Precipitates Evolution During Processing,serving Conditions And Its Effect On The Mechanical Behaviors Of Inconel 718 Alloy

Inconel 718 is a widely used nickel-based cast/wrought superalloy in the world,which can be applied in the fabrication of rotating and static components in aero-engines,such as turbine disks,casings,high-strength fasteners,etc.In this dissertation,the precipitates evolution during multiple processing,serving conditions and resultant elevated-temperature mechanical behaviors of Inconel 718 alloy were systematically investigated.The precipitation behaviors of?,??,??phases during various hot deformation processes were characterized.The underlying mechanisms responsible for the degradation of hot ductility caused by cold working and the effect of?phase on the hot tensile properties at 650 ^oC regarding its differential morphologies were revealed.The creep behaviors controlled by??,?characteristics and grain morphologies at 650 ^oC were studied.The disparate transitions of microstructure-properties during long-term static serving condition tailored by original structural configurations were clarified.Some main conclusions are summarized as follows:(1)The precipitation behavior of?phase was affected by the dynamic recrystallization degrees,and reducing the dynamic recrystallization degree in prior forging stage can promote the formation of needle-shaped?precipitates in the Delta Processing.Hot deformation changed the precipitation kinetics of?,??phase,and cooling fast to below the peak precipitation temperature of?phase after finish forging shall be sufficient in avoiding excess?precipitation in forged turbine disks.During isothermal deformation,the?-matrix exhibited coupled plastic deformation mechanisms defined as twinning and dislocation planar slipping.??phase was found to nucleate at the??/?interface,possessing parallel crystallographic orientation relationships with adjoining??particles.The spherical??particle showed la_—ttice distortion anisotropy and was found to be sheared by paired gliding{100}a/2<110>dislocations during deformation,which determined its APB(anti-phase boundary)-type ordered strengthening role in Inconel 718 alloy.(2)Cold working lowered the structural stability and advanced the necking occurrence,resulting in the deterioration of hot tensile ductility.The effects of?phase on the hot tensile performance were tailored by its differential morphologies,that the needle-shaped?precipitates generated along grain boundary can enhance the strength but degrade the ductility,and the granular?precipitates distributed on the grain boundary as well as within grains played an inverse role of improving ductility but reducing strength.(3)The morphological transition of??particles from equivalently perpendicular intersecting disc-shapes to refined ellipsoidal ones with modified one-single variant contributed to the increase of stress rupture life from 1217 h to 2539 h under 650^oC/650?700 MPa,as well as the decrease of steady-state creep rate from4.19×10^-4%/h to 2.74×10^-4%/h,thereby improved the creep resistance of Inconel718 alloy significantly.The GB(grain boundary)-?precipitates and duplexing grains were proved detrimental to creep properties.Due to its benefits of retaining the fine grains and excluding the negative GB-?precipitation,direct aging treatment enabled the wrought Inconel 718 alloy superior creep properties at 650 ^oC.(4)The long-term static serving performance of Inconel 718 alloy was primarily influenced by the grain characteristics and GB-precipitates.The specimens with fine grains in average size of 14.7±2.9?m were identified with high notch sensitivity,in which the GB-?and?-Cr precipitation led to the degradation of strength and enhancement of ductility.Larger grain-size of 17.4±3.8?m and more GB-?precipitates were available in improving hot ductility and eliminating notch sensitivity,thus improving the mechanical stability.However,the formation of needle-shaped?precipitates along grain boundary was found to result in the acute ductile degradation of heterogeneous duplexing-grain structure with varied grain sizes of 24.2?130.6?m.It can be drawn that appropriate sub-solution+aging treatments can enable wrought Inconel 718 alloy the optimum long-term static serving properties.