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After nitrogen permeation heat treating the Al and Cu alloyed 13wt%Cr stainless steels in the nitrogen gas pressure of 1㎏/㎟ mainly at 1200℃, the surface nitrogen content,, phase changes, hardness variations and corrosion properties were investigated. The nitrogen dissolution and exhaustion of Al owing to the nitride precipitation cause to $quot;change the microstructure of the nitrogen permeated surface layer from α+γ(martensite) phase to γ single phase. The surface austenite phase contains round type aluminum nitrides and needle type carbonitrides. The nitrogen content of the surface layer increases after showing low content at the outmost surface due to the hindrance effect of carbon diffusing from interiors to surface. The Al and Cu move from γ phase to precipitate and Cr moves from precipitate to γ phase during nitrogen permeation heat treatment. At the nitrogen permeation temperature, the specimen representing γ single phase shows gradual decrease in the hardness with increasing depth below surface. while the specimen representing α+γ phase shows abrupt decrease in hardness due to the obvious nitrogen permeation layer boundary. The corrosion resistance of nitrogen permeated surface layer increases and remarkablely decreases in the 5%HCl solution and the 5%H₂SO₄ solution, respectively. compared to that of matrix.

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The precipitation kinetics of the detrimental sigma phase in Udimet 720Li has been characterized and quantified by varying exposure temperatures between 725℃ and 775℃ and exposure period up to 5000 hours. The electrochetemiyal extraction and the X-ray diffraction combined with the Rietveld analysis have been employed for the quantification of sigma precipitation. The sigma formation rate is increased and the incubation time for sigma nucleation is decreased with increasing temperatures. Sigma formation behavior has been analyzed by comparing the activation energy for sigma formation with the activation energy for the diffusion of sigma forming elements such as Cr, Co, Mo. Sigma precipitation is controlled by the diffusion of Cr segregated in the γ matrix adjacent to the primary γ′ in the early stage. and then controlled by the diffusion of Mo in the steady state. Sigma morphology was also investigated by observing the extracted residue. As the aging time increases, sigma morphology is changed from the globular shape to the dendritic. From the analysis mentioned above. a model of sigma precipitation behavior was schematically suggested.

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Creep-rupture properties of a cast type Superalloy, Mar-M247LC. were studied in 760℃ and 980℃. The shape and size of γ′ particles in grains were known to have significant influence on the creep behavior and rupture life in relation to the micro-mechanism of plastic deformation. It was found that the creep properties were also affected by the cast structure which is composed of dendrite matrix and eutectic boundaries. The hardening effect of boundaries by solute segregation and eutectic phases may improve the creep resistance of grain or eutectic boundaries during creep-rupture.

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The effect of transition elements and thermomechanical processing(TMP) on the superplastic behavior of Al-7%Mg-0.3%Cr alloys were examined and correlated to the microstructural evolution during superplastic deformation. To establish an optimum manufacturing process for the enhanced superplastic properties, different processing methods such as warm rolling and cold rolling were used. In cold rolled Al-7%Mg-0.3%Cr alloys with intermediate annealing. the maximum elongation (close to 730%) was observed at 530℃. On the other hand, low temperature superplasticity was not observed in Al-7%Mg-0.3%Cr alloys with the TMP developed by McNelley because of the suppression of continuous recrystallization due to the dispersion of intermetallics. However, when these TMPed Al alloys were tested at high enough temperatures above the recrystallization temperature, the maximum elongation reached approximately 400% In this study, the possible effect of solute-dislocation interaction on the high temperature ductility was also examined. The predicted strain rate range of high ductility associated with high strain rate sensitivity due to solute strengthening coincides with the range of high temperature superplasticity of Al-Mg alloys, suggesting solute strengthening may be contributory to high temperature superplasticity of Al-Mg alloy.

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The present research was performed to evaluate the long-term isothermal temper embrittlement resistance and carbide behaviors. The material used in this study is SA336F22, the forged heavy wall 2.25Cr-1Mo steel. Temper embrittlement susceptibility after isothermal aging for 36,000h at 430-515℃ was assessed with the shift of the 50% fracture appearance transition temperature (FATT_(50)), 40ft-lb transition temperature (vTr_(40)) and grain boundary segregation of phosphorus by AES. XRD and TEM-EDS examined the behaviors of carbide. The shift of FATT and vTr_(40) of 2.25Cr-1Mo steel were significantly increased up to 10.000h aging then slowly saturated at 515℃. At lower temperature, however. saturation occurred slowly. The type and composition of carbides were changed to the Mo-rich stable ones(Mo-rich Mo₂C and M_6C), which reduces the Mo content in matrix. And P-segregation was detected on the prior austenite grain boundary after isothermal aging. These results suggested that the temper embrittlement be related with P-segregation caused by Mo-consuming at the matrix.

