Trans. Nonferrous Met. Soc. China 31(2021) 512-520

Effects of La on microstructure and mechanical properties of NbMoTiVSi_0.2 refractory high entropy alloys

Qin XU¹, De-zhi CHEN², Cong-rui WANG¹, Wen-chao CAO², Qi WANG², Hong-zhi CUI ³, Shu-yan ZHANG⁴, Rui-run CHEN^2,3

1. School of Mechanic and Electrical Engineering, Henan University of Technology, Zhengzhou 450001, China;

2. National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China;

3. School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China;

4. Centre of Excellence for Advanced Materials, Dongguan 523808, China

Received 29 March 2020; accepted 18 December 2020

Abstract: To study the effects of La on the microstructure and mechanical properties of refractory high entropy alloys, NbMoTiVSi_0.2 alloys with different La contents were prepared. Phase constitution, microstructure evolution, compressive properties and related mechanisms were systematically studied. Results show that the alloys with La addition are composed of BCC solid solution, eutectic structure, MSi₂ disilicide phase and La-containing precipitates. Eutectic structure and most of La precipitates are formed at the grain boundaries. Disilicide phase is formed in the grains. La can change the grain morphologies from dendritic structure to near-equiaxed structure, and the average grain size decreases from 180 to 20 μm with the increase of La content from 0 to 0.5 at.%. Compressive testing shows that the ultimate strength and the yield strength increase with the increase of La content, which is resulted from the grain boundary strengthening. However, they cannot be greatly improved because of the formation of MSi₂ disilicide phase with low strength. The ductility decreases with the increase of La content, which is due to the La precipitates and brittle MSi₂ disilicide phase.

Key words: high entropy alloy; lanthanum; eutectic structure; refractory metal; disilicide phase

1 Introduction

High entropy alloys (HEAs) containing at least five principal elements were first put forward in 2004 [1,2]. They have attracted much attention due to their high thermal stability and excellent mechanical properties [3-6]. HEAs were originally defined as homogeneous solid solutions with a single BCC, FCC or HCP crystal structure. Among them, refractory high entropy alloys (RHEAs)  with BCC solid solution structure are the most potential candidates for high temperature structural applications [6-9]. RHEAs are composed of refractory elements such as W, Ta, Nb, Mo, Hf, Zr, Ti and V, and are designed in equimolar or near equimolar ratios [10-14]. Due to the high melting point, RHEAs show high strength and excellent performance at high temperatures.

The purpose of designing RHEAs is to extend the service temperature of structural materials, and most of RHEAs show the potential to extend the service temperature of blades and disks  beyond current superalloys. Many RHEAs such  as Al_0.4Hf_0.6NbTaTiZr, AlMo_0.5NbTa_0.5TiZr and NbTiVTaAl_x (x=0-1, molar ratio) have sufficient room-temperature compressive ductility (ε ≥10%), but there is a degree of cleavage fracture [15,16]. Other refractory high entropy alloys (AlNb-TiV [17], HfMoNbTiZr [18], AlNbTiZr [19] and NbMoTiV [20]) have attractive yield strength with good enough ductility, which can be further improved by alloy composition modification and microstructure control. LIU et al [21] reported that HfMo_0.5NbTiV_0.5Si_x can achieve excellent strength at elevated temperature and reasonable ductility at room temperature by adding silicon. Many investigations have confirmed that the addition of rare earth element La can refine the microstructure of steel, TiAl and many other alloys [22-28], thus improve their mechanical properties. However, to the best of our knowledge, the effects of La on the microstructure and mechanical properties of RHEAs have not been studied.

Previous study [20] showed that NbMoTiVSi_0.2 refractory high entropy alloy exhibited good room-temperature compressive ductility (ε >16%) and high yield strength (σ_0.2>1760 MPa). In this study, the NbMoTiVSi_0.2 refractory high entropy alloys were modified by adding different contents of La, and the alloys were prepared by vacuum arc melting process. The effects of rare element La on the phase constitution, microstructure evolution, mechanical properties and the related mechanisms were thoroughly studied.

2 Experimental

The raw materials were sponge titanium (purity ≥99.98%), niobium sheet (purity ≥99.98%), molybdenum block (purity ≥99.98%), vanadium block (purity ≥99.98%), particle silicon (purity ≥99.98%) and lanthanum powder (purity ≥99.99%). The NbMoTiVSi_0.2 refractory high entropy alloys with different La contents (0, 0.1, 0.2, 0.3, 0.4, and 0.5 at.%) were prepared by the vacuum non-consumable arc melting with tungsten electrode. Before melting, the furnace was evacuated to 3 × 10^-3 Pa and filled with argon as protective atmosphere, and then the alloys were melted into button ingots. The ingots were melted 5 times to ensure the composition homogeneity.

