Strengthening and interface-mediated plastic co-deformation in an ultrafine Cr-Ni eutectic: A nanomechanical investigation

Abstract

Ultrafine eutectic heterostructures provide a stringent test of plasticity in high-strength materials, where deformation must be accommodated through interfaces and strain gradients. Room temperature ductility is typically limited by premature fracture of the hard phase, leaving open the fundamental questions regarding the interface spacing, atomic structure and local chemistry that enable plastic co-deformation. Here we address this question using a model system of Cr-Ni binary alloy, processed via electron-beam powder bed fusion that produces a lamellar eutectic microstructure of Cr-rich BCC and Ni-rich FCC phases, with an average interlamellar spacing of ~450 nm. Atomic-resolution STEM revealed a stepped semi-coherent FCC/BCC interface with the Kurdjumov-Sachs orientation relationship and Ni-enrichment confined to a few atomic planes on the BCC side. In situ micro-scale compression and tension tests in SEM demonstrate high flow stresses coupled with large plastic strains without cracking, indicating stable accommodation of plastic incompatibility. Correlative TEM/HR-STEM establishes a deformation sequence: initial plasticity is dominated by strain-gradient driven dislocation accumulation in the FCC lamellae adjacent to interfaces, followed by deformation twinning in FCC and local interfacial shear and reorientation. The BCC phase subsequently develops a high density of mobile dislocations. Atomistic modeling has been employed to understand the influence of the FCC/BCC interface atomic structure and chemistry on the slip activation in the hard phase. These findings show that nanoscale confinement, stepped K-S interfacial structure, and interfacial chemistry collectively promote dislocation glide in a hard phase below its monolithic brittle to ductile transition temperature, and plastic codeformation at high flow strengths.