Elevated Temperature Creep Behavior And Mechanism Of The Third-generation Ni-based Single Crystal Superalloy DD33

Ni-based single crystal superalloys are the suitable materials for serving for the critical components in advanced aero-and land-based gas turbine engines,because of their excellent composition combability,and superior creep-,fatigue-,oxidation-and hotcorrosion resistances up to near 90%melting temperature and beyond.One of the most common applications is the base material of the turbine blade,which are the most critical components in engines.During services,Ni-based single crystal superalloys are mainly subjected to the elevated temperature invasion generated from the combustion of kerosenes,and to the creep and fatigue deformation from the centrifugal force from turbine blades' rotation.Therefore,the creep performance of Ni-based single crystal superalloys is one of the main research objectives.Besides,their developments are strongly dependent upon and limited by the addition of refractory elements,that approachs to the solubility limit in Ni-matrix.In addition to bringing about the density and cost of single crystal alloys,it also brings the disadvantage of microstructural instablity.As a concequence,the design of a new generation of Ni-based sigle crystal superalloys is unpractical without the intrinsic understanding of the relationship between composition,micro structure and performance.Moreover,the more prevaling ideal of environmental protection,thrust and fuel economy needs us pay more attention to studying the intrinsic strenthening and toughening mechanism of Ni-based single crystal superalloys.Therefore,the creep behavior and mechanism of Ni-based single crystal superalloys have become a research hotspot.For example,the formation and uncoupling of the dislocation networks in elevated temperature and low stress creep,their interaction with cavities,and the degradation mechanism of ?-?' microstructure and its role in elevated temperature creep,etc.are the most cutting-edge research content.The present paper is aiming at a third generation of Ni-based single crystal superalloy DD3 3 containing 4 wt%Re,which was developed in China.Elevated temperature creep behaviors have been analyzed.Moreover,the characteristics of the evolution of cavity,?-?' microstructure and dislocation networks,especially,the coupling mechanism between dislocation and other factors,were discussed in detail applying multiple analytical testing techniques.The paper provided elaborate description about the bahavior and mechanism of the elevated temperature creep of single crystal superalloy DD33.The main results are summaried as follows:(1)The dimension,volume fraction and shape of the cast porosity,and the dendrite microstructures as a function of withdrawal rate were studied.The minimum level of porosity are observed at an optimized withdrawal rate 125 ?m/s,where the maximum diameter and number of porosity is about 10 ?m and 42,respectively.Moreover,the close-packed and well-distributed dendrite arms are found either,where the primary and secondary dendritic spacing is 315 ?m and 32 ?m,respectively.Further,the threshold value for the level of cavities can be set as about 0.1%to fufill the acceptance requirement in our laboratory scale.(2)The primary creep regime is found to be operative at 980?/260 MPa and 1050?/200 MPa,where the period of steady-state creep with a constant strain rate is completely absent.Simultaneously,the ?'precipitates still keep the cubic shape until the late creep.However,the creep behavior at 1100?/137 MPa is the typical rafting creep regime,where a completely rafted ?-?' micro structure with dislocation networks can be observed on the end of primary creep stage.The composite ?/?' interfaces can be kept until the late steady stage.The evolution of the pore shape reveals that the shape changes from irregular to the hexahedral and strip-like.And the hexahedral pores account for the main part in crept sample after rupture at 1100?,which may be the result of the diffusion and aggregation of cavities along the crystallographic direction.However,on basis of the analysis the stress distribution around pores using the finite element software ABAQUS,the strip-like pores are the result of lateral expansion of micro-cracks caused by the stress concentration.Further,the interaction between dislocations and pores promotes the transition of creep deformation from steady to tertiary creep stage.(3)The analysis of the elevated temperature creep bahavior at 1100? under a broad stress regime of 120?174 MPa indicates that the formation of the dislocation network and the degradation of ?-?' micro structure are strongly dependent on the stress.The micro structural transformation process is speeded up due to a higher diffusion rate under the aid of the dense dislocation network under a higher stress.The distribution of the stable dislocation networks at the end of primary creep stage presents a splitting phenomenon.Two kinds dislocation networks,close-(?100 nm)and sparse-spaced(>120 nm)networks,are observed,which is more obvious under a higher stress.The spacings of the close-spaced networks is believed to be determined by the ?/?' lattice misfit,but the sparse-spaced networks are induced by the higher plastic flows in local regions under a higher stress.The final microstructural morphology is strongly dependent on the temperature,less on plastic folws and least on creep time.(4)The degraded ?-?' micro structure has almost no signigicant effect on the creep rate and creep rupture life at 1100? and non-isostress creep.Various degraded ?-?'microstructures with different dislocation networks are approaching to the same state within an additional 15 h at 1100? and 137 MPa.On basis of the measure results of yand y'-phase compositions using 3-dimensional atom probe(3DAP)tomography,the concentration gradients of element Re,Ta,Ni,Al,Co and Cr are found on ?-?' interface.Moreover,the compositional transition zone across the ?-?' interface decreases from?9 nm in the heat treated specimen to?7 nm at the degraded state.The analysis on the rupture micro structure reveals the same configuration as the specimen crept at 1100? and a constant load 137 MPa,showing a large number of a<011>superdislocations with short lines and a mall mount of a<010>superdislocations with long lines in y' rafts as a result of insufficient driving force for the shearing process.(5)On basis of the calculation results on the critical resolved shear stresses(CRSSes)for dislocation climbing,Orowan looping and precipitates shearing mechanisms,it is found that the dominating creep mechanism is changing from the dislocation slipping in the primary stage to the dislocation climbing in the secondary stage,and finally to the precipitates shearing mechanism in the late tertiary stage at for constant creep(137?174 MPa)at 1100?.However,the favorable mechanism is still the dislocation climbing mechanism in the late creep stage at 1100? under 120 MPa,which is consistent with the micro structural analysis showing numbers of a<010>superdislocations with low mobility.