Static recrystallization and grain growth behaviour of Al0.3CoCrFeNi high entropy alloy

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Abstract

Single phase Al0.3CoCrFeNi (Al0.3) FCC high entropy alloy (HEA) synthesized through arc melting was subjected to cold rolling and subsequent annealing to study its recrystallization and grain growth behaviour. For the as-cast alloy, only partial recrystallization was achieved after cold deformation followed by annealing at 800 and 900 °C, which was attributed to the pining effect of Alsingle bondNi rich precipitates formed during annealing. The activation energy of static recrystallization (549 kJ/mol) for the as-cast Al0.3 HEA was significantly higher than that of conventional alloys due to the distorted lattice, sluggish diffusion and most importantly the interaction of Alsingle bondNi rich precipitates with the recrystallization. Homogenization heat treatment on Al0.3 HEA increased the static recrystallization kinetics and was associated with the lower volume fraction of precipitates formed during annealing. An increased grain growth exponent (n ≈ 5) and activation energy (583 kJ/mol) compared with conventional alloys, particularly at low annealing temperatures, was observed for the Al0.3 HEA mainly due to the pinning effect of Alsingle bondNi rich precipitates. Finally, through investigating the mechanical performance of the alloy after annealing in the precipitation-free (1050 and 1000 °C) and precipitation-containing (900 °C) temperature ranges, the relative contributions for grain boundary and precipitation strengthening were estimated for the annealed Al0.3 HEA.

Introduction

High Entropy Alloys (HEAs) are a relatively new concentrated multi-element alloying concept which offer single or duplex phase structures consisting of simple solid solutions [1,2]. To form solid solutions a series of empirical rules must be satisfied, such as multiple principal elements (at least five) with atomic percentage of 5–35% and variation of atomic radii less than 12% among mixing elements [2]. Correct multi-principal alloy design leads to several advantages, such as simple single or duplex stable phases rather than complex intermetallic phases [1,2], lattice distortion that enhances mechanical behaviour [3], sluggish diffusion that provides thermal stability [2], and a composite effect that produces an advantageous combination of properties [2,4]. Consequently, HEAs are candidate materials in applications that require improved strength/formability [5,6], wear [7,8], corrosion [9] and fatigue resistance [10].

FCC structured HEAs have high ductility with low yield strength, while BCC structured HEAs have high strength with limited ductility [11]. Thermomechanical processing of metals and alloys can enhance both the strength and ductility by refining the grains via recrystallization processes. For FCC HEAs, grain boundary migration has been suggested as the main recrystallization mechanism [[12], [13], [14]]. It is expected that low vacancy concentration and slow diffusion would limit the grain boundary migration [15,16], and subsequently affect the recrystallization behaviour of HEAs. However, the kinetics of recrystallization and grain growth of HEAs has received little attention to date.

The AlxCoCrFeNi and CoCrFeNiMn systems are among the most extensively studied HEAs, with work focussing mainly on microstructure and mechanical properties in the as-cast condition [[17], [18], [19], [20], [21]]. Some studies investigated the cold deformation and annealing behaviour of FCC HEAs [[22], [23], [24], [25]], highlighting the sluggish diffusion effect resulting from the formation of a complex solid solution [15,16,[22], [23], [24], [25], [26]]. For instance, annealing was performed on deformed as-cast Al0.3CoCrFeNi (Al0.3) and Al0.25CoCrFeNi HEAs at very high temperature of 1100 °C for 1 h and 10 h, respectively, to obtain a fully recrystallized microstructure [27,28]. Another recrystallization study was performed on highly deformed homogenized Al0.3 HEA by annealing between temperatures 700 and 1100 °C for 1 h to obtain refined grain size [29]. Although a refined microstructure was produced through static recrystallization in the above-mentioned studies, a detailed investigation of kinetics of recrystallization and grain growth behaviour of these alloys is lacking in the literature. In particular, there has not been a systematic attempt to study the kinetics and activation energies of recrystallization and grain growth behaviour in Al0.3 HEA. Combined effect of precipitation and grain boundary strengthening on the mechanical properties have been studied in Al0.3 HEA after annealing at temperatures T ≤ 1000 °C [27,29]. However, the effect of precipitation on the kinetics of recrystallization and grain growth behaviour of this alloy system is lacking in the literature.

It is important to understand the recrystallization behaviour of this new alloy class in both as-cast and homogenized conditions to direct the design of thermomechanical schedules to obtain target microstructures and properties. Consequently, a systematic study is presented on the effects of the initial microstructure, degree of cold deformation, and annealing conditions on the static recrystallization behaviour of FCC Al0.3 HEA. To reveal the interaction of precipitates with recrystallization and grain growth, a detailed analysis of the kinetics and activation energy of static recrystallization and grain growth was conducted at annealing temperatures below and above the precipitation temperature range. Tensile testing of thermo-mechanically processed HEA was performed to determine the relative contributions of grain boundary and precipitation strengthening in the recrystallized Al0.3 HEA.

Section snippets

Experimental methods

Arc melting was used to prepare Al0.3 HEA of ∼40 g ingots with a chemical composition (wt.%) of 3.5% Al, 24.9% Co, 22.5% Cr, 24.1% Fe and 24.9% Ni determined by Glow-discharge optical emission spectroscopy (GDOES). Melting was carried out in a high purity argon atmosphere to minimise contamination. Remelting was performed at least five times to create a homogeneous chemical composition. A homogenization heat treatment was conducted at 1100 °C for 24 h in a muffle furnace purged with argon. The

As-cast, homogenized and cold deformed microstructures of Al0.3 HEA

A columnar dendritic structure was observed along the solidification direction in the as-cast alloy (Fig. 1a). It has been well documented that the growth direction of columnar grains is parallel to heat flow direction. In this FCC alloy, {100} planes are of a low coordination number, and therefore, the largest rate of growth occurs along the 〈100〉 directions [31]. At low and high magnifications the microstructure appears single phase (Fig. 1a and b), and EDS analysis revealed local segregation

Discussion

Annealing of cold rolled Al0.3 HEA resulted in the precipitation of a second phase which according to EDX and XRD analysis were BCC Alsingle bondNi rich precipitates, which is consistent with the results obtained by Yasuda et al. [29]. The formation of these precipitates is mainly due to high negative enthalpy of mixing of Alsingle bondNi, relative to other elemental pairs (ΔHmix of Alsingle bondNi = −22 kJ/mol) [35]. It was shown that the volume fraction of precipitates increased with a decrease in the annealing temperature.

Conclusions

Cold rolling and subsequent annealing of as-cast and homogenized Al0.3CoCrFeNi HEAs were investigated. Through a detailed study of the microstructure, recrystallization and grain growth behaviour as well as final mechanical properties of the annealed samples, the following conclusions were derived:

  • For the as-cast Al0.3 HEA subjected to 70% deformation, only partial recrystallization was achieved even after 600 min of annealing at 900 and 800 °C mainly due to the supressing effects exerted by Alsingle bond

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Acknowledgement

The present work was carried out with the support of the Deakin Advanced Characterisation Facility.

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