Investigations on Microstructures and Properties of (Fe, Cr, W)₇C₃ Carbides by First Principles and Experiments

Highlights

• The structural and elastic properties of (Fe, Cr, W)₇C₃ were investigated by first principles.
• Tungsten doping can improve the ductility and the indentation modulus of (Fe, Cr)₇C₃ carbides.
• The elastic anisotropy of M₇C₃ became weaker after tungsten doping.
• A new carbide (Fe_3.27Cr_2.99W_0.74) C₃ was found to be a combination of mechanical properties.

Abstract

Tungsten doping is critical for the wear resistance and application of High-Chromium Cast Iron (HCCI). A series of investigations of (Fe, Cr, W)₇C₃ carbides were performed by first principles calculations and experimental analysis. The calculated results showed that with the increase in tungsten content in M₇C₃, the equilibrium cell volumes and the density gradually increased, and the formation energy of M₇C₃ carbides gradually decreased. The TEM results showed that the (Fe, Cr, W)₇C₃ carbides were (Fe_3.27Cr_2.99W_0.74) C₃ with a hexagonal structure after adding 2.13 wt % tungsten into laser cladding coatings of High-Chromium Cast Iron with a composition of Fe-26.8 wt % Cr-3.62 wt % C. These results from calculations and in situ nanoindentation show that tungsten doping could improve the ductility and indentation modulus of (Fe, Cr)₇C₃ carbides, and the composition of (Fe, Cr, W)₇C₃ was expected to be a high hardness and softness material. The wear test results showed that the wear resistance of tungsten-bearing HCCI was better than ordinary HCCI.

1. Introduction

Hypereutectic High-Chromium Cast Iron (HCCI) is an important wear resistance material, widely used in the mining and crushing field due to its easy production, low cost and excellent wear resistance [1,2,3]. The microstructure of HCCI primarily contains hard M₇C₃-type carbides (with a high hardness of 1300–1800 HV) and martensitic matrix [4]. There are three types of HCCI: hypoeutectic HCCI, eutectic HCCI and hypereutectic HCCI [5]. For hypereutectic HCCI, the M₇C₃ eutectic carbides solidify firstly, and could improve the hardness and the wear resistance of HCCI. HCCI could be prepared by casting [6], an Electrothermal Exial Plasma Accelerator (EAPA) [7] and so on. Heydari et al. [6] prepared HCCI with 22% Cr, 10%–12% tungsten by casting. The results showed that the coarse chromium carbides are distributed in the matrix when the carbon content was low (2.3 wt %). With the increase in carbon content, the carbides will become finer. Efremenko et al. [7] studied the effect of layered morphology and heat treatment on the microstructure and hardness after the pulsed plasma deposition of Fe-C-Cr-W coating on HCCI. Post-deposition heat treatment resulted in the precipitation of M₇C₃ carbides, the carbide precipitation led to a substantial increase in the coating hardness to 1240–1445 HV_0.05. Thus, M₇C₃ carbides are important for HCCI.

M₇C₃ (M = Fe, Cr or other element) carbides are the main hardening phases in HCCI, showing a high hardness, a high strength, a high elastic moduli and a ductile property [8]. Furthermore, M₇C₃ carbides enhance the mechanical properties and oxidation resistance of HCCI at high temperatures [9]. The effect mechanism of alloy elements on the structure and mechanical properties of M₇C₃ has been comprehensively studied by first principles calculations [10,11,12,13,14]. Experiments from Coronado [15] showed that M₇C₃ carbides in HCCI were rod-like single crystals with strong anisotropy. The abrasion resistance of the M₇C₃ carbides in the transverse section ([0001] direction) was higher than in the longitudinal section (non-[0001] direction) [15]. Moreover, the wear resistance of ZTA_P/HCCI composites with a honeycomb structure was higher than High-Chromium Cast Iron [16,17]. Therefore, the extensive application of M₇C₃ multi-component carbides is of great significance to future wear-resistant materials.

