Phosphorus in Al–Si cast alloys: Thermodynamic prediction of the AlP and eutectic (Si) solidification sequence validated by microstructure and nucleation undercooling data

A self-consistent thermodynamic description of the Al–Si–P ternary system was developed. Based on that, the first consistent and experimentally supported Al–Si–P phase diagram was calculated. The P-threshold – excluding the formation of potent AlP nucleants before eutectic (Si) – was elaborated using both normal Scheil and equilibrium solidification simulation. Its quantitative dependence on the Si content of the alloy was determined and a precipitation sequence map, covering all Si and P compositions relevant for Al–Si cast alloys, was predicted from these thermodynamic calculations. The predicted map was validated by independent experimental studies on hypoeutectic Al–Si alloys and it is in perfect agreement with microstructure observation and undercooling measurements. Moreover, a constrained Scheil solidification simulation technique was applied to predict the undercooling under clean heterogeneous nucleation conditions and these data are in perfect agreement with dedicated experimental observations from the entrained droplet technique. Even for extremely low P-content and large undercooling the direct nucleation of (Si) on primary (Al) does not occur but is triggered by the formation of AlP under clean conditions. The P-threshold is not trivial since it varies from zero to 7.4 ppm P for hypoeutectic alloys, which is just in or below the commercial purity range of regular Al–Si cast alloys