Effect of annealing heat treatment on microstructural evolution and tensile behavior of Al0.5CoCrFeMnNi high-entropy alloy

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Abstract

In this work, the mechanical characteristics and microstructural evolution of Al0.5CoCrFeMnNi high-entropy alloy (HEA) were studied after annealing at various temperatures (1000, 1100, and 1200 °C). X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy analyses were performed to reveal the phase and microstructural variations. The mechanical properties related to different microstructures of the alloy were characterized using tensile testing with digital image correlation. Annealing at lower temperatures led to a higher fraction of B2 phase and finer grain size of FCC (face-centered cubic) phase. A good combination of strength and ductility in this alloy was attributed to the ductile FCC matrix and hard secondary B2 phase. The alloy showed the active evolution of deformation twinning due to the low stacking fault energy when Al was added to CoCrFeMnNi to make the HEA. However, for alloy annealed at lower temperatures, twinning activity was suppressed by the smaller size of grains and depletion of Al content in the FCC matrix. The correlation between the microstructure and mechanical properties was also explored using a simple composite model.

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.

  • (1)

    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).

References (55)

  • J. Moon et al.

    Effects of homogenization temperature on cracking during cold-rolling of Al0.5CoCrFeMnNi high-entropy alloy

    Mater. Chem. Phys.

    (2018)
  • C. Ng et al.

    Entropy-driven phase stability and slow diffusion kinetics in an Al0.5CoCrCuFeNi

    Intermetallics

    (2012)
  • A.K. De et al.

    Quantitative measurement of deformation-induced martensite in 304 stainless steel by X-ray diffraction

    Scr. Mater.

    (2004)
  • M. Herbig et al.

    Crain boundary segregation in Fe-Mn-C twinning-induced plasticity steels studied by correlative electron backscatter diffraction and atom probe tomography

    Acta Mater.

    (2015)
  • Q. Li et al.

    On the calculation of annealing twin density

    Scr. Mater.

    (2006)
  • P.P. Bhattacharjee et al.

    Microstructure and texture evolution during annealing of equiatomic CoCrFeMnNi high-entropy alloy

    J. Alloy. Compd.

    (2014)
  • G. Grewal et al.

    Modeling matrix grain growth in the presence of growing second phase particles in two phase alloys

    Acta Mater.

    (1990)
  • J.C. Rao et al.

    Secondary phases in AlxCoCrFeNi high-entropy alloys: an in-situ TEM heating study and thermodynamic appraisal

    Acta Mater.

    (2017)
  • T. Yang et al.

    Effects of Al addition on microstructure and mechanical properties of AlxCoCrFeNi high-entropy alloy

    Mater. Sci. Eng. A

    (2015)
  • T.S. Reddy et al.

    Severe plastic deformation driven nanostructure and phase evolution in a Al0.5CoCrFeMnNi dual phase high entropy alloy

    Intermetallics

    (2017)
  • D. Broek

    The role of inclusions in ductile fracture and fracture toughness

    Eng. Fract. Mech.

    (1973)
  • Y.H. Jo et al.

    Role of brittle sigma phase in cryogenic-temperature-strength improvement of non-equi-atomic Fe-rich VCrMnFeCoNi high entropy alloys

    Mater. Sci. Eng. A

    (2018)
  • G. Laplanche et al.

    Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi

    Acta Mater.

    (2017)
  • N. Kumar et al.

    High strain-rate compressive deformation behavior of the Al0.1CrFeCoNi high entropy alloy

    Mater. Des.

    (2015)
  • J.W. Bae et al.

    Trade-off between tensile property and formability by partial recrystallization of CoCrFeMnNi high-entropy alloy

    Mater. Sci. Eng. A

    (2017)
  • M.A. Meyers et al.

    The onset of twinning in metals: a constitutive description

    Acta Mater.

    (2001)
  • I.V. Kireeva et al.

    Orientation dependence of twinning in single crystalline CoCrFeMnNi high-entropy alloy

    Mater. Sci. Eng. A

    (2017)
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