Abstract
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 dissimilar aluminum/steel joints with respect to processing history (e.g., the pressure and temperature profiles) and chemical composition, where the 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 and ductile (judged by the presently predicted elastic constants) Al6Fe is a pressure (P) favored IMC observed in processes involving high pressures. The MoSi2-type Al2Fe is brittle and a strong P-favored IMC observed at high pressures. The stable, brittle η-Al5Fe2 is the most observed IMC (followed by θ-Al13Fe4) in almost all processes, such as fusion/solid-state welding and additive manufacturing (AM), since η-Al5Fe2 is temperature-favored, possessing high thermodynamic driving force of formation and the fastest atomic diffusivity among all Al–Fe IMCs. Notably, the ductile AlFe3, 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 Al2Fe and Al5Fe2 were also examined by machine learning based datamining together with first-principles verifications and structure predictions. All the IMCs that are not P-favored can be identified using the conventional equilibrium phase diagram and the Scheil-Gulliver non-equilibrium simulations.
Highlights
The present work aims to unveil the forming mechanism of equilibrium and non-equilibrium intermetallic compounds (IMCs) in dissimilar aluminum to steel joints based on thermodynamic knowledge in the Al–Fe system from (1) the present first-principles and phonon calculations based on density functional theory (DFT) and (2) the CALPHAD modeling by Sundman et al.[9] and based on kinetic knowledge reported in the literature[42,58,59]
All DFT-based first-principles calculations in the present work were performed by the Vienna Ab initio Simulation Package (VASP)[81] with the ion–electron interaction described by the projector augmented wave method[82] and the X-C functional described by the generalized gradient approximation (GGA) developed by Perdew, Burke, and Ernzerhof (PBE)[83]
It can be seen that (i) the DFT-predicted ΔH0 values agree well with the experimental data which are scattered; (ii) Al6Fe is close to but above the convex hull, indicating that it is metastable at T = 0 K and P = 0 GPa, and more attentions need to be paid to its phase stability at high temperatures and high pressures; (iii) Al9Fe2 is an unstable structure due to the existence of imaginary phonon modes and ignored in the present work; (iv) Al5Fe2 is a metastable phase at T = 0 K and P = 0 GPa, various configurations have been examined in the present work
Summary
According to Pugh’s c riterion[11,12], i.e., the ratio of bulk modulus versus shear modulus (B/G) based on the present first-principles calculations (cf., “Details of first-principles calculations” section). The metastable, ductile Al6Fe was observed in the processes of direct chill casting (example #1 in Table 1)[10], high-pressure die casting (#2)[13], equal channel angular extrusion (#3)[14], tungsten inert gas (TIG) welding-brazing (#4)[15], and additive manufacturing (AM) via laser powder bed fusion (#5)[16]. These observations suggest that A l6Fe is an IMC existing at high pressures. Al alloy 5754 with coated DP600 or 22MnB5 steel Al alloy 5083 and steel (< 0.1 wt.% C) sheets Al alloy 6061-T6 and AISI 1018 steel Al sheet (6016) and galvanized IF-steel sheet Al alloy (surfalex 6 s) and ultrahigh strength steel Al alloy (1050) sheets and Fe particles Al sheet (6061 T4) and coated steel sheet Al alloy wire (ER5356) and Zn-coated steel Pure Al and Fe Pure Al and Fe Pure Al and Fe Pure Al plate and pure Fe sheet Pure Al and Fe rods (diffusion couples)
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