Quenching Studies of Lattice Vacancies in High-Purity Aluminium

Abstract

The experimental techniques for obtaining reliable enthalpies of formation and migration of vacancies in pure metals and the importance of achieving high accuracy are critically discussed, with emphasis on studies based on the quenching-in of ‘thermal’ vacancies. From measurements of the residual electrical resistance introduced into high-purity Al foils (thickness 0.1 mm) by ultrafast quenches (initial quenching rate≈2×10⁶ K s⁻¹) from temperatures T between 800 K and 530 K and the literature data on high-temperature differential dilatometry, the enthalpy, H_1V^F=(0.65±0.01) eV, and the entropy, S_1V^F=(0.76±0.04)k_B (k_B=Boltzmann’s constant), of monovacancy formation as well as the resistivity ρ_1V=(1.9±0.1) \\microΩm per unit atomic concentration of vacancies are derived. Combining these results with the Al self-diffusion data deduced from nuclear magnetic resonance leads to the migration enthalpy H_1V^M=(0.61±0.02) eV and the pre-exponential factor D_1V⁰=6×10⁻⁶ m²s⁻¹ of the monovacancy diffusivity D_1V=D_1V⁰exp(−H_1V^M⁄k_BT). The divacancy binding enthalpy is found to be H_2V^B=(0.17₅±0.02₅) eV . This is in full agreement with earlier determinations by Doyama and Koehler and by Levy, Lanore and Hillairet, who employed a different technique, but in stark contrast to the recent assertion H_2V^B≈0 of Carling et al.