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Article

Mechanical, Corrosion, and Wear Properties of TiZrTaNbSn Biomedical High-Entropy Alloys

by
Xiaohong Wang
1,
Tingjun Hu
1,
Tengfei Ma
1,
Xing Yang
1,
Dongdong Zhu
1,
Duo Dong
1,
Junjian Xiao
1,* and
Xiaohong Yang
2,3,*
1
Key Laboratory of Air-Driven Equipment Technology of Zhejiang Province, Quzhou University, Quzhou 324000, China
2
Academician Expert Workstation, Jinhua Polytechnic, Jinhua 321017, China
3
Key Laboratory of Crop Harvesting Equipment Technology of Zhejiang Province, Jinhua 321017, China
*
Authors to whom correspondence should be addressed.
Coatings 2022, 12(12), 1795; https://doi.org/10.3390/coatings12121795
Submission received: 21 October 2022 / Revised: 18 November 2022 / Accepted: 21 November 2022 / Published: 22 November 2022
(This article belongs to the Special Issue Mechanical, Corrosive and Tribological Degradation of Metal Coatings and Modified Metallic Surfaces)
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Abstract

:
The phase composition, microstructure, mechanical, corrosion, and wear behaviors of the Ti15Zr35Ta10Nb10Sn30 (Sn30) and Ti15Zr30Ta10Nb10Sn35 (Sn35) biomedical high-entropy alloys (BHEAs) were studied. We found that the Ti–Zr–Ta–Nb–Sn BHEAs showed hyper-eutectic and eutectic structures with body-centered cubic (BCC) and face-centered cubic (FCC) solid-solution phases. The Sn30 BHEA exhibited a high Vickers hardness of approximately 501.2 HV, a compressive strength approaching 684.5 MPa, and plastic strain of over 46.6%. Furthermore, the Vickers hardness and compressive strength of Sn35 BHEA are 488.7 HV and 999.2 MPa, respectively, with a large plastic strain of over 49.9%. Moreover, the Sn30 and Sn 35 BHEA friction coefficients are 0.152 and 0.264, respectively. Sn30 BHEA has the smallest and shallowest furrow-groove width, and its wear rate is 0.86 (km/mm3); at the same time, we observed the delamination phenomenon. Sn35 BHEA has a wear rate value of 0.78 (km/mm3), and it displays wear debris and the largest–deepest furrow groove. Sn30 BHEA has the highest impedance value, and its corrosion current density Icorr is 1.261 × 10−7 (A/cm2), which is lower than that of Sn35 BHEA (1.265 × 10−6 (A/cm2)) by 88%, and the passivation current density Ipass of Sn30 BHEA and Sn35 BHEA is 4.44 × 10−4 (A/cm2) and 3.71 × 10−3 (A/cm2), respectively. Therefore, Sn30 BHEA preferentially produces passive film and has a small corrosion tendency, and its corrosion resistance is considerably better than that of the Sn35 BHEA alloy.

