Characterization, corrosion resistance and hardness of rapidly solidified Ni–Nb alloys

Abstract In this research various Ni(-x)Nb (x = 10, 15, 20, 30, 40, 45, 52 and 57 wt %) alloys samples were rapidly solidified (RS) by using copper mold casting. High purity elements were used to produce the RS Ni–Nb samples. Even though Ni–Nb alloys have been extensively investigated over the past decades regarding thermodynamics, phase diagram and heat treatment; as-solidified microstructures and properties generated under non-equilibrium conditions remain undetermined for this alloy system. The Ni–Nb system is characterized by two eutectic reactions (termed E1: 22.5 wt % Nb and E2: 52 wt % Nb) and three Ni3Nb, Ni6Nb7 and Ni8Nb intermetallics, being the later barely reported in literature. The E1 group of alloys (i.e., x = 10, 15, 20 wt%) showed the microstructures constituted by Ni-fcc cellular/dendritic matrix enveloped by Ni + Ni3Nb eutectic. In contrast, the higher niobium (Nb) containing compositions (E2 group: x = 40, 45, 52 and 57 wt%) exhibited plate-like morphologies associated with either Ni3Nb or Ni6Nb7 phases. Characterization of the microstructure (cell/dendrites) spacing associated with the E1 group of alloys allowed the cooling rates to be estimated. A mixture of irregular γ″-Ni3Nb and needle-like δ-Ni3Nb morphologies is present in the Ni–20%Nb alloy solidified under a cooling rate of 1.7 × 103 K/s. The passive range of the Ni–30%Nb or higher Nb content alloys was demonstrated to be at least two-fold higher when compared to that of the commercial 625 Ni-based superalloy. The concurrent evaluation of key requirements for Ni–Nb alloys such as Vickers microhardness and passivability were determined as function of Nb content. They are both strongly affected by the Nb containing.