1 Introduction

High entropy alloys (HEAs), which are regarded as one kind of innovative materials, have received a lot of attention [1,2,3,4,5]. In general, HEAs consist of multiple elements and are in a single-phase solid solution [6,7,8]. The solid-solution structure can not only contribute to the solid-solution hardening caused by the multi-element interactions but also modify the ductility due to lacking interphase boundaries [9]. Consequently, HEAs show many desired properties, namely, high strength and hardness, suitable ductility, and excellent thermal stability, etc. [8, 10,11,12,13,14,15,16,17,18,19,20]. For example, AlCoCrFeNi HEA possesses a Vickers hardness higher than HV 520 and a compressive yield strength between 1138 and 1702 MPa [11]. Based on the Archard rule under dry wear conditions [21], there is an inversely proportional relationship between wear rate and material hardness (or strength). Therefore, HEAs are expected to exhibit excellent anti-wear properties [22,23,24,25,26,27,28,29,30].

Investigations on wear-resistant HEA materials manifest that the improved wear properties of these HEAs are mainly associated with multiple phases in materials, and precipitated phase strengthening plays an essential role in different strengthening mechanisms [31,32,33,34,35,36]. For instance, Chuang et al. [31] assessed the effects of Al and Ti contents on phases, microstructures, and wear properties of AlxCo1.5CrFeNi1.5Tiy HEAs. Al and Ti could tailor the phase compositions of the HEAs, particularly in the volume fraction of hard η phase, thus improving their wear resistance. Furthermore, the formation of a boride phase [37] or TiC [38] ceramic–metal composite enhances the HEA hardness, thus, the wear resistance.

Because of good overall performance of AlCoCrFeNi containing A2 and B2 phases, it catches numerous attention [39,40,41,42,43]. Many metallic elements, i.e., Nb [44] and Zr [45], are added into AlCoCrFeNi HEA to produce the hard precipitated phase to modify the mechanical performance. The atomic radius of Hf is almost the same as that of Zr but larger than that of Nb [46]. Meanwhile, the mixing enthalpies between Hf and other constituent elements in AlCoCrFeNi HEA are a little more positive than those between Zr and constituent elements and more negative than those between Nb and constituent elements [47]. What occurs when add Hf into AlCoCrFeNi HEA?

In this work, we design a group of (AlCoCrFrNi)100−xHfx (x = 0, 2, 4 and 6; at%) HEAs. The effect of Hf addition on phases and microstructures of the HEAs is revealed. And the wear properties of (AlCoCrFrNi)100−xHfx HEAs are studied. This work opens other doors on the potential microstructure features that cause the superior wear-resistant properties of AlCoCrFeNi HEA.

2 Experimental

The pure Al, Co, Cr, Fe, Ni and Hf particles with the purity higher than 99.9% (DM Material Inc., Beijing, China) were received as raw materials and compounded through arc melting in an argon atmosphere. For getting the uniform composition, each ingot (Φ45 mm × 10 mm) was re-melted at least six times. The crystal structures of HEAs were determined by X-ray diffraction (XRD, Empyrean, Netherland) with Cu Kα radiation scanning from 20° to 100° at a rate of 4 (°)·min−1. The microstructure morphology of each HEA ingot was observed using scanning electron microscopy (SEM, Merlin Compact, Zeiss, Germany) operated at 20 kV. Transmission electron microscopy (TEM) and EDS high-angle annular dark-field (EDS-HAADF) analyses were carried out by using an FEI-Talos F200X microscope operated at 200 kV.

The Vickers hardness (HV-1000, Shanghai Lianer Testing Equipment Co., China) was recorded at a load of 300 g for 10 s. For dry wear tests, the specimens with dimensions of Φ25 mm × 3 mm were fabricated by wire cutting from the as-cast ingots. Prior to the wear test, all specimens were firstly ground against 400-, 800-, 1200- and 2000-grit sandpaper and then mechanically polished with diamond paste (W1.5). The surface roughness (Ra) values determined by atomic force microscopy (AFM, Dimension Fastscan, Bruker, Germany) for Hf-0, Hf-2, Hf-4 and Hf-6 HEAs are around 4.34, 2.59, 4.62 and 5.10 nm, respectively. The dry wear test parameters (MS-T3000, Lanzhou Huafeng Technology Co., Ltd. China) were load of 5 N, sliding speed of 300 r·min−1, track radius of 6 mm, sliding time of 30 min and Si3N4 ball (Φ6 mm), at room temperature. The three-dimensional morphologies and profiles of (AlCoCrFrNi)100−xHfx HEAs after dry wear tests were obtained by white light interferometer (Contour GT-X3, Bruker, Tucson, AZ, USA).

