Trans. Nonferrous Met. Soc. China 25(2015) 2958-2964

Effect of minor B addition on microstructure and properties of AlCoCrFeNi multi-compenent alloy

Qiu-shi CHEN, Yi-ping LU, Yong DONG, Tong-min WANG, Ting-ju LI

School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China

Received 17 November 2014; accepted 25 March 2015

Abstract: The influences of slight amount of B element on the microstructure and properties of AlCoCrFeNiB_x high entropy alloys (x = 0, 0.01,…, 0.09 and 0.1, mole fraction) were investigated. The AlCoCrFeNi high entropy alloy exhibits equiaxed grain structures with obvious composition segregation. However, with the addition of B element, the alloys exhibit dendrite structures. Inside the dendrites, spinodal decomposition structure can be clearly observed. With the addition of B element, the crystal structures change from (B2 + BCC) to (B2 + BCC + FCC) structures, and the hardness firstly increases from HV 486.7 to HV 502.4, then declines to HV 460.7 (x ≥ 0.02). The compressive fracture strength firstly shows a trend of increasing, and then declining (x ≥ 0.08). The coercive forces and the specific saturation magnetizations of the alloys decrease as B addition contents increase, the decreasing coercive forces show a better soft magnetic behavior.

Key words: high entropy alloy; B element; microstructure; compressive properties; hardness; magnetic properties

1 Introduction

Conventional alloy-design strategy is mainly based on one or two principal components, while other elements are regarded as minor components to improve the properties [1,2]. Different from the conventional one, a new alloy-design strategy was proposed by CANTOR et al [3] and YEH et al [4], and the alloys were named by high-entropy alloys (HEAs) or multi-principal element alloys. At present, HEAs have become a new research hotspot in the field of materials science [5].

A lot of researches based on the AlCoCrFeNi equiatomic alloy have been done, such as FeCoNiCrCu_0.5Al_x [6], FeNiCoCrAl₃ [7], FeTiCoNiVCr- MnCuAl [8], AlCoCrFeNiNb_x [9], AlCoCrFeNiC_x [10] and AlCoCrFeNiV_x [11] alloys. However, in these alloy systems, the amounts of additions were mostly above 0.1 in molar ratio. Few researches about HEAs with a small amount of element additions have been studied before. And in this study, B element was selected for the following reasons: firstly, B and other elements have large negative mixing enthalpies (the values of B and Co, B and Cr, B and Fe, B and Ni atomic pairs are -24, -31, -26 and -24 kJ/mol, respectively) [12,13], which leads to the formation of borides, and the borides can improve the compression strength and wear resistance of alloys; secondly, the atomic radius of B element is much smaller than that of other elements, which can result in the diffusion during solidification more easily. And diffusion is a key factor of controlling phase transition [14]. The transition of the morphologies and microstructure is crucial to mechanical properties. Next, a small amount of B element is usually able to refine the grain size [15]. At last, some alloys containing B element may possess good soft magnetic property [16]. Therefore, in this work, the effects of B element addition from 0 to 0.1 in molar fraction on the microstructures and properties of the AlCoCrFeNiB_x HEAs were investigated in detail.

2 Experimental

The HEAs ingots of AlCoCrFeNiB_x (x: mole ratio, x=0, 0.01, …, 0.09 and 0.1, denoted by BM0, BM1, …, BM9 and BM10, respectively) were prepared by vacuum arc furnace under a Ti-gettered high-purity argon atmosphere. The purity of all the raw materials (Al, Co, Cr, Fe, Ni, B) was above 99.9%. The alloys were flipped and remelted at least five times. The 30 g button ingots were directly solidified in a water-cooled cold copper hearth.

X-ray diffractometer (XRD, Shimadzu XRD-6000) with Cu K_α radiation was used for the phase analysis, with scanning angle ranging from 20° to 100° at a speed of 4 (°)/min. Microstructures, phase compositions and fracture surfaces were observed by using a scanning electron microscope with energy dispersive spectrometry (SEM, Zeiss supra55). The size of the samples for the tests of compressive properties was d5 mm × 10 mm. The tests were conducted at a strain rate of 1.0 s^-1. The hardness test was conducted through Vickers hardness tester model (MH-60) with 1 kg for 15 s. Five measurements were made for each sample to obtain the averaged experimental data. The magnetization curves were measured with a JDM-13T vibrating sample magnetometer.

3 Results and discussion

3.1 X-ray diffraction analysis

Figure 1 shows the XRD patterns of the AlCoCrFeNiB_x HEAs. When x=0, the alloy exhibits the mixing structures of B2 and BCC. That is consistent with previous research results [17]. As the B content increases, the intensity of BCC phase diffraction peaks decreases. In addition, the diffraction peaks of FCC phase appear from BM2 alloy, and the intensity gradually increases with the increase of B content. So, a conclusion can be inferred that the transformation from BCC phase to FCC phase occurs as B content increases.

