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Title: Hot forging of the nickel-base superalloy ATI 718Plus
Author: Kienl, Christiane
ISNI:       0000 0004 8501 4590
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Nickel-base superalloys are widely applied in the hot section of aero engines due to their high temperature capabilities and excellent mechanical properties. For turbine discs, polycrystalline nickel-base superalloys are used where they experience temperatures up to 700ºC and stresses as high as 1000MPa. The manufacturing process, especially the forging and heat treatment, plays a pivotal role towards achieving the final microstructure and mechanical properties needed to withstand this harsh environment. ATI 718Plus® (718Plus) is a polycrystalline multi-phase strengthened nickel-base superalloy for turbine disc applications in aero engines. Its main strengthening phase is γ' but it was also found to precipitate the hexagonal η phase. This alloy is used for studying the microstructural changes due to recrystallisation and deformation occurring during forging. The body of work includes the testing and characterisation of 718Plus with and without η. High temperature compression tests were performed in the temperature range of 850 to 1025ºC, strains between 0.4 and 1.2 and strain rates from 0.01 to 1s-1. To analyse the materials behaviour during meta-dynamic recrystallization, a second set of samples was kept in the furnace for 120s after the compression at high temperature before they were water-quenched. The microstructural changes were investigated using both scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis. Dynamic and meta-dynamic recrystallisation were analysed upon their sensitivity on the testing parameters. Interestingly, an increase in recrystallised fraction with strain rate was found, which is in line with reported values on 718Plus but in contrast to similar nickel-base superalloys such as IN718 or Waspaloy. A detailed analysis of TEM specimens revealed that the higher fraction in recrystallised area was due to meta-dynamic recrystallisation during the transfer time between deformation and quench. In the γ\η microstructure the same mechanism could be observed. In addition, lamellar packets of η phase were found to inhibit recrystallisation and promote a narrower grain size distribution. The deformation mechanisms of 718Plus during high temperature compression testing was analysed by an extensive TEM study. A number of grains formed deformation twins. This mechanism is commonly found in materials with a low stacking fault energy at low temperatures and/or high strain rates. Initial stages of twinning could be captured and allowed a detailed analysis of the underlying processes. In the γ\η microstructure the deformation characteristics of the η were studied. η phase adjusted to the strain by breakage and realignment and also by severe bending.
Supervisor: Rae, Catherine Sponsor: Rolls-Royce plc ; EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Nickel-base superalloy ; recrystallization ; deformation twinning ; DRX ; MDRX ; 718Plus ; high temperature compression test ; TEM ; EBSD