Enhancing the mechanical properties via HCP phase induction in CrFeNi 2 La x medium-entropy alloys by alloying La

Abstract

La−modified CrFeNi2Lax (x = 0, 0.05, 0.15, 0.25) medium−entropy alloys were investigated to reveal La's effects on phase evolution and mechanical properties. Thermodynamic and experimental analyses demonstrated that La addition transformed the single FCC phase into dual FCC + HCP phases, with HCP volume fraction increasing alongside La content. Grain refinement occurred from 412 to 245 μm with higher La concentrations. Mechanical properties improved significantly: hardness rose from 125 to 325 HV, yield strength surged from 110 to 839 MPa, and optimal fracture strength (1317 MPa) was achieved at 3.6 at.% La. These enhancements stemmed from combined effects of HCP phase formation, grain refinement, and La−induced strengthening mechanisms.

Keywords

CrFeNiLa MEAs microstructure phase composition mechanical properties strengthening mechanism

Introduction

Metal materials, serving as foundational components in aerospace engineering, precision electronics, and biomedical devices, drive technological breakthroughs through their exceptional mechanical properties and extreme temperature stability, while simultaneously elevating human living standards via energy-efficient applications and sustainable infrastructure solutions. Since the Industrial Revolution, a series of metal alloy systems have been developed, such as stainless steel, titanium alloys, copper alloys, and cobalt alloys. The design concept of these traditional alloys is centered on a primary component, which serves as the solvent, along with a minor quantity of additives that function as solutes.^1–3 However, with the in-depth research conducted by scientists, the performance of these traditional composite materials has reached its limits and fails to meet the demands of modern society.⁴ It was not until 2004 that medium and high entropy alloys (M/HEAs) garnered significant attention from researchers due to their entirely new design concept,^4,5 which included four or more principal components. Due to the special four core effects^6–8: high entropy effect, slow diffusion effect, lattice distortion effect, and the cocktail effect, M/HEAs exhibit comprehensive properties, including high strength and hardness, excellent ductility, corrosion resistance, wear resistance, and high-temperature stability.^9–11 Due to their enormous potential for enhanced performance, M/HEAs have a broad range of applications in the industrial field.^12–14

Although M/HEAs exhibit excellent comprehensive properties, there are still many issues to be resolved in practical applications. Single-phase FCC-structured M/HEAs, such as CrCoFeNi HEAs and CrCoNi MEAs, have excellent ductility but insufficient strength.^15–19 Moreover, the high cost of Co significantly increases the alloy's production cost, hindering its widespread application in structural engineering. To address the issues of low strength and high cost, researchers have undertaken extensive studies. Elemental addition is an effective approach to improve the properties of single-phase M/HEAs. Lu et al.²⁰ achieved a strength-ductility balance in CrCoFeNi HEA by introducing a BCC phase through the addition of Al. In this system, solid solution strengthening and secondary phase precipitation strengthening were the main mechanisms for improving alloy performance. Gao et al.²¹ induced a second σ phase in CrCoFeNi HEA by alloying Si. The σ phase refined the grain size, leading to a significantly increasing in the strength. In addition, elements such as W, Mo, Nb, and Ti have been utilized to further improve the performance of HEAs.^17,^22–24 Simultaneously, the cost of M/HEAs can be effectively reduced by eliminating Co or replacing it with other cost-effective elements. This has led to the development of a series of new Co-free alloy systems, such as CrFeNi,²⁵CrFe₂Ni,²⁶ and CrFeNi₂²⁷ MEAs. These researches indicated that the addition of specific strengthening elements to these systems can further enhance their comprehensive properties, even outperforming some Co-containing M/HEAs, thereby attracting significant attention from researchers in recent years.

Rare earth elements, known as “industrial gold,” possess excellent physical properties that can significantly enhance the quality and performance of materials.²⁸ In recent years, rare earth elements have been successfully applied to improve the strength of M/HEAs. Zhang et al.²⁹ investigated the effect of the rare earth element Y on CoCrFeNiY _x HEAs and found that as the Y content increased, the yield strength improved from 202 (x = 0) to 1440 MPa (x = 0.3), the hardness increased from 146 (x = 0) to 400 HV (x = 0.3). Zhang et al.³⁰ studied the influence of the rare earth element Gd on the microstructure and mechanical properties of CoCrCuFeNiGd _x HEAs, reporting that the addition of Gd increased the yield strength from 320 (x = 0) to 1132 MPa (x = 0.3). Hong et al.³¹ enhanced the yield strength and ultimate tensile strength of (CrCoNi)_100−xY _x MEAs (x = 1) by approximately 87.5% and 49%, respectively, through the addition of a small amount of Y to CrCoNi MEA. Li et al.³² also examined the impact of Y on the mechanical properties of FeCoNi_1.5CuBY _x HEAs, reporting improvements of 75.4%, 19.9%, and 37% in yield strength, compressive strength, and maximum compressive strain, respectively, after Y addition. In addition, Chan et al.¹⁹ improved the strength of CrCoNi MEA by adding trace amounts of La, increasing the yield strength, ultimate tensile strength, and hardness of (CrCoNi)_100−xLa _x MEAs(x = 0.2) by approximately 56%, 26.4%, and 8.4%, respectively. Qu et al.³³ demonstrated that with increasing La content, the grain size of CoCrFeMnNiLa _x HEAs decreased, resulting in an gradual increasing in mechanical properties. In addition, other studies have also shown a grain refinement effect with the addition of La in Al-Cu and Al-Si alloys, leading to improved mechanical properties in both alloy systems.^34,35 Therefore, enhancing the strength of M/HEAs by adding rare earth elements is a feasible and effective design strategy.^36–38

Inspired by the aforementioned research findings, in this study, we prepared a serials of novel Co-free CrFeNi₂La _x (x = 0, 0.05, 0.15, 0.25) MEAs alloy by adding La. We replaced the expensive Co with Ni to reduce production costs. Meanwhile, we increased strength by introducing a second HCP phase into the FCC-structured CrFeNi₂ MEA through the adding of La. Combining of the thermodynamic calculations and experimental characterization, we thoroughly investigated the effects of La on the microstructural evolution and mechanical properties of CrFeNi₂ MEA. Furthermore, the corresponding strengthening mechanisms were also explored.