Fig.1 XRD patterns of AlCoCrFeNiB_x HEAs

3.2 Microstructure and characterization

Figures 2 and 3 display the SEM images of the microstructures of the AlCoCrFeNiB_x HEAs. BM0 alloy exhibits typical equiaxed grain morphology, accompanying with obvious intragranular segregation, as shown in Fig. 2(a). The bright segregation area and the dark segregation area are denoted as A and B, respectively. Typical spiondal decomposition microstructure can be observed in the interior of the grains. According to the previous research, the bright section and the dark section of interconnected microstructure were composed of (Fe, Co, Cr)-rich element and (Al, Ni)-rich element whereas section A has a higher amount of Al and Ni than section B [18].

Fig. 2 SEM images of AlCoCrFeNiB_x HEAs in low magnification

Fig. 3 SEM images of AlCoCrFeNiB_x HEAs in high magnification

With the B element addition, BM1-BM10 alloys all exhibit dendritic morphology as seen in Fig. 2. The atomic radius of B element is far smaller than thoes of Al, Co, Cr, Fe and Ni elements, which may lead to the large strain energy of the crystal lattice when B atoms are further added. And during the solidification, the B atoms are discharged in front of the solid-liquid interface to reduce the strain energy, which facilitates the constitutional undercooling that is beneficial to the formation of dendrites.

The morphology of the dendrites in AlCoCrFeNiB_x HEAs is sensitive to alloying contents. The interdendritic regions become larger obviously as B contents increase, and many precipitated phases, as indicated in Fig. 3, can be detected in interdendritic regions. The precipitated phases in the matrix, rich in Cr and B elements, belong to ordered intermetallics phases. From the XRD and EDS results (as shown in Table 1), the matrix of interdendrite is rich in Fe, Co and Cr elements. According to the previous research, regions rich in Fe, Co and Cr tend to form FCC solid-solution phases. And combined with the XRD analysis, the amount of FCC solid-solution phases increases with the addition of B. Therefore, the matrix structure in interdendritic regions is deduced to be FCC solid-solutions. When x≤0.07, the precipitates are mainly in particle-shape as seen in Figs. 3(b)-(e). When x exceeds 0.09, the “particles” assemble together, and form in stripe-shape, as seen in Fig. 3(f). The dendritic region is a mixture of rod-like and interlaced spinodal decomposition structures, marked by DA and DB, respectively.

Table 1 Chemical compositions of AlCoCrFeNiB_x HEAs

Fig. 4 Compressive properties of AlCoCrFeNiB_x HEAs

With the addition of B element, distinct refinement of crystalline grain size occurs, such as sample BM0, 200-300 μm, sample BM4, 50-100 μm, until to sample BM10, 20-50 μm, as shown in Fig. 2.

3.3 Compressive properties

Figure 4 shows the compressive stress-strain curves of AlCoCrFeNiB_x alloys with different B contents at the strain rate of 1.0×10^-3 s^-1, and the compressive test results are listed in Table 2. The curve in Fig. 4 firstly shows a trend of increasing, and then declining. BM2 alloy exhibits the highest compression strength (2643.475 MPa), while from BM2 to BM8, the alloys show similar values (about 2600 MPa), subsequently drastically decrease to about 2100 MPa. As for the compression ratio, BM0 reveals a value of 24%. BM2-BM8 alloys show an increase (~30%), but afterwards, when x exceeds 0.08, the compression ratio declines to 27%, as shown in Table 2.

Table 2 Mechanical properties of AlCoCrFeNiB_x alloy

All of the tested samples exhibit typical intergranular fracture surface morphology. A small amount of B element segregates at crystal boundaries, which leads to the bonding strength of crystal boundaries much lower than that of internal regions of grains.

In this research, the compression strength of BM2 is higher than that of BM0, which can be due to the effect of B atomic solution strengthening in dendrites. According to the EDS results, B element cannot be detected neither in dendritic nor interdendritic regions of BM0-BM2, but can be detected in interdendritic regions of BM3-BM10. A conclusion can be obtained that the solid solubility of B atom in dendritic regions is rather small, between 0.4%-0.6% in mole fraction. As a result, when x exceeds 0.02, redundant B atoms are discharged from the dendrites. The FCC structural interdendritic region increases as B content increases, which enhances the plasticity, but is unfavorable to the compression strength improvement. However, as a result of further addition of B element, the formation of more and more (Cr,B)-rich intermetallic phases can improve the compression strength. And the similar compression strength (x= 0.02-0.08) may be due to the combined effects of the above two factors. The decrease of the compression strength (x ≥ 0.08) is attributed to too much FCC phase.

