Mesoscopic Plastic Deformation Mechanism Of GH4169 Nickel-based Superalloy Thin Sheet

Plastic microforming technology has become a key technology for the manufacture of thin sheet microstructural parts due to its advantages of mass production,high productivity,high accuracy,high reliability,low cost,no pollution,and good performance of micro-components.The existence of size effect in mesoscopic plastic forming limits the direct application of the relatively mature processing technology and theory of traditional macro forming in the field of mesoscopic plastic forming.With the development of miniaturization of products in the fields of aerospace,nuclear industry,weapon equipment and energy,nickel-based superalloy thin sheets with excellent comprehensive performance have become the key materials for micro-components in these fields.Due to the complexity of the microstructure and strengthening mechanismthe mesoscopic plastic deformation behavior of nickel-based superalloys under the influence of multiple factors such as sample thickness,grain size,precipitate size and content has not been thoroughly studied.This paper aims to reveal the mesoscale plastic deformation mechanism of nickel-based superalloys,provide a theoretical basis for the forming process of nickel-based superalloy thin sheet microstructural parts,which has practical application value for promoting its wide application in the mesoscopic forming field.In this paper,GH4169 alloy thin sheet samples with different microstructures were prepared by designing high temperature solution treatment scheme,high temperature aging treatment scheme and two-step aging heat treatment scheme.The microstructure evolution behavior was studied based on optical microscope,EBSD,SEM and TEM.It is found that the grain size increases significantly with the increase of the solution temperature,and the particle size and volume content of the ? phase and the ??/?? phase increase with the aging time.The room temperature microtensile tests of GH4169 alloy thin sheet were carried out.The effects of thickness,grain size,precipitate size and content on the microtensile mechanical properties were analyzed.It was found that the flow stress decreased significantly with the increase of grain size.The ? phase and ??/?? phases lead to an increase in microtensile flow stress.The microtensile flow stress decreases as the specimen thickness decreases.The effects of sample thickness,grain size,unshearable ? phase and shearable ??/?? phases on the microtensile flow stress size effect of GH4169 alloy thin sheet were systematically studied.The flow stress of GH4169 alloy in solid solution state decreases with the decrease of the thickness-to-diameter ratio(t/d),and the size effect phenomenon of "smaller is weaker" appears.The flow stress of GH4169 alloy with ? phase increases with the decrease of t/d,and the size effect phenomenon of "smaller is stronger" appears.The flow stress of GH4169 alloy with ??/?? phases decreases with the decrease of t/d,and the size effect phenomenon of "smaller is weaker" appears.Based on the surface layer model,considering the effects of grain boundary strengthening,precipitation strengthening,and solution strengthening,the microtensile material constitutive model of nickel-based superalloys was constructed,and the size effect mechanism of microtensile flow stress of nickel-based superalloys was revealed.The orientation evolution behavior of the grains during microtensile deformation was studied by EBSD.It was found that the orientation at the grain boundaries was the largest,indicating that strain localization occurred at the grain boundaries.Surface coarsening behavior of sample during microtensile deformation was studied by means of laser scanning confocal microscope.It was found that the surface roughness of the sample increases with the decrease of t/d.The surface roughness of the sample decreases with the increase of the ? phase content.The local strain evolution behavior of in-situ microtensile deformation was studied based on digital image correlation(DIC)technique.It was found that severe inhomogeneous deformation occurs on the sample surface with the increase of the grain size.The hindrance of ? phase and ??/?? phases to dislocation motion exacerbates strain localization.Based on in-situ synchrotron radiation X-ray computed tomography(SR-CT),the 3D visualization of void evolution behavior of GH4169 alloy in microtensile process was realized.It is found that the void defects have been formed during the uniform plastic deformation stage.The density and size of voids increase with the plastic strain,while the number of voids decreases with the increase of grain size.Coupled fiber loading mechanism and dislocation theory,the fracture model of ? phase is established,and the fracture mechanism of ? phase in microtensile process is revealed.It was found that the needle-shaped ? phase whose long axis direction is close to the loading axis direction is most likely to fracture.The localization of strain caused by grain boundaries and phase boundaries hindering dislocation motion,coarsening of the sample surface,and uncoordinated deformation during plastic deformation leads to the initiation and propagation of voids at the grain boundaries and phase boundaries of the alloy,causing microtensile fracture of GH4169 alloy.The mesoscopic forming limit testing device was designed and fabricated.The test platform of mesoscopic Holmberg uniaxial tensile test and Nakazima hemispherical domed punch bulging test based on DIC technique with high precision strain measurement was established.The effect of microstructure on the mesoscopic forming properties of GH4169 alloy sheet was studied.The forming limit diagrams of GH4169 alloy with different microstructures were drawn.It is found that the size effect appears in the mesoscopic forming limit.As the t/d decreases and the particle size and volume content of the ??/?? phases increase,the formability of the alloy decreases.The influence of the microstructure on the plastic deformation behavior of the mesoscopic forming limit loading process is revealed.It is found that the loading path is gradually inclined to the plane strain path directionwith the plastic deformation.As the t/d decreases,the sample surface coarsening intensifies,the plastic deformation coordination of the alloy decreases,and the influence of the deformation characteristics and orientation of the surface grains on the overall deformation behavior of the alloy increases.The presence of the precipitates limits the movement of dislocations in the plastic deformation process,causing strain localization and reducing the formability of the alloy.