Effect of Al content on microstructure and mechanical properties of Alx(Nb3TaTi3Zr)100-x refractory high-entropy alloys

In this study, seven refractory high-entropy alloys of Al x (Nb3TaTi3Zr)100-x (x = 0, 5, 10, 15, 20, 25, 30) were synthesized by vacuum arc melting and characterized by various techniques, including XRD, SEM, EDS, compression tests at both 1200 °C and room temperature, as well as hardness tests. The analysis revealed that the alloys exhibit a single BCC structure when the Al content is between 0 and 15%, while the B2 phase appears at 20% and a mixed structure of BCC + B2 + σ phase at 30%. As the Al content increases, the hardness, stiffness, and room temperature yield strength of the alloy increases, while the plasticity decreases. Notably, the alloy with 25% Al content displayed the highest yield strength of 1400 MPa and Young’s modulus of 75.94 GPa at room temperature. Moreover, the hardness of the seven alloys increased from 247 HV to 490 HV with Al content from 0 to 30%. Furthermore, the alloy containing 30% Al exhibited a notable elevated-temperature yield strength of 143 MPa at 1200 °C.


Introduction
High-entropy alloys (HEAs) are unique alloys consisting of five or more elements, typically in equimolar or near-equimolar ratios, with each element's atomic fraction ranging from 5% to 35% [1][2][3] .These alloys derive their remarkable properties, including high strength, hardness, high-temperature softening resistance, corrosion resistance, wear resistance, and oxidation resistance, from the synergistic interaction of multiple principal elements [1][2][3][4][5][6][7][8][9] .Notably, the solid solution phases of the alloy are retained at high temperatures due to the unique delayed diffusion effect and high-entropy effect, resulting in a greater tendency to form simple single-phase solid solutions, such as those with body-centered cubic (BCC) structure, face-centered cubic (FCC) structure, or close-packed hexagonal (HCP) structure [1][2][3][4] .In particular, refractory high-entropy alloys (RHEAs) have shown exceptional performance at elevated temperatures, making them promising materials for lightweight superalloys in aerospace applications and other fields.
Recently, research on lightweight refractory high-entropy alloys has gained significant attention [4][5][6][7][8][9] .Stepanov et al. [7] developed a new lightweight high-entropy alloy, AlNbTiV, with remarkable high-temperature properties.This alloy features a single BCC phase structure and demonstrates outstanding mechanical characteristics, including a density of 5.59 g/cm3, high hardness measuring 448 HV, and a compressive strength of 685 MPa at 800℃ [7] .Subsequently, Stepanov et al. [8] examined how varying Al content influenced the microstructure and properties of Al x NbTiVZr alloys (x = 0, 0.5, 1, 1.5).The AlNbTiVZr alloy demonstrated a significant compressive yield strength of 1320 MPa; however, the ductility of Al x NbTiVZr lightweight high alloys decreased with increasing Al content, accompanied by the appearance of a dual-phase structure comprising BCC phases and Laves phases [7][8] .To design RHEAs with exceptional specific strength and reduced density, refractory elements can be combined with low-density elements.Based on this concept, a series of Al x (Nb 3 TaTi 3 Zr) 100-x refractory high-entropy alloys were designed (x = 0, 5, 10, 15, 20, 25, 30; hereafter denoted as Al 0 , Al 5 , Al 10 , Al 15 , Al 20 , Al 25 , Al 30 ).The objective of this study is to analyze how Al content influences the microstructure and mechanical properties of Al x (Nb 3 TaTi 3 Zr) 100-x HEAs and to provide insights for further research and application of the alloy system.

Experiments
In this study, seven different compositions of Al x (Nb 3 TaTi 3 Zr) 100-x (x = 0, 5, 10, 15, 20, 25, 30) alloys were synthesized using pure metal particles of Al, Nb, Ta, Ti, and Zr.Prior to melting, the raw materials were polished to remove surface oxides and cleaned ultrasonically in anhydrous ethanol before being weighed according to the required ratio with an electronic balance.The treated raw materials were melted in a vacuum arc melting furnace at a pressure of 3×10 -3 Pa for 7~9 cycles.Microstructure observation and composition analysis were conducted using a Phenom-XL scanning electron microscope.Physical phase analysis of the alloys was conducted using a Rigaku XRD-6100 X-ray diffractometer, utilizing a copper target and scanning angles ranging from 20° to 90°.The microhardness of the seven alloys was measured by an HV-1000A microhardness tester, applying a 1-kilogram load for 10 seconds.The room temperature compression testing was performed on an E45-105 electronic universal testing machine, and the compression deformation was 40 % at the strain rate of 1×10 -3 /s.The size of the compression specimens is Φ6 mm × 9 mm.High-temperature compression tests were performed on the Gleeble-3500 thermal simulation testing machine, and the size of the compression specimens is the same as Φ6 mm × 9 mm.The test procedure is illustrated in Figure 1.The temperature was elevated at a rate of 10 °C/s in an environment with a vacuum level of 1×10 -3 Pa.Once reaching 1200 °C, the temperature was maintained for 5 minutes.Subsequently, the compressed sample was deformed to 50% at a strain rate of 1×10 -3 /s and then rapidly quenched with water.These experimental measurements comprehensively evaluated the mechanical properties of the Al x (Nb 3 TaTi 3 Zr) 100-x alloys synthesized in this study.

