Effects Of Rolling And Annealing On The Microstructure And Mechanical Properties Of A Ni-Cr-W Alloy

Nanostructured metallic materials have been wildly investigated owing to their novel properties.Plastic deformation has been considered as an efficient pathway to refine coarse grains into extremely small grains(<100 nm).Ni-based alloys have been considered as good candidates for high temperature applications,such as jet engines.In this work,cold rolling was utilized to refine the coarse grains by introducing defects,such as stacking faults,nano-twins,into the grain interiors of a Ni-Cr-W alloy with low stacking fault energy.Scanning electron microscope,transmission electron microscope and other characterization techniques were performed to study the microstructural evolution of the Ni-based alloy with increasing rolling strain.Tensile and microhardness tests were carried out to investigate the mechanical properties of the Ni-based alloy with increasing rolling strain,and the strengthening mechanisms were analyzed as well.Thermal annealing was further performed on the rolled 45% sample to investigate the thermal stability of the nano-twins.The main results are seen below:(1)With the plastic strain increases,the dominant microstructures in the sample is characterized by stacking faults,nano-twins and nano-grains,respectively.The average thickness of nano-twin/matrix lamellae is ? 39.2 nm in the sample with 45% rolling strain,and the characteristic size of the nanostructures in the sample with 80% rolling strain is ?38.2 nm.This indicates that the bulk nanostructured Ni-based alloy could be fabricated by means of cold rolling.(2)Grain refinement was achieved by the introduction of high density of nano-twins into the coarse grain interior,and the subsequent interaction of dislocations with nano-twins in the sample with large rolling strain.(3)With increasing rolling strain,the strength of rolled sample increases rapidly,while the tensile ductility reduced significantly.As the amount of deformation increases,the yield strength increases from ?308 MPa to ?1456 MPa,and the uniform elongation decreases from ?43.4% to ?2.3%.Based theoretical calculations,the contribution of various strengthening mechanism is determined to be solution strengthening ?15%,nanostructure strengthening ?60%,and additional strengthening on grain boundaries from solute atoms?25%.(4)No significant change in the microstructure is observed in the rolled 45% sample annealed below 500 ?,while nano-sized carbides are observed in the sample annealed at600 ?.Recrystallization starts to occur in the sample annealed at 700 ?,and a fully recrystallized microstructure is obtained in the sample annealed at 800 ?,and the grain size is ?2.5 ?m.(5)With increasing annealing temperature,it is found that the microhardness of the rolled 45%sample increases up to the annealing temperature of 500 ?,and then starts to drop.The microhardness of the sample annealed at 700 ? is still comparable with the rolled sample,indicating that the superior thermal stability of nano-twins.The low twin boundary energy,segregation of solute at twin boundaries and the subsequent precipitation of nano-sized carbides all contribute to the superior stability of nano-twins up to 700 ?.(6)After annealing,the strength of the rolled 45% sample is increased due to the segregation of solute atoms at the twin boundary is ? 251 MPa,and the precipitation of carbides is ?364 MPa.