3 Results and discussion

3.1 Phase composition and microstructures

Figure 1a presents XRD patterns of (AlCoCrFrNi)100−xHfx HEAs. A2 and B2 body-centered cubic (BCC) phases are found in each HEA. And some new diffraction peaks originating from (Ni, Co)2Hf-type Laves phase appear in three Hf-doped HEAs. By adding Hf, the peak intensity of Laves phase increases, manifesting that the volume fraction of Laves phase increases. Besides, the (110)BCC diffraction peak shifts towards a lower reflection angle, as learnt from Fig. 1b. This suggests that the dissolution of Hf atom in BCC phase can induce a more serious lattice distortion compared with Hf-0 HEA.

Fig. 1
figure 1

a, b XRD patterns and c–f SEM images of HEAs as well as g elemental distribution maps of Hf-6 HEA

Full size image

For SEM observation of Hf-0 HEA, Fig. 1c illustrates the single-phase morphology. When Hf is introduced (Fig. 1d–f), i.e., Hf-2, -4 and -6 HEAs, the hypoeutectic structure composed of BCC phase and eutectic phase is obtained. The eutectic phase is composed of alternating BCC phase and Laves phase. More Hf addition causes a higher volume fraction of Laves phase, according well with XRD results.

Figure 1g shows the chemical distribution maps of Hf-6 HEA. In the primary BCC phase, the dendrite core (DR) region has (Ni, Al)-rich composition, whereas the inter-dendrite (ID) region displays (Fe, Cr)-rich composition. In Laves phase region, Ni, Co and Hf segregate. Table 1 shows the chemical compositions of various regions in Hf-6 HEA. The primary BCC phase (Zone 1) is enriched with Fe, Cr and Al but depleted of Ni, Co and Hf, which is opposite to the Laves phase (Point 3). In general, the chemical composition of each element in Zone 2 is almost half of the sum of these in Zone 1 and Point 3. This proves that the eutectic phase consists of the BCC phase and Laves phase. The higher contents of Ni, Co and Hf in Laves phase are due to the negative mixing enthalpies (Table 2 [46, 47]) and large electronegativity (Table 3 [48]) of Ni–Hf and Co–Hf atomic pairs.

Table 1 Chemical distribution of various regions in Hf-6 HEA (at%)
Full size table
Table 2 Mixing enthalpy (kJ·mol−1) between two components and atomic radius (nm)
Full size table
Table 3 Electronegativity of various constituent elements in Al–Co–Cr–Fe–Ni–Hf HEA system
Full size table

Figures 2, 3 show TEM morphologies, SAED patterns and chemical composition maps of Hf-0 and Hf-2 HEAs. As learnt from Fig. 2a, b, d, e, the cubic or rod-like A2 precipitates (disordered phase) and continuous B2 matrix (ordered phase) are found. A difference in nm-sized morphologies of DR and ID regions is ascribed to the type of order of the domains and strain [49]. Figure 2c, f reveals the elemental distribution maps of Hf-0 HEA. The A2 phase is enriched with Fe and Cr but depleted of Ni and Al, which is opposite to the B2 phase. The Co is uniformly distributed. Figure 3a–c shows TEM images of Hf-2 HEA. In the eutectic region, the alternately grown BCC and Laves phases are further testified by SAEDs (Fig. 3d1–d3). And the numerous stacking faults and twins are discovered in Laves phase of an as-casted HEA, which is in accordance with Refs. [50,51,52]. As seen in Fig. 3e, f, Laves phase is (Hf, Ni and Co)-rich, agreeing well with EMPA results (Fig. 1g).

Fig. 2
figure 2

a–d TEM images, b1, b2, e1, e2 SAED patterns, and c, f elemental composition maps of Hf-0 HEA

Full size image
Fig. 3
figure 3

a–c, e TEM images, d1–d3 SAED patterns, and f elemental composition maps of Hf-2 HEA

Full size image

3.2 Mechanical properties

The Vickers hardness values of (AlCoCrFrNi)100−xHfx HEAs are collected in Fig. 4a. By increasing Hf addition, the Vickers hardness enhances from ~ HV 512.3 to ~ HV 734.1. The hardness enhancement is resulted from three strengthening mechanisms: solid-solution strengthening, fine-grain strengthening, and precipitated phase strengthening. As learnt from Fig. 1b, the dissolution of Hf atoms in BCC phase can cause a more serious lattice distortion relative to Hf-0 HEA. Among the four HEAs, the most serious lattice distortion takes place in Hf-6 HEA. However, the dissolution content of Hf atoms in Hf-6 HEA is only 1.25 at% (Table 1), which is too lower to be responsible for the total Vickers hardness increments of (AlCoCrFrNi)100−xHfx HEAs.

