Analysis and modelling of the dislocation response during non-isothermal creep in Ni-SX superalloys

Nickel-based single-crystal superalloys are commonly used as the material of choice for turbine blades in modern aircrafts. The temperatures and stresses in the turbine are subject to continuous change during in-flight operation. The deformation rates of the turbine blades during operation are currently approximated based on isothermal creep tests. This dissertation seeks to understand how the high temperature creep response of the single-crystal nickel-based superalloys CMSX-4 and CMSX-10 changes when subjected to a non-isothermal test cycle varying between the tertiary and rafting creep regimes. For this purpose, creep tests cycling between a base temperature of 900 ◦C and a peak temperature of 1050 ◦C under a constant load (σ = 200 MPa) were carried out and the samples were subse- quently analyzed using TEM dislocation analysis. The creep results displayed significantly faster non-isothermal strain rates than would be expected when adding up the isothermal strain rates at each temperature. By studying tests interrupted at different stages of creep, it is argued that the faster strain accumulation results from a higher dislocation activity compared to isothermal studies which are driven by the non-isothermal evolution of the interfacial lattice misfit. Furthermore, the thermal cycling creep rate under these conditions depends on the creation of interfacial dislocation networks that can take two shapes (a classical edge-type network, or one in which the dislocations are paired) and their disintegration by the γ′-shear of dissimilar Burgers vector pairs. The experimental findings were used to create a non-isothermal creep model to better understand the dislocation-based creep response. In particular, the role of γ′-shear and dislocation glide and climb in the γ-phase on the overall creep resistance of the alloys were examined with the model. Furthermore, the rate of non-isothermal micro-structural transfor- mation (via rafting and coarsening) and its impact on the creep response was examined. The non-isothermal creep model is based on the evolution of key creep parameters during thermal cycling. These parameters include the effective γ-interdiffusivity, the γ-Orowan resistance, the interfacial lattice misfit, the anti-phase boundary energy, the γ′-critical resolved shear stress and the solid solution hardening in both phases. The computation of the key creep parameters was based on an in situ measured phase fraction evolution of the alloy CMSX-4.EPSRC and Rolls-Royce plc