In-situ Measurement of Recovery and Recrystallisation in Ni-base Single Crystal Superalloy via Electrothermal Mechanical Testing

Single crystal Ni-base superalloys are an advanced family of super strength alloys designed for use in modern gas turbine engines. These alloys exhibit superior performance at elevated temperatures compared with conventional alloys but this comes with the need for state-of-the-art manufacturing methods to produce viable components. Inevitably, this comes with a high cost. Recrystallisation during high temperature solution treatment is one of the main drivers of scrap costs for the aerospace casting sector. Several key factors remain unknown including the exact conditions leading to secondary grain nucleation and the search continues for reliable techniques which provide valuable experimental data for use in process modelling.

The work presented here demonstrates the capability of electrothermal mechanical testing as an experimental technique for the measurement of recrystallisation and recovery behaviour of superalloys at temperatures exceeding the γ′ solvus. Key understanding of the importance of the historical strain path on the consequent recrystallisation behaviour has been studied. A key finding is that high temperature deformation does not induce recrystallisation behaviour akin to that seen when strain accumulates over a dynamic temperature range, the latter being conditions far more akin to those seen during industrial casting.

Furthermore, eletro-thermal mechanical testing (ETMT) has proven to be an effective measure of material recovery during isothermal holding at super-solvus temperatures. Recovery is strongly reflected in the electrical properties of the material when held at super-solvus temperatures. Another key observation was a clear difference in the magnitude of these observations between the recovered material and the recrystallised material. These studies have set out several findings from which future research can be built to further the understanding of the mechanisms governing super-solvus recrystallisation during heat treatment of single crystal Ni-base superalloys.