The evolution of damage in tritium exposed copper
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Severe microstructural damage has been observed in polycrystalline OFHC copper specimens thermally exposed to high pressure tritium gas at temperatures ^200 °C, but not at 300 °C. No such damage occurs in single crystal specimens exposed under identical conditions, regardless of temperature. In the polycrystals, the damage takes the form of very flat, crack-like intergranular cavities. It is found that the cavitation evolves slowly with time. For short exposure times, cavities as small as 0.1 jum are observed. In specimens subjected to the longest aging times, the cavities grow and link until entire grain boundary facets fail. The driving force for the growth of these cavities is attributed to the internal gas pressure of helium-3 generated by the decay of tritium. The growth kinetics of the cavity microstructure are described by a coupled grain boundary, surface self-diffusion process. The tritium exposure profoundly affects the mechanical properties of the polycrystalline material, 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.
I. INTRODUCTION
Oxygen-free, high conductivity (OFHC) copper does not suffer from the kind of large-scale hydrogen induced damage to which tough pitch and other high oxygen content grades of copper are susceptible.1"7 The origin of this damage (called hydrogen attack) has been known for some time and involves the interaction of solute oxygen and the reduction of copper or other impurity oxides with dissolved hydrogen to form water vapor. The water vapor then forms bubbles, which can be many microns in diameter, at the site of the initial particles. This response to hydrogen exposure is typically a high temperature phenomenon with H2O(V) bubble formation being most pronounced at temperatures above 375 °C where the reduction of copper oxides and other impurity particulates are both thermodynamically and kinetically favorable.6'7 Even at lower temperatures, the reduction of these oxides and impurities can remain thermodynamically favorable, although their reaction with hydrogen may be exceedingly slow. Associated with the formation of this bubble microstructure can be a degradation in mechanical properties, especially when deformation occurs at high temperatures. This is especially so when the bubbles reside on the grain boundaries of the host material (see, for example, Ref. 3). With its low oxygen content, OFHC copper is essentially immune to the kind of hydrogen embrittlement described above (indeed, oxygen-free electrolytic grades of copper were developed in order to minimize their susceptibility to hydrogen induced degradation). OFHC copper suffers from neither the extensive cavi-
tation nor from the degradation in mechanical properties that has been observed in hydro
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