Sustainable Strengthening of CoCrFeNiMn High-Entropy Alloy Using NbC-Nb(C) Composite Synthesized from Recycled Carbon Precursors

This study presents a sustainable approach to the synthesis of niobium carbide (NbC) using carbon obtained from thermal plasma valorisation of waste polypropylene (PP), followed by its application as a ceramic reinforcement in CoCrFeNiMn high-entropy alloy (HEA) produced via powder metallurgy. Pure niobium powder and carbon soot were mechanically alloyed as raw materials in Nb: C ratios of 1:1 and 1:2 under high-energy ball milling for up to 8 h, both with and without n-heptane as a process control agent (PCA). Phase formation was monitored using X-ray diffraction (XRD), while scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were employed to characterize the particle morphology and composition. The absence of n-heptane favoured NbC formation and prevented pyrophoric oxidation, with a Nb: C ratio of 1:1, yielding approximately 19 wt.% NbC after only 4 h of milling, while the remaining fraction consisted predominantly of carbon-supersaturated Nb(C), serving as a highly reactive precursor for subsequent NbC formation. The 4 h NbC powders were subsequently incorporated into CoCrFeNiMn HEA at 5, 10, and 15 wt.% and consolidated by spark plasma sintering (SPS) at 1000 °C under a pressure of 48 MPa. Mechanical testing revealed an increase in compressive yield strength (CYS) with increasing NbC content, starting already at 5 wt.% NbC and reaching an approximately 40% improvement at 15 wt.% NbC compared to the unreinforced alloy. At the highest investigated loading, the composite achieved a yield strength of 717 MPa and a hardness of 310 HV30, corresponding to a nearly 15% increase in hardness, while retaining excellent compressive plasticity (>40%).