Temperature-modulated dilatometry as tool for studying precipitation kinetics: Case example Al-Mg

Temperature-modulated dilatometry is implemented as a tool for studying phase equilibration processes in alloys. The potential of this method is demonstrated using a two-phase Al-Mg alloy consisting of the saturated solid solution Al(Mg) and intermetallic Al3Mg2. The relative length change upon sinusoidal temperature modulation around 380∘C with amplitude of 6∘C is monitored for different modulation periods between 1 h and 8 h. The temperature modulation and the thereby varying solubility of Mg in Al imposes a modulation of the relative fractions of the two phases, which can sensitively be detected by dilatometry owing to the different atomic volumes of the two phases. The phase equilibration process causes a characteristic phase lag of the periodic length variation with respect to the temperature modulation. From the variation of the phase shift with the modulation frequency, a reaction rate of k = 1.85 × 10−4 s−1 for the equilibration of the phase equilibrium can be deduced in the framework of a simple kinetic model. This value is in quantitative agreement with the kinetics expected for an equilibration process that is controlled by the diffusion of Mg in Al upon modulation-induced growth or shrinkage of the intermetallic Al3Mg2-phase.