The development of a dual microstructure heat treated Ni-base superalloy for turbine disc applications

• Main Author: Connor, Leigh
• Published: University of Cambridge 2009
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597898

This thesis considers the effect of applying a dual microstructure heat treatment (DMHT) process to two turbine disc forgings. Each for the forgings has been manufactured from a different polycrystalline Ni-base superalloy. Particular emphasis is placed on fully characterising the microstructure of each forging. The subsequent evaluation of mechanical properties can then by compared to the microstructure of that region. In chapters 3 and 4, the microstructural response of both forgings to the DMHT process is established. Chapter 3 looks at the effect of the heating rate and maximum temperature achieved during the DMHT process and compares it to a detailed characterisation of the primary γ’ precipitate population and the γ grains. Chapter 4 evaluates the effect of the cooling cycle from the DMHT process and compares it to the development of intra-granular secondary and tertiary γ’ populations. The work contained in chapter 5 examines the tensile performance throughout both of the forgings, and remarks on whether any negative effect on mechanical properties has developed due to the DMHT process. Comparisons are made between measured tensile properties and microstructure, a post mortem review of the failed fracture surfaces and TEM analysis of the failed tensile specimens are also undertaken. Any change in deformation behaviour between a fine γ grain microstructure in the bore and a coarse γ grain microstructure in the rim can then be observed. In chapter 6 the effect of creep deformation on the RR1000 DMHT forging is determined. The microstructural variations that occurred as a result of the DMHT process are compared to the measured creep properties to determine the advantages and disadvantages of the DMHT process. This will ensure not only it is not detrimental to the creep properties, but the process also provides a foundation for improved temperature capabilities at the rim of the disc. A post mortem TEM analysis of the failed tensile specimens is also undertaken to discover if a change in the creep behaviour occurs between the microstructures observed in the bore and rim of the forging.