Tension–compression creep asymmetry in a turbine disc superalloy: roles of internal stress and thermal ageing

The tension and compression creep behaviour of an as-received and pre-aged IN100 disc alloy have been characterised in order to validate a previous hypothesis that the unusual response of low and even negative initial creep rates in tension was caused by the presence of an internal stress field within the alloy. Absolute values of initial creep rates in compression were found to be much greater than in tension and this asymmetric creep response is conclusive proof of the presence of an internal compressive stress field in the alloy matrix. The asymmetry was virtually removed by pre-ageing prior to creep and this is attributed to the decay of the internal stress. These features have been simulated using a microstructure-based creep model incorporating an evolving internal stress field. The model also simulates the additional (and complicating) reduction in general creep strength that is thought to be due to coarsening and dissolution of the smallest particles of the tri-modal γ ′ distribution in the alloy. The net consequence of these two competing thermal processes is that the short-term creep response is dominated by the initial magnitude of the internal stress field whereas coarsening and dissolution of the smallest γ ′ particles determines the long-term behaviour.