Equilibrium vacancy concentration measurements on aluminum

Abstract

Thermal expansion on high purity aluminum has been measured to study defects in thermal equilibrium by the (ΔL/L ₀−Δa/a ₀) method. Measurements of the changes in macroscopic length ΔL and lattice parameter Δa were made from room temperature to 637° C. Length changes were measured by a laser interferometer and lattice parameter changes by a X-ray diffractometer technique on the same crystal at identical temperatures. At temperatures above 400° C ΔL/L ₀ becomes greater than Δa/a ₀, indicating the generation of a noticeable amount of new lattice sites due to vacancy formation. Extrapolation gives a vacancy concentration ΔN/N ₀=9.8·10⁻⁴ at the melting point (660° C). The experimental findings can be explained by assuming formation of monovacancies and divacancies. Values for the enthalpy and entropy of formation for mono- and divacancies are given.