Elsevier

Materials Science and Engineering: A

Volume 713, 24 January 2018, Pages 134-140
Materials Science and Engineering: A

Precipitation and its strengthening of Cu-rich phase in CrMnFeCoNiCux high-entropy alloys

https://doi.org/10.1016/j.msea.2017.12.060Get rights and content

Abstract

The effects of Cu addition on the phase change and mechanical properties of CrMnFeCoNiCux high-entropy alloy were investigated. The results indicated that there was an evolution of phase structure from fcc1 to fcc2 with the increase of Cu content. The fcc2 was identified to be (Cu, Mn)-rich phase by scanning electron microscopy and transmission electron microscopy. In the alloys with high amount of Cu, Cu-depleted dendrites and (Cu, Mn)-rich inter-dendrites was observed, due to the positive mixing enthalpy between Cu and other elements in the alloy. The yield strength and microhardness increased with increasing Cu content. The yield strength increased from 188.04 MPa for Cu0 to 350.63 MPa for Cu1 alloy. The microhardness increased from 165.35 Hv to 215.84 Hv. The high strength of CrMnFeCoNiCu alloy was attributed to the uniformly dispersed nano-size Cu-rich precipitates in the matrix, which impeded the dislocation motion during alloy deformation. The Cu-added CrMnFeCoNiCux alloys also showed excellent strain hardening ability during compression test.

Introduction

Since the concept of high-entropy alloys (HEAs) was put forward, it had attracted considerable research interests, because of their interesting and unexpected microstructure and properties [1], [2], [3], [4], [5]. For example, though Co (hcp), Cr (bcc), Fe (bcc) and Ni (fcc) had different crystal structures, the CoCrFeNi HEA formed a simple fcc phase structure and exhibited outstanding ductility and fracture toughness at room temperature [5]. Similar to the conventional alloys, the crystal structure, microstructure and mechanical properties has a strong relation with the compositions of the HEAs. Considering that the strength of HEAs depended mainly on the solid-solution hardening, elements with large atomic radius difference were usually added into the HEAs. On the other hand, precipitation or formation of hard phases was found to be more effective to increase the strength. For instance, addition of Al in the CoCrCuFeNiAlx [6] and CoCrFeMnNiAlx HEAs [7] could significantly increase the hardness and strength of the alloys, due to the formation of Ni-Al-rich phases in the ductile matrix. However, a significant lost in ductility of the alloy was observed if the content of Al exceeded a critical value. Therefore, development of a way to improve the strength while keeping the good ductility is important for the potential engineering applications of HEAs.

Copper has been used as a precipitation-hardening element in steels due to its low solubility Fe-base alloys. For example, super304H austenitic heat resistance steel, which is based on 304 stainless steel and alloyed mainly with about 3 wt% Cu, exhibits outstanding properties at elevated temperatures, mainly attributing to the precipitation strengthening effect of nanoscale Cu-rich phase in the austenitic matrix [8], [9], [10]. Recent microstructure researches in HEAs also showed that Cu element has a tendency to promote the liquid-phase separation before solidification and to form a Cu-rich supersaturated solid solution with nano-size precipitates. It has been reported [11] that additions of Cu increased the strength in the CoCrFeMoNiCux HEAs, for the reasons that Cu element has large positive mixing enthalpy and weak bonds with many other metal elements [12], [13], [14], [15], making it readily segregate to inter-dendrite region and subsequently form Cu-rich precipitation phases.

Following the idea of Cu-rich phase precipitation in the increasing strength of alloy but without the loss of ductility, in this paper, the CrMnFeCoNi HEA [6], [7], [16], a typical and widely investigated HEA, was selected as the baseline alloy and Cu was the incorporation element, in order to achieve the optimal strength and ductility combination. The precipitation behavior of nano-size Cu-rich phase, microstructure evolution, phase constitution and mechanical properties of the alloys have been studied. Moreover, mixing enthalpy has been calculated to investigate the phase formation in CoCrFeMoNiCux alloys. The results indicated that Cu was a promising element for improving the strength and ductility simultaneously in ductile CrMnFeCoNiCux HEA.

Section snippets

Experimental

The six-component CrMnFeCoNiCux (x = 0, 0.25, 0.5, 0.75 and 1, respectively, denoted as Cux hereafter) ingots were prepared by vacuum arc-melting with pure elements (> 99.9%) under argon atmosphere on a water-cooled copper mold, and subsequently furnace-cooled. Ingots with a weight of 30 g were remelted at least five times to ensure chemical homogeneity. Differential scanning calorimetry (DSC) was used to study the phase transition during heating in Ar atmosphere from 25 °C to 1400 °C. The crystal

Crystal structure and phase constituent

Fig. 1 showed the XRD diffraction patterns of the as-cast CrMnFeCoNiCux alloys. Cu0 alloy had a fcc crystal structure and the lattice constant was estimated to be 0.3604 nm, which was similar to the result reported by Cantor [16]. With increasing the content of Cu, some diffraction peaks appeared around at the angle of 43° and 51°, suggesting the formation of another phase with a fcc structure (denoted as fcc2 hereafter). The relative intensity of the fcc1 diffraction peaks reduced while that of

Conclusions

The effects of Cu on the evolution of phase structure, microstructure, and mechanical properties of high-entropy CrMnFeCoNiCux (x = 0, 0.25, 0.5, 0.75 and 1) alloys were investigated in this paper. The following conclusions could be drawn:

  • (1)

    Addition of Cu caused the formation of fcc2 phase in the CrMnFeCoNiCux alloys, forming a dual phase structure (fcc1 and fcc2 phases). With the increase of Cu content, the volume fraction of fcc1 phase decreased, while that of fcc2 increased.

  • (2)

    The high positive

Acknowledgement

The authors gratefully acknowledge the financial support from the National Magnetic Confinement Fusion Science Program of China (No. 2015GB121003) and the National Natural Science Foundation of China (No. 51401071).

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1

X. Xian and L.J. Lin contributed equally to this work.

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