1. Introduction

Pure Ti has the advantages of non-toxicity, light weight, high strength, good biocompatibility, etc. Therefore, in the 1950s, the United States and the United Kingdom started to apply it for use with living organisms [1]. In the 1960s, Ti alloys (first Ti-6A1-4V [2,3] and later Ti-5Al-2.5Fe and Ti-6A1-7Nb) began to be widely used in clinical practice as a human implant material [4,5,6,7]. In the 1970s and 1980s, researchers began to prepare Ti alloys with V-free implants because of its toxic and potentially harmful effects on the human body; furthermore, in the mid-1980s, new types of α+β Ti alloys, namely Ti-5Al-2.5Fe and Ti-6Al-7Nb, were developed in Europe [3]. The mechanical properties of these alloys are similar to Ti-6Al-4V [7], albeit with higher biocompatibility and corrosion resistance properties. However, these alloys still contain Al, which can cause organ damage and harmful symptoms, such as osteomalacia, anemia, and neurotin disorder [4,5]. Based on the above reasons, new types of alloys, which are free of both V and Al but with the addition of Nb, Zr, Ta, and Sn (Ti-13Nb-13Zr, Ti-35Nb-5Ta7Zr, Ti-24Nb-4Zr-7.9Sn), have been developed in recent years [4,5,6], and their elastic moduli are closer to that of natural human bone, and their strength is also higher than that of pure Ti. Consequently, Ti alloys are being rapidly developed for human implant materials; however, their strength, friction and wear, and corrosion resistance need to be studied further.
Traditional alloy systems usually consist of one or two main elements, and the content of the other elements is much lower. In 2004, Yeh et al. [8,9,10] first proposed high-entropy alloy, a class of materials containing five or more elements in relatively high concentrations (5–35 at.%) [11,12]. Due to their unique high-entropy, sluggish diffusion, lattice distortion, and cocktail effects [13,14,15], HEAs show excellent comprehensive properties compared with traditional alloys, such as high hardness [16], high strength [17], corrosion resistance, wear resistance [18,19], etc. At the same time, high-entropy alloys break the traditional alloy design concept of using only one principal element. Despite this, researchers are actively exploring the possibility of applying high-entropy alloys in new products [20], such as biomedical, magnetic, hydrogen storage materials, etc. [21,22,23,24,25]. At present, the comprehensive mechanical properties of medical alloys are still to be improved in clinical practice. Therefore, the design concept of high-entropy alloy can be used to prepare biomedical high-entropy alloy materials with low modulus, high strength, corrosion resistance, and other excellent comprehensive properties to meet demand.
In the past decade, a series of Ti-Zr-Hf-Nb-Ta [21,22,23,24,25], Ti-Mo-Ta-Nb-Zr [26,27,28,29], and Ti-Nb-Hf-Ta-Zr-Mo [30,31] HEAs with considerable mechanical and chemical properties suitable for biomedical applications have been designed. Researchers are also improving the performance of HEAs by alloying with O, Si, Al, and Cr [32,33,34,35]. However, we noticed that adding Sn to Fe-Co-Cu-Ni(-Mn) HEAs can improve elongation strain and tensile strength by 16.9% and 476.9MPa, respectively [36,37]. In addition, Sn is non-cytotoxic and widely present in β-Ti alloys [38,39,40,41]. Previously, we studied the effects of atomic ratios on as-cast microstructural evolution, and the mechanical and electrochemical properties of Ti30Zr20Ta20Nb20Sn10 high-entropy alloy [42]. While its elastic modulus is relatively high, with a value of 110GPa, it does not match the elastic modulus of human bones (30–50 GPa). Moreover, Zr-based Ti0.5Zr1.5Ta0.5NbSn0.2 (Ti13.5Zr40.5Ta13.5Nb27Sn5.5) high-entropy alloys display an elastic modulus value of about 40 GPa [43]. Therefore, in our study, we designed a new Zr-based high-entropy Ti-Zr-Ta-Nb-Sn alloy based on metastable β-titanium alloy, which is based on the four elements of Ti-Zr-Ta-Nb. We prepared two kinds of biomedical-grade high-entropy alloys, namely Sn30 BHEA and Sn35 BHEA, by vacuum arc melting, and we systematically studied the feasibility of preparing biomedical-grade high-entropy alloy with five elements (Ti, Zr, Ta, Nb, and Sn), paying specific attention to morphology, compressive strength, electrochemical corrosion, and friction and wear properties. Our work will provide data for the future development of biomedical-grade high-entropy alloy, in addition to guidance for further scientific research on Ti alloys.

