Thermophysical and magnetic properties of Co-Ni-Mo-Al-Ta class of tungsten free Co-based superalloys

Abstract

Cobalt-based superalloys strengthened by precipitation of the ordered γ′-phase is extensively investigated in recent times. Among these superalloys, those containing tungsten are widely studied. However, relatively less information is available for the recently developed Mo-containing cobalt-based superalloys stabilized by the addition of niobium or tantalum. The thermophysical properties of this class of alloys are sparse in the literature. The present paper attempts to fill this gap and discusses the thermal and magnetic properties of Ta/Mo stabilized superalloys of compositions Co-30Ni-10Al-5Mo-2Ta-xTi-yCr (x = 0, 2; and y = 0, 2, 5, 10 at%) and to correlate the microstructural parameters with the properties. Increase in γ′ volume fraction achieved through the addition of 2 at% Ti increases the coherent interface area, which may influence the thermal properties. The thermal conductivity and magnetic properties initially tend to decrease in the temperature range of 25–1000 °C with chromium (Cr) to the Ti-containing alloys. However, the alloy with 10 at% Cr shows a reverse trend in thermal properties. The lattice and electronic contribution calculations indicate that a significant contribution to the total thermal conductivity comes from the electrons. The Cr addition causes a decrease in Curie temperature leading to a paramagnetic behavior when the concentration reaches 10 at%. Finally, attempts are made to rationalize the thermal and magnetic properties in terms of microstructures, precipitates shapes, and volume fractions and compare them with well-studied nickel-base superalloys.