Microstructure Formation and Dry Reciprocating Sliding Wear Response of High-Entropy Hypereutectic White Cast Irons

Abstract

White cast irons (WCI) are widely used in industries requiring high wear resistance due to their microstructure consisting of hard carbides dispersed within a metallic matrix. This study focuses on developing wear-resistant multi-component hypereutectic high chromium cast irons, merging concepts of high entropy alloys with the conventional metallurgy of white cast irons, specifically exploring the influence of carbide-forming elements such as V, Mo, and Ni on solidification behavior, microstructure, and wear performance. The research investigates the solidification process of the alloys using Computer-Aided Cooling Curve Analysis (CA-CCA) and characterizes the microstructures through X-ray diffraction (XRD) and scanning electron microscopy (SEM). The wear behavior of the developed alloys is evaluated through reciprocating sliding wear tests, revealing the impact of varying chemical compositions on wear resistance. The results demonstrate that high-entropy white cast iron (HEWCI), particularly those enriched with carbide-forming elements, exhibit superior abrasion resistance compared to conventional high-chromium cast irons. The alloy with 2 Mo and 4 V content showed the best performance, presenting the lowest wear rate (61.5% lower than HCCI alloy) and CoF (values ranging from 0.20 to 0.22) due to the highest concentration of V carbides.

1. Introduction

White cast iron (WCI) has been extensively used in many industries due to its exceptional wear resistance properties. This material’s unique characteristics are reflected from its microstructure, which consists of a high-volume fraction of hard carbides dispersed in a metallic matrix. The presence of these carbides, typically iron carbides or chromium carbides, imparts superior hardness and abrasion resistance to WCIs, making them ideal for applications involving severe wear conditions. WCIs used in conditions where wear resistance is required can be categorized into four groups: pearlitic, Ni-hard (with M₃C eutectic carbides or M₇C₃ eutectic carbides), high-chromium cast iron (HCCI), and multi-component cast iron (MCCI) [1,2,3].

Ni-Hard and HCCI have been extensively used in many industrial applications that require wear resistance. Ni-Hard is a type of white cast iron that contains Ni (nickel) and Cr (chromium) and is known for its high hardness and excellent resistance to abrasion. HCCI is unmatched in situations where toughness, abrasion resistance, and sometimes corrosion resistance are important. Among the Ni-Hard family, Ni-Hard 4 has the highest levels of Ni, Cr, and Si (silicon). In contrast, the HCCI family exhibits Cr content that is 2 to 3 times greater than Ni-Hard 4. Despite their different chemical compositions, both alloys have similar microstructures with M₇C₃ eutectic carbides [1,4].

MCCI is composed of Cr and other alloying elements, such as Ti (titanium), V (vanadium), Nb (niobium), Mo (molybdenum), and W (tungsten), which tend to form additional carbides. The carbon content of MCCI falls within the range of carbon values observed in high-speed tool steel, Ni-Hard, and HCCI. During the solidification process, specific carbides, namely, MC and M₂C, develop within the microstructure of these alloys. After heat treatment, MCCI exhibits enhanced durability against abrasive wear compared to HCCI, despite containing a lower proportion of carbides [5,6].

The main microstructural features that affect wear performance are the morphology, fraction and type of second phases, and the presence and distribution of hard carbides, as well as the properties of the supporting matrix. The addition of carbide-forming elements such as Mo, W, V, and Nb to the alloy is widely acknowledged to improve its wear resistance. The development of new abrasion-resistant cast iron based on a multi-component approach is a well-recognized research topic, with numerous studies providing evidence in favor of this hypothesis [5,6,7,8,9,10,11].

Currently, the development of new wear-resistant alloys has advanced significantly with the introduction of high entropy alloys (HEAs). HEAs are commonly composed of five or more alloying elements, in concentrations ranging from 5 to 35 at. %. In general, these alloys should only present a solid solution and configurational or mixing entropy (ΔS_conf) greater than 12.5 kJ/mol (ΔS_conf ≥ 1.5R, where R is the gas constant) [12,13]. The configurational entropy (ΔS_conf) can be calculated from Equation (1):

$$∆S_{conf}=-R{\sum }_i{c}_i.ln{c}_i$$

(1)

where c_i is the molar content of the ith component.

