Effects Of Element W Content On Microstructure Stability And Creep Properties Of Nickel-Base Superalloys

By means of Md and Nv methods, nickel-base superalloys with different tungsten content have been designed to predict the precipitated tendency of TCP phase. The influence of tungsten content on the precipitated tendency of TCP phase is investigagted by means of the preparing alloys and long term aging treatment. And the effects of the element W content on the microstructure stability and creep properties of the single crystal nickel-base superalloys are investigated by means of the measuring creep properties and microstructure observation.Results show that no TCP phase is precipitated when the maximum of Md and Nv values were calculated to be 0.9836 and 2.210, respectively, in the composition designed of the alloys. After aged for 300 h at 1080℃, theγ' phase in the alloy still remains the cubical configuration with the increase of the element W content, therefore, the alloy displays a better microstructure stability. Compared to the alloy with 7.5% W,9%W alloy has a better creep resistance, in the ranges of the applied stress and temperature, the activation energy of the alloy during steady state creep is calculated to be Q= 465kJ/mol. In the initiation period of high temperature creep, theγ' phase in the alloy is transformed into the N-type rafted structure, and the deformation mechanism of the alloy during steady state creep is that dislocation climbs over the raft-likeγ' phase, thereinto, the raftedγ' phase which is perpendicular to the applied stress axis may effectively hinder the dislocation movement, which is thought to be the main reason of the alloy possessing the better creep resistance. The various morphology of the raftedγ' phase is displayed in the different regions of the creep samples, the raftedγ' phase which is vertical to the applied stress axis appears in the region far from the fracture, and the twisted raftedγ' phase appears in the region near the fracture, which is attributed to the servere plastic deformation occurred in the region. During middle temperature creep, no rafted of theγ' phase is detected in the alloy, and the deformation mechanism in the alloy is that the cubicalγ' phase is sheared by the<110> super-dislocation which may be decomposed into the the configuration of (1/3)<112> partials+stacking faults, which may hinder the cross-slipping of the dislocations to improve the creep resistance of the alloy. And the deformation mechanism of the alloy in the later stage of creep at high temperature is that<110> super dislocation shears into the raftedγ' phase. The uniformity composition and creep lifetimes of the alloy may be improved by enhancing the solution temperature to decrease the segregation in the dendrite /interdendric regions. The creep resistance of the alloy at high temperature may be enhanced with the increase of the element W content. And the microstructure defection may obviously deteriorate the creep properties of the alloy.