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Prolonged exposure of oxygen-free, high conductivity (OFHC) copper to high pressure tritium gas has been found to result in the development of severe microstructural damage. The damage takes the form of very flat, crack-like intergranular cavities. At the same time, tritium exposure profoundly affects the mechanical properties of the alloy, inducing a severe loss in ductility. In concert with the observed ductility loss is a change in fracture morphology from transgranular ductile rupture to intergranular fracture. Examination of the resulting grain boundary facets reveals a dimple structure. The spacing of these dimples can be correlated with the spacing of the exposure-induced grain boundary cavities. The extent of cavitation and the subsequent embrittlement is found to be sensitive to both the specimen grain size as well as the tritium charging and aging temperature. The nucleation and growth of these cavities is attributed to the helium-3 born in the metal as the result of tritium decay. Being insoluble in the metal lattice, the helium migrates to grain boundaries where it is trapped and agglomerates to form small clusters (or cavity nuclei). These cavities then grow by both the precipitation of tritium dissolved in the metal and by the precipitation of additional helium generated from the continuous decay of the tritium within the metal.
tritium exposure, helium, cavitation, embrittlement, intergranular fracture
Member of technical staff, Sandia National Laboratories, Livermore, CA