystallization and Growth under Applied Stress

In general, plastic strain occurs over a certain stress, called yield stress. However, it has been reported that the permanent strain could happen during boundary migrating even under the extremely slight externally applied stress. In this study, we performed dilatometry experiments under the various compressive stresses and measured the amount of recrystallization and growth induced permanent strain. A new empirical constitutive equation was suggested to describe the recrystallization and growth induced plasticity. This equation was verified by comparing the calculated values with dilatometric experimental data under the various compressive stresses.