Microtructural Evolution And Tensile Behavior Of Ni-Based Alloy Processed By Surface Mechanical Rolling Treatment

In this paper,surface mechanical rolling treatment(SMRT)technique was utilized to fabricate a nickel-based alloy with low fault energy to obtain the gradient nanostructures.Microhardness,scanning electron microscope and transmission electron microscope were performed to characterize the evolution of microstructure and mechanical property of SMRT sample.The contribution of various strengthening mechanism was also estimated based on theoretical models and compared with experimental results.The tensile properties of SMRT samples at intermediate temperatures were characterized,and compared with the untreated samples.TEM observation was performed on the SMRT sample tensile tested at 873 K and compared with the sample without loading to reveal the deformation mechanism during loading at 873 K.The main results are as follows:(1)Gradient nanostructure with a thickness of ～300 μm is formed on the surface of the nickelbased alloy.With decreasing distance from the treated surface,the main structure of SMRT nickel-based alloy varies from dislocations/stacking faults,nano-twins and finally to nanograins.(2)The size of nanograins in the topmost surface is ～30 nm,and the microhardness at the topmost surface is as high as ～5.5 GPa.The strength of nanograins arises solid solution strengthening,nanostructure strengthening and additional strengthening on grain boundaries from solute atoms.The contribution of various strengthening mechanisms was calculated to be～36.5% for solid solution strengthening,～45.9% for nanostructure strengthening,and ～17.6%for additional strengthening on grain boundaries from solute atoms.(3)As the tensile temperature increased from 873 K to 973 K,the engineering yield strength of SMRT samples decreases from ～395 MPa to ～338 MPa.Meanwhile,the tensile ductility of SMRT samples shows a significant enhancement from ～32.5% to ～87.8%.The engineering yield strength and tensile ductility of the coarse-grained sample at 973 K are ～235 MPa and～92.7%,respectively.SMRT sample shows a superior combination of strength and tensile ductility at 973 K.(4)Microhardness on the topmost surface of SMRT sample after tensile test at 873 K is found to be enhanced as compared to SMRT sample without tensile test.Microstructure characterization on the SMRT sample after tensile tests indicates that the size of nanograins increases ～30 to 45 nm,while a significant amount of nanotwins is observed in the nano-twins.Thus the enhanced microhardness is attributed to the formation of high density nano-twins within grain interior to provide strengthening during tensile deformation.