3.4 Microhardness of AlCoCrFeNiB_x HEAs

The microhardness of AlCoCrFeNiB_x alloys is shown as Fig. 5. All of the AlCoCrFeNiB_x HEAs possess high hardness. As B content increases from 0 to 0.1, the hardness firstly increases from HV 486.7 to HV 502.4, then, it declines to HV 460.7. Corresponding to the compressive test, the lattice distortion led by B element addition plays a dominating role in the hardness increase when x is in the range of 0-0.02. When x exceeds 0.02, the formation of more and more FCC phases could be the key reason for the decrease of the hardness. And this change is exactly consistent with the XRD and SEM analyses.

Fig. 5 Microhardness of AlCoCrFeNiB_x HEAs

3.5 Magnetic properties

The magnetization curves of AlCoCrFeNiB_x HEAs are shown in Fig. 6. Obviously, all of the alloys show typical ferromagnetic behavior. The specific saturation magnetizations of the alloys under the magnetic field of 3.98×10³ A/m decrease from 53.91 to 43.75 A·m²/kg with the increase of B contents. According to the following equation of the classification of magnetic properties [9]:

                                                                             (1)

where χ is the permeability, M is the magnetization, H is the magnetic field intensity, σ is the specific saturation magnetization, and ρ is the density of alloys. The decline of ferromagnetic behavior can be explained by the addition of the non-magnetic B element into the alloy system. Moreover, the coercive forces are 3664.5×10³, 3358.9×10³, …, 3023.9×10³ A/m when x = 0, 0.02×10³, …, 0.1, respectively, which reveals better and better soft magnetic behavior. When the content of B element increases, all alloys still exhibit ferromagnetic property but the permeability shows little difference (5.6×10^-3-5.8×10^-3), according to Eq. (1).

Fig. 6 Magnetization curves of AlCoCrFeNiB_x HEAs

4 Conclusions

1) XRD and SEM results revealed that the crystal structures of AlCoCrFeNiB_x alloys transformed from B2 + BCC to B2 + BCC + FCC structures with the addition of B element. The addition of B element facilitated the morphology changing from equiaxed grain to dendritic structure, accompanied with obvious refinement.

2) Both the fracture strength and compressive ratio exhibited a tendency of first increasing and then declining. When x=0.02, the values reached the highest value, 2643.475 MPa and 30.6%, respectively.

3) The hardness of the alloy exhibited a tendency of first increasing from HV 486.7 to HV 502.4, and then declining to HV 460.7 with the addition of B element.

4) With the increase of B element content, a decrease of coercive forces and the specific saturation magnetizations can be observed. And the decrease of coercive forces benefits the soft magnetic behavior.

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微量硼元素的添加对AlCoCrFeNi多主元合金显微组织和性能的影响

陈秋实，卢一平，董 勇，王同敏，李廷举

大连理工大学 材料科学与工程学院，大连116024

摘  要：研究微量硼元素对AlCoCrFeNiB_x(x = 0, 0.01, 0.02…0.09, 0.10, x为摩尔分数)高熵合金微观组织和性能的影响。AlCoCrFeNi高熵合金呈现典型的等轴晶结构，晶内伴随着明显的成分偏析。然而，随着硼元素的添加，合金呈现明显的树枝晶结构。在树枝晶内可以观察到典型的调幅分解结构。晶体结构也随着硼元素的增加由B2 + BCC结构转变为B2 + BCC + FCC的混合结构；合金硬度呈先升后降趋势，由HV 486.7升高至HV 502.4，最后降低至HV 460.7；压缩断裂强度在x=0.08前呈上升趋势，之后呈下降趋势；合金呈软磁性，其矫顽力和饱和磁化强度均随着硼元素含量增加而下降，表明硼元素对合金软磁性有提高作用。

关键词：高熵合金；硼元素；显微组织；压缩强度；硬度；磁性能

(Edited by Yun-bin HE)

Foundation item: Projects (51134013, 51104029, 51471044) supported by the National Natural Science Foundation of China; Project supported by the Fundamental Research Funds for the Central Universities, China; Project (LZ2014007) supported by the Key Laboratory of Basic Research Projects of Liaoning Province Department of Education, China; Project (2014028013) supported by the Natural Science Foundation of Liaoning Province, China

Corresponding author: Yi-ping LU; Tel: +86-411-84709400; Fax: +86-411-84708940; E-mail: luyiping@dlut.edu.cn

DOI: 10.1016/S1003-6326(15)63922-X