Structural and Mechanical Properties of Directionally Solidified Al-Si Alloys S.P. Nikanorov, V.N. Osipov, and L.I. Regel (Submitted April 19, 2018; in revised form September 20, 2019) This review covers research aimed at finding the optimum composition and growth rate to obtain a highly modified Al-Si alloy using directional solidification. Investigations of microstructure and mechanical properties as a function of Si content and growth rate are analyzed. These works show that the composition yielding a eutectic microstructure changes considerably with increasing solidification rate in the range of 10 2 -10 4 lm/s. The increase in ultimate tensile strength with increasing Si content up to that giving a completely eutectic microstructure is explained by a redistribution of volume content of a-Al and eutectic. The increase in tensile strength with increasing rate is explained by a decrease in microstructural scale accompanying the transformation of flake-to-fiber eutectic microstructure. The optimal fine fiber structure without any primary crystals of Al-Si alloy at a given Si content is obtained at the solidification rate giving a completely eutectic microstructure at that composition. Hypereutectic alloys can be fully modified using rapid cooling at such solidification rate that causes coupled growth of the eutectic for given composition of the alloy. Additional Sr modification results in a super-modified structure, high tensile strength and record high elongation. Keywords Al-Si alloy, directional solidification, elongation, eu- tectic point, microstructure, tensile strength 1. Introduction The binary Al-Si system is the basis of aluminum-silicon casting alloys in which silicon is the major alloying element. These alloys are called ‘‘silumins.’’ The manufacture of these alloys constitutes over 90% of all aluminum-based castings. Al- Si alloys are used as construction materials for different purposes, as well as solder for aluminum like Al-Ge. Heat- resistant aluminum alloys are used for car engine parts such as connecting rods, rocker arms, cylinders, pistons and valve retainers (Ref 1, 2). Construction alloys usually have a hypoeutectic content of silicon. Alloys for other applications are eutectic or hypereutectic in Si. This is connected primarily to ductility decreasing with increasing Si content. Other elements are added to these alloys to increase the strength and improve some other properties. The search continues for optimal additions and optimal Si content for different applica- tions of cast alloys based on the Al-Si system. This search will be aided by analysis of published information on the depen- dence of structural and mechanical properties on Si content, solidification rate and modification of Al-Si alloy. The effects of Si content in hypoeutectic alloys and low rates of solidification have been well studied. There are also many results for very high rates of solidification, 10 5 -10 6 lm/s. Presented here are the results of such investigations obtained predominantly during the last decade for alloys of near eutectic and hypereutectic compositions at rates up to 10 4 lm/s. Such rates are the most interesting for applications. The regions of Si content under consideration are determined by conventional casting processes: For slow cooling rate processes (sand, plaster, investment), the range is 5-7 wt.%, for permanent molds 7-9 wt.%, and for die castings 8-12 wt.% Si. Hypereu- tectic Al-Si alloys of 13 wt.% and higher Si content are obtained by die casting and thixo-forging technologies. That is why in this review investigation of 7-15 wt.%, Si alloys are mainly described. Conclusions on improving technology based on the results of these investigations focus on hypereutectic alloys. Here, results of directional solidification of alloys are analyzed because in this case there is opportunity to control the thermodynamic parameters of solidification. 2. Structure of Binary Alloys 2.1 Component Concentration Effect Physical and mechanical properties of alloys are determined by microstructure, which depends on composition and condi- tions of solidification and cooling such as rate, temperature gradient and convection. According to the equilibrium phase diagrams, at low freezing rates coupled solidification of Al-Si and Al-Ge components takes place at eutectic compositions equal to 12.6 wt.% Si (Ref 3) and 52 wt.% Ge (Ref 4). This coupled solidification results in a fine microstructure consisting of the two phases, sometimes as alternating lamellae and sometimes as fibers of one phase in the other. At fast solidification rates, coupled solidification moves toward increasing Si or Ge content. Earlier, this displaced point had been called the quasi-eutectic point because the microstructure is like that which is formed at low freezing rates from the eutectic composition. Following current practice, here we call S.P. Nikanorov and V.N. Osipov, Ioffe Institute, Politekhnicheskaya Str. 26, Saint Petersburg 194021, Russia; and L.I. Regel, Clarkson University, Potsdam, NY 13699-5700. Contact e-mail: s.nikanorov@mail.ioffe.ru. JMEPEG ÓASM International https://doi.org/10.1007/s11665-019-04414-3 1059-9495/$19.00 Journal of Materials Engineering and Performance