Microstructure Evolution And Element Migration (Diffusion) Behavior Of A Ni-based Alloy Processed By Surface Mechanical Rolling Treatment

Plastic deformation tends to induce significant grain refinement in metallic materials.Nanostructured metallic materials with extremely small characteristic microstructure（<100nm）have drawn significant attention during the past decades,due to its novel properties.Ni-based alloys have been widely used for many applications,such as jet engines.People have been dedicating to optimize the mechanical properties by tailoring microstructure of Ni-based alloys in the past several decades.It raised a question that whether the microstructure of theγ’strengthened Ni-based alloy could be refined by plastic deformation,and what the refinement mechanisms ofγ’andγphase are.In this work,Surface Mechanical Rolling Treatment（SMRT）technique was utilized to obtain gradient nanostructures in aγ’strengthened Ni-based alloy with low stacking fault energy.Scanning Electron Microscope,Transmission Electron Microscope and Energy Dispersive Spectroscopy were performed to characterize the grain refinement mechanism of the Ni-based alloy during SMRT processing.Accompanied with the grain refinement,the redistribution of solute atoms betweenγ’andγphase and the associated diffusion mechanism were elucidated as well.The main conclusions are as follows:（1）Gradient nanostructure with a thickness of～450 nm is formed in the surface of Ni-based alloy with 8-pass SMRT treatment.The strain rate on the topmost surface is～1.6×10³ s^–1,which deceases with increasing the distance from the topmost surface.Transmission Electron Microscope observation indicates that the microstructure of the SMRT sample on the various depth can be divided into four categories,i.e.,nano-grains,nano-twins,stacking faults and the original coarse-grained structure.Energy Dispersive Spectroscopy characterization shows that the nano-sizedγ’phase is further refined due to shearing of stacking faults and nano-twins,and the extent of ordering ofγ’phase decreases rapidly with the decrease of the distance to the topmost surface.（2）Due to the low stacking fault energy of Ni-based superalloy,the dominant deformation substructure is stacking faults and nano-twins,playing critical roles on the grain refinement.The formation of nano-sized grains on the topmost surface is due to the dense intersection of the nano-twins on conjugate{111}planes,and the mean size of nano-sized grains is～13 nm.（3）The observed interdiffusion of solute atoms betweenγ’andγphase is induced by the stacking faults shearing throughγ’phase.In the depth with nano-twins as the dominating substructure,the extent of the interdiffusion of solute atoms betweenγ’andγphase is reduced as compared to that in the depth of stacking faults as dominant substructure.Overall,interdiffusion of solute atoms betweenγ’andγphase is observed at various depth in the gradient nanostructures,however,it becomes weaker with decreasing the distance to topmost surface.（4）The equivalent temperature for the interdiffusion of solute atoms during SMRT processing is more than 1600 K.The diffusion coefficients of Co during SMRT processing is higher than or close to those of Al and Ti,which is contrary to the case for the thermal activated diffusion.（5）The extent of interdiffusion betweenγ’andγphase is determined by the deformation mechanism locally in the SMRT processed sample.There are no variations on the average composition in the various depth of SMRT sample,indicating that the interdiffusion occurs locally between γ’and γ phase.