# Crystallization Kinetics of Sr-Modified Precipitation Hardening Al-Si-Cu Alloys
## 1 Introduction  
Al alloying additives cause the formation of intermetallic phases, which affect the microstructure and properties of metal alloys. The size of the phases formed, their volume and morphology are important factors affecting mechanical and fatigue properties [1]. Copper in aluminum reduces the grain growth rate by segregation and alters the surface tension at the liquid/grain interface. It also affects the distribution and surface contribution of the eutectic in the alloy [1]. The presence of elements such as Cu or Mg contributes to the strengthening of aluminum alloys through the formation of phases Al\({}_{2}\)Cu and Mg\({}_{2}\)Si, especially after the precipitation hardening process. The resulting phases are Al\({}_{5}\)Cu\({}_{2}\)Mg\({}_{8}\)Si\({}_{6}\) and Al\({}_{2}\)CuMg, and, when added, both of these elements also have an effect on the mechanical properties of the aluminum alloy [2, 3]. The most damaging intermetallic phases are primarily those rich in iron, such as Al\({}_{5}\)FeSi. They are characterized by a lamellar morphology. Their sharp edges cause stress concentrations, which increase the conditions for crack initiation and propagation. In addition, brittle phases of Al\({}_{5}\)FeSi promote the increase in the porosity of castings by blocking the flow of liquid metal in interdendritic spaces. Other common Fe-rich phases are \(\alpha\)-Al\({}_{5}\)Fe\({}_{2}\)Si, and, after adding Mg, and \(\pi\)-Al\({}_{5}\)FeMg\({}_{3}\)Si\({}_{6}\) phases, with a morphology similar to \"Chinese writing\". The phase transformation of \(\beta\) into the \(\alpha\)-Al\({}_{15}\)(Fe,Mn)\({}_{3}\)Si\({}_{2}\) phase is made possible by the addition of Mn, which neutralizes thenegative effect of Fe. The effect caused by the addition of Mn is associated with a change in the morphology of the harmful plate-shaped phases into a morphology similar to writing or multi-walled crystals [4; 5; 6; 7]. The purpose of the modification of aluminum-silicon alloys is to transform the silicon crystals in both size and shape. Thus, the effect of the modification of subeutectic alloys is a different structure and a reduced interfacial distance of the eutectic [8]. Silicon crystals are brittle, and their morphology also takes the form of plates, which leads to cracks in the material [9; 10; 11]. Strontium is a strong multiple action modifier--repeated action during crystallization [12]--and its addition leads to a change in the morphology of eutectic silicon from lamellar to fibrous, which is associated with improved mechanical properties [9; 10; 13; 14]. The modification with strontium is due to the accumulation of Sr atoms in areas at the border of silicon crystals with an aluminum matrix and their absorption by the crystallization front of phases with silicon. The stresses created by the lattice distortion increase the density of twins in the silicon crystals. Their growth is hindered, and the subcooling effect during eutectic crystallization is enhanced. The modification induces the isotropic growth of silicon phases at reduced eutectic crystallization temperatures [8; 13; 15; 16]. Strontium causes an increase in the porosity of aluminum-silicon alloys because Sr oxides are pore nucleation sites [9]. As the temperature decreases, corresponding phases are formed as a result of reactions occurring during the solidification process of aluminum alloys. Due to the variable number of elements, temperatures and reactions can vary. The temperature of \(\alpha\)-phase dendrite formation depends on the silicon and copper contents of Al-Si-Cu alloys. At temperatures in the range of 580-610 \({}^{\circ}\)C, a network of \(\alpha\)-phase dendrites is formed, followed by the crystallization of phases such as Al\({}_{15}\)(Fe,Mn)\({}_{3}\)Si\({}_{2}\) or Al\({}_{5}\)FeSi. These reactions are followed by crystallization of the eutectic \(\alpha+\beta\), causing the remaining liquid to be enriched in copper [1; 17; 18]. Elements such as Mg or Cu can also lower the eutectic temperature of the aluminum alloy [17; 19].