Microstructural homogenization and substantial improvement in corrosion resistance of mechanically alloyed FeCoCrNiCu high entropy alloys by incorporation of carbon nanotubes

Abstract

FeCoCrNiCu high entropy alloy (HEA) powder was produced by mechanical alloying. Carbon Nanotubes (CNTs) were mixed in the HEA powder (0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 3.0, 5.0, 7.0 wt.% of CNT) and then the mixture was consolidated by spark plasma sintering at 800°C. Phase constitution of the pellets was sensitive to the CNT amount. With smaller quantities of CNTs (upto 2 wt.%), the pellets contained a mixture of two different face centred cubic (fcc) phases along with σ phase. With increase in the CNT amount (till 2 wt.%), a gradual decrease in the volume fraction of one of the fcc phase occurred leading to microstructural homogenization. Higher CNT additions (3, 5 and 7 wt.%) led to re-evolution of inhomogeneous microstructure containing chromium carbide phase and the two different fcc phases. Corrosion behaviour measurements showed that with continued addition of CNTs, reduction in the corrosion rate happened till 2wt.% CNT. Beyond this, a significant enhancement in the corrosion rate was noticed for pellets with 3, 5 and 7 wt.% CNT additions. Between the pristine HEA pellet and the pellet with optimum CNT amount (2 wt.%), 88.6 % reduction in the corrosion rate was measured. The enhancement in the corrosion resistance was attributed to an increase in the chemical homogeneity, which reduced the possibility of galvanic coupling. The decrease in the corrosion resistance beyond the optimum CNT (2 wt.%) was attributed to the re-appearance of chemical heterogeneity and evolution of chromium carbide (Cr23C6) phase, which causes intergranular corrosion in these systems.