Experimental methods

Sample preparation

We prepared four non-equiatomic CrFeNi₂La _x (x = 0, 0. 05, 0.15, 0.25) MEA ingots with a diameter of 40 mm by using a vacuum arc melting furnace. The raw materials of Cr, Fe, Ni, and La had a purity of >99.95 wt%. To ensure compositional homogeneity, the alloy ingots were flipped and melted at least 5 times during the melting process, and subsequently subjected to homogenization treatment to ensure uniformity. Homogenize in a pure Ar atmosphere at 1273 K for 24 h, followed by water quenching. Two samples were cut from the center of each alloy ingot. One sample, with dimensions of 10 × 10 × 5 mm³, was used for hardness, XRD, and SEM/EBSD tests, while the other, with dimensions of Ø4 × 6 mm, was used for compression tests. For simplicity, the samples were denoted as La₀, La_0.05, La_0.15, and La_0.25 according to their La content.

Tissue characterization

The phase composition and lattice constants of the La₀, La_0.05, La_0.15, and La_0.25 samples were analyzed using a TD-3500X X-ray diffractometer (Cu target, wavelength 0.15406 nm with a scanning range of 20°–100° and a step size of 4°/min). To further investigate the effect of different La contents on the microstructure and composition of the samples, the microstructure was analyzed using an APREO scanning electron microscope (SEM) with EBSD capabilities. The chemical composition of the samples was determined using energy-dispersive spectroscopy (EDS) attached to the SEM. Point scans were performed to analyze the phase compositions in the samples, with 10 measurements for each phase, and the average value was used as the final result. The volume fractions of each phase in the SEM images were statistically analyzed using ImageJ software. Additionally, the samples were mechanically polished and subsequently vibratory polished to prepare EBSD specimens. The EBSD test selects two different multiples for testing. The low-magnification test step size is selected as 6 μm for observing the grain size and orientation. The high-magnification test step size is selected as 0.2 μm for observing the phase distribution and orientation. The obtained data were analyzed using OimA software. The structure of phases in all four samples were by analyzed by Transmission Electron Microscopy (TEM).

Performance testing

The microhardness of the La₀, La_0.05, La_0.15, and La_0.25 samples was measured using an HVS-1000 micro-Vickers hardness tester. Ten measurements were performed for each sample, and the average value was taken as the final result. The test load was 300 g, with a dwell time of 15 s. Prior to hardness testing, all samples were polished to remove surface scratches. At room temperature, compression tests were conducted to each sample using an AGX-V precision universal testing machine, with a strain rate of 5 × 10⁻⁴/s. Each composition was tested in three times, and the average value was used as the final result.

Result

Effect of La on the structure and composition

Figure 1(a) showed the XRD patterns of the La₀, La_0.05, La_0.15, and La_0.25 samples over a scanning angle range of 20° to 100°. The results indicated that the La₀ sample, without La addition, contained only an FCC phase. After the addition of La, HCP precipitates were detected in the La0.05, La0.15, and La0.25 samples. Some researchers reported that the volume fraction of phase was increased as the intensity of the diffraction peak increased.³⁹ Our results showed that with increasing La content, the intensity of the HCP diffraction peaks gradually increased, indicating a corresponding increase in the volume fraction of the HCP phase. To analyze the changes in the lattice constant of the FCC phase after La addition, the scanning angle was set to 40°～50°. The results showed that the FCC diffraction peaks shifted downward with the addition of La, and the shift became more pronounced as the La content increased, indicating the lattice constant of the FCC matrix phase increased with increasing La content. Zhaolong et al. reported that alloyed larger atomic radius Mo to FeCrCo alloys resulted in an increase in the lattice constant of the modulated structure.⁴⁰ Similarly, in our alloys, the addition of La, which has a larger atomic radius (Table 1), is the primary reason for the observed increase in the FCC lattice constant after La addition.

Figure 1.

The XRD spectra of samples La₀, La_0.05, La_0.15, and La_0.25 and lattice constants of the FCC phase in samples La₀, La_0.05, La_0.15, and La_0.25. (a) XRD pattern, scanning angle ranging from 20° to 100°; (b) XRD pattern, scanning angle of 40° to 50°; (c) Lattice constant of FCC phase in each sample.

Table 1.

Physical parameters of each element in CrFeNi₂La_x MEAs and the mixing enthalpy (ΔHmix AB, kJ/mol) of different binary components.

Element (Melting point, atomic radius)	ΔHmix AB, kJ/mol
Element (Melting point, atomic radius)	La	Cr	Fe	Ni
La (920°C, 187 pm)	-	11	−1	−27
Cr (1907°C, 128 pm)	-	-	−1	−7
Fe (1538°C, 126 pm)	-	-	-	−2
Ni (1455°C, 124 pm)	-	-	-	-

Table 1 presented the physical parameters of each element and the mixing enthalpy between different atomic pairs. The atomic radius of La is significantly larger than that of Cr, Fe, and Ni, which induced substantial lattice distortion in CrFeNi₂La _x MEAs. Additionally, the more negative the direct mixing enthalpy between elements, the more likely it is for a phase structure to form.⁴¹ This was the reason why the volume fraction of the second HCP increased gradually with the addition of La.

