Design of Fe-Based Bulk Metallic Glasses with Improved Wear

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Design of Fe-based Bulk Metallic Glasses with Improved Wear Resistance Yi-Cheng Li, Cheng Zhang, Wei Xing, Sheng-Feng Guo, and Lin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11561 • Publication Date (Web): 13 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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ACS Applied Materials & Interfaces

Design of Fe-based Bulk Metallic Glasses with Improved Wear Resistance Yi-Cheng Li †, Cheng Zhang †, Wei Xing †, Sheng-Feng Guo ‡ and Lin Liu † †State Key Laboratory of Material Processing and Die & Mould Technology and School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ‡Faculty

of Materials and Energy, Southwest University, Chongqing 400715, China

ABSTRACT:

The application of bulk metallic glasses (BMGs) as advanced wear-resistant materials has remained limited despite optimistic expectations. In this work, we develop a series of novel Febased BMGs with improved wear resistance by altering the Cr and Mo contents in an Fe-Cr-MoC-B-Y glass forming system. Experimental results demonstrate that increasing Cr and Mo contents enhance the thermal stability and hardness (Hv) of the resulting BMGs without reducing their fracture toughness (Kc). The enhanced hardness is mainly attributed to the increased fraction of the stiff (Cr, Mo)-C covalent bonds in the resultant BMGs, as revealed by X-ray photoelectron spectrograph measurement. Worn surface/subsurface observations and stress field modeling reveal the activity of two kinds of wear mechanisms, i.e., hardness-controlled abrasion wear and toughness-controlled fatigue wear. We further clarify the enhanced wear performance of the Febased BMGs according to an effective indicator Kc3/4Hv1/2 that correlates positively with the wear resistance of the samples. The optimal Fe-based BMG sample possessed versatile properties, including a strong glass-forming ability (i.e., a minimum critical diameter of 8 mm), a high hardness of 1335 Hv, and a very low specific wear rate of ~1.3×10-6 mm3 N-1m-1, which represents one of the most outstanding Fe-based BMGs reported thus far. KEYWORDS: Fe-based bulk metallic glass; Hardness; Toughness; Wear.

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1. INTRODUCTION Bulk metallic glasses (BMGs) are a special class of materials with unique combinations of properties, including high strength, high hardness, and excellent corrosion resistance, that are attributed to their disordered atomic arrangement and their lack of typical crystalline defects

1-3.

From this perspective, BMGs could be assumed to have very good wear resistance according to Archard equation

4, 5,

which predicts that wear rate of a material is inversely proportional to its

hardness (or strength). However, this interesting prospect has been recently questioned by the work of Hofmann et al. 6, who demonstrated that the majority of BMGs possessed inferior wearresistance relative to the crystalline steels even though the former has a higher hardness. Furthermore, some specific BMGs suffered catastrophic fracture during repeated sliding wear tests owing to their low fracture toughness. Required by the great potential applications of BMG materials in the field of precision gear, advanced robotics, surface coatings, etc., it is of great significance to develop new BMGs with enhanced wear resistance for advanced engineeringrelated applications. A number of earlier studies have focused on improving wear resistance of BMGs by nanocrystallization 7, 8 or by the fabrications of BMG composites 6, 9-12, in which the strengthening effect relies on the introduction of hard in situ or ex situ second phases. While these approaches have improved the tribological properties of BMGs, the increase comes at the expense of destruction of the single-phase of glassy structure, and also leads to an undesirable compromise in corrosion resistance

13.

Other studies have focused on tailoring the composition of monolithic

BMGs to improve their wear resistance. For example, Maddala et al. 9 found that alloying an FeCr-Mo-C-B system with 2 at% Er resulted a ~55% improvement in the wear resistance due to a 14% increase in hardness. Villapun et al. 14 documented that a minor addition of 1–2 at% Ni in a


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CuZr-based BMGs could reduce the wear rate during repeated sliding wear tests at high loading as a result of complex surface reactions that included surface crystallization, and oxidation and martensitic transformation, which, in turn, hardened the worn surface in situ. While these microalloying approaches for increasing the wear resistance of BMGs have provided positive results, an in-depth understanding of the relationship between chemistry composition and the wear properties for BMG materials is still lacking. Among the families of BMG materials, Fe-based BMGs have become particularly attractive due to their high hardness, high strength, good magnetic properties, excellent corrosion and wear resistance, and relatively low cost

15-19.