The concept of HEAs challenges traditional alloy design principles, opening new pathways for creating materials with exceptional performance in demanding environments. The original proposal had a narrow definition of HEAs due to the challenges in fully defining HEAs based on a single definition and microstructure; new terms like complex concentrated alloys (CCAs) have been introduced to address these limitations and cover a wider range of alloy chemical compositions and microstructures [12,13,14].

Recent studies have explored the integration of HEA concepts into HCCI to develop high-entropy white cast iron (HEWCI). This alloy combines the benefits of HCCI with the high entropy effect, leading to refined microstructures and improved wear properties. For instance, the addition of elements such as V and Mo in HEWCI has been shown to significantly enhance the wear resistance by refining carbide phases and improving the overall hardness of the material [7,8,11,15].

Further research has demonstrated that modifying high-Cr cast irons with HEA principles can lead to the development of hybrid high-entropy high-Cr cast irons. These alloys exhibit superior sliding wear resistance due to their elevated hardness, although their performance under impact wear conditions may be limited by reduced toughness [8,15]. The addition of multiple carbide-forming elements such as Ti, V, Mo, and W in these alloys results in the formation of various fine carbides, which contribute to the improved wear resistance by suppressing the growth of primary carbides and enhancing the overall microstructural stability [7,15].

In this study, five different chemical compositions of high-alloyed white cast iron were proposed, encompassing a hypoeutectic HCCI alloy, an MCCI alloy and three HEWCI alloys, to investigate the influence of different V, Mo, and Ni contents on the solidification path, microstructure formation, and sliding wear behavior. These HEWCI alloys can be classified as CCAs due to their complex chemical composition and multi-phase microstructure, while their design incorporates HEA principles, as they meet the configurational entropy criterion.

2. Materials and Methods

2.1. Alloy Design, Melting, and Thermal Analysis

The base alloy, a hypoeutectic High-Chromium Cast Iron (HCCI), was modified to create the MCCI and HEWCI alloys. V, Mo, and Ni were chosen as the alloying elements, since these elements have significant effects on microstructures. Ni tends to form M₇C₃ eutectic carbides in combination with Cr and allows for obtaining a pearlite-free matrix. V and Mo are strong carbide formers and are widely used in high-speed and tool steels. The objective of adding high amounts of these elements to the alloys is to enhance the hardening capability [3,16].

To produce the HEWCI alloys, pure Ni, C, Fe-Si, Fe-Cr, Fe-Mn, Fe-V, and Fe-Mo ferrous alloys were added to the hypoeutectic HCCI base alloy, hereinafter called HEWCI-A, HEWCI-B, and HEWCI-C, according to their chemical compositions.

The casting process consists of melting alloy in a coreless induction furnace at 1450 °C. After melting, the slag was removed from the liquid surface and the molten metal was poured into two shell-molding sand cups instrumented with type-K thermocouples. The computer-aided cooling curve analysis (CA-CCA) method was used to analyze the solidification path and phase transformations of the HEWCI alloys during solidification under near-equilibrium conditions. The chemical compositions of the alloys were determined using optical emission spectrometry, and the obtained chemical compositions can be found in

Table 1. Chemical compositions of the hypereutectic white cast iron alloys (at. %).

Alloys	C	Mn	Si	Cr	Mo	V	Ni	ΔSmix
HCCI	2.5	1.7	1.4	18.9	0.02	0.1	0.1	0.96R
MCCI	3.7	1.9	1.8	25.9	0.02	0.1	6.2	1.30R
HEWCI-A	4.0	2.1	2.1	28.4	5.4	1.8	6.1	1.52R
HEWCI-B	3.8	1.9	2.0	27.1	2.1	4.7	6.1	1.51R
HEWCI-C	3.8	2.0	2.8	27.1	5.9	3.8	5.7	1.56R

.

