Effect of Cr on microstructural evolution of aluminized fourth generation Ni-base single crystal superalloys

An effect of Cr on microstructural evolution of aluminized fourth generation Ni-base single crystal superalloys was investigated. Aluminized fourth generation alloys, having different amounts of Cr in substrates, were examined at 1100 °C up to 300 h. After aluminizing, all alloys exhibited similar phase constitutions, however, they showed distinctive morphologies of secondary reaction zones (SRZs): an alloy containing the highest Cr content developed continuous SRZs under an inter-diffusion zone (IDZ), whereas the other alloys hardly developed SRZ formation. The highest Cr containing alloy particularly exhibited higher coverage of SRZ after aluminizing, and this suggests higher Cr in substrate encourages SRZ formation. All alloys developed more SRZ formation after 300 h, and higher Cr containing alloys precipitated more topologically close-packed (TCP) phases in substrate. All alloys demonstrated increases of SRZ coverage, whereas penetration depth of SRZ in each alloy was not significantly different after 300 h. Higher Cr containing alloys precipitated TCP phases in substrate after 300 h, and this prevents further growth of SRZ.