Effects of Alloyed Aluminum and Tantalum on the Topological Inversion Behavior of Ni‐Based Single Crystal Superalloys at High Temperature

The application of Ni‐based superalloys with a high γ′ volume fraction at high temperatures can induce topological phase inversion, where the γ′ phase topologically becomes the matrix phase (Acta mater. 49 (2001) 4017), resulting in subpar material properties. Topological inversion behaviors during ageing at 1200 °C of five experimental single‐crystal alloys (as‐cast and heat‐treated) that contain various γ′‐forming elements, namely, Al and Ta, are investigated. Under the same temperature condition, the topological inversion tendencies in the high‐Al and high‐Ta alloys are stronger than those in the low‐Al and low‐Ta alloys, respectively. The high‐Al alloy shows a stronger tendency toward topological inversion than the high‐Ta alloy do under the same γ′ volume fraction condition, that is, element Ta can decelerate the topological inversion. Meanwhile, the topological inversion of the γ/γ′ microstructure takes place when the total contents of Al and Ta elements in the alloys are higher than (10.5–11.0) wt%, and it can be suppressed effectively by slightly regulating the γ′‐forming elements Al and Ta. Moreover, the narrow γ channel can promote the topological inversion of the γ/γ′ microstructure after the loss of coherency in the γ/γ′ interfaces.