Nickel-based Superalloy Powder Oxidation Behavior And Its Effect On Microstructure And Properties Of Alloys

Powder metallurgy(PM)nickel-based superalloy is treated as the preferred materials for high performance aero-engine turbine disk due to its excellent mechanical properties,corrosion resistance,and high temperature oxidation resistance.Obtaining high quality nickel-based superalloy powder is the foundation and guarantee of alloy and turbine disk.In order to improve the quality of nickelbased superalloy powders,this paper focuses on the process of original powderpowder storage-powder degassing-powder consolidation.On the basis of clarifying the evolution of powder surface state during storage and the dynamic degassing behavior of powders,the influence mechanism of powder storage state and vacuum degassing on the microstructure and properties of alloys are studied.The formation mechanism of prior particle boundaries(PPBs)defects is clarified from the perspective of powder high temperature oxidation.The oxidation behavior of nickel-based superalloy powders and its effect on the microstructure and properties of alloys are fully revealed,providing theoretical basis and data support for obtaining high performance turbine disks.The main results are as follows:Nickel-based superalloy powders are covered by homogenous NiO/Ni(OH)2 layer.The thickness of NiO/Ni(OH)2 layer decreases with the increase of active metallic(Al+Ti)content(IN625-IN718-FGH96)and the decrease of particle size((106-150μm)-(48-106μm)-(<48μm)).Affected by local oxygen partial pressure and surface passivation behavior,the NiO/Ni(OH)2 layer thickness and oxygen content of powders stored under relatively dry atmospheres(vacuum,high-purity argon,ambient air,and high-purity oxygen)increase from initial values(～3.8nm,～120ppm)to stable values(～10nm,～200ppm)after a certain period of storage(715 days),in which the powders stored in oxygen atmosphere possess the highest oxygen content(maximum 213ppm)while the lowest in vacuum,the gap can reach 25ppm.As absolute humidity increases,the NiO/Ni(OH)2 layer thickness and oxygen content of powders stored under 90℃/95%RH atmospheres increase to the stable values of(～13.9nm,～450ppm)in shorter storage time(3 days).Storage time and humidity are the key factors affecting powder storage state.After hot isostatic pressing(HIP)consolidation of powders with different storage states,the increase of low angle grain boundaries(LAGBs)fraction and decrease of average misorientation angle leads to higher activation energy and narrower hot-working window of as-HIPed parts with higher oxygen content.In addition,higher oxygen content generates PPBs defects in as-HIPed parts,which limits the grain growth and increases the grain boundary strengthening strength.As the fracture mode changes from transgranular fracture to inter-particle fracture,the ductility is significantly reduced.The above results not only optimize the hot processing parameters,but also provide theoretical basis for obtaining PM nickelbased superalloy with good combination of strength and ductility.In-situ TPD-MS was applied to study the dynamic degassing behavior on the basis of high temperature powder desorption principle.The results indicate that the in-situ degassing behavior mainly occurs between 100℃ and 650℃,in which the peak temperature lies in the range of 300℃ and 550℃.Due to the obstruction effect of surface oxide layer on molecular diffusion,the degassing amounts decrease with the increase of NiO/Ni(OH)2 layer thickness.Based on this,the effects of vacuum degassing parameters on the microstructure and properties of alloys were studied.The results indicate that the vacuum degassing treatment gradually reduces oxygen content(minimum value of～113ppm)and PPBs severity(optimal level of grade 1)according to the order of 25℃-300℃-600℃-300℃+600℃.At the same time,the hot workability and ductility are remarkably improved without reducing the yield strength.The tensile and stress rupture deformation mechanism of 300℃+600℃ desorbed FGH96 alloy shows that higher density stacking faults and thicker microtwins significantly promote the increase of ductility,while the cross-cutting of stacking faults and microtwins prevent the reduction of strength.After vacuum degassed at 300℃+600℃,the(HIP+HT)state PM FGH96 superalloy has excellent tensile,stress rupture and low cycle fatigue properties,which provides support for the further development of high-performance PM nickel-based superalloy.The formation mechanism of PPBs is revealed by simulating the powder oxidation process during HIP,and the reaction process of PPBs is deduced.The PPBs precipitates are composed of large-sized γ’,MC carbides-(Ti,Nb)C,A12O3 and ZrO2 oxides,where the pre-existing high bond energy oxide/oxide interface on powder surface provides structural condition for carbide nucleation.The above research clarifies the internal relationship between powder and bulk alloy,and then provides a theoretical basis for controlling the formation of PPBs defects and improving the comprehensive mechanical properties of PM nickel-based superalloy.