Elasto-plastic behaviour of a columnar structure of nanocrystalline iron with sharp 〈011〉 fibre texture

Abstract

We explore, using molecular dynamics (MD), the strength and mechanical stability at 298 K of pure bcc Fe with an oriented columnar nanostructure of long grains randomly distributed about 〈011〉 and average size ranging from 20 to 5 nm in the cross-section. According to the simulations, the tensile flow stress of such aligned nanostructure spans from 4 GPa to 6 GPa as grain size decreases from 20 nm to 5 nm. The axially oriented tilt boundaries are very stable to axial plastic elongations as large as 0.5 true total (elastoplastic) tensile strain. The structure shows an important Bauschinger effect in compression after tensile elongation, but remains stable for relatively large compressive deformations. However, moderate plastic transverse shear deformations, which promote shear-coupled tilt boundary migrations, destabilize it, by provoking grain growth and softening. Our simulations connect with real world materials: very fine wires obtained by large-strain drawing bcc metals or alloys develop a similar columnar structure of 〈110〉 axially oriented grains of nanometric cross section and extraordinary mechanical strength.