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Positron-annihilationS-parameter measurements in thermal equilibrium on pure and carbon-doped (50 and 750 at. ppm) a-iron are presented. It is shown that trapping of positrons in both monovacancies and carbon-vacancy pairs occurs, even far above the dissociation temperature of the vacancy pairs. Therefore, a three-state trapping model is used in the analysis of the measured S curves. The vacancy-formation enthalpy in both the paramagnetic and ferromagnetic state is deduced: It is found to be 1.79%0.10 eV in the paramagnetic state and 2.0+0.2 eV in the ferromagnetic state. These values are larger than those published so far. The activation enthalpy for vacancy migration obtained by combining the values cited above with recently published self-diffusion enthalpy values confirms the applicability of the one-interstitial model in a-iron.

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Electron-irradiated high-purity a-iron doped with various amounts of interstitial carbon impurities has been studied by positron-lifetime measurements. It is shown that during vacancy migration at 220 K an asymmetric vacancy-carbon pair is formed, where the carbon atom is located off the center of the vacancy.

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