Figure 2 showed the SEM images of the La₀, La_0.05, La_0.15, and La_0.25 samples. The results indicated that prior to the addition of La, sample La₀ contained only a single FCC phase. When we added 0.05 at.% La into sample La_0.05, a new second phase precipitated along the grain boundaries. Based on the XRD results, the crystal structure of this new phase was identified as HCP.¹⁹ With further increases in La content, more HCP phases were observed to precipitate along the grain boundaries in samples La_0.15, and La_0.25 (Figure 2(b)–(e)). Statistical analysis of the volume fraction of the HCP phase revealed that it increased from 10.32% in the La_0.05 sample to 36.91% in the La_0.25 sample (Figure 2f). The gradual increase in the volume fraction of the HCP phase with increasing La content also led to an increase in the width of the HCP phase.

Figure 2.

SEM images of the samples La₀, La_0.05, La_0.15, and La_0.25, and the volume fractions of the FCC and HCP phases: (a) Sample La₀; (b) La_0.05; (c, d) La_0.15; (e) La_0.25; (f) Volume fractions of FCC and HCP phases.

To determine the composition of the FCC and HCP phases, SEM-EDS mapping analysis was performed on the La₀, La_0.05, La_0.15, and La_0.25 samples, as shown in Figure 3. The results indicated that La and Ni were enriched in the HCP phase, which was associated with the significantly negative mixing enthalpy between Ni and La (Table 1).⁴²

Figure 3.

SEM images and SEM-EDS surface scanning maps of the samples La₀, La_0.05, La_0.15, and La_0.25: (a) Sample La₀; (b) Sample La_0.05; (c) Sample La_0.15; (d) Sample La_0.25.

To further quantify the composition of the FCC and HCP phases in the La₀, La_0.05, La_0.15, and La_0.25 samples, EDS point analyses were conducted on all samples (Table 2). The results indicated that in the La₀ sample, the FCC matrix phase contains 24.11 at.% Cr, 23.14 at.% Fe, and 52.75 at.% Ni. After the addition of La, the formation of the HCP phase altered the elemental distribution in the FCC phase. With an increasing in La content, the Cr and Fe concentrations in the FCC phase gradually increased, while the Ni concentration decreased. In the La_0.25 sample, the Cr and Fe contents reached 27.42 at.% and 27.69 at.%, respectively, while the Ni content decreased to 44.38 at.%. These results indicated that an increasing amount of Ni segregated into the HCP phase, leading to a redistribution of atoms within the FCC phase. Furthermore, the La content in the FCC phase increased from 0.14 at.% in the La_0.05 sample to 0.51 at.% in the La_0.25 sample. This increasing in La concentration in the FCC phase is a key factor contributing to the observed increase in the FCC lattice constant (Figure 1).

Table 2.

Compositional analysis (at.%) of the FCC and HCP phases in the samples La₀, La_0.05, La_0.15, and La_0.25.

Alloys	phase	Cr(at.%)	Fe(at.%)	Ni(at.%)	La(at.%)
La₀	FCC	24.11 ± 0.20	23.14 ± 0.27	52.75 ± 0.25	–
La_0.05	FCC	22.58 ± 0.21	24.47 ± 0.19	52.81 ± 0.31	0.14 ± 0.02
La_0.05	HCP	2.59 ± 0.06	8.32 ± 0.12	66.36 ± 0.51	22.73 ± 0.21
La_0.15	FCC	24.57 ± 0.15	24.07 ± 0.16	50.83 ± 0.36	0.42 ± 0.08
La_0.15	HCP	3.52 ± 0.08	6.24 ± 0.12	67.59 ± 0.54	22.65 ± 0.18
La_0.25	FCC	27.42 ± 0.21	27.69 ± 0.25	44.38 ± 0.36	0.51 ± 0.05
La_0.25	HCP	3.11 ± 0.03	5.04 ± 0.12	67.46 ± 0.76	24.39 ± 0.32

It is well known that grain size has a significant impact on the mechanical properties of alloys. To investigate the effect of La content on grain size, inverse pole figure (IPF) analyses were performed on the La₀, La_0.05, La_0.15, and La_0.25 samples using SEM-EBSD techniques (Figure 4). The results showed that prior to the addition of La, the La₀ sample exhibited a large grain size. As the La increased, an increasing number of small HCP phases precipitated from the matrix and along the grain boundaries or within the grains. The measured grain sizes based on Figure 4 were presented in Figure 4(e). The results showed that, the La₀ sample exhibited a large grain size of approximately 412 ± 20 µm, which is unfavorable for the alloy's overall performance. As La increased, the grain size of the samples decreased significantly, particularly in the La_0.15 and La_0.25 samples. Compared to the La₀ sample, the grain size was reduced by 33.5% and 40.5% in the La_0.15 and La_0.25 samples, respectively. Theses results indicated that the addition of La could significantly refined the grain structure of the CrFeNi₂ alloy. Especially in sample La_0.25, the average grain size was only 245 µm, approximately half that in sample La₀. Furthermore, as shown in Figure 4, the grain boundaries of the alloy exhibited irregular morphologies. This is attributed to phase transformation induced by the addition of La, which caused distortion and displacement of the grain boundaries. As depicted in Figure 4, the volume fraction and average grain size of the HCP phase exhibited opposite trends as La increased.

Figure 4.

IPF maps of the samples La₀, La_0.05, La_0.15, La_0.25: (a) La₀; (b) La_0.05; (c) La_0.15; (d) La_0.25;(e) Analysis of average grain size and volume fraction of HCP phase in samples La₀, La_0.05, La_0.15, and La_0.25.

