Isotope effect for diffusion of iron in vanadium

The diffusion coefficients of iron, using Fe59 as the radioactive tracer, and the isotope-effect parameter, using Fe55 and Fe59 as tracers, have been measured in vanadium over a temperature range of more than 800°C by standard sectioning and counting techniques. The diffusion of Fe59 in vanadium exhibits the anomalous behavior of having not one but two straight lines in a plot of lnD versus 1T, with a change of slope occurring near 1350°C. The data have been analyzed into the sum of two exponentials: D=2280 exp(-101 500RT) at high temperatures, and D=0.0936 exp(-67 600RT) at low temperatures. The values of the isotope-effect parameter for the Fe59 and Fe55 isotopes are approximately constant near 0.7 at low temperatures, but decrease rapidly with increasing temperature above 1350°C. The measurements are interpreted to indicate that two mechanisms of diffusion operate in vanadium. The mechanism predominant at low temperatures is identified as motion of a single vacancy; that at high temperatures is probably motion of divacancies. © 1968 The American Physical Society.