Accordingly, Fe-based BMGs have been applied as

structural components and wear and corrosion resistant coatings used in earth excavation, drilling, shipping and tunnel-boring applications

20-22.

Owing to the intrinsically high hardness and wide

application potentials of Fe-based BMGs, the present work adopted a composition design method to develop a new series of Fe-based BMGs based on a well-known Fe-Cr-Mo-C-B-Y glass forming system by a composition design method. These alloys were designed according to the following two steps: i) Target the composition in the BMG system with the highest glass-forming ability (GFA) from the standpoint of the maximum critical diameter (Dcrit) to enable extending the potential range of composition modification. Accordingly, a BMG composition of Fe48Cr15Mo14C15B6Y2 with Dcrit=9 mm was selected as the starting alloy 23. ii) Partially replace the Fe component with Cr and Mo, which provide bonds with greater stiffness and have superior chemistry inertness. As a result, the substitution of Cr and Mo is predicted to provide the BMGs with improved hardness, fatigue strength and corrosion resistance. The composition-dependent thermal and mechanical properties, and wear mechanisms of the developed BMGs were systematically investigated via experimental observation and finite element modelling. The results


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demonstrate that the thermal stability and hardness of the Fe-based BMGs increase with increasing Cr and Mo contents without reducing their ability to withstand crack initiation and propagation (i.e., without reducing their fracture toughness). The superb combination of hardness and fracture toughness of the newly developed BMGs yields superior wear resistance.

2. EXPERIMENTAL SECTION Alloy ingots with the composition of Fe77-x-yCrxMoyC15B6Y2 (at%) were prepared by arc-melting a mixture of Fe (99.8 wt%), Cr (99.9 wt%), Mo (99.9 wt%), C (99.99 wt%), Y (99.99 wt%) and FeB alloy (B=18.5 wt%) under an Ar atmosphere. Cylinder samples with diameters of 3-10 mm were fabricated by copper-mold casting under a Ti-gettered argon atmosphere. The alloy ingots were melted six times to ensure compositional homogeneity. The amorphicity and thermal behaviors of as-cast specimens were examined by X-ray diffraction (XRD, χ’ Pert PRO) with Cu Kα radiation (λ=0.15405 nm), differential scanning calorimeter (DSC, TA SDT Q600) and transmission electron microscopy (TEM, Tecnai G2, 200 kV), respectively. Thin foil specimens for TEM were prepared by mechanical polishing and Ar ion milling (Gatan). The bonding states of the samples with a different Cr and Mo concentration were evaluated by X-ray photoelectron spectrograph (XPS) using a VGMultilab 2000 instrument with a monochromic Al Kα X-ray source (hv = 1486.6 eV). Prior to XPS measurements, the specimens were etched by Ar+ ion for 60 s to remove possible surface contaminants. All spectra were calibrated using the binding energy (BE) of C 1s (284.6 eV) as a reference.


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The Vickers hardness (Hv) was measured at a load of 300 g and a dwell time of 10 s. The fracture toughness (Kc) was calculated based on indentation method using a high applied load of 20 kg according to the following equation 24:  Hv(a )0.5   2.5 E  K c  0.035     2.5   Hv 

0.4

l   a 

0.5

(1)

Where E is Young’s modulus, l is the crack length and a is the half-length indentation diagonal. At least 5-10 indents were carried out to obtain the average value. The elastic constants, including Young’s modulus E, shear modulus G, bulk modulus B, Poisson’s ratio υ, of various BMG samples were derived from the acoustic velocities, and the density (𝜌) data was measured with Archimedes method using a microbalance with a sensitivity of 0.01 mg. The acoustic longitudinal velocity (vl) and shear velocity (vs) of the samples were measured with an ultrasonic system (UMS-100) using a carrying frequency of longitudinal wave and shear wave of 10 MHz and 5 MHz, respectively. 𝜌(𝑣2𝑙 ― 𝑣2𝑠 )

4

Then, the G, B, 𝜐 and E were calculated as follows 25: 𝐺 = 𝜌𝑣2𝑠 ; 𝐵 = 𝜌(𝑣2𝑙 ― 3𝑣2); 𝜐 = 2(𝑣2 ― 𝑣2); 𝐸 𝑠

