Characterization and modelling of the mechanical properties of the isolated phases in γ/γ’ microstructures

Ni superalloys are a class of alloys most often used for high temperature applications such as critical hot section engine components in aircraft gas turbine engines in aerospace industry. These materials are suitable for such applications due to their excellent high temperature properties such as strength, creep resistance, low cycle fatigue, oxidation and corrosion resistance. Such unique properties are mostly attributed to the microstructure arrangement: ordered L12 structured Ni3Al based intermetallic γ′ precipitates distribute in disordered γ matrix. It is known that the applied heat treatment can have a significant impact on the γ′ distribution and therefore the mechanical properties. The main objective of this research is aimed at finding a relationship between the microstructures and macro-mechanical properties in nickel based superalloys using various characterisation and computer simulation. Different cooling paths from a supersolvus temperature have been applied to FGH96, a polycrystalline nickel base superalloy for turbine disc applications, in order to simulate the different microstructures that exist through the thickness of a disc following an industrial heat treatment. The microstructures have been evaluated in terms of secondary and tertiary γ’ size distribution, morphology and volume fraction for the different heat treatments using SEM and post digital image software. Detailed compositional data for both γ’ precipitate and γ matrix that was obtained from APT analysis has been presented and compared to equilibrium compositions calculated by Thermo-Calc. For the heat-treatments studied, the secondary γ’ composition indicates a shell of differing composition to that towards the precipitate core. From sequential equilibria compositional calculations, it is suggested that the ‘shell’ forms at a lower temperature than the precipitate core. The fine tertiary precipitates do not show the core-shell compositional differences on continuous cooling. W peaks are noted at the γ/γ’ interfacial region, which is of significance for retarding coarsening. A γ’ depletion zone surrounds the secondary precipitates, within which the γ matrix composition differs significantly to the γ far-field values. A precipitate nucleation and growth mechanistic model is also suggested based on the experimental data and Thermo-Calc calculations. Mechanical properties of this material were studied by means of nano-indentation and high temperature creep tests. The nano-indentation results show that the higher hardness and reduced modulus values for both secondary γ’ precipitates and γ matrix are found in faster cooling rate samples. Tensile creep tests show that the creep response is inversely proportional to secondary γ’ size. Tertiary γ’ volume fraction was also found having a great impact on the material’s creep properties. The TEM micrographs show that matrix dislocations could be the precursor of microtwin formation and creep deformation modes are closely related to tertiary γ’ precipitates. Tertiary γ’ size determines whether a/2 dislocations shear or dissociate before entering tertiary γ’ and the tertiary γ’ volume fraction determines how matrix dislocation dissociates. A CPFE model has been developed based on the experimental results. The simulation results showed that the modelled secondary stage creep rate is in very good agreement with the experimental results. In another attempt of applying this model to a turbine disc shows that different locations experience different creep strain rate which could potentially cause damage nucleation and component distortion. Finally, long term ageing experiments have been performed on this material with times up to 3000 hours over temperatures ranging from 700 to 850°C. The coarsening data from each ageing condition was analysed and a coarsening model was proposed based on the data. Among the available theories, particle size distributions during ageing were found to agree with BWEM model. Coarsening rate constants are acquired from measured secondary γ’ particle size. Both cube law (r3 vs. t) and square law (r2 vs. t) analyses have been attempted to describe the evolving particle radius as a function of ageing time. The rate constants derived from experimental data are compared with the calculation values from two models: volume diffusion and trans-interface diffusion through the γ/γ’ interface. The results indicate that both Al and/or Cr volume diffusion and Al trans-interface diffusion could be the rate limiting steps in coarsening process.Open Acces