Iron (Fe) is the most common and the most detrimental impurity element in Al alloys due to the formation of Fe-containing intermetallic compounds (IMCs), which are harmful to mechanical performance of the Al-alloy components. In this paper we investigate the formation of Fe-containing IMCs during solidification of an Al-5Mg-2Si-0.7Mn-1.1Fe alloy under varied solidification conditions. We found that the primary Fe-containing intermetallic compound (P-IMC) in the alloy is the BCC α-Al₁₅(Fe,Mn)₃Si₂ phase and has a polyhedral morphology with {1 1 0} surface termination. The formation of the P-IMCs can be easily suppressed by increasing the melt superheat and/or cooling rate, suggesting that the nucleation of the α-Al₁₅(Fe,Mn)₃Si₂ phase is difficult. In addition, we found that the IMCs with a Chinese script morphology is initiated on the {1 0 0} surfaces of the P-IMCs during the binary eutectic reaction with the α-Al phase. Both the binary and ternary eutectic IMCs are also identified as the BCC α-Al₁₅(Fe,Mn)₃Si₂ phase. Furthermore, we found that the Fe content increases and the Mn content decreases in the Fe-containing intermetallic compounds with the decrease of the formation temperature, although the sum of the Fe and Mn contents in all of the IMCs is constant.

Control of Fe in Al alloys is a severe challenge for the full metal circulation to produce the recycled alloys with mechanical and physical performance as high as the primary alloys. The high restriction of Fe content is mainly due to the deterioration caused by the large-scale Fe-containing intermetallic compounds (FIMCs) in Al alloys. In this paper, recent knowledge gained regarding nucleation, formation, and technical developments on microstructural control and refinement of FIMCs are overviewed. Specific characteristics of the multiple types of FIMCs in Al alloys are presented in two- and three- dimensional (2D and 3D) form. Phase relationships between the FIMCs in different structures, such as primary phase, binary eutectic, and ternary eutectic, formed at different solidification stages are studied. Phase transformations between the FIMCs with or without intermediate phases during the solidification process are examined in different Al alloys, with the mechanisms being clarified. Various approaches to microstructural control of FIMCs are proposed and validated. Significant refinement of FIMCs has been achieved through inoculation of TiB₂ particles that had been previously modified with deliberately interfacial segregation of desirable alloying elements, leading to the development of the novel “compositional templating” concept.

Heterogeneous nucleation is a widespread phenomenon in both nature and technology. However, our current understanding is largely confined to the classical nucleation theory (CNT) postulated over a century ago, in which heterogeneous nucleation occurs stochastically to form a spherical cap facilitated by a substrate. In this paper, we show that heterogeneous nucleation in systems with negative lattice misfit completes deterministically within three atomic layers by structural templating to form a two-dimentional template from which the new phase can grow. Using molecular dynamics (MD) simulations of a generic system containing metallic liquid (Al) and a substrate of variable lattice misfit (fcc lattice with fixed Al atoms), we found that heterogeneous nucleation proceeds layer-by-layer: the first layer accommodates misfit through a partial edge dislocation network; the second layer twists an angle through a partial screw dislocation network to reduce lattice distortion; and the third layer creates a crystal plane of the solid (the 2D nucleus) that templates further growth. The twist angle of the solid relative to the substrate as a signature of heterogeneous nucleation in the systems with negative lattice misfit has been validated by high resolution transmission electron microscopic (HRTEM) examination of TiB₂/Al and TiB₂/α-Al₁₅(Fe, Mn)₃Si₂ interfaces in two different Al-alloys.

Refinement of intermetallic compounds (IMCs) through enhancing heterogeneous nucleation during casting process is an important approach to improve the properties of aluminium alloys, which greatly increases the economy value of recycled Al-alloys. However, heterogeneous nucleation of IMCs is inherently more difficult than that of a pure metal or a solid solution. It requires not only creation of a crystal structure but also the positioning of 2 or more different types of atoms in the lattice with specific composition close to that of the nucleated IMCs. Previous understanding on heterogeneous nucleation is based on structural templating, usually considering the small lattice misfit at the interface between the nucleating solid and substrate. In this work, we proposed a hypothesis and demonstrated that composition templating plays a critical role in heterogeneous nucleation of IMCs. The experimental results revealed that segregation of Fe atoms on the AlB₂ surface, i.e., the Fe modified AlB₂ particle, provides the required composition templating and hence enhances heterogeneous nucleation of α-Al₁₅(Fe, Mn)₃Si₂, resulting in a significant refinement of the α-Al₁₅(Fe, Mn)₃Si₂ particles in an Al-5 Mg-2Si-1.0Mn-1.2Fe alloy.

During solidification of metallic materials, heterogeneous nucleation occurs on substrates, either endogenous or exogenous. The potency of the substrates for nucleation is mainly dependent upon the atomic arrangements on the substrate surface, which are affected by the lattice misfit between the substrate and the nucleated solid, the surface roughness at atomic scale, and the chemical interaction between the substrates and the melt. Extensive examinations on metal/substrate (M/S) interfaces at atomic scale by the state-of-the-art aberration (Cs) corrected STEM and associated EDS and EELS have shown that alloying elements in liquid melts tend to segregate at the interfaces, leading to the formation of various 2-dimensional compounds (2DCs) or 2-dimensional solutions (2DSs), depending upon segregation behavior of the elements. For instance, Al₃Ti 2DC and Ti₂Zr 2DC at the Al/TiB₂ interface, Y₂O₃ 2DC at the Mg/MgO interface, and a Si-rich 2DS layer at Al-Si/TiB₂ interface have been identified. Such interfacial segregations significantly affect nucleation potency of the substrates, resulting in either promoting or impeding the heterogeneous nucleation process during solidification. In this paper, we present an overview of the current studies of interfacial segregation behavior, the structure and chemistry of interfaces, and their impacts on the subsequent heterogeneous nucleation and grain initiation processes. Our focus is on the advances made in the understanding of the mechanisms for nucleation and grain refinement. It is demonstrated that it is feasible to manipulate heterogeneous nucleation by modifying nucleation potency of a substrate through deliberate interfacial segregation of desirable elements, achieving effective control of the grain structure of cast metallic materials.