Outward diffusion and external oxidation of thorium in iridium alloys

Grain growth in thorium doped Ir + 0.3% W alloys is greatly inhibited by thorium segregation to grain boundaries and formation of thorium rich precipitates at grain boundaries. These alloys have been observed to undergo exaggerated growth of near surface grains when annealed at 1330° C in low pressure (1.3 × 10^-3 Pa) oxygen. Experimental observations of thorium depletion at near surface grain boundaries, using Auger electron spectroscopy, are reported. Thorium oxide precipitates on external surfaces are observed using scanning electron microscopy. X-ray fluorescence analysis, and X-ray diffraction. These phenomena appear to result from the selective external oxidation of thorium at the metal surface, depleting thorium from near surface grain boundaries. The thorium depleted grain boundaries can then migrate freely, allowing growth of the near surface grains. Rough estimates of the thorium diffusivities required to explain these phenomena suggest that thorium diffuses to the free surface primarily along grain boundaries. © 1981.