2023 Volume 64 Issue 10 Pages 2523-2529
This work explores the effect of the Hf concentration on the constituent phase, microstructure, and room temperature mechanical properties of TiVZrNbHfx (x = 0–1.0) light weight refractory high entropy alloys prepared by the arc melting technique. The results show that the TiVZrNbHfx alloys possess a single disordered bcc phase and dendritic structure, with densities ranging from 6.47 ± 0.03 to 8.69 ± 0.02 g/cm3. Elemental scan mapping reveals a relatively homogeneous distribution of all elements, although local concentrations of elements can be observed in the microstructures. The addition of Hf enhances the Vickers hardness, ultimate strength, and the 0.2% proof stress, which could be attributed to solid-solution-like strengthening mechanism. The TiVZrNbHf alloy exhibits the highest values of Vickers hardness (449.0 ± 11.3 HV1), ultimate strength (1682.7 MPa), and the 0.2% proof stress (1291.8 MPa). However, the fracture strain decreases with the increase in Hf composition, from 39.9% (for TiVZrNb) to 18.1% (for TiVZrNbHf).
Refractory high entropy alloys (RHEAs) have recently attracted significant attention due to their outstanding mechanical properties at high temperatures and microstructure stabilization.1–4) RHEAs are composed of high melting point elements, such as W, Ta, Mo, Hf, Nb, Re, and Os.1,5) The first two equimolar RHEAs including WNbMoTa and WNbMoTaV, have been developed with remarkable yield strength exceeding 400 MPa at 1600°C.1,6) However, the high densities of these alloys, 12.36 g/cm3 and 13.75 g/cm3 for WNbMoTa and WNbMoTaV, respectively, limit their use in various engineering applications.5,7–11) To address this challenge, researchers have explored various alternative lighter weight metals and lower density refractory elements to replace the heavy metals, W and Ta, while maintaining the high-temperature strength.5) The class of light-weight RHEAs was then developed in a large number of researches with various proposed compositions.12–14) Previous research reports that the densities of obtained light-weight RHEAs are in the range of 4.75 to 8.861 g/cm3 for single-phase, from 5.05 to 7.785 g/cm3 for dual-phase, and from 4.93 to 8.6 g/cm3 for multi-phase RHEAs.14) Due to lower density and remaining good properties at high temperature, light-weight RHEAs have been broaden the researches for variety applications such as in high temperature, irradiation resistance, oxidation resistance and superconductivity materials, etc.14–17)
Recently, ligh-weight RHEAs has been received more attention in the field of hydrogen storage energy.12,18–21) Some reports reveal that the Hf contained light-weight RHEAs exhibit excellent hydrogen absorption properties with hydrogen absorbed capacity H/M ratio closed to 2.0,12,19,20) even reached the value of 2.5 for the equimolar TiVZrNbHf RHEA alloy,22) which show a good candidate of these RHEAs for energy applications. Many works have been carried out to explore the structure, synthesis process, hydrogen storage properties, thermal stability of these alloys.20,22–24) However, few researches have been done to investigate the mechanical behaviors of these alloys, which, indeed, have an important role for their technical applications. Therefore, this work was carried out to study the mechanical properties alloys at room temperature of TiVZrNbHfx light-weight RHEAs produced by vacuum arc melting route. In this work, the effect of Hf concentration on the phase component, microstructure and mechanical properties in terms of Vickers hardness and compression strength of TiVZrNbHfx light-weight RHEAs was investigated.
High-purity elemental metals (Ti, V, Zr, Nb and Hf) with a minimum purity of 99.8% were used to prepare TiVZrNbHfx (x = 0, 0.25, 0.50, 0.75, and 1.0, denoted to TiVZrNb, TiVZrNbHf0.25, TiVZrNbHf0.5, TiVZrNbHf0.75 and TiVZrNbHf) alloys using a laboratory electric arc melting furnace. Prior to the arc melting process, the chamber was evacuated and refilled with high-purity Ar gas (99.99%) several times, and the high-purity Ti getter was melted beforehand to eliminate residual oxygen in the chamber. The arc melting was conducted in a water-cooled copper cavity under a high-purity Ar atmosphere. Each alloy underwent six re-melting cycles and was flipped for each melt to ensure homogenization.
