Aluminium and cast iron alloys are used in the automotive industry in a broad range of applications. To acquire specific characteristics of the materials and the relation with their manufacturing processes, such as casting, exhaustive studies have been performed. The usual goal of this studies is to understand and predict the behavior of mechanical properties to optimize materials and processes. 

During the past decades, new models have been developed to determine the ultimate tensile strength (UTS) of lamellar graphite cast iron (LGI) as a function of morphologcial parameters connected to the surface area of primary microstructures, such as the hydraulic diameter of the interdendritic region (D^Hyd_ID). In Al-Si alloys, the morphology of primary microstructures are usually characterized in terms of secondary dendrite arm spacing (SDAS) when used in the models of tensile properties of the material. 

The main objective of this work is to apply the existing knowledge in cast iron to understand if the stereologically-based relations are applicable to the characteziation of the UTS in Al-Si alloys.

In this study, four different Al-Si alloys have been studied; Al-7Si, Al-9Si Al11-Si and Al13-Si in as-cast condition and after gradient solidification. To study the relationship between the microstructure of the primary phase and the UTS, the SDAS was measured with the help of the Olympus software.

From the relations found between the stereological interpretations and the UTS, it is found that the most influential  parameters to UTS in Al-Si alloys are the Hydraulic diameter of intradendritic region (D^Hyd_INTD), and the Modulus of primary dendrite (M_PD). In both cases, the lower values of D^Hyd_INTD and M_PD represented an increment in UTS values.

It is therefore suggested, that the influence of primary microstructure morphology, in terms of the stereological parameters, D^Hyd_INTD and the M_PD, have similar effects on the UTS, for both cast iron and aluminium silicion alloys.