Tailoring formation and proportion of strengthening phase in non-equiatomic CoCrFeNi high entropy alloy by alloying Si element
Graphical abstract
Introduction
High entropy alloys (HEAs) containing multiple principal elements have attracted great attention since a novel concept was proposed in 2004 by Cantor and Yeh [1,2]. HEAs break the traditional alloy design concept and show high performance, which makes them a potential candidate for structural material applications [3,4]. Therefore, a large number of HEAs with different component systems have been explored [[5], [6], [7], [8], [9]]. Originally, HEAs are defined as an alloy with an equal atomic ratio or near equal atomic ratio and the atomic ratio of an element is between 5 and 35 at%, which has the structure of single-phase solid solution, such as the face-centered cubic (FCC) structure, body-centered cubic (BCC) structure, and hexagonal (HCP) structure [[10], [11], [12]]. Co–Cr–Fe–Ni HEAs with FCC phase structure is one of the most widely studied alloys, which has high plasticity and excellent cryogenic temperature properties [[13], [14], [15]]. But, FCC phase structure alloys have a low strength of about 200 MPa at room temperature [16]. Therefore, the design of equal atomic ratio and the structure of single-phase solid solutions in HEAs limit the design range and performance requirements of structural materials.
Recently, a large number of HEAs of dual-phase and multi-phase structures with non-equiatomic composition have been developed [17,18]. This greatly expands the design space and further improves the properties of HEAs. The strengthening mechanisms in traditional metals, such as solution strengthening and precipitation strengthening have been applied to HEAs by changing the content of principal elements and microalloying strengthening elements [19,20]. The phase stability and properties of HEAs are strongly dependent on the atomic radius and type of elements. Fang et al. [21] found the non-equiatomic Co35Cr25Fe40-xNix (x = 0–15 at.%) HEAs present an excellent combination of strength and plasticity, which is attributed to the solution strengthening of Co and Cr elements. The increase of Cr content promotes the formation of brittle and hard intermetallic compounds [22]. The strength of non-equiatomic Co30Cr40Fe10Ni20 HEAs is obviously improved by solid solution strengthening and rich Cr precipitated phase strengthening [23]. At present, a large number of studies show that alloying non-metallic elements into HEAs can significantly improve structural properties. Li et al. [24] investigated the mechanical behavior of interstitial CoCrFeMnNi HEA by adding small atomic sized carbon. They found that the yield strength and tensile strength at carbon content of 0.8% are ∼1030 MPa and ∼1170 MPa, but the fracture strain is ∼11%, which is attributed to nano-sized carbide strengthening. The interstitial boron element has been introduced into CoCrFeNiMn HEA with a single-phase FCC structure by Jae Bok Seol et al. [25]. The strength is dramatically improved due to the enrichment of the B element in the grain boundaries. However, the interstitial atoms are separated at the grain boundary or form massive precipitate phases with the matrix elements, which seriously reduces the plasticity.
The solid solution phase is easy to form when the non-metallic Si is added in the FCC phase structure CoCrFeNi HEAs due to the smaller atomic-size difference with the matrix elements. The Si element and matrix elements exist in negative mixing enthalpy, implying that the atomic binding force between elements is stronger. Therefore, the Si element has different effects on the microstructures and properties of HEAs. Huang et al. [26] studied the phase evolution and mechanical properties of CoCrFeNiSix HEAs and found the micro-hardness increased from 89.52 HV to 653.71 HV with the addition of the Si element. Zuo et al. [27] found that the effect of adding Si on the mechanical properties of CoFeNiSix HEAs is more significant for new compounds. However, there are few studies on the alloying of Si in HEAs. The effect on microstructure evolution and the deformation mechanism of tensile properties has not been clarified. In this work, the non-metallic Si element is selected for addition in non-equiatomic Co30Cr30(FeNi)40 HEA with a single FCC phase structure. The microstructure, tensile properties, and deformation behavior of Co30Cr30(FeNi)40-xSix HEAs are systematically investigated.
Section snippets
Materials and methods
The pure metal Co, Cr, Fe, Ni and Si particles with purity higher than 99.9% (wt.%) were selected as raw materials to prepare Co30Cr30(FeNi)40-xSix HEAs. The ingots were prepared by arc-melting in a high-purity argon atmosphere. The mixed pure metal particles placed into a water-cooled copper crucible were melted. The ingots were turned over and re-melted five times to obtain a homogeneous structure. The crystal structures were characterized by X-ray diffraction (XRD) with Cu Kα radiation
Microstructure of Co30Cr30(FeNi)40-xSix HEAs
The XRD curves of Co30Cr30(FeNi)40-xSix HEAs by using the finer scanning speed are shown in Fig. 1. The results show that HSi2 presents a single FCC phase structure. With the increase of Si content, the rich Ni–Si phase is detected in HEAs and the diffraction peak of the second phase gradually increases. This indicates that the Si element can promote the precipitation of rich Si phases in Co–Cr–Fe–Ni HEAs. The (311) diffraction peak shifts to a higher 2θ with the increase of Si content,
Phase formation of Co30Cr30(FeNi)40-xSix HEAs
Previous studies have shown that the strengthening phase significantly improves the properties of HEAs [[29], [30], [31], [32]]. The phase formation of HEAs has been predicted by calculation and experiment [22,[33], [34], [35]]. Some physical parameters have been proposed to predict phase formation and structural stability in HEAs, such as atomic-size difference (δ), enthalpy of mixing (ΔHmix), entropy of mixing (ΔSmix), and the ratio of ΔSmix to ΔHmix (Ω) [36,37]. Guo et al. [38] investigated
Conclusions
In this paper, non-metallic Si has been added to non-equiatomic CoCrFeNi HEA to improve the strength of HEAs with single phase FCC. The microstructure evolution, tensile properties, and deformation mechanism of Co30Cr30(FeNi)40-xSix HEAs were investigated in detail. The main conclusions are summarized, as follows:
- (1)
The microstructures are FCC, rich Cr–Si, and rich Ni–Si phases in HSi2 and HSi4, but the rich Cr–Si phase disappears and the rich Ni–Si phase grows gradually with the increase of Si
Author contributions
Xuefeng Gao: Executor of the experiment, Writing - original draft.
Yao Chen: Investigation, Writing-review & editing.
Ruirun Chen: Project administration, Designer of the experiment, Investigation, Funding acquisition.
Tong Liu: Writing - review & editing, Formal analysis.
Hongze Fang: Editing, Investigation, Funding acquisition.
Gang Qin: Writing-review & editing.
Yanqing Su: Supervision, Investigation.
Jingjie Guo: Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No. 51825401), the Postdoctoral Foundation of Heilongjiang Province (LBH-Z19154) and National Natural Science Foundation of Heilongjiang Province (LH2020E031).
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