Microstructure and phase structure of the alloys
Figure 2 presents the XRD patterns of seven cast refractory high-entropy alloys.Alloys containing less than 20% Al exhibit a single BCC structure, while the introduction of 20% Al gives rise to a second BCC structure phase called the B2 phase.When the Al content is 30%, the σ phase appears.Furthermore, the proportion of the B2 phase increases with higher Al content, indicating that the precipitation of this phase is promoted by the increase in Al content.However, the brittle σ phase appears in the Al 30 alloy, which adversely affects the properties of the alloy.The lattice constant of the alloy decreases as the Al content increases, as shown in Table 1.This decrease can be attributed to the interaction of p-d hybrid orbitals between Al and transition metal elements, resulting in the establishment of a significantly shorter covalent bond compared to the sum of the atomic radii of the corresponding elements.These strong polar bonds foster orderliness within the alloy system, leading to a reduction in bond length and consequently the lattice constant.Moreover, the microstrain of the alloys decreases with an increase in Al content, primarily due to the decrease in atomic radius difference within the alloy, resulting in reduced lattice distortion.
At present, there is a lack of a comprehensive theoretical framework to understand the phase formation mechanism and factors influencing the performance of high-entropy alloys.In response to this issue, several criteria have been introduced for identifying the formation of a solid solution phase in alloys, considering both thermodynamics and the Hume-Rothery criterion [10] .These factors include valence electron concentration (VEC), mixing enthalpy (ΔH mix ), mixing entropy (ΔS mix ), electronegativity difference (∆χ), thermodynamic parameter Ω, and atomic radius difference (δ) [10] .It has been found that when Ω ≥ 1.1 and δ ≤ 6.6, a high-entropy alloy typically develops a simple solid solution structure, and when VEC < 6.87, the HEAs typically develop a BCC solid solution [10] .According to this criterion, the seven alloys listed in Table 1 should form a BCC solid solution.However, the brittle σ phase appears in Al 30 alloy.This can be attributed to the increase in the absolute value of the mixing enthalpy with increasing Al content.Specifically, the stronger bond cooperation between Al and other alloying elements results in the formation of intermetallic compounds.In Figure 3, SEM images of Al x (Nb 3 TaTi 3 Zr) 100-x alloy ingots with seven different Al contents are displayed.All alloys exhibit a coarse dendrite morphology, indicating a broad solidification temperature range.Notably, increasing Al content leads to a refined dendrite morphology, while 15% Al content results in coarse grains and a coarsening dendrite morphology with the further increase of Al content.While the increase in Al content reduces the volume fraction of the dendrite region, leading to dendrite refinement, when the Al content is less than 15 %.This is due to the strong atomic binding ability resulting from the large absolute value of the ΔH mix between Al and other elements, which hinders diffusion of alloying elements during solidification, reduces the degree of segregation of the alloy, and impedes grain growth.However, for Al contents at or above 15%, dendrite coarsening is mainly due to the adhesion of adjacent secondary dendrites.The driving force of secondary dendrite arm bonding is mainly the decrease in the total solid-liquid interface area of the dendrite.Table 2 presents the elemental composition analysis of the dendritic and interdendritic regions in the alloys.The dendritic region is enriched with Ta and Nb elements, which have high melting points, meanwhile, the interdendritic region exhibits an enrichment of Ti, Al, and Zr elements, which have comparatively lower melting points.This segregation phenomenon occurs because the high melting-point elements solidify and crystallize first, leading to their enrichment in the dendritic region.Simultaneously, Al, Ti, and Zr are gradually repelled to the interdendritic region as the dendrites grow.The slow diffusion of Al, Ti, and Zr, attributed to the hysteresis diffusion effect of HEAs, resulting in the formation of a phase rich in these elements between dendrites, thus explaining the emergence of the B2 phase or σ phase.

