Engineering transformation pathways in an Al_0.3CoFeNi complex concentrated alloy leads to excellent strength–ductility combination

Abstract

Guided by thermodynamic modeling, engineering phase transformation pathways via thermomechanical processing, in a complex concentrated alloy/high entropy alloy (HEA) of composition Al 0.3 CoFeNi, lead to a novel multi-scale microstructure consisting of fine-scale FCC + L1 2 grains mixed with B2 + BCC grains. The two-step pathway comprises initial decomposition of the parent single-phase FCC to form a fine-grained FCC + B2 microstructure, which further decomposes in the second step into the complex four-phase mixture, exhibiting an excellent combination of tensile yield stress of ∼ 1490 MPa, ultimate tensile strength of ∼ 1663 MPa, with a good ductility of ∼ 12% at room temperature. IMPACT STATEMENT This paper reports a novel multi-phase microstructure in a HEA/CCA similar to the microstructure observed in dual-phase stainless steel. This report exemplifies the novelty of HEA/CCA compositional space.

The Effects of Heat Treatment on the Microstructure and Mechanical Properties of a Selective Laser Melted AlCoFeNi Medium-Entropy Alloy

“…Two distinct precipitates can be seen in Figure 10c: one is enriched with Fe and Co, and the other is enriched with Ni and Al. This can be compared with the results of [23], which include B2 grain-enriched Ni and Al and the FCC grain-enriched Fe and Co. Therefore, the phase pointed by the blue arrows may be the B2 phase, while the phase pointed by the orange arrows may be the FCC phase, as shown in the Figure 10b.…”

“…This may be due to the fact that the addition of small amounts of Al may form little or no L1 2 phase. Another reason for this is the inaccuracies in solution thermodynamic modeling [23]. However, the results of the calculations and experiments show the same variation in FCC precipitates with an increase in aging temperature.…”

“…Consequently, the amount of the FCC phase is less. Meanwhile, Fe + Co enrichment (and Ni + Al depletion) in the FCC phase [23] resulted in some B2 grains located at FCC grain boundaries.…”

“…This microstructure consisted of fine-scale FCC + L1 2 grains interspersed with B2 + BCC grains. This resulted in a YS of around 1490 MPa, a UTS of around 1663 MPa, and an El of around 12% at room temperature [23]. Gwalani et al studied the tensile strength of an Al 0.3 CoCrFeNi HEA.…”

Insights into Defect-Mediated Nucleation of Equilibrium B2 Phase in Face-Centered Cubic High-Entropy Alloys

“…The microstructure resulting from such defect mediated discontinuous precipitation leads to improved mechanical properties as well. Similarly, in the case of Al0.3CoFeNi, solutionizing after cold rolling followed by annealing at 600°C, leads to eutectoid decomposition into (FCC+L12) and (BCC+B2) lamellae whereas direct annealing at 700°C followed by 600°C after cold rolling, results in ultrafine grained microstructure of FCC grains containing L12 precipitates, and BCC grains with B2 precipitates [13]. This also results in improved yield strength along with reasonable ductility.…”