On the dislocation density in aluminum during Harper-Dorn creep

In research on high temperature, low stress, creep in aluminum, Lacombe and Beaujard's etchant has been used to determine dislocation densities. This work has led to the suggestions that the density is about 10⁸ m⁻² or less and independent of stress and that for Harper-Dorn creep the density must be below a critical value of about 2×10⁹ m⁻². Here, the characteristics of the etchant have been reviewed and it has been shown that they are complex and that pit densities are generally unreliable measures of dislocation populations. The purpose of this note is to point this out and to suggest that further work on dislocation populations during Harper-Dorn creep is warranted.