Specimens with dimensions of 10 mm × 10 mm × 8 mm were cut from the ingots by electrical-discharge wire cutting. After grinding and polishing, phase analysis and microstructure observation were carried out. The crystal structure of the alloys was identified by X-ray diffractometry (XRD, Panalytical Empyrean) with Cu K_α radiation, and the scanning angle (2θ) ranged from 20° to 90° with a scanning rate of 8 (°)/min. Scanning electron microscopy (SEM, FEI, Quanta 200FEG) was used to observe the microstructure morphology in the backscatter electron (BSE) mode. The compositions of different phases were examined by energy dispersion spectroscopy (EDS). The average grain size of alloys was quantified by linear intercept method with Image-Pro Plus software. Compressive testing was conducted at room temperature with an Instron 5569 universal tester. The samples with sizes of d4 mm × 6 mm were tested with an initial strain rate of 1×10^-3 s^-1, and at least 3 samples were tested for each alloy.

3 Results and discussion

3.1 Phase constitution of (NbMoTiVSi_0.2)_100-xLa_x alloys

Figure 1 shows the XRD patterns of alloys (NbMoTiVSi_0.2)_100-xLa_x (x is molar fraction, %). The results show that the crystal structure of the NbMoTiVSi_0.2 alloy without La is composed of BCC solid solution and silicide phase M₅Si₃ (M=Nb,Mo,Ti,V). The (NbMoTiVSi_0.2)_99.9La_0.1, (NbMoTiVSi_0.2)_99.8La_0.2 and (NbMoTiVSi_0.2)_99.7La_0.3 alloys are also composed of BCC solid solution  and M₅Si₃ silicide phase. When the La addition is increased to 0.4 and 0.5 at.%, the disilicide   phase MSi₂(M=Mo,Nb,Ti,V) appeared in XRD patterns. Therefore, the (NbMoTiVSi_0.2)_99.6La_0.4 and (NbMoTiVSi_0.2)_99.5La_0.5 alloys are composed of BCC solid solution, M₅Si₃ silicide phase and MSi₂ disilicide phase.

Fig. 1 XRD patterns of (NbMoTiVSi_0.2)_100-xLa_x alloys

Besides, the peaks of BCC phase shift slightly to the higher 2θ direction with the increase of La content, which shows the decrease of the lattice parameters of BCC phase. This change is resulted from the bigger atomic radius of La (2.740 ) compared with that of the other alloying elements (shown in Table 1). Furthermore, the diffraction peaks of the BCC phase become shorter after adding La element, which indicates that the amount of the BCC phase decreases with the increase of La content.

The diffraction peaks of M₅Si₃ silicide phase for the NbMoTiVSi_0.2 alloy are much stronger than those of the alloys with La. This indicates that the addition of La can decrease the amount of silicide phase in the studied alloys. When the addition of La is higher than 0.4 at.%, the amount of the silicide phase will decrease and the disilicide phase will be formed.

3.2 Microstructures of (NbMoTiVSi_0.2)_100-xLa_x alloys

Figures 2 and 3 show the microstructures of the (NbMoTiVSi_0.2)_100-xLa_x alloys with different magnifications. Figure 4 shows the positions for EDS measurement of (NbMoTiVSi_0.2)_99.9La_0.1 alloy with higher magnification. The results in Figs. 2-4 show that there are five different phases in the alloys after adding different contents of La. They are the light gray phase and the black phase at the grain boundaries, the dark grey phase, the white phase and the black phase in the light grey phase. These phases were examined by EDS, and the results are summarized in Table 2. Combining the XRD and EDS results, it is clear that the light gray phase marked with A in Fig. 4 is the primary BCC solid solution, which is the matrix, the black phase marked with B is M₅Si₃ silicide phase, the dark grey phase marked with C is the eutectic BCC solid solution, the white phase marked with D is the precipitate that is rich in La, and the black phase marked with E is the MSi₂ disilicide phase that is formed in the grains of the alloy (rich in  Mo). However, the MSi₂ disilicide phase cannot be detected by X-ray diffraction (XRD) in (NbMoTiVSi_0.2)_99.9La_0.1, (NbMoTiVSi_0.2)_99.8La_0.2 and (NbMoTiVSi_0.2)_99.7La_0.3 alloys because the volume fraction is too low.

Phases in Fig. 4 show that eutectic structure is composed of eutectic BCC solid solution (dark grey phase marked with C) and M₅Si₃ silicide phase (black phase marked with B) formed at the grain boundaries. Most of the La precipitates are also formed at the grain boundaries, especially at the interface of the silicide phase and the primary BCC solid solution (shown in Fig. 3). The MSi₂ disilicide phases with irregular shape are formed in the primary BCC phase by addition of La. Therefore, the (NbMoTiVSi_0.2)_100-xLa_x alloys with addition of La consist of primary BCC solid solution, eutectic structure, La precipitates and MSi₂ disilicide phase. With the increase of La addition, more La precipitates and MSi₂ disilicide phase are formed, but the size of MSi₂ disilicide phase is almost not changed.

