Abstract
Recent studies indicate that eutectic high-entropy alloys can simultaneously possess high strength and high ductility, which have potential industrial applications. The present study focuses on Al0.7CoCrFeNi, a lamellar dual-phase (fcc + B2) precipitation-strengthenable eutectic high entropy alloy. This alloy exhibits an fcc + B2 (B2 with bcc nano-precipitates) microstructure resulting in a combination of the soft and ductile fcc phase together with hard B2 phase. Low temperature annealing leads to the precipitation of ordered L12 intermetallic precipitates within the fcc resulting in enhanced strength. The strengthening contribution due to fine scale L12 is modeled using Orowan dislocation bowing and by-pass mechanism. The alloy was tested under quasi-static (strain-rate = 10−3 s−1) tensile loading and dynamic (strain-rate = 103 s−1) compressive loading. Due to the fine lamellar microstructure with a large number of fcc-bcc interfaces, the alloy show relatively high flow-stresses, ~1400 MPa under quasi-static loading and in excess of 1800 MPa under dynamic loading. Interestingly, the coherent nano-scale L12 precipitate caused a significant rise in the yield strength, without affecting the strain rate sensitivity (SRS) significantly. These lamellar structures had higher work hardening due to their capability for easily storing higher dislocation densities. The back-stresses from the coherent L12 precipitate were insufficient to cause improvement in twin nucleation, owing to elevated twinning stress under quasi-static testing. However, under dynamic testing high density of twins were observed.
Highlights
The results clearly indicate that with suitable heat treatments the fcc phase in the lamellar two-phase microstructure is further strengthened by forming ordered L12 nano precipitates, without compromising the tensile ductility
While the dual phase eutectic Al0.7CoCrFeNi was precipitation strengthenable by introducing hard-coherent intermetallic phase in the fcc phase via suitable heat treatment, post deformation transmission electron microscopy (TEM) examination was used to investigate the modes of deformation under various strain rates
The following can be established from the current study: (1) Phase composition The alloy was cast, homogenized, rolled and heat treated at two different temperatures i.e. 1100 °C and 580 °C
Summary
A recently developed new class of eutectic HEAs (EHEAs), have attracted a lot of attention due to their promising mechanical properties[10,11,12,13,14,15,16,17,18] These eutectic HEAs typically exhibit a lamellar fcc + B2 microstructure, with the ability to tune the composition of both phases over a wide range due to their inherent complexity. The present study investigates the influence of such heat-treatments on the mechanical behavior of this alloy under both quasi-static low strain rate tensile loading conditions, as well as dynamic strain rate compression conditions. Microstructural assessment after deformation revealed substantial deformation twinning in this alloy at higher strain rates
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