Effect of precipitates on the corrosion–fatigue crack initiation of ISO 5832-9 stainless steel biomaterial

The correlation between the microstructure and fatigue crack initiation behavior (in air and 0.9% NaCl solution) of ISO 5832-9 austenitic stainless steel biomaterial was investigated. A combination of techniques such as extraction of precipitates, SEM, EDS and X-ray diffraction was found to be very useful to characterize Z-phase precipitates, which are abundant in this steel. An analysis of fatigue crack initiation, using SEM, revealed that fatigue and corrosion–fatigue cracking initiate preferentially in the elongated Z-phase precipitates, which are parallel to the tensile direction, and in coarse precipitates usually associated with Al-rich nonmetallic inclusions. Crack initiation is caused by particle rupture rather than by separation of the particle/matrix interface. The findings indicate that coarse Z-phase precipitates and nonmetallic inclusions are detrimental to the fatigue properties of the steel. These microstructural constituents accelerate the initiation of fatigue and especially corrosion–fatigue cracks.