발간논문

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The microstructure and texture of TiN coatings on a Ni-based super-alloy were characterized by the automated version of electron backscatter diffraction (EBSD), EBSD techniques were used to investigate the very fine TiN grain shape and crystal orientation. This study confirmed that EBSD techniques provide a very useful tool for characterization of coating materials. The TiN grains had a special texture, a {001}-fiber texture in the coating layer. It was also found that, in severe environments, the coating performance of equiaxial and randomly oriented TiN is superior to that with columnar structures.

keyword : TiN, thin films, coating, texture, electron backscattering diffraction(EBSD)

Dual Phase (DP) steel which has a soft ferrite phase and a hard martensite phase reveals both high strength and high ductility and has received increased attention for use in automotive applications. To conduct structural analysis to verify vehicle safety, highly credible experimental results are required. In this study, tensile tests were performed in a strain rate range from 10-4/s to 300/s for Sink Roll-Less (SRL) hot-dip metal coated sheets. Collision properties were estimated through simulation by LS-DYNA using the stress-strain curve obtained from the tensile test. The simulation results were compared with the actual crash test results to confirm the credibility of the simulation. In addition, a tensile test and a crash test with 2% prestrain and a baking (PB) specimen were evaluated identically because automotive steel is used after forming and painting. The mechanical behaviors were improved with an increasing strain rate regardless of the PB treatment. Thus, plastic deformation with an appropriate strain rate is expected to result in better formability and crash characteristics than plastic deformation with a static strain rate. The ultimate tensile strength (UTS) and absorbed energy up to 10% strain were improved even though the total elongation decreased after PB treatment, The results of the experimental crash test and computer simulation were slightly different but generally, a similar propensity was seen.

keyword : metals, galvanization, strain rate, computer simulation, crash test

Carbon diffusion of ultra low carbon steel treated at 880℃ and 930℃ for 10, 30, 60 and 120 minutes was investigated using optical microscopy, SAM, EPMA, and Micro Vickers. The martensite patterns of the specimens treated at 880℃ and 930℃ were different. Martensite in the ferrite region was found in the specimen treated at 880℃ because of grain boundary diffusion. Such phenomena is explained by a carbon diffusion model.

keyword : HPF (hot press forming), carbon diffusion, ultra Low carbon steel, martensite, carbon diffusion model

Nanopowders of MgO and TiO2 were made by high energy ball milling. The rapid synthesis and sintering of the nanostructured MgTiO3 compound was investigated by the high-frequency induction heated sintering process. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition grain growth. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. As nanomaterials possess high strength, high hardness, excellent ductility and toughness, undoubtedly, more attention has been paid for the application of nanomaterials. A highly dense nanostructured MgTiO3 compound was produced with simultaneous application of 80 MPa pressure and induced current within 2 min. The sintering behavior, gain size and mechanical properties of MgTiO3 compound were investigated.

keyword : compounds, nanostructured materials, sintering, mechanical properties

Directional solidification experiments were carried out in a hypoeutectic Al-11.3Si-3.5Cu system to investigate the microstructural evolution with the solidification rate. At a fixed temperature gradient, a dendritic microstructure was observed at a constant speed of more than 25 ㎛s-1, a cellular interface developed at 5 ㎛s-1 and the growth rate of 0.5 ㎛s-1 led to the stability of the planar interface. The results revealed that primary silicon phases formed among cells, even though the studied Al-Si alloy system formed the composition within a hypoeutectic silicon composition. This suggests that the liquid concentration among cells during solidification reached a higher concentration, i.e., the eutectic concentration. It is, however, interesting that primary silicon phases did not form during a dendritic growth of more than 25 ㎛s-1. These experimental observations are explained using the theoretical models on the interface temperatures.

keyword : Al-Si alloy, directional solidification, solidification rate, segregation, dendrite spacing

High purity silicon can be obtained from Al-Si alloys by a combination of solvent refining and centrifugation. Silicon purification by crystallization of silicon from an Al-Si alloy melt was carried out using 2N and 4N purity aluminum and 2N purity silicon as raw materials. The effect of the purity of raw materials on the final silicon ingot purity by centrifugation was investigated for an Al-50 wt% Si alloy. Alloys were melted using an electrical resistance furnace, and then poured into a centrifuging apparatus. A silicon lump like foam was obtained after centrifugation and was leached by an acid in order to get pure silicon flakes. Then silicon flakes were melted to make a silicon ingot using an induction furnace. The purities of the silicon flakes and silicon ingot were enhanced significantly compared to those of the raw materials of silicon and aluminum. The silicon ingot made of 4N aluminum and 2N silicon showed the lowest impurities.

keyword : alloys, purification, solidification, image analysis, centrifugation, solvent refining

