| With the increase of the turbine inlet temperature of aero-engine,higher requirements are put forward for the temperature bearing capacity and mechanical properties of turbine disk materials.Alloying is the main method to improve the properties of nickel-based wrought superalloys and the continuous addition of alloy elements content can improve the strength of the alloy.However,the development of highly alloyed tendency of nickel-based wrought superalloys is prone to bring disadvantages such as poor microstructure stability and large deformation resistance.It means that the high alloying development of nickel-based superalloys has fallen into the bottleneck of easy strengthening and difficult deformation.The nanotwin structure based on the concept of plainification can simultaneously improve the strength and plasticity of the material,and experiments show that the composite structure of "?' phase and nanotwin" can greatly improve the mechanical properties of superalloys in the field of superalloys.The generation of twins largely depends on the low stacking fault energy of the alloy.Therefore,a nickel-based wrought superalloy with low alloying and low stacking fault energy was designed based on the interface control concept of "?' phase and nanotwin" composite structure,which was used as a synergistic strengthening model alloy for studying plainification on the basis of structure of "?' phase and nanotwin".Alloy ingot is melted by vacuum induction melting(VIM)and electroslag remelting(ESR)and homogenized.Hot deformation behavior and microstructure evolution of as-cast alloys are studied.Furthermore,attempts were made to introduce nanotwins and ?' phases into fee metals by using deformation and subsequent annealing,aging of as-forged alloy,so as to provide evidence for the design of low stacking fault energy alloys and the control of the "?'phase and nanotwin".A new-type nickel-based wrought superalloy with low alloying and low stacking fault energy is designed by JMatPro thermodynamic software.In the Ni-Cr-Co-W-Ti-Al-C alloy system,the increase of W significantly increases the volume fraction of ?' phase(f?'),Ti/Al have the opposite effect,while the change of Co and Cr contents have no effect on f?'.All the above alloying elements decrease the stacking fault energy(SFE)of the alloy,especially with the addition of Cr.Combining the various elements' effects on f?' and SFE,in order to design the alloy with appropriate amount ?' phase(25%?30%)and lower SFE,18.5Cr-18.5Co-5.6W-3.2Ti-1.4A1-0.01C-Ni(mass fraction,%)was determined.The JMatPro calculation results show that f?' and SEF are 27.64%and 93.1 mJ/m2,respectively.Besides,a larger hot-working window is found in the new alloy.By studying the element segregation and segregative phases,the ingot is prepared by vacuum-induction-melting(VIM)and electroslag-remelting(ESR),then the segregation elimination and mechanical properties of as-cast alloy are measured after homogenization at 1180? for 8?56 h,in the end,the most appropriate homogenization treatment process is determined.The results show that dendrite and element segregation exist in the as-cast alloy,Ti is the principal segregated element,and the segregation coefficients of Ti and W elements are 2.37 and 0.62,respectively.There are nubbly TiC and needle-like ? phases in the ingot.Moreover,the average size of ?' phases at interdendritic region is significantly larger than that at dendritic region.With the increase of homogenization time,dendrites gradurally decrease and ?phases dissolve to the matrix,as a result the elements also diffuse more evenly.The elongation of the alloy greatly increased to 40%after homogenization treatment,which is about twice as much as that of as-cast alloy.Considering grain size and the elimination of element segregation,the appropriate homogenization treatment is finally determined as 1180?×40 h.Under this condition,the elements of Ti and W almost diffuse uniformly with the value of KTi and KW are 1.03 and 0.93.Besides,grain size is uniform without abnormal coarsening after homogenization treatment for 40 h.JMatPro can well predict the segregation of element in as-cast alloy and the effect of homogenization treatment on eliminating element segregation.The hot deformation behavior and microstructure evolution of as-cast alloy were studied during hot compression at 5 s-1,the strain is 60%during different temperatures(1050 ??1250 ?).The results show that the dynamic recrystallization characteristics of the true stress-true strain curves of hot compression are obvious,and the serrated flow behavior all exist in the curve.The higher hot compression temperature is,the dynamic recrystallization softening more obvious,and the ?' phase in as-cast alloy gradually dissolve.Therefore,the peak stress decrease.When the hot compression temperature rises to 1200? and 1250?,the ?' phase completely dissolve to the matrix,and the flow stress decreases significantly.With the increase of hot deformation temperature,the higher the proportion of recrystallized grains,the average grain size and the twin fraction increase.In addition,at 1250? the twin boundary fraction increases to 22.7%.The nucleation of dynamic recrystallization is dominated by the discontinuous dynamic recrystallization mechanism in a form of boundary bulging.As-forged alloy is continuously cold rolled with 80%deformation,annealed(1080?/30 min,1 h/AC)and aged(760?/1 h,5 h/AC),its microstructure evolution and mechanical properties were studied.The results show that the tensile yield strength is as high as 1.7 GPa after severe deformation rolling due to the high dislocation density,however the elongation is only 2.8%.With the extension of annealing time,due to the decrease of dislocation density and the formation of a large number of recrystallized grains,the strength reduced and the plasticity greatly improved.After annealing at 1080? for 1 h,the tensile yield strength of the sample is 1210.4 MPa,and the fracture elongation increased to 27%.In addition,a large number of annealing twins formed during the recrystallization process,in which sample annealed for 1h and aged for 1h,and the fraction of ?3 twin boundary increased to 50.9%.The twins with smaller thickness and fine grains have obvious strengthening effect,and the contribution to the yield strength is 204.6 MPa and 431.6 MPa,respectively.The results show that the composite structure of "?' phase and nanotwin" can obtained in the new alloy with this approach,which is to induce nanotwins firstly and the subsequent aging is executed to achieve ?' phase.Besides,the movement of the dislocation can be prevented though the ?' phase and the nanotwin,and there exist a cutting phenomenon between ?' phase and nanotwin. |
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