CREEP STRENGTH AND MICROSTRUCTURE OF POLYCRYSTALLINE γ′-STRENGTHENED COBALT-BASE SUPERALLOYS

The influence of several alloying elements on the microstructure and creep properties of polycrystalline Co-base superalloys hardened with the ternary Ll(2) compound, gamma'-Co-3( Al, W), are presented in this work. The stability of the gamma/gamma'-microstructure was investigated by long term aging at temperatures of 750 degrees C and 900 degrees C. High contents of chromium and iron destabilize the gamma/gamma'-microstructure. Additions of nickel widen the gamma/gamma'- two phase field. Discontinuous precipitation is observed only in the iridium-containing alloy during aging. In all other alloys no formation of further intermetallic phases are found even after 1000 hrs aging at 750 degrees C and 900 degrees C. Boron has been added for the formation of grain boundary strengthening intermetallic phases which leads to an adjacent gamma'-depleted zone. Nanoindentation shows that titanium improves its hardness. The creep strength of the studied alloys is sufficiently better than that of commercial carbide hardened polycrystalline Co-base superalloys and already close to gamma'-hardened Ni-base superalloys. An alloy containing 2 at.% titanium exhibited a creep strength comparable with the Ni-base alloy IN 100. The microstructure after creep tests at 950 degrees C showed rafting perpendicular to the compressive load axis, confirming the positive lattice misfit of the alloys at high temperatures which was determined with high energy X-ray diffraction.