Extrusions and intrusions in fatigued metals. Part 2.AFM and EBSD study of the early growth of extrusions andintrusions in 316L steel fatigued at room temperature

摘要

Early stages of surface relief evolution of persistent slip markings (PSMs),formed in areas where persistent slip bands (PSBs) intersect the free surface,in polycrystalline 316L stainless steel cycled with constant plastic strainamplitude were studied using atomic force microscopy (AFM) and electronbackscattering diffraction (EBSD). Focused ion beam (FIB) technique wasemployed to obtain additional, more detailed information on the shape ofPSMs. To reveal true qualitative and quantitative information on thesimultaneous growth of intrusions and extrusions within individual PSMs,identical areas both on the specimen surface and on its inverse copyobtained via plastic replica were studied using AFM. Intrusions arepreceded by extrusions regardless of orientation of individual grains of thepolycrystal. The first intrusions appear largely around 1% of fatigue life atthe moment whenstatic' extrusions are developed. They appearpredominantly but not exclusively at the side of extrusions where theemerging active slip plane is inclined to the surface at an acute angle. Theygrow faster than the stage of stable extrusion growth. Typical morphologyof mature PSMs developed at 15% of fatigue life consists of ribbon-likeextrusions accompanied by two thin parallel intrusions along both PSB-matrix interfaces. Experimental data on the morphology and growth ofextrusions and intrusions are discussed in relation to the theoretical modelsand computer simulations of surface relief evolution leading to fatiguecrack initiation.