Modeling the age-hardening behavior of Al-Si-Cu alloys

We describe a new approach for modeling the age-hardening behavior of Al-Si-Cu cast alloys, that utilizes recently proposed micromechanical models of precipitation strengthening which connect key microstructural parameters for realistic precipitate morphologies (e.g., {100} plates) with the age-hardening response. This approach is illustrated and tested for a series of 319-type Al alloys (which we refer to as W319), where the microstructural parameters of theta' plates measured by transmission electron microscopy and a first-principles/computational-thermodynamics model of theta' volume fraction are used in the micromechanical model to predict precipitation strengthening. Thus, the precipitation-strengthening contribution contains no free parameters in our approach. An aging temperature- and time-dependent component that describes the strengthening of the Guinier-Preston (GP) zones and solid-solution copper, as well as a constant intrinsic strength, is combined with the calculated precipitation strength to predict the yield strength with a minimum of fitting parameters. This yield-strength model provides a good predictor of the yield strength of W319, and the methodology should be more generally applicable to all industrial cast alloys strengthened primarily by theta'. We also discuss limitations of the present approach and point to areas for improvement in future studies.