Effect of Mo Addition on Tribological Properties of Al

Abstract

Tribological properties of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x (x=0, 1, 2, 3, 4, 5) eutectic high-entropy alloys (EHEAs) were investigated in this research. Results show that EHEAs with trace Mo addition can form the face-centered cubic (fcc)+B2 eutectic microstructure, whereas EHEAs with relatively higher Mo content can form fcc+B2+μ dendritic microstructure. Mo element is beneficial to the strength enhancement of L1₂ phase and the ductility improvement of B2 phase. However, with increasing the Mo content to x>2, the resultant Mo-rich μ phase degrades the strength and plasticity of EHEAs. Al₁₉Fe₁₈Co₁₈Ni₄₁Mo₄ EHEA has the optimal combination of high strength and high ductility. Increasing Mo content can improve the oxidation resistance of EHEAs. With increasing the Mo content, EHEA forms a tribo-oxide layer with improved oxidation resistance during sliding process, and the friction coefficient is monotonically decreased. This research provides guidance for the investigation of tribological properties of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs.

Keywords

eutectic high-entropy alloys; tribological property; strength; plasticity

Combining eutectic alloy with high-entropy alloys (HEAs), the eutectic high-entropy alloys (EHEAs) ^[1–5] with eutectic microstructure and HEA composition have been proposed, which have many attractive properties, such as excellent mechanical properties^[6–9], good wear resistance^[10–13], and 3D printability^[14–16]. It is reported that the AlCoCrFeNi_2.1 EHEA has fine lamellar microstructure with optimal combination of strength and ductility, and the additive-manufactured AlCo-CrFeNi_2.1 EHEAs show better mechanical properties than most other additive-manufactured alloys do^[14]. The directionally solidified Al₁₉Fe₂₀Co₂₀Ni₄₁ EHEA presents herringbone micro-structure with a strong crack buffering effect, implying ultra-high uniform tensile elongation without strength degradation^[9].

The eutectic microstructure of Al₁₉Fe₂₀Co₂₀Ni₄₁ EHEA consists of the ordered face-center cubic (fcc) phase (L1₂ phase) and ordered body-centered cubic (bcc) phase (B2 phase). It is found that the Mo addition is beneficial to the strength enhancement of L1₂ phase^[17] and the ductility improvement of B2 phase^[18–19] in the intermetallic compounds. Additionally, the minor addition of Mo element can even ameliorate the oxidation resistance by forming a dense and continuous oxide scale during high-temperature oxidation^[20]. Therefore, the Al₁₉Fe₂₀Co₂₀Ni₄₁ EHEAs with Mo addition show great potential as structural materials.

Tribological performance is an important mechanical property for structural materials. It is reported that the forma-tion of eutectic lamellar microstructure is beneficial to increase the hardness, thereby improving the wear resis-tance^[21–27]. Besides, the oxidation resistance can be improved by the minor addition of Mo element in the Al₁₉Fe₂₀Co₂₀Ni₄₁ EHEA^[20], which also contributes to the enhancement in wear resistance. Therefore, in this research, a series of Al₁₉Fe₂₀-Co₂₀Ni₄₁ EHEAs with Mo addition were prepared to inve-stigate their tribological properties.

1 Experiment

The Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs with x=0, 1, 2, 3, 4, and 5 (at%) were denoted as Mo₀, Mo₂, Mo₄, Mo₆, Mo₈, and Mo₁₀ specimens, respectively. These EHEAs were synthesized by vacuum arc melting in furnace under high-purity argon protection. The purity of all raw materials was above 99.99%. The raw materials were put into the water-cooled copper hearth and remelted at least six times to ensure the chemical homogeneity. The mass of each alloy ingot is about 25 g and the central part of the ingot was selected for analysis. The specimens were cut by the electric discharge machine wire cutter into the ones with size of 5 mm×5 mm×5 mm. All specimens were ground by SiC sandpaper (2000#) and cleaned by ethanol through the ultrasonic cleaner.

The phase structure analysis was conducted through Rigaku SMARTLAB9 X-ray diffractometer (XRD, 40 kV, 150 mA). The morphology and composition were investigated by FEI Nova Nano 450 scanning electron microscope (SEM) coupled with energy dispersive spectrometer (EDS). Compression tests were conducted by AG-X Plus 250 kN/50 kN universal testing machine under deformation rate of 2 and 0.5 mm/min at room temperature. The reciprocating dry friction tests were conducted at room temperature (25 °C). The Si₃N₄ ceramic balls with 5 mm in diameter were used as the friction pair, the friction load was 5 N, the friction amplitude was 5 mm, the friction frequency was 2 Hz (linear velocity was 0.02 m/s), and the test duration was 30 min. Before tests, the alloy surface was polished. The wear morphology was observed by SEM.

