발간논문

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At 1073K the creep rate ε˚ of the Ni-base alloy Inconel 617 has been measured as a function of strain at different stresses. Several characteristics of the dislocation structure have been observed by TEM as a function of strain for three stresses of 107, 180 and 320 ㎫: in particular free dislocations (not bound into subgrain boundaries and tangles around carbide particles), subgrains and dislocation configuration in the subgrain boundaries. At 1073K the first creep rate increases with strain as like in solution hardened materials and subsequently decreases as like in pure materials. These creep curves have also a good agreement with results of measured values of dislocation structures at various stresses. Concluded that 1) creep behavior of IN 617 shows transition phenomena from class M to A with increasing stress and 2) creep rate and microstructure results reveal a interdependent relationship.

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The influence of gas stirring on mass exchange without chemical reation in the LD converter was studied as a function of the number and arrangement of the tuyeres, gas flow rate and blowing method of gas. In terms of mass exchange, dephosphorizaticn in LD converter was simulated in a water model using thymol as a tracer element. The most suitable number and arrangement of the tuyeres was decided from the better value of the volumetric mass transfer coefficient kw`a. The average kw`a value for the combined blowing process was found to be between that of the top and bottom blowing processes. The better liquid/liquid contacts in combined blowing process is mainly attributed to entrainment of oil for slag droplets.

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Tungsten heavy alloys are successful combinations of strong W grain and ductile Ni base matrix. Its high density and strength combined with excellent ductility are the principal advantages of these alloys. The purpose of this study was to understand the effects of cold working and post-ageing treatments on the mechanical properties using the classical approach of sintering compacts from a mixture of 93W-4. 9Ni-2.1Fe system. By using the ageing treatment, the strength and hardness were increased with ageing temperatures and showed maximum strength at 600℃ for the 15% cold worked specimens; whereas, ductility was minimum. Strengthening by the ageing treatment might be attributed to the very fine dispersed precipitates in W grains.

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An analytical model is developed to describe the plasma deposition process in which average solidified thickness and coating and substrate temperatures are obtained. During deposition process solidification rate is periodically varied due to the impingement of liquid splats and the amount of liquid in the coating layer increases. Periodical variation of the solidification rate causes temperature fluctuation in coating and substrate. The nature of interfacial structure of plasma-sprayed NiCrBSi MA powder is compared with the result predicted using our model, which indicates that the liquid, which residue at the coating surface during deposition, causes discontinuous boundaries within the coating. And spraying rate and solidification rate are reverse periodically with spraying process.

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The oxidation of nickel has been studied in a temperature range of 800℃ to 1200℃ in the air. The oxidation kinetics obeyed a parabolic time relationship, and the estimated activation energy for growth of the oxide layer is 34,3 ㎉/mole. According to the marker test, nickel ions migrated much faster than oxygen ions and NiO formed on the oxide surface. The new oxide formed along grain boundaries, where oxygen ions diffusing inward along grain boundaries combines with nickel ions diffusing outward through the Ni grains. The structure of the interface between NiO and Ni substrate has been studied by using a transmission electron microscope and interfacial stuctures are discussed.

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The effect of the degassing treatment before HIPing on the microstructures and the mechanical properties were investigated using prealloyed PREP Ti-6Al-4V powder. Degassing condition was determined from the quantitative analysis of the extracted gases as a function of temperature and time. The determined degassing condition is heating up to 500℃ and then holding 2 hours in vacuum. The principal gases extracted by the treatment were H₂O and H₂. The compacts produced with cold degassing and HIPing exhibited slightly improved strength due to oxygen embrittlement with similar ductility and reduction of area to the cold degassed compacts. Dynamic properties, like fatigue strength, fracture toughness, fatigue crack propagation rate, were also not decreased compared to hot degassed compact. It was interpreted due to the high solubility of gas elements into the titanium matrix. Pores were, however, existed at prior powder particle boundaries due to dissolution of the adsorbed gases into the matrix in the cold degassed compact. The oxygen entered with powder particles located at α/βphase boundaries, and the hydrogen formed titanium hydride phase inside of the a phase.

