발간논문

Home > KJMM 논문 > 발간논문

This study examined a fundamental issue as to if a compressive elastostatic loading can induce structural evolution of amorphous alloys. It was found that the elastic strain of amorphous alloys consisted of the strain components characterized by ideal elasticity, anelasticity, and viscoelasticity depending on their deformation characteristics. The strain component associated with viscoelasticity caused irreversible structural change even upon the removal of the applied stress, resulting in the generation of excess free volume. With the application of elastostatic loading, the plasticity of the pre-loaded amorphous alloy increased, while the yield strength decreased. These behaviors were analyzed using HRTEM and DSC from the perspective of the microsturctural changes characterized by the free volume generation.

keyword : bulk amorphous alloy, plasticity, free volume, static Loading

The cyclic softening behavior in low cycle fatigue of disk-forged Alloy718 by direct aged process was investigated at high temperatures of 500℃ and 650℃ It resulted that the softening phenomena during fatigue cycles should be related to precipitates and dislocation behavior in the microstructure of DA Alloy718. The slip bands by shearing matrix were composed of the main deformation mode in both of the an forged and the cyclic tested. The dislocations in the slip bands might get distangled in progressing the repeated cyclic motion, resulting the diffused configuration of the slip band lines. The precipitates of γ″ moreover, have made an appearance of shearing by the dissolved dislocations.

keyword : Alloy718, cyclic softening, Low cycle fatigue, slip band

A study has been made of the grain growth in IF steel during strain annealing. A 92.3% cold rolled and annealed IF steel sheet was temper rolled by 0.4% and subsequently annealed at 670, 750 and 850℃ for various time periods in 25%H2 + 75%N2. The grains occasionally started to grow abnormally near the surface and grew into the center during strain annealing. The pole figure and EBSD measurements showed that the abnormally grown grains did not have any orientation relation with the primary recrystallization texture before temper rolling. Thus, the major coarsening mechanism was regarded to be the strain induced grain boundary migration rather than grain boundary characteristics. The permeability increased initially and then decreased along with the increased of grain size, therefore there existed an optimum grain size showing maximum value of permeability. The maximum permeability was measured up to be 13,000, then optimum grain size is 83 μm at 850℃ for 30 min.

keyword : strain annealing, grain coarsening, permeability, SIGM, EBSD

The effects of CH4 content at nitriding step in the low temperature two-step plasma treatment (carburizing+nitriding) on the surface characteristics of AISI304L stainless steel were investigated. The low temperature plasma carburizing was carried out at 550℃ for 5h in a gas mixture of H2, Ar and CH4. The thickness of a carburized layer increased up to about 30 μm and corrosion resistance of the layer decreased due to the precipitation formed at the grain boundary. After carburizing, a low temperature plasma nitriding was subsequently performed in the same chamber at 400℃ for 15 h to improve corrosion resistance and to further increase the surface hardness. The surface hardness of a N-enriched layer after nitriding reached up to 1,200 HV0.1, which is much higher than that of as-carburized layer (750 HV0.1). The post nitriding process had a beneficial effect on reducing the precipitates in the C-enriched layer. The thickness of the N-enriched layer was about 10 μm without containing CH4 in the nitriding atmosphere. With increasing the CH4 level in the atmosphere, the uniformity and thickness of the N-enriched layer decreased and finally reached 5 μm with 8% CH4. The amount of the precipitates in the N-enriched layer increased with increasing CH4 composition.

keyword : austenitic stainless steel, plasma carburzing, plasma nitriding, expanded austenitie

The electro-reduction of titanium dioxide (TiO2) to titanium metal (Ti) was performed in molten calcium chloride. Partially reduced samples during the reduction process were collected and characterized by X-ray diffraction analysis. The reaction pathway has been derived from the time-dependent reduction phases of TixOy, which is concluded to the pathway of Ti6O11, Ti4O7, Ti3O5, Ti2O3 and Ti2O.

keyword : titanium sponge, electro-reduction, titanium dioxide, molten salt, calcium chloride

The mixing ratio effect of the powder W, Mo, Ta and B insert metal powder on Transient Liquid Phase bonding phenomenon and on the mechanical properties were investigated. The N5, MN5 and MN6 insert metal powder was eliminated by isothermal solidification which was controlled by the diffusion of B into the base metal and the insert metal powder was created some polycrystal in bonded interlayer but The solids in the bonded interlayer grew epitaxially from the base metal near the bonded interlayer inward the insert metal during the isothermal solidification. The number of grain boundaries formed at the bonded interlayer corresponded with those of base metal. When the isothermal solidification was finished, the concentration of the boned interlayer was similar with base metal. The tensile strength of N5 insert metal powder was the highest in insert metal powder as 1,070 MPa. At the creep rupture, The fracture time of N5 insert metal powder was similar to base metal by 81 hours (stress 345 MPa, 870℃) but elongation, reduction area was lower than base metal. At the mixing ratio of MN7 insert metal powders, the polycrystal in the bonded interlayer was formed when the isothermal solidification finished.

keyword : transient Liquid phase bonding, directionally solidified superalloy, GTD-111, wide-Gap brazing, powder, isothermal solidification, blade

ITO thin films were sputtered to evaluate the preferred orientation, the microstructure and crystallinity of the films on corning 1737 glass by systematically varying the process conditions including O2/(Ar+O2) ratio, film thickness and substrate temperature, respectively. As O2/(Ar+O2) ratio increased, ITO thin film was confirmed to be combined in addition to (400) and (440) plane. ITO thin film grew to the crystalline with preferred orientation of (400) plane as the substrate temperature and the film thickness increased. And the microstucture of ITO thin film also changed from amorphous phase to columnar structure which was composed of nano columns. Consequently, the preferred orientation of (400) plane of ITO thin film depended on the film thickness and the substrate temperature.

keyword : sputtering, ITO, thin film, preferred orientation, microstructure

In this study, we prepared Cu-Zr binary alloys reinforced with an ultra-fine eutectic microstructure. The alloys consisted of alternating layers of a hard superlattice phase and a ductile Cu phase with a very fine interlamellar spacing of ~60 nm. The hard superlattice phase attributed the enhancement in the strength, while the laminated eutectic structure helped improve the plasticity of the alloys, making their mechanical properties comparable to the earlier reported high strength alloys. This paper discussed the fundamental microstructural aspects that influence the unique mechanical properties of the alloys.

keyword : composite, Cu-Zr binary alloy, eutectic structure, superlattice