The effect of phase continuity on the fatigue and crack closure behavior of a dual-phase steel
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I.
INTRODUCTION
THE microstructure
of a dual-phase steel consists of a strong phase, martensite, in conjunction with a ductile phase, ferrite. The morphology of the phases influences the mechanical properties of the steel. ~-5Generally a dual-phase steel consists of martensite dispersed in a ferrite matrix; however, by controlling the heat treatment process, this morphology can be inverted to a continuous martensite microstructure. ~-3,6 Suzuki and McEvily ~have shown that this inversion of phase morphology results in an increase in strength and an increase in fatigue crack growth resistance. The phase connectivity has a major influence on the mechanical properties of a dual-phase steel. A continuous martensite network results in higher strengths and threshold stress intensities by constraining the plastic deformation in the ferrite at the crack tip.2 The higher thresholds in a continuous martensite microstructure have been attributed to higher closure levels caused by a combination of Mode I and Mode I1 crack growth. 2 Other studies have shown that the martensite morphology in dual-phase steels can cause significant differences in crack deflections 3~4and roughnessinduced crack closure. 4 A Fe-C-Mn steel was used to investigate the effects of phase continuity on the fatigue behavior of a dual-phase steel. Low cycle fatigue tests were conducted to determine the crack initiation sites in both microstructures. TEM investigations were conducted on the LCF samples to characterize the dislocation substructures. Fatigue crack propagation tests in conjunction with closure measurements were performed to determine the effect of phase continuity on fatigue crack growth. Chakrabortty's model, which incorporates microstructural and low-cycle fatigue parameters, was used to predict the intrinsic crack growth rates of the dual-phase steel. R . M . RAMAGE, Graduate Student, K.V. JATA, Research Assistant Professor, G.J. SHIFLET, Associate Professor, and E. A STARKE, Jr., Earnest J. Oglesby Professor and Dean, are with the Department of Materials Science, University of Virginia, Thornton Hall, Charlottesvdle, VA 22901. Manuscript submitted July 31, 1986.
METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURES
The alloy used for this research has a nominal composition of 1 wt pct Mn and 0.2 wt pct C (Table I) received as a 20 mm plate from Armco, Inc. To eliminate the macrosegregation, "banding", of alloying elements 7 the asreceived material was homogenized at 1473 K before heat treating for specific microstructures. Following the homogenization, the steel was analyzed for C and Mn to insure that significant changes in composition did not occur. Although many other studies disregard a homogenization step, it was determined to be necessary for the control of some of the complex microstructures. The heat treatments were determined with the aid of a pseudo-binary phase diagram developed using the HillertStaffanson analysis. 8'Isothermal transformations followed by rapid quenching were conducted to ensure predictable microstruc
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