Quantitative analysis of the precipitation kinetics in dilute alloys by isothermal in-situ high-precision dilatometry

High-precision dilatometry serves as sensitive tool for quantitatively characterizing precipitation processes even if they are associated with only tiny relative length changes in the range of 10^-5 and even if they occur on long time scales exceeding 10⁵ s. For a dilute Al-based Al-Mg-Si alloy the amount of the metastable coherent β‘‘-phase and the semicoherent β‘-phase as well as of the stable phase β could be determined on an absolute scale from in-situ relative length change measurements upon long-time isothermal annealing.

The quantitative analysis allows an assessment on the various contributions of the length change, i.e., the volume excess of the precipitates, the volume change of the matrix upon precipitation of solute atoms, and the length change due to the formation of interfaces.

The time- and temperature-dependence of the length change could quantitatively be analysed in the framework of the Johnson-Mehl-Avrami-Kolmogorow model, which yields a detailed understanding of the precipitation kinetics. Bringing together all those findings, the industrial highly relevant time-temperature-precipitation diagrams, which, for these types of alloys, were hard to ascertain with common experimental techniques, are now easily accessible.