The development of Co-free low-cost easy-to-deform Ni-based superalloys

The development of a new precipitation-strengthened Ni-based superalloy relies on a seemingly contradictory yet crucial trade-off between ‘easy-to-deform’ and ‘high strength’. Commercial Haynes 282 (Ni-20Cr-10Co-8.5Mo-2.1Ti-1.5Al) has a good balance of mechanical properties and formability, but it still faces growing concerns regarding material cost and applicability due to the high concentration of Co (∼10 wt%). Therefore, it is necessary to develop Co-free, easy-to-deform and low-cost alloys with thermomechanical processing capabilities and mechanical properties comparable to those of Haynes 282. Both Ni and Fe are selected to replace Co, because the atomic sizes of Fe, Ni and Co are almost identical (∼0.125 nm). Moreover, the moduli of Co and Fe are all 82 GPa, which is only 7% higher than that of Ni (76 GPa). The effects of Co substitution by Ni/Fe on the alloy microstructures and mechanical properties are investigated. Substitution of Co with Ni/Fe does not change the size and volume fraction of the γ′ phase, nor does it change the partitioning behavior of elements between the γ matrix and γ′ phase. The substitution of Co with Ni/Fe only slightly changes the type of precipitates on grain boundaries (GBs). The newly developed ‘Co-free 282’ low-cost alloys are not susceptible to stress-accelerated GB oxidation and exhibit great mechanical properties (tensile and creep), which are comparable to the 282 alloy. Current findings not only broaden the alloy selection for applications in extreme environments, but also represent an important trend in the field of precipitation-strengthened superalloys and high/medium entropy alloys to reduce the dependence on strategic high-cost metal Co.