2. Experimental

The Ti, Zr, Ta, Nb, and Sn raw materials with a purity of more than 99.9 wt.% were used to prepare BHEAs with the atom ratio of Sn30 BHEA and Sn35 BHEA. The master alloy ingot was arc melted and cooled on a water-cooled copper crucible in a high-purity argon protective atmosphere at least 5 times to ensure the chemical homogeneity. Wire cut electrical discharge machining (WEDM, DHL-500) was used to cut samples from the core region of the master alloy ingot (Buttonhole, maximum diameter Φ32mm, maximum height 16 mm). The surface of the alloy sample was ground with silicon carbide sandpaper up to 2000 grit. The phase composition and microstructure of the alloy sample were characterized via an X-ray diffractometer (XRD, Rigaku D/max-RB) and a Hitachi S-4300 scanning electron microscope and HITACHI SU8010 scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDS) was used to analyze the chemical composition of the HEAs.
Cylinder-shaped (Φ4 mm × 6 mm) samples were cut via WEDM, and used for Vickers hardness and room-temperature compression stress and strain test. The microhardness test was carried out on an HYHVS-1000T Vickers hardness tester using an applied load of 1000 g force (gf) and a dwell time of 15 s. A 370.1 type mechanical testing system (MTS) was employed to record the room-temperature compression stress and strain curves of HEA samples. The compressive strain rate was set as 0.5 mm/min. After the compression test, the lateral surface morphologies of the compressed specimens were examined by SEM.
Corrosion behavior was obtained through the electrochemical workstation (CHI660E) using a three-electrode-cell system. Saturated mercuric chloride was used as reference electrode, platinum electrode as counter electrode, a 1 mm thick sample was embedded in copper wire as working electrode, test temperature was 26 ℃, scanning speed was 1 mV/s, and scanning voltage range was from −1.5 to +1.5 V. Before corrosion experiments, the surface of the sample was polished with 2000 grit silicon carbide sandpapers. Then, as-polished specimens were ultrasonically cleaned in deionized water, acetone, and ethanol. Before the potentiodynamic polarization test, the alloy sample was immersed in 3.5% NaCl solution until the open circuit potential (OCP) reached a stable state. After the polarization experiment, TAFEL and IMP were used to measure the polarization curve and AC impedance of Sn30 and Sn35 BHEAs, respectively. Then, Corrview software was used to analyze the Tafel curve and Zview software to analyze the impedance spectrum. The corroded morphologies on the sample surface were examined by SEM, and the composition of the corroded surface was determined by EDS.
When performing the friction and wear test, we ensured each specimen of the two materials was wet-ground and polished using a polishing machine (UNIPOL-1502, Shenyang Kejing Auto-Instrument Co., Ltd., Shenyang, China) with a series of silicon carbide papers of P320, P400, P800, P1200, P1500, P2000, and P3000 grits (Matador Starcke, Germany) under water cooling. Finally, after ultrasonically cleaning for 10 min in deionized water, we fine-polished all specimens with a diamond velour polishing pad under a flowing cerium oxide solution (particle size: 1.5 μm) (Shenyang Kejing Auto-Instrument Co., Ltd., China) before finishing with a mirror-like surface. Then, we tested the wear behaviors of the HEAs by a VHX-2000 tribology tester using a Si3N4 ball (4 mm in diameter) as the couple-pair. In our study, the test parameters were as follows: load 10 N, time of 30 min, sliding velocity 600 r/min, and friction reciprocating motion amplitude 2 mm. We recorded the friction coefficient during the sliding process. After the wear test, we determined the wear volume (WV) of the alloy samples by an MT-500 Probe-type material surface profile measuring instrument. We examined the morphologies and compositions of the HEAs’ wear scars by SEM and EDS, respectively.

3. Results

3.1. Phase Composition and Microstructure of Sn30 BHEA and Sn35 BHEA

Figure 1 shows the XRD diffraction patterns of Sn30 BHEA and Sn35 BHEA. We found that there is an obvious peak correspondence relationship by comparing with the standard PDF database. The diffraction peaks of 36.8° and 38.4° correspond to BCC Zr, FCC Nb, and BCC Ti solid solutions, respectively. At the same time, 64.8° corresponds to the FCC Ti solid solution, while 75.9° and 80.2° correspond to the diffraction peaks of the BCC Zr and BCC Ti solid solutions, respectively. We also observed that 41.8° corresponds to the diffraction peak of the BCC Ti solid solution in Sn30 BHEA. Furthermore, the 37.6° (Sn35 BHEA) and 35.9° (Sn30 BHEA) diffraction peaks correspond to the HCP Zr5Sn3 phase. Our results show that both alloys contain BCC and FCC phases. Table 1 shows the corresponding thermodynamic parameters. The values of ΔH, ΔS, and δ are in the range where the solid-solution phase is likely to occur. Table 1 shows the calculated values of the entropy–enthalpy quotient parameter (Ω), the valence electron concentration (VEC) criterion, and the mean square deviation of the atomic radius of elements (δ) of these two high-entropy alloys. The thermodynamic parameter VEC further proves that FCC and BCC phases co-exist in the alloy.
We used SEM and EDS to analyze the chemical compositions and microstructures of HEAs. Figure 2 shows the microstructures of Sn30 BHEA and Sn35 BHEA. Sn30 BHEA is a typical hypoeutectic structure composed of long-strip gray phases, a bright white small-block phase, and an interphase rod eutectic microstructure (Figure 2a,b). Moreover, the eutectic microstructure’s volume fraction is 33.8%. Sn35 BHEA is a typical eutectic microstructure with lamellar distribution; however, there are some bright white blocky phases distributed on the dark gray phase. Additionally, the solidification mode looks more equiaxed for Sn35 BHEAs, while it seems more columnar dendritic for Sn30 BHEAs. With changes in atomic ratio, the eutectic structure’s content and morphology also considerably changed. In order to analyze the element content of each phase in Figure 2, EDS analysis was carried out on eutectic microstructure (area 1), gray phases (area 2), and bright white blocky phases (area 3). The results are shown in Table 2. We discovered a high content of Zr and Sn in the dark gray phase of both high-entropy alloys. Combined with our XRD analysis results, we deduced that this phase is a Zr solid solution with a BCC crystal structure. The bright-colored small-block area contains a large amount of Ti and Ta, which exceeds the initial composition of the alloy; therefore, it is rich in Ta and Ti FCC phase. Based on our above analysis, we determined that the bright-colored phase in the remaining eutectic structure is a Ti solid-solution phase with a BCC crystal structure.
To further analyze the element distribution, the map scanning results of its element distribution are shown in Figure 3. Figure 3 shows the map scanning results of the element distribution. The Zr, Nb, and Sn elements are mainly distributed in the dark gray phase, while Ti tends to be uniformly distributed in the dark gray phase, and is eutectic in structure; however, Ti is segregated in the bulk phase close to the eutectic microstructure (Figure 3(a1–a5)). For Sn35 BHEA, the distribution of elements tends to be consistent with that of Sn30 BHEA.

