Mechanical and tribological performance of CoCrNiHfx eutectic medium-entropy alloys

doi:10.1016/j.jmst.2021.03.023

Abstract

Abstract:

The excellent properties of the multi-principal element alloys (e.g., the CoCrNi medium-entropy alloy) make them a perfect candidate for structure materials. Their low strength and poor wear-resistance, however, limit considerably their applications. In this study, a lamellar eutectic microstructure was introduced by addition of Hf into CoCrNi alloy to produce a series of CoCrNiHf_x (x = 0.1, 0.2, 0.3 and 0.4) eutectic medium-entropy alloys. A homogeneous eutectic microstructure with an alternate array of the soft FCC solid-solution phase and the hard Laves phase was identified for the as-cast CoCrNiHf_0.3 alloy. After an investigation of the microstructure, mechanical and tribological properties, it was found that the hardness (plasticity) increases (decreases) with the increasing volume fraction of the Laves phase and the CoCrNiHf_0.3 eutectic alloy exhibits both good plasticity and high strength. The wear behavior is strongly dependent on the applied normal load. For a low normal load, its tribological behavior follows the Archard's equation and a higher hardness due to Hf addition can resist plastic deformation and abrasive wear. When the normal load is high enough, the hypoeutectic or hypereutectic alloy, which possessing either high strength or good ductility but not at the same time, exhibit a poor wear resistance. In comparison, the full eutectic CoCrNiHf_0.3 alloy with a superior combination of strength and toughness shows the best wear performance, as it can significantly reduce fracture during wear.

Key words: Medium-entropy alloy, Lamellar eutectics, Hardness, Plasticity, Wear-resistance

Yin Du, Xuhui Pei, Zhaowu Tang, Fan Zhang, Qing Zhou, Haifeng Wang, Weimin Liu. Mechanical and tribological performance of CoCrNiHf_x eutectic medium-entropy alloys[J]. J. Mater. Sci. Technol., 2021, 90: 194-204.

Figures/Tables 15

Fig. 1. XRD patterns of the CoCrNiHfx medium-entropy alloys (x?=?0, 0.1, 0.2, 0.3 and 0.4).

Fig. 2. Representative back-scattered electron microscopy images of the CoCrNiHfx medium-entropy alloys (x?=?0, 0.1, 0.2, 0.3 and 0.4). (a) Microstructure of a single FCC phase for Hf0 alloy. (b) Primary FCC phase and secondary lamellar eutectics for Hf0.1 alloy. (c) Primary FCC phase and secondary lamellar eutectics for Hf0.2 alloy. (d) Homogeneous lamellar eutectics for Hf0.3 alloy. (e) Primary Laves phase and secondary lamellar eutectics for Hf0.4 alloy. (f) Volume fraction of lamellar eutectics as a function of the addition of Hf. (g) Average lamellar eutectic spacing as a function of the addition of Hf. Inset of Fig. 2(b) shows the definition of lamellar eutectic spacing.

Fig. 3. (a) Transmission electron microscope images of the lamellar eutectics and the corresponding selected area electron diffraction (SAED) patterns of the Laves phase and the FCC phase for the Hf0.3 alloy. (b) Energy-dispersive X-ray spectroscopy (EDS) elemental mappings of the lamellar eutectics.

Fig. 4. True compressive stress-strain curves of the CoCrNiHfx medium-entropy alloys (x?=?0, 0.1, 0.2, 0.3 and 0.4) at room temperature.

Fig. 5. Micro-hardness of the CoCrNiHfx medium-entropy alloys (x?=?0, 0.1, 0.2, 0.3 and 0.4).

Table 1 The microstructures and mechanical properties (e.g., micro-hardness (Hmicro), yielding strength (σy) and fracture strain (εu)) of the CoCrNiHfx eutectic medium-entropy alloys (x?=?0, 0.1, 0.2, 0.3 and 0.4).

Samples	Microstructure	Mechanical properties
Samples	Microstructure	H_micro (GPa)	σ_y (MPa)	ε_u (%)
Hf0	FCC	3.12 ± 0.24	161.2 ± 5.1	61±0.2
Hf0.1	FCC+Eutectic Lamellar	4.06 ± 0.26	429.6 ± 19.5	54.4 ± 0.3
Hf0.2	FCC+Eutectic Lamellar	4.78 ± 0.26	630.9 ± 17.4	33.5 ± 1.2
Hf0.3	Eutectic Lamellar	5.66 ± 0.27	907.8 ± 10.2	32.6 ± 1.6
Hf0.4	Laves+Eutectic Lamellar	8.23 ± 0.35	990.5 ± 31.8	15.9 ± 2.8

Fig. 6. Wear characteristics of the CoCrNiHfx eutectic medium-entropy alloys (x?=?0.2, 0.3 and 0.4) upon a dry sliding test against a commercial ball of Al2O3 under an applied normal load of 10?N. (a) 3D surface profiles of the wear track. (b) Coefficient of friction as a function of the sliding distance. (c) Cross-section profiles of the wear track along the dotted line in (a). (d) Wear volumes of the wear tracks for different alloys.

