Effect of Ta/W Ratio on Microstructural Features and Segregation Patterns of the Single Crystal PWA1483 Ni-Based Superalloy

Microstructure and microsegregation pattern of four Ni-based single crystal (SX) PWA1483 superalloys with different Ta/W ratios of 0.75, 1.0, 1.32, and 1.5 (in wt.%) were investigated. The alloys were directionally solidified by the Bridgman method with a withdrawal rate of 3 mm/min and thermal gradient of about 7 K/mm, followed by an age hardening heat treatment. The experimental observations showed that the Ta/W ratio influenced microstructures and elemental microsegregation patterns of the SX superalloys. Based on the EDS results, W, Mo, Cr, and Co segregated to the dendrite core regions, while Ta, Ti, and Al to the γ/γ′ eutectic and interdendritic zones. The microsegregation degree of Ta and Ti was increased with increasing Ta/W ratio. In addition, Ta and Ti tended to partition to γ′ phase while Mo, W, Co, and Cr to γ matrix phase. By increasing the Ta/W ratio, microporosity was decreased, while the volume fraction of TCP phase and γ/γ′ eutectic structure increased. The superalloy with the Ta/W ratio of 1 showed the best homogeneous distribution of alloying elements in both γ and γ′ phases with no sign of secondary grain defect.