Forming Mechanism of Equilibrium and Non-equilibrium Metallurgical Phases in Dissimilar Materials: Illustrated With Aluminum/steel (Al-Fe) Joints

Forming metallurgical phases has a critical impact on the performance of dissimilar materials joints. Here, we shed light on the forming mechanism of equilibrium and non-equilibrium intermetallic compounds (IMCs) in the dissimilar aluminum/steel joints with respect to processing history (e.g., the pressure- and temperature-profiles) and chemical composition, where the used knowledge of free energy and atomic diffusion in the Al-Fe system was taken from first-principles phonon calculations and data available in the literature. We found that the metastable while ductile (judged by the presently predicted elastic constants) Al 6 Fe is a pressure ( P ) favored IMC observed in the processes involving high pressures. The MoSi 2 -type Al 2 Fe is a brittle and a strong P -favored IMC observed at high pressures. The stable, brittle h-Al 5 Fe 2 is the most commonly observed IMC (followed by q-Al 13 Fe 4 ) in almost all processes, such as fusion/solid-state welding and additive manufacturing (AM), since h-Al 5 Fe 2 is temperature-favored, possessing high thermodynamic driving force of formation and the fastest atomic diffusivity among all Al-Fe IMCs. Notably the ductile AlFe 3 , the less ductile AlFe, and most of the other IMCs can be formed during AM, making AM a superior process to achieve desired IMCs in dissimilar materials. In addition, the unknown configurations of Al 2 Fe and Al 5 Fe 2 were also examined by machine learning based datamining together with first-principles verifications and structure predictor. All the IMCs, which are not P- favored, can be identified using the conventional equilibrium phase diagram and the Scheil-Gulliver non-equilibrium simulations.