The surface phase changes, the hardness variations, the nitrogen contents and the corrosion resistances of 17-4 PH stainless steel have been investigated after nitrogen permeation(solution nitriding) at a temperature ranges from $1050^{\circ}C$ to $1150^{\circ}C$ The phases appeared at the nitrogen-permeated surface layer were shown to martensite plus austenite and austenite, depending on the variation of nitrogen and chromium contents. And the surface hardness was also depended on the phases appeared at the surface layer from 370 Hv to 220 Hv. The precipitates exhibited at the nitrogen-permeated surface layer were niobium nitride, niobium chromium nitride and carbo-nitride in the austenite and martensite matrices. The surface nitrogen contents were followed by the Cr contents of the surface layers, representing 0.55% at the temperatures of $1050^{\circ}C$ and $1150^{\circ}C$ respectively, and 0.96% at $1100^{\circ}C$ at the distances of $60{\mu}m$ from the outmost surface. From the comparison of the corrosion resistances between nitrogen-permeated and solution-annealed steels, nitrogen permeation remarkably improved the corrosion resistance in the solution of 1 N $H_2SO_4$ due to the increase of nitrogen content in the surface austenite phase.

Nitriding or carburizing of carbon steels results in good mechanical properties such as high surface hardness and wear resistance but it has no affection on the corrosion resistance. Corrosion properties of nitriding and carburizing steels could be deteriorated. So, recently, there have been great demand for oxi-nitriding to enhance both mechanical properties and corrosion resistance. In this study, the corrosion resistance of carbon steel, S35C, and free cutting steel, SUM222, are prepared by vacuum nitriding and vacuum post-oxidation were compared with those treated by nitriding. After vacuum post-oxidation, $5{\mu}m$ oxide layer was formed on the nitride layer with $20{\sim}30{\mu}m$ depth. Potentio-dynamic polarization curve in corrosion test showed that the corrosion potential after post oxidation was increased from 200 mV to 800 mV in S35C and from 600 mV to 1200 mV in SUM222. SEM analyses showed that pores was increased and surface roughness became rougher with post oxidation. However, the formation of $Fe_3O_4$ resulted in the enhanced corrosion resistance of steels.

Development of Rotation-Cylinder Method(RCM) is given for Mg-Al-Zn-X(X=Sr) alloys, identifying some of the key factors that need to be controlled and indicating subsequent casting processes. Effects of Sr addition on the microstructure and mechanical properties of as-cast and T6 heat-treated Mg-Al-Zn-X alloys fabricated by the RCM were investigated. The microstructural and mechanical characterizations were performed by utilizing optical microscope, scanning electron microscope, transmission electron microscope, hardness test and ultimate tensile test. The solution and aging treatment time was varied to optimize the T6 heat treatment conditions, and experimental results were discussed. The grain size of Sr containing alloys was refined by increasing Sr content, but the tensile and yield strengths were increased by addition up to 1wt% Sr.

Microstructural evolution of Pt-aluminide coated Ni-based superalloy has been investigated with ductilization heat treatment. The Pt coat was prepared on the superalloy and then aluminide coating was conducted using a pack cementation process. Samples were heat-treated at $1050^{\circ}C$ for 2 hrs and the microstructure and element analysis were preformed. A various precipitated compounds were observed within the coating layer and the diffusion region in the Pt-aluminide coating and heat treatment, indicating that the bi-phase compounds of $PtAl_2$ and NiAl were performed during the Pt-aluminide coating, whereas $M_{23}C_6$, MC, $Ni_3Al$ and ${\sigma}$ phases were precipitated in the inter-diffusion region. The bi-phase compounds of $PtAl_2$ and NiAl were transformed into the single phase compound of $PtAl_2$ with the heat treatment, increasing the amount of carbide and ${\sigma}$ phase.