Damage and rupture mechanisms in an austenoferritic duplex steel

The influence of ageing on damage and rupture mechanisms in an austenoferritic duplex stainless steel is studied using conventional straining and impact toughness testing at 20 degrees C and 320 degrees C, and in situ SEM straining at 20 degrees C. While the as-received alloy fails in a ductile mode, damage in the aged material starts with cleavage nucleation in ferrite. We show that, owing to the bipercolated topology of the alloy, these cleavage cracks can propagate while passing round austenite ligaments whose plastic stretching controls the crack extension. The variations with strain of both the crack size and the average crack separation are computed analytically and their comparison gives a good prediction of ductility. (C) 1997 Acta Metallurgica Inc.