𝑙

𝑠

= 2𝐺(1 + 𝜐). Linear reciprocating wear experiments were conducted on a UMT-3 wear machine using a 6.35 mm Si3N4 ball as the counter body. A load of 40 N, a sliding speed of 5 mm s-1, an oscillating stroke of 3 mm and a total sliding time of 30 min, were adopted for all wear experiments. New balls were used for each test. The wear tracks were analyzed using an optical interferometer (VK-X200) to calculate the wear volume loss. The surface and subsurface morphology of worn tracks was observed by SEM. Wear resistance was then calculated by the equation of Rw  NS / Vw 26, where N is the normal load, S is the total sliding distance and Vw is the wear volume loss. Finite element modelling (FEM) was performed using the ABAQUS software to calculate distribution of stress induced by contact and sliding of a spherical indenter. The mesh the specimen consisted of 9375 nodes with a high concentration of elements (200 × 200


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× 200 μm3) in the vicinity of the contact. The type and shape of elements are coupled temperature−displacement and hex, respectively. The corrosion rate of the BMG rod with the dimensions of Φ 4×20 mm was measured by immersion tests in aqua regia for 3 days. Before and after the immersion, the samples were weighted using an electronic balance with a sensitivity of 0.01 mg. The average corrosion rate (Vc, in mm/a) was calculated by Vc=(W0-W1)/tρA, where W0 and W1 are the original and final weights of samples (g), t represents the immersion time (a), ρ is the density (g cm-3) and A is the exposed surface area (cm2). Potentiodynamic polarization tests were performed in a 3.5% NaCl solution using a standard three-electrode system consisting of a Pt counter-electrode and an Ag/AgCl solid reference electrode at 298±1 K. The scanning rate is 0.333 mV s-1. All electrochemical tests were repeated at least three times to ensure data reliability.

3. RESULTS 3.1 Microstructure and thermal stability Figure 1a illustrates the compositional design map adopted in this work. As discussed, the strong GFA of the Fe48Cr15Mo14C15B6Y2 alloy system (i.e., Dcrit= 9 mm) enables the possibility of widely altering its composition. Basically, we kept the overall content of elements (C+B+Y) to be constant, and gradually change the Mo content (from 14 to 18 at%) and/or Cr content (from 15 to 23 at%) with a fixed interval (4 at%), and Fe is balance. In this process, we adjusted Mo and Cr contents by changing Fe for the balance. Hereafter, BMG specimens are referred as CrxMoy, where x and y represent Cr and Mo content, respectively, in at %. Figure 1b presents the X-ray diffraction (XRD) patterns obtained for the BMG samples fabricated with a diameter of 4 mm. The broad diffraction peaks and the absence of detectable crystalline peaks indicate that all the as-cast alloys


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have an amorphous structure within the XRD resolution limit. The DSC curves presented in Figure 1c show distinct glass transition and multistep crystallization peaks, which further confirms the glassy structure of all samples. Increasing each Cr or Mo content (while keeping the other to be constant) leads to increase of the glass transition temperature (Tg) (inset in Figure 1d) and crystalline onset temperature (Tx) (not shown here) of the BMGs, which is suggestive of an enhanced thermal stability. A monotonic increase of Tg as a function of total amount of Cr and Mo content in the alloys is further revealed in Figure 1d, wherein an extraordinarily high thermal stability (Tg = 900 K) is achieved for the Cr23Mo18 alloy with the maximum Cr and Mo content investigated. In addition, the crystallization activation energy (Ec) of two representative alloys with the lowest and highest Tg values (i.e., Cr15Mo14 and Cr23Mo18) was also measured by DSC (not shown here), which provided Ec values of 339 kJ mol-1 and 438 kJ mol-1, respectively. This further verifies the enhanced stability against crystallization for the BMGs with increased Cr and Mo content. In consideration of the ultrahigh thermal stability of the Cr23Mo18 alloy, the GFA of this alloy was further investigated. Figure 2a shows XRD patterns obtained from 8–10 mm diameter cylinder samples of Cr23Mo18 and Cr15Mo14 alloys. As can be observed, the XRD patterns of the 8 mmdiameter samples exhibit a perfect halo without any detectable Bragg peaks, indicating a single amorphous structure. However, the XRD pattern of the 10 mm-diameter Cr23Mo18 alloy sample shows a broad halo peak superimposed with a few sharp crystalline peaks, which indicate the presence of a mixed structure of amorphous and crystalline phases [i.e., (Fe, Cr)23(C, B)6 and Fe3Mo3C]. In contrast, the amorphous halo completely disappears in the XRD pattern obtained for the 10 mm-diameter Cr15Mo14 alloy sample, indicating a critical GFA of 8–9 mm, which is consistent with the results of a previous study 23. To further evaluate the microstructure of the alloy


