Investigation of phases in Al23Co15Cr23Cu8Fe15Ni16 and Al8Co17Cr17Cu8Fe17Ni33 high entropy alloys and comparison with equilibrium phases predicted by Thermo-Calc
Highlights
► High entropy alloys are investigated by three dimensional atom probe. ► The brittle Al23Co15Cr23Cu8Fe15Ni16 alloy decomposes into two body centred cubic phases. ► Ductile Al8Co17Cr17Cu8Fe17Ni33 shows two prominent face-centred cubic phases. ► Results are compared with equilibrium phases predicted by Thermo-Calc.
Introduction
High entropy alloys have been known as a new type of materials and have been defined as having five or more principal elements, each one having a concentration between 5 and 35 at.%. [1], [2], [3]. They are crystalline materials which predominantly may form simple solid solutions, mainly of face-centred cubic (fcc) or body-centred cubic (bcc) structure. The general absence of a single base element distinguishes them from alloys based on one principal element with a content of up to 80 wt.% or more [4], [5]. High entropy alloys promise an interesting combination of properties, such as oxidation resistance, thermal stability, high strength and soft magnetic behaviour. [3], [6], [7], [8], [9].
The most studied high entropy alloy is AlCoCrCuFeNi with equiatomic composition [4], [5], [10], [11]. This high entropy alloy solidifies dendritically within wide casting conditions. The dendrites mainly consist of two phases, an Al–Ni-rich and a Fe–Cr-rich phase, whereas interdendritic regions are enriched in Cu [4], [10]. It has also been found that the Fe–Cr-rich phase decomposes into Fe-rich and Cr-rich domains. A length scale between these domains of just a few nanometres indicates spinodal decomposition as the formation process [8], which is often observed in Fe–Cr systems [12], [13]. In addition, several types of Cu-rich precipitates, differentiated by their morphologies, have been observed within the Al–Ni matrix [10]. Furthermore, the microstructure of AlxCoCrCuFeNi is very sensitive to the Al level in the high entropy alloy. While alloys with an Al content less than x = 0.5 are composed of a simple solid solution with a fcc structure, a mixtures of fcc and bcc phases have been observed in the alloys with x ⩾ 0.8 and finally at x = 2.8 and higher a simple bcc ordered structures were obtained [5]. Mechanical properties of high entropy alloys are correlated to their microstructures. An increase of the Al content in the AlxCoCrCuFeNi alloy provokes multi phase microstructure which results in a hardness increase [3], [11]. However, alloys with high Al contents are more brittle [5], [11].
In order to optimize the behaviour of high entropy alloys at high-temperatures for structural and functional applications, the knowledge of the microstructure and phase composition of a material is essential for the understanding and prediction of its macroscopic mechanical properties. The first step of this work therefore was to investigate the microstructure of selected alloys on the micro and nano scale. The concept was to modify the composition of equiatomic AlCoCrCuFeNi alloy in order to reduce the number of phases in the alloy. Hence two alloys with different Al and Ni composition and reduced Cu content, Al23Co15Cr23Cu8Fe15Ni16 and Al8Co17Cr17Cu8Fe17Ni33, were selected. The work has focused on microstructure investigations. X-ray diffraction (XRD), transmission electron microscopy (TEM) and three-dimensional atom probe (3D-AP) were used for this study. The phase characterization using 3D-AP analysis is combined with thermodynamic calculations using Thermo-Calc [14]. The observed phases in as-cast alloys are compared with the equilibrium phases predicted by Thermo-Calc.
Section snippets
Experimental
Al23Co15Cr23Cu8Fe15Ni16 and Al8Co17Cr17Cu8Fe17Ni33 alloys were prepared in a vacuum induction furnace. The alloy constituents were of 99.99% purity. In the following, the Al23Co15Cr23Cu8Fe15Ni16 alloy will be called “the Al-rich alloy” and the Al8Co17Cr17Cu8Fe17Ni33 alloy will be called “the Al-poor alloy”. The ingots were re-melted at least three times to achieve a better homogenization. Specimens for XRD analysis and SEM observations were mechanically grinded and polished down to 50 nm using
Results
The investigated alloys have first been observed by light microscope, where both of them show very large elongated grains with dimensions up to 1 mm in length and 200 μm in width (not shown here). Fast cooling on the water cooled copper hearth in the arc furnace results in dendritic solidification of the alloys in all cases. The as-cast Al-rich alloy has an average micro-hardness of ∼580 HV and the as-cast Al-poor alloy has ∼280 HV.
Discussion
It is obvious that slight composition changes of AlCoCrCuFeNi high entropy alloys may have a strong influence on phase formation. Higher amounts of Al and Cr in the Al-rich alloy enhance the formation of bcc phases (Fig. 1) whereas a higher amount of Ni in the Al-poor Al8Co17Cr17Cu8Fe17Ni33 alloy enhances the formation of fcc phases (Fig. 6). The present results are in good agreement with observations reported previously [4], [5], [10].
The microstructure of the Al-rich alloy is similar to that
Summary
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The Al8Co17Cr17Cu8Fe17Ni33 and Al23Co15Cr23Cu8Fe15Ni16 high entropy alloys have been investigated by SEM, TEM, XRD and 3D atom probe.
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The brittle Al23Co15Cr23Cu8Fe15Ni16 alloy shows formation of three types of phases, namely an Al–Ni-rich matrix whose volume fraction as estimated from the 3D-AP analysis is ∼46%, Fe–Cr-rich cubes and parallelepipeds whose volume fraction is ∼43%, and Cu-rich precipitates (platelets at grain boundaries and spherical precipitates inside the Al–Ni-rich phase) whose
Acknowledgements
The authors are grateful to the German Research foundation (DFG) for the financial support by WA 1378/15-1 and GL 181/25-1.The authors would like to thank C. Förster for sample preparation and H. Kropf for help with FIB-tomography.
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