Microstructure And Mechanical Optimization Of GH783 Alloy

Low thermal expansion superalloy as critical materials of construction in gas turbine engines can maintain close tolerance, which greatly enhances their fuel efficiency, thrust-weight ratio and reduces specific fuel consumption. A new alloy, GH783 alloy, is still Ni-Fe-Co base magnetic superalloy. Compared with conventional Inco's 900 series, GH783 alloy is the culmination in the development of an alloy system with very high Al content that, in addition to formingγ′, causeβaluminide phase precipitation in the austenitic matrix. It was discovered that this type of structure could be processed to resist both SAGBO and general oxidation, while providing low thermal expansion and useful mechanical properties up to 700℃.In this thesis, phase characterization and microstructure study on GH783 alloy at as casting, hot deformation, solid solution, aging and long-term aging conditions, respectively, have been studied by means of SEM and TEM examination. Whether GH783 alloy has notch sensitivity or not under tensile and stress rupture conditions in high temperature and the influence of constituent phases on the changing velocity of coefficient of thermal expansion (CTE) are also studied. Results show that:(1) GH783 alloy contains blocky and granularβphase, disperseγ′andγaustenite and don't have notch sensitivity under tensile and stress rupture conditions at 650℃.βphase precipitates along grain boundary which makesγ′phase poor. That plays the role of microplastic zone and is benefit for redistributing and relaxing stress.(2) The tensile strength of GH783 alloy at room temperature and 650°C increased after aging at 650°C for 3000 hours because of theγ-γ′coherent strengthening. After aging at 700°C and 750°C, respectively, for 1000 hours, the relationship ofγ-γ′coherent strengthening was destroyed and the tensile strength decreased. Especially, after aging at 750°C for 1000 hours theγ′phase coarsened severely, which lead to the strength and stress rupture life decreased significantly. Ni-rich Laves phase was found inβ(NiAl)of GH783 alloy after long-term aging. And that did not have evident influence on mechanical properties.(3) If the influence of the variation ofαγand Eγin GH783 alloy in different heat treatments on CTE value can be ignored, the CTE value for GH783 alloy can be expressed as:In addition, the decrease in changing velocity of CTE with temperature for GH783 alloy is caused by the increasing volume fraction ofγ′andβprecipitates. That is an innovative idea and has been proved.