Fatigue crack propagation in dual-phase steels: Effects of ferritic-martensitic microstructures on crack path morphology
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INTRODUCTION
T H E role of microstructure in influencing the fatigue crack growth behavior in steels has been a subject of considerable research interest for many years. Recent studies ~-7 on lower growth rate behavior in ferritic, pearlitic, bainitic, and martensitic steels have indicated that maximum resistance to fatigue crack extension can generally be achieved with coarse-grained microstructures of low cyclic yield strength, although the beneficial influence of grain size and strength level is seldom retained at high load ratios. Such microstructural effects on cyclic crack advance are found to be most significant in the near-threshold regime, below typically ~10 -6 mm per cycle, where stress intensity levels approach the threshold stress intensity range, AKo, for no detectable growth of long cracks* (see References 1 *The term "long crack" here refers to macro-cracks which are large compared to the scale of microstructure and/or the scale of local plasticity. When crack sizes approach such dimensions, so-called "short crack effects" have been observed where initiation and growth can occur at stress intensity levels below AKo (for review, see Reference 8).
through 5). A particularly promising class of steels, where superior near-threshold fatigue resistance has been obtained without compromising strength, is the low carbon dual-phase steel. In this system, intercritical heat treatments in the two phase V.B. DUTTA, Graduate Student Research Assistant, and R.O. RITCHIE, Professor, are with Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. S. SURESH, formerly Lecturer and Research Engineer in the Department of Materials Science and Mineral Engineering, University of California, is now Assistant Professor, Division of Engineering, Brown University, Providence, RI 02912. Manuscript submitted September 26, 1983. METALLURGICALTRANSACTIONS A
(a + y) region lead to duplex structures consisting of ferrite (a) as the ductile matrix with a strong and tough second phase of low carbon martensite (a'). Such duplex steels, particularly those containing dislocated martensitic microstructures, show highly desirable tensile properties, such as continuous yielding from a low yield strength, high initial strain hardening rates, and an excellent combination of tensile strength with ductility.8-12 In terms of fatigue properties, the most dramatic results on the fatigue crack growth resistance of dual-phase steels were shown by McEvily and co-workers 14'15who developed two different duplex microstructures in an AISI 1018 steel; one where a continuous ferrite matrix encapsulated islands of martensite (termed FEM or Type A in McEvily and co-workers' terminology~4'lS), and the other where a continuous martensitic phase encapsulated islands of ferrite (termed MEF or Type B). The former FEM microstructure showed properties similar to conventionally normalized AISI 1018 steel with a yield strength of 293
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