Table 1 Physical properties of elements in studied alloys

Fig. 2 Microstructures of (NbMoTiVSi_0.2)_100-xLa_x alloys with lower magnifications

The microstructure of the NbMoTiVSi_0.2 alloy shows typical dendritic morphology (shown in Fig. 2(a)), and the average grain size is about 180 μm. The grains of the alloys are refined after adding La, and the morphology changes from the dendritic structure to near-equiaxed structure. The eutectic structure is also formed at the grain boundaries of the (NbMoTiVSi_0.2)_100-xLa_x alloys. The eutectic structure of the alloys is also refined by the addition of La. The average grain size of the alloy decreases with the increase of La content (shown in Fig. 5). The average grain size is the smallest in the alloys when the addition of La is 0.5 at.%, which is 20 μm.

The addition of rare earth element can effectively affect the nucleation rate of the melt and the subsequent growth of the nucleus. According the classical nucleation theory [29,30], the addition of rare earth element La reduces the surface tension at the phase interface and the energy fluctuation required to generate crystal nucleus. Consequently, the nucleation rate of the NbMoTiVSi_0.2 alloy melt can be improved by the addition of La. Meanwhile, the rare earth oxides at the frontier of the solid/liquid interface are ultra-micro particles, and their melting points are higher. They can prevent the growth of crystals and the movement of the grain boundaries, thus the grains and the eutectic structure of the (NbMoTiVSi_0.2)_100-xLa_x alloys are refined.

Fig. 3 Microstructures of (NbMoTiVSi_0.2)_100-xLa_x alloys with higher magnifications

The addition of rare earth element into Nb-Si alloys can propel the formation of NbSi₂ disilicide phase from Nb₅Si₃ silicide phase [31], and the phase transformation from Nb₅Si₃ silicide phase to NbSi₂ disilicide phase during cooling leads to the formation of microcracks [32,33]. In this study,  the addition of La can lead to the formation of  MSi₂ disilicide phase distributing in the primary BCC solid phase, and the amount of the MSi₂ disilicide phase increases with the increase of La addition.

Fig. 4 Positions for EDS measurement of (NbMoTiVSi_0.2)_99.9La_0.1 alloy with higher magnification

Table 2 EDS results of five positions in Fig. 4

Fig. 5 Average grain size of (NbMoTiVSi_0.2)_100-xLa_x alloys

3.3 Mechanical properties of (NbMoTiVSi_0.2)_100-x-La_x alloys

The compressive stress-strain curves of (NbMoTiVSi_0.2)_100-xLa_x alloys are shown in Fig. 6, and the fracture strain, ultimate strength and yield strength of the alloys are summarized in Table 3.

The results show that the ultimate strength of the alloys with different additions of La ranges from 2085 to 2157 MPa. The yield strength of the alloys ranges from 1766 to 1929 MPa. The ultimate strength of the alloys increases with increasing addition of La except the (NbMoTiVSi_0.2)_99.6La_0.4 alloy, and yield strength of the alloys increases by addition of La. Both the ultimate strength and yield strength of (NbMoTiVSi_0.2)_99.5La_0.5 alloy are the highest. The ultimate strength and yield strength  of (NbMoTiVSi_0.2)_99.5La_0.5 alloy are 2157 and 1929 MPa, respectively. The yield strength of (NbMoTiVSi_0.2)_99.5La_0.5 alloy is increased by 9.3% compared with that of NbMoTiVSi_0.2 alloy. The ultimate strength of (NbMoTiVSi_0.2)_99.6La_0.4 alloy is the lowest (2085 MPa).

The fracture strains of the (NbMoTiVSi_0.2)_100-xLa_x alloys decreases with the addition of La. The fracture strain of NbMoTiVSi_0.2 alloy is 16.47%. The fracture strain of (NbMoTiVSi_0.2)_99.6La_0.4  alloy decreases to 12.83%, and that of the (NbMoTiVSi_0.2)_99.5La_0.5 alloy with the highest ultimate and yield strengths is 15.28%.

Fig. 6 Compressive strength-strain curves of (NbMoTiVSi_0.2)_100-xLa_x alloys

Table 3 Mechanical properties of (NbMoTiVSi_0.2)_100-x- La_x alloys

As has been stated above, the addition of La to NbMoTiVSi_0.2 alloys can prevent the growth of crystals and the movement of the grain boundaries, and refine the grains of the NbMoTiVSi_0.2 alloys. Therefore, the mechanical properties of the NbMoTiVSi_0.2 alloys can be improved by the addition of La for the grain boundary strengthening effect.