2 Results and Discussion

2.1 Microstructure analysis

Mo has a negative binary enthalpy of mixing and relatively low valence electron concentration (VEC), compared with other constitutional elements. Fig.1 shows the VEC values, mixing enthalpy ΔH_mix, mixing entropy ΔS_mix, and atomic size diffe-rence δ of the Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x (x=0, 1, 2, 3, 4, 5) EHEAs. It can be seen that with increasing the Mo content, the mixing enthalpy ΔH_mix, mixing entropy ΔS_mix, and atomic size difference δ are increased, whereas VEC value is decreased. Hence, the microstructure and phase stability of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs may be changed with the composition variation. With increasing the Mo content, the Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs have relatively large atomic size difference. Based on the solid-solution phase formation rules of HEAs, with increasing the Mo content, the ordered phase tends to precipitate in the solid solution, such as the intermetallic compounds in the matrix^[28]. The minimum VEC value is still greater than 8, indicating that the phase composition of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs is mainly composed of fcc phase^[29].

  

  

Fig.2 shows XRD patterns of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs with x=0, 1, 2, 3, 4, 5. It can be seen that the as-prepared Mo₀ EHEA is composed of B2+fcc dual-phase microstructure^[20], which is very similar to the B2+L1₂ phase^[9]. The fcc phase is disordered, because the (100) diffraction peak cannot be observed. With the Mo addition, the B2+fcc dual-phase microstructure remains in the Mo₂ and Mo₄ EHEAs, suggesting that the minor Mo can be dissolved into the dual-phase microstructure. When the Mo content is higher than 4at% (x>2), the μ phase can be observed in the Mo₆ EHEA, and the peak intensity of μ phase is increased with further increasing the Mo addition. The variation in peak intensities of fcc and B2 phases is probably caused by the texture difference of the tested specimens. The results of the microstructure characterization of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs are in good agreement with the calculated results, inferring that the solid-solution phase formation rules of HEAs are suitable to predict the phase composition of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs.

Fig.3 and Table 1 show SEM morphologies and EDS point analysis results of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs with x=0, 1, 2, 3, 4, 5, respectively. The Mo₀ EHEA specimen presents typical fine eutectic microstructure with interlamellar spacing of approximately 2 μm, which is consistent with the results in Ref.[9]. EDS analysis results show that B2 phase is composed of Al and Ni-rich structures, whereas the fcc phase is composed of Fe and Co-rich structures. With increasing the Mo addition, the microstructure is transformed from the fine lamellar eutectic microstructure to the coarse lamellar microstructure in Mo₂ EHEA specimen, and further to the dendritic microstructure in Mo₄ EHEA specimen. EDS results show that the fcc phase is composed of Mo-rich structures, which is consistent with the result in Ni-Al systems^[30]. With further increasing the Mo addition, the dendritic micro-structure becomes coarser, and the Mo-rich μ phase can be observed along the phase boundary. The proportion of the μ phase is increased with increasing the Mo addition, and the μ phase separates the fcc and B2 phases in the Mo₈ EHEA specimen. The microstructure suggests that the composition of EHEAs with high Mo addition is different from the that of eutectic compound.

Fig.3  SEM morphologies of different Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs: (a) x=0; (b) x=1; (c) x=2; (d) x=3; (e) x=4; (f) x=5

Table 1  EDS point analysis results of fcc phase, B2 phase, and μ phase of different Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs in Fig.3 (at%)

Specimen	Phase	Element
		Al	Fe	Co	Ni	Mo
Mo0	fcc	13.72	24.96	20.68	40.64	-
	B2	21.52	19.67	17.53	41.28	-
Mo2	fcc	11.38	24.03	20.97	40.27	3.35
	B2	25.61	16.54	15.42	42.07	0.36
Mo4	fcc	10.14	23.94	20.83	40.79	4.29
	B2	22.13	16.70	15.79	44.39	0.99
Mo6	fcc	10.33	23.83	20.10	37.77	7.97
	B2	21.90	16.32	14.58	44.80	2.40
	μ	5.33	20.91	20.57	24.12	29.07
Mo8	fcc	12.62	20.41	17.59	37.56	11.82
	B2	24.18	14.15	12.60	45.16	3.91
	μ	6.15	21.88	19.14	24.60	28.23
Mo10	fcc	10.87	22.64	18.89	38.91	8.69
	B2	23.79	15.48	13.50	45.53	1.70
	μ	7.13	21.52	19.43	28.82	23.10

2.2 Mechanical properties

Fig.4 shows the compressive engineering stress-engineering strain curves of the Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs at room temperature. The mechanical compressive properties of Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs at room temperature are shown in Table 2. Mo₀ EHEA has the yield strength (σ_s) of 751 MPa, ultimate compressive strength (σ_p) of 2593 MPa, and compressive strain at failure (namely elongation ε_p) of 40.5%,

