This study has investigated the effect of aging treatment on the microstructure and mechanical properties of Mg-6Al-xZn(x = 1.5, 2.5) alloys fabricated by the squeeze casting process. The microstructures of as-squeeze cast were composed of pro-eutectic ${\alpha}$, super saturated ${\alpha}$ and ${\beta}(Mg_{17}Al_{12})$ compound. Aged at both $200^{\circ}C$ and $240^{\circ}C$, Mg-6Al-xZn alloys showed the peak hardness due to the formation of ${\beta}(Mg_{17}Al_{12})$ precipitates. The discontinuous precipitates of the lamella type are predominant at $200^{\circ}C$ aging treatment, while the finely dispersed continuous precipitates were dominant at $240^{\circ}C$ aging treatment. Mg-6Al-xZn alloys fabricated by the squeeze casting process had the better combination of tensile strength and elongation compared to the conventionally cast alloys. As zinc contents increased, the tensile strength was increased by the solid solution strengthening effect of zinc.

The effect of post oxidation, water-quenched after holding in air for 5~420 seconds or cooling or furnace cooling, on corrosion resistance and phase formation characteristics of the surface layer of SM20C and SM45C carbon steels after gas nirtrocarbursing in the $NH_3-5%CO_2-N_2$ gas atmosphere at $580^{\circ}C$ for 3hours is studied. The compound layers of two steels consist of ${\varepsilon}-Fe_{2-3}N$, ${\gamma}^{\prime}-Fe_4N$ and $Fe_3O_4$, phases, however, the quantity of ${\gamma}^{\prime}-Fe_4N$ phase increases for the furnace cooled specimen compared to that of air cooling specimen. With increasing $NH_3$ content in the gas mixture and also increasing the keeping time in the air after gas nitrocarburising, the ${\varepsilon}-Fe_{2-3}N$ phase of compound layer increases, while the decreased current density recognizing the improvement of corrosion resistance are shown. the passive current density of SM45C steel is lower than that of SM20C steel at the same nitrocarburising conditions.

The present study were investigated changes of precipitation behaviour of laves phase in ferrite single phase and ferrite-martensite dual phase and precipitation of laves phase under stress. Hardness changes in ferrite phase appeared two hardness peaks by precipitation of initial fine precipitator and laves phase in 3Mo-0.3Si and 3Mo-0.3Si-C specimens, respectively. Hardness changes in martensite phase of 3Mo-0.3Si-C specimen was lower in the initial stage of aging by carbide precipitation and after this, increased by re-hardening due to precipitation of laves phase. In the ferrite phase, laves phase was mainly precipitated, whereas in the martensite phase, carbide was preferentially formed during the initial stage of aging and with increasing aging time, laves phase and carbide were simultaneously precipitated by precipitation of laves phase at around carbide. In the ferrite-martensite interface, laves phase was mainly precipitated and carbide was mainly formed at boundary of lath martensite than grain boundary. Adding the stress in aging, fine precipitator of inital precipitation of laves phase precipitated in (100) of perpendicular to tensile direction and has grown to only followed<010>direction and also, volume fraction of laves phase increased. Consequently, the stress added was accelerated initial precipitation of laves phase.

In the present study, oxidation behavior of 304 and 316 stainless steels was investigated. After solution treatment, specimens were polished up to $1{\mu}m$ $Al_2O_3$ grade and then subjected to oxidation treatment in dry air. The range of temperature was used for oxidation treatment at $300^{\circ}C{\sim}500^{\circ}C$ and TEM was used for analyzing the components and structure of oxide film. Also, these results were compared with the results of ESCA and TG. According to the results of TEM analysis, it was found that Cr oxide film was formed on top of the surface after room temperature oxidation but amorphous Fe oxide was formed on top of the surface and polycrystalline $(Cr,Fe)_2O_3$ was formed below the amorphous Fe oxide layer after $500^{\circ}C$ oxidation treatment. The oxidized specimens at $500^{\circ}C$ showed that 316 stainless steel resists more strongly to grain and grain boundary oxidation than 304 stainless steel. These results suggested that Mo component resolved in 316 stainless steel matrix suppressed the formation of Cr carbide which may results in local Cr deplete area.

Rotary bending fatigue tests were carried out to investigate the improvement of fatigue limit in annealed and austempered spheroidal graphite cast iron. Main results obtained are as follows. (1) The tensile strength(hardness) of Series C is higher than that of Series B, and fatigue limits are 245MPa in Series C, 230MPa in Series B and 195MPa in Series A, respectively. (2) The fatigue limits of Series B and Series A are mainly governed by the resistance to fatigue crack initiation. Whereas, the fatigue limit of Series C is governed by the resistance to fatigue crack initiation and growth. The defect size and the resistance to crack initiation and growth should be considered to clarify the fatigue properties in spheroidal graphite cast iron. (3) Improvement of fatigue limit by half-austempering is more reasonable than that of full-austempering treatment in multi defective materials as spheroidal graphite cast iron.

Microstructure of Al-Si coated PWA 1426 alloy was studied. Diffusion coated specimens were heat treated for 4hr at $870{\sim}1087^{\circ}C$ and then were examined the changes of microstructure and interfacial compound by optical microscopy, SEM and EDS. According to the result of EDS, it is supposed that the coated layer was composed of $Ni_2Al_3$. When diffusion treatment was conducted at $1087^{\circ}C$, coated layer varied from $Ni_2Al_3$ to NiAl phase and composed of mixed, denuded and inter-diffusion layer.

Graphitization of cementite in high carbon steels at subcritical temperature is reported. We have studied about the effects of Ni in these steels on graphitization. The chemical compositions of the specimens were Fe-(0.54, 0.7)%-C-1.0%Si-0.1%Mn-(0.2~1.0)%Ni. After annealing at $650^{\circ}C$, $680^{\circ}C$ and $700^{\circ}C$ during various time the microstructures and hardness change were observed. In order to recover the initial hardness of high carbon steel, dissolution treatment of graphite was performed at $870^{\circ}C$. In case of 0.7%C steel, graphitization was accelerated rather 1.0%Ni addition steel than 0.2% and 0.54%Ni addition steels but the graphite is coarser. In case of 0.54%C-0.2%Ni steel, graphite particles were distributed relatively homogeniously and finely. Nickel addition promotes graphitization of these steels but makes graphite blocky.