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The strength of partially crystalline Al-Ni-Y alloys was found to exhibit the maximum when the size of Al particle is 10㎚. Partially crystallized Al-Ni-Y alloys were also found to be brittle when the size and the volume fraction of the Al particles exceeded the optimum values. On the assumption that the diffusivity of Y is so low that Y atoms rejected from FCC-Al particles are limited near interface region and the diffusivity of rejected Ni atoms are so high that the perfectly mixed solution is readily attained in the matrix, the reaction in three regions, Al particles, Y accumulated interlayers and amorphous matrix, are derived by balancing the quantity of each element. Al-Y-Ni alloys were assumed to become brittle when the solute content of interface layers reached the critical value (20%). The predictions based on the Y accumulation model are in good agreement with the published data available.

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Al-1wt%Mg was unidirectionally solidified to investigate the effect of enhanced gravity on the planar interface morphology. Planar solid/liquid interface morphology became unstable with increasing centrifugal force up to 15G but stabilized again at 20G. Surface stabilization of this alloy with increasing centrifugal force can be explained by thermosolutal convection.

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MC carbide trapping in Mar-M 247LC alloy was systemically investigated by directional solidification and quenching method. It was found that several kinds of differently shaped carbides formed according to the distance from the dendrite tip and solidification rates. The trapping behaviors between relatively higher solidification rate and slower solidification rate were different. The calculated maximum carbide nuclei size was close to the observed value when the sample solidification rate was slow, but it was smaller than the observed value when the sample solidification rate was high. The inter-secondary dendrite arm region was the preferred trapping site. At very slow solidification rate, the bottom of the mushy zone was also a trapping site while at high solidification rate the dendrite side wall became a trapping site. Reduction of carbide volume fraction at the slow sample solidification rate was caused by the low trapping ability of the solid-liquid interface and lack of suitable solidification site in the solid-liquid interface at, the slow solidification rate. At relatively higher solidification rate, the solid-liquid interface trapped the carbide nuclei in front of the moving interface In contrast. some difficulty to trap the carbide nuclei occurred at the slower solidification rate.

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Magnesium alloys have received an increasing amount of attention in recent years due to their exceptionally low densities and high specific strengths. Among various manufacturing methods for magnesium alloys, semi-liquid forming has advantages of near net shape and the safety over inflammable magnesium alloys. The objective of this paper was to produce a electroplating base for semi-liquid formed AZ91HP magnesium alloy, consisting of activations and zinc immersion deposition. The variations of the surface potentials with time were conducted both during activations and zinc immersion deposition. The results indicate that oxalic/chromic activation sequence can produce a good foundation for zinc immersion deposition and by the double-dip technique, thin and homogeneous zinc immersion deposits can be obtained in shorter time compared with the normal single-dip technique. The growth mode of zinc immersion deposits largely depends on the surface potential deviations caused by the differences of microstructures and phases.

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CrN_x film was deposited on S45C by cathodic arc ion plating process. The deposition of CrN_x layer was carried out at the various N₂ partial pressure of 8×10^(-4) torr to 2×10^(-2) torr under the bias voltage of -200V. All the coated specimens were analyzed by XRD and SEM for structural characterization and the wear and corrosion behavior was compared with that of hard Cr coated specimens by ball-on-disc wear test and electrochemical polarization measurement. The CrN_x coatings at all deposition conditions appeared to be very dense and fine granular structure. The CrN_x coating deposited at 1×10^(-2) torr showed the highest CrN(220) intensity and a maximum hardness of Hk 2700 which is twice of that of the hard Cr coating. The wear resistance of CrN_x coated S45C against AISI 52100 steel balls was highly improved compared to that of the hard Cr coating. The CrN_x coatings also enhanced the resistance of pitting and crevice corrosion. This improvement was associated with the formation of the CrN_x coatings with dense and fine granular structures without microcrack which exists in all the electroplated hard Cr coating.

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