Microstructure Evolution And Thermal Stability Of A Ni-based Alloy Processed By 4-pass Surface Mechanical Rolling Treatment

Ni–based alloys have been considered as good candidates for the applications at extreme environments.However,the application of Ni–based alloys has also been limited by the degradation of the microstructure and mechanical properties when the materials are exposed to intermediate to high temperature environments.The present work tends to provide a strategy to improve the mechanical property and thermal stability of Ni–based alloys by engineering nanotwin structures by means of plastic deformation.Surface mechanical rolling treatment(SMRT)was performed on a single–phase Ni–based alloy with low stacking fault energy to obtain gradient nanostructures on the surface of the processed sample.Micro-hardness tester,Scanning Electron Microscope,Transmission Electron Microscope,High-Resolution Transmission Electron Microscope and Energy Dispersive Spectrometer was mainly used.Microhardness test and microstructure characterization were carried out to study the variations of mechanical property and microstructure with the distance to the topmost surface.Furthermore,thermal annealing was performed to investigate the thermal stability of the SMRT processed sample.The main results in this work are as follows:(1)A gradient nanostructure was fabricated on the surface of the SMRT processed sample.The microstructure is characterized by high density of nano-twins,indicating that nanotwinning dominate the entire grain refinement process.(2)With increasing the distance from the topmost surface,the microstructures in the gradient layer are characterized by nano-grains,nano-twins,stacking faults and original coarse grains.(3)The microhardness at the top surface is ?5 GPa,which is ?82% higher than the that of coarse-grained counterpart.The microstructure is characterized by high density of the nano–twins with an average thickness of ?15.6 nm,which is densely distributed in the depth of ?20– 70 ?m from the surface.The significant strengthening from the nano-twins is consistent with the traditional mechanism of structural strengthening in the materials processed by plastic deformation.(4)The average thickness of nano-twins slightly increases to ?23.6 nm after annealing at600 ? for 2 h,indicating the superior thermal of stability of nano-twins.(5)After ageing at 600?,carbides precipitate at the nano-twin boundaries as the second phase,leading to the enhancement of microhardness from?4.7 to?5.3 GPa.The precipitation of carbides suggests the segregation of solute atom Cr to twin boundaries during ageing at 600?.The superior thermal stability of nano-twins at 600? is attributed to the segregation of solute atom Cr to twin boundaries and the subsequent precipitation as carbides.