TEMPERATURE-INDUCED TRANSITION IN DUCTILE FRACTURE APPEARANCE OF A NITROGEN-STRENGTHENED AUSTENITIC STAINLESS-STEEL

A nitrogen-strengthened austenitic stainless steel was tested in uniax ial tension at room temperature (295 K) and in liquid nitrogen (76 K). A transition in ductile fracture appearance from a cup-cone fracture at room temperature to shear fracture at cryogenic temperature is obse rved and correlated to deformation behavior and micromechanisms (void nucleation and strain localization) of fracture. The flow stresses, fr acture stresses, and strain hardening rates are all higher at liquid n itrogen temperature compared to those at room temperature, and the sig nificant increases in plastic flow stresses are accompanied by planar deformation mechanisms. At both temperatures, primary void nucleation is observed mainly at scattered, large patches of sigma phase, and ini tial primary void growth is associated with tensile instability (necki ng) in the specimen. Postuniform elongation at 295 K leads to secondar y void nucleation from small, less than 1 mum in diameter, microalloy particles, leading directly to failure; the strain required for second ary void growth and coalescence is highly localized and does not contr ibute to macroscopic elongation. At 76 K, uniform strain increases, to tal strain decreases, and strain localization into shear bands between the primary voids and the surface of the neck leads directly to failu re. Secondary void nucleation, growth, and coalescence are limited to shear bands and also do not contribute to the macroscopic elongation. The observations of void nucleation are characterized in terms of a co ntinuum analysis for the interfacial stress at void-nucleating particl es. The critical interfacial stress for void nucleation at the lower t emperature correlates with the increased flow properties of the matrix .