The Influence of Centerline Sigma (σ) Phase on the Through-Thickness Toughness and Tensile Properties of Alloy AL-6XN

The J-integral fracture toughness and tensile behavior of AL-6XN (ATI Properties Inc., Pittsburgh, PA, USA) plate material in the short-transverse (S-T) orientation was studied. The material was tested with respect to the presence or absence of microstructural ‘packets’ of brittle sigma phase (σ-phase) particles within the austenite matrix, located near the centerline of the plate. The J-integral-fracture resistance curve (J-R-curve) qualitatively indicated that the presence of the σ-phase packets is a detriment to the fracture toughness of the material in the S-T orientation. Tensile specimens containing the packets of σ-phase particles exhibited reduced yield and tensile strengths as well as pronounced reduction in ductility compared to specimens nominally free of σ-phase packets. Particle cracking was observed within the σ-phase packets, which lead to premature fracture. This limited the ability of the material to plastically deform and work-harden, thereby accounting for the observed reductions in ductility, toughness, and ultimate tensile strength.