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samples, high-resolution TEM was applied to investigate the microstructure of the as-cast 8 mmdiameter Cr23Mo18 alloy sample. The random atomic arrangement in the HRTEM image in Figure 2b and the diffuse halo ring pattern without diffracted spot shown by the selective area electron diffraction (SAED) pattern in the inset of Figure 2b confirm the fully amorphous nature of the ascast sample of Cr23Mo18 alloy. Figure 2c-2f presents the EDX mapping of elements Fe, Cr, Mo and C in the 8 mm-rod of Cr23Mo18 alloy, all of which distributed homogeneously throughout the sample, indicating chemical homogeneity of this BMG sample.

3.2 Hardness and fracture toughness As shown in Figure 3, the hardness (Hv) of the BMG samples was dramatically enhanced with increasing Cr and Mo in the alloys, and the hardness of the Cr23Mo18 alloy was 1335 Hv0.3, which represents about 12% increase with respect to that for the Cr15Mo14 alloy (1188 Hv0.3). A similar trend was also found with respect to the compressive strength of the BMG samples, where the values obtained for the Cr15Mo14 and Cr23Mo18 alloys were 3.4± 0.1 GPa and 4.1± 0.2 GPa, respectively. The values of fracture toughness (Kc) of all the BMG samples fell into a narrow range of 4.8-6.0 MPa m1/2, with no evidence of dependence on the Cr/Mo content. The highest Kc value of 5.7 MPa m1/2 was obtained for the Cr19Mo14 alloy while the lowest Kc value of 4.9 MPa m1/2 was obtained for the Cr19Mo18 alloy. In support of the validity of the fracture toughness measurements, we note that perfect corner cracks were successfully produced around the indentations obtained under a high applied load of 200 N (not shown here). The Kc values obtained here for the developed BMGs are slightly greater than those obtained for other amorphous steels reported in the literature, e.g., 1.9-2.1 MPa m1/2 for Fe47-xCr20Mo10WxC15B6Y2 BMG 27, 2.2 MPa


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m1/2 for Fe41Co7Cr15Mo14Er2C15B6 BMG 28, 3.2-3.8 MPa m1/2 for Fe50-xCr15Mo14C15B6Erx BMG 9, 29,

4.9 MPa m1/2 for Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 BMG 30.

3.3 Friction and Wear resistance The friction and wear properties of the developed BMGs were measured using pin-on-disk experiments run against an Si3N4 ball under a constant normal load of 40 N. As shown in Figure 4a, the coefficient of friction (COF) values obtained during all of our sliding wear tests exhibited transient increase from near zero to a steady-state value after approximately 200 s of testing. The average steady-state COF values obtained for the BMG samples are summarized in Figure 4b, which indicates that very similar COF value of approximately 0.5 were obtained for all BMG samples, which is indicative of an equivalent dominant wear mechanism being operative for all samples. However, the wear rates of the developed BMGs are considerably less than that of the Cr15Mo14, and the results generally demonstrate an enhanced wear resistance with increasing Cr/Mo content. For example, the lowest wear rate of 1.3 × 10-6 mm3 N-1m-1 was obtained for the Cr23Mo18 alloy, and this value is less than half that obtained for the Cr15Mo14 alloy (2.8 × 10-6 mm3 N-1m-1). It is of interest here that the wear rate is not inversely proportional to the hardness of the BMG samples. For example, while the Cr19Mo18 alloy exhibited a greater hardness than those of the Cr19Mo14, Cr23Mo14 and Cr15Mo18 alloys, the wear rate of Cr19Mo18 was greater than those of the other alloys. This indicates that the wear resistance for the developed Fe-based BMGs cannot be simply determinated by their hardness, as predicted by Archard equation. The wear behaviors of the BMG samples were evaluated by examining their worn surfaces and subsurface morphologies by optical microscopy (OM) and SEM. As shown in Figure 5, the


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surface roughness (Ra) of the worn regions is actually small (20%, probably due to the reduction of p-d hybridization by Ni addition, or formation of ultrafine nanocrystals on the order of 1-1.5 nm in the glassy matrix 49.

