Abstract
This paper reports a novel eutectoid nano-lamellar (FCC + L12)/(BCC + B2) microstructure that has been discovered in a relatively simple Al0.3CoFeNi high entropy alloy (HEA) or complex concentrated alloy (CCA). This novel eutectoid nano-lamellar microstructure presumably results from the complex interplay between Al-mediated lattice distortion (due to its larger atomic radius) in a face-centered cubic (FCC) CoFeNi solid solution, and a chemical ordering tendency leading to precipitation of ordered phases such as L12 and B2. This eutectoid microstructure is a result of solid-state decomposition of the FCC matrix and therefore distinct from the commonly reported eutectic microstructure in HEAs which results from solidification. This novel nano-lamellar microstructure exhibits a tensile yield strength of 1074 MPa with a reasonable ductility of 8%. The same alloy can be tuned to form a more damage-tolerant FCC + B2 microstructure, retaining high tensile yield stress (~900 MPa) with appreciable tensile ductility (>20%), via annealing at 700 °C. Such tunability of microstructures with dramatically different mechanical properties can be effectively engineered in the same CCA, by exploiting the complex interplay between ordering tendencies and lattice distortion.
Highlights
A novel eutectoid-like nano-lamellar microstructure consisting of complex hierarchically decomposed alternating lamellae of FCC + L12/BCC + B2, develops in a simple Al0.3CoFeNi HEA/CCA, due to the competing tendencies of L12 ordering within the FCC lattice versus BCC/B2 phase formation, due to Al addition
The experimentally observed phase stability as a function of annealing temperature could be largely rationalized based on CALPHAD modeling (ThermoCalc with TCHEA3 database)
Optimization of the balance of mechanical properties in the same Al0.3CoFeNi alloy has been successfully demonstrated by tuning the microstructure in the FCC + B2 two-phase field via higher temperature annealing at 700 °C
Summary
Al0.3CoFeNi, with the nominal chemical composition 9.1Al30.3Co30.3Fe30.3Ni (at. %) was produced using conventional arc melting. Al0.3CoFeNi, with the nominal chemical composition 9.1Al30.3Co30.3Fe30.3Ni %) was produced using conventional arc melting. The composition of the cast ingot was measured with Energy dispersive spectroscopy in Scanning electron microscope (SEM-EDS) and was found to be 9.4Al−30.1Co−31.9Fe−28.6Ni The cast alloy was homogenized at 1250 °C for 30 min before rolling and annealing treatments. All samples were cold rolled to 85% and encapsulated in quartz tubes backfilled with argon and solutionized at 1250 °C for 5 min, solutionized and annealed at 600 °C or 700 °C for 50 hrs or directly annealed at 700 °C for 50 hrs and water quenched.
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