1. The reason for the low plasticity, and in some cases the hardness, of Silumins is the brittleness of silicon particles of lamellar form. These platelets break by splitting even with small plastic deformation of the α matrix. Because of the substantial volume of these silicon particles in the structure of the alloy they have a large effect on crack initiation and propagation. The matrix — α phase — is subjected to the effects of numerous cracks in silicon and fails with ductile fracture or slip, and therefore the ductile characteristics of Silumins are low. 2. A second factor impairing the properties of Silumins is dendritic porosity. 3. Modification of Silumin AlSi10Mn with sodium improves its properties.