Short crack growth during cycling in an aged superduplex stainless steel

M BALBI; S HEREÑU; I PRORIOL SERRE; J.-B VOGT; A ARMAS; I ALVAREZ

Brno

Conferencia; 6th International Cnference on Materials Structure & Micromechanics of Fracture; 2010

New generation of duplex austenitic-ferritic stainless steels are advanced materials with increasing number of applications in a range of industries. The kinetics of short crack growth during cycling has been studied in a superduplex stainless steel in the aged condition (100hr at 475ºC). Cylindrical specimens with shallow notch were subjected to plastic strain controlled tests. Crack nucleation and growth was monitored by a powerful optical system.   After few cycles, slip lines appear heterogeneously distributed in both phases but the preferred site for micro-crack nucleation is the ferrite. Contrary to the exponential behavior observed in the as-received material, the growth rate of micro-cracks in this condition follows a rather linear law. Internal dislocation structures were studied in thin foils taken from slices cut at different depths, parallel to the specimen axis, from the shallow-notched area of the specimen on the side opposite. It was found that micro-bands are formed at approximately 45º of the tensile axis on ferrite grains, which extends over several grains crossing austenite ones. The origin of these microbrands corresponds to the formation of twist boundaries on (110) plane. These boundaries are the result of the dislocation reactions giving rise to dislocations parallel to the [001] direction with b3= [001] burgers vector.