발간논문

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A nanoindentation model for analyzing thin-film stress has been developed by considering stress interaction and relaxation concepts. It was applied to the stress characterization of diamond-like carbon and Au thin films in previous studies(Y.-H. Lee et al., J. Kor. Inst. Met. & Mater., 39 (2001) p. 1101 and Y.-H. Lee et al., J. Kor. Inst. Met. & Mater., 40 (2002) p. 744). However, the validity of the proposed model cannot be directly confirmed by the thin film testing with various affecting factors. Nanoindentation on thin film can be affected by additive deformation of free-standing film, surface roughness and microstructural inhomogeneity. Therefore, we tried to verify the proposed model experimentally through the nanoindentation tests on an artificially stressed monolithic material instead of thin film. An electropolished (100) tungsten single crystal, which is free from the microstructural inhomogeneity, was used for a nanoindentation specimen. Nanoindentation curves obtained from the stress-free and the artificially stressed states using specially designed jigs were compared and analyzed based on the proposed stressanalysis model. The analyzed stress from the proposed model was consistent with the artificially applied stress measured from the strain gage attached on the specimen surface.

keyword : Residual stress-analyzing model, Nanoindentation, Tungsten single crystal, Artificially applied stress

A new model is presented for measuring equi-biaxial stress on thin-film using nanoindentation. The model was focused on the analysis of a depth-shift in the nanoindentation curve by stress effect. A load-controlled relaxation of the shifted indentation depth was chosen to avoid the variation of the contact area occurred in a depth-controlled relaxation. Nanoindentation experiments on (100) W single crystal at artificial equi-biaxial stress states verified that the proposed load-controlled relaxation model is capable of analyzing the residual stress within the standard deviation of ±35.9 MPa.

keyword : Residual stress, Nanoindentation, Load-controlled relaxation, Tungsten single crystal

The critical fracture stresses(σ^*_f (sp)) in various low alloy steels for reactor pressure vessel(RPV) have been evaluated by finite element analysis(FEA) of small punch test in the cleavage temperature region(-150 ∼ -196℃). In order to improve the accuracy of the FEA results on small punch tests, the uniaxial stress-strain properties beyond the necking were optimized by comparing the results of uniaxial tensile test and its finite element analysis. The loaddisplacement curves and distances from center to fracture surface(D_f) of FEA results are in good consistence with the experimental results. The σ^*_f(sp) from small punch test in various reactor pressure vessel(RPV) steels were found to have a linear relationship to the values from the precracked specimens(σ^*_f(PCVN)). This result indicates that small punch tests could be a useful method to evaluate the cleavage fracture behavior of the low alloy steels.

keyword : Small punch test, Finite element analysis, Critical fracture stress, Pressure vessel steel, Low alloy steel

In this work, tensile and fatigue properties of the metal injection molded 316L stainless steel were studied with the variation of sintering conditions. It was found that increasing sintering temperature and time resulted in a decrease in the porosity along with an increase in the grain size. With decreasing porosity, tensile strength and elongation increased, while yield strength maintained constant, which was attributed to the balanced effects of decrease of porosity and grain coarsening. Tensile strength and elongation were found to be largely dependent on the porosity rather than on the grain size since the fracture occurred mainly by microvoid growth and coalescence. A quantitative analysis on yield strength was made by considering the classical Hall-Petch equation and mechanistic models for yield strength of porous materials. The fatigue crack growth resistance at low ΔK level was not largely decreased when the porosity was increased up to 8%, but was significantly decreased when the porosity was increased to 17%. It was discussed in relation to the intrinsic nature of pore morphology and the extrinsic nature of crack closure.

keyword : Metal injection molding, Stainless steel, Sintering, Fatigue crack growth resistance, Hall-Petch equation

Effects of vibration(V) and inoculation(I) during casting on the microstructure and tensile properties of nickel-base superalloy CM247LC were investigated. The vibration treatment with a frequency of 10 ㎐ was effective to refine the grain size and decrease the microporosity level to some extent. The vibration treatment for the inoculated mold developed the grains of the casting into fine equiaxed grains compared with columnar grains of the casting with vibration free inoculated mold. The existing phases of CM247LC alloy such as finely dispersed cuboidal γ`, MC, and grain boundary precipitates were not influenced by the vibration and/or inoculation treatment. The fine equiaxed grained CM247LC had excellent tensile properties compared with those of the columnar grains. The tensile behavior of CM247LC alloy at 760℃ showed different aspects with respect to the grain size change and which was carefully analyzed by fractographic investigations.

keyword : Vibration, Inoculation, Microporosity, Superalloy, Carbide, Tensile property

The optimum conditions of soft reduction were determined to minimize centerline segregation in the continuously cast slabs using a solidification model and a series of tests at Pohang Works of POSCO. A solidification model reflecting microsegregation model that can precisely predict surface temperature and the crater end, was developed on the basis of surface temperature measured by thermocouple and solid shell thickness measured by pin shooting method. Optimum soft reduction conditions such as reduction zones and casting speeds for various carbon steel grades were determined on the basis of the simulation results. Changing test of casting speed was carried out to validate optimum casting speed predicted by a solidification model. In a series of tests for soft reduction, the center segregation degree of manganese was reduced from 1.08 to 1.04 and the area fraction of phosphorus segregation was reduced from 0.77 to 0.18%.

