TEM (Transmission Electron Microscopy) Studies of Crack Tip Deformation in BCC Molybdenum at Low Temperatures.

摘要

Electron microscope studies have been made of the slip systems that are activated at the crack tip in bcc molybdenum during in situ tensile deformation at various temperatures (100 approx. 300 exp 0 K). At ambient temperature, all three modes of crack tip deformation have been observed under an applied stress. Dislocations are emitted from the tips of propagating cracks, pass rapidly through a dislocation-free zone (DFZ) and pile up in a plastic zone. The observed crack tip slip systems were analyzed in terms of the crack tip Schmid factor obtained from the resolved shear stress modified by the presence of a crack. Results show that crack tip deformation occurs by the emission of dislocations from the crack tip, and that the shear crack of mode III is most prevalent mode. Therefore, the choice of slip system is primarily dictated by the crack tip Schmid factor, along with the critical stress intensity factor for dislocation emission from the crack tip. The temperature dependence of crack tip deformation and its relationship to the ductile-brittle transition have also been examined. The slip systems possessing the maximum crack tip Schmid factors are usually activated from the crack tip at temperatures above 150 exp 0 K. At temperatures below 140 exp 0 K, a brittle crack (cleavage) is preceded by mixed mode crack tip deformation, although the extent of the deformation is markedly reduced. The ductile-brittle transition of the fracture mode observed in the electron microscope agreed well with the mechanical behavior of the metal observed from bulk tensile tests performed at low temperatures. (ERA citation 10:044588)