Further optimizing the composition of the current BMG system is our ongoing work.

4.3 Wear mechanisms for Fe-based BMGs As stated above, the wear behavior of the Fe-based BMGs was simultaneously dominated by several mechanisms, i.e., abrasive wear, subsurface cracking and delamination wear. The deviation


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from Archard scaling observed for some of the BMGs with low Kc (e.g., Cr19Mo18, Cr15Mo18) highlights that the fracture toughness plays a crucial role in the wear performance of BMGs. Similarly, Rahaman et al.

50

also found that the Cu-based BMG with a higher ductility (with a

plastic strain of 11%) possessed much higher wear resistant than the Ti-based BMG (with a plastic strain of only 3%), although these two BMGs have almost the same hardness (540 Hv), pointing out the important role of fracture toughness in controlling the wear properties of BMGs. However, how cracks initiated in BMGs under tribological conditions still remain elusive. Rosenfield

51

stated that the driving force for a locally weak region below the surface to undergo shear instability peaks at some depth below the surface, and a crack will grow when it exceeds its fracture toughness. Contrarily, other studies 52, 53 indicate that the maximum shear stress would appear at the surface rather than below it, and the observed cracks originated at the surface and then propagated in the subsurface region before they emerged again on the surface. To verify whether the cracking originates from the large contact stress induced by normal loading, we theoretically calculated the stress state with a ball-on-plate contact configuration according to the Hertzian contact model 54, 55. On the basis of the materials parameters (e.g., Poisson ratio vSi3N4 =0.26 and vBMG=0.316; ESi3N4=320 GPa and EBMG=223 GPa) and wear conditions (normal load of 40 N and sliding speed of 5 mm s-1) adopted in this study, the maximum contact stress and shear stress were calculated to be 2.47 GPa and 755 MPa, respectively, at a position of around 40 μm beneath the surface. The values of the maximum contact and shear stress are much less than the fracture strength of the BMGs studied (~4.0 GPa). As such, crack initiation and propagation may be prohibited under a high, yet static contact stress. Considering that the friction force generated during sliding wear testing may additionally contribute to the stress state of the material and affect the stress distribution, we further explore the stress field developed under a sliding condition by


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finite element modelling (FEM) with the measured COF value of 0.5 adopted between the tribological system. The results are shown in Figure 11. In Figure 11a, one can see that the maximum von Mises stress of 5.3 GPa, which is located at two typical regions, i.e., the contact region directly underneath the Si3N4 ball and the front edge of the sliding contact, readily exceeds the fracture strength of the modeled BMG. In contrast, we note from Figure 11b that the region with the highest shear stress appears at the sliding surface, and that this value is greatly increased to 2.7 GPa under the sliding wear condition relative to the shear stress obtained under a static contact condition (755 MPa). The region with maximum shear stress is located within 30 μm below the topmost surface, which agrees well with the experiential observations (Figure 6b). These results indicate that a very high strain is imposed in the topmost layer under a high sliding load, and that this level of strain cannot be accommodated in the confined surface layer of the developed BMGs in the absence of plasticity. As such, surface damage and subsurface cracking inevitably occur due to cohesive failure. In previous studies, micro-cracking has been commonly proposed as a dominating wear feature of Fe-based BMGs during two-body wear 9, 56. In fact, it has been reported that BMGs normally have a low fatigue ratio (σw/σB) 57, defining as the ratio of the fatigue limit (σw) to the tensile or compressive strength (σB). For high-strength Fe-based BMGs, the value of σw/σB is around 0.5 58. Consequently, the value of σw is expected to be about 2 GPa. Therefore, the locally high shear stress generated during sliding wear testing exceed the fatigue limit of the BMG alloys, and result in fatigue crack initiation and growth. Accordingly, either increasing the strength to enhance the fatigue limit and to prevent crack initiation, or improving the damage tolerance to hinder crack propagation, would provide superior wear resistance.

4.4 Correlation between wear resistance and the hardness and fracture of BMGs


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Generally, the wear resistance increases with hardness improvements, but the fracture toughness will predominantly play a role when the wear mechanism follows subsurface cracking or delamination wear

1, 9.

