Abstract
The influence of several alloying elements on the microstructure,and creep properties of polycrystalline Co-base superalloys,hardened with the ternary L12 compound, γ’-Co3(Al,W), are,presented in this work. The stability of the γ/γ’-microstructure was,investigated by long term aging at temperatures of 750 °C and,900 °C. High contents of chromium and iron destabilize the γ/γ’-,microstructure. Additions of nickel widen the γ/γ’- two phase,field. Discontinuous precipitation is observed only in the iridiumcontaining,alloy during aging. In all other alloys no formation of,further intermetallic phases are found even after 1000 hrs aging at,750 °C and 900 °C. Boron has been added for the formation of,grain boundary strengthening intermetallic phases which leads to,an adjacent γ’-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 γ’-,hardened Ni-base superalloys. An alloy containing 2 at.% titanium,exhibited a creep strength comparable with the Ni-base alloy,IN100. The microstructure after creep tests at 950 °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.
