Abstract
During heating of Al alloys, typically a sequence of precipitation and dissolution reactions occurs and the single (partly opposing) reactions superimpose. Differential scanning calorimetry (DSC) is one common technique to analyse the kinetic development of precipitation and dissolution in Al alloys, but the superposition of the exothermic precipitation and endothermic dissolution reactions complicates the DSC signal interpretation, as DSC measures the sum of any heat effect. Synchrotron high-energy X-ray diffraction (HEXRD) allows the kinetic development of phase transformations to be obtained and can support the separation of superimposed DSC signals. HEXRD results from this work offer a new approach to separate part of the superimposed reactions and their kinetic development for the equilibrium phases β-Mg2Si in EN AW-6082 and η-Mg(Zn,Cu,Al)2 in EN AW-7150. Comparing DSC and HEXRD results confirms serious overlap issues. Common DSC evaluation methods alone, using zero crossing between endothermic and exothermic heat flow or peak positions can be misleading regarding individual reaction start and finish temperatures as well as regarding reaction intensities, which can be unambiguously determined by in situ HEXRD.
Highlights
The kinetic investigation of phase transformations during heating of aluminium alloys is of importance, for instance, to adjust process parameters during production
Exothermic precipitation reactions are more suppressed compared to dissolution reactions—this is predominantly as the diffusion ways are shorter for dissolution reactions compared to precipitation from homogenous solid solutions [3], which leads to a seemingly larger fraction of the dissolution reaction seen in the differential scanning calorimetry (DSC) sum signal at faster heating rates
The diffraction peak areas as well as the phase transformation rates for all three heating rates considered are given as a function of temperature for b-Mg2Si in EN AW-6082
Summary
The kinetic investigation of phase transformations during heating of aluminium alloys is of importance, for instance, to adjust process parameters during production. Exothermic precipitation reactions are more suppressed compared to dissolution reactions—this is predominantly as the diffusion ways are shorter for dissolution reactions compared to precipitation from homogenous solid solutions [3], which leads to a seemingly larger fraction of the dissolution reaction seen in the DSC sum signal at faster heating rates This general issue was already addressed in previous works [1, 3]. It directly refers to one single reaction, is exactly the basis of most interpretations of heating DSC signals from Al alloys, see for instance [4,5,6] This is even more doubtful if the DSC peaks are used for the determination of activation energies for the precipitation of specific phases, such as in many works, We compare HEXRD and DSC data on heating of two Al alloys, assessing precipitation and dissolution of two equilibrium phases, b-Mg2Si in EN AW-6082 and g-Mg(Zn,Cu,Al) in EN AW-7150
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