Yong-Seog Kim

Constrained groove pressing and its application to grain refinement of aluminum

Abstract The new intense plastic straining technique, named ‘constrained groove pressing’ (CGP), was developed for fabrication of plate-shaped ultrafined grained metallic materials without changing their initial dimensions. The principle of CGP is that a material is subjected to the repetitive shear deformation under the plane strain deformation condition by utilizing alternate pressing with the asymmetrically grooved die and flat die constrained tightly by the cylinder wall. A submicrometer order grain structure was obtained in pure aluminum by utilizing this technique. The grain refinement sequences during pressing were examined by transmission electron microscopy. The enhancement of the mechanical properties of submicrometer order grained pure aluminum fabricated by this technique was comparable to that produced by other intense plastic straining techniques at the similar accumulated strains.

Microstructure development during equal-channel angular pressing of titanium

Abstract The development of microstructure during equal-channel angular pressing (ECAP) of commercial-purity titanium was investigated to establish the mechanisms of grain refinement and strain accommodation. Samples were deformed at 623 K via three different processing routes: A, B, and C. After the first pass, transmission electron microscopy (TEM) revealed that the strain imposed by pressing was accommodated mainly by 10 1 1 deformation twinning. During the second pass, the deformation mechanism changed to dislocation slip on a system which depended on the specific route. For route C, prism (a) and pyramidal ( c + a ) slip occurred within alternating twin bands. For route B, prism a slip was the main deformation mechanism. For route A, deformation was controlled by basal a slip and micro-twinning in alternating twin bands. The variation in deformation behavior was interpreted in terms of the texture formed during the first pass and the Schmid factors for slip during subsequent deformation.

Thermal stability and mechanical properties of ultrafine grained low carbon steel

Ultrafine grained low carbon steel manufactured by equal channel angular pressing was annealed at 753 K, where negligible grain growth occurred, up to 72 h and the microstructural change and the mechanical properties were examined. This investigation was aimed at providing the guiding information for the effective use of ultrafine grained low carbon steel manufactured by severe plastic deformation processes. Under the present annealing conditions, the microstructural change was dominated by recovery. The tensile behavior of annealed ultrafine grained steel was characterized by much higher strength and the absence of strain hardening compared with that of large grained steel. In addition, the present ultrafine grained steel became mechanically stable by 24 h annealing treatment although recovery was in progress. The microstructure of the deformed sample of annealed ultrafine grained steel exhibited the elongated grains and dislocations distributed densely in the vicinity of grain boundaries. This finding indicated that dynamic recovery during deformation was associated with the absorption of dislocation by grain boundaries. The mechanical behavior of the present ultrafine grained low carbon steel was discussed in light of the recent development explaining that of nanocrystalline materials, i.e. the dislocation bow-out mechanism for high strength and the spreading kinetics of trapped lattice dislocation into grain boundary for the absence of strain hardening.

On the formation of MoSi2 by self-propagating high-temperature synthesis

The formation mechanism of MoSi{sub 2} by self-propagating high-temperature synthesis (SHS) was studied by quenching the reaction front into liquid nitrogen or in a copper block. The microstructural analysis of the front indicated that the formation of MoSi{sub 2} occurs via dissolution of Mo into Si melt followed by MoSi{sub 2} precipitation. The effects of processing variables including Mo particle size, preheating temperature, and diluent content on the MoSi{sub 2} particle size were discussed, based on the mechanism proposed in this study.

Formation of fine cementite precipitates by static annealing of equal-channel angular pressed low-carbon steels

Abstract In this study, a static annealing of low-carbon steels severely deformed by equal-channel angular pressing was conducted. Employment of the processing route on a low-carbon steel containing 0.06% vanadium resulted in the formation of cementite precipitations as well as the refinement of ferrite grains to submicrometer size. During the static annealing treatment, rod-like cementites in the pearlite colonies of the sample decomposed to form spherical cementite precipitates distributed uniformly in ferrite grains. The decomposition phenomenon of the cementite is discussed on the basis of the dislocation–cementite interaction.

Effect of heating rate on the combustion synthesis of intermetallics

Effects of heating rate on the temperature profile of the thermal explosion mode of combustion synthesis and on the homogeneity of the reaction product are investigated. Analysis of the temperature profile using a simple mathematical model indicated that a significant fraction of reactants is consumed during the precombustion duration, forming intermetallic compounds at the reactant interface, which was confirmed experimentally. The relationship between the parameters measured from the temperature profile and the degree of conversion of reactants to products is discussed.

Surface treatment effects of indium–tin oxide in organic light-emitting diodes

Abstract Effects of mechanically polished and annealed indium–tin oxide (ITO) substrate in tris-(8-hydroxyquinoline) aluminum (Alq3)-based light-emitting diodes were studied. It was found that roughness and oxygen content at ITO surface were reduced after the mechanical polishing. The organic light-emitting diodes (OLEDs) fabricated on the polished ITO surface showed 10 times better electroluminescence and current injection than the OLEDs based on as-received ITO surface, due to the increase of smooth contact area between the ITO surface and organic layer. The nucleation of TPD on ITO shows different feature according to the surface condition of ITO. As a result, the mechanical polishing is an effective treatment to enhance the performance of the OLEDs.

Formation of fine cementite precipitates in an ultra-fine grained low carbon steel

Abstract Low-carbon steel containing 0.06 wt.% vanadium was deformed by a severe plastic deformation technique and annealed in a temperature range from 420 to 600 °C. The treatment resulted in a formation of nano-sized cementite precipitates as well as refinement of ferrite grains to a submicrometer size. This phenomenon is discussed based on dislocation-cementite interactions.

Stability of channels at a scalloplike Cu 6 Sn 5 layer in solder interconnections

The thermodynamic stability of the solder channels at a scalloplike Cu 6 Sn 5 layer formed between Sn-containing solders and Cu substrate was evaluated by studying the penetration behavior of the liquid solders into the grain boundaries of a Cu 6 Sn 5 substrate. The orientational relationship between the grains of the Cu 6 Sn 5 layer formed during reflow soldering was also analyzed using the electron backscattered diffraction technique. The results showed that liquid solders penetrate into the grain boundaries at an order of faster speed than the growth rate of the layer, which provided a direct evidence of thermodynamic stability of the channel.

Effects of Cu and Ni additions to eutectic Pb–Sn solders on Au embrittlement of solder interconnections

Effects Cu and Ni additions on Au embrittlement of eutectic Sn–Pb solder interconnections during a solid-state aging treatment were evaluated using a ball shear testing method. The addition resulted in a formation of Au-containing ternary intermetallic compounds, either Au–Sn–Cu or Au–Sn–Ni phase, inside the solder matrix during aging treatment. The fracture energy of the solder interconnection containing 2.9 wt% Cu remained almost the same up to 200 h of aging treatment at 150 °C, demonstrating the possibility of suppressing the Au embrittlement by forming ternary intermetallic phases inside the solder matrix.