Abstract
The surface self-diffusivity of α-Fe was measured by the grain boundary grooving method in hydrogen atomospheres with dew points of −60 and +13°C. The groove width measurement was made through all runs with the same region of grain boundary of a coarse-grained specimen. The diffusion anneal was carried out at 800, 850 and 890°C.
In dry hydrogen (dew point −60°C) purified through a palladium foil, the surface self-diffusivity of α-Fe can be expressed by the following equation:
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The value of activation energy agrees well with that obtained by Blakely et al.
In moist hydrogen (d.p.+13°C), however, the rate of groove development was varied, depending on whether or not the specimen had been equilibrated with the atmosphere. When the specimen, which had been in equilibrium with dry hydrogen, was annealed in moist hydrogen, the development rate of groove was larger than that in dry hydrogen in the early stage, but it decreased with annealing time. On the other hand, if the specimen had been equilibrated with the same atmosphere, the grooving rate was smaller than that in dry hydrogen. In the former case, surface roughening would occur more than in thermal equilibrium owing to the adsorption and solution reactions of oxygen at the surface, which would lead to an enhancement of surface diffusivity. On the contrary, on the surface of the specimen, which is equilibrated with moist hydrogen, the blocking of kink and adatom would occur by adsorption of oxygen, which would lower the surface diffusivity.
A marked decrease in surface diffusivity in hydrogen with a dew point of +13°C was observed at 800°C with the specimen equilibrated with the same atmosphere, which could be attributed to an incidental adsorption of impurities during the sample preparation.
