Effects Of Alloying Elements On The Microstructure And Mechanical Behavior Of ?'-Strengthed Co-base Superalloys

Co-Al-W system superalloys have attracted extensive attention from superalloy researchers due to the ?'-Co3(Al,W)phase with L12 structure.However,in the Co-Al-W alloy system,a large amount of W element is required to stabilize the ?'-Co3(Al,W)phase,resulting in an excessive density which limits its application in the practical aerospace production.How to reduce the density of the alloy while maintaining the stability of the microstructure especially the y' phase at elevated temperatures is an urgent problem to be solved in the development of Co-Al-W based superalloy system.Based on the previous studies on Co-Al-W ternary alloys and multi-component Co-Al-W alloys,the additions of trace elements C and B,solution strengthening elements Cr and Re as well as precipitation strengthening elements Ta and Hf were added into ternary alloy systems.The addition of Mo replacing part of W in multi-component Co-Al-W alloys systems and several alloys systems with different Ni content are also designed to systematically investigate the effects of C,B,Mo,Ni etc.on the microstructure and mechanical properties.The additions of C in ternary alloy systems can slowly increase the solvus temperature of the ?' phase,which also reduces the misfits of ?/?'.And it improves the yield strength of the alloy,but the elongation decreases with the C additions.The additions of the B element has little effect on the solvus temperature of the ?' phase and reduces the mismatch of ?/?'.The additions of B also reduces the yield strength as well as the ductility of the alloys.The addition of Cr reduces the solvus temperature of the?' phase and the yield strength.However,the ductility of the alloys increases with Cr contents.From the results of the coarsening rate,we find that the increasing C content will increase the coarsening rate of the ?' phase at 900 ?,while the addition of B element will reduce the coarsening rate of the ?' phase at 900 ?.Both C and B keep the coarsening rate of the y'phase at a relatively higher level than other ternary Co-Al-W alloys.Mo addition decreases the y'phase solvus temperature and the mass density but does not influence the volume fraction of the ?' phase.The formation of a surrounding?' phase in the heat-treated alloys was caused by the diffusion of elements between secondary phases and the substrate during the heat treatments.Although the melting point of the ? phase is decreased by Mo addition,the solidification paths remain the same.The yield strength decreases with Mo addition at room temperature because more of the ? phase precipitated in the interdendritic regions.During the fracturing process,the ?' precipitate around the ? phase play important roles:blocking the extension of microcracks.As the Ni content in the alloy system increases,the ?' phase solvus temperature increases,and the morphology of ?' phase changes from spherical to cuboidal.And the volume fractions of secondary phases also reduce with Ni additions.However,the content of Ni has little effect on the mass density and the volume fraction of the ?' phase.The yield strength of the alloys at room temperature and 900 ? increase significantly with Ni additions.This is because the addition of Ni in the alloy changes the element distribution,resulting in the change of the ?' phase morphology.The increasing mismatch stress between the ?/?' phases ultimately increases the yield strength.