Solidification Microstructure and Mechanical Properties of Directionally Solidified Ni3Al Alloys = 일방향 응고한 Ni3Al 합금 의 응고조직 과 기계적 성질에 관한 연구

다국어 초록 (Multilingual Abstract)

Ni₃Al has been considerable research area due to its high temperature behavior increasing strength with increasing temperature. However, it is not well established whether the eutectic is composed of γ /γ´, γ´/β, or γ/ β. In order to understand solidification behavior of the eutectic structure, directional solidification experiments have been carried out with solidification rates near the Ni₃Al composition in this study. The effects of the solidification rate and composition on formation of the equilibrium and metastable eutectics have been discussed by using Jackson-Hunt eutectic model.
The (γ '+ γ) band and cellular coupled structure were formed in hypoperitectic Al23 alloy at low solidification rates. The effect of convection, sample length and solidification rates on the structure development have been calculated qualitative.
To increase both tensile ductility and strength of Ni₃Al alloys, the ductile phase of γ (Ni-rich) dendrite fibers or strong phase of β (NiAl) dendrite fibers are often arrayed in the γ´-(Ni₃Al) matrix by directional solidification. Dendrite spacing could be controlled by solidification rate, and the volume fraction of γ or β phase could be altered by alloy composition, for example, from 23 at.% Al to 27 at.% Al. With increasing Al content, the γ dendritic microstructure transformed to the β dendrite in γ´ matrix, reducing the tensile ductility by increasing the volume fraction of brittle β dendrites in γ´ matrix. With increasing solidification rate, the dendrite spacing decreased and the tensile properties of Ni₃Al varied in a complex manner. The microstructural evolution affecting the tensile behavior of directionally solidified Ni₃Al alloy specimens with different solidification rates and Al contents have been discussed.

목차 (Table of Contents)

• General Introduction = 1
• Chapter 1 Analysis on Eutectic near Ni₃Al Compositions = 3
• 1. Introduction = 3
• 2. Experiments methods and theory = 6
• 3. Results and Discussion = 10
• 4. Conclusions = 40
• Reference = 41
• Chapter 2 Band and Coupled Peritectic Microstructure Formation in Ni-Al alloy = 43
• 1. Introduction = 43
• 2. = Experiments47
• 3. Results = 47
• 4. Discussion = 56
• 5. Conclusions = 76
• Reference = 77
• Chapter 3 Mechanical Properties of Directionally Arrayed Dendrites in the Ni₃Al Matrix = 79
• 1. Introduction = 79
• 2. Experiments = 81
• 3. Results and Discussion = 81
• 4. Conclusions = 103
• Reference = 103
• General Summary = 105
• Abstract = 107
• Acknowledgements = 109
• Appendix A: Program for Calculation Undercooling vs. Eutectic Spacing Curves = 110
• Appendix B : Calculation Program for Band Structure = 112