Maddala and Hebert

9, 59, 60

have proposed that the combined effects of

hardness and toughness should be considered for the wear behavior of BMGs. Considering the similarly brittleness of Fe-based BMGs and ceramic materials, we adopted a quantitative model describing the tribological behavior of brittle ceramics as a means of establishing the correlation between the wear resistance (RW) of the developed Fe-based BMGs and their values of Hv and Kc obtained from testing. The model 61 is given as RW∝αKc3/4Hv1/2, where α is an empirical constant. This model emphasizes that the wear resistance depends inversely on both hardness and fracture toughness, where the exponent applied to Hv and Kc reflect the competition between shear deformation and cracking processes. The relationships between RW and Kc3/4Hv1/2 for various Febased BMGs are illustrated in Figure 12, which includes the data obtained in the present study and in the literature. It can be observed that an increasing value of Kc3/4Hv1/2 correlates an increasing value of RW for Fe-based BMGs. This agrees well with previous works, which indicated that the wear resistance of BMGs peaked for hardness and fracture toughness combinations is optimal 9, 58. These findings also clarify why the Cr23Mo18 and Cr19Mo14 exhibited superior wear resistance relative to the other BMGs studied, in that Cr23Mo18 exhibited the greatest value of Hv and a moderate value of Kc; while Cr19Mo14 exhibited the greatest value of Kc and a moderate value of Hv. In addition, we note from Figure 12 that the wear resistance of the newly developed BMGs are substantially greater than that of the majority of previously developed Fe-based BMGs 1 and other BMG systems, including Zr-based, Ti-based, Hf-based, Pd-based and Cu-based systems. To the best of our knowledge, the present developed Cr23Mo18 BMG provides one of the highest wear resistance among all Fe-based BMG materials reported to date. Interestingly, a transition appears


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in Figure 9 when the value of Kc3/4Hv1/2 is too high (i.e., >35), which corresponds with a regime where the wear resistance of the relatively tough BMGs is largely dominated by Hv, rather than Kc. In the “hardness control” regime, the fracture toughness dose not predominantly impact the wear behavior, since these BMGs are too tough for crack initiation and propagation under tribological conditions. As such, the wear behavior of the tough BMGs generally follows the classic Archard’s wear law, which predicts that wear loss is inversely proportional to material hardness. As a matter of fact, the work of Hofmann et al. 6 indeed verified that the general trend of Archard’s wear law fitted well to those tough BMGs. Thus, this finding suggests a potentially productive avenue for directing further efforts toward exploring new wear-resistant BMGs near the transition regime by regulating the hardness and toughness. It is worth considering the wider impacts of the current work. Generally, nanocrystallization by heat treatment is a very common method used to enhance the hardness and wear resistance of BMGs. For example, we also obtained a nanocrystallized Cr23Mo18 alloy by heat treatment at 973 K for 1 h that provided an ultrahigh hardness of 1618 Hv0.3. However, the wear resistance of the nanocrystallized alloy was far less than its glassy counterpart (see Figure 12), which is mainly ascribed to the fairly low fracture toughness of only 2.68 MPa m1/2 caused by the crystallization effect.

CONCLUSIONS To summarize, we developed a serials of novel Fe-based BMGs with enhanced thermal stability and superb wear resistance via appropriate partially replacing Fe component with Cr and Mo content in a Fe-Cr-Mo-C-B-Y glass forming alloy system. We found that increasing Cr and Mo enhanced the thermal stability, hardness, strength and Young’s modulus of the resultant BMGs


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without reducing their fracture toughness (Kc), which is associated with the stiff (Cr, Mo)-C covalent bond provided a strengthening effect. Abrasion wear and subsurface cracking (fatigue wear) were detected for the developed Fe-based BMGs via worn surface observations and stress field modelling. We further clarify the enhanced wear performance of the Fe-based BMGs according to an effective indicator Kc3/4Hv1/2 that correlates positively with the wear resistance of the samples. The optimal Fe-based BMG sample possessed versatile properties, including a strong GFA (i.e., Dcrit =8 mm), a high hardness of 1335 Hv, very low specific wear rate of ~1×10-6 mm3 N-1m-1 and significantly improved corrosion resistance, which represents one of the most outstanding Fe-based BMGs reported thus far. The findings here provide an alternative approach to design novel BMGs materials with superb wear resistance.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] and [email protected] ORCID Id (Cheng Zhang): https://orcid.org/0000-0001-5321-0624. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS C. Z. received funding from National Natural Science Foundation of China (Grant No. 51771077) and National Key Research and Development Plan (Grant No. 2016YFB1100101). L. L. received funding from National Natural Science Foundation of China (Grant Nos. 51471074, 51531003 and


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51871102) and Technical Innovation Project of Hubei Province (Grant No. 2017AAA018). The authors are grateful to the Analytical and Testing Center, Huazhong University of Science & Technology for technical assistance.

