Microstructure Evolution And Thermal Stability Of Ni-Based Alloy By Surface Mechanical Rolling Treatment

In the present work,gradient nanostructures were fabricated on the surface of aγ′phase strengthened Ni-based alloy with low stacking fault energy by means of surface mechanical rolling treatment（SMRT）.Transmission electron microscopy（TEM）,scanning electron microscopy（SEM）,X-ray diffraction（XRD）were utilized to characterize the microstructure on the various depths from topmost to 250μm to elucidate the microstructure evolution during SMRT,and the results demonstrate that the microstructures are characterized by nano-grains,nano-twins and intersections of stacking faults（SFs）on the depth ranging from topmost to 150μm.Ageing treatments on the gradient nanostructures were performed at temperatures ranging from 600℃to 825℃,and the thermal stability of high densities of intersected SFs were investigated in terms of the microstructure evolution and microhardness.The conclusion is as following:（1）Gradient nanostructured layers with a thickness of 250μm were fabricated on the surface of theγ′phase strengthened Ni-based alloy,and the nanostructures exhibit a gradient variation on the nano-grain sizes,SFs density and extent of ordering ofγ′phase.TEM results demonstrate that grain refinement was achieved by deformation twinning and subsequent evolution of nano-twins to nano-grains.（2）The results demonstrate that the microstructures from topmost surface to the inside are characterized by nano-grains,nano-twins and intersections of stacking faults（SFs）and dislocation interactions withγ′phase.（3）The dominant microstructure on the depth ranging from 20μm to 60μm is high densities of interested SFs.The average spacing of neighboring SFs is about 6 nm,and the microhardness is up to 6.26 GPa,which can be explained by the Hall-Petch relationship.（4）The high densities of SFs have both ultrahigh hardness and superior thermal stability,with the increase of aging temperature from room temperature to 700℃,the average spacing of neighboring SFs range from 6 nm to 12 nm.The average spacing of neighboring SFs of the samples at 780℃/50 h is 18 nm.（5）TEM observations indicate that theγ′phase lost its L1₂ superlattice structure with the high densities of SFs cutting throughγ′phase.Ageing treatment induces the re-ordering withinγ′phase due to atom diffusion at medium to high temperatures,which leads to an enhancement on the microhardness.The microhardness measurements demonstrate that microhardness reaches to peak value of 6.88 GPa at ageing temperature of 700℃/50 h,and the microhardness of the samples aged at 780℃/50 h will be drop back to that of the SMRT sample.