Cyclic deformation and precipitation behaviour of NiCr22Co12Mo9 during thermal fatigue

Specimens of the nickel-base superalloy NiCr22C12Mo9 (corresponding to Inconel 617) are thermally fatigued with a constant minimum temperature Tmin=473 K and maximum temperatures Tmax=873, 1123, 1323, and 1473 K. Cryclic hardening is observed in test with Tmax≤1323 K, whereas cyclic softening occurs in tests with Tmax=1473 K. Coherent and incoherent γ'-precipitates develop during cycling with Tmax=873 K. In experiments with Tmax≤1323 K the major secondary precipitates are found to be M23C6 carbides. In tests with Tmax=1323 K, carbide bands are produced in former twin areas. Almost no carbides are observed after thermal cycling with Tmax=1473 K. The thermal cyclic deformation behaviour of the material is discussed taking into account the occurring precipitation features and other microstructural changes. A model to explain the development of carbide bands is suggested. (orig.)