Creep Behaviors And Effecting Factors Of A Nickel-Based Single Crystal Superalloy

In this paper,4%W-6%Mo/free-Ru,6%W-6%Mo/free-Ru and 6%W-6%Mo-2%Ru nickel-based single crystal superalloys are designed and prepared,respectively.By means of measuring the creep properties,microstructures observation and contrast analysis of the dislocation configuration,the creep behaviors and effect factors of single crystal nickel-based superralloy is investigated to reveal the deformation and damage mechanism of the alloys during creep.And the effect mechanism of Ru improving the creep resistance of nickel-based single crystal superalloy is studied by measuring the concentration distribution of elements in ?/?' phases and lattice parameters and mismatch of ?/?' phases.The results show that the primary/secondary dendrite arm spacing and component segregation in the alloy are decreased and the creep resistance of the one is improved with the increase of withdrawal rate.The primary/secondary dendrite arm spacing of the alloy is diminished as the pouring temperature enhances,which may enhance the segregation coefficients of the Al,Ta in the dendrite/inter-dendrite regions and reduce the segregation coefficients of the W,Cr and Mo in the ones.And the better creep resistance and longer creep life of alloy may be obtained when the withdrawal rate of 0.08mm/s and pouring temperature of 1550? are used.The(001)plane of the as-cast alloy with different compositions displays the typical "+" dendrite pattern.And the Ta and W which are largest positive and negative segregated elements,respectively,are segregated in the inter-dendrite and dendrite regions.No obvious the element Ru is segregated in the dendrite/inter-dendrite regions,but the Ru may reduce obvious the segregation of other elements in the dendrite/inter-dendrite regions.After solid solution treated at 1315?,the segregation extent of elements in the dendrite/inter-dendrite regions is obviously reduced to improve the creep properties of alloy.Wherein,the creep life of 4%W-6%Mo nickel-based single crystal superalloy at 760?/800 MPa is measured to be354 h,the creep life of the one at 1040?/137 MPa is measured to be 556 h,in the range of the applied temperatures and stresses,the creep activation energy of the alloy during steady state creep are calculated to be Q = 458.3kJ/mol and Q = 420.9kJ/mol,respectively.After the concentration of the W increases from 4wt% to 6wt%,a large number of needle-like ? phase are precipitated in the alloy during the long-term aging and creepproperty measuring.The ? phase enriching the refractory elements destroys the continuity of the microstructure in the alloy,which is the weaker link of the creep strength in alloy due to the one causing easily the stress concentration to promoting the initiation and propagation of crack.Wherein,the precipitated needle-like ? phases may also greatly reduce the creep properties of the alloy,compared with the 4%W-6%Mo alloy,the creep lifes of 6%W-6%Mo alloy at 1040? and 800? reduced about 67% and 41.1%,respectively.Adding 2%Ru to 6%W-6%Mo alloy can promote the more Al,Ta atoms of ?' former dissolving into matrix phase,and can promote the more Cr,Co,Mo,W atoms of ? former dissolving into the ?' phase,which is thought to be the main reason of the element Ru restraining the precipitation of TCP phase in the alloy.Compared with 4%W-6%Mo alloy,the creep life of 2%Ru-6%W-6%Mo alloy at 1100?/137 MPa increases from 68 h to 125 h.Wherein,the element Ru may enhance the alloying extent of elements in ?/?' phases and misfits,because the one can restrain the precipitation of TCP phase,which is thought to be the main reason of the Ru-containing alloy having a better creep resistance at high temperature.The deformation and damage mechanism of alloy at during creep at intermediate temperature are dislocations slipping in ? matrix and shearing into the ?' phase,wherein,the dislocations of shearing into ?' phase be decomposed to form the configuration of the partials plus stacking fault,or may be cross-slipped from {111} plane to {010} plane to form the K-W locks.In the later stage of creep,the alternated activation of the primary/secondary slipping system results in the twisted ?/?' phases to promote the micro-holes appearing in the interface.As the creep goes on,the twisted degree of ?/?' phases is aggregated for assembling the much more micro-holes to form the cracks,which may propagate along the interface perpendicular to the stress axis.The creep fracture of alloy occurs once the cracks at various cross-section are connected by the tearing ridges,which is thought to be the damage and fracture mechanism of alloy during creep at intermediate temperature.The deformation mechanism of the alloy during steady state creep at high temperature is dislocation slipping in ? matrix and climbing over the rafted ?' phase.In the later stage of creep,the alternated activation of the primary/secondary slipping dislocations can cause the dislocation slipping in ? matrix to shear into the rafted ?' phase,which may decrease the strength of ?' phase to promote the initiation and propagation of cracks up to the occurrence of creep fracture,which is thought to be the damage and fracture mechanism of the alloyduring creep at high temperature.