One method to improve the hardness and the wear resistance would be to add alloying elements in HCCI, such as vanadium, tungsten, titanium and niobium [18]. As a strong carbide-forming element, tungsten could improve the wear resistance of HCCI [19]. The extensive experiments on the effect of tungsten on microstructure and properties of HCCI have been published over the past few decades [18,19,20,21]. Cortés-Carrillo et al. [18] analyzed the effects of tungsten on the microstructure, hardness, microhardness and abrasive wear of High-Chromium Cast Iron with 17 wt % Cr. The results showed that when tungsten content was 4 wt %, the hardness of the alloys increased due to the dispersion of tungsten into the matrix and the M₇C₃ carbides. When the added amount of tungsten exceeded 4 wt %, M₂C and M₆C carbides appear in the microstructure of the alloy. The work from Lv et al. [19] found that tungsten considerably improved the wear resistance of HCCI, and the wear resistance of HCCI with 1.03 wt % W increased by 205% compared to HCCI without tungsten. Results from Anijdan et al. [20] also showed that the wear resistance of High-Chromium Cast Iron increased after adding the tungsten. The experimental results from Guerra et al. [21] verified that tungsten partially distributed in the different phases, increasing the microhardness and refining the eutectic carbides. However, the effects of tungsten on the structure and mechanical properties of M₇C₃ are not yet clear and should be further explored to reveal the mechanism.

First principles calculations are an effective way to reveal the mechanism. Zhang et al. [8] investigated the optimization of mechanical properties of Fe_7-xCr_xC₃ carbides by first principles calculations, including Fe₆CrC₃, Fe₄Cr₃C₃, Fe₃Cr₄C₃ and FeCr₆C₃, but they ignored M₇C₃ carbides. Chong et al. [22] designed the anisotropic mechanical properties of M₇X₃ (M = Fe, Cr, W, Mo. X = C, B) by multi-alloying. The results showed that the ductility could be increased by doping of W + B and W + Mo without sacrificing the mechanical modulus of Cr₄Fe₃C₃, and the hardness of Cr₄Fe₃C₃ could be improved by doping of Mo + B and Mo + W + B with a finite decrease in ductility. The anisotropy of M₇C₃ carbides is important in the performance of High-Chromium Cast Iron [23]. However, the effect of tungsten on the anisotropy has not been researched and the mechanism is not clear to date. In addition, unfortunately, the stability, properties and electronic structure of M₇C₃ (M = Fe, Cr, W) carbides are seldom comprehensively investigated in the literature.

Therefore, the electronic structures, stability, chemical bonds and existing form of M₇C₃ (M = Fe, Cr and W) in Fe—26.8 wt % Cr—3.62 wt % C—2.13 wt % W High-Chromium Cast Iron were thoroughly investigated by first principles calculation experiments, which are helpful to improve the whole performance of HCCI.

2. Experimental Details

2.1. Calculation Details

First principles calculations are an effective way of investigating the electronic structures, stability and chemical bonds of M₇C₃ carbides in HCCI. The Density Functional Theory (DFT) calculation based on the pseudopotential plane-wave within the Generalized Gradient Approximation (GGA), as implemented in the Cambridge Serial Total Energy Package (CASTEP), was performed in the present work. The Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm [24] was applied in the relaxation process of models to optimize the structures. In the structural optimization process, the maximal displacement was 11.0 × 10⁻³ Å, the largest force was 0.03 eV/Å and the energy change convergence value was less than 1 × 10⁻⁵ eV/atom. After convergence tests, the cutoff energy of 360 eV and k-point of 8 × 8 × 8 were selected for these carbides. Although Fang et al. [25] demonstrated that orthorhombic Fe₇C₃ is more stable than an orthorhombic or hexagonal structure, Chong et al. [22] found that the crystal structure of (Cr, Fe)₇C₃ carbides is hexagonal with the space group of P6₃mc (No. 186) by XRD and TEM analysis. Therefore, a hexagonal structure was selected in this work. Zhang et al. [8] investigated the mechanical properties of Fe_7−xCr_xC₃ carbides based on first principles calculations, including Fe₆CrC₃, Fe₄Cr₃C₃, Fe₃Cr₄C₃ and FeCr₆C₃, but they neglected to calculate the other type, M₇C₃ carbides. Therefore, the properties of Fe₅Cr₂C₃, Fe₂Cr₅C₃ and Fe₃Cr₄C₃ (the crystal structure shown in Figure 1) were calculated by first principles, and the results were compared to the calculated results from references [8,22]. To study the influencing mechanism of tungsten on the structure and mechanical properties of M₇C₃, different amounts of tungsten were doped into M₇C₃ carbides. The M₇C₃ structures consist of nonequivalent Fe (Cr) atoms, and the lattice parameters, elastic moduli, etc., depend strongly on the substitution sites. We calculated all the formation energy of various sites of doped atoms in the unit cell, and then the unit cell obtained by the minimum formation energy was selected as the final result. Some unit cells of M₇C₃ carbides with crystal structures were built, including of Fe₃Cr_3.5W_0.5C₃, Fe₃Cr₃W₁C₃, Fe₃Cr_2.5W_1.5C₃ and Fe₃Cr₂W₂C₃, as shown in Figure 1.