3.2. Mechanical Properties of Sn30 BHEA and Sn35 BHEA

The mechanical properties of Sn30 BHEA and Sn35 BHEA are shown in Figure 4 and Figure 5. Figure 4a shows the compression stress–strain curves of Sn30 BHEA and Sn35 BHEA at room temperature. Compared with Sn35 HEAs, the Sn30 HEAs exhibit double-yielding behavior, which is often observed in shape memory alloys as a stress-induced phase transformation and has relatively lower plasticity [44]. Figure 4b displays the Vickers hardness results for both alloys. Hardness is one of their mechanical properties, and it has a considerable impact on the application of alloys. At the same time, it is also one of the factors that affect alloys’ friction and wear properties. Sn30 HEA has the highest hardness level, and the average value of 5 measurements is 501.2HV, while the hardness of alloy Sn35 HEAs is 488.72HV, which is slightly lower. Figure 4c–f show the fracture morphologies of these two high-entropy alloys after compression testing. Figure 4c,d show that Sn30 BHEA’s macrofracture morphology is relatively flat, while its micromorphology has river patterns, showing obvious shear failure characteristics. On the gray strip, it also shows typical brittle fracture characteristics. Furthermore, after increasing the magnification, we observed tear edges and dimples on the eutectic structure, showing the mixed characteristics of quasi-cleavage and ductile fractures. Generally, the samples show brittle quasi-cleavage fractures [45]. Figure 4e,f show that the compression fracture surfaces of Sn35 HEAs are uneven and considerably fluctuate, and there are obvious signs of shear tear on the fractures. After increasing the magnification, we observed shallow and slender dimples on the relatively flat shear plane, and we determined that the compression fracture is generally a brittle cleavage fracture. Figure 5 shows the maximum strain and compressive strength values corresponding to the two alloys. The maximum strain value and compressive strength of Sn30 HEAs are 46.6% and 684.5 MPa, respectively, and the maximum strain value and compressive strength of Sn35 HEAs are 49.9% and 999.2 MPa, respectively. Therefore, Sn35 HEAs exhibit better mechanical properties.

