Development of Low-Cost Polycrystalline Nickel-Base Superalloys for Gas Turbine Applications
Polycrystalline nickel-base superalloys have found extensive application throughout the aerospace, petrochemical and power generation sectors, due to their exceptional balance of high temperature mechanical properties, thermal stability and oxidation resistance. Extensive alloy development has produced a diverse class of alloys suited to a wide range of operating conditions. This work details research performed to develop new low-cost polycrystalline Ni-base superalloys, intended for intermediate temperature and loading environments. There is currently significant industrial demand for alloys of this type, that offer improved thermal stability and mechanical properties compared to current commercial alloys such as Inconel 718 and ATI 718Plus, whilst retaining good amenability to deformation processing and joining operations. The important properties required of polycrystalline Ni-base superalloys are discussed alongside an initial comparison of current commercially available materials and a potential class of candidate alloys suitable for further development. Compositional modifications were made to these alloys in order to improve their thermal stability and reduce their cost. Characterisation was performed after a standard ageing heat treatment, and after long term thermal exposure at 700˚C, revealing significantly improved resistance to precipitation of the δ phase in these alloys compared to Inconel 718. Despite this, comparable measurements of the hardness values indicated similar mechanical strength. The oxidation resistance of the candidate alloys was also assessed through 1000~hour exposures in air at 700˚C and 800˚C. The alloys were found to be chromia forming, with internal oxidation of Al forming a discontinuous subscale. Significant microstructural degradation was observed at 800˚C due to the formation of the δ phase, with the differences in oxidation behaviour rationalised using Wagnerian diffusional analyses. So that further investigation of fundamental alloy behaviour could be performed, industrial quantities of three candidate alloys were produced. The initial characterisation of these materials is described, including a detailed study of the ageing response after a controlled solution heat treatment performed using a quenching dilatometer. The observed precipitate coarsening was well described using classic models, and was correlated to the hardening response by comparison to the contributions from strong- and weak-pair dislocation coupling. The deformation behaviour of small-scale thermomechanical compression specimens of the preferred candidate alloy was studied under both sub- and super-solvus forging operations. Subsequent microstructural examination was used to assess the extent of recrystallisation, and characterise the microstructural evolution that took place during deformation. In the supersolvus trials, fully recrystallised microstructures were obtained, with partially recrystallised microstructures evident in the sub-solvus trials. These results, combined with the flow curve data were used to specify large-scale forging of the industrially produced compacts. In an exploratory study of the weldability of these alloys, electron beam welding was used to produce autogenous bead-on-plate welds using three different welding conditions. All the welds produced were fully penetrating, with no deleterious defects characterised in any of the conditions studied. Using scanning electron microscopy and large area hardness contour mapping, the effects of a post-weld heat treatment on the as-welded microstructure were assessed. The restoration of mechanical properties across the weld cross-section was successfully correlated with the local distribution of precipitates within the fusion zone after heat treatment. The results indicated that the alloys were amenable to joining via this process. The research described details the development of new low-cost polycrystalline Ni-base superalloys from initial compositional design, through to industrial production, including detailed studies of the phase equilibria, thermal stability, microstructural evolution and oxidation resistance. In addition to demonstrating superior microstructural stability compared to current commercial alloys, the alloys developed appear to be readily formed via conventional deformation processing routes, and can be joined without producing defects using electron beam welding. With additional work to verify the detailed mechanical response of the alloys produced, it is hoped that they will find industrial application in the near future.
Forging, Gas Turbines, Metallurgy, Microstructure, Ni-base Superalloys, Oxidation, Precipitation, Recrystallisation, Welding
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