Change of critical events of cleavage fracture with variation of microscopic features of low-alloy steels
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I. INTRODUCTION
ON the essential importance of the critical event of cleavage fracture, Smith clarified “In developing a realistic physical model of the cleavage process it is of paramount importance to ascertain the nature of the critical event in the formation of a cleavage crack. . . .”[1] A critical event, being the most important link in a process of cleavage, means the most difficult step among the three successive steps of a cleavage process, i.e., a crack nucleation in a second-phase particle, the just nucleated crack passes through the boundary between the second-phase particle and the matrix grain, and the propagation of the grainsized crack. Before the early 1950s, the crack nucleation was considered to be the most difficult step, i.e., the critical event of a cleavage process. This was based on the Stroh model,[2] which suggested that the crack could be nucleated by a dislocation pileup at an energy balance higher than that needed for crack propagation. Yet, a number of experiments confirmed the important influence of the hydrostatic tension stress on the cleavage fracture,[3,4] which implied that the controlling step was crack propagation rather than crack nucleation. Observation of grain-sized microcracks remaining in fractured specimens[5] supported this argument and exemplified grain boundary control. On the basis of the dislocation pileup model and the preceding observation of the remaining ferrite grainsized crack, in the late 1950s to the early 1960s, ferrite grain-sized crack propagation was identified as the critical event. The grain size was identified as the dominant microstructural dimension for cleavage fracture.[6,7] Yet, this model could not explain the influence of second-phase particles on the variation of toughness for similar grain sizes and yield strength, which was observed by McMahon and Cohen.[8] They also observed that the majority of the ferrite microcracks originated at carbide cracks. Smith proposed a wellJ.H. CHEN and G.Z. WANG, Professors, and Q. WANG, Postgraduate Student, are with the Welding Research Institute, Gansu University of Technology, Gansu 730050, People’s Republic of China. Contact e-mail [email protected] Manscript submitted November 2, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
known model:[9] a grain boundary carbide particle was cracked by an impinging dislocation pileup and the microcarbide crack acted as a Griffiths crack. The critical event was the propagation of the carbide crack into the neighboring ferrite grain as a result of the combined action of the dislocation pileup and the applied normal stress. This model supplied a mechanism for an increase in effective surface energy subsequent to crack initiation, which supported the model of propagation control. Curry and Knott modified this model further and concluded that the cleavage strength depended on the carbide thickness rather than the ferrite grain size.[10] Thus, from the late 1960s to 1980s, the critical event of cleavage was held as the propagation of a second-phase particle-sized crack
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