Mechanical properties of seven alloys
Figure 4(a) illustrates the compression stress-strain curve of seven cast alloys at room temperature, while Table 3 provides a summary of the compression experiment and hardness test results.The alloy without any Al addition exhibits a room-temperature yield strength of 729 MPa, Young's modulus of 40.60 GPa, and a hardness of 247 HV.As the Al content increases, the solid solution strengthening effect becomes more pronounced, leading to an enhancement in yield strength, Young's modulus, and microhardness of the alloys at room temperature.This strengthening effect is due to the hindrance of the relative dislocation movement during the deformation process, resulting in dislocation plugging and increased alloy strength.
The negative ΔH mix between Al and elements such as Zr, Ti, Ta, and Nb indicates strong bond formation.With increasing Al content, the absolute value of the mixing enthalpy gradually increases, further reinforcing the strong bonding between Al and other alloying elements, consequently increasing the alloy's hardness.The alloy with 25% Al content exhibits the highest room-temperature yield strength and Young's modulus, measuring 1400 MPa and 75.94 GPa, respectively.However, increasing the Al content to 30% leads to a reduction in yield strength and Young's modulus, despite hardness reaching its peak at 490 HV.This decrease can be attributed to the segregation of the σ phase, which impedes dislocation movement, resulting in localized deformation and the initiation of cracks, thereby compromising both plasticity and yield strength.The plasticity of the alloy decreases with increasing Al content due to solid solution strengthening, enhancing yield strength while diminishing plasticity.The introduction of Al results in electronic hybridization with other main elements, thereby forming a strong polar covalent bond and promoting the formation of ordered alloy phases.As the Al content rises, the B2 phase and σ phase appear.The σ phase not only increases the hardness of the alloy but also renders it brittle, thereby reducing its plasticity.Moreover, the large magnitude of the ΔHmix between Al and the other main elements creates a strong binding force.This impedes the diffusion of atoms, culminating in the refinement of the alloy structure and disruption of the matrix continuity.Consequently, the grain refinement obstructs the movement of dislocations during deformation, resulting in enhanced strength and increased brittleness of the alloy.Overall, the interaction between Al and transition metals significantly influences the structural and mechanical properties of HEAs.
Figure 4(b) illustrates the compressive stress-strain curve of seven alloys at 1200 °C.The elevated-temperature yield strength of the alloy without Al element at 1200 °C is 62 MPa, which increases to 74 MPa with the addition of 5% Al.However, surpassing 5% Al content results in a decline in elevated-temperature yield strength, which is attributed to the lower melting point of Al.The theoretical melting point of the alloys diminishes as the Al content increases, dropping from 2164 °C for A l0 to 1713 °C for Al 30 .Consequently, the reduction in the melting point results in a decrease in the elevated-temperature yield strength of the alloys.Interestingly, when the Al content reaches 20%, the elevated-temperature yield strength increases, with the Al 30 alloy exhibiting a high-temperature yield strength of 143 MPa.Based on previous studies on crystal structure and microstructure, the precipitation of a second phase occurs once the Al content surpasses 20%, resulting in a higher occurrence of the σ phase in the Al 30 .This increased σ phase content likely accounts for the enhancement in high-temperature yield strength.Notably, the impact of the B2 and σ phases on the high-temperature strength of the alloy exceeds that of the decrease in the melting point.

Conclusion
As can be drawn from the experiments and analysis mentioned above, as the Al content in the alloy increases, a shift from a single BCC structure to a multiphase structure occurs.Specifically, an Al content of less than 15% corresponds to a single BCC structure, while the B2 phase appears in Al 20 .Al 30 exhibits a mixed structure composed of BCC+ B2 + σ phase.At room temperature, the Young's modulus, yield strength, and microhardness of the alloy all increase as the Al content increases.However, when Al content surpasses 30%, both Young's modulus and yield strength decrease.Notably, Al 25 exhibits the highest Young's modulus and yield strength, at 1400 MPa and 75.94 GPa, respectively.
At a temperature of 1200 °C, the high-temperature yield strength initially increases as the Al content increases, but then decreases before increasing again.The highest high-temperature yield strength of 143 MPa is observed at an Al content of 30%, followed by an Al content of 5% with a high-temperature yield strength of 74 MPa.This indicates that the yield strength of Al x (Nb 3 TaTi 3 Zr) 100-x alloys is relatively low at 1200 °C.To enhance the elevated-temperature yield strength of the alloy, the addition of a high melting-point Mo element could be considered.This would require further research to investigate and expand the potential applications of these alloys.

Figure 3 .
Figure 3. SEM images of the seven alloys.

Table 1 .
Calculation results of alloy parameters, lattice constants (a BCC ), and microstrain.

Table 2 .
The results of EDS energy spectrum point measurement of the seven alloys.