Study of the brittle behaviour of annealed Fe-6.5 wt%Si ribbons produced by planar flow casting

Rapidly solidified Fe-6.5wt%Si ribbons are fully ductile in the as-quenched state. However, the annealing process, required for optimization of the magnetic properties, generally leads to final brittle materials. In this article, a study is made of the embrittlement process, through mechanical tests and structural investigations on ribbons subjected to different cooling rates (1°C min-1 ≤ Ṫ ≤ 1500°C min-1). A ductile-to-brittle transition is observed for Ṫ ∼ 1000°C min-1, where long range B2 ordering takes place and the dislocation character changes from unitary to superlattice. An increase in the work hardening coefficients is correspondingly found, in association with enhanced hindering of the dislocation motion within the grains. The restricted glide and cross-slip capability of the dissociated superdislocations is consequently identified as the chief mechanism responsible for the buildup of internal stresses and eventual brittle fracture of the material.