2.2. Experimental Data

In this work, Q235 carbon steel with a chemical composition of Fe-0.18% wt % C-0.22% wt % Si-0.45% wt % Mn-0.02% wt % P-0.02% wt % S was used as the substrate, and the cladding materials were High-Chromium Cast Iron powders and tungsten powders (70 wt % W and 29 wt % Fe). The diameter of the above powders was 75–105 μm. The cladding layer was made by an IPG fiber laser system (YLS-6000) with a continuous wave, with a laser beam size of 5 mm × 5 mm. The parameters were as follows: powder feeding rate k = 15 g/min, scanning speed ν = 4 mm/s, laser power P = 2000 W, flow rate of high-purity argon shielding gas = 15 L/min.

The microstructures were observed using a JEM-2100F Transmission Electron Microscope (TEM, manufacturer, city, country) with an energy-dispersive X-ray (EDX, manufacturer, city, country). TEM samples were prepared by ion milling. The wear resistance of the cladding layer was tested by a MM-200 block-on-ring wear testing machine (manufacturer, city, country, the working principle of machine can be found in reference [26]). In situ nanoindentation experiments were performed using the NanoFlip InForce 50 (manufacturer, city, country) to investigate the hardness and modulus of M₇C₃ carbides.

For the wear resistance of HCCI, the reported value of weight loss is the average of five results. All data graphs were drawn by Origin 8.0 software, and show the formation energy, mechanical properties, hardness and modulus and weight loss.

3. Results and Discussion

3.1. The Equilibrium Lattice Constants and Stability

To ensure the accuracy of the calculation, the optimized lattice constants were calculated, and the calculated results were compared to other calculated results and experimental values, as shown in

Table 1. The lattice parameters of different M₇C₃ carbides.

Species	Space Group	a (Å)	b (Å)	c (Å)	Volume (Å3)	Density (g/cm3)
Fe5Cr2C3	P63mc (186)	7.0054	7.0054	4.3188	183.00	7.61
Fe2Cr5C3	P63mc (186)	6.7795	6.7795	4.6500	187.21	7.23
Fe3Cr4C3	P63mc (186)	6.8358	6.8358	4.5471	185.61	7.37
Fe3Cr4C3 [11]	P63mc (186)	6.8258	6.8258	4.4948	180.0	7.48
Fe3Cr4C3 [8]	P63mc (186)	--	--	--	181.09	--
Fe3Cr4C3Exp. [11]	P63mc (186)	6.9	--	4.52	--	--
Fe3Cr3.5W0.5C3	P63mc (186)	6.7556	6.7558	4.8070	190.12	8.34
Fe3Cr3W1C3	P63mc (186)	6.8609	6.8609	4.8098	195.45	9.23
Fe3Cr2.5W1.5C3	P63mc (186)	6.8998	6.9721	4.7962	199.98	10.12
Fe3Cr2W2C3	P63mc (186)	6.9818	6.9818	4.8382	203.78	11.00

. The optimized lattice constants of Fe₃Cr₄C₃ were 6.8358, 6.8358 and 4.5471 Å, which are similar to other calculated values [8] and experimental values [9] (error less than 1.2%). Considering the differences in experimental equipment and calculation methods, it was proved that the parameters adopted were reliable. With the increase in Cr content, the equilibrium lattice parameter a increased and c decreased, meaning that a decreases when c increases, which is consistent with the conclusion of reference [22]. With the increase in tungsten content, the equilibrium lattice parameters a and b gradually increased. The calculated result of equilibrium cell volume (185.61 Å) for Fe₃Cr₄C₃ was similar to the calculated values (181.09 Å) from Zhang et al. [8] (error less than 3%). With the increase in tungsten content, the equilibrium cell volumes and the density gradually increased.

