Mechanical Creep Behaviors And Deformation Mechanism Of 4.5Re/3Ru Nickel-based Single Crystal Alloys

It is found that adding 3Ru based on the research of 4.5Re nickel-based single crystal alloy can improve the precipitation of TCP phase in the 4.5Re alloy,reduce the degree of element segregation,solve the main defects such as large number and size of eutectic structure,and increase the alloying degree and creep resistance of γ’/γ phases considerably.Therefore,further study of the mechanical creep behavior and deformation mechanism of 4.5Re/3Ru nickel-based single crystal alloy is significant in terms of improving the deformation and dislocation movement mechanism during the tension/creep stage,it can provide additional technical support and form a theoretical basis for the application of nickel-based single crystal alloy in the aerospace field,which is of very significance to the scientific research and social value.In this thesis,we formulate three different heat treatment processes by the differential thermal analysis of 4.5Re/3Ru nickel base single crystal alloy.After undergoing EDS analysis and three-dimensional atomic probe,the composition segregation between the dendrites/inter-dendrites and the concentration distribution of elements in γ/γ’ phases of alloy(as-cast and fully heat-treated)are measured.The scheme was determined as the optimal heat treatment process.The mechanical creep behaviors and deformation mechanism of fully heat-treated alloys are studied,indicating the deformation and damage mechanism of crack initiation and propagation at different temperatures.Finally,the stress distribution of alloy during creep are simulated by the elastic-plastic finite element analysis method,and the following conclusions are obtained:1.While below the initial melting temperature,the eutectic structure and element segregation decreases drastically in the as-cast alloy as the solution temperature increases to 1332℃,at which the homogenization degree of alloy are eliminated.After two aging heat treatments at 1180℃and 870℃,the reasonable distribution of the γ’phase shape and size are obtained,which can greatly improve the creep resistance of alloy.Before and after creep,the concentrations of Al,Cr,CO and Ru atoms in γ/γ′ two phases have no significantly change,which measured by three-dimensional atomic.But the hybridization of elements Re,W,Mo,Co and Ru,which make the W,Re,Mo and Ru atoms enriched in γ matrix of the near γ’ phase,forming a peak concentration,leading to the γ phase producing lattice distortion,which can hinder the dislocation movement and the delay shear of the γ′ phase.This is one of the reasons why the alloy has excellent high-temperature creep properties.2.Through the diffraction analysis of dislocation configuration,it is determined that the alloy can form K-W lock during high-temperature deformation and inhibit dislocation slip and cross-slip during the tensile/creep test,which is one of the reasons why the alloy has good deformation resistance.The threshold of dislocation shear theγ’ phase was calculated and decreased with increasing temperature and the peak temperature of abnormal yield strength is 900 ℃ in alloy.It reveals that the hybridization of between Ru and Re,W atoms in the γ′ phase is the main reason for the retention temperature of K-W lock increases from 900 ℃ to 1100 ℃.When the temperature exceeds 1160 ℃,the dislocations in the K-W lock are reactivated and released,which makes the dislocations cross slip to the {111} plane again,reducing the creep-resistance,and it is the main reason contributing to the short life of alloy during ultra-high temperature creep.3.During creep at 760℃-880℃,most of the γ′ phase still maintains cubic shape,a small portion of the cubic γ′ phase is characterized by passivation or globalization phenomenon and mutual annexation along the direction perpendicular to the stress axis to form an intermittent raft structure.As the temperature rises to more than 950℃,the cubic γ′ phase has transformed into raft structure.Particularly,when the temperature exceeds 1160℃,the γ′ phase is dissolved within the alloy,which gradually reduces the size and volume fraction and the γ′ phase strength.Additionally,with the alternating activation of the primary / secondary slip system,it makes the γ/γ′ phases twisted and coarser,and even fractured to form sub-crystals,resulting in the reduction of alloy creep-resistance,which is the damage and fracture mechanism of alloy during high-temperature creep.4.The alloy deformation mechanism below 920℃ is that dislocation sliding in the γ matrix channel and cut into the γ’ Phase and decompose on {111} plane to form <112 > incomplete dislocation + SISF configuration or a / 2 < 110 > incomplete dislocation + APB configuration,respectively.It may also be that the dislocation cut into the γ’ phase slides from the {111} plane to the {100} plane to form K-W lock plus APB configuration,which can inhibit the dislocation slip and cross-slip.As the creep temperature rises,the stacking fault energy of alloy increases,and the probability of dislocation decomposition to form stacking faults decreases.The K-W lock formed can still improve the creep resistance of alloy and promote the dislocation to climb over the raft γ’ Phase,which is the deformation mechanism during the alloy high-temperature steady-state creep.5.In comparison with the "I-shaped" sample,the creep life of the "U-shaped" sample is significantly reduced because the sample transforms into the state of multi-axial stress and the stress distribution is more complicated.Among them,the shoulder or neck stress is small.As the extension of creep time,the maximum stress of the "U-shaped" sample shifts from the complex region to the constraint end at the bottom of the sample,showing an inverted "V" or "M" distribution,necking and fracture.It reveals the microstructure evolution law of U-shaped sample and the damage and fracture characteristics of alloy in the late creep stage of 780℃,980℃and 1070℃.Overall,the mechanical creep behavior and deformation mechanism of alloy during the tension/creep period are deeply studied,it revealed the correlation between the temperature and the deformation mechanism.In addition,the generation mechanism of K-W lock is proposed,and the elastic-plastic creep constitutive model is established.The effect of stress distribution on creep behavior is analyzed by the elastoplastic finite element analysis method,which provides a theoretical basis for the application of single crystal alloy in the aerospace field.