Abnormal Grain Coarsening in Inconel 718 Aero-Engine Disc Forgings

  • Richard Watson

Student thesis: Phd

Abstract

Regions of abnormally coarse grains, have been observed in an industrial forgingduring a routine metallurgical investigation. These abnormally large grains are notpredicted by current recrystallization models and appear seemingly at random.There is currently great uncertainty about the effect on mechanical properties thatthese microstructures have and consequently the appearance of abnormallycoarse grains leads to the scrapping of the forging process and re-tooling of dies,both at great expense.This work aims to improve the understanding of this microstructural phenomenonas well as providing guidance in avoiding and preventing abnormally coarse grainsin future industrial forgings. Preliminary experiments showed that abnormal graincoarsening formed during a 980°C solution anneal though it was not clear if thiswas due to a recrystallization or grain growth process. The initial investigationgave an early indication that grew from newly recrystallized grains, inferred fromthe stored energy difference between normal and abnormal microstructuresassessed using EBSD data. The first set of results also indicated that delta-phasevolume fraction or morphology might be to blame as there was clear evidence thatthe delta-phase was not pinning grain boundaries as precipitates were clearly visibleinside grains away from grain boundaries.Further work, utilising isothermal compression, Gleeble compression andisothermal torsion tests, was able to confirm that this phenomenon occurs over aset range of parameters that can be plotted on a processing map. This work hasalso shown that all observable grain growth comes from new grains, with the asdeformed microstructure showing little change as a result of recovery orsubsequent grain growth. It is because of this that abnormally coarse grainformation should be considered a recrystallization process with new grains formingthrough strain induced boundary migration (SIBM). What sets this process apartfrom normal recrystallization is the presence of 2nd phase particles which hindergrain growth of some newly formed grains leading to an 'abnormalrecrystallization' process.This investigation has also shown that abnormal grain coarsening can beprevented by careful manipulation of 2nd phase particle precipitation through priorheat treatment before performing the standard solution anneal, pining the newlyformed grains before they can grow. The most promising prior anneal was found tobe 1hr at 850°C however the stored energy remains within the microstructure.Further tests are required to assess the impact of this prior anneal on the materialperformance.
Date of Award31 Dec 2016
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMichael Preuss (Supervisor) & Joao Quinta Da Fonseca (Supervisor)

Keywords

  • Grain Growth
  • Recrystallization

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