In order to better describe the distribution and orientation of the HCP phase in alloys. EBSD tests were conducted on La_0.05, La_0.15 and La_0.25 alloys at high multiples. As shown in Figure 5, (a1), (b1) and (c1) are the IPF diagrams of the three alloys, and the orientation color diagram is on the left. (a2), (b2) and (c2) are phase diagrams of three alloys, among which the green area represents the FCC phase and the yellow area represents the HCP phase. It can be found that its phase diagram distribution is similar to that of the SEM image (Figure 3). Unlike Figure 4, the HCP phase can be clearly observed in Figure 5. It is speculated that the reason for this is that the average grain size of this series of alloys is relatively large, and the scanning step size in Figure 4 is 6 μm, while the width of the HCP phase is approximately 6 μm. Therefore, the HCP phase was not clearly observed in Figure 4 of the low-magnification scan. (a3), (b3) and (c3) show the KAM diagrams of the alloy. The KAM plots provide a visual depiction of internal deformation irregularities and the corresponding distribution of dislocation or defect density.⁴³ It can be observed that the distribution of KAM values is relatively uniform, and the areas with high KAM values are in good agreement with the phase boundaries. This reflects the ideal homogenization effect of the alloy

Figure 5.

EBSD IPF, KAM and Phase maps of three alloys: (a1–a3) La_0.05; (b1–b3) La_0.15; (c1–c3) La_0.25.

Using TEM, we further investigated the precipitation behavior of precipitated phase and phase structure in sample La_0.15 (Figure 6(a)). The results clearly revealed that the precipitated second phase was precipitated along grain boundaries or within the grain. The selected area diffraction patterns (SADPs) showed the structure of precipitated phase is HCP, while matrix is FCC, which was consistent with the results of XRD. Based on the indexing and symmetry of the diffraction spots, the HCP structure can be identified as the 2H-type HCP (ABAB stacking, space group P6₃/mmc).⁴⁴ Moreover, the TEM analysis further confirmed that the HCP phase was precipitated within the grain interiors or at the grain boundaries. The presence of the HCP phase effectively hindered grain boundary migration, thereby suppressing grain growth. In conclusion, the addition of La significantly refined the grains of the CrFeNi₂ alloy by introducing the HCP phase, and grain refinement is beneficial for enhancing the mechanical properties of the alloy.

Figure 6.

TEM results of the sample La_0.15: (a) TEM image; (b) and (c) Corresponding selected area electron diffraction (SAED) patterns of selected areas A and B, respectively, as indicated in Fig. a.

Prediction of phase transition based on thermodynamic calculations

It is well known that the phase composition of an alloy has a significant impact on its performance. Therefore, predicting the phase transformation and phase stability of CrFeNi₂La alloys provides important theoretical guidance for designing CrFeNi₂La medium-entropy alloys with excellent mechanical properties. In this study, the phase stability of CrFeNi₂La alloys will be predicted using the following thermodynamic formulas^42,45:

{Δ H}_{m i x} = \sum_{i = 1, i \neq j}^{n} Ω_{i j} c_{i} c_{j}

(1)

{Δ S}_{m i x} = - R \sum_{i = 1}^{n} (c_{i} \ln c_{i})

(2)

Ω = \frac{T_{m} {Δ S}_{m i x}}{| Δ H_{m i x} |}

(3)

T_{m} = \sum_{i = 1}^{n} c_{i} {(T_{m})}_{i}

(4)

\begin{aligned} δ = \sqrt{\sum_{i = 1}^{n} c_{i} (1 - \frac{r_{i}}{\bar{r}})} \end{aligned}

(5)

where c_і corresponds to the atomic percentage of the i component; Ω_іj = (4ΔHmix AB) is the interaction parameter between the ith and јth elements, ΔHmix AB represents the mixing enthalpy; ΔSmix AB denotes the mixing entropy of the n element, R is the gas constant (8.314 J/mol/K); (T_m) _i is the melting point of the i element; δ is the atomic size difference;

\bar{r} = \sum_{i = 1}^{n} c i r i

is the average atomic radius of all constituent elements, and r_i is the atomic radius of the ith element (see Table 1). Experimental results indicated that the formation range for a single-phase solid solution is −15 < ΔH_mix < 5kJ mol⁻¹, 1 < Ω < 6.6, δ < 6.6%.⁴⁵ Outside these ranges, intermetallic compounds might form. For the current CrFeNi₂La _x alloy system, the values of ΔHmix AB, ΔSmix AB, Ω, and δ were summarized in Table 3. As seen from the table, La₀ and La_0.05 both fall within the single-phase solid solution formation range, but previous microstructural analysis revealed that the La_0.05 alloy exhibits an FCC and HCP dual-phase structure. Therefore, this prediction criterion could not successfully predict the phase transitions in La_0.05, La_0.15, and La_0.25 alloys. Notably, the results in Table 3 showed that La_0.15 and La_0.25 alloys fall within the intermetallic compound formation range. Similar to La, Nb (146pm), Zr (160pm), and Y(187pm) elements possessed larger atomic radii and have more negative mixing enthalpies with Ni. It has been reported that the ΔH_mix, Ω, and δ criteria have been successfully applied to M/HEAs with added Nb, Zr, and Y.^17,46 Furthermore, a new criterion for predicting the formation of Laves phases was proposed based on the effect of Nb and Zr elements on the alloy phase structure, suggesting that δ > 5% predicts Laves phase formation.²⁹ As can be seen from Table 3, the HCP phase could be observed when δ > 5.02%. More importantly, since the crystal structure of the Laves phase was closely related to the HCP phase, the δ > 5% value could be considered an effective criterion for predicting the formation of the HCP phase.

Table 3.

Calculated parameters ΔS_mix, ΔH_mix, Ω, δ, VEC, Δχ, and Λ for the CrFeNi₂La_x MEA studied.