Fig. 4
figure 4

a Vickers hardness, b calculated hardness increase and c volume fraction of Laves phase of (AlCoCrFrNi)100−xHfx HEAs

Full size image

Meanwhile, fine-grain strengthening has also made some contributions due to the “Hall–Petch” relationship [53, 54] as follows:

$${\sigma }_{\mathrm{y}}={\sigma }_{0}+\frac{{k}_{\mathrm{y}}}{{d}^{1/2}}$$
(1)

where σy is the yield stress, σ0 stands for the lattice friction stress, ky (182 MPa·μm1/2 [55]) denotes the strengthening coefficient and d is the mean grain diameter. According to Eq. (1), an increase in yield strength ascribed to the grain size difference (ΔσG) is described as:

$$\Delta {\sigma }_{\mathrm{G}}\text{=}\,{{k}}_{\text{y}}\left({d}_{\mathrm{H}}^{-1/2}-{d}_{0}^{-1/2}\right)$$
(2)

where dH denotes the grain size of the Hf-doped HEA (Hf-2, Hf-4 and Hf-6 HEAs), and d0 is the grain size of Hf-0 HEA. Based on Fig. 1c–f, the mean grain sizes of Hf-0, Hf-2, Hf-4 and Hf-6 HEAs are d0 = 141.6 μm, dHf-2 = 61.7 μm, dHf-4 = 45.9 μm, and dHf-6 = 31.1 μm, respectively. The strength contribution values of fine-grain strengthening on Hf-2, Hf-4 and Hf-6 HEAs are 7.88, 11.58, and 17.35 MPa, respectively. It should be noted that the strength of an alloy follows a linear relationship with the hardness of an alloy, which fits with Tabor’s findings [56], i.e.,

$$H=c\times {\sigma }_{\mathrm{y}}$$
(3)

where c (3.3) is the proportionality factor based on Ref. [57]. Therefore, the calculated hardness increments corresponding to the fine-grain strengthening for Hf-2, Hf-4, and Hf-6 HEAs are ~ HV 2.60, ~ HV 3.82, and ~ HV 5.73, respectively (Fig. 4b). The calculated hardness increments are too small to account for the total hardness increase.

Figure 4c elucidates the relationship between the volume fraction of Laves phase and Hf addition. By adding Hf from 0 to 6 at%, the volume fraction of Laves phase increases from 0 to 29.7 vol%. In consequence, the Vickers hardness is enhanced from HV 512.3 to HV 734.1. This indicates that precipitated phase strengthening acts as a vital role in the total Vickers hardness increase of (AlCoCrFrNi)100−xHfx HEAs.

3.3 Wear features

The coefficient of friction (COF), regarded as a basic parameter of a friction system, is related to the resistance between an alloy and a friction pair. When the test is conducted under the same conditions, the microstructure and surface roughness play a vital role on the COF. The COF curves of (AlCoCrFrNi)100−xHfx HEAs are depicted in Fig. 5a. Some severe fluctuations during the running-in process are caused by the ragged grinding between HEA and friction pair. After this running-in process of appropriately 10 min, there are four relatively stable COF curves. During the relatively stable process, both instrument vibration and measurement accuracy lead to the fluctuation of COF value [58]. To accurately compare COF values of (AlCoCrFrNi)100−xHfx HEAs, Fig. 5b shows the average COF values collected from the relatively stable period of COF curves. The COF values of Hf-0, -2, -4 and -6 HEAs are 0.53, 0.46, 0.35 and 0.28, respectively. By adding Hf, the COF value decreases. Tailoring the alloy microstructure results in the increase of Vickers hardness and thereby reduces the COF value [10, 58], agreeing well with this work.

Fig. 5
figure 5

a Coefficient of friction (COF) curves and b mean COF values of (AlCoCrFeNi)100−xHfx HEAs

Full size image

The 3D worn morphologies of (AlCoCrFrNi)100−xHfx HEAs are shown in Fig. 6. When Hf addition increases from 0 to 6 at%, the distance and depth of wear track of Hf-doped HEAs gradually decrease, indicating the anti-wear property improvement of Hf-doped HEAs. Figure 7 displays the wear rates of (AlCoCrFrNi)100−xHfx HEAs. With increasing Hf element, the Vickers hardness increases (Fig. 4a), and the wear rate decreases from 6.71 × 10−5 to 0.97 × 10−5 m3·N−1·m−1, agreeing well with Archard’s rule [21].

