Investigation of microstructural evolution in nano-structured bainite during Paris law regime of fatigue crack growth

Two different blocks of nano-bainitic specimens, NSB_250 and NSB_350 were prepared by austenitization at 950°C for 40 min, followed by austempering at 250°C and 350°C respectively. Lower austempering temperature led to thinner bainitic laths, smaller fraction of retained austenite (RA) and higher fatigue crack growth rates. This study investigated the microstructural evolution in both specimens close to the cracked surface when the crack growth was in the Paris law regime. Samples were extracted using focussed ion beam (FIB) milling and the microstructure was examined using transmission electron microscopy (TEM) and compared with that of undeformed specimens. Formation of deformation twins in RA were observed in NSB_350 while severe deformation of bainitic ferrite and micro-twin formation in RA was observed for NSB_250.