In situ high-energy X-ray diffraction of precipitation and dissolution reactions during heating of Al alloys

During heating of Al alloys, typically a sequence of precipitation and dissolutionreactions occurs and the single (partly opposing) reactions superimpose. Differentialscanning calorimetry (DSC) is one common technique to analyse thekinetic development of precipitation and dissolution in Al alloys, but thesuperposition of the exothermic precipitation and endothermic dissolutionreactions complicates the DSC signal interpretation, as DSC measures the sum ofany heat effect. Synchrotron high-energy X-ray diffraction (HEXRD) allows thekinetic development of phase transformations to be obtained and can supportthe separation of superimposed DSC signals. HEXRD results from this workoffer a new approach to separate part of the superimposed reactions and theirkinetic development for the equilibrium phases b-Mg2Si in EN AW-6082 andg-Mg(Zn,Cu,Al)2 in EN AW-7150. Comparing DSC and HEXRD results confirmsserious overlap issues. Common DSC evaluation methods alone, usingzero crossing between endothermic and exothermic heat flow or peak positionscan be misleading regarding individual reaction start and finish temperatures aswell as regarding reaction intensities, which can be unambiguously determinedby in situ HEXRD