Effect of Al Addition on Microstructure and Creep Properties of TiZrNbTaMo Refractory High-Entropy Alloys
Release time:
2025-10-28
Amid surging demand for ultra-high-temperature structural materials in aerospace, advanced energy, and other fields, refractory high-entropy alloys have emerged as ideal candidates to replace conventional high-temperature alloys due to their outstanding high-temperature strength, thermal fatigue resistance, and oxidation resistance [1]. However, their creep behavior during service severely limits engineering applications. Therefore, elucidating the creep deformation mechanism of refractory high-entropy alloys and optimizing their creep properties have become current research focal points. Alloy composition control is a key approach to improving the creep performance of refractory high-entropy alloys. Due to its small atomic radius and high reactivity, Al can significantly influence alloy microstructure and properties through mechanisms such as solid solution strengthening and inducing second-phase precipitation [2]. Additionally, Al addition refines grain size and enhances oxidation resistance, though its effect on creep performance remains controversial. Al can improve high-temperature stability and reduce creep rates; however, excessive Al leads to brittle phase formation, accelerating creep and causing alloy failure [3]. This controversy stems from the limitations of traditional testing methods in precisely analyzing the influence of micro-area composition and microstructural evolution on creep behavior.
Nanoscale indentation technology, with its high spatial resolution and in-situ testing capabilities, enables quantitative analysis of localized mechanical behavior at the nanoscale [4], providing a microscopic perspective for investigating the effect of Al addition on the creep behavior of refractory high-entropy alloys. Currently, systematic studies on the relationship between Al and alloy creep behavior based on nanoscale indentation technology are scarce. This study employs nanoindentation testing to investigate the mechanism by which Al addition affects the creep behavior of TiZrNbTaMo-based refractory high-entropy alloys, providing theoretical support for alloy composition optimization and engineering applications.
1 Experimental Materials and Methods
Pure Ti, Zr, Nb, Ta, and Mo blocks with a purity exceeding 99.95% were selected as raw materials. Alloys were prepared by chemically stoichiometric mixing according to the composition (Ti₃₉.₅Zr₃₉.₅Nb₉.₇Ta₄.₃Mo₇)_(100−x)Al_x (x = 0, 5, 10, 15, 20, molar fraction, %) and prepared alloy specimens via arc melting. These were designated as Al0, Al5, Al10, Al15, and Al20, respectively. During melting, the furnace vacuum was maintained at 5×10⁻³ Pa and purged three times with high-purity argon gas. The melt was then repeatedly inverted over six cycles before being cast into ingots using a water-cooled copper mold.
The ingots were cut into 10 mm × 10 mm × 5 mm thin blocks using electrical discharge wire cutting. The blocks were sequentially polished with 400-grit, 800-grit, 1200#, 1500#, and 2000# sandpaper. After degreasing and ultrasonic cleaning, samples were etched with Kroll's acid (2% HF + 3% HNO₃ + 95% C₂H₅OH) for 15 seconds. Microstructural morphology and elemental distribution were analyzed using a SU5000 thermal field emission scanning electron microscope equipped with an energy dispersive spectrometer. The phase composition of the specimens was analyzed using a D8ADVANCE X-ray diffractometer. The X-ray source was CuKα (λ=0.15406 nm), operating at 40 kV, with a scanning speed of 5°/min and a scanning range (2θ) of 20°–90°.
Nanoindentation hardness testing of different alloy specimens was performed using a G200 nanoindenter equipped with a Berkovich indenter. Tests were conducted at room temperature in constant loading rate mode (1 mN/s), with peak loads of 10, 50, and 100 mN and a dwell time of 40 s. Six indentation points were tested per specimen to minimize experimental error. Load-displacement and creep displacement-time data were recorded during testing and plotted.
