Understanding of relationship between fracture and slip behaviour in Permendur alloys using multiscale mechanical testing of poly- and single-crystalline specimens

In this study, we employed multi-scale tensile testing to correlate the brittle fracture behaviour of polycrystals with the slip behaviour of single crystals in FeCo and FeCo-2V Permendur alloys with focus on the effects of V addition and disordering. In the ordered-phase state, single-crystalline specimens exhibited moderate ductility with specimen bending under uniaxial tensile loading conditions. This may be associated with shear banding due to the activation of multiple slip systems. Disordering caused the FeCo-2V alloy to exhibit discontinuous yielding with slip localisation and the following cross-slipping. For poly-crystalline specimens, the ordered-phase alloys exhibited some plasticity prior to brittle failure such as intergranular and cleavage cracking. The addition of V to FeCo increased the fracture strength since the more plastic accommodation reduced the local stress concentration in FeCo-2V. For ordered-phase multi-grains specimens, the bamboo-structured FeCo specimen behaved like a single-crystalline specimen in deformation behaviour, whereas in the FeCo-2Vspecimen with multiple grains in its transverse direction, plastic constraint by the neighbouring grains caused brittle fracture.