On the formation of Al2Si2Sr phase and its behavior in Al–Si–Sr alloys

Abstract

Sr element is commonly used to modify eutectic Si in Al–Si alloys, while Al2Si2Sr phase is quite easy to precipitate when Sr content is high to tens of ppm level. In this study, the growth models of Al2Si2Sr crystal as well as its behavior in Al–xSi–2 Sr (x=5, 8, 12, 18, 25 and 40, wt%) alloys were investigated. Polyhedral, triangular pyramid and dendritic morphologies of Al2Si2Sr crystals were detected, and their growth models were discussed. Based on the crystallographic parameters, the disregistry between (0001)Al2Si2Sr and (111)Si is calculated to be 9.7%. Transmission electron microscopy analysis confirms that the Al2Si2Sr phase is not likely to be a good nucleation site for Si. Differential scanning calorimetry analysis indicates that Al2Si2Sr phase is firstly precipitated in Al–5, 8, 12, 18Si–2 Sr alloys, while its precipitating temperature is close to primary Si in Al–25Si–2 Sr alloy and it falls behind to primary Si in Al–40Si–2 Sr alloy. Interestingly, amounts of Al2Si2Sr dendrites were engulfed by primary Si in Al–40Si–2 Sr alloy. Based on the widely–known impurity–induced twinning mechanism in Sr modified Al–Si alloys, the Al2Si2Sr dendrites were deduced to be in–situ evolved from Sr–rich clusters, which are absorbed on {111}Si planes. This work may provide a new perspective for understanding the modification mechanisms of Sr element on Si crystal in Al–Si–Sr alloys.