The phase formation of the as-cast samples was studied using an X-ray diffractometer (XRD, Kα-Cu: 1.5406 Å, D8 ADVANCE Brucker), while the microstructure was observed using a field-emission scanning electron microscope (FE-SEM, Hitachi S-4800). For microstructure observation, the samples were prepared by grinding with SiC paper, polishing with a colloidal diamond suspension, and finally etching with a solution of HF, HNO3, and H2O with a volume ratio of 1:3:7. The element distribution and chemical composition of the unetched samples were observed using the elemental scan mapping mode on the scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDS, Jeol JSM 6490). The density of the alloys was measured following Archimedes’ principle using a density determination kit (AND GR 202). Each alloy was measured ten times, and the average value was recorded. The room temperature mechanical properties were characterized in terms of Vickers hardness measurement and compression test. The hardness of the samples was measured using a Vickers hardness tester (Mitutoyo AVK-C0) at a load of 1 kgf (HV1) with a dwelling time of 15 s, and the average of ten indents was recorded for each composition. For the compression test, the samples were sectioned into cylindrical pellets with a diameter of 4.0 mm and a height of 6 mm. The room temperature compression tests were performed at a constant ram speed corresponding to an initial strain rate of 10−4 s−1 on the AGX-50kNV testing machine (Shimadzu).
Figure 1(a) shows the XRD patterns of as-cast TiVZrNbHfx high-entropy alloys. In the case of TiVZrNb alloy, the main diffraction peaks appeared at 2 theta positions of approximately 38.37°, 55.38°, 69.38°, and 82.17°, corresponding to the 110, 200, 211, and 220 diffraction peaks, respectively, of a single-disordered bcc crystal structure. This finding is in agreement with previous research.25–27) The lattice parameter (a) was calculated by the Rietveld method using the Fullprof software and found to be 3.315 Å, which is within the range of previous reports.7,12,27) The XRD patterns also showed that the single-disordered bcc crystal structure was maintained with the addition of Hf. No diffraction peaks of intermetallic phases were detected. However, a slight shift in diffraction peaks toward lower 2θ angles was observed with increasing Hf content (as shown in the inset in Fig. 1(a)), indicating a change in the lattice of the bcc structure due to the addition of Hf. Figure 1(b) plotted the calculated lattice parameters for all alloys. The lattice parameter increased with the concentration of Hf, indicating an expansion of the unit cell volume, particularly with the x value of 1.0. The increase in lattice parameter with Hf content may be due to the atomic size of the Hf atom (1.58 Å), which has the second-largest atomic radius in TiVZrNbHfx RHEAs, second only to that of Zr atom.28)
(a) XRD patterns and (b) the calculated lattice parameter of as-cast TiVZrNbHfx RHEAs.
Table 1 presents the calculated values for the valence electron concentration (VEC), atomic size mismatch (δr), mixing enthalpy (ΔHmix), mixing entropy (ΔSmix), and parameter Ω for all designed alloys, which were obtained using the following equations:
| \begin{equation} \mathit{VEC} = \varSigma c_{\text{i}}\mathit{VEC}_{\text{i}} \end{equation} | (1) |
| \begin{equation} \varDelta S_{\text{mix}} = -Rc_{\text{i}}\,\mathit{ln}\,c_{\text{i}} \end{equation} | (2) |
| \begin{equation} \varDelta H_{\text{mix}} = \varSigma 4\omega_{\text{ij}}c_{\text{i}}c_{\text{j}} \end{equation} | (3) |
| \begin{equation} \varOmega = T_{\text{m}}\varDelta S_{\text{mix}}/|\varDelta H_{\text{mix}}| \end{equation} | (4) |
| \begin{equation} \delta_{\text{r}} = 100\%[\varSigma c_{\text{i}}/(1-r_{\text{i}}/\bar{r})^{2}]^{1/2} \end{equation} | (5) |
These criteria are commonly used to predict phase formation in high entropy alloys or intermetallic compounds.30–36) Zhang and Yang found that a solid solution would form if the atomic size mismatch is less than 6.6%, the calculated values of ΔHmix are in the range of −20 to 5 kJ/mol, and Ω is larger than 1.1.31,34) Additionally, Guo et al. reported that the bcc structure would be stable if VEC values are lower than 6.87.37) Table 2 shows that all calculated values, including VEC, δr, ΔSmix, ΔHmix, and Ω, are suitable for the formation of a bcc structure in TiNbZrVHfx RHEAs with x value in the range of 0 to 1.0. This indicates that the arc melting technique produced a single bcc phase of TiVZrNbHfx RHEAs, following the above rules.
Figure 2 shows SEM images of as-cast TiVZrNbHfx samples after polishing and etching using a solution of HF, HNO3, and H2O in a volume ratio of 1:3:7. Uniform dendritic structures are observed for all specimens. Figure 3 presents elemental mapping images of TiVZrNb, TiVZrNbHf0.5 and TiVZrNbHf RHEA specimens (without etching). Generally, the images show a relatively good distribution of metal elements throughout the microstructure of the RHEA samples. However, local concentrations of individual elements are also observed in all the microstructures of the as-cast samples, demonstrating the formation of the highly disordered bcc structure and the localization of each element in the microstructure of the RHEA samples. The images also show a greater appearance of Hf elements in the microstructure with an increase in its amount in the RHEA composition.
