발간논문

Home > KJMM 논문 > 발간논문

This study investigated the effect of excess free volume generated by the application of elastic shear stress on the mechanical properties of bulk amorphous alloys. A shear deforming device that can create excess free volume without fracturing samples was designed. With repeated elastic stress, the plasticity of shear-deformed amorphous alloys increased, while the yield strength decreased. These behaviors were analyzed from the perspective of the microsturctural changes characterized by the free volume generation, and quantified using the effective activation energy for crystallization.

keyword : bulk amorphous alloy, plasticity, shear stress, free volume, ECAP

Thermal expansion property was investigated on Fe-42% Ni alloy strip added by alloying element of Si of 0~1.5wt.%. The strip was fabricated by a melt drag casting process. Addition of Si enlarged the solid-liquid region and reduced the melting point which leads to the increase of the formability of a strip. The alloy containing 0.6wt.% Si showed the lowest thermal expansion ratio in the temperature range between 20 to 350℃. The grain size was increased with reduction ratio and annealing temperature, which resulted in the decrease of the thermal expansion coefficient of strip. Because of grain refining by precipitation of Ni3Fe, the alloy strip containing 1.5wt.% Si showed higher thermal expansion ratio compared with the alloy containing 0.6wt.% Si.

keyword : thermal expansion ratio, melt drag casting, Fe-Ni Alloy, Si content

Copper coating was deposited by cold spraying and especially two kinds of powders with different particle size (powder A: 9~53μm, powder B: 4~23μm) were used for the spraying. Annealing heat treatment was also applied to the coating to examine its effect on the microstructure and properties of the cold-sprayed copper coating. It was found that the use of fine powder could decrease the surface roughness and porosity of coating layer. After annealing, the micro-hardness of the cold-sprayed copper coating decreased significantly with increasing annealing temperature. The examination of coating microstructure evidently revealed that coating was constituted by the elongated particles and interfaces were clearly observed between the deposited particles. Annealing treatment led to the recovery and the evident recrystallization of deformed grains and the remarkable grain growth as well. The abrupt decrease in hardness was mainly attributed to recrystallization. The electrical conductivity and thermal conductivity of powder B coating were higher those of powder A. It was also apparent that the electrical and thermal conductivities of cold-sprayed copper coating could be improved through annealing treatment.

keyword : cold spraying, copper (Cu) coating, particle size, annealing heat treatment, porosity, micro-hardness, electrical resistivity, thermal conductivity

Characteristics of crevice corrosion on pure Ni and Ni-(15, 30%)Cr alloys were investigated, using the specially designed crevice cell and the micro Luggin capillary in 1N H2SO4 solution with and without chloride ion. The potential profiles inside the crevice and the cell currents were measured with the variation of alloying element, Cr, applied potential, chloride ion concentration and immersion time. In order to examine repassivation characteristics, the abrading electrode measurements were performed. Experimental results showed that the repassivation kinetics of pure Ni and Ni-(15, 30%)Cr alloys after the passive film breakdown can be described as i=At(-n) where n is the repassivation rate parameter. An attempt was made to establish the relationship between the occurrence of crevice corrosion and the repassivation rate parameter, n.

keyword : crevice corrosion, passive film, repassivation kinetics, repassivation rate parameter

Contact material is widely used as electrical parts. Ag-CdO has a good wear resistance and stable contact resistance. But the disadvantages of Ag-CdO alloy are coarse Cd oxides and harmful metal, Cd. Then Ag-Sn alloy that has stable and fine Sn oxide at high temperature has been developed. In order to investigate the effect of Bi addition that affects the formation of the oxide layer on the surface and the formation of oxide in matrix Ag, we studied the microstructures and properties of Ag-Sn-In(-Bi) material fabricated by rapid solidification process. The experimental procedures were melting using high frequency induction, melt spinning, and internal oxidation. We have shown that the size of Sn oxide at no additional Bi is 20nm and the effect of Bi for high hardness is 0.3wt%Bi. Bi was concentrated to grain boundary and Bi made precipitation free zone (PFZ). Then Bi caused poor properties of alloy.

keyword : contact material, internal oxidation, oxide particle, Ag-Sn, rapid solidification process, Bi

The desulfurized slag during hot metal pretreatment station has lots of calcium sulfide, and iron oxide dust generated during oxygen blowing from the BOF contains high contents of iron and calcium sulfate. The calcium sulfide in the desulfurized slag and calcium sulfate in the CBP(cold bonded pellet) are the main source of sulfur reversal in the converter process. The behaviors of calcium sulfate and sulfide have been studied by the use of 50kg high frequency induction furnace, and the actual plant tests were carried at the BOF. The results show that evaporation of decomposed sulfate increases with increasing oxygen content in the slag with low carbon metal. In the case of ladle slag, desulfurization ratio is higher than 98% with blowing mixture gas which has more than 10% oxygen. The desulfurization time is shorter as the oxygen content in the blown gas increases. As above result, sulfur contents in the molten steel and slag can be controlled efficiently by oxygen blowing in slag

keyword : BOF slag, sulfate reduction, recycling of iron oxide dust, desulfurization

Sn doped Indium oxide (ITO) films coated by Sb doped tin oxide (ATO) films with varying thickness were deposited by radio frequency magnetron sputtering, and their temperature dependence of electrical and optical properties were examined. ITO and ATO films deposited at 300℃ by using optimized oxygen content yielded resistivity values of 1.04 x 10(-4) and 3.37 x 10(-3)Ωcm, respectively. Annealing of monolithic ITO film in atmospheric environment up to 500℃ resulted in increase of sheet resistance of about 3.5 times. It was shown that the ATO films deposited at 300℃ could retard the increase of sheet resistance of dual layer and reduce the final amount of degradation to some extent, but could not give satisfactory protection. The ITO layer covered with ATO film deposited at 500℃, however, resulted in the increase of sheet resistance of only 1.4times, and could be used as the substrate with excellent conductivity for dye-sensitized solar cell, in which the sintering of TiO2 nano-particles at elevated temperature as high as 500℃ in atmosphere is inevitable to form electrodes.

keyword : transparent conducting film, magnetron sputtering, indium tin oxide, dye-sensitized solar cell, thermal stability

The dry sliding wear tests were performed on A356/20vol.% SiCp composites against organic pad materials which is generally used in brake lining for middle speed train. The samples were tested by pin-on-disc wear test equipment with different applied load and sliding velocity. Wear behavior of A356/20vol.% SiCp against organic pad is examined through the observations and analysis of the worn surface, worn subsurface and wear debris. In the low applied load region, thin layer has formed on the worn surface to enhance the lubrication during sliding. In this region, wear has occurred mainly by oxidation. With increasing the applied load, fracture has occurred on the thin layer formed on worn surface and thus wear rate has increased sharply. A356/20vol.% SiCp composites has exhibited stable friction coefficient values against organic pad materials in the range of 0.3~0.4 regardless of the testing conditions.

keyword : aluminum, silicon carbide, composites, sliding wear, organic pad