3.3. Friction and Wear Properties of Sn30 BHEA and Sn35 BHEA

As hardness is one of the primary factors that affect the friction and wear properties of the alloys, there are certain requirements regarding the friction and wear properties of materials implanted into the human body. The friction and wear mechanical properties tests of Sn30 BHEA and Sn35 BHEA are shown in Figure 6. Wear marks can be clearly seen in Figure 6a, which shows the sample used in the friction and wear test, including its wear marks and approximate dimensions. Figure 6b displays the friction coefficient curves (COFs). The COFs of Sn30 and Sn35 are 0.152 and 0.264, respectively. Moreover, the friction coefficients first display a sharp increase and then decrease before becoming flat. The main reason is that at the initial stage, the sample and the Si3N4 sphere will undergo a point-to-surface contact process. First, the friction coefficient is sensitive to the roughness of the sample surface, causing the friction coefficient to rapidly increase. When the ball makes complete contact with the sample and the contact condition is stable, the friction coefficient tends to be flat. The Sn30 BHEA presented the minimum average COFs. The mechanical properties (hardness, strength, and plasticity) have a considerable effect on tribological properties [46,47,48]. Normally, the relationship between tribological and mechanical properties can be described as W = k(P/H) (W is wear rate, P is applied load, k is relative to plasticity, and H is hardness) [46]. Figure 4 shows there are no considerable differences in plasticity; however, Sn30 BHEA’s hardness is much higher. Hence, the tribological properties of Sn30 BHEA are superior to Sn35 BHEA.
To further obtain the wear volume (Wv) of the Sn30 BHEA and Sn35 BHEA after the wear test, the section profile morphologies of the worn surfaces were characterized by a Keyence surface profilometer (VHX-2000), and the results are illustrated in Figure 6c,d. Sn30 BHEA showed finer grooves owing to its higher hardness; therefore, it resists abrasive wear (Figure 6c). Abrasive wear was aggravated in Sn35 BHEA, which was confirmed by its widened grooves (Figure 6d). The corresponding wear width and depth in Figure 6a can be obtained directly from the images. Table 3 displays the calculated wear properties, where E is wear resistance, Kv is specific wear rate, and K2 is linear wear rate. The Kv of Sn30 BHEA is 2.27 × 10−4 (mm3/nm), the K2 is 1.163 (mm3/km), E is 0.86 (km/mm3), and the Wv is 0.51 mm3. Furthermore, the Kv of Sn35 BHEA is 2.49 × 10−4 (mm3/nm), and the values of K2, E, and Wv are 1.277 (mm3/km), 0.78 (km/mm3), and 0.56 mm3, respectively. A higher material loss rate will also lead to slight work hardening (Figure 7). Therefore, our comprehensive analysis shows that Sn30 BHEA has better friction and wear morphologies.
We carried out our SEM characterizations of the worn surfaces to explore the wear mechanisms of Sn30 BHEA and Sn35 BHEA, the results are displayed in Figure 8, and the plastic deformation and delamination traces showed severe wear for both alloys. Furthermore, the wear surfaces of all Sn30 BHEAs and Sn35 BHEAs showed typical furrow characteristics, and there are grooves and wear debris along the sliding direction due to the micro cutting and furrow effects of the Si3N4 ball, which indicates that furrow wear is the main wear mechanism [47,49].
Sn30 BHEA’s worn surfaces were smoother with less debris and smaller grooves compared with Sn30 BHEA, as shown in Figure 8a–d. Moreover, Figure 8a shows that the surface morphology of the Sn30 BHEA alloy’s friction structure tends to form a regular pit structure, and the wear surface is almost free of wear debris. Figure 8c,d show that there are wear debris generated on Sn35 BHEA’s surface morphology, resulting in poor surface quality and cracks. Furthermore, we found severe delamination in Sn35 BHEA. The subsurface generates work hardening owing to its dislocation motion, rearrangement, and grain refinement. However, once deformed, dislocation accumulation occurred to a certain extent, resulting in crack formation and, finally, delamination. The absence of obvious work hardening for Sn30 BHEA (Figure 7) was due to its higher material loss rate. This consecutive deformation can be easily induced through delamination [50].

