Deciphering deformation mechanisms of hierarchical dual-phase CrCoNi coatings

Abstract

Hierarchical CrCoNi medium entropy alloy (MEA) thin films with a dual-phase face-centred cubic (FCC) and hexagonal closed-packed (HCP) nanostructure were prepared on M2 steel substrates by closed field unbalanced magnetron sputtering. Nanoindentation tests show an ultra-high hardness of 9.5 GPa, attributable to large amounts of innate planar defects (i.e., growth twins and stacking faults) impeding dislocation motion in the coatings. A deep analysis of undeformed and post-mortem samples reveals grain refinement as the dominant deformation mechanism in FCC dominated regions, while phase transformation and shear banding played major roles in regions occupied by HCP phase. The grain refinement was facilitated by twin/matrix lamellae, with dislocations piling up and arranging into interconnecting grain boundaries. The shear banding was accelerated by innate planar defects in the HCP phase due to a lack of slip systems. Of particular interest is the observation of HCP → FCC phase transformation, which was catalysed by deformation-induced grain reorientation with innate stacking faults acting as embryos to grow the FCC phase. The results of this work suggest that multiple deformation pathways could be activated in CrCoNi coatings with assistance of growth defects, thereby imparting these technically important coatings appreciable ductility.