Evolution of surface deformation during fatigue of PH 13-8 Mo stainless steel using atomic force microscopy L. CRETEGNY 1 and A. SAXENA 2 1 GECorporateR&DCenter,P.O.Box8,Schenectady,NY12301,USA 2 Professor and Chair, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA Receivedinfinalform05November2001 ABSTRACT TherelationshipbetweenmicrostructureandnucleationoffatiguecracksinPH13-8Mo stainlesssteelwasexploredwiththeuseofatomicforcemicroscopy(AFM)thatallowedan accurate quantitative characterization of the surface features. Fully reversed strain- controlled fatigue tests were performed at 0.4 and 0.6% strain amplitudes, and the evolution of the surface deformation was observed at various fractions of life. At 0.4% strainamplitude,fatiguesurfacedamageoccurredfirstintheshapeofstreaksabout4nm deep that formed at the interface between martensite laths and at prior austenite grain boundaries, and eventually coalesced to form crack nuclei. The increase in strain ampli- tudeto0.6%ledtotheformationoflargeextrusions,onaveragebetween2and5 mmlong withheightsbetween10and200nm,whichwerethepreferredcracknucleationsites. Keywords atomic force microscopy; extrusion; fatigue; microstructure; stainless steel; surface roughness. NOMENCLATURE AFM  atomic force microscopy FOV  field of view N  number of cycles N f  number of cycles to failure SEM  scanning electron microscopy TEM  transmission electron microscopy De  strain range De pl  plastic strain range INTRODUCTION A detailed understanding of both fatigue crack initiation and growth is essential for predicting remaining life, for improving the total fatigue life and for reliably detecting cracks in order to take corrective actions to avoid cata- strophic failures in critical components. The PH 13-8 Mo stainless steel is an alloy used in applications that require corrosion resistance, high strength, high fracture toughness and oxidation resistance up to 425 8C. 1 Despite excellent mechanical and chemical properties that are desirable for structural application in a marine environment, little research has been performed on the fatigue failure mechanisms for this alloy. Hochanadel et al. 2 and Seetharaman et al. 3 have performed detailed transmission electron microscope (TEM) investigations of the microstructure related to the various heat treat- ments and the resulting basic mechanical properties, and the fatigue crack growth behaviour was characterized by Patel etal. 4 However, the fatigue crack nucleation mech- anisms in this material remain unknown. Therefore, the objective of the current investigation was to explore the relationship between microstructure and nucleation of fatigue cracks in PH 13-8 Mo stainless steel. The surface deformation features were analyzed with the use of the scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The latter has been suc- cessfully used recently in order to observe the formation of surface upset in specimens subjected to cyclic loading (see Refs [5±10]). It is applied here to a previously unex- plored material in order to identify fatigue crack initi- ation mechanisms. This both validates this relatively new technique and demonstrates its potential for enhancing our understanding of the fatigue crack initiation mech- anisms. ß 2002 Blackwell Science Ltd. Fatigue Fract Engng Mater Struct 25, 305±314 305 Correspondence: Laurent Cretegny, GE Corporate R & D Center, P.O. Box 8, Schenectady, NY 12301, USA. E-mail: cretegny@crd.ge.com