3.4. Corrosion Property Characterization of Sn30 BHEA and Sn35 BHEA

The corrosion resistance properties of biomedical materials during their implantation into the human body is another aspect that needs to be studied. Figure 9 shows the impedance spectroscopy of Sn30 BHEA and Sn35 BHEA, Figure 9a is the Nyquist diagram, and Figure 9b shows the Bode diagram. The electrochemical AC impedance value can reflect the corrosion resistance of the alloy to a certain extent. The greater the impedance value, the better the corrosion resistance. It can be seen from Figure 9a that the capacitive arc curvature radius of Sn30 BHEA decreases significantly, indicating that among the two high-entropy alloys, Sn30 BHEA has the best corrosion resistance; furthermore, it suggests that the decrease in Sn content and the increase in Zr content will improve the corrosion resistance of Sn30 BHEA and Sn35 BHEA. In the Bode diagram, the phase angle of Sn30 BHEA is close to 75° in a wide frequency range, while the phase angle of Sn35 BHEA is about 60°. Sn30 BHEA has a large resistance value and forms a passive film.
The potentiodynamic polarization curve and the corrosion morphology of Sn30 BHEA and Sn35 BHEA are shown in Figure 10. The key parameters derived by the Tafel method such as the corrosion current density (Icorr) and the corrosion potential (Ecorr) are listed in Table 4. It was clearly found that Sn30 BHEA exhibited an Icorr value smaller than that of Sn35 BHEA. Moreover, for passivation current density Ipass, the smaller the value, the easier it is to enter the passivation state. The Ipass value of Sn30 BHEA is 4.44 × 10−4 A/cm−2, which is 88% lower than that of Sn35 BHEA—a significant improvement. Another noteworthy fact is that in the anode and cathode area, the value of βa and βb of Sn30 BHEA is lower than that of Sn35 BHEA, demonstrating that Sn35 BHEA has stronger corrosion resistance properties. The corrosion potential (Ecorr) can determine the corrosion trend of the alloy. According to the thermodynamic principle, the smaller the Ecorr, the greater the corrosion tendency. The Ecorr of Sn30 HEAs is −0.96, which is about 43.3% smaller than that of Sn35 BHEAs. However, the data that most directly reflect the corrosion resistance of the alloy are the corrosion rate, which can be obtained by the following equation.
C R = 0.13 I corr ∗ EW d ,
where EW is the equivalent weight, and d is the density of the metal (g/cm3). The calculation results are shown in Table 4. The corrosion rate of Sn30 BHEA is 1.37 × 10−4 mm/a. The corrosion rate of Sn35 BHEA is 1.20 × 10−3 mm/a, which is nearly 88.6% greater than that of ambient pressure. Therefore, Sn30 BHEA has stronger corrosion resistance properties.
The corrosion surface morphology was observed by scanning electron microscopy (SEM) after the potentiodynamic polarization test, as shown in Figure 10b,e. The surfaces of these two high-entropy alloys show certain levels of corrosion after electrochemical corrosion for 2400s and the corrosion degree of (Zr, Sn)-rich phase is high. Meanwhile, the corrosion of Sn35 BHEA also mainly occurs in the gray phase, which is accompanied by many irregular corrosion pits and some cracks. There are also some electrochemical corrosion products on the surface, as shown in Figure 10d,e. The volume fractions of large dendritic Zr-rich zones within Sn30 BHEA and Sn35 BHEA are 77.7% and 90.8% (Figure 11), respectively. Then, from the corrosion morphology and analysis of the above two high-entropy alloys, it can be concluded that the corrosion resistance of Sn30 BHEA is better than that of Sn30 BHEA. However, both alloys exhibit high levels of corrosion resistance [51], indicating that Sn30 BHEA and Sn35 BHEA are a suitable alternative material considering corrosion resistance.

4. Discussion

According to our SEM results for the surface of the corroded coatings, all of the corroded alloys exhibited similar surface morphologies, i.e., corrosion at the (Zr, Sn)-rich location. The volume fraction of the (Zr, Sn)-rich phase in Sn30 BHEA is smaller than in Sn35 BHEA (Figure 11). Combining the results of our electrochemistry corrosion analysis, we discovered that decreases in the anti-corrosion properties of Sn35 BHEA were due to the increases in the volume fraction of the (Zr, Sn)-rich phase. Moreover, the hardness of Sn30 BHEA is greater than Sn35 BHEA. However, as Sn35 BHEA exhibits a near-eutectic microstructure, it shows excellent plasticity, which, in turn, leads to the best compression strength.
The anti-wear properties are concerned with the alloy’s surface quality, hardness, toughness, and loading capability [52,53,54,55]. Besides the different anti-wear properties of Sn30 BHEA and Sn35 BHEA, the hardness of each alloy is the key factor related to wear resistance. The hardness of Sn30 BHEA is greater than that of Sn35 BHEA because the volume fraction of the Ta-rich BCC phase is extensively generated (Figure 11). Moreover, during the sliding wear process, the hard Si3N4 ball will act similar to a cutter tool and peel off the surface materials, leading to severe plastic deformation and a high wear rate. In addition, the surface’s severe wear and damage is accompanied by oxidation under the high-speed wear process. However, for Sn30 BHEA, wear resistance is considerably improved because of the large number of BCC phases generated, which could act as strengthening support sites and carry loads during the wear process. To sum up, Sn30 BHEA shows excellent comprehensive properties. We also conclude that too many Sn elements are not conducive to TiZrTaNbSn HEA design.