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2014, 615, s2-s8. (18) Zhang, C.; Liu, L.; Chan, K.C.; Chen, Q.; Tang, C.Y. Wear Behavior of HVOF-sprayed Febased Amorphous Coatings. Intermetallics 2012, 29, 80-85. (19) Zhang, C.; Wang, W.; Li, Y.C.; Yang, Y.G.; Wu, Y.; Liu, L. 3D printing of Fe-based Bulk Metallic Glasses and Composites with Large Dimensions and enhanced Toughness by Thermal Spraying. J. Mater. Chem. A 2018, 6, 6800-6805. (20) Farmer, J.C.; Choi, J-S.; Saw, C.K.; Haslam, J.J.; Day, S.D.; Hailey, P. D.; Lian, T.; Rebak, R. B.; Perepezko, J. H.; Payer, J.H.; Branagan, D. J.; Beardsley, B.; D’amato, A.; Aprigliano, L. F. Iron-based Amorphous Metals: High-performance Corrosion-Resistant Materials Development. Metall. Mater. Trans. A 2009, 40, 1289-1305. (21) Liu, L.; Zhang, C. Fe-based Amorphous Coatings: Structures and Properties. Thin Solid Films 2014, 561, 70-86. (22) Zhang, C.; Zhou, H.; Liu, L. Laminar Fe-based Amorphous Composite Coatings with Enhanced Bonding Strength and Impact Resistance. Acta Mater. 2014, 72, 239-251. (23) Gu, X.J.; Poon, S.J.; Shiflet, G.J. Mechanical Properties of Iron-based Bulk Metallic Glasses. J. Mater. Res. 2007, 22, 344-351. (24) Abbas, S.Z.; Khalid, F.A.; Zaigham, H. Indentation Fracture Toughness Behavior of FeCobased Bulk Metallic Glass Intrinsic Composites. J. Non-Crys. Solid. 2017, 457, 86-92. (25) Zhang, B.; Wang, R.J.; Zhao, D.Q.; Pan, M.X.; Wang, W.H. Superior glass-forming ability through microalloying in cerium-based alloys. Phys. Rev. B 2006, 73, 092201. (26) Huang D.; Li, R.; Huang, L.; Ji, V.; Zhang, T. Fretting Wear Behavior of Bulk Amorphous Steel. Intermetallics 2011, 19, 1385-1389. (27) Liang, D.D.; Wei, X.S.; Chang, C.T.; Li, J.W.; Wang, X.M.; Shen, J. Effect of W Addition


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C.;

Pang,

S.J.;

Cheng,

Y.Y.;

Zhang,

T.

A

Centimeter-size

Zr40Hf10Ti4Y1Al10Cu25Ni7Co2Fe1 Bulk Metallic Glass with High Mixing Entropy Designed by Multi-substitution. J. Non-Cry. Solids 2015, 410: 39-42.


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Figure 1. (a) Composition design map in the Fe-Cr-Mo-C-B-Y glass forming system. (b) XRD patterns and (c) DSC plots for BMG samples in a diameter of 4 mm. (d) the glass transition temperature (Tg) as a function of total Cr and Mo content in the alloys. Inset show the Tg variation as a function of single Cr or Mo content in the alloys.


1

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Figure 2. (a) XRD patterns for Cr23Mo18 and Cr15Mo14 BMG samples with diameters of 8 and 10 mm; (b) TEM image and the selective area electron diffraction (SAED) pattern of the Cr23Mo18 BMG sample with a diameter of 8 mm; (c-f) EDX mapping of the 8 mm-sample of Cr23Mo18 alloy.


