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Directional solidification studies have been carried out at the solidification rates of 0.8㎛/s-61㎛/s in the Ni₃Al composition. The microstructures formed at the solid/liquid interface were observed by the directional solidification technique that allowed quenching of the solidification interface. The dendritic interface was found at relatively high solidification rates and the planar eutectic interface at rates. Using this interface stability, the diffusion coefficient of liquid was estimated by the critical constitutional supercooling condition and it was in the order of 10^(-4)㎤/s which is about 10 times higher than usual. The equilibrium and non equilibrium eutectic structures were observed and the characteristics of these eutectic structures were also discussed.

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The present study is concerned with microstructural analyses of the surface layer of a high speed steel (HSS) roll modified by irradiation of accelerated electron beam. HSS roll specimens were irradiated using an electron accelerator, and the microstructures of the irradiated surface layers were examined. Upon irradiation, the irradiated surface layer was melted because of the large heat input, resulting in microstructural change from coarse cell structure of primary carbides to a mixture of fine carbide cell and retained austenite. This microstructural modification might improve fracture or fatigue properties of the irradiated surface layer, although its hardness was decreased and quench cracks were found occasionally in the heat affected region. In order to solve the problems for applying the accelerated electron beam technique to HSS rolls, appropriate processing methods such as pre-or post-irradiation were suggested.

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In order to prevent the cracking and fracture generated in the alumina/Inconel 600 joints after cooling, three types of insert materials with Ag-Cu-Ti filler metals was used in the joints. Effect of insert materials on the crack formation of the joints was investigated. The joints free from cracks were obtained when the laminates consisting of niobium of 250 ㎛ thick and copper of 50 ㎛ thick were used as insert materials. For the joints cooled at the rate of 4.8 ℃/min after brazing at 1113 K for 1.2 ks, the average shear strength was 75.7 MPa and the strength distribution showed the mixed mode.

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The α-Fe based Nd-Fe-B magnet with 4at% Nd content have been investigated for the development of the new type rare earth magnets. Amorphous Nd₄Fe_(85.5)B_(10.5) alloy prepared by rapid quenching technique crystallized to form only two phases, α-Fe as main phase and Nd₂Fe_(14)B as secondary phase. However, the alloy showed a poor magnetic properties (₁H₁= 12 kOe, B₁= 1.2 T). The additions of Mo and Cu to Nd₄Fe_(85.5)B_(10.5) alloy reduced the average grain size and increased the coercivity up to 2.6 kOe. Furthermore, the substitution of Co far Fe in Nd₄Fe_(82)B_(10)Mo₃Cu₁ alloy to 16 at% entirely improved the magnetic properties. The optimally annealed Nd₄Fe_(74)C_5B_(10)Mo₃Cu₁ alloy showed the best magnetic properties among the perpared alloys. The coercivity, remanence and energy products, of the alloy was 2.75 kOe. 1.42 T and 13 MGOe respectively.

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A small amount of rare earth metals(REM) was added in melts of 5Cr hot working tool steel in order to investigate the changes in cleanliness, shape control of nonmetallic inclusions, and mechanical properties in comparison with those of typical hot working tool steel. The important results obtained are summarized as follows. 1) REM addition in 5Cr hot working tool steel resulted in significant decreases in final oxygen and sulfur, and total nonmetallic inclusions content. 2) RE-complex inclusions in the form of fine globular less than 3㎛ were composed of RE-oxysulfide surrounded by RE-sulfide. 3) Impact toughness, machinability, and chipability were improved by REM addition, owing to the refining of prior austenite grain size and insoluble carbide, and RE-complex inclusions.

