Analysis of hardness–tensile strength relationships for electroformed nanocrystalline materials

Tensile and hardness data from 198 electrodeposited nanocrystalline Ni- and Co-based specimens of varying shape, composition, microstructure and quality were analyzed in order to reveal whether the established hardness–strength relationships in frequent use for conventional coarse-grained metals and alloys can be applied to these ultrafine-grained materials. It was concluded that the HV = 3·σUTS expression is a reliable predictor of the relationship between hardness and strength for nanocrystalline electrodeposits, provided the material is ductile enough to sustain tensile deformation up to a discernible peak load and does not instead fracture before the onset of necking instability. On the other hand, the widely used relationship HV = 3·σY was found to be inapplicable to this class of materials owing to the fact that they do not deform in an ideally plastic manner and instead exhibit plastic deformation that is characteristic of strain hardening behaviour.