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In order to identify experimentally the microstructural characteristic distance and the critical effective strain of the 2124-T6 aluminum composite reinforced with 15 v/o SiC whiskers, the effect of the notch root radius on the measured apparent fracture toughness was analyzed using a strain controlled fracture model based on the strain field ahead of blunt notch tip. In-situ SEM fracture test and fractographic observation were also conducted to investigate micromechanisms of fracture process. The calculated microstructural characteristic distance was nearly consistent with the average spacing between the observed fracture initiation sites. The SiC whisker clusters and the microcracks located at brittle intermetallic particles were found to act as major fracture initiation sites, thereby leading to the blocky roughness in the range from 10 to 20 ㎛ on fracture surfaces., These foundings suggest that fracture toughness of the composites is affected primarily by the local distribution of SiC whiskers and the existance of the brittle matrix constituents which are much larger microstructural parameters than the average whisker spacing.

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In polycrystalline Fe-30Mn-5Si alloys, the effects of deformation temperature and strain at various Co content on the shape memory effect(S.M.E.) and formation of εmartensite were studied. The deformation below M_d was governed by the r⇒ε mrtensite transformation, whereas it was due to the conventional slip above M_d without regard to Co concentration: The effect of Co might be the reduction of stacking fault energy of austenite, which resulted in increasing S.M.E. S.M.E, according to deformation temperature and strain decreased with increasing strain at the same temperature, while it increased with decreasing deformation temperature at the same strain. The amount of εmartensite decreased as deformation temperature increased and it changed little with Co contents. Regardless of Co content, Ma was not changed and A₃, A_f increased with Co contents.

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The magnetic phase transition taking place in melt-spun NdFe₂ alloys during annealing has been discussed. The high intrinsic coercive force(iHc=2∼3.5kOe) of NdFe_7(rhombohedral system of hexagonal cell) plus a Nd-rich(85∼90 at% Nd) matrix. After annealing the NdFe_7, phase exhibiting a Curie temperature of 230∼235℃ was found to form in the melt-spun NdFe₂ alloy. The NdFe_7 phase transforms into Nd_5Fe_(17) plus Nd₂Fe_(17) together with Nd-rich phase under a vacuum atmosphere. Under an oxygen-controlled atmosphere, the NdFe_7+(Nd-rich) phases transform directly into Nd₂O₃ plus α-Fe. The Nd_5Fe_7 phase is observed to be magnetically soft(iHc=0.18 kOe) and exhibits a higher Curie temperature(Tc=240∼245℃) than that of the NdFe_7, phase. The melt-spun NdFe₂ alloy shows a magnetic anomaly indicating the spin reorientation of NdFe_7 crystals at 325∼350 K.

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A new model for the continuous structural evolution during rapid solidification which includes the interactions between the external cooling, release of latent heat, solute diffusion and the capillary effect of solid-liquid interface was suggested. This model where the heat transfer and the solute diffusion procedure were calculated by a numerical and an analytic method each other, was based on the facts that the boundary layer thickness of the thermal diffusion field is much larger than the cell or dendrite spacing, while that of the solute diffusion field is smaller. The microstructural evolutions in the gas -atomized Al-Fe(2∼8wt%) droplets were obtained by this model. The main results are as follows. The discontinuous microstructural change from `Zone A` to `Zone B` with increasing solid fraction was predicted and their microstructural scales were calculated quantitively. In `Zone A`, the interface velocity is governed mainly by the solutal diffusion and the non-steady state solidification occurs. In `Zone B`, the interface velocity is determined by the heat removal rate of external coolant rate irrespective of the solute concentration and initial undercooling, and the steady state solidification occurs. Irrespective of the initial undercooling, droplet size(＞10㎛) and Fe Concentration(2∼8wt%), the `Zone A` and `Zone B` structures appear simultaneously during gas-atomization of AI-Fe alloys.

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