PLASTIC DEFORMATION OF BODY-CENTERED CUBIC METALS. VOLUME II. FRACTURE OF TUNGSTEN.
Technical rept. 15 Feb 63-30 Jun 66,
LIVERPOOL UNIV (UNITED KINGDOM) DEPT OF METALLURGY AND MATERIALS SCIENCE
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A comprehensive investigation has been made of the deformation modes and fracture mechanisms in tungsten. It has been shown that high purity single crystals of tungsten are not inherently brittle but are extremely notch sensitive. Tests on pre-cracked crystals indicate that crack propagation can occur by different mechanisms depending on the testing temperature. At low temperatures where crack propagation occurs in a completely brittle manner, the fracture stress obeys a Griffith type criterion. At higher temperatures the crack extends by a slow mode of growth due to the concentration of strain energy in front of the crack tip. Prestraining at high temperatures effectively blunts the crack and increases the low temperature brittle fracture stress. The characteristic markings on the cleavage faces depend primarily on the stress level at fracture and are essentially independent of initial dislocation density and sharpenss of the microcrack. Tests on polycrystalline tungsten indicate that at temperatures below about 350 K fracture is completely brittle. At temperatures from 350 K to about 460 K the specimens yield prior to fracture but the strain to fracture is low after a region of high work hardening. At temperatures above 460 K, grain boundary parting on the specimen surface occurs immediately after yielding, resulting in increased strain to fracture. Fracture over the whole temperature range occurs predominantly by cleavage with some inter-granular separation. The amount of intergranular fracture increases slightly with temperature. Crack propagation in pre-cracked polycrystalline specimens shows similar temperature dependence to the single crystals. Author
- Metallurgy and Metallography