Samples	ΔS_mix(J/mol/K)	ΔH_mix(kJ/mol)	Ω	δ (%)	VEC	Λ	Δχ
La₀	8.64	−4.75	3.38	1.39	8.5	4.507	0.102
La_0.05	9.06	−5.02	3.32	5.63	8.43	0.29	0.129
La_0.15	9.64	−5.54	3.18	9.19	8.3	0.11	0.169
La_0.25	9.98	−5.98	3.02	11.42	8.17	0.076	0.198

From the perspective of elemental chemical compatibility, the electronegativity difference can be used as a characterization parameter to evaluate the phase formation potential in the CrFeNi₂La _x MEAs system. The electronegativity difference (Δχ) is defined by the formula $Δ χ = \sqrt{\sum_{i = 1}^{n} {c i (χ_{i} - \bar{χ})}^{2}}$ ,⁴⁷ where χ_i represents the Pauling electronegativity of the ith element, and $\bar{χ} = \sum_{i = 1}^{n} c i r i$ represents the average Pauling electronegativity. Wang et al.⁴⁸ indicated that a Δχ≥0.133 favors the formation of TCP phases. From Table 3, it could be seen that the minimum electronegativity difference in the current CrFeNi₂La _x MEAs was very close to this value. Additionally, Table 3 also provided the calculations of VEC (valence electron concentration) and Λ(Λ = ΔS_mix/δ²) to predict the phase formation in the current alloy system. According to the VEC phase prediction criterion, all alloys should belong to the FCC single-phase structure (VEC ≥ 8). Therefore, the VEC criterion fails to predict the phase formation. Previous studies also used the VEC criterion for predictions, but the results showed that the addition of elements such as Nb, Y, and Gd could not predict the formation of HCP, Laves, TCP, and other phases, requiring further research.^29,30,46 Singh et al.⁴⁹ established three ranges for the Λ-parameter to predict intermetallic compound formation. Based on the three ranges of the Λ-parameter, all alloys (Λ < 0.231) (see Table 3) correspond to the unique compound region. However, as shown in Figure 2, all La-containing alloys exhibit an FCC and HCP dual-phase structure. Therefore, the Λ-parameter criterion cannot predict the phases in the current alloy system.

Effect of adding La on the mechanical properties

To investigate the effect of La addition on the mechanical properties of CrFeNi₂La _x alloys, Vickers hardness tests and room-temperature compression experiments were conducted on samples La₀, La_0.05, La_0.15, and La_0.25. Figure 7 showed the Vickers hardness values of the samples La₀, La_0.05, La_0.15, and La_0.25. The results indicated that the hardness of the La₀ sample is 125 HV. After the addition of La, the hardness of the samples significantly increased. Compared to the La₀ sample, the hardness of the La_0.05, La_0.15, and La_0.25 samples increased by 34.4%, 96%, and 160%, respectively.

Figure 7.

Vickers hardness of samples La₀, La_0.05, La_0.15, and La_0.25.

Figure 8 showed the compression engineering stress-strain curves of the samples La₀, La_0.05, La_0.15, and La_0.25. The compression yield strength (σ_y), fracture strength (σ_f), and fracture plastic strain (ε_p) values were obtained from the curves, as shown in Table 4. The results indicated that the yield strength of the La₀ sample was only 110 MPa, but it exhibited good compressive plasticity, and could be compressed to 60% strain without fracture. After adding with 1.2 at.% La, the La₀.₀₅ sample exhibited an increased yield strength of 295 MPa while maintaining excellent ductility, with no fracture observed during compression. As the La content increased, the yield strength demonstrated a progressive enhancement, the La₀.₁₅ sample achieved a yield strength of 559 MPa, a fracture strength of 1317 MPa, and retained 46% plastic strain. When the Co content was increased to 5.9 at.%, The yield strength of the La_0.25 sample increased to 839 MPa, but the fracture strength and plasticity decreased by 2.9% and 10%, respectively. The changes in these properties were primarily related to the volume fraction of the HCP phase, which will be discussed in detail in Discussion.

Figure 8.

Compression engineering stress-strain curves of samples La₀, La_0.05, La_0.15, and La_0.25.

Table 4.

Compression yield strength σ_y, fracture strength σ_f, and fracture plastic strain ε_p values of samples La₀, La_0.05, La_0.15, and La_0.25.

Alloys	σ_y(MPa)	σ_f (MPa)	ε_p (%)
La₀	110 ± 8	No fracture	60%
La_0.05	295 ± 10	No fracture	60%
La_0.15	559 ± 12	1317 ± 14	46 ± 0.7
La_0.25	839 ± 15	1279 ± 15	10 ± 0.6

Discussion

The mechanical properties data for the samples La₀, La_0.05, La_0.15, and La_0.25 showed that, after the addition of La, both the hardness and strength increased, and gradually increased as La content creased. The addition of La might have contributed to the enhancement in mechanical properties by improving the volume fraction of the HCP phase, solubility, and the grain size of the FCC phase. Ma et al. found that as the Hf content increased, the Vickers hardness and compressive yield strength of the CoCrFeNi HEA improved due to second-phase strengthening.⁵⁰ Lin et al. observed that the ɛ-martensitic transformation from the FCC phase to the HCP phase in an Fe17.5Mn10Co12.5 Cr5Ni5Si MEA, leading an increasing in mechanical properties.⁵¹ In addition, Huang et al. found that the introduction of the HCP phase enhanced the yield stress, yield strain, and plastic flow stress of CoCrFeMnNi HEAs.⁵² Therefore, we speculated that the increasing in hardness and strength in our alloys was closely related to the formation of the HCP phase. Figure 9 showed the curve fitting relationship between yield strength and hardness as a function of volume fraction of the HCP phase. The fitting results indicated that both yield strength and hardness exhibited a good linear relationship with the volume fraction of the HCP phase, indicating that the volume fraction of the second-phase HCP was the main reason for the enhancement of hardness and strength in CrFeNi₂La _x MEAs.

Figure 9.

Linear fitting relationship between yield strength and hardness of samples La₀, La_0.05, La_0.15, and La_0.25 and HCP phase volume fraction.