Table 2  Mechanical compressive properties of Al₁₉Fe_20-xCo_20-xNi₄₁-Mo_2x EHEAs near eutectic HEAs at room temperature

Specimen	Yield strength, σs/MPa	Ultimate compressive strength, σp/MPa	Elongation, ɛp/%
Mo0	751	2593	40.5
Mo2	770	2679	40.9
Mo4	703	2793	39.6
Mo6	931	2545	30.7
Mo8	1056	2486	26.1
Mo10	1291	2439	22.1

  

which are all higher than the tensile properties in Ref.[9]. This result indicates the strong tension/compression asymmetry effect. With 2at% Mo addition, the Mo₂ EHEA shows slightly higher σ_s and σ_p of 770 and 2679 MPa, respectively, and its ε_p value is also slightly increased to 40.9%, inferring the enhancement effect of Mo addition on mechanical properties. The Mo₄ EHEA presents the high σ_p value of 2793 MPa, but its σ_s and ε_p show a slighter decrease. With further increasing the Mo addition, the σ_s value is continuously increased, whereas the σ_p and ε_p values are significantly decreased. This phenomenon is caused by the formation of hard and brittle μ phase along the phase boundary.

2.3 Tribological properties

Fig.5 shows the coefficient of friction (COF) curves of different Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs at room temperature. A transition period occurs in the beginning followed by a steady period for all the EHEA specimens, which is consistent with the typical COF curves^[31]. With increasing the Mo content, COF is monotonically decreased. Fig.6 shows the cross-sectional and 3D confocal laser scanning images of the wear tracks of different Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs. It can be seen that Mo₀ specimen has the largest amount of oxide film. With increasing the Mo content, the amount of oxide film in EHEAs is firstly decreased and then maintained at a certain level. When the Mo content increases from 0at% to 4at%, the amount of oxide film decreases; when the Mo content increases from 4at% to 10at%, the amount of oxide film barely changes.

Fig.5  Coefficients of friction of different Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs

Fig.6  Cross-sectional morphologies (a) and 3D confocal laser scanning images (b–g) of wear tracks of different Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs: (b) Mo₀; (c) Mo₂; (d) Mo₄; (e) Mo₆; (f) Mo₈; (g) Mo₁₀

Fig.7 shows SEM microstructures of the wear tracks of different Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs. In Fig.7, area A represents the exposed substrate caused by the peeling of oxide film; area B represents the thinner oxide film; area C represents the thicker oxide film. The wear tracks of EHEAs present the typical ploughing grooves and flake-shape wear debris, indicating the abrasive wear and delamination wear mechanisms, respectively. The oxides (white flakes with oxygen content>60at%) can be observed in all wear tracks of EHEAs. The oxidation is mainly caused by the temperature rise at the sliding interface between the ball and disk. Compared with the wear surface of Mo₀ EHEA specimen, less wear debris caused by delamination and less mixed oxides exist on the surface of Mo₄ EHEA specimen, indicating the better wear resistance of Mo₄ EHEA specimen^[32]. With further increasing the Mo content, the amount of wear debris on the EHEA specimen surface is increased again. The fact that COF is decreased with increasing the Mo content is probably attributed to the better oxidation resistance caused by Mo addition and the formation of more protective tribo-oxide layer during friction and wear process. The wear resistance of Mo₄ EHEA improves, which is caused by the higher hardness of fcc+B2 near-eutectic microstructures and the formation of a more protective tribo-oxide layer during sliding process. With further increasing the Mo content, the wear resistance is decreased due to the appearance of μ phase.

Fig.7  SEM microstructures (a–c, g–i) and magnified images (d–f, j–l) of wear tracks of different Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs: (a, d) Mo₀; (b, e) Mo₂; (c, f) Mo₄; (g, j) Mo₆; (h, k) Mo₈; (i, l) Mo₁₀

3 Conclusions

1) The Mo addition can significantly improve the yield strength and wear resistance of Al₁₉Fe₂₀Co₂₀Ni₄₁ eutectic high-entropy alloys (EHEAs).

2) With x=0–2, the Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs show face-centered cubic (fcc)+B2 near-eutectic microstructures. With x=3–5, the Al₁₉Fe_20-xCo_20-xNi₄₁Mo_2x EHEAs present fcc+B2+μ dendritic microstructures.

3) The strength and plasticity simultaneously increase with minor Mo addition, whereas the plasticity significantly decreases after the appearance of μ phase.

4) Al₁₉Fe₁₆Co₁₆Ni₄₁Mo₈ EHEA has good mechanical properties with fine wear resistance, which is attributed to the optimal addition of Mo element.