In order to predict whether M₇C₃ (M = Fe, Cr and W) carbides were easy to compound, the formation energy was calculated by the following equation [8]:

ΔE_{M7C3 (M = Fe, Cr, W)} = E_tot(M₇C₃) − xE_tot(Fe) − yE_tot(Cr) − zE_tot(W) − 3E_tot(C)

(1)

where E_tot(M₇C₃), E_tot(Fe), E_tot(Cr), E_tot(W) and E_tot(C) are the total energies of M₇C₃ carbides, Fe, Cr, W and C systems, respectively. ΔE_{M7C3 (M = Fe, Cr, W)} is the formation energy of M₇C₃ carbides. The formation energy can describe the relative stability of these carbides. If ΔE_M7C3 > 0, the crystal is unstable or metastable; if ΔE_M7C3 < 0, the crystal can exist stably [27]. Additionally, if the stability of carbides is better, the carbides may be synthesized easily; if the M₇C₃ carbides are not stable, it may not be easy to compound them in the experimental stage [28]. According to Equation (1), the formation energies of M₇C₃ carbides were calculated, as shown in Figure 2. Except for Fe₇C₃, the formation energies of other carbides are less than zero, indicating that they are stable crystal structures. With the increase in W and Cr content, the formation energy was increased gradually, indicating that the stability of carbides increased with W and Cr content.

3.2. Mechanical Properties

M₇C₃ carbides are the main hard phases in wear-resistant material, so the elastic of M₇C₃ compounds has an important role in the application of wear-resistant material. The elastic constants C_ij depend mainly on the response of the crystal to external forces, and can be calculated by the bulk, shear and Young’s modulus, Poisson’s ratio, etc. The bulk modulus and the shear modulus can be calculated by the following method [29]:

$$B_{\mathrm{V}}=(\frac{1}{9})[2(C_{11}+C_{12})+4C_{13}+C_{33}]$$

(2)

$$G_{\mathrm{V}}=(\frac{1}{30})[(C_{11}+C_{12}+2C_{33}-4C_{13}+12C_{44}+12C_{66})$$

(3)

B_R = [(C₁₁ + C₁₂) C₃₃ − 2C₁₃²]/(C₁₁ + C₁₂ + 2C₃₃ − 4C₁₃)

(4)

$$G_{\mathrm{R}}=(\frac{5}{2})[(C_{11}+C_{12})C_{33}-2{C_{13}}^2]{\mathrm{C}}_{44}{\mathrm{C}}_{66}/\{3B_{\mathrm{V}}{C}_{44}{C}_{66}+[C_{11}+C_{12})C_{33}-2{C_{13}}^2](C_{44}+C_{66})\}$$

(5)

B = (B_V + B_R)/2

(6)

G = (G_V + G_R)/2

(7)

E = 9BG/(3B +G)

(8)

σ = (3B − 2G)/(6B + 2G)

(9)

where B and G are the bulk modulus and the shear modulus, respectively. E and σ are Young’s modulus and Poisson’s ratio, respectively. C₁₁, C₂₂ and C₃₃ represent the uniaxial deformation along the [$\overline{1}$2$\overline{1}$0], [2$\overline{1}$$\overline{1}$0] and [0001] directions, respectively. C₄₄, C₅₅ and C₆₆ represent the pure shear deformation on ($\overline{1}$2$\overline{1}$0), (2$\overline{1}$$\overline{1}$0) and (0001) crystal planes, respectively. C₁₂ is the shear deformation on the ($\overline{1}$100) crystal plane along the [1$\overline{1}$00] direction.

For the hexagonal phase, the mechanical stability criteria are given by [29]:

C₄₄ > 0, C₁₁ > |C₁₂|, (C₁₁ + 2C₁₂)C₃₃ > 2C₁₃

(10)

The calculated elastic constants of M₇C₃ carbides satisfied the above formula, indicating that these M₇C₃ carbides were stable structures.

Table 2. The elastic constants (C_ij, GPa) of different M₇C₃ carbides.

Species	C11	C33	C44	C12	C13	C66
Fe5Cr2C3	534.6	560.8	123.8	193.5	255.4	152.3
Fe2Cr5C3	562.1	559.2	157.9	159.2	258.3	193.0
Fe3Cr4C3	549.2	531.6	122.5	179.6	248.1	179.6
Fe3Cr4C3 [9]	550.7	532.8	110.6	185.2	229.0	182.7
Fe3Cr3.5W0.5C3	546.1	523.6	100.9	251.5	246.0	166.5
Fe3Cr3WC3	543.2	504.9	99.1	236.8	243.6	155.8
Fe3Cr3WC3 [9]	565.5	415.4	87.8	252.0	249.2	156.8
Fe3Cr2.5W1.5C3	578.9	403.4	92.3	276.5	279.5	149.1
Fe3Cr2W2C3	581.5	377.5	90.9	300.1	288.1	166.9

lists the calculated elastic constants (C_ij) of different M₇C₃ carbides, which showed good agreement with the data of other researchers [22]. The largest C₁₁, C₁₂ and C₁₃ appeared on Fe₃Cr₂W₂C₃, but the largest C₆₆ and C₄₄ appeared on Fe₂Cr₅C₃. Because the hardness of M₇C₃ carbides is large, the relatively simple semi-empirical equation of hardness can be used, which is [30]:

