TEMPERATURE-DEPENDENCE OF DISLOCATION-STRUCTURE AT CRACK TIP OF FRACTURED SINGLE-CRYSTALS OF MOLYBDENUM

Dislocation structures at the crack-tip plastic zones of {100} oriented molybdenum crystals fractured at 77, 198 and 293 K have been examined using etch pitting and tensile testing techniques. Experimental data of specimens that showed little plastic relaxation at the crack tip allow an estimate of the intrinsic fracture surface energy to be ∼ 3500 ergs/cm2. A model adopted previously by these authors (Liu and Bilello 1977 a) to account for crack tip plasticity in tungsten at 77 K has been modified to reflect better the dependence of the size of the crack-tip dislocation loops on the observed size of the plastic zones at higher temperatures, and the dependence of the stress field of the crack on the radius of curvature at the crack tip. The fracture stress at room temperature has been found to be independent of the major direction of crack propagation on the {100} plane. This has been explained in terms of the control of plastic flow at the crack tip by multiple slip behaviours. © 1978 Taylor & Francis Group, LLC.