Twinning-Induced Plasticity and Ultrahigh Hardness in Additively Manufactured Ni-Nb

Abstract

The hardness of a two-phase intermetallic with a nanoscale microstructure found within an additively manufactured near-eutectic Ni-Nb specimen was investigated. This alloy was found to have a peak hardness of  at temperatures between ambient and 400 °C, above which permanent softening was observed. Microstructural characterization of the peak-hardness regions revealed the presence of a coarse-grained, chemically ordered matrix of orthorhombic (D0a) μ-Ni6Nb7with nanoscale inclusions of a secondary ordered rhombohedral (D85) δ-Ni3Nb phase. Plastic deformation was largely confined to nanotwining in the μ-Ni6Nb7. Molecular dynamics simulations were used to further examine phase- dependent strengths and deformation behavior. The simulations show that deformation for single-phase nanocrystalline volumes was primarily intragranular in δ-Ni3Nb, via slip band formation, and intergranular in μ-Ni6Nb7, via grain boundary sliding. High resolution transmission electron microscopy of severely deformed, fine-grained, nanostructured two-phase regions indicate that phase boundaries act as nucleation sites for dislocations, promoting twinning induced plasticity (TWIP) confined to the μ-Ni6Nb7 grains. This work highlights that (1) additively manufacturing techniques enable formation of unique microstructures that exhibit superior mechanical properties, and (2) multi-phase intermetallic compounds provide a route to mitigate brittle fracture though the promotion of twinning-induced plasticity.

Keywords: Additive manufacturing, intermetallics, nanostructure, TWIP, twinning