Recent progress in dilatometry for quantitative analysis of precipitation kinetics

A high-stability non-contact dilatometer based on two-beam Michelson interferomtry was developed recently, enabling isothermal measurements of relative length changes in the sub-10−5 regime with minimized drift over times as long as 106 s. This advanced technique opens up novel potentials for quantitative studies of precipitation phenomena, which are associated with tiny relative length changes that have not been accessible before. Using a dilute Al-Mg- Si alloy as a case study, the amount of the metastable coherent β″- and of the semi-coherent β'-phase, as well as the amount of the stable β-phase could be determined quantitatively from isothermal length change measurements. This is accomplished by the distinct length change features that arise from the volume excess due to the precipitates, on the one hand, and the contraction of the matrix upon precipitation of solute atoms, on the other hand. Moreover, reaction-rate analyses of isothermal length change variations yield the kinetic parameters for the formation and transformation of the precipitates.