Elsevier

Wear

Volumes 428–429, 15 June 2019, Pages 293-301
Wear

The scratch behaviour of AlXCoCrFeNi (x=0.3 and 1.0) high entropy alloys

https://doi.org/10.1016/j.wear.2019.03.026Get rights and content

Highlights

  • The scratch behaviour of Al1.0CoCrFeNi (BCC) and Al0.3CoCrFeNi (FCC) was studied

  • For the FCC materials, the fraction of material removed from the groove (f) fell in the range of 0.2–0.7

  • The f value for the BCC material lied in the range of 0.7–0.9 implying dominance of cutting mechanism.

  • A wear map was constructed which allows for alloy design and development for different wear conditions

  • At high loads hardness plays a major role in wear performance.

  • For low loads, a combination of hardness and scratch ductility dictates the material loss.

Abstract

The wear behaviour of as-cast Al1.0CoCrFeNi with a body-centred cubic (BCC) structure and Al0.3CoCrFeNi with a face-centred cubic (FCC) structure in the as-cast, homogenized and recrystallized conditions was studied by determining the intrinsic response to abrasive scratch testing. For the FCC material, the wear mechanism was qualitatively the same for the as-cast, homogenized and recrystallized conditions. The wear rate increased with increasing load, and the fraction of material removed from the groove (f) fell in the range of 0.2–0.7. The wear rate for the as-cast BCC material was significantly lower, which was ascribed to its high hardness. The f value for the BCC material lay in the range of 0.7–0.9, which implies dominance of the cutting wear mechanism. A wear map was constructed to enable comparison of the wear performance of materials under different loading conditions. It was found that the importance of hardness in wear is dependent on load. At high loads the wear performance of alloys highly depends on the scratch hardness. For low loads, however, the combination of scratch hardness and scratch ductility dictates total material loss.

Introduction

High entropy alloys (HEAs) enjoy the simultaneous presence of topological order and chemical disorder. Due to their high entropy (thermal and configurational), multicomponent HEAs often form simple solid solution phases with FCC or/and BCC structures rather than complicated intermetallic phases [[1], [2], [3]]. Their excellent mechanical properties, high temperature stability and resistance to oxidation make them ideal candidates for a wide range of applications from the automotive industry to wind turbines and construction machinery [[4], [5], [6]]. Their high strength and good fracture toughness [2,3] also makes them attractive for mining applications. However, such applications also require high resistivity to wear. This is the focus of the present work.

The AlXCoCrFeNi alloy system has been widely studied and displays a broad range of microstructures and properties. FCC equiatomic CoCrFeNi has good plasticity and low strength under tension [7,8] and compression [9,10]. The addition of aluminium increases the strength. This is due to its large atomic radius, which leads to distortion of the crystal lattice, the formation of ordered (L12) nano-precipitates and the presence of an ordered B2 phase (in BCC grades) [11]. It has been reported that for an arc-melted AlXCoCrFeNi alloy system, the crystal structure is FCC for alloys containing Al up to 0.45 (molar ratio) while a dual phase FCC/BCC structure forms in alloys containing Al between 0.45 and 0.88 (molar ratio). Al contents exceeding 0.88 (molar ratio) stabilises the BCC structure [12]. The alloy system exemplifies the breadth of properties achievable in a HEA and will be employed in the present study.

The wear behaviour of HEAs has been investigated mostly by sliding wear. For instance, Wu et al. [13], studied the adhesive wear behaviour of AlXCoCrCuFeNi high-entropy alloys. They found that for Al contents of 0.05 and 1.0 M fraction, the wear mechanism is predominantly delamination wear. An increase in aluminium content to 2.0 M changed the wear mechanism to predominantly oxidative wear. Hsu et al. [14], studied the influence of adding iron to AlCoCrFeXMo0.5Ni on the sliding wear resistance and reported a decrease in wear rate with increasing iron due to an increase in the oxidation and sigma phase formation. Chuang et al. [15], examined the microstructure and wear behaviour of AlXCo1.5CrFeNi1.5TiY high-entropy alloys. They suggested that the wear mechanism of the Al00Ti05 and Al02Ti05 alloys was delamination wear, whereas the Al00Ti10 and Al02Ti10 alloys displayed mild oxidational wear, evident from oxides and Fe in debris. Joseph et al. [16], pointed to the suitability of Al1.0CoCrFeNi HEAs for high temperature wear applications due to the combined action of high hot-strength due to the precipitation of σ-phase and the presence of an alumina scale that minimises delamination wear. Wang et al. [17], investigated the microstructure and wear properties of nitrided AlCoCrFeNi alloy reporting an enhancement in the hardness/wear resistance by nitriding. In these studies sliding wear has been mainly studied, while less attention has been paid to abrasive conditions, and in particular the early stages of abrasive wear, which can be studied through scratch testing.

