Mechanical properties and deformation behavior of dual-phase Al_0.6CoCrFeNi high-entropy alloys with heterogeneous structure at room and cryogenic temperatures

Highlights

• Al_0.6CoCrFeNi HEAs with heterogeneous structure have a combination of high strength and good plasticity.

• The SFE of the fcc phase in Al_0.6CoCrFeNi is calculated to be 49.33 mJ/m² and 28.69 mJ/m² at 298 K and 77 K, respectively.

• Based on the critical stress formula, deformation twins prevail when true train is over 3.8% at 77 K.

• Nanoscale deformation twins induced by 77 K loading realize the excellent strength-ductility combination.

Abstract

Dual-phase Al_0.6CoCrFeNi high-entropy alloys (HEAs) with heterogeneous structure are obtained by cold rolling and annealing. Tensile experiments show that this alloy exhibits excellent combinations of yield strength, tensile strength and tensile elongation at room (298 K) and cryogenic (77 K) temperatures. Through the thermodynamic formula, the stacking fault energy of face-centered cube (fcc) phases in Al_0.6CoCrFeNi alloys are calculated to be 49.33 mJ/m² and 28.69 mJ/m² at 298 K and 77 K, respectively. Combining with EBSD (electron backscattering diffraction) and TEM (transmission electron microscopy), it is concluded that the deformation of dual-phase Al_0.6CoCrFeNi alloys with heterogeneous structure at 298 K is mainly dominated by the dislocation slip in fcc plus bcc (body-centered cube) phases, and the deformation mode at 77 K can be divided into two stages: (1) when the true strain is between 0 and 3.8%, the plasticity is mainly caused by dislocation slip of fcc and bcc phases; (2) when the true strain is between 3.8% and samples fracture, the plasticity is mainly induced by deformation twins in fcc phase and dislocation slip in bcc phase.

Introduction

It is well known that conventional metallic alloys usually consist of one or two principal elements. In order to overcome this limitation, a new kind of alloys named multi-principal component alloys or high-entropy alloys (HEAs) has been proposed recently. They generally consist of five or more principal elements, and the concentration of each element ranges from 5 to 35 at. % [1].Due to their high mixing entropy of HEAs, it is easy to form solid solutions of face-centered cube (fcc), body-centered cube (bcc) or hexagonal close-paced (hcp) structures under as-cast conditions. In addition, HEAs have exhibited many potential properties, such as high hardness, high strength, thermal stability and excellent wear resistance [2]. Because of its excellent physical and chemical properties, HEAs have the potential to be used as structural materials in military, aerospace, automotive and other industries.

Generally, fcc-single phase HEAs exhibit a good ductility but a low strength, while bcc-single phase HEAs have good strength but poor plasticity. Nowadays, many scholars are based on single-phase alloys, in order to obtain HEAs with good plasticity and high strength, many measures are implemented. Such as, precipitation-hardening [3], twinning-induced plasticity effect (TWIP) [4], and transformation-induced plasticity effect (TRIP) [5]. Although alloys with better comprehensive properties can be obtained by these strategies, it is important that the matrix materials have excellent comprehensive performance and the fcc plus bcc dual-phase HEAs conforms to this characteristic. For example, the yield strength, tensile strength and tensile elongation of the heat-treated duple-phase Al_0.5CoCrFeNi HEAs are 834 MPa, 1220 MPa and 25%, respectively. This excellent property is very rare in fcc or bcc single-phase alloys. In addition, researchers usually study alloys with fully recrystallized microstructures, that is, alloys with homogeneous structure. However, alloys with homogeneous structure generally have a good ductility and a low yield strength. Recently, some scholars have found that heterogeneous microstructure with recrystallization and non-recrystallization regions can be obtained by cold rolling, followed by partial recrystallization annealing. Non-recrystallized grains can cause strain hardening, thereby can contribute to a high strength, while recrystallized grains can increase the ductility in materials with heterogeneous structure [6]. Therefore, alloys with heterogeneous structure tend to have high strength without sacrificing the ductility.

At present, there are many studies on the low temperature mechanical properties of fcc or bcc single phase HEAs [7,8]. In their view, for cubic structure, a decrease in temperature leads to a decrease in the stacking fault energy (SFE) of the materials, thus activating more deformation twins to improve the plasticity of the alloys [9]. This theory is called the “twinning-induced plasticity” effect (TWIP) [4]. In the present study, the dual-phase Al_0.6CoCrFeNi HEAs with heterogeneous structure was obtained by cold rolling 50%, and followed by an annealing at 1000 °C for 1 h. The newly-developed alloys with a heterogeneous structure exhibit high yield and tensile strength, together with good uniform elongation and fracture elongation at room and low temperatures. Microstructure evolution, tensile properties, and deformation mechanisms of dual-phase Al_0.6CoCrFeNi HEAs with heterogeneous structure at room and low temperatures are shown in the results and discussion.