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Noise problem arising from transmission gears of automobiles is well known to be related to the oxidation of gear materials during crburizing and distortion in the subsequent quenching and tempering process. In order to develop new grade steels with minimal internal oxidation layer which has detrimental effects on gear life and noise reduction, the effects of alloying element, Mn, Cr and Si on the internal oxidation behavier during carburization were studied in comparision to the characteristics of internal oxidation for two commmom transmission gear materials, SCM415H, SCR420H. The chemical composition of the new grade is 0,17C -Si$lt;0.13-0.75Mn -1.0Cr -0.40Mo. The hardenability characteristics of new grade steel is equivalent to that of SCM415H and SCr420H and internal oxidation layer depth after case hardening was smaller than those of SCM415H and SCr420H. The depth of internal oxidation layer was decreased with decreasing the amount of Si which has the highest affinity for oxygen.

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Electrochemical methods were employed as nondestructive means of diagnosis of thermal aging embrittlement of cast duplex stainless steels of CF8 and CF8M grade. In polarization experiments conducted in 5wt% CH₃ COOH solution and the mixture of 0.5M H₂SO₄ and 0.01M KSCN, the anodic current density peak increased, with the aging time. SEM observations to investigate what gives rise to the increase of the anodic current density peak, however, indicated that such change was not related to the spinodal decomposition that was known to be the main cause of thermal aging embrittlement. In polarization experiments for CF8 grade specimens conducted in the mixture of 0.05M H₂SO₄ and 0.01M KSCN, two current density peaks, i_(peak1) and i_(peak2_ appeared. i_(peak1) was attributed to preferential dissolution of the ferrite phase and decreased by aging. The decrease of i_(paek1) was caused by the decrease of ferrite dissolution in aged specimens and reflected the degree of spinodal decomposition occurring in the ferrite phase. A correlation was established between the increase of ferrite hardness and Δi_(peak1)/ferrite%. It was suggested that thermal aging embrittlement of duplex stainless steels of CF8 grade could be evaluated nondestructively by electrochemical method.

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The experiments were carried out in order to provide the statisitical data with strength evaluation methods : shear, tension, and three - and four - point bending tests of Si₃N₄/Inconel 600 alloy joints. Average strength values of shear, tension, and three - and four - point bending were 178, 233, 321, and 344 MPa, respectively. In the viewpoint of Weibull moduli, tension and three - point bending tests showed smaller strength scattering than shear and four - point bending test. Differently from the monolithic ceramic, the average strength of Si₃N₄/Inconel 600 alloy joints decreased as the effective volue of test specimen increased.

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This study has been carried out to investigate for the dephosphorization equilibrium between Cao-SiO₂-CaF₂, CaO-Na₂O-SiO₂-CaF₂ stags and molten steel containing chromium in the temperature range of 1550℃∼1600℃. The results obtained from the experiment are as follows. The solubility of Cr₂O₃ is shown to be increased by increasing MgO content due to the MgO·Cr₂O₃ formation regardless of basicity in the slags. The solubility of oxygen in the CaO-SiO₂-CaF₂ slags system is decreased in low [Cr] region, increased with increasing [Cr] content. The solubility of oxygen in the CaO-Na₂O-SiO₂-CaF₂ slags system is about 250 to 450ppm in low [Cr] region and 250 to 400ppm in high [Cr] region. The observed values of L_o are in fairly well agreement with the theoretical values calculated.

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This study has been carried out to investigate for the dephosphorization equilibrium between CaO-SiO₂-CaF₂, CaO-Na₂O-SiO₂-CaF₂ slags and molten steel containing chromium in the temperature range of 1550℃∼1660℃. The distribution ratio of phosphorous is higher in the low[Cr] region than high[Cr] region in the CaO-SiO₂-CaF₂ slags system. And also, the distribution ratio of phosphorous in the CaO-Na₂O-SiO₂-CaF₂ slags system is higher than CaO-SiO₂-CaF₂ slags system. The values for phosphate capacity, C_P were discussed by using the slag compositions. The carbon content of molten steel containing chromium had a remarkable effect on the dephosphorization degree by increasing chromium, is due to the formation of excess Cr₂O₃ in the slags which reduces its phosphate capacity, as well as to the reduction of a_o, with increasing chromium content when carbon content is kept constant.

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