Ductile fracture initiation in pure α-Fe: Part I. Macroscopic observations of the deformation history and failure of crystals

Single crystalα-Fe whiskers, grown by the reduction of ferrous chloride by hydrogen have been strained to fracture in an Instron tensile testing machine and in a bench straining device at various elongation rates at room temperature. Whiskers were found to exhibit macroscopic slip behavior strongly dependent upon elongation rate while the geometric reduction in area and the fracture mode remained in all cases identical. Ductile rupture of iron whiskers produces a characteristically shaped chisel-edge fracture whose geometry is sensitive to crystal orientation, due to the geometry of active slip systems, but which isnot a function of strain rate. The micromechanisms of ductile rupture of these single crystals are strongly affected by dislocation dynamics. The development of dislocations necessary to accomodate an extensive reduction in area appears to be independent of the nature of surface slip observed. Dislocation structures form small volume elements which are separated from one another by dislocation cell walls. The accommodation of large strains as well as the reduction in area is determined by the movement of dislocations on the order of a distance equal to that of the dislocation cell size. The boundaries of the cell and/or the cell volume could then be expected to be specifically related to the site where the initiation of fracture occurs.