SEM images of the TiVZrNbHfx samples etching with HF, HNO3 and H2O solution.
Elemental mapping images of the TiVZrNbH, TiVZrNbHf0.5 and TiVZrNbHf RHEAs.
The SEM-EDX analysis results on the samples without etching reveal the composition of the as-cast samples, as shown in Table 2. Some deviation in the chemical composition of the designed alloys and the experimental results is observed, which could be attributed to the localization of elements in the microstructure and the sensitivity of the detector. However, it is evident that all the alloy compositions are relatively close to the designed alloys.
3.2 The densities and room temperature (RT) mechanical properties of as-cast RHEAsThe measured density (ρexp.) of the as-cast samples is shown in Fig. 4(a). The density of the TiVZrNbHfx RHEAs gradually increases from 6.47 ± 0.03 to 8.69 ± 0.02 g/cm3 with the increase of Hf concentration, due to the higher density of Hf metal compared to the other components. Figure 4(a) also displays the densities of the disordered solid solution for the designed HEAs, calculated from the densities of the constituent elements and the rule of mixture, using the following eq. (1) for disordered solid solutions.7)
| \begin{equation} \rho_{\text{mix}} = \varSigma A_{\text{i}}c_{\text{i}}/(\varSigma A_{\text{i}}c_{\text{i}}/\rho_{\text{i}}) \end{equation} | (6) |
(a) Measured densities and density of mixing calculation, and (b) Vickers harness of as-cast TiVZrNbHfx RHEAs.
Figure 4(b) presents the measured Vickers hardness of the as-cast specimens which show the hardness increase with the concentration of Hf. Specifically, the measured hardness of the as-cast RHEAs specimen’s increases gradually from 350.5 ± 7.6 to 449 ± 11.3 HV1 as the Hf concentration increases from 0 to the x value of 1.0. With the x value of 1.0, the Vickers hardness of TiVZrNbHf alloys is about 28% higher than the hardness of TiVZrNb alloy. The increase of hardness with the addition of Hf can be attributed to a solid-solution-like strengthening mechanism resulting from elastic interactions among the local stress fields of solute atoms and dislocations which is widely accepted for multi-component alloys.26,32,38,39)
Figure 5 shows the typical compressive stress-strain curves of the RHEAs samples, while Fig. 6 presents the ultimate strength (σU), 0.2% proof stress (σ0.2) and fracture strain (εf) obtained from the compression test. The ultimate strength and yield strength of the TiVZrNb alloys were 1239.1 and 1021.3 MPa, respectively. Both ultimate strength and 0.2% proof stress increased with the addition of Hf, with the highest values of 1682.7 MPa and 1291.8 MPa, respectively, obtained for the TiVZrNbHf alloy. However, the ductility showed a decreasing trend with increasing Hf concentration. For the TiVZrNb alloy, the fracture strain reached a maximum value of approximately 39.9%, after which it gradually decreased with increasing Hf content and dropped to 18.1% for the equimolar TiVZrNbHf composition. The reduction of compressive ductility could be resulted from the increase of the strength which hinders the plastic deformation.28,34) Table 3 listed the 0.2% proof stress, ultimate strength and the fracture strain at room temperature of several reported RHEAs and the TiVZrNbHfx alloys fabricated in this study. It show that the mechanical properties of light-weight RHEAs in this work are in the mid-range in comparison to those of other current RHEAs indicating that these TiVZrNbHfx alloys could be potential candidate for technical applications.
Compression stress-strain curves of as-cast TiVZrNbHfx RHEA specimens.
The 0.2% proof stress, ultimate strength and fracture strain of as-cast TiVZrNbHfx RHEAs specimens.
Based on the investigations carried out in this work, it can be concluded that the addition of Hf in TiVZrNbHfx (x = 0–1.0) refractory high entropy alloys produced by arc-melting method leads to a reduction in lattice parameter and an increase in density towards higher Hf contents, without altering the single bcc phase with dendrite structure. The room temperature mechanical tests showed that the Vickers hardness, ultimate strength, and the 0.2% proof stress of the alloys increased with increasing Hf composition due to the solid-solution-like strengthening mechanism. The TiVZrNbHf alloy obtained the remarkable Vickers hardness, ultimate strength, and the 0.2% proof stress values of 449 ± 11.3 HV1, 1682.7 MPa, and 1291.8 MPa, respectively. However, the compressive ductility decreased with increasing Hf content, from 39.9% for the alloy without Hf to 18.1% for the equimolar TiVZRNbHf alloy. The reduction of compressive ductility could be attributed to the enhancement of strength which hinders the plastic deformation of the alloys.
The authors gratefully acknowledge to the Vietnam Academy of Science and Technology for the financial support of this research under the projects: No. VAST03.04/21-22 and QTSK.01.02/20-21.
