Creep Behavior And Mechanism Of A Co-Al-W-Ta-Ti Single Crystal Superalloy At 1000?/137MPa

?'-strengthened Co-based superalloys are considered as a promising candidate material for high temperature applications due to their analogous ?/?' microstructure and higher melting temperatures(>100?)compared to the widely used Ni-based superalloys.Creep performance is one of the most important mechanical properties that must be considered in the design,selection and safety evaluation of high temperature components.?'-strengthened Co-based single-crystal superalloys have possessed comparable,if not better,creep properties than first-generation Ni-based single-crystal superalloys at 900?.Until now,most researches on the creep mechanism of the y'-strengthened Co-based superalloys were conducted at 900?and below.More work performed at higher temperatures(>950?)is required in order to further improve of the temperature capacity of y'-strengthened Co-based superalloys.In this study,Co-7Al-8W-1Ta-4Ti(at.%)single-crystal superalloy was selected as the experimental alloy and its creep behavior,microstructural and deformation substructural evolution were investigated at 1000?/137MPa in order to identify the creep mechanism of ?'-strengthened Co-based superalloys under high temperature and low stress condition.Subsequently,formation mechanisms of featured creep defects and their interaction configurations during the creep process were analyzed in atomic scale.Moreover,the effect of such configurations on the creep resistance of y'-strengthened Co-based superalloys was investigated.Finally,considering the composition of the experimental alloy,the influence of elemental segregation at creep defects on the creep resistance was studied.The investigation of creep behavior shows that the entire creep process of experimental alloy at 1000?/137MPa can be divided into four stages:deceleration creep stage(?),minimum steady-state creep stage(?),global steady-state creep stage(?)and acceleration creep stage(?).The rapid decrease of creep rate in stage ? is ascribed to the accumulation of dislocations in ? channels.The formation of rafts and interfacial dislocations in stage ? keep the creep rate stable.The decrease of the ?' phase volume fraction and shearing of ?' phases by partial dislocations of stacking faults(SFs)increase the minimum to the global steady-state creep rate.After the topological inversion,the interactions of SFs in ?' phases are responsible for the stage ?.As the creep proceeds,partial dislocations of SFs shear ?' phases greatly,and the microcracks nucleate,which leads to the rapid rise of creep rate in stage ?.The research on the formation mechanism of featured creep defects and their interaction configurations suggests that two types of SFs,namely superlattice intrinsic and extrinsic stacking faults(SISF&SESF),exist in ?' phases,and the formation mechanism of the fundamental SSF can be summarized as:a/2<011>?a/6<121>+SISF+a/6<112>.Two types of SSFs interact into three types of configurations(V-,T-and X-types),and V-and T-type configurations account for big scale in interaction structures,almost no X-type configurations being observed.The V-type configuration mainly forms when a partial dislocation bends from one glide plane to another,and two types of dislocation reactions are involved,such as a/6<211?a/3<001>+SISF+a/6<211>and a/6<121>?a/6<110>+SISF+a/6<211>.The a/3<001>and a/6<110>are sessile stair-rod dislocations,which improves creep resistance by preventing further expansion of SFs in ?' phases.The T-type configuration forms when the expansion of SF is blocked by the coherent SF interfaces on the other slip plane,improving creep resistance.The study on the elemental segregation behavior at creep defects shows that the enrichment of W at SSFs triggers the local ?'??/? phase transformation and reduces the SSF energy.This makes the SSF easier to expand in ?' phases and reduces the creep resistance.The Co segregation slows down the slip rate of partial dislocations,dominates the expansion of SSFs and contributes to the formation of V-and T-type configurations,thus improving the creep resistance.Moreover,a local ?'?? phase at the partial dislocation is active due to the Co segregation,which generates more ?/?' interfaces,thus trapping the dislocation moving in the ?' phase and improving the creep resistance at low stress condition.However,under high stress condition,the y phase transformation may decrease creep resistance due to the promotion of misfit dislocation generating anti-phase boundary in ?' phases,which decreases creep resistance.In summary,this work further the study on the creep behavior and mechanism of ?'-strengthened Co-based single crystal superalloys in multi-scale at a higher temperature(>950?),and the influence of elemental segregation at creep defects on the creep resistance is also studied,providing a foundation for the strength design and composition optimization of the next generation of ?'-strengthened Co-based single crystal superalloys.