H_V = 1.92 K^1.137G^0.708, K = G/B

(11)

Figure 3 shows the mechanical properties of M₇C₃ carbides. The shear modulus and Young’s modulus of Fe₂Cr₅C₃ are the largest. With the addition of tungsten, the shear modulus and Young’s modulus began to decrease, indicating that the alloying tungsten decreases the strength of M₇C₃. Poisson’s ratio can reflect the flexibility; the larger Poisson’s ratio is, the softer the material is. Poisson’s ratio of Fe₃Cr_2.5W_1.5C₃ is the largest, indicating that the Fe₃Cr_2.5W_1.5C₃ compound is the softest. Pugh’s modulus ratio B/G and Cauchy pressure (C₁₁–C₄₄) can explain the ductile/brittle properties. When G/B is smaller than 0.571, the M₇C₃ has good ductile property [8]. With the addition of tungsten, G/B is reduced, indicating that tungsten can improve the toughness of M₇C₃. Cauchy pressure of M₇C₃ carbides increased with tungsten content, as shown in Figure 3f, which indicates that tungsten doping can improve the ductility of (Fe, Cr)₇C₃ carbides. The composition of (Fe, Cr, W)₇C₃ is expected to be a high hardness and softness material; thus, the synthesis of this carbide would be of great interest.

Anisotropy of M₇C₃ effects the wear resistance of HCCI [8]. Therefore, the analysis of anisotropy is important for understanding the properties of M₇C₃ carbides. The anisotropy of Young’s modulus for hexagonal M₇C₃ carbides along different directions is expounded by the 3D surface contours, following [22]:

1/E = (1 − l₁²)² S₁₁ + l₁⁴S₃₃ + l₁² (1 − l₁²) (2S₁₃ + S₄₄)

(12)

where S_ij is the elastic compliance constant. S_ij is the inverse matrix of C_ij. l₃ = cosφ is the directional cosine. The results are shown in Figure 4. In Figure 4a,b,d,e, it can be seen that Fe₅Cr₂C₃, Fe₃Cr₄C₃, Fe₃Cr_3.5W_0.5C₃ and Fe₃Cr₃W₁C₃ showed strong anisotropy. As the Cr content increased, the elastic anisotropy of (Fe, Cr)₇C₃ became weaker. Similarly, as the tungsten content increased, the elastic anisotropy of (Fe, Cr, W)₇C₃ became weaker, as shown in Figure 4f,g. The results indicate that alloying could weaken the elastic anisotropy of M₇C₃ carbides, which is in agreement with other calculated results [22].

3.3. The Electronic Structures

As is known, the properties of carbides are associated with electronic states [31]. In order to gain some insight into the nature of bonding in M₇C₃ carbides, the band structure and the Partial Density of States (PDOS) were calculated. Figure 5 shows the band structure of M₇C₃, and the dashed line indicates the Fermi level. All calculated M₇C₃ carbides in this work exhibited metallic properties. Figure 6 shows the Total Density of States (TDOS) and the Partial Density of States (PDOS). PDOS can analyze the electronic hybridization states quantitatively with chemical bonding. Fe₅Cr₂C₃, Fe₃Cr₄C₃, Fe₂Cr₅C and Fe₃Cr₃W₁C₃ carbides show large shifts, because the up and down spin channels are not symmetric. However, Fe₃Cr_3.5W_0.5C₃, Fe₃Cr_2.5W_1.5C₃ and Fe₃Cr₂W₂C₃ are symmetric, which may indicate non-magnetic characteristics of these carbides. In Figure 6, the DOS on both sides of the Fermi level were determined mainly by the Fe-d and Cr-d. From −15 to −10 eV, TDOS mainly consists of C-s orbit, but from −7.5 to 10 eV, TDOS of M₇C₃ mainly consists of W-d, Fe-d, Cr-d and C-p orbit, as shown in Figure 6d–g. From −7.5 to −2.5 eV, TDOS mainly consists of Fe-d, W-d and Cr-d orbit, and their peak shape and peak intensity are similar, indicating that there is orbital hybridization. The d orbit of Cr, Fe, W and the p orbit of carbon have strong hybridization, suggesting a covalent bond between the Cr, Fe, W atom and carbon atom. The total electron density distribution is shown in Figure 7. For an ideal single crystal, the magnitude of the mechanical modulus is related to the chemical bond strength. In Figure 7a, Fe-C-Cr and Fe-C-Fe covalent chains can be observed in Fe₃Cr₄C₃ carbides, which is in agreement with other calculated results [8]. In Figure 7b, Cr-W-C, Fe-C-Cr and Fe-C-W covalent chains can be observed in Fe₃Cr₂W₂C₃ carbides, explaining the decrease in formation energy after adding tungsten.