2 Experimental Results and Analysis
2.1 Phase Composition and Microstructure
Figure 1 shows the XRD patterns of the four high-entropy alloys. As depicted in Figure 1, the TiZrNbTaMo refractory high-entropy alloy exhibits a single body-centered cubic (BCC) solid solution structure. At low Al content (x=10), the alloy presents a single BCC phase, consistent with the composition without Al addition [5], indicating that a small number of Al atoms can be well-dissolved into the BCC lattice. As the Al content increases to 15%, B2 and Al3Zr phases emerge in the alloy. Typically, the B2 phase precipitates as extremely fine spherical nanoparticles on the BCC matrix, with sizes ranging from 10 to 30 nm [6]. In similar high-entropy alloys, the intermetallic compound Al₃Zr typically precipitates as nanoparticles. This is primarily due to the low elemental diffusion rates in high-entropy alloys and the suppression of coarse phase formation during solidification by rapid cooling. Particularly after Al addition, its strong negative mixing enthalpy with Zr (-44 kJ/mol) promotes the formation of fine precipitates [7].
Additionally, the diffraction peak positions (2θ) corresponding to the (110) crystal planes for the Al0, Al10, Al15, and Al20 specimens were 37.42°, 37.70°, 37.93°, and 38.06°, respectively. This indicates that the (110) peak shifts to higher angles with increasing Al content. the (110) peak shifts toward higher angles. This occurs because, in addition to forming B2 and Al3Zr phases, some Al solid-solves into the BCC matrix. According to Bragg's law, the lattice constant a of the BCC phase can be calculated using the following equation [8]:
where: θ is the diffraction angle; λ is the wavelength, 0.15406 nm; (hkl) is the crystal plane index; d is the interplanar spacing; n is the optical path difference between the two X-ray beams undergoing constructive interference relative to the incident wavelength λ (an integer multiple), typically n=1. Calculations reveal lattice constants of 0.3404 nm, 0.3377 nm, 0.3351 nm, and 0.3339 nm for the Al₀, Al₁₀, Al₁₅, and Al₂₀ specimens, respectively, indicating a decrease in the alloy matrix lattice constant upon Al addition. This is attributed to the smaller atomic radius of Al (0.143 nm) compared to the average atomic radius of the Al0 sample (0.151 nm), which induces lattice contraction upon solid solution incorporation into the matrix.
Figure 2 shows the BSE morphology and EDS elemental distribution maps of the TiZrNbTaMo high-entropy alloy without Al addition and with 10%, 15%, and 20% Al additions. As seen in the figure, all alloys exhibit distinct dendritic structures composed of Ta-rich dendrite arms and Zr- and Al-rich inter-dendritic regions. The distribution differences among elements correlate with their inherent properties and kinetic factors during solidification. Tantalum (melting point 3017°C), being the highest-melting-point component in the system, preferentially nucleates and enriches in dendrite arms during early solidification. Zirconium (melting point 1855°C) and aluminum (melting point 660°C), with lower melting points and slower diffusion rates, are repelled into the dendrite-arm-interval regions [9]. Furthermore, the negative mixing enthalpy between Al and Zr further promotes their segregation to form the intermetallic compound Al₃Zr (see Figure 1).
2.2 Creep Properties
Figure 3(a–e) shows the load-displacement curves of five high-entropy alloys under different peak loads. As shown, during the loading phase, the load-displacement curves for each alloy at different peak loads nearly overlap, attributed to the identical loading rate. The indentation depth increases with rising peak load for all alloys; however, at the same load, the indentation depth decreases with increasing Al content, indicating that Al addition enhances the alloy's deformation resistance. Figure 3(f) shows the nanohardness of the five high-entropy alloys under different peak loads. As peak load increases, alloy hardness exhibits a decreasing trend, primarily attributed to the nanoindentation size effect [10]. Additionally, the nano-hardness of the alloys increases with rising Al content. For example, at a peak load of 50 mN, the nano-hardness values for the Al0, Al5, Al10, Al15, and Al20 specimens were 4.95, 5.23, 5.71, 6.62, and 6.85 GPa, respectively. This is primarily due to Al solid solution strengthening. Furthermore, high Al content promotes the formation of B2 and Al₃Zr phases, achieving secondary phase strengthening to a certain extent.