2.2. Microstructure Characterization

For microstructure analysis, HCCI and MCCI samples were extracted from the thermal analysis ingots near the type-K thermocouple and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The samples were ground with #240 to #1200 SiC sandpapers, followed by polishing with alumina suspension. In SEM analyses, backscattered electron (BSE) and secondary electron (SE) imaging modes were used, as well as energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) analysis was carried out on each alloy (CuKα1 radiation, 40 kV, 30 mA, scanning range 2θ: 20 to 100°). A PANalytical Empyrean X-ray diffraction system (Malvern Panalytical Ltd, Great Malvern, United Kingdom) was used to identify the phase constituents, and the phase compositions of the alloys were determined using HighScore software (version 5.2).

2.3. Wear Tests and Hardness

In accordance with the ASTM G133 standard test method [17], the sliding wear performance of the HCCI, MCCI, and HEWCI alloys was investigated through reciprocating sliding wear testing. A high-frequency reciprocating rig (Ducom, HFRR 4.2) (Ducom Instruments, Bohemia, NY, USA) was used, with a sphere as a counterbody and the body being evaluated as a disc. Dry reciprocating sliding tests were conducted at room temperature. As counterbody, a sphere of alumina (d = 6 mm, 74/80 HRC hardness) was utilized. A 2 N load was applied, the reciprocating frequency was set at 50 Hz, and the stroke distance was maintained at 1 mm. The as-cast specimens, which were extracted using electrical discharge machining (EDM), were ground with alumina papers of grit sizes #220, #320, #400, #600, and #1200. They were sequentially polished with an alumina suspension containing particles of 1 μm and 0.3 μm. Frictional force data were collected directly from the load cell and utilized to calculate the coefficient of friction (CoF). The wear volumes (V) and wear scar profiles were determined following the ASTM D7755 standard [18]. The linear wear rate (W_l) and specific wear rate (κ) were also calculated. The Rockwell C scale was used to determine the hardness of the alloys. At least 6 measurements were taken along the cross section of the ingot following the recommendations of the ASTM E18 standard method [19].

3. Results and Discussion

3.1. X-Ray Diffractograms and SEM Micrographs

To obtain additional information about the solidification process of the HCCI, MCCI, and HEWCI alloys, X-ray diffraction (XRD) and backscattered electron (BSE) analyses were utilized. Figure 1 illustrates the results of phase identification for the as-cast alloys, HCCI, MCCI, and HEWCI-B, acquired from XRD spectra (Figure 1a–c) and BSE images (Figure 1d–f). The XRD analyses indicated the existence of γ (austenite phase) and M₇C₃ ((Cr,Fe)₇C₃) in HCCI and MCCI alloys, while complex carbides, including M₂C carbides (Mo₂C and V₂C), were identified in HEWCI alloys. It is important to note that all the HEWCI alloys showed similar peaks corresponding to the presence of γ and M₇C₃, Mo₂C, and V₂C carbides. For the HEWCI-B alloy, Figure 1g shows an example of EDS analysis allowing the identification of the γ-phase, primary and eutectic M₇C₃ carbides, and M₂C carbides (V-rich and Mo-rich), as indicated in the microstructure and presented in the spectrum.

3.2. Computer-Aided Cooling Curve Analysis (CA-CCA)

The temperatures at which phase transitions occurred during the cooling process of the alloys were identified using the Newtonian analysis method, employing the first and second derivatives of the cooling curve. To enhance comprehension, a comparative analysis was conducted between the zero curve, illustrating the alloy’s cooling rate in the absence of phase transformations, and the first derivative of the cooling curve. This methodology facilitates the identification of peaks and inflections in the cooling rate curves, corresponding to specific phase transformations. The occurrence of these transformations is influenced by the heat transfer between the dissipated heat to the environment and the latent heat released during solidification [20].

The cooling curve of the HCCI alloy, as shown in Figure 2a, exhibits three distinct reaction or arrest points labeled as R1 (beginning of solidification), R2 (eutectic reaction), and T_sol (end of solidification), occurring at temperatures of 1324 °C, 1247 °C, and 1233 °C, respectively. In the cooling curve of the MCCI alloy, shown in Figure 2b, three distinct reaction points labeled as R1, R2, and T_sol are observed at temperatures of 1345 °C, 1252 °C, and 1226 °C, respectively. For the HEWCI-C alloy, four distinct arrests are evident, as shown in Figure 2c, labeled as R1, R2, R3 (complex carbides formation), and T_sol, occurring at temperatures of 1346 °C, 1208 °C, 1182 °C, and 1159 °C, respectively.

