Hierarchical nanostructure of CrCoNi film underlying its remarkable mechanical strength

Abstract

A CrCoNi medium entropy alloy thin film is fabricated using magnetron sputtering. It exhibits a unique hierarchical nanostructure, featuring (1) a high density of planar defects (mostly stacking faults plus a small number of twin boundaries), (2) a dual-phase configuration (a mix of face-centred-cubic and hexagonal-close-packed), and (3) vertically aligned, textured nanocolumns, each with a width of ∼100 nm. The hierarchical nanostructure in this study is original, especially for its dual phase combination, since the bulk CrCoNi medium entropy alloy generally presents a single phase face-centred-cubic structure. The CrCoNi film shows a hardness quadruple that of its face-centred-cubic structured counterpart. The formation and its role of the hierarchical nanostructure in producing such mechanical strength are discussed.

Designing Transformation-Induced Plasticity and Twinning-Induced Plasticity Cr-Co-Ni Medium Entropy Alloys: Theory and Experiment

“…Bearing these facts in mind, for the well-characterized CrCoNi MEA, the primary deformation mode is predicted to be DIMT at room temperature, which is further promoted at 77K. This result is in line with the experimental observations at both room and cryogenic temperatures [4,70]. For example, Miao et al found that in the equiatomic CrCoNi MEA hcp lamellae appear at large strain levels and its volume fraction increases at cryogenic temperature [4].…”

Size-dependent deformation behavior of dual-phase, nanostructured CrCoNi medium-entropy alloy

“…At the atomic scale, typical STEM images of two columnar 'grains' (Fig. 1b, c [14,17], endowing it with ultrahigh hardness and unique deformation behavior [29].…”

“…More recently, the design principles are dedicated to incorporating other strengthening mechanisms into this kind of novel alloys, such as microstructure modification [18][19][20], precipitation strengthening [21,22], ductile multicomponent intermetallic nanoparticles [23], and interstitial solid solution strengthening (by adding boron [24], carbon [25], nitrogen [26,27] and oxygen [28]), to push the property boundary of possibility apart from their inherent substitutional solid solution strengthening. Similarly, we successfully fabricated a nanostructured dual-phase (HCP and FCC) CrCoNi MEA using magnetron sputtering, which possesses ultrahigh nanohardness of~10 GPa obtained by nanoindentation that is much higher than that of conventional FCC structured counterpart [29,30]. It was revealed that this ultrahigh hardness originated from its unique hierarchical microstructures enabled by the special processes of magnetron sputtering [29,30].…”

“…Similarly, we successfully fabricated a nanostructured dual-phase (HCP and FCC) CrCoNi MEA using magnetron sputtering, which possesses ultrahigh nanohardness of~10 GPa obtained by nanoindentation that is much higher than that of conventional FCC structured counterpart [29,30]. It was revealed that this ultrahigh hardness originated from its unique hierarchical microstructures enabled by the special processes of magnetron sputtering [29,30]. However, the detailed mechanical behaviors of the newly fabricated dual-phase CrCoNi MEA in terms of stress-strain curves, deformability and underlying deformation mechanism have not been systematically explored and especially, the ductility achievable under such high hardness remains mysterious, both of which will be essentially uncovered in the current investigation.…”

Can Experiment Determine the Stacking Fault Energy of Metastable Alloys?

“…The pressure-induced hcp martensite was shown stable at ambient conditions after the release of the high pressure and transferring to the high temperature fcc phase after heating [108]. Additionally, Chen et al found the coexistence of the hcp and fcc phases in the epitaxial film where the hcp phase can reach very high fraction [110]. As discussed above, it is likely the large lattice friction stress [103,104] that prevents the spontaneous nucleation of hcp martensite in these alloys at room temperature, despite that the hcp structure is energetically more stable than the fcc one.…”