Figure 3(a–e) shows the indentation depth (displacement) after the onset of load holding, which can be fitted using the following empirical equation [11]:
where: hʹ is the indentation depth, nm; h₀ is the creep-initiation indentation depth, corresponding to the indentation depth at the onset of load holding on the load-displacement curve (stage marked by the red box in Fig. 3), nm; tʹ is the loading time, s; t₀ is the creep-initiation loading time, s; a, b, k are fitting parameters. To normalize the fitted curve for analyzing the alloy's creep behavior, Equation (3) can be rewritten as:
Where: h is the creep displacement, nm; t is the creep time, s. Figures 4(a–e) show the creep displacement-time curves of five high-entropy alloys under different peak loads. As shown in the figure, the creep of the alloy indentation can be divided into an initial transient creep stage and a steady-state creep stage. During the early creep phase, the creep displacement increases rapidly. As the load-holding time extends, the growth rate of creep displacement gradually slows down, indicating the transition from the transient to the steady-state creep stage. Figure 4(f) shows the total creep displacement of five high-entropy alloys under different peak loads. As seen in the figure, higher peak loads result in greater total creep displacement. For example, the total creep displacements of the Al10 specimen under peak loads of 10, 50, and 100 mN were 5.23, 9.00, and 11.77 nm, respectively. Additionally, the total creep displacement exhibited a decreasing trend with increasing Al content. Under a 50 mN peak load, the total creep displacement decreased from 12.08 nm (Al0 specimen) to 7.85 nm (Al20 specimen), representing a 35% reduction. This indicates that Al addition enhances the alloy's creep resistance.
Differentiating the creep displacement-time curve in Figure 4 yields the creep rate-time curve:
where ε. denotes creep rate, s⁻¹. Creep rate-time curves under different peak loads are shown in Figure 5. The figure reveals that all alloys exhibit similar creep rate trends across varying peak loads. During the transient creep stage, the creep rate is relatively high, primarily governed by dislocation slip and grain boundary shear mechanisms. As creep time increases, the creep rate rapidly decreases and stabilizes, at which point creep is controlled by atomic diffusion.
The characteristic stress σ during the steady-state creep stage can be calculated using the following equation [12]:
where: P is the peak load; c is the geometric constant, typically set to 24.56 for Berkovich indenters [13].
The strain rate sensitivity index m is a key parameter characterizing a material's rate-dependent deformation behavior. A smaller value indicates a more gradual change in strength across different strain rates, typically corresponding to superior creep resistance. m can be determined using the following equation [14]:
Figure 6 presents the logarithmic stress-logarithmic strain rate curves and strain rate sensitivity index m for five high-entropy alloys under different peak loads. The figure shows that at a given peak load, the strain rate sensitivity index m gradually decreases with increasing Al content. Conversely, at a fixed Al content, m progressively increases with higher applied loads. This is primarily because Al addition promotes the formation of B2 and Al3Zr phases, enhancing lattice friction. This process significantly increases dislocation motion resistance through the size mismatch effect [15-16], reducing the sensitivity of rheological stress to strain rate changes. This manifests as a decrease in the strain rate sensitivity index m with increasing Al content. At fixed compositions, increasing applied load (stress) sharply elevates dislocation density and activates more strain-rate-sensitive slip mechanisms (e.g., cross-slip) [17]. Consequently, under high stress, the variation of rheological stress with strain rate becomes more pronounced, resulting in an increase in the strain-rate sensitivity index m.
3 Conclusions
1) At low Al content (≤10%), the refractory high-entropy alloy (Ti₃₉.₅Zr₃₉.₅Nb₉.₇Ta₄.₃Mo₇)_(100-x)Al_x exhibits a single BCC phase, with Al atoms solid-solving into the BCC lattice, resulting in a reduced lattice constant. When Al content increases to 15% or higher, B₂ and Al₃Zr phases appear in the alloy.
2) The microstructure of the (Ti39.5Zr39.5Nb9.7Ta4.3Mo7)100−xAlx refractory high-entropy alloy exhibits a dendritic structure composed of Ta-rich dendrite arms and Zr/Al-rich dendrite interiors. With increasing Al content, the alloy's nanohardness gradually rises, reaching 6.85 GPa at a peak load of 50 mN when 20% Al is added.
3) Al addition significantly enhances the alloy's creep resistance. Under a fixed peak load, the total creep displacement decreases substantially with increasing Al content, while the strain rate sensitivity index m gradually decreases.
Reference: DOI:10.19990/j.issn.1004-0536.20250811.1012 Chinese Library Classification: TG132.32 Document Code: A Effect of Al Addition on Microstructure and Creep Properties of TiZrNbTaMo Refractory High-Entropy Alloys
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