Previous studies have shown that the HCP phase exhibited higher strength and hardness compared to the FCC phase.⁵³ The strength increment of the alloy can be expressed as:

σ_{y} = σ_{F C C} V_{F C C} + σ_{H C P} V_{H C P}

(6)

In the equation, σ_FCC and σ_HCP represent the yield strengths of the FCC and HCP phases, respectively. V_FCC and V_HCP represent the volume fractions of the FCC and HCP phases, respectively. Considering that this alloy system contains only FCC and HCP phases, i.e., V_FCC + V_HCP = 1, Equation (6) can be rewritten as:

σ_{y} = σ_{F C C} + (σ H C P - σ F C C) V_{H C P}

(7)

The variation relationship between the HCP phase volume fraction and σ_y expressed by Equation (7) matched well with the data in Figure 9, indicating that Equation (7) can effectively explained the yield strength increment of CrFeNi₂La _x MEAs.

Some research groups found that the strength of HEAs was closely related to solubility.^54,55 In our alloy system, we considered the FCC matrix phase as the solid solution phase, while La atoms were regarded as solute elements. When the larger La atoms were dissolved into the FCC solid solution phase, lattice distortion occurred, leading to an increase in the hardness and strength of the alloy. In this study, the effect of the solubility of La on the hardness and strength of the alloy was calculated using the dislocation-solute elastic interaction model, and the calculation formula was as follows²⁹:

Δ σ s = 3^{3 / 2} \cdot \frac{G \cdot ε_{s}^{3 / 2} \cdot c^{1 / 2}}{700}

(8)

where Δσ_s represents the yield stress increment caused by solid solution strengthening, 3^3/2 is an approximate conversion parameter, c is the molar ratio, and G is the shear modulus. The value for G is derived as follows:

G = E / 2 (1 + v)

(9)

where v represents Poisson's ratio, with a value of 0.3. ε_s is the interaction parameter, is derived as follows :

ε_{s} = | \frac{ε_{G}}{1 + 0.5 ε_{G}} - 3 \cdot ε_{α} |

(10)

where, ε_G represents the elastic misfit parameter, which can be expressed as

ε_{G} = \frac{1}{G} \frac{\partial G}{\partial C}

. ε_α is the atomic size difference, which can be expressed as

ε_{α} = \frac{1}{α} \frac{\partial α}{\partial c}

, where α is the lattice parameter, and the value of ε_α can be calculated from the XRD results (Figure 1(c)). By substituting the required parameter values into Equation (8), we calculated the values of Δσ_s for samples La_0.05, La_0.15, and La_0.25 were all less than 9.41 MPa. Our calculations showed that the solid solution strengthening contributed only a modest increase to the hardness and yield strength for CrFeNi₂La _x MEAs.

Other research groups found that grain refinement could improve the strength of alloys.^56,57 The SEM results revealed that the addition of La led to the refinement of the FCC grains in the samples La_0.05, La_0.15, and La_0.25 (Figure 5). According to the classical Hall-Petch theory, the relationship between the yield stress increment ( $Δ σ_{G}$ ) and grain size can be expressed as⁵³:

Δ σ G = k ({d x}^{- 1 / 2} - {d 0}^{- 1 / 2})

(11)

where k is the Hall-Petch constant, the value of k in the above equation was 600 MPa µm^1/2 in CrFeNi and CrFeNi₂ alloys,^58,59 and

d x

and

d 0

represented the initial and final average grain sizes, respectively. The average grain sizes of the samples La_0.05, La_0.15, and La_0.25 were 362, 274, and 245 µm, respectively (Figure 5). We then calculated the strength increments for the samples La_0.05, La_0.15, and La_0.25 were 1.97 MPa, 6.69 MPa, and 8.77 MPa, respectively. The calculations showed that grain boundary strengthening played a limited role in the increasing for yield stress.

In summary, we analyzed three strengthening mechanisms for enhancing the strength of CrFeNi2Lax MEAs, i.e. solid solution strengthening of the FCC matrix phase, grain refinement strengthening, and second−phase strengthening. Among these, second−phase strengthening was the primary strengthening mechanism, while solid solution strengthening and grain boundary strengthening contribute relatively little to the overall strength.

Conclusion

In this study, four novel CrFeNi2Lax (x = 0, 0.05, 0.15, 0.25) medium−entropy alloy ingots were prepared by vacuum arc melting. The effect of La on the microstructure and mechanical properties of the alloy was investigated. Our major results are as follows:

The addition of La had a significant impact on the phase composition of CrFeNi2 MEAs, transforming the structure from a single FCC phase to a dual−phase structure consisting of FCC and HCP phases. Meanwhile, the volume fraction of the HCP phase increased with the La content. The presence of the HCP phase effectively hindered grain boundary migration, resulting in a refinement in grain size, the refined grain size is approximately 245 µm.

The hardness test results indicated that with the increase in La content, the hardness gradually increased. The increase in the volume fraction of the HCP phase may have contributed to a corresponding increase in hardness. Compression experiments showed that as the La content increased, both the yield strength and fracture strength were significantly enhanced. La0.15 has the best performance, with strength and plasticity of 1317MPa and 46%, respectively. However, the volume fraction of the brittle HCP phase in La0.25 continuously increased, leading to a reduction in both strength and plasticity.

Three strengthening mechanisms, i.e. solid solution strengthening of the FCC matrix phase, grain refinement strengthening, and second−phase strengthening were applied for analyzed the increasing in strength for CrFeNi2Lax MEAs. Among these, second−phase strengthening played a primary role in contributing to the strength, while solid solution strengthening and grain boundary strengthening contributed relatively little to the strength.

Footnotes

Acknowledgements

This study was financially supported by Natural Science Foundation of Shandong Province (NO. ZR2020ME001, ZR2024ME238), the National Natural Science Foundation of China (No. 12272392), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB22040303), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2022020).

