Effect of compositions and processing parameters on eutectoid reaction in AlCoFeNi multi-component alloys

Suitable chemical compositions for lamellar microstructures (LMs) via eutectoid reaction in multi-component alloys (MCAs) is a challenging task. This study employed Thermo-Calc (TCHEA3 database) to calculate phase diagrams by systematically varying Al, Co, Fe and Ni content within the AlCoFeNi system. Based on these calculations, selected compositions were arc-melted and thoroughly investigated for the optimal chemical compositions, initial phase microstructures and phase transformation to LMs. The optimal compositions of MCAs for LMs were determined to be Al₇(CoFeNi)₉₃ and Al₁₀(CoFeNi_1.5)₉₀ at.%, which are in single face centered cubic (FCC) phase. SEM analysis confirmed that aging the solid-solution MCAs at 500–600 °C induces LMs transformation. For the Al₇(CoFeNi)₉₃ MCA, aging at 500, 560, and 600 °C yielded LMs spacings of 75 ± 13 nm, 138 ± 29 nm, and 364 ± 102 nm, respectively; under the same conditions, the Al₁₃(CoFeNi)₈₇ MCA gave spacings of 103 ± 51 nm, 188 ± 18 nm, and 347 ± 51 nm. The tensile properties of Al₇(CoFeNi)₉₃ MCA were governed aging temperatures transforming the FCC phase into LMs. Aging at 500 and 560 °C increased strength (YS: 414–620 MPa; UTS: 493–799 MPa) but reduced ductility (UE: 0.94–0.50%). Aging at 600 °C (24 h, WQ) provide balanced tensile properties (UTS: 603 MPa; UE: 29.12%). Aging for 48 h gives enhanced strength (749 MPa) at the cost of ductility (4.60%). Furthermore, Johnson–Mehl–Avrami-Kolmogorov (JMAK) analysis at 560 °C yielded transformation kinetics parameters (n, k) of (1.08, 0.028) for Al₇(CoFeNi)₉₃ MCA and (1.47, 0.15) for Al₁₃(CoFeNi)₉₀ MCA. The corresponding activation energies for the eutectoid LMs determined via the Arrhenius model, are 199.71 and 268 kJ/mol.