Quantitative metallographic methods are used to relate alloy ductility to microstructural fracture mechanisms for ten tungsten-nickel-iron alloys with Ni/Fe ratios of 7 : 3 and 1 : 1 and over the tungsten composition range 87.5 to 97.5 weight percent. These techniques are used to characterize the crack nucleation state at the rupture strain. The observations are consistent with fracture by load shedding and a cascade of nucleation events within a narrow zone in the sample. An instability analysis is presented which includes reduction in area due to both tensile plastic strain and to the development of crack nuclei within the sample.