Microstructure And Texture In The Surface Gradient Nanostructured Nickle And Aluminum Alloys

Ni-based superalloy?GH series?and ultra-high strength aluminum alloy?7×××series?are two important aeronautical materials used for airplane engine and fuselage,respectively.For the industrial practice,the performance of the structural components of airplanes is not only closely associated with the mechanical,physical and chemical properties of the original materials,but also significantly affected by the surface integrity after mechanical machining.Generally,under the themomechanical effect induced by machining the surface-adjacent layer tends to be severely plastic deformed,leading to great changes in microstructures and mechanical properties as well as final performance of the structural component such as fatigue lifetime.Therefore,a systematic and in-depth investigation of the microstructure and crystallographic texture in the surface severe plastic deformion?SSPD?layer induced by various surface treatment is of great significance to fully understand and further improve the performance of the structural component.In this study,three typical aeronautical materials,namely,GH4169 superalloy,7055 and7050 aluminum alloys were surface-treated through high speed grinding,milling and surface sliding friction treatment?SFT?,respectively.The scanning electron microscope and electron backscatter diffraction,high resolution electron microscopy,high angle annular dark field imaging,precession electron diffraction assisted orientation mapping and nanoindentation were used to characterize grain structures,precipitates,crystallographic texture and mechanical properties in the SSPD layer.Moreover,the corresponding processes,characteristics and mechanisms were analyzed and discussed.After high speed grinding,a gradient SSPD layer with a thickness of a few micometers was inroduced at the surface of the GH4169 superalloy.The nanograin size can be refined down to 5 nm in diameter.Apart from dislocation activity and deformation twinning,the dislocation-twin interaction is another dominated grain refinement mechanism,which includes deformation twinning,dislocation-twin reaction,localized thinning of nanotwin lamellae and final fracture.Different from the coarse grain region with a cube texture{100}<001>,the crystallographic texture of the SSPD layer is composed of rotated cube{100}<011>,cube,copper{112}<111>and Goss{110}<001>.The intrinsic thermomechanical effects induced by shear deformation under high strain,high strain rate and high temperature conditions are responsible for crystallographic texture evolution in the topmost SSPD layer.The SSPD layer of the GH4169 superalloy after SFT processing is much thicker than its counterpart after high speed machining.The crystallographic texture includes rotated cube,E{111}<110>?F{111}<112>and weak Goss texture.With increasing depth from sample surface,the intensity of rotated cube component become weaker and weaker.Milling of 7055 aluminum alloy can fabricate a gradient nanostructured deformation layer which is composed of nangrain and ultrafine grains with equiaxed and lamellar mophologies.The crystallographic texture of this deformation layer is a mixture of brass texture{110}<112>,cube and weak rotated cube,while that of the coarse grain matrix is pure brass texture.Compared with the coarse grain matrix,the mophorlogy and distribution characteristics of grain boundary precipitates?GBPs?and grain interor precipitates?GIPs?in the SSPD layer are distinctly different,which is considered as an indicative of precipitate redistribution.This process can be accomplished via mechanically induced precipitate dissolution,solute diffusion and reprecipitation.The precipitate dissolution is attributed to shear deformation of precipitates by intense dislocation activities,while the reprecipitation and growth can be greatly affected by the specific chemical,structural and energetic conditions at grain boundaries.For 7050 aluminum alloy after SFT processing,the SSPD layer is characterized by both equiaxed and lamellar ultrafine grains and high density of coarse GBPs.Comparatively,these coarse GBPs are located at high angle grain boundaries?HAGBs,?15o?rather than low angle grain boundaries?LAGBs,2o¹5o?,and grain boundary triple junction becomes the most favorable nucleation site for GBPs.Texture analysis indicates that the surface ultrafine grain layer shares a shear-type texture which includes rotated cube and F texture components.Three samples processed by different SFT parameters are concentrated,namely 100N/50cycles,100N/100 cycles and 300N/100 cycles.The result indicates that the grain structures and precipitate distribution in the samples after 100N/50cycles and 100N/100 cycles SFT processing are almost the same,while that after 300N/100 cycles SFT processing shows different GBPs and GIPs in the subsuface ultrafine grain layer,which presumably results from the decrease in volume fraction of grain boundary triple junctions.After 100N/100 cycles SFT processing of 7050 aluminum alloy,a large number of depth-dependent gradient deformation bands?DBs?approximately parallel to{111}_Al can be observed in the coarse grain matrix.Compared with precipitates in the surrounding matrix,the precipitates at DB boundaries?DBBs?are much coarser,whereas those within DBs are smaller and scarcer.Deformation banding can introduce grain refinement and precipitate redistribution,which are closely related with intense dislocation activities and possible dynamic recovery and recrystallization within DBs,and can be significantly affected by the gradient shear strain component as well as the randomly encountered coarse dispersoids and precipitates.Additionally,a variant selection phenomenon during mechanically induced precipitate dissolution process was pointed out,and the crystallographic orientation of equivalent variants with respect to the DBB plane was considered to be responsible for this orientation dependent precipitate dissolution.