Quench-induced precipitates in Al–Si alloys: Calorimetric determination of solute content and characterisation of microstructure

• A new DSC method gives access to the entire cooling rate range of physical interest.
• Two precipitation reactions were identified during cooling of pure binary Al–Si.
• A physical model for solute and precipitation fraction is presented and validated.
• The model allows to control phase transformations by heat treatments.
• This enables to produce and mechanically test targeted microstructures.

The present study introduces an experimental approach to investigate mechanical properties of well-defined non-equilibrium states of Al–Si alloys during cooling from solution annealing. The precipitation behaviour of binary Al–Si alloys during the cooling process has been investigated in a wide cooling rate range (2–0.0001 K/s) with differential scanning calorimetry (DSC). To access the low cooling rate range close to equilibrium an indirect DSC measurement method is introduced. Based on the enthalpy change measured by DSC a physically-based model for the calculation of remaining solute Si amount as function of temperature and cooling rate is presented. Microstructural analyses via light optical microscopy, scanning electron microscopy, atom probe tomography and X-ray diffraction have been performed to evaluate the introduced model and for information on cooling rate dependent precipitate formation. It was found that quench-induced particles of different morphology are formed during cooling. Thermomechanical analyses on clearly distinct undercooled Al–Si states show that flow stress during cooling is dependent on temperature as well as cooling rate. The mechanical behaviour is therefore influenced by solute Si content and quench-induced precipitates.