By employing scratch testing on virgin material, the intrinsic scratch response of a material can be assessed. In a real wear situation, the surface will become work hardened, increasing its flow strength and reducing its ductility. The effect of a worked (worn) surface upon the behaviour in a subsequent scratch/gouging event will depend upon the degree to which the scratch penetrates into the surface. A shallow scratch will interact with the brittle work hardened surface layer. A deeper scratch will tend to interact more with the virgin subsurface material. Thus the situation is rather complex. The basic intrinsic material response to scratching is, however, a starting point to understanding the complexity. For example, scratch testing has been used to show that the crystallographic orientation of grains strongly affects the corresponding mechanisms of plastic ploughing and wear resistance in Ti [18]. In another study, high temperature scratching has been performed on unworn surfaces of an austenitic steel, a cast steel with a carbide network and a Ni-based alloy with carbide network to reveal their wear behaviour [19]. Scratch testing has also been used to shed light on the dominant wear mechanisms in Hadfield and construction steels [20,21].

To the best of our knowledge, there are only two studies using scratch testing to examine the wear of HEAs. In one of them, the effect of addition of 5 at.% C on a FeCoCrNiW0.3 coating was assessed [22]. The work showed that addition of 5 at.% C significantly increased the scratch resistance of this HEA, where it became comparable to the commercial wear resistant Stellite® 6 alloy. In the other study, the effect of the carburization of CoCrFeNi HEA was studied through nano-scratch testing [23]. It was shown that the carburized layer shows higher wear resistance compared with the matrix. The scratch behaviour of more common HEAs systems such as AlXCoCrFeNi appears not to have been studied. In the current work, the scratch resistance of AlXCoCrFeNi alloys containing 0.3 and 1.0 (molar ratio) aluminium is investigated using micro and macro scratch testing. A wear map for HEAs is generated. The findings are important in further consideration of these alloys for critical parts where scratch resistance plays a role.

Section snippets

Experimental procedure

Nominal alloys Al0.3CoCrFeNi and Al1.0CoCrFeNi (molar ratio) were cast with the exact compositions (wt. %) of “Al 3.4, Co 24.3, Cr 23.2, Fe 23.3, Ni 25.0” and “Al 10.9, Co 21.8, Cr 22.6, Fe 21.8, Ni 22.4”. Casting was performed using a vacuum arc melting furnace to melt a total weight of ∼100 g using a water-cooled Cu hearth. The samples were cast as rectangular rods ∼10 × 10 × 80 mm size. Melting was performed in a pure argon atmosphere with a Ti-getter. Ingots were inverted and remelted at

Results

The results obtained from XRD showed that the as-cast microstructure of the alloy containing 3.4 at.% Al has an FCC crystal structure, while the alloy containing 10.9% Al has a fully BCC crystal structure (Fig. 1). This is consistent with previous investigations on arc-melted AlXCoCrFeNi where alloys with low Al concentrations (less than 0.4 M fractions) exhibited a single-phase FCC structure, and those with molar fractions higher than 0.7 possessed a single-phase BCC structure [12].

The SEM

Discussion

In the materials studied in the present work, the lowest wear rates are seen in the hardest materials, as expected. However, for the FCC structures, the effect is only evident at higher loads. At lower loads the slightly higher f values in the harder recrystallized FCC samples offset its shallower grooves enabled by the high hardness. The wear volume for the BCC microstructure is almost the same as for the FCC case for loads up to 200 mN. It is, however, substantially lower compared to that of

Conclusions

The purpose of this research was to study the single-point abrasion properties of AlXCoCrFeNi (x = 0.3 and 1.0) high entropy alloys. This was carried out employing scratch testing using 25 μm and 0.8 mm spherical indenters under different loads. Through incorporating the parameter ‘scratch ductility’, a wear map was constructed for different high entropy alloys with the different microstructures. An analytical approach was used to calculate the materials wear loss based on the groove geometry.

Acknowledgements

The financial support through the Australia-India Strategic Research Fund (AISRF53731-Advanced manufacturing of new high entropy alloys) is acknowledged. The authors are grateful to the Deakin Advanced Characterisation Facility and Mr. Mohan Setty for the technical support. NH sincerely acknowledges the financial support by the government of Australian state of Victoria and VESKI through the Victoria Fellowship.

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