Fig. 7. Wear characteristics of the CoCrNiHfx eutectic medium-entropy alloys (x?=?0.2, 0.3 and 0.4) upon a dry sliding test against a commercial ball of Al2O3 under an applied normal load of 30?N. (a) 3D surface profiles of the wear track. (b) Coefficient of friction as a function of the sliding distance. (c) Cross-section profiles of the wear tracks along the dotted line in (a). (d) Wear volumes of the wear tracks for different alloys.

Fig. 8. SEM images of the worn surfaces of the CoCrNiHfx eutectic medium-entropy alloys (x?=?0.2, 0.3 and 0.4) upon a dry sliding test against a commercial ball of Al2O3 under an applied normal load of 10 N: (a) Hf0.2 hypoeutectic alloy, (b) Hf0.3 eutectic alloy, (c) Hf0.4 hypereutectic alloy.

Fig. 9. SEM images of the worn surfaces of the CoCrNiHfx eutectic medium-entropy alloys (x?=?0.2, 0.3 and 0.4) upon a dry sliding test against a commercial ball of Al2O3 under an applied normal load of 30 N: (a) Hf0.2 hypoeutectic alloy; (b) Hf0.3 eutectic alloy; (c) Hf0.4 hypereutectic alloy. SEM magnified image of the worn surfaces: (a1) Hf0.2 hypoeutectic alloy, (b1) Hf0.3 eutectic alloy, (c1) Hf0.4 hypereutectic alloy.

Table 2 Chemical compositions (at.%) of the marked area on the worn surfaces of the CoCrNiHfx eutectic medium-entropy alloys (x?=?0.2, 0.3 and 0.4) upon dry sliding tests against a commercial ball of Al2O3 under an applied normal load of 10?N and 30?N.

Load (N)	Alloys	Point	Co	Cr	Ni	Hf	O
10	Hf0.2	1	22.47	22.27	21.23	4.62	29.41
	Hf0.3	2	25.31	25.25	27.71	6.85	14.88
	Hf0.4	3	25.46	23.57	22.19	10.51	18.27
30	Hf0.2	4	20.89	21.39	6.78	4.12	46.82
	Hf0.3	5	19.00	20.52	19.86	5.77	26.63
	Hf0.4	6	10.42	10.97	13.19	8.87	56.55

Fig. 10. Representative load-displacement curves with a fixed peak-load of 10 mN and different strain rates for the (a) primary FCC phase, (b) primary Laves phase, and (c) lamellar eutectic. (d) A double logarithmic plot of the nano-hardness and the strain rates under a peak-load of 10 mN.

Table 3 The measured micro-hardness (Hmicro), nano-hardness (Hnano), strain rate sensitivity (m), activation volume V* (b3) of the primary FCC phase, lamellar eutectic and primary Laves phase at room temperature.

Structure	H_micro (GPa)	H_nano (GPa)	m	V* (b³)
FCC	3.12±0.04	3.25±0.12 ($\dot \varepsilon$=0.01)	0.0571	7.5
		3.56±0.19 ($\dot \varepsilon$=0.05)
		3.76±0.15 ($\dot \varepsilon$=0.1)
		4.05±0.18 ($\dot \varepsilon$=0.5)
Laves	8.22±0.10	8.80±0.12 ($\dot \varepsilon$=0.01)	0.0007	231
		8.81±0.12 ($\dot \varepsilon$=0.05)
		8.82±0.15 ($\dot \varepsilon$=0.1)
		8.83±0.18 ($\dot \varepsilon$=0.5)
Eutectic Lamellar	5.65±0.07	5.36±0.10 ($\dot \varepsilon$=0.01)	0.0175	13.5
		5.53±0.15 ($\dot \varepsilon$=0.05)
		5.60±0.13 ($\dot \varepsilon$=0.1)
		5.74±0.07 ($\dot \varepsilon$=0.5)

Fig. 11. XPS spectra of the individual elements on the worn surfaces of the CoCrNiHfx eutectic medium-entropy alloys (x?=?0.2, 0.3 and 0.4) after a dry sliding test against a commercial ball of Al2O3 under an applied normal load of 10 N: (a) Co element, (b) Cr element, (c) Ni element, (d) Hf element.

Fig. 12. XPS spectra of the individual elements on the worn surfaces of the CoCrNiHfx eutectic medium-entropy alloys (x?=?0.2, 0.3 and 0.4) after a dry sliding test against a commercial ball of Al2O3 under an applied normal load of 30 N: (a) Co element, (b) Cr element, (c) Ni element, (d) Hf element.

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