Quantitative description of the T₁ formation kinetics in an Al–Cu–Li alloy using differential scanning calorimetry, small-angle X-ray scattering and transmission electron microscopy

Abstract

The object of the present study is to design a methodology to follow the kinetics of T₁ precipitation, in an AA2198 alloy, in terms of precipitate size, morphology (thickness, diameter) and volume fraction, during a two-temperature isothermal heat treatment. We used in situ small-angle X-ray scattering (SAXS) as a way to measure the evolution of the T₁ mean thickness and diameter during the heat treatment. Transmission electron microscopy (TEM) was then performed in order to calibrate these evolutions. Furthermore, we demonstrate that the volume fraction evolution can be described successfully using a simple analysis of the differential scanning calorimetry (DSC) thermograms. The latter was calibrated by selected observations in high angular annular dark field scanning transmission electron microscopy (HAADF-STEM). Microstructure evolution during DSC heating ramps was analysed using in situ SAXS: the T₁ phase transformation is found to consist in a two-step thickening process explained by two consecutive diffusion stages. The enthalpy of formation of the T₁ phase is deduced from the DSC measurements.

Keywords:

• Aluminium alloys
• precipitation
• differential scanning calorimetry

Acknowledgements

We are very grateful for the help and support we received from the technical staff of beamline BM02-D2AM of ESRF. We would like to express our appreciation to Dr P. Donnadieu for fruitful discussions; her expertise in TEM has been of great benefit to the present work. We also convey special thanks to G. Renou for conducting some of the TEM observations.