Study On The Strengthening And Toughening Of Ni-based Wrought Superalloy Strengthened By "Twin+γ’ Phase" Induced By Cold Rolling Deformation

As high-temperature structural materials,Ni-based wrought superalloys have important applications in hot components such as aero-engines.With the continuous development of aero-engines,further improving the high-temperature strength and temperature-bearing capacity of alloys has become an urgent problem.Traditional Ni-based wrought superalloys can enhance the effects of solid-solution strengthening,precipitation strengthening,and grain boundary strengthening by increasing the number of alloy elements,resulting in higher high-temperature strength and good temperature-bearing capacity.Although,the strength of the alloy will increase,its plasticity will reduce by introducing amounts of solid-solution andγ′phase-strengthening elements.In the current situation of strength-plasticity inversion,it is difficult to solve the bottleneck problem of alloy strengthening.It is noteworthy that due to the novel concept of material homogenization,it is determined that introducing a low-energy stable twin boundary in the material can improve the high-temperature strength and temperature-bearing capacity of alloy,and the“twin+γ?phase”composite structure in the precipitation strengthened superalloy can dramatically improve the mechanical properties of the superalloy.Given this,the article investigates the construction of a"twin+γ?phase"structure in the self-designed Ni-based wrought superalloy with low stacking fault energy and appropriate volume fraction ofγ?phase,providing a novel strategy for high-temperature strength-plasticity synergy of Ni-based wrought superalloy.The main conclusions are as follows.For forged Ni-based wrought superalloys,the evolution law of the microstructure was studied by adjusting the temperature and time of the solution treatment.The results show that a recrystallization texture formed in the alloy after the solution treatment,preparing for the subsequent plastic deformation of the Ni-based wrought superalloy.According to the mechanical properties at room temperature,the optimal solution treatment system was determined to be 1080℃/2 h/WC.The recrystallization fraction of the specimens reached 96.66%,and annealing twin boundaries（TBs）accounted for27.81%of the whole grain boundary.The average grain size with twins(d_eff)and without twins（d）were 16.59 and 27.15μm,respectively.In addition,the yield strength（σ_y）,tensile strength(σ_uts),and fracture elongation（ε_f）of the alloy at room temperature were 751 MPa,1152 MPa,and 42.86%,respectively.The high temperature（760℃）yield strength（σ_y）,tensile strength(σ_uts)and fracture elongation were 733 MPa,793MPa,and 4.23%,respectively.Thermomechanical treatment was utilized to control the content of the annealing twins in the alloy,and the evolution of the microstructure and mechanical properties during cold deformation and annealing was also studied.The results show that when the amount of cold deformation was small（ε≤45%）,the grains were elongated along the rolling direction and distributed in a flat shape,the stress mainly concentrated near the grain boundary and twin boundary.The high-angle grain boundaries（HAGBs）and twin boundaries（TBs）gradually transform into sub-grains（Sub-GBs）and low-angle grain boundaries（LAGBs）,causing the appearance of a large number of deformation textures.When the amount of rolling reduction exceeded 70%,the grain shape gradually changed from flat to fibrous,the deformation uniformity of the grains gradually improved,and the stress distribution became uniform.LAGBs began to dominate,and the strength of texture increased,but the type of texture in the alloy remains unchanged.The strength of the alloy increased,and its plasticity decreased significantly at room temperature.After the cold-rolling alloy was annealed at 1120℃for 15 mins,Sub-GBs and LAGBs gradually transformed into HAGBs and TBs.The number of annealing twins produced increased significantly,and the length fraction of the annealing twins increased with the rolling reduction.Simultaneously,the deformation texture transformed into a recrystallization texture,which increased the texture types,but weakened the texture strength.As a result,the alloy strength decreased,and the plasticity increased.The stability of annealed twins under thermal action was studied by ex-situ EBSD technology,and the morphology,size,and distribution ofγ?phase were studied by SEM technology.The results show that the annealed twin boundary in annealed Ni-based wrought superalloy exhibited good thermal stability after aging treatment at 650°C/24h/AC+760°C/16 h/AC,due to the low-energy stable TBs,which have lower mobility as TBs energy is much lower than the of ordinary HAGBs.After the aging treatment,a large number of nanoscale sphericalγ?phase precipitated in the alloy and dispersed on theγmatrix to form a"twin+γ?phase"structure.Tensile experiments,EBSD and TEM were employed to quantitatively characterized the contribution of TBs to the room-temperature yield strength of Ni-based wrought superalloys.Additionally,the high-temperature strengthening mechanism of Ni-based wrought superalloy with"twin+γ?phase"structure strengthened was revealed.The results show that the"twin+γ?phase"structure can improve the mechanical properties of the alloy.After cold rolling 45%,annealing at1120℃for 15 mins,and two-stage aging treatment,the proportion of annealing twins was 25.49%,the thickness was 2.49μm,and theγ′phase average size was 67.42 nm.Compared with the solid solution alloy,its yield strength（σ_y）at room-temperature increased from 751 MPa to 1479 MPa,and tensile strength(σ_uts)increased from 1152MPa to 1704 MPa,maintaining an elongation of 14.76%.The yield strength（σ_y）at760℃increased from 733 MPa to 1220 MPa,tensile strength(σ_uts)increased from 793MPa to 1272 MPa,maintaining an elongation of 13.44%.These findings indicate that the"twin+γ?phase"structure can improve the yield strengths of the alloy at room-temperature and high-temperature,which can be attributed to the grain boundary,annealing twin boundary and interaction betweenγ?phase and dislocation.In addition,pre-rolling and subsequent heat treatment performed to form deformation twins during the tensile process,led to a synergistic improvement in the high-temperature strength and plasticity of the alloys.