5. Conclusions

Taking β-type titanium alloy as the design conception, the comprehensive properties of biomedical high-entropy alloy based on Ti-Zr-Nb-Ta and Sn element were systematically discussed. The main conclusions are as follows:
  • Sn30 BHEA and Sn35 BHEA are typical hypo-eutectic and eutectic structures, respectively, and both alloys are composed of BCC and FCC phases;
  • the two high-entropy alloys have brittle fractures at room temperature. The maximum strain value and compressive strength of Sn30 BHEA are 46.6% and 684.5 MPa, respectively, while the maximum strain value and compressive strength of Sn35 BHEA are 49.9% and 999.2 MPa, respectively. Sn30 HEA’s strength and yield strength are better than those of Sn35 HEA;
  • the friction coefficient of Sn30 BHEA is 0.152, the specific and linear wear rates (Kv and K2, respectively) are 2.27 × 10−4 (mm3/nm) and 1.163 (mm3/km), respectively, while the width of the furrow groove is the smallest and shallowest, with almost no wear debris. Furthermore, the friction coefficient of Sn35 BHEA is 0.264, and the values of specific and linear wear (Kv and K2, respectively), in addition to wear resistance E, are 2.49 × 10−4 (mm3/nm), 1.277 (mm3/km), and 0.78 (km/mm3), respectively. Furthermore, the width of the furrow groove is the largest and deepest, and there are wear debris. In conclusion, the Sn30 HEA has excellent wear resistance and rates compared with Sn35 HEA;
  • Sn30 BHEA has the highest impedance value. The corrosion current density Icorr is 1.261 × 10−7 (A/cm2), which is lower than that of Sn35 BHEA by about 88%. The capacitive arc curvature radius of Sn30 BHEA also considerably decreases. Therefore, Sn30 HEAs preferentially produce passivated film with a small corrosion tendency, indicating that its corrosion resistance is considerably better than that of Sn35 BHEA alloy.

Author Contributions

X.W.: Conceptualization, Investigation, Formal analysis, Writing—original draft. T.H.: Investigation, Writing—original draft. T.M.: Conceptualization. X.Y. (Xing Yang): Investigation. D.Z.: Conceptualization. D.D.: Software. J.X.: Investigation, Methodology, Writing—review and editing. X.Y. (Xiaohong Yang): Conceptualization, Formal analysis, Supervision, Funding acquisition, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Zhejiang Province Natural Science Foundation of China (No.: LQ20E010003), the National Natural Science Foundation of China (Nos. 52071188, 52171120, 52001262, 52071165), and Research Funding Project of Education Department of Zhejiang Province (No. Y202045618).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 12 01795 g001 550
Figure 1. XRD patterns of Sn30 BHEA and Sn35 BHEA.
Figure 1. XRD patterns of Sn30 BHEA and Sn35 BHEA.
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Figure 2. The microstructure of Sn30 BHEA and Sn35 BHEA. (a,b) Sn30 BHEA, (c,d) Sn35BHEA.
Figure 2. The microstructure of Sn30 BHEA and Sn35 BHEA. (a,b) Sn30 BHEA, (c,d) Sn35BHEA.
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Figure 3. The map scanning of Sn30 BHEA and Sn35 BHEA: (a)–(a5) Sn30 BHEA, (b)–(b5) Sn35 BHEA.
Figure 3. The map scanning of Sn30 BHEA and Sn35 BHEA: (a)–(a5) Sn30 BHEA, (b)–(b5) Sn35 BHEA.
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Figure 4. Compressive stress–strain curves and corresponding fracture morphologies of Sn30 BHEA and Sn35 BHEA: (a) stress–strain curves, (b) HV of Sn30 BHEA and Sn35 BHEA, (c,d) fracture morphology of Sn30 BHEA, (e,f) fracture morphology of Sn35 BHEA.
Figure 4. Compressive stress–strain curves and corresponding fracture morphologies of Sn30 BHEA and Sn35 BHEA: (a) stress–strain curves, (b) HV of Sn30 BHEA and Sn35 BHEA, (c,d) fracture morphology of Sn30 BHEA, (e,f) fracture morphology of Sn35 BHEA.
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Figure 5. Compression properties of Sn30 BHEA and Sn35 BHEA.
Figure 5. Compression properties of Sn30 BHEA and Sn35 BHEA.
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Figure 6. Friction and wear test results of Sn30 BHEA and Sn35 BHEA (a) samples, (b) friction coefficient curves, and profiles of the worn surfaces for sintered composites and corresponding 2D cross-section profiles of wear tracks: (c) Sn30 BHEA, (d) Sn35 BHEA.
Figure 6. Friction and wear test results of Sn30 BHEA and Sn35 BHEA (a) samples, (b) friction coefficient curves, and profiles of the worn surfaces for sintered composites and corresponding 2D cross-section profiles of wear tracks: (c) Sn30 BHEA, (d) Sn35 BHEA.
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Figure 7. Hardness of the worn surface for the Sn30 BHEA and Sn35 BHEA.
Figure 7. Hardness of the worn surface for the Sn30 BHEA and Sn35 BHEA.
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Figure 8. SEM images of Sn30 BHEA and Sn35 BHEA: (a,b) Sn30 BHEA, (c,d) Sn35 BHEA.
Figure 8. SEM images of Sn30 BHEA and Sn35 BHEA: (a,b) Sn30 BHEA, (c,d) Sn35 BHEA.
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Figure 9. EIS of Sn30 BHEA and Sn35 BHEA: (a) Nyquist plots, (b) Bode plots.
Figure 9. EIS of Sn30 BHEA and Sn35 BHEA: (a) Nyquist plots, (b) Bode plots.
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Figure 10. Corrosion properties characterization for Sn30 BHEA and Sn35 BHEA: (a) potentiodynamic polarization curves, (b,c) corrosion morphology of Sn30 BHEA, (d,e) corrosion morphology of Sn35 BHEA.
Figure 10. Corrosion properties characterization for Sn30 BHEA and Sn35 BHEA: (a) potentiodynamic polarization curves, (b,c) corrosion morphology of Sn30 BHEA, (d,e) corrosion morphology of Sn35 BHEA.
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Figure 11. Volume fraction of Zr-rich phase in Sn30 BHEA and Sn35 BHEA.
Figure 11. Volume fraction of Zr-rich phase in Sn30 BHEA and Sn35 BHEA.
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Table
Table 1. Thermodynamic parameters of Sn30 BHEA and Sn35 BHEA.
Table 1. Thermodynamic parameters of Sn30 BHEA and Sn35 BHEA.
AlloyΩδΔH/(KJ/mol)ΔS/J (K/mol)VEC
Sn30 BHEA 1.043.78−21.212.257.2
Sn35 BHEA 0.953.64−2212.37.7
Table
Table 2. EDS analysis of the phase in Figure 2.
Table 2. EDS analysis of the phase in Figure 2.
CompositionPlaceColorTi/at%Zr/at%Ta/at%Nb/atSn/at%
Sn30 BHEAarea 1-19.5126.568.2116.8028.93
area 2Gray15.9639.713.758.5232.06
area 3Bright white28.6919.1412.0715.3824.72
Sn35 BHEAarea 1-19.5827.837.8220.0224.73
area 2Gray15.6934.010.988.8340.50
area 3Bright white24.5518.3914.6514.5527.86
Table
Table 3. Wear properties of Sn30 BHEA and Sn35 BHEA at room temperature.
Table 3. Wear properties of Sn30 BHEA and Sn35 BHEA at room temperature.
Sample L/mWv/(mm3)Kv/(mm3/nm)K2/(mm3/km)E/(km/mm3)
Sn30 BHEA 226.20.512.27 × 10−41.1630.86
Sn35 BHEA 226.20.562.49 × 10−41.2770.78
Table
Table 4. The key electrochemical parameters of Sn30 BHEA and Sn35 BHEA.
Table 4. The key electrochemical parameters of Sn30 BHEA and Sn35 BHEA.
Composition (at.%)Icorr (A/cm2)Ecorr (V)βaβcIpass (A/cm−2)Corrosion Rate (mm/a)
Sn30 BHEA 1.261 × 10−7−0.960.360.214.44 × 10−41.37 × 10−4
Sn35 BHEA 1.265 × 10−6−0.670.760.883.71 × 10−31.20 × 10−3
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Wang, X.; Hu, T.; Ma, T.; Yang, X.; Zhu, D.; Dong, D.; Xiao, J.; Yang, X. Mechanical, Corrosion, and Wear Properties of TiZrTaNbSn Biomedical High-Entropy Alloys. Coatings 2022, 12, 1795. https://doi.org/10.3390/coatings12121795

AMA Style

Wang X, Hu T, Ma T, Yang X, Zhu D, Dong D, Xiao J, Yang X. Mechanical, Corrosion, and Wear Properties of TiZrTaNbSn Biomedical High-Entropy Alloys. Coatings. 2022; 12(12):1795. https://doi.org/10.3390/coatings12121795

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Wang, Xiaohong, Tingjun Hu, Tengfei Ma, Xing Yang, Dongdong Zhu, Duo Dong, Junjian Xiao, and Xiaohong Yang. 2022. "Mechanical, Corrosion, and Wear Properties of TiZrTaNbSn Biomedical High-Entropy Alloys" Coatings 12, no. 12: 1795. https://doi.org/10.3390/coatings12121795

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