Microstructure And Creep Behaviours Of A Re-Free Single Crystal Ni-Base Superalloy

In this paper, the single crystal nickel based superalloys are prepared in a vacuumdirectional solidification furnace according to different temperature gradients. By means ofheat treatment at different regims, SEM/EDS composition analysis and creep propertiesmeasurementtest, influence of the pouring and solution temperature on microstructure andcreep property of the alloy are investigated. The features of microstructure evolution anddeformation mechanism of the alloy during creep at moderate and elevated temperature arediscussed.Results show that, as the pouring temperature enhances, the primary and secondarydendrite spacing of the alloy decrease to minish the composition segregation of theelements Al, Ta, W and Mo in the dendrite/inter-dendrite regions. In as-cast alloy, theelements Al, Ta and Co are enriched in the inter-dendrite region, while the elements W, Moand Cr are enriched in the dendrite region. After full heat treatment, the absolute values ofthe segregation coefficients of the elements reduce with the increase of the solutiontemperature, which may improve the creep resistance of the alloy. The creep activationenergies of the nickel base superalloy under conditions of the moderate temperature/highstress and high temperatures/lower stress are measured to be Q=568.3kJ/mol and Q=484.6kJ/mol, respectively.The deformation mechanism of the alloy during creep at moderate temperature aredislocations slipping in the J matrix and shearing into the cubic J cphase, thereinto, thesuper-dislocations shearing into the J cphase may be decomposed to form the configurationof partial dislocations plus stacking faults. In the latter stage of creep under condition of moderate temperature, the primary/secondary slipping systems are alternatively activatedto result in the initiation of micro-crack occurred on the interface of the J/c J phases alongthe direction perpendicular to the stress axis, and the crack may propagated along <110>direction until creep fracture up to the occurrence of creep fracture, which is thought to bethe main reason of the fracture displaying the square cleavage plane on (001) plane.During the primary stage of creep at elevated temperature, the J cphase hastransformed into the N-type rafted structure along the direction that perpendicular to thedirection of stress axis, and the deformation mechanism of the alloy during steady statecreep is identified to be the dislocations climbing over the rafted J cphase. During the latterstage of creep, the deformation mechanism of the alloy is that the dislocations shearinginto the rafted J cphase. As creep goes on, the primary/secondary slipping systems arealternatively activated to make the micro-crack initiated on the interface of the rafted J/c Jphases and propagated along the direction perpendicular to the stress axis until theoccurrence of creep fracture, which is thought to be the fracture mechanism of the alloyduring creep at elevated temperature.