keyword : Centerline segregation, Soft reduction, Crater end, Solidification, Reduction zone

Recently, copper electrodeposits having more fine grains are required in the lead frame manufacturing process such as lead bending and wire bonding. These processes could not be achieved with a conventional DC copper electrodeposits, which resulted in a cracking or poor bonding with lead wire. In this study, square pulse current was conducted in order to overcome these problems. Prior to plating, the substrate was electropolished, cleaned in deionized water, activated in 5% H_2SO_4 and then transferred immediately to the plating solution. The deposition thickness was 1 ㎛. Cu was electro-plated onto A42(Fe 58% + Ni 42 alloy) sheet from cyanide-Cu bath by DC and high frequency pulse current. Morphology of plating surface was studied by the scanning probe microscope and optical microscope. The corrosion resistance of each plated specimens were studied by A.C Impedance test. Some of our results are summarized as follows; The cathode current efficiency was increased with increasing pulse frequency. XRD analysis shows that microstructure of high frequency pulse deposits develops into more anisotropic pattern than that of DC deposit. Corrosion resistance increased with increase in pulse frequency.

keyword : Copper, Electrodeposits, Pulse current, High frequency, Leadframe

This study is concerned with the improvement of abrasive and sliding wear properties of (TiC, SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron beam irradiation. The mixtures of TiC, SiC, SiC+Ti, or SiC+TiC powders and CaF_2 flux were deposited on a Ti-6Al-4V substrate, and then electron beam was irradiated on these mixtures. The surface composite layers of 1.2∼2.1 ㎜ in thickness were homogeneously formed without defects, and contained a large amount (30∼66 vol.%) of hard phases such as TiC and Ti_5Si_3 in the martensitic matrix. This microstructural modification including the formation of hard phases in the surface composite layer improved hardness and abrasive wear resistance. Particularly in the surface composite fabricated with TiC+SiC powders, the abrasive wear resistance was greatly enhanced by 25 times higher than that of the Ti alloy substrate because of the formation of 66 vol.% of TiC and Ti_5Si_3 in the hardened martensitic matrix. During the sliding wear process, hard and coarse TiC and Ti_5Si_3 were fallen off from the matrix, and their wear debris worked as abrasive particles, thereby reducing the sliding wear resistance. On the other hand, needle-shaped Ti_5Si_3 decelerated the sliding wear because their wear debris were fine and worked as solid lubricants instead of abrasive particles. These findings indicated that high-energy electron beam irradiation was economical and useful for the development of Ti-base surface composites with improved abrasive and sliding wear properties, although the abrasive and sliding wear data should be interpreted by different wear mechanisms.

keyword : Surface composite, Electron beam irradiation, Ti-6A1-4V, TiC, SiC, Flux, Abrasive wear, Sliding wear, Hardness

The roles of the hydride precipitation and integrity of the oxide layer in the accelerated corrosion of Zircaloy-4 are investigated by determining the critical ratio of P_H2/P_H20 using TGA(Thermo-Gravimetric Apparatus) and studying the effect of hydride precipitation on the oxidation kinetics. Under hydrogen-water mixture environment hydriding reaction takes place much faster than the oxidation reaction at early stage and soon later growing oxide begins to retard the hydriding reaction and completely prevent from the hydriding in the second stage. It turns out that even in this stage the oxidation rate is enhanced with increasing P_H2/P_H20 ratio. In the final stage, the oxide breakdown occurs, thus, massive hydriding takes place again. Intermittent weight gain measurements using autoclaves also confirm these results.

keyword : Nuclear fuel clad materials, Zircaloy-4, Accelerated oxidation, Hydride precipitates, Hydrogen pick-up, Massive hydriding

With the object of developing a ferromagnetic filtering media used in electromagnetic filtering process of iron & steel industries, addition effects of Cr and Ni on magnetic properties and corrosion resistance for Fe_80P_20 amorphous ribbon alloy have been investigated. In (FeCr)_80P_20 ternary alloys, the soft magnetic properties and corrosion resistance were largely enhanced upon Cr addition of about 5 at%, although considerable decrease in saturation magnetization occurred. However the corrosion rate was thought to be still high for practical use. If the Fe was substituted by some Ni to form a quarternary Fe-Ni-Cr-P alloys, decrease of the saturation magnetization was strongly suppressed. Moreover the soft magnetic properties and corrosion resistance were improved to a great extent as compared to Fe_75Cr_5P_20 amorphous alloy. For Fe_75Ni_5Cr_5P_20 amorphous alloy, the values of B_10, M_s and H_c were 6.9 kG, 110 emu/g and 0.24 Oe, respectively. The corrosion rate was measured to be very low(less than 0.2 ㎜/ yr) in 5 wt% HCl solution(room temperature) and 2 wt% NaOH solution(60℃). The good soft magnetic properties and corrosion resistance of Fe_75Ni_5Cr_5P_20 amorphous alloy were attributed to the decrease in saturation magnetostriction and more active formation of CrO_X(OH)_(3-2x)·nH_2O passive films by the combined addition of Ni and Cr.

keyword : Amorphous alloys, Filtering media, Magnetic properties, Corrosion resistance