High-entropy alloys (HEAs) introduce a new concept of developing advanced metallic materials with properties that conventional alloys, based on one principal element, cannot achieve [1]. Other multi-component metallic systems are quasicrystals (QCs), complex metallic alloys (CMAs) and bulk metallic glasses (BMGs).

HEAs are multicomponent mixtures of 4 to 9, and occasionally up to 20 chemical elements, in similar concentrations, ranging from 5 to 35 at.% each, where the high entropy of mixing can stabilize disordered solid-solution phases with simple crystal structures like a body-centered cubic (bcc), a face-centered cubic (fcc) and a hexagonal close-packed (hcp), in competition with ordered intermetallic phases and phase-segregated mixtures [2]. Though the average crystal structure of a HEA is generally simple, microstructure might be highly complex, as will be shown later in this presentation.

In our research, the HEA series CoCrFeNiZr_x (x = 0.40, 0.45, and 0.50) has been investigated. Various SEM techniques were employed, including backscattered-electron (BSE) imaging, EDS point analysis, chemical mapping and EBSD.

The results reveal high complexity of the HEAs’ microstructure and helped us to determine the grains’ composition. Brief outline is presented in the four figures shown below. Figs. 1 and 2 show BSE SEM images. The coarsest interpretation is that the samples are composed of two different types of microstructures: (1) rounded dark phases and (2) fine interweaving of the light and dark phases. Higher magnification of the image in Fig. 2 shows the “invisible borders”, where the fine microstructure changes its character. EBSD investigations at lower magnification (2.000x) revealed that the “invisible borders” correspond to grain boundaries of larger grains (see Fig. 3 – showing different area than Figs. 1 and 2.), whereas more detailed EBSD results at higher magnification of 10.000x (Fig. 4) explained the fine grain structure inside the main grains. Therefore, grains’ sizes at two different scales are present in these samples. The effect of Zr content on the HEA microstructure was thus traceable in great detail.

[1] Y. Zhang, et al., Prog. Mater. Sci. 61, 1-93 (2014).

[2] P. Koželj, S. Vrtnik, A. Jelen, S. Jazbec, Z. Jagličić, S. Maiti, M. Feuerbacher, W. Steurer, and J. Dolinšek, Phys. Rev. Lett. 113, 107001 (2014).

Figures:

Fig. 1: BSE image of the HEA at 2.000x.

Fig. 2: BSE image of the HEA at 10.000x (magnified yellow box of the Fig. 1).

Fig. 3: Grains of the HEA at 2.000x by EBSD.

Fig. 4: Grains of HEA at 10.000x by EBSD (magnified white box of the Fig. 3).

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EMC Abstracts - https://emc-proceedings.com/abstract/correlative-sem-techniques-for-resolving-complex-microstructure-of-cocrfenizrx-high-entropy-alloys/