Trimodal hierarchical structure in the carbonaceous hybrid (GNPs plus CNTs) reinforced CoCrFeMnNi high entropy alloy to promote strength-ductility synergy

The concept of adding reinforcement phase to high entropy alloys (HEAs) is relatively new. Introducing the carbonaceous hybrid reinforcements (graphene nano plates (GNPs) and carbon nano tubes (CNTs)) into traditional metal matrix has initiated a cumulative attention as they show cost-effective and superior performance compared to individual ones. Herein, carbonaceous hybrid (GNPs + CNTs) reinforced CoCrFeMnNi (Cantor) high entropy alloy was fabricated using mechanical alloying (MA) and spark plasma sintering (SPS). Based on the XRD, XPS, Raman, FESEM and STEM results, carbonaceous reinforcements could dissolve in the Cantor matrix alloy and also promoted carbides formation, where more content of ones were represented as reinforcement phase. It was also revealed the fairly uniform dispersed carbon element in the Cantor-(0.2 wt%) GNPs + CNTs composite via three distinct morphologies as plate like, tube and structural shapes. The average grain size achieved for the Cantor alloy and Cantor-(0.2 wt%) GNPs + CNTs composite was 260 nm and 157 nm, respectively. The 0.2 wt% addition of GNPs + CNTs into the Cantor alloy resulted in the increase of the micro-hardness from 490 HV to 540 HV, compressive yield strength (CYS) from 1332 MPa to 1697 MPa with over-coming the strength and fracture strain trade-off effect. Dislocation strengthening was acting as main strengthening mechanism. In the Cantor-(0.2 wt%) GNPs + CNTs composite, the promotion of strength-ductility synergy was affected by trimodal hierarchical structure due to presence of nano sized carbonaceous hybrid reinforcements following by two-dimensional structure of graphene, sub-micron sized grains and nano sized areas such as twins, precipitates and grains. Besides, bridging and pulling-out of reinforcement phase in the fracture surfaces of composite were response for advanced performance. This study provided high entropy alloy play-ground, where carbonaceous hybrid reinforcements can lead to the formation of a trimodal hierarchical structure, resulting to novel property opportunities.