Influence of Hafnium Addition on the Microstructure, Microhardness and Corrosion Resistance of Ti20Ta20Nb20(ZrMo)20−xHfx (where x = 0, 5, 10, 15 and 20 at.%) High Entropy Alloys

Abstract

The presented work aimed to investigate the influence of the hafnium/(zirconium and molybdenum) ratio on the microstructure, microhardness and corrosion resistance of Ti20Ta20Nb20(ZrMo)20−xHfx (where x = 0, 5, 10, 15 and 20 at.%) high entropy alloys in an as-cast state produced from elemental powder and obtained via the vacuum arc melting technique. All studied alloys contained only biocompatible elements and were chosen based on the thermodynamical calculations of phase formation predictions after solidification. Thermodynamical calculations predicted the presence of multi-phase, body-centered cubic phases, which were confirmed using X-ray diffraction and scanning electron microscopy. Segregation of alloying elements was recorded using elemental distribution maps. A decrease in microhardness with an increase in hafnium content in the studied alloys was revealed (512–482 HV1). The electrochemical measurements showed that the studied alloys exhibited a high corrosion resistance in a simulated body fluid environment (breakdown potential 4.60–5.50 V vs. SCE).

Influence of Zirconium on the Microstructure, Selected Mechanical Properties, and Corrosion Resistance of Ti20Ta20Nb20(HfMo)20−xZrx High-Entropy Alloys

“…However, for the Mo_0 sample, this phase was Ti-, Nb-, and Zr-enriched. The comparison of the chemical composition of the BCC2 phases revealed higher concentrations of Ti and Hf for the Zr_0 sample, Zr and Nb for the Hf_0 sample, and Ti, Nb, and Zr for the Mo_0 sample [27,28]. The as-cast state of the studied high-entropy alloys contributed to the segregation of the alloying, similar to our previous results of different Hf and Mo content [27,28].…”

“…The phases denoted as BCC1 and BCC2 corresponded to the dendritic and interdendritic regions, respectively (Figure 6). It should be underlined that the microstructure presented in the current work is similar to the microstructure of high-entropy alloys with different Hf and Mo content, described in our published work [27,28]. Moreover, the recorded micrograph stays in high agreement with the XRD phase analysis, where the presence of diffraction of peaks corresponding to the dual-BCC phases was confirmed (earlier in the text and Figure 3).…”

“…However, a further decrease in this phase was revealed. The average phase contribution was also compared with the average phase contribution calculated for the literature-reported Hfcontaining HEAs (Figure 7) [28]. According to the results above, it can be confirmed that the average contribution of the BCC2 phase increased up to 15% of the Zr concentration from 23(5) to 56(10) %.…”

“…However, a further decrease in this phase was revealed. The average phase contribution was also compared with the average phase contribution calculated for the literature-reported Hf-containing HEAs (Figure 7) [28]. The results reveal that the average phase contribution of BCC1 for Zr_0, Zr_10, Zr_15, and Zr_20 was smaller than that of the Hf-containing samples with similar concentrations [28].…”

“…The average phase contribution was also compared with the average phase contribution calculated for the literature-reported Hf-containing HEAs (Figure 7) [28]. The results reveal that the average phase contribution of BCC1 for Zr_0, Zr_10, Zr_15, and Zr_20 was smaller than that of the Hf-containing samples with similar concentrations [28]. On the other hand, only for the Zr_5 sample was the increase in the contribution of the BCC1 phase measured compared with the BCC1 phase for the Hf_5 sample [28].…”