3.3. Wear and Hardness Results

The ASTM G133 and ASTM D7755 standards describe a method for evaluating material wear through a linear reciprocating ball-on-flat sliding wear test. The results are summarized in

Table 2. Coefficient of friction (CoF), wear volume (V), wear rate (κ), and hardness of the cast irons.

Alloys	CoF	V (mm3)	κ (mm3/Nm)	Hardness (HRC)
HCCI	0.302	0.0026	1.87 × 10−8	46 ± 0.3
MCCI	0.309	0.0013	0.89 × 10−8	50 ± 1.2
HEWCI-A	0.309	0.0011	0.79 × 10−8	53 ± 1.2
HEWCI-B	0.208	0.0009	0.57 × 10−8	51 ± 0.6
HEWCI-C	0.225	0.0012	0.81 × 10−8	53 ± 1.2

.

Table 2 shows that CoF decreases with increasing hardness for those alloys with higher Mo, V, and Ni contents. Figure 3a shows examples of CoF behavior as a function of sliding distance for the alloys measured during the sliding wear tests. An initial transient friction regime is observed at the first few meters, revealing an initial rapid increase, and a quasi-steady-state behavior is attained after a 10 m sliding distance. The CoF presents higher values in the HCCI, MCCI, and HEWCI-A alloys when compared to the HEWCI-B and HEWCI-C alloys. In the case of the HEWCI-A alloy, a slight variation is observed after approximately 40 m, with CoF varying from 0.35 to 0.27. The HEWCI-B alloy presented values ranging from 0.20 to 0.22 and the HEWCI-C alloy from 0.22 to 0.24. The lowest CoF values for alloys with higher V content can be attributed to the higher concentration of V carbides compared to the other alloys. These carbides facilitate sliding motion by reducing body–counterbody interaction. On the other hand, the higher CoF value for the HEWCI-A alloy can be associated with to the lower V content and, consequently, the lower amount of V-rich carbides. A three-dimensional optical profilometry (Bruker (Billerica, MA, USA) 3D Optical Profilometer—GTK M, CONTOUR GTK) was applied to characterize the wear tracks. Using the three-dimensional surface profile, the depth and width of each track were measured (Figure 3b), and the wear volume was determined.

3.4. Discussion

The identification of the solidification reactions at each arrest point was conducted by examining both the final microstructure of the alloys, XRD diffractograms, and the literature review. Concerning HCCI, MCCI, and HEWCIs alloys, it is possible to compare the results shown in Figure 2 to the findings of the previous studies on the Fe-Cr-C system [21]. The thermodynamic simulations were performed using FACT SAGE software (version 8.2) to describe the solidification path under non-equilibrium conditions.

In the HCCI alloy, the first arrest point (R1) on the cooling curve (Figure 2a) corresponds to the formation of primary γ-phase, while the second arrest point (R2) represents the beginning of the eutectic reaction γ + M₇C₃. The T_sol point, indicating the end of solidification or the eutectic formation, corresponds to the minimum value of the second derivative of the cooling curve or the cross-point between the first derivative and the zero curve. The results from HCCI show good agreement with the findings presented by Thorpe and Chicco [21].

For the MCCI alloy, the first arrest point (R1) on the cooling curve (Figure 2b) corresponds to the formation of primary M₇C₃, and the second arrest point (R2) represents the eutectic reaction γ + M₇C₃. The T_sol point indicates the end of solidification. Comparing these results with the Fe-Cr-C system in the literature, it can be concluded that the addition of Ni increases the M₇C₃ reaction. For an Fe-26Cr-3.7C alloy, the R1 for M₇C₃ primary precipitation is estimated to occur around 1300 °C, and the R2 eutectic is in agreement with the estimates by Thorpe and Chicco [21].

