EFFECTS OF Ru AND Cr ON γ/γ' MICROSTRUCTURAL EVOLUTION OF Ni--BASED SINGLE CRYSTAL SUPERALLOYS DURING HEAT TREATMENT

Abstract

The influences of Ru and Cr as well as their interaction on the elemental partitioning ratio and microstructural evolution have been investigated in six Ni-based single crystal experimental superalloys with various levels of Ru(0—5.1%) and Cr(0—5.7%) additions(mass fraction).The results indicate thatγ′precipitates are nearly spherical in the dendrite core of the base alloy(Ru and Cr-free),which has a low Re partitioning ratio and near zero lattice misfit,after aging treatment at 1100℃for 8 h.The lattice misfit and Re partitioning ratio increase slightly and theγ′precipitates change to be more cuboidal with the addition of 5.1%Ru in both Cr-free and Cr-containing alloys. Meanwhile,the Re partitioning ratio increases significantly with increasing the Cr content in both Rufree and Ru-containing alloys,which in turn results in more negative lattice misfit and more cuboidalγ′precipitates.After long-term thermal exposure at 1100℃,the nearly sphericalγ′precipitates with near zero lattice misfit in the alloy have no change in morphology,and are coarsened after a longer exposure time,while the alloy with intermediateγ′precipitates and low lattice misfit is coarsened more severely.However,a nearly-rafted structure tend to form in the alloy with nearly cuboidalγ′precipitates and intermediate misfit after heat treatment for 800 h.The time to form the rafted structure is significantly reduced in the alloys containing both Ru and Cr with high Re partitioning ratio and high lattice misfit as well as cuboidal or rectangularγ′precipitates.The alloy containing high Ru and intermediate Cr exhibits a rafted trend after heat treatment for 200 h while the rectangularγ′precipitates are rafted after heat treatment for only 50 h in the alloy containing high levels of Ru and Cr additions with the highest lattice misfit.