The phase composition, character of nonequilibrium solidification, structure, and strengthening of alloys based on the Al–8% Zn–3% Mg (wt %) composition, which are cooperatively and separately alloyed with silicon and calcium, are studied in this work using calculations and experimental methods. In the Al–Zn–Mg–Ca–Si system, the (Al,Zn)₄Ca, Al₂CaSi₂, Mg₂Si and T (Al₂Mg₃Zn₃) phases can be in equilibrium with the (Al) solid solution. Conditions of the two-stage annealing, which is used for the dissolution of eutectic inclusions of the Т and Mg₂Si phases formed upon nonequilibrium solidification, are substantiated. The Al₂CaSi₂ phase is characterized by needle-like morphology and will not take on a spherical shape upon heating up to 520°С. Upon the individual introduction of silicon (0.5%) and calcium (1 and 2%), the effect of precipitation strengthening upon aging is seen to decrease. This is due to the decrease in the magnesium and zinc concentrations in the (Al) solid solution because of fixing these elements to form the Mg₂Si and (Al,Zn)₄Ca, phases, respectively. Upon the cooperative introduction of Si and Ca, the Al₂CaSi₂ phase forms; this allows us, to a considerable extent, to retain unchanged the initial composition of the (Al) solid solution and, therefore, the effect of precipitation strengthening.

In the present paper, Natural Metal-Matrix Composites (NMMC) based on multicomponent hypereutectic Al-Ca-(Mn)-(Ni) alloys were studied in as-cast, annealed and rolled conditions. Thermo-Calc software and microstructural observations were utilised for analysing the equilibrium and actual phase composition of the alloys including correction of the Al-Ca-Mn system liquidus projection and the solid phase distribution in the Al-Ca-Mn-Ni system. A previously unknown Al₁₀CaMn₂ was discovered by both electron microprobe analysis and X-ray studies. The Al-6Ca-3Mn, Al-8Ca-2Mn, Al-8Ca-2Mn-1Ni alloys with representative NMMC structure included ultrafine Ca-rich eutectic and various small-sized primary crystals were found to have excellent feasibility of rolling as compared to its hypereutectic Al-Si counterpart. What is more, Al-Ca alloys showed comparable Coefficient of Thermal Expansion values due to enormous volume fraction of Al-based eutectic and primary intermetallics. Analysis of tensile samples’ fracture surfaces revealed that primary intermetallics may act either as stress raisers or malleable particles depending on their stiffness under deformation. It is shown that a compact morphology can be achieved by conventional casting without using any refining agents. Novel hypereutectic Al-Ca NMMC materials solidifying with the formation of Al₁₀Mn₂Ca primary compound have the best ductility and strength. We reasonably propose these materials for high-load pistons.