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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 agehardening 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 Ј plates measured by transmission electron microscopy and a first-principles/computational-thermodynamics model of Ј volume fraction are used in the micromechanical model to predict precipitation strengthening. Thus, the precipitationstrengthening 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 Ј. We also discuss limitations of the present approach and point to areas for improvement in future studies.

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presented in Figs. 3(B) and (C), indicate that the phase in bright contrast in Fig. 3(A) contains both yttrium. and aluminum while the phase in dark contrast in Fig. 3(A) is rich in aluminum but devoid of yttrium.The work of Loudjani et al., ' un ublished work of McCune and Donlon, and the YZ03-A1203 phase diagram3 all suggest that the phase in bright contrast is Y3AI5Ol2. Apparently the solubility of yttrium in N203 is extremely low and the dopant was rejected to the melt during crystal growth until the local concentration was sufficient to solidify the eutectic.In summary, these results indicate that even in single-crystal alumina with an yttrium content roughly 2 orders of magnitude lower than that investigated by Loudjani et al. virtually no yttrium goes P into solution. The emission spectroscopy results indicate that the upper limit for the solubility of yttrium is

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