Influence Of Element Re On Creep Behavior Of Single Crystal Nickel-base Superalloy

In this paper, the4.5%/free Re single crystal nickel-base superalloys are prepared byvacuum directional solidification furnace. By means of heat treatments with different regimes,SEM/EDS composition analysis and creep properties measurement test are used to investigatethe influence of the element Re and different heat treatment regimes on creep properties of thealloy. And the features of microstructure evolution and deformation mechanism of the alloyduring creep at different temperatures are observed by electron microscope and deeplydiscussed.Results show that, compared with the elements Cr and Co, a bigger segregation extent ofthe elements Al, Ta, W, Mo and Re appear in as-cast superalloy. Thereinto, the element W, Cr,Mo and Re are enriched in the dendrite arm region, while the elements Al, Ta and Co areenriched in the inter-dendrite region. After treated by solution for longer time and aging, theelements segregation extents in the dendrite arm and inter-dendrite regions may be obviouslyreduced, which can improve the creep property of the alloy. After heat treated with differentregimes, the creep activation energies of the nickel-base superalloy during steady state creep athigh temperature are measured to be Q=452.5kJ/mol and Q=530.7kJ/mol, respectively.And the creep activation energy of the nickel-base superalloy during steady state creep atintermediate temperature is measured to be Q=574.4kJ/mol.In the latter stage of creep at moderate temperature, the primary/secondary slippingsystems in the alloy are alternately activated to promote the initiation of micro-crack occurringon the interface of the γ′/γ phases along the direction perpendicular to the stress axis, and thecrack may propagate along <110> direction until the occurrence of creep fracture. It is thoughtto be the main reason of the creep fracture that the fracture displaying a square-like cleavageplane is on (001) plane. On the other hand, when the propagation of the crack along the <110>direction is hindered to change the direction, it may result in the formation of the irregularcleavage plane on (001) plane. In the initial stage of creep at high temperature, the cubic γ′phase transformes into the N-type raft structure, which is perpendicular to the stress axis. Andthe deformation mechanism during creep steady state is the dislocations slipping in the γmatrix and climbing over the rafted the γ′phase.Under the conditions of the applied higher stress at medium temperature, significant amount of dislocations activated in the matrix channels, and the (1/2)<110> super-dislocations shear into the cubical γ′phases, which is thought to be the deformationmechanism of the superalloy during creep. Thereinto, the <110> super-dislocations shearinginto γ′phase will be decomposed to form the configuration of (1/3)<112> partial dislocationsplus stacking faults, which can hinder the dislocations cross-slipping to improve the creepresistance. During creeping under the temperatures of760oC and980oC, the dislocationsshearing into the γ′phase may cross-slip from {111} planes to {100} planes to form the K-Wlocks with the dislocation configuration of the non-plane core structure, which is thought caneffectively restrain the dislocations slipping on {111} planes. As the temperature enhances to1100oC, the dislocations in the K-W locks may be re-activated for slipping on {111} planes torelease the locks under the action of thermal activation. It is thought to be the main reason ofno K-W locks detected in the alloy.According to the creep properties measurement tests, the creep-life of the4.5%Re alloy is427h which is much longer than the creep-life245h of the free-Re alloy under the conditionof760oC/800MPa. During creeping, the γ′phases in the free-Re alloy transform into thestring structure and the dislocations forms no K-W locks. While the γ′phases in4.5%Re alloycan keep cubical morphology until creep fracture. When at980oC/300MPa, the free-Re alloydisplays the creep-life72h while the4.5%Re alloy increases to168h. During creeping at980oC, the γ′phases in both alloys transform into the raft structure. Compared with the free-Realloy,4.5%Re alloy displays a higher lattice mismatch, which can reduce the amount of thedislocations shearing into the γ′phases, dues to the shorter distance of the dislocations on theboundary of γ/γ′phases. It is thought to be the main reason that the4.5%Re alloy displays abetter creep resistance.