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.
