Recrystallization and precipitation of gamma prime in the nickel based superalloy AD730 during forging

Project aim

Propose and validate a mechanism for managing the occurrence of large, deformed grains in the nickel superalloy AD730 at the end of the industrial forging sequence, and produce a thermo-mechanical treatment that results in the desired material microstructure.

Project background

The nickel-based superalloy AD730 was patented in 2012, and intended for use in jet engine turbine discs. Due to relatively low amounts of expensive alloying elements, AD730 was seen as a cheaper alternative to other superalloys. However, the industrial forging of AD730 has proved challenging and has not produced the required microstructure. An ideal microstructure would consist of small, recrystallized γ grains, with fine and dense precipitation of γ’ precipitates. Instead, several large, deformed grains are present alongside small, recrystallized grains. This inhomogeneous structure is not only deleterious in terms of mechanical properties but also causes the alloy to fail industry-standard ultrasonic testing. The occurrence of these large, deformed grains has attracted some attention in the literature, but as yet there are no theories concerning their formation, or more importantly, their elimination.