Fig. 6
figure 6

3D worn morphologies of (AlCoCrFeNi)100−xHfx HEAs

Full size image
Fig. 7
figure 7

Wear rate values of (AlCoCrFeNi)100−xHfx HEAs

Full size image

Figure 8 shows the worn morphologies of (AlCoCrFrNi)100−xHfx HEAs. On the wear track surface of Hf-0 HEA, the wear debris island and scattered wear debris are detected. The formation of the wear debris island is caused by the accumulation and compaction of wear debris [59]. Si and O are found on the surface of wear debris island. Meanwhile, O is obtained on the wear track surface of Hf-0 HEA (Table 4). Hence, the wear mechanism of Hf-0 HEA is affirmed to be abrasive wear, adhesive wear, and mild oxidative wear. For Hf-2, -4 and -6 HEAs, many wear grooves and wear debris are present on their wear track surfaces instead of the wear debris island. It is noted that O content on the wear track surfaces increases by adding Hf, manifesting that oxidative wear plays an increasingly crucial role. As a result, the wear mechanisms of Hf-2, -4 and -6 HEAs are oxidative wear and adhesive wear.

Fig. 8
figure 8

SEM images of worn morphologies of (AlCoCrFeNi)100−xHfx HEAs: a Hf-0; b Hf-2; c Hf-4; d Hf-6

Full size image
Table 4 Compositions (at%) of wear track surfaces in (AlCoCrFeNi)100−xHfx HEAs by EDS
Full size table

In Table 4, the chemical compositions of wear track surfaces of (AlCoCrFrNi)100−xHfx HEAs are testified by EDS. An increasing O content suggests that more oxides are produced on the wear track surfaces of Hf-doped HEAs. In order to identify the phase compositions of oxides on worn surfaces, Raman spectra of four (AlCoCrFrNi)100−xHfx HEAs are carried out and displayed in Fig. 9a. On the outside of wear track surface, no oxides are observed. For Hf-0 HEA, there is only a small number of Al2O3 and Cr2O3 oxides on the inside of the worn surface. The Vickers hardness of Hf-doped HEAs is higher than that of Hf-0 HEA. Therefore, Hf-doped HEAs are strong enough to resist the wear against the Si3N4 ball, and the alloy surface will not be worn off quickly during the wear test. Indeed, they can withstand being worn for a long time and are thus continuously heated due to friction. Such frictional heating raises the temperature of the local contact areas on the surface, causing serious surface oxidation [31, 38]. As a result, for Hf-6 HEA, there are a large number of Al2O3 and Cr2O3 oxides. Al2O3 and Cr2O3 oxides protect the Hf-doped HEAs from direct contact with friction pair and reduce the degree of adhesive wear [60, 61]. Thus, the wear rate of the Hf-doped HEAs is further decreased. The reason why Al2O3 and Cr2O3 oxides form rather than other oxides is correlated with the lowest Gibbs free energy of the formation of Al2O3 and Cr2O3 oxides, as shown in Fig. 9b.

Fig. 9
figure 9

a Raman spectra of (AlCoCrFeNi)100−xHfx HEAs and b Ellingham diagram of Al2O3, Cr2O3, Fe3O4, CoO and NiO oxides [62]

Full size image

4 Conclusion

To summarize, the phase compositions, microstructure features and wear properties of (AlCoCrFrNi)100−xHfx HEAs are studied. Hf addition changes the original phase constitution, which yields the formation of (Ni, Co)2Hf-type Laves phase besides the original solid solution phase. This causes the microstructure evolution from a BCC solid solution to a hypoeutectic structure. By increasing the Hf addition, the Vickers hardness enhances from HV 512.3 to HV 734.1, which is related to solid-solution strengthening, fine-grain strengthening and precipitated phase strengthening. The elevated hardness of Hf-doped HEAs and the formed oxides (Al2O3 + Cr2O3) resulting from the wear heating contribute to a downward wear rate, thus changing the wear mechanism from abrasive wear, adhesive wear and mild oxidative wear to oxidative wear and adhesive wear. The high-performance AlCoCrFeNiHf HEAs have broad application prospects in the mechanical equipment which are used in extreme conditions in the future.