The addition of La to the NbMoTiVSi_0.2 alloys leads to the formation of MSi₂ disilicide phase, which is distributed in the primary BCC phase. However, the transformation from Nb₅Si₃ silicide phase to NbSi₂ disilicide phase during cooling can result in the formation of microcracks. The strength of MoSi₂ phase is only 350 MPa [33]. Thus, the strength of NbMoTiVSi_0.2 alloy cannot be greatly improved due to the lower strength MSi₂ disilicide phase.

La precipitates appear in NbMoTiVSi_0.2 alloy by addition of La, and the amount of La precipitates increases with the increase of La content. The La precipitates distributed at the grain boundaries lead to stress concentration during testing. Consequently, the La precipitates and the brittle MSi₂ disilicide phase bring about the brittle fracture and reduce the ductility of alloys.

Therefore, the ultimate strength of NbMoTiVSi_0.2 alloys can be improved by adding rare earth element La, which is attributed to the grain boundary strengthening effect. However, the strength of the alloy cannot be greatly improved due to the formation of low strength MSi₂ disilicide phase. But the fracture strain and the ductility of the alloys decrease because of the La precipitates distributed at the grain boundaries and the formation of brittle MSi₂ disilicide phase.

4 Conclusions

(1) The microstructure of (NbMoTiVSi_0.2)_100-x- La_x RHEAs with different La contents consists of primary BCC solid solution phase, eutectic structure, disilicide phase MSi₂ (M=Mo, Nb, Ti and V) and La precipitates. Eutectic structure and most of La precipitates are formed at grain boundaries, and the disilicide phases are distributed in the primary BCC solid solution.

(2) The La precipitates and disilicide phase are formed in the (NbMoTiVSi_0.2)_100-xLa_x RHEAs when La is added, and their contents increase with the increase of La content.

(3) The grains of (NbMoTiVSi_0.2)_100-xLa_x RHEAs are refined by adding La. With the increase of La content, the grain morphology is changed from dendritic structure to near-equiaxed structure. The average grain size of the alloys decreases with the increase of La content, and the average grain size of (NbMoTiVSi_0.2)_99.5La_0.5 alloy is the smallest (20 μm).

(4) The ultimate strength and the yield strength of the (NbMoTiVSi_0.2)_100-xLa_x RHEAs increase with the increase of La content, which is resulted from the grain boundary strengthening. But they cannot be greatly improved due to the formation of low strength disilicide phase. The ductility is reduced because of the La precipitates and the brittle disilicide phase.

Acknowledgments

The authors are grateful for the financial supports from the National Natural Science Foundation of China (51825401 and 52001114), the State Key Laboratory of Materials Processing and Die & Mould Technology (P2020-023) and  the Guangdong Introducing Innovative and Entrepreneurial Teams, China (2016ZT06G025).

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镧对NbMoTiVSi_0.2难熔高熵合金显微组织与力学性能的影响

徐 琴¹，陈德志²，王聪锐¹，曹文超²，王 琪²，崔洪芝³，张书彦⁴，陈瑞润^2,3

1. 河南工业大学 机电工程学院，郑州 450001；

2. 哈尔滨工业大学 金属精密热加工国家级重点实验室，哈尔滨 150001；

3. 山东科技大学 材料科学与工程学院，青岛 266590；

4. 广东(东莞)材料基因高等理工研究院，东莞 523808

摘  要：为了研究La对难熔高熵合金显微组织与力学性能的影响，制备不同La含量的NbMoTiVSi_0.2合金，并对其相组成、显微组织演变、压缩性能及相关机理进行系统分析。结果表明，不同La含量的合金由BCC固溶体、共晶组织、MSi₂二硅化物和La析出相组成。共晶组织及大部分La析出相在晶界处形成，而二硅化物相在晶粒内部形成。La的添加使合金晶粒形态由树枝晶转变为近等轴晶，且随着La含量从0增加到0.5 at.%，合金的平均晶粒尺寸由180减小到20 μm。压缩性能测试结果表明，由于晶界强化作用，合金的压缩强度和屈服强度均随着La含量的增加而增加；但是由于低强度MSi₂相的形成，其提高幅度有限。由于La析出相和脆性MSi₂相的形成，因此合金的韧性随着La含量的增加而降低。

关键词：高熵合金；镧；共晶组织；难熔金属；二硅化物

 (Edited by Wei-ping CHEN)

Corresponding author: Rui-run CHEN; Tel/Fax: +86-451-86412394; E-mail: ruirunchen@hit.edu.cn

DOI: 10.1016/S1003-6326(21)65513-9

1003-6326/ 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press