Modeling the age-hardening behavior of Al-Si-Cu alloys
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11/7/04
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Modeling the Age-Hardening Behavior of Al-Si-Cu Alloys S.C. WEAKLEY-BOLLIN, W. DONLON, C. WOLVERTON, J.W. JONES, and J.E. ALLISON 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 plates measured by transmission electron microscopy and a first-principles/computational-thermodynamics model of 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 . We also discuss limitations of the present approach and point to areas for improvement in future studies.
I. INTRODUCTION
AGE-HARDENABLE aluminum alloys are important engineering materials used in many applications. The aging time and temperature are often chosen to optimize the alloy strength, or other mechanical properties of interest. The functional relationship between aging schedule and strength for a given material can be determined via an empirical approach; however, every distinct alloy composition requires repeated measurements to develop an empirical aging/strength database for each new material. A more efficient way of optimizing properties associated with aging would be possible if a model capable of predicting strength as a function of aging time, temperature, and alloy composition existed. Therefore, the investigation and modeling of strengthening mechanisms in aluminum alloys remains an active research area.[1–13] Precipitates can impede the motion of dislocations through a wide variety of mechanisms, which have been reviewed by Ardell.[14,15] Building on the equations governing these mechanisms and including other contributions to the strength of a material, a process-model approach was developed by Shercliff and Ashby[1] to describe the age-hardening response of the yield strength and hardness of aluminum alloys. Although the original Shercliff-and-Ashby model was calibrated to 2000- and 6000-series alloys, subsequent researchers have applied this process-model approach to a wide variety of wrought and, recently, cast Al alloys.[2–7,16] The process model[1] describes the ambient-temperatur
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