For HEWCI-A, B, and C alloys, R1 corresponds to the primary M₇C₃ phase, while R2 indicates the eutectic reaction γ + M₇C₃. Similar to MCCI, the addition of Ni increases the intensity of the R1 reaction. However, the additions of Mo and V induce a reduction in temperatures at R2 and T_sol, and additional peaks are present (R3), suggesting the precipitation of complex carbides, specifically M₂C. Similar temperatures were reported in the literature by Ikeda et al. [22] and Pasini et al. [11].

Wear Behavior and Hardness

Concerning sliding wear behavior, Figure 4 presents the wear volume (V) and specific wear rate (κ) of the alloys. Analyzing Table 2 and Figure 4, it is possible to notice an improvement in wear resistance in alloys with the addition of C, Cr, V, Mo, and Ni. With increasing alloying element content, the specific wear rate decreases for all HEWCI alloys. The MCCI alloy has a specific wear rate that is 53% lower than that of the base HCCI alloy, while the HEWCI alloys have an average wear rate that is 61.5% lower than HCCI. The HEWCI-B alloy showed the lowest wear rate and CoF despite a lower Mo content. This indicates that a higher amount of V-rich carbides improves the CoF, while a lower amount of Mo-rich carbides decreases the volume loss. This behavior can be attributed to the higher matrix hardness due to the increase in solid solution strengthening, which reduces the probability of abrasion when compared to the softer matrix of those alloys with higher carbide formers, reducing alloying elements in solid solution.

The results presented highlight the superior abrasion resistance of multi-component cast irons, particularly those containing carbide-former elements, which have been shown to outperform many other cast irons and steels. For instance, multi-component white cast irons with Nb exhibit significantly lower wear loss compared to those with V, with the Nb-containing alloys showing enhanced resistance to abrasive wear due to the formation of larger eutectic MC carbides [5]. Additionally, the incorporation of Nb and Mo in high-chromium cast irons has been found to improve abrasion resistance under mild wear conditions, emphasizing the synergistic effect of these elements [23]. Furthermore, high-chromium cast irons with refined microstructures, achieved through controlled cooling rates, exhibit reduced wear coefficients, indicating that microstructural refinement plays a crucial role in enhancing wear resistance [24,25]. Overall, multi-component cast irons, particularly those with carbide-forming elements, offer superior abrasion resistance compared to traditional cast irons and steels such as high carbon steels, tool steels, stainless steels, and others, making them highly suitable for applications requiring high wear resistance.

Concerning the CoF values presented by HEWCI-B and HEWCI-C, they are approximately 30% lower than HCCI, MCCI, and HEWCI-A. When analyzing the hardness, the HCCI alloy presented the lowest value, while the HEWCI alloys exhibited the highest values, without significant differences between them. The CoF in sliding wear tests is closely correlated with the abrasion resistance of materials, as demonstrated in various studies on steels, cast irons, and iron-based alloys. For instance, in the study of 52100 steels and ductile and white cast irons, it was found that under severe abrasion conditions, the CoF exceeded 0.4, indicating a predominance of the micro cutting wear mechanism [26]. This illustrates that CoF is a critical parameter in determining the abrasion resistance of materials, with lower CoF values generally indicating better wear resistance across different types of steels, cast irons, and iron-based alloys.

4. Conclusions

This study demonstrates that multi-component high-chromium cast irons exhibit superior abrasion resistance compared to conventional high-Cr cast irons. The addition of Cr, Mo, V, and Ni resulted in improved wear resistance through the formation of larger primary M₇C₃ carbides and fine complex eutectic carbides M₂C (V-rich and Mo-rich). Among the developed alloys, HEWCIs exhibited the lowest wear volumes and friction coefficients, indicating a significant enhancement in wear performance.

Concerning the effect of the alloy elements on the solidification path, the Ni addition appears to increase the M₇C₃ liquidus temperature, with minimum effect on the eutectic reaction (γ + M₇C₃ carbides). The addition of carbide-formers Mo and V, however, contributed to a decrease in the temperature for the eutectic reaction and solidus temperature.

In summary, multi-component cast irons, particularly those incorporating carbide-forming elements with HEA and CCA concepts, provide a promising material choice for industries requiring exceptional wear resistance. This research supports the potential of multi-component strategies to achieve advanced material properties, driving innovation in wear-resistant alloy design.