ORCID iD

WenQing Wei

Author contribution(s)

YunFei Cheng: Data curation; Investigation; Methodology; Software; Writing – original draft.

WenQing Wei: Formal analysis; Methodology; Resources; Writing – original draft; Writing – review & editing.

Jing Sun: Software; Supervision; Writing – review & editing.

YuKun Miao: Data curation; Resources.

JingXiang Mao: Investigation; Resources; Writing – original draft.

YongKang Zhang: Formal analysis; Investigation; Project administration.

ZhaoLong Xiang: Conceptualization; Resources.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Natural Science Foundation of Shandong Province (grant number ZR2020ME001).

Declaration of conflicting interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

References

Wang

, et al. High-entropy alloy: challenges and prospects. Mater Today 2016; 19: 349–62.

Tsai

Yeh

. High-entropy alloys: a critical review. Mater Res Lett 2014; 2: 107–23.

Miracle

Senkov

. A critical review of high entropy alloys and related concepts. Acta Mater 2017; 122: 448–511.

Yeh

Chen

Lin

, et al. Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Adv Eng Mater 2004; 6: 299–303.

Xiao

Yin

. Properties and preparation of high entropy alloys. MATEC Web Conf 2018; 142: 3003–5.

Zhang

, et al. Recent progress in high-entropy alloys. Adv Mat Res 2013; 631–632: 227–32.

Zheng

, et al. Tailoring nanoprecipitates for ultra-strong high-entropy alloys via machine learning and prestrain aging. J Mater Sci Technol 2021; 69: 156–67.

Guo

Pei

, et al. Predict the phase formation of high-entropy alloys by compositions. J Mater Res Technol 2023; 22: 3331–39.

Liu

Tsai

. Effect of Ge addition on the microstructure, mechanical properties, and corrosion behavior of CoCrFeNi high-entropy alloys. Intermetallics 2021; 132: 107167.

10.

Wei

Zhang

, et al. A sunflower-like eutectic microstructure in the AlCrFeMnNi2 high-entropy alloy with excellent compressive mechanical properties. Mater Lett 2023; 339: 134107.

11.

Liang

Yang

, et al. Study on the wear resistance and mechanism of AlCrCuFe2NiTix high-entropy surfacing alloys. J Alloys Compd 2024; 971: 172510.

12.

Nascimento

Donatus

Ríos

, et al. A review on corrosion of high entropy alloys: exploring the interplay between corrosion properties, alloy composition, passive film stability and materials selection. Mater Res 2022; 25: 1516–1439.

13.

Liu

, et al. Development of high-entropy shape-memory alloys: a review. Metals (Basel) 2023; 13: 1279.

14.

Muhammad Nadzri

Halin

DSC

Al Bakri Abdullah

, et al. High-entropy alloy for thin film application: a review. Coatings 2022; 12: 1842.

15.

Xia

, et al. Microstructures and mechanical properties of CoCrFeNiHfx high-entropy alloys. Mater Sci Eng A 2020; 792: 139820.

16.

Yang

Xia

Liu

, et al. Effects of AL addition on microstructure and mechanical properties of Al CoCrFeNi High-entropy alloy. Mater Sci Eng A 2015; 648: 15–22.

17.

Wang

Cheng

, et al. Designing eutectic high entropy alloys of CoCrFeNiNb x. J Alloys Compd 2016; 656: 284–89.

18.

Wei

Gong

Tsuru

, et al. Si-addition contributes to overcoming the strength-ductility trade-off in high-entropy alloys. Int J Plast 2022; 159: 103443.

19.

Chan

S-N

Hsueh

C-H

. Effects of La addition on the microstructure and mechanical properties of CoCrNi medium entropy alloy. J Alloys Compd 2022; 894: 162401.

20.

Luo

Yang

, et al. Effects of Al addition on structural evolution and mechanical properties of the CrCoNi medium-entropy alloy. Mater Chem Phys 2019; 238: 121841.

21.

Gao

Chen

, et al. Tailoring formation and proportion of strengthening phase in non-equiatomic CoCrFeNi high entropy alloy by alloying Si element. Intermetallics 2022; 147: 107617.

22.

Karimzadeh

Malekan

Mirzadeh

, et al. Effects of titanium addition on the microstructure and mechanical properties of quaternary CoCrFeNi high entropy alloy. Mater Sci Eng A 2022; 856: 143971.

23.

Mukarram

Munir

Mujahid

, et al. Systematic development of eutectic high entropy alloys by thermodynamic modeling and experimentation: an example of the CoCrFeNi-Mo System. Metals (Basel) 2021; 11: 1484.

24.

Yang

Liang

Wang

, et al. Improving mechanical properties of (Co1.5FeNi)88.5Ti6Al4R1.5 (R = Hf, W, Nb, Ta, Mo, V) multi-component high-entropy alloys via multi-stage strain hardening strengthening. Mater Des 2022; 222: 111061.

25.

Zhang

, et al. Cobalt-element-free eutectic medium-entropy alloys with superior mechanical performance and processability. Mater Sci Eng A 2021; 801: 140421.

26.

Song

Chai

Zheng

, et al. Design Fe-based eutectic medium-entropy alloys Fe2NiCrNbx. Acta Metall Sin (Engl Lett) 2021; 34: 1103–08.

27.

Lee

W-H

M-G

, et al. Microstructures and mechanical properties of CrFeNi2Nbx eutectic multicomponent alloys. J Alloys Compd 2021; 860: 158502.

28.

Chen

, et al. Recent advances in selective separation technologies of rare earth elements: a review. J Environ Chem Eng 2022; 10: 107104.

29.

Zhang

Zhou

, et al. Effects of rare-earth element, Y, additions on the microstructure and mechanical properties of CoCrFeNi high entropy alloy. Mater Sci Eng A 2018; 725: 437–46.

30.

