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.

The phase transformations in the Al-Ca-Mg-Si system have been studied using thermodynamic calculations and experimental methods. We show that at 10% Magnesium (Mg), depending on the concentrations of calcium (Ca) and silicon (Si), the following phases crystallize first (apart from the aluminum (Al) solid solution): Al₄Ca, Mg₂Si, and Al₂CaSi₂. We have found that the major part of the calculated concentration range is covered by the region of the primary crystallization of the Al₂CaSi₂ phase. Regardless of the Ca and Si content, the solidification of the aluminum-magnesium alloys ends with the following nonvariant eutectic reaction: L → (Al) + Al₄Ca + Mg₂Si + Al₃Mg₂. With respect to the temperature and composition of the liquid phase, this reaction is close to the eutectic reaction in the Al-Mg binary system. The addition of Ca and Si to the Al-10% Mg base alloy increases its hardness, reduces its density, and has no negative influence on its corrosion resistance. We have also established that the near-eutectic alloy containing about 3% Ca and 1% Si has the optimum structure.

In this paper, we study a group of alloys based on the Al–Mg–Ca–(Zn) system with various magnesium and calcium contents using computational (in Thermo-Calc software environment) and experimental (optical and scanning microscopy and X-ray microanalysis) methods. The calculated liquidus surfaces show that all alloys fall into the hypoeutectic region. According to the data of nonequilibrium crystallization, as the calcium concentration increases the liquidus temperature of the alloys decreases, while the temperature of the nonequilibrium solidus does not change. There is good agreement between the calculated and practical results. For example, there are no primary phases of crystallization origin in the structure, and the basis is an aluminum solid solution (Al) surrounded by veinlets of nonequilibrium eutectic containing magnesium, calcium, and zinc in its composition. At a joint increase in the concentration of magnesium and calcium, eutectic colonies coarsen and thicken. It was found that quenching allowed almost complete dissolution of the Al₃Mg₂ phase and the equilibrium double eutectic (Al) + Al₄Ca acquired a more fragmented form. Upon joint doping with calcium and zinc, the eutectic structure becomes more fragmented, especially in the Al6Mg2Ca2Zn alloy. Subsequent heat treatment for solid solution leads to even greater fragmentation of the eutectic inclusions, as well as the dissolution of the Al₃Mg₂ phase. Due to the high solubility of zinc in the Al₄Ca phase, inclusions of the Al₂Mg₃Zn₃ phase, which is present in the calculated isotremic cross sections, are not detected. In the Al–Mg–Ca–Zn system, under conditions close to equilibrium, at magnesium and calcium contents of at least 6 and 2%, respectively, a stable Al₂(Mg,Ca) phase can exist in alloys along with stable Al₄Ca with zinc dissolved in the latter.

Investigating the effect of Fe and Si is essential for any new Al-based composition, as these impurities can be easily found both after primary production and recycling. This study is dedicated to filling the gap in revealing the phase composition of an Al-6%Mg-2%Ca-2%Zn alloy after the combined and separate addition of Fe and Si. This was addressed by permanent mold casting and solid solution heat treatment. The investigation of slowly solidified samples also contributed to understanding potential phase transitions. It was found that the alloy containing 0.5%Fe can have nearly spherical intermetallics after heat treatment, whereas a higher Fe content brought the formation of a needle-shaped Al₃Fe intermetallic. We explain this by the formation of a ternary α-Al + Al₁₀CaFe₂ + Al₄Ca eutectic, which is more compact in as-cast condition compared to divorced binary α-Al + Al₄Ca and α-Al + Al₃Fe eutectics. Similarly, 0.5%Si readily incurred the formation of a needle-shaped Al₂CaSi₂ intermetallic, probably also by a binary reaction L → α-Al + Al₂CaSi₂. In the solidified samples, no Mg₂Si phase was found, even in slowly solidified samples. This is contrary to the thermodynamic calculation, which suggests a peritectic reaction L + Al₂CaSi₂ Mg₂Si. Interestingly, the addition of 0.5%Si caused an even coarser microstructure compared to the addition of 1%Fe, which caused the appearance of a primary Al₃Fe phase. We conclude that the new alloy is more tolerable to Fe rather than Si. Specifically, the addition of 0.5%Fe can be added while maintaining a fine morphology of the eutectic network. It was suggested that the morphology of eutectic and solid solution hardening governed the mechanical properties. The strength of the alloys containing separate 0.5%Fe (UTS = 215 ± 8 MPa and YS 146 ± 4 = MPa) and the combined 0.5%Fe and 0.5%Si additions (UTS = 195 ± 14 MPa and YS ± 1 = 139 MPa) was not compromised compared to the alloy containing 0.5%Si (UTS 201 ± 24 = MPa and YS = 131 ± 1 MPa).

Расчетными и экспериментальными методами, включая построение поверхностей ликвидуса, проведен сравнительный анализ фазового состава и морфологии первичных кристаллов в заэвтектических сплавах системы Al–Ca–Ni–X (где Х – Fe, Si, Mn). Показано, что дополнительное легирование базового сплава Al–6%Ca–3%Ni железом и кремнием приводит к формированию грубых вытянутых первичных кристаллов размером до 100 мкм в длину. Установлено, что добавка марганца, напротив, приводит к образованию сравнительно небольших (около 20 мкм) компактных первичных кристаллов двух 4-компонентных фаз – предположительно на основе тройных соединений Al9CaNi и Al10CaMn2. Определен состав эвтектик в четверных сплавах. Все алюминиево-кальциевые эвтектики характеризуются повышенной долей вторых фаз и более тонким строением по сравнению с алюминиево-кремниевой эвтектикой в силумине АК18, а также способностью к сфероидизации при нагреве начиная с 500 °С. Сочетание компактной и сферической морфологий частиц после отжига в сплаве 63-2Mn представляется благоприятным для деформации. Сравнение технологичности экспериментального сплава Al–8%Ca–1%Ni–2%Mn и марочного силумина АК18 показало преимущество первого. По совокупности характеристик он может рассматриваться в качестве основы для разработки заэвтектических сплавов нового поколения как альтернативы поршневым силуминам типа АК18. Экспериментальный сплав, микроструктура которого характеризуется компактной морфологией, малыми размерами первичных кристаллов и тонким строением эвтектики, в отличие от заэвтектических силуминов не требует специального модифицирования.