3.4. TEM Analysis

To study the existence form of carbides after adding tungsten to HCCl, the microstructures of High-Chromium Cast Iron with a composition of 3.44C-26.7Cr-1.25Mn-2.3 wt % W was characterized by TEM. Figure 8a shows the bright-field TEM micrographs, and Figure 8b shows the Selected Area Diffraction Pattern (SADP) of M₇C₃. The results show that the carbides are M₇C₃ phase with a hexagonal structure, and the space group is P6₃mc (No. 186). According to the accurate measurement using EDX at 10 different areas of M₇C₃, the calculated analysis suggests that (Fe, Cr, W)₇C₃ has a stoichiometry of (Fe_3.27Cr_2.99W_0.74) C₃. Moreover, the lattice constant of (Fe_3.27Cr_2.99W_0.74) C₃ carbides is a = 0.6833 nm, b = 0.6833 nm, c = 0.4796 nm.

3.5. Nanoindentation Experiments

The hardness and modulus of carbides were investigated by a NanoFlip InForce 50. Figure 9a,b shows the indentation hardness HIT and the indentation modulus EIT of M₇C₃ carbides, respectively. With indentation depths larger than 100 nm, EIT and HIT of M₇C₃ phase reached a constant level, which indicated that the intrinsic material properties of the hard phases were measured in this experiment. Furthermore, the crack formation was not observed at an indentation depth of 200 nm, and the triangular indentations could be observed on M₇C₃ carbides, indicating that the credibility of the data is high. The indentation hardness values of (Fe, Cr)₇C₃ and (Fe, Cr, W)₇C₃ were 17.55 and 17.39 GPa, respectively. The indentation modulus values of (Fe, Cr)₇C₃ and (Fe, Cr, W)₇C₃ were 367.87 and 385.48 GPa, respectively.

3.6. Wear Resistance

The wear resistance of HCCI was tested by a MM-200 block-on-ring wear testing machine. It was apparent that the wear resistance of HCCI after adding tungsten exceeds that of HCCI without tungsten. With the increase in wear load, the wear loss increased, as shown in Figure 10. The wear resistance of HCCI was closely related to the hardness of carbides, but also to the hardness of the matrix. Therefore, the abrasion resistance slightly increased after tungsten was added.

To better reflect the wear resistance of HCCI after the addition of tungsten, the worn surface was characterized by a JSM-6510 Scanning Electron Microscope (SEM) and a VK-9710 color 3D laser scanning microscope. Figure 11a,b is the SEM images of ordinary HCCI and HCCI with tungsten, respectively. Figure 11c,d is the 3D laser morphologies of the worn surfaces of ordinary HCCI and HCCI with tungsten, respectively. Some obvious scratches were found on the specimen surface, and the wear surfaces of both HCCIs were consistent [32]. However, the groove scratches and fine wear of ordinary HCCI were obviously deep, indicating that the wear resistance of HCCI is better after adding tungsten.

4. Conclusions

In this work, we added tungsten to High-Chromium Cast Iron to investigate the microstructures and properties of M₇C₃ carbides by first principles and experiments.

(a) With the increase in W and Cr content in M₇C₃ carbides, the formation energy of M₇C₃ carbides gradually decreased. Tungsten doping can improve the ductility of (Fe, Cr)₇C₃ carbides, and the composition of (Fe, Cr, W)₇C₃ is expected to be a high hardness and softness material.

(b) TEM results showed that the (Fe, Cr, W)₇C₃ carbides are (Fe_3.27Cr_2.99W_0.74) C₃ with a hexagonal structure after adding 2.13 wt % W into Fe—26.8 wt % Cr—3.62 wt % C High-Chromium Cast Iron.

(c) Wear test results showed that the wear resistance could be improved after adding tungsten to HCCI.