Zhang

, et al. Microstructure and mechanical behaviors of GdxCoCrCuFeNi high-entropy alloys. Mater Sci Eng A 2017; 707: 708–16.

31.

Hong

X-W

Hsueh

C-H

. Effects of yttrium addition on microstructures and mechanical properties of CoCrNi medium entropy alloy. Intermetallics 2022; 140: 107405.

32.

Liu

Wang

, et al. Effect of the rare earth element yttrium on the structure and properties of boron-containing high-entropy alloy. Jom 2020; 72: 2332–39.

33.

Hou

Ren

, et al. Grain refinement of the CrMnFeCoNi high entropy alloy cast ingots by adding lanthanum. Metall Mater Trans B 2021; 52: 1194–99.

34.

Zheng

Zhang

Jiang

, et al. Effect mechanisms of micro-alloying element La on microstructure and mechanical properties of hypoeutectic Al-Si alloys. J Mater Sci Technol 2020; 47: 142–51.

35.

Lin

Feng

, et al. Effects of La–Ce addition on microstructure and mechanical properties of Al–18Si–4Cu–0.5Mg alloy. Trans Nonferrous Met Soc China 2019; 29: 1592–600.

36.

Wang

T-H

Liao

Y-C

, et al. Hardness and strength enhancements of CoCrFeMnNi high-entropy alloy with Nd doping. Mater Sci Eng A 2019; 764: 138192.

37.

Lin

Y-S

Y-C

Hsueh

C-H

. Strengthening of CoCrNi medium entropy alloy with gadolinium additions. Vacuum 2023; 211: 111969.

38.

Guo

L-Y

Y-C

Hsueh

C-H

. Effects of erbium addition on microstructure and mechanical properties of CoCrNi medium entropy alloy. J Mater Eng Perform 2024; 33: 4856–66.

39.

Xiang

Zhang

Xin

, et al. Ultrafine microstructure and hardness in Fe-Cr-Co alloy induced by spinodal decomposition under magnetic field. Mater Des 2021; 199: 109383.

40.

Xiang

Wang

, et al. Improvement of magnetic properties and hardness by alloying Mo to a FeCrCo alloy. Acta Mater 2024; 281: 120388.

41.

Zhang

Chen

Jayalakshmi

, et al. Factors determining solid solution phase formation and stability in CoCrFeNiX0. 4 (X = Al, Nb, Ta) high entropy alloys fabricated by powder plasma arc additive manufacturing. J Alloys Compd 2021; 857: 157625.

42.

Zhang

Zhou

Lin

, et al. Solid-solution phase formation rules for multi-component alloys. Adv Eng Mater 2008; 10: 534–38.

43.

Liu

Fang

Xiong

, et al. The response of dislocations, low angle grain boundaries and high angle grain boundaries at high strain rates. Mater Sci Eng A 2021; 822: 141704.

44.

Wang

Huang

, et al. Seeded synthesis of unconventional 2H-phase Pd alloy nanomaterials for highly efficient oxygen reduction. J Am Chem Soc 2021; 143: 17292–99.

45.

Yang

Zhang

. Prediction of high-entropy stabilized solid-solution in multi-component alloys. Mater Chem Phys 2012; 132: 233–38.

46.

Huo

Zhou

Fang

, et al. Microstructure and mechanical properties of CoCrFeNiZrx eutectic high-entropy alloys. Mater Des 2017; 134: 226–33.

47.

. Relative effect of electronegativity on formation of high entropy alloys. Int J Cast Met Res 2015; 28: 229–33.

48.

Liu

Peng

, et al. The effect of Ti addition on phase selection of CoCrCu0.5FeNi high-entropy alloys. Mater Sci Technol 2018; 34: 473–79.

49.

Singh

Kumar

Dwivedi

, et al. A geometrical parameter for the formation of disordered solid solutions in multi-component alloys. Intermetallics 2014; 53: 112–19.

50.

Shek

. Effects of Hf on the microstructure and mechanical properties of CoCrFeNi high entropy alloy. J Alloys Compd 2020; 827: 154159.

51.

Lin

Chen

S-C

Lin

H-C

, et al. Enhancement in mechanical properties through an FCC-to-HCP phase transformation in an Fe-17.5 Mn-10Co-12.5 Cr-5Ni-5Si (in at%) medium-entropy alloy. J Alloys Compd 2022; 898: 162765.

52.

Huang

Yao

Peng

, et al. Plastic deformation and strengthening mechanism of FCC/HCP nano-laminated dual-phase CoCrFeMnNi high entropy alloy. Nanotechnology 2021; 32: 505724.

53.

Zhou

Mao

Jiang

, et al. Effect of rare-earth element Y addition on microstructure and mechanical properties of CrFeNi2 medium entropy alloy. Intermetallics 2023; 163: 108079.

54.

Cai

Chen

, et al. Phase composition and solid solution strengthening effect in TiZrNbMoV high-entropy alloys. Mater Des 2015; 83: 651–60.

55.

Jiang

Lin

, et al. Studies on the microstructure and properties of AlxCoCrFeNiTi1-x high entropy alloys. J Alloys Compd 2018; 741: 826–33.

56.

Wang

Pan

Xie

, et al. Grain refinement and strength enhancement in Mg wrought alloys: a review. J Magnes Alloys 2023; 11: 4128–45.

57.

Easton

Qian

Prasad

, et al. Recent advances in grain refinement of light metals and alloys. Curr Opin Solid State Mater Sci 2016; 20: 13–24.

58.

Liu

, et al. Grain growth and the Hall–Petch relationship in a high-entropy FeCrNiCoMn alloy. Scr Mater 2013; 68: 526–29.

59.

Liang

Wei

Ren

. Introducing Laves phase strengthening into an ultrafine-grained equiatomic CrFeNi alloy by niobium addition. Mater Sci Eng A 2021; 806: 140611.