Ductile fracture is generally considered to occur via void nucleation, growth, and colaescence mechanisms that result in a dimpled fracture appearance. For many materials, void initiation and growth take place at locations where there are hard particles or inclusions. However, ductile fracture involving void formation has been observed in planar slip materials that contain neither hard particles nor inclusions. Two different crack growth processes have also been identified: (1) direct crack extension into the crack-tip plastic zone and (2) indirect crack extension by microcrack nucleation ahead of the main crack. In this paper, the near-tip fracture processes in several structural alloys, inclduing Al-, Ti-, and Nb-base alloys, that exhibit ductile fracture in a variety of microstructures with and without hard particles are summarized. These fracture mechanisms are correlated with the microstructure and the fracture resistance curve to provide a basic understanding of the role of microstructure in the crack-tip fracture process, the transition from ductile to brittle fracture, and the source of fracture resistance. A possible mechanism for he formation of dimpled fracture in the planar slip materials is also proposed.
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