Effects of σ phase embrittlement and Al addition on the ductile-brittle transition in super ferritic stainless steels

The precipitation evolution and mechanical behaviour of Al-modified and Al-free super ferritic stainless steels (SFSSs) subjected to hot-rolling and isothermal aging at 850 °C were investigated comparatively. Experimental results revealed that hot-rolled samples devoid of Al exhibited elongated fibre microstructure including many sub-grain boundaries and shear bands, whereas a surface recrystallization layer was observed in Al-modified samples. During isothermal aging, σ, χ, and Laves phases were observed in Al-free samples, whereas primarily Laves phases and a small σ phase were identified in 1.0 wt%Al samples. Rolling deformation accelerated the σ phase precipitation kinetic, whereas the addition of Al inhibited σ phase precipitation and promoted Laves-phase precipitation. The bulky σ phase precipitation caused the cleavage fracture in SFSSs owing to the high critical resolved shear stress for dislocation glide on {110}<001>, {100}<001>, {100}<010>, and {111}<0-11> slip systems and the easy crack initiation and propagation in the σ phase during tensile test. Despite being aged for 16 h, Al-modified SFSSs exhibited a significant elongation of 22.6 %. This study highlights that Al addition would facilitate not only the low-temperature hot rolling of SFSSs but also SFSS production without the need for solution treatment prior to cold rolling, thereby offering a promising short-process approach for industrial manufacturing.