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On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high-entropy alloys
Accepted Manuscript On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high-entropy alloys Xiaotao Liu, Wenbin Lei, Lijuan Ma, Jing Liu, Jinling Liu, Jianzhong Cui PII: DOI: Reference:
S0925-8388(14)02733-9 http://dx.doi.org/10.1016/j.jallcom.2014.11.085 JALCOM 32639
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
8 July 2014 9 November 2014 12 November 2014
Please cite this article as: X. Liu, W. Lei, L. Ma, J. Liu, J. Liu, J. Cui, On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high-entropy alloys, Journal of Alloys and Compounds (2014), doi: http:// dx.doi.org/10.1016/j.jallcom.2014.11.085
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On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high-entropy alloys
Xiaotao Liu1, Wenbin Lei1, Lijuan Ma2, Jing Liu2, Jinling Liu1, Jianzhong Cui1 1.
The Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang, 110819, China 2.
The School of Science of Northeastern University, Shenyang, 110004, China
Abstract: The microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix (x = 0, 0.4, 0.8) high-entropy alloys are studied in this paper. Alloy with x = 0 is a simple FCC solid solution. As Si content x increases, alloys are gradually changed from FCC to BCC. As Si content x increases, compressive strength of the alloys increases, while ductility decreases. The parameters representing simple solid-solution formation ability in Al0.5CoCrCuFeNiSix alloys are estimated. The addition of Si destabilizes the FCC structure and introduces the transition from a closed-packed FCC structure to a loose-packed BCC structure to relax the lattice distortion energy due to the smaller radius of Si. The improved mechanical properties of the alloys are attributed to the basic structure changes from FCC to BCC phases and solution hardening mechanism. Keywords: High-entropy alloys; Microstructures; Mechanical properties; Solution strengthening 1. Introduction The high-entropy alloys (HEAs) with multiple principal elements in equiatomic or close-toequiatomic ratios in one alloy system were developed base on a novel alloy design concept beyond traditional metallurgy by Yeh et al. in 1995 [1, 2]. HEAs exhibits superiority in the formation of simple solid solutions over that of intermetallic compounds and terminal solid solutions as expected in conventional alloys system. Due to their simple solid solution structures 1
and promising properties, including high strength/hardness, outstanding wear resistance, exceptional high-temperature strength, good structural stability, good corrosion and oxidation resistance and combinations of aforementioned properties, HEAs have received more and more attentions in the past decades and become an attractive field of metallic materials [3]. The phase selection in multicomponental alloy systems is related to fundamental properties of the constituent elements [4]. Thus, from the alloy design point of view, phase selection and competition are important topics, which is critical for developing multicomponent HEAs with solid-solution phases rather than amorphous phase or intermetallic compounds. To design appropriate alloy compositions with required properties and optimize the properties for the existing HEAs can widen their potential applications. Even though the high mixing entropy in HEAs facilitates the formation of simple solid-solution phases, as felt from the name, it is not the sole parameter in this regard. The mutual interaction between constituent elements is a key issue for the prediction of phase formation in HEAs. So far, several criteria for predicting phase formation in HEAs were proposed as a guideline for alloy design based on the standpoint of enthalpy of mixing (∆Hmix), entropy of mixing (∆Smix), atomic size difference (δ), and valence electron concentration (VEC) [5, 6]. Zhang et al. related the formation of simple phases (i.e. FCC, BCC or their mixtures) to the mixing enthalpy (∆Hmix), the mixing entropy (∆Smix) and atomic size difference (δ) and proposed a solid-solution formation criteria for HEAs where two parameters, Ω, the entropy of mixing timing the average melting temperature of the elements over the enthalpy of mixing, and δ, the mean square deviation of the atomic size of elements, were used to estimate the phase formation behavior of HEAs [7, 8]. These two parameters were defined by the following equations, respectively.
2
Ω=
δ=
Tm ∆S mix ∆H mix
− ci 1 − ri / r i =1
∑
n
2
Where ci is the atomic percentage of the ith component, −
∑
n
c = 1 , and ri is the atomic radius of
i =1 i
−
the ith component and r is the average atomic radius, r =
∑
n
cr i =1 i i
. ∆Hmix, ∆Smix and Tm are
calculated as follows: n
∆H mix = 4
∑ ∆H
mix ij ci c j
i =1,i ≠ j
∆H ijmix is the enthalpy of mixing of binary liquid alloys, ci and cj are the atomic percentage of the
ith and jth component, respectively. n
∆S mix = − R∑ (ci ln ci ) i =1
Where R ( = 8.314 J⋅K-1mol-1) is gas constant. n
Tm = ∑ ci (Tm )i i =1
Where (Tm)i is the melting point of the ith component. The mixing entropy indicates the tendency for the formation of random solid solution while the enthalpy of mixing indicates the tendency for ordering and clustering. The value of Ω indicates the relative predominance of Tm∆Smix and ∆Hmix. The value of δ represents the atomic size difference between components which causes lattice distortion and subsequently the
3
corresponding strain energy which lower the stability of solid solution. Therefore, Ω and δ are the key determinants for the selective formation of a random solution or an intermetallic compound. Simple crystal structures tend to form in HEAs when Ω ≥ 1.1 and δ ≤ 6.6%. Guo et al. further confirmed that the valence electron concentration (VEC) is the key parameter to control the phase stability for solid solutions and thus can be used to determine whether an alloy crystallizes into FCC or BCC structure. FCC phases are found to be stable at higher VEC ( ≥ 8 ) and BCC phases are stable at lower VEC ( < 6.87 ). At VEC values between 6.87 and 8, FCC and BCC phase coexist [9]. The valence electron concentration, VEC, which has been proven useful in determining the phase stability of intermetallic compounds, is defined by: n
VEC = ∑ ci (VEC )i i =1
Where ci is the atomic percentage and (VEC)i is the VEC for the ith element which counts the total electrons including the d-electrons accommodated in the valence band [10, 11]. Elemental composition affects the alloys behaviors, phase assemblages and properties of HEAs [12, 13]. Up to now, more than 30 elements have been employed to prepare more than 300 reported HEAs [14]. The most commonly used elements in HEAs that have been investigated are Al and transition metal elements of the fourth period, such as Co, Cr, Cu, Fe, Ni, and Ti, etc. Addition of the minor alloying elements with a large atom size difference with the other constituent elements into HEAs can result in the change in the lattice distortion and even lattice types, thereby altering their mechanical properties, such as strength, hardness and plasticity. If a multicomponent alloy contains atomic pairs that are prone to form phase with each other, it would be generally reflected in the microstructure of the resultant multiple component alloy as well [15, 16]. For example, Al with the largest atomic radius has obvious influence on phase transformation and mechanical properties of AlxCoCrCuFeNi alloys. With increasing Al content, 4
the microstructure of the alloys showed gradual changes from