Elsevier

Materials Science and Engineering: A

Volume 684, 27 January 2017, Pages 552-558
Materials Science and Engineering: A

Deformation mechanisms of Al0.1CoCrFeNi at elevated temperatures

https://doi.org/10.1016/j.msea.2016.12.110Get rights and content

Abstract

Deformation mechanisms of a high-entropy alloy with a single face-centered-cubic phase, Al0.1CoCrFeNi, at elevated temperatures are studied to explore the high temperature performances of high-entropy alloys. Tensile tests at a strain rate of 10−4 s−1 are performed at different temperatures ranging from 25 to 700 °C. While both yield strength and ultimate tensile strength decrease with increasing temperature, the maximum elongation to fracture occurred at 500 °C. Transmission electron microscopy characterizations reveal that, at both 25 and 500 °C, most of deformation occurs by dislocation glide on the normal face-centered-cubic slip system, {111}〈110〉. In contrast, numerous stacking faults are observed at 600 and 700 °C, accompanied by the decreasing of dislocation density, which are attributed to the motion of Shockley partials and the dissociation of dislocations, respectively. According to the Considere's criterion, it is assumed that the dissociation of dislocations and movement of Shockley partials at higher temperatures significantly decreases the work hardening during tensile tests, promoting the early onset of necking instability and decreasing the high-temperature ductility.

Introduction

High-entropy alloys (HEAs) are multicomponent alloys, which contain at least five major metal elements in a nearly equimolar ratio [1]. Generally, as the amount of the alloying components increases, intermetallic compounds are gradually formed. This trend will cause the difficulty in the microstructure analyses and the inherent brittleness [2]. However, HEAs tend to form simple solid-solution phases, such as face-centered-cubic (FCC) and body-centered-cubic (BCC) phases due to the high mixing entropy [3]. The high mixing entropy can facilitate the formation of disordered solution states and suppress the formation of intermetallic compounds [4]. The simplification of their structures avoids the disadvantages of conventional multicomponent alloys which usually exhibit complicated microstructures and intrinsic brittleness, and the concept of HEAs greatly expands the number of applicable alloy systems.

HEAs have attracted great attention due to their excellent mechanical properties. Based on previous research, it has been found that HEAs can possess high hardness, strong fatigue resistance, good oxidation resistance, and good age-softening resistance [5], [6], [7], [8], [9], [10], [11], [12], [13]. Furthermore, Kuznetsov et al. [14] studied the microstructures and mechanical properties of the AlCoCrCuFeNi HEA, and found that the forged alloy exhibited superplastic behavior in the temperature range of 800–1000 °C. The maximum tensile ductility can reach 860% in the experiment. Bernd Gludovatz et al. found that the HEA, CrMnFeCoNi, exhibits a remarkable fracture toughness at cryogenic temperatures, which is attributed to the nanotwinning mechanisms [15]. Much work has been devoted to studying the mechanical properties and microstructures of different HEA systems. However, the deformation mechanisms of HEAs at different temperatures, especially at elevated temperatures, which are of important for understanding their mechanical behavior, have not been fully understood.

The AlxCoCrFeNi HEA is a well-studied system, and its microstructures and mechanical properties can be well modified by changing the content of Al and fabrication process [11], [16], [17], [18], [19], [20], [21], [22], [23], [24]. In the current study, AlxCoCrFeNi was chosen as a model system for several reasons: (a) its microstructures and mechanical properties have been well-studied; (b) it can form different microstructures as a variation of the Al content, which is convenient for a further systematic investigation; (c) a simple FCC single phase can be formed when the Al content is low (x<0.45), which facilitates the investigation of fundamental mechanisms; and (d) it exhibits good mechanical properties, which make it a very promising system for applications.

In this study, the Al0.1CoCrFeNi HEA was fabricated by vacuum induction-melting and hot isostatic pressing (HIP) sintering. The microstructure of the as-prepared sample was characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that only a single FCC phase is formed without nano-precipitates, and the grain size is on the order of millimeter. The hardness was measured using a Vickers hardness tester, and then tensile tests at a strain rate of 10−4 s−1 were performed at different temperatures ranging from 25 to 700 °C. The microstructures of the HEA samples tested at different temperatures were characterized by TEM and the serration behavior in the tensile tests was also analyzed. The aim of current study is to investigate the microstructure evolution of HEAs following deformation at different temperatures, which helps understand the plastic-deformation mechanisms of HEAs and opens practical routes for the improvement of mechanical properties.

Section snippets

Experimental procedures

The raw elemental materials with at least 99 (weight percent-wt%) purity were used. Repeated vacuum induction-melting for at least 5 times was carried out to improve the chemical homogeneity of the alloy. The solidified plate was HIP sintering at 1200 °C and 100 MPa for 4 h. Argon (Ar) gas was used to supply the high pressure. After the HIP sintering, the chamber temperature decreased from 1200 °C to 340 °C in 3 h, and then decreased to 190 °C in one hour. The prepared sample was cut into small pieces

Results

Fig. 1(a) displays the microstructure of the prepared Al0.1CoCrFeNi alloy. Typical dendrite and interdendrite structures were not observed in this material and it only exhibits a single-phase structure with a large grain size (>1 mm). EDS was performed on ten different regions. The average chemical compositions are listed in Table 1. The EDS results indicate that different elements are evenly distributed in the sample, which confirm a single-phase structure, and element concentrations are close

Effect of temperature on yield stress

Generally, the evolution of yield stress versus temperature can be divided into thermal and athermal parts [34]. The thermal part occurs at low temperatures when the thermal activation and atom diffusion is low and dislocation movement is limited by barriers. As the temperature rises, the dislocation motion is enhanced by thermal fluctuation. Therefore, the yield stress exhibits a significant decrease with increasing temperature in the thermal part region. The athermal part occurs when the

Conclusions

An Al0.1CoCrFeNi high-entropy alloy was fabricated by vacuum induction-melting and hot isostatic pressing (HIP) sintering. Only a single FCC phase was formed with a large grain size in the millimeter scale. Tensile experiments at four different temperatures, 25 and 500, 600, 700 °C, were performed. It was found that both yield strength and ultimate tensile strength decrease with increasing temperature, but the highest value of elongation to fracture is obtained at 500 °C. To interpret the

Acknowledgements

GY and PKL very much appreciate the financial support from the US National Science Foundation (DMR-0909037, CMMI-0900271, and CMMI-1100080). TY, ZT, and YZ thank the support of the Department of Energy (DOE). Office of Nuclear Energy's Nuclear Energy University Program (NEUP) 00119262. KAD and PKL appreciate the support from DOE, Office of Fossil Energy, National Energy Technology Laboratory (DE-FE-0008855 and DE-FE-0011194). Yugang Wang and Tengfei Yang thank the support of the National

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