Effect of annealing heat treatment on microstructural evolution and tensile behavior of Al0.5CoCrFeMnNi high-entropy alloy
Introduction
High-entropy alloys (HEAs) are defined as multi-component alloys of which most contain five or more principal elements in equiatomic or near-equiatomic composition with high configurational entropy. In contrast, most conventional (low-entropy) metallic alloys are composed of one or two principal elements and some minor elements [1], [2]. The high configurational entropy of HEAs promotes the formation of simple single-phase crystalline materials of face-centered cubic (FCC) [3], body-centered cubic (BCC) [4], [5], or sometimes hexagonal close-packed (HCP) structures [6]. These multicomponent alloys have been attracting considerable attention all over the world [2], [3], [4], [5], [6]. This is because many HEAs reportedly exhibit good mechanical properties, excellent corrosion resistance, high thermal stability, and resistance to fatigue [7], [8], [9], [10].
In general, the FCC-HEAs are ductile and soft, while the BCC-HEAs have high strength but relatively low tensile ductility [11]. From this point of view, the FCC-HEAs need improved yield strength and the BCC-HEAs need more plasticity. Therefore, there have been several attempts to enhance the mechanical properties of HEAs using secondary phases to reach a reasonable balance between strength and ductility to broaden the applications for HEAs [12], [13], [14], [15].
Recent studies showed that addition of Al to FCC-HEAs could have strong effects on the formation of secondary phases resulting in the variation of mechanical properties [16], [17]. He et al. [18] studied the alloying effect of Al on the structure and tensile properties with respect to an FCC CoCrFeMnNi HEA. They found that the crystalline structure changed from the initial single FCC structure to an FCC + BCC structure in the range 8–11 at% of Al. This dual phase alloy showed composite behavior with sharp increases in strength and hardness due to the hard BCC phase. However, a certain problem arose as the Al content was increased in the alloy: a large fraction of disordered and ordered (A2 and B2, respectively) BCC phases could lead to poor ductility. Moon et al. [19] investigated cracking phenomenon in Al0.5CoCrFeMnNi HEA during cold-rolling. They reported, from the results of microstructural analysis and thermodynamic calculation, that the cracking behavior was induced by the formation of an AlNi-rich B2 phase. They proposed that homogenization heat treatment at high temperature might minimize the poor workability of this alloy by reducing the B2 phase which is known to be brittle in the ingot [11]. However, no studies on the microstructural and mechanical properties of the Al0.5CoCrFeMnNi alloy after annealing heat treatment and recrystallization have been reported.
In this study, we explored the microstructural evolution of the Al0.5CoCrFeMnNi HEA after cold rolling followed by annealing heat treatment. The characterization of tensile behaviors of the annealed alloy was also investigated in parallel. In addition, the strengthening behavior associated with the microstructure was described using a simple composite model to quantify the effect of the annealing heat treatment in the present alloys.
Section snippets
Sample preparation
Al0.5CoCrFeMnNi HEA ingots were fabricated using vacuum induction melting (VIM) of the pure elements (purity above 99.9%). The homogenization heat treatment for the cast ingots was carried out at 1200 °C for 6 h to reduce the inhomogeneity of the chemical compositions. This heat treatment also contributed to dissolving the secondary phase and improving their workability for cold rolling in accordance with recent publication [19]. After the homogenization treatment, the ingot was cold rolled for
Phase evolution behaviors of the Al0.5CoCrFeMnNi HEAs after annealing
Fig. 1(a) indicates the XRD patterns for the samples, which reveal the presence of two phases designated as FCC and BCC in the H10 and H11 alloys, while the H12 alloy indicates a single FCC crystal structure. The measured volume fraction of the BCC phase (shown in Fig. 1b) was compared with the volume fraction calculated using the thermodynamic method [19]. In the H10 and H11 alloys, the volume fraction of the BCC phase measured, was ~10.56 and ~4.07%, respectively.
Fig. 2 shows the SEM-BSE
Conclusions
In this study, an Al0.5CoCrFeMnNi HEA, after cold rolling and subsequent annealing heat treatment at different temperatures, was investigated to explore the effects of microstructural evolution on tensile deformation behavior at RT.
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After the annealing heat treatment, the secondary B2 phase was clearly observed in the H10 and H11 alloys, while no secondary phase was observed in the H12 alloy. The lower annealing temperature caused a greater B2 phase fraction and smaller grain size of FCC phase.
Acknowledgement
This work was supported by the Future Material Discovery Project of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea (NRF-2016M3D1A1023383).
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