In spite of some problems in processability and bondability, Au wires in the microelectronics industry are gradually being replaced by copper wires to reduce the cost of raw material. In this article, the effects of surface roughness enhanced capillaries on thermosonic Cu wire bonding were evaluated. The roughness-enhanced zirconia toughened alumina (ZTA) capillaries were fabricated via a thermal grooving technique. As a result, the shear bond strength of first bonds (ball bonds) bonded using the roughness-enhanced capillary was enhanced by 15% as compared with that of normal bonds due to more effective plastic deformation and flow of a Cu ball. In the pull-out test of second bonds (stitch bonds), processed at two limit conditions on combinations of process parameters, the bond strength of bonds formed using the roughness-enhanced capillary also resulted in values higher by 55.5% than that of normal bonds because of the increase in the bonding area, indicating the expansion of a processing window for Cu wire bonding. These results suggest that the adoption of roughness-enhanced capillaries is a promising approach for enhancing processability and bondability in Cu wire bonding.

keyword : Cu wire, metals, bonding, strength, scanning electron microscopy (SEM)

The pulsed current activated sintering method (PCAS) is a new rapid sintering method that was developed recently for fabricating ceramics and composites. This method combines a high temperature for a short time with pressure application. In this work, PCAS was used to fabricate WC-5wt%Mo2C-5wt% Co hard material using WC, Mo2C, and Co. The WC-Mo2C-Co was almost completely dense with a relative density of up to 100% after the simultaneous application of a pressure of 60 MPa and electric current for 11 min without grain growth. The average grain size of WC that was produced through PCAS was about 0.5-0.6 ㎛. The vickers hardness and fracture toughness of the WC-5wt%Mo2C-5wt%Co hard materials were about 2453.5 kg/mm2 and 7.9 MPa · m1/2, respectively, for 60 MPa at 11200℃.

keyword : pulsed current activated sintering method, WC-Mo2C-Co, hard material, mechanical property, rapid sintering

The microstructure of Cu-15 wt%Ag composites fabricated by equal channel angular pressing (ECAP) with intermediate heat treatment at 320℃ was investigated by transmission electron microscopy (TEM) observations. Ag precipitates with a thickness of 20-40 nm were observed in the eutectic region of the Cu-15 wt%Ag composite solution treated at 700℃ before ECAP. The Cu matrix and Ag precipitates had a cube on cube orientation relationship. ECAPed composites exhibited ultrafine-grained microstructures with the shape and distribution dependent on the processing routes. For route A in which the sample was pressed without rotation between each pass, the Cu and Ag grains were elongated along the shear direction and many micro-twins were observed in elongated Cu grains as well as in Ag filaments. The steps were observed on coherent twin boundaries in Cu grains. For route Bc in which the sample was rotated by 90 degrees after each pass, a subgrain structure with misorientation of 2-4 degree by fragmentation of the large Cu grains were observed. For route C in which the sample was rotated by 180 degrees after each pass, the microstructure was similar to that of the route A sample. However, the thickness of the elongated grains along the shear direction was wider than that of the route A sample and the twin density was lower than the route A sample. It was found that more microtwins were formed in ECAPed Cu-15 wt%Ag than in the drawn sample. Grain boundaries were observed in relatively thick and long Ag filaments in Cu-15 wt%Ag ECAPed by route C, indicating the multi-crystalline nature of Ag filaments.

keyword : Cu-Ag composite, plastic deformation, equal channel angular pressing (ECAP), transmission electron microscopy (TEM)

A 3-ply clad metal consisting of aluminum and copper was fabricated by roll bonding process and the microstructures and mechanical properties of the roll-bonded and post-roll-bonding heat treated Cu/Al/Cu clad metal were investigated. A brittle interfacial reaction layer formed at the Cu/Al interfaces at and above 400℃. The thickness of the reaction layer increased from 12 ㎛at 400℃ to 28 ㎛at 500℃. The stress-strain curves demonstrated that the strength decreased and the ductility increased with heat treatment up to 400℃. The clad metal heat treated at 300℃ with no indication of a reaction layer exhibited an excellent combination of the strength and ductility and no delamination of layers up to final fracture in the tensile testing. Above 400℃, the ductility decreased rapidly with little change of strength, reflecting the brittle nature of the intermetallic interlayers. In Cu/Al/Cu clad heat treated above 400℃, periodic parallel cracks perpendicular to the stress axis were observed at the interfacial reaction layer. In-situ optical microscopic observation revealed that cracks were formed in the Cu layer due to the strain concentration in the vicinity of horizontal cracks in the intermetallic layer, promoting the premature fracture of Cu layer. Vertical cracks parallel to the stress axis were also formed at 15% strain at 500℃, leading to the delamination of the Cu and Al layers.

keyword : heat treatment, interfacial fracture, copper, aluminum, clad