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A series of directional solidification experiments have been conducted to elucidate the formation mechanism of eta and Cr-rich phases in the Ni-base superalloy IN792 + Hf. Both eta and Cr-rich phases were found to be the final solidification products developed from the remaining liquid after c/c¢ eutectic reaction. The (Ti + Ta + Hf)/Al ratio in the residual liquid played a significant role in the nucleation of eta phase. During the solidification of c/c¢ eutectic, the continual increase of (Ti + Ta + Hf)/Al ratio in the residual liquid eventually led to the completion of c/c¢ eutectic reaction and caused the nucleation of eta phase. The results of electron probe microanalysis and transmission electron microscopy revealed that the Cr-rich phase was Cr, Mo, and W containing M 5 B 3 and M 3 B 2 type borides. The formation of these boride phases was found to be strongly influenced by the formation of c/c¢ eutectic. Because of the limited solubility of Cr, Mo, and W in c¢ phase, these elements were enriched in the residual liquid during the solidification of c/c¢ eutectic. In addition, boron would preferentially segregate into liquid due to its very limited solubility in both c and c¢ phases so that the possibility of boride formation in the residual liquid ahead of the c/c¢ eutectic was increased. A modified Scheil model was adopted to explain the influence of solidification rate on the formation of eta phase and borides, and the results were discussed.

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Microstructural evolution during high temperature exposure and its effects on tensile and stress rupture properties of the Ni-base superalloy B1900 have been studied. Tensile deformation of the as-cast specimen was concentrated in the localized slip bands in general. Stacking faults and deformation twins were observed in the as-cast tensile specimen tested at 871°C where the alloy exhibited the lowest ductility. Dense dislocation network formed at γ/γ´ interface during thermal exposure caused homogenous deformation in the thermally exposed tensile specimen. Thermal exposure did not have significant effect on the stress rupture lives of the alloy at 760°C and at and above 871°C but it reduced stress rupture life of the alloy at 816°C γ´ coarsening and coherency loss at the γ/γ´ interface during thermal exposure were primarily responsible for the deterioration of mechanical properties and characteristic deformation behavior of the alloy.

Mechanical and corrosion properties were evaluated in the nitrided and non-nitrided specimens of ASTM 355 and SUS422 steels. The oxides formed on the specimen were analyzed using by XRD and the weight change was measured after oxidation test at high temperature, high pressure, and steam conditions. The oxidation behaviors and rates between two alloys and nitrided/non-nitrided specimens were clearly different. XRD analysis showed that the nitrides formed on the nitrided specimen were composed of CrN, Fe4N, and Fe2-3N. Also, it was found that the hardness on the nitrided specimen at 283°C decreased from Hv 1150 of room temperature to Hv 425.