Cobalt-free Nickel-base Superalloys

Abstract

Cobalt-free nickel-base cast superalloys hava been developed. Cobalt is considered to be a be-neficial element to strengthen the alloys but should be eliminated in alloys to be used for direct cycle helium turbine driven by helium gas from HTGR ( high temp. gas reactor). The elimination of cobalt is required to avoid the formation of radioactive ⁶⁰Co from the debris or scales of the alloys. Cobalt-free alloys are also desirable from another viewpoint, i. e. recently the shortage of the element has become a serious problem in industry.
Cobalt-free Mar-M200 type alloys modified by the additions of 0.15-0.2wt% B and 1-1.5wt% Hf were found to have acreep rupture strength superior or comparable to that of the original Mar-M200 alloy bearing cobalt. The ductility in tensile test at 800°C, as cast or after prolonged heat-ing at 900°C ( the tensile test was done without removing the surface layer affected by the heatlng), was also improved by the additions of 0.15-0.2% B and 1-1.5% Hf.
The morphology of grain boundaries became intricated by the additions of 0.15-0.2% B and 1-1.5% Hf, to such a degree that one can hardly distinguish grain boundaries by microscopes.
The change in the grain boundary morphology was consldered, as suggested previously by one of the authors (M.Y.), to be the reason for the improvements in the creep rupture strength and tensile ductility.