2


1400 6.0 5.8 1300

5.6 5.4 5.2

1200

Toughness (MPa m1/2)

Hardness Toughness

Hardness (Hv0.3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 40

5.0 4.8 1100

4 4 8 4 8 8 Mo 1 Mo 1 Mo 1 Mo 1 Mo 1 Mo 1 9 3 r 3 5 1 9 r 2 C C C Cr 2 Cr 1 Cr 1

r 15

Figure 3. Hardness and fracture toughness for the BMG samples.


3

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Figure 4. (a) COF as a function of sliding duration of the BMG samples under a constant load of 40 N. (b) averaged wear rate and COF values of the BMG samples.


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Figure 5. Optical micrographs of wear tracks for various BMG samples. (a) Cr15Mo14; (b) Cr19Mo14; (c) Cr23Mo14; (d) Cr15Mo18; (e) Cr19Mo18 and (f) Cr23Mo18.


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Figure 6. (a) The surface worn morphology and (b) subsurface worn morphologies for the Cr23Mo18 alloy after 30 min-sliding under a high load of 40 N, revealing two kinds of wear mechanisms, hardness-controlled abrasive wear and toughness-controlled subsurface cracking.


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Figure 7. Corrosion resistance of the Fe-based BMGs. (a) Corrosion rate of the various BMG samples after immersion in aqua regia for 3 days; (b) Potentiodyamic polarization curves of the BMG samples in a 3.5 % NaCl solution open to air at room temperature.


7

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Figure 8. Young’s modulus dependence of hardness for Fe-Cr-Mo-C-B-Y BMGs.


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Figure 9. XPS spectra of the (a) C 1s; (b) Cr 2p; (c) Mo 3d and (d) B 1s for various Fe-based BMGs.


9

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a

C-O bond 100

8% 6%

Carbides 7% 8%

9% 7%

C-C bond

9%

9%

9%

7%

8%

9%

b 100

60

86%

40

85%

83%

84%

83%

82%

Cr-O bond

60

12%

51%

50%

12%

14%

63%

53%

30%

33%

50%

33% 40

20

20

Cr-C bond

7%

39%

80

Fraction (%)

Fraction (%)

Cr-Cr bond 17%

80

37%

33%

28%

38%

0

0

Cr 1

4 Mo 1 5

c

Cr 1

4 Mo 1 9

Cr 2

Mo-O bond

Cr 1

8 Mo 1 5

Cr 1

8 Mo 1 9

Cr 2

8 Mo 1 3

Mo-C bond

Cr 1

4 Mo 1

5

d

80

41%

Fraction (%)

38% 34%

60

40

45%

4 Mo 1 5

4 Mo 1 9

4 Mo 1

3

Cr 1

8 Mo 1

5

Cr 1

8 Mo 1

9

Cr 2

8 Mo 1

3

Borides

30%

32%

31%

46%

42%

Cr 2

B-O bond

80

20

4 Mo 1

9

12%

15%

22%

46%

Cr 1

100

6%

9%

12%

4 Mo 1 3

Mo-Mo bond

100

Fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


43%

47%

8 Mo 1 5

8 Mo 1 9

57%

58%

54%

48%

43%

42%

46%

60

40

70%

68%

57%

53%

52%

20

0

0

Cr 1

Cr 1

Cr 2

4 Mo 1 3

Cr 1

Cr 1

Cr 2

8 Mo 1 3

Cr 1

4 Mo 1

5

Cr 1

4 Mo 1

9

Cr 2

4 Mo 1

3

Cr 1

8 Mo 1

5

Cr 1

8 Mo 1

9

Cr 2

8 Mo 1

3

Figure 10. Comparison of bond information regarding the elements of (a) C, (b) Cr, (c) Mo and (d) B for the various BMGs obtained from the XPS measurements.


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Figure 11. The distribution of (a) von Mises stress and (b) shear stress (S12) under normal loading plus sliding wear for the Cr23Mo18 alloy.


11

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Figure 12. Correlation between wear resistance and the parameter of Kc3/4Hv1/2 for various kinds of BMGs. Data from a fully crystallized Fe36Cr23Mo18C15B6Y2 alloy and the other BMGs reported in the literatures are also included for comparison. [CuZrAlBe

6

; FeCrMoCB(Er)

9

;

FeCoCrMoCBY 28, 63; CuHfAl 59; ZrTiCuNiBe 1, 62; PdNiCuP 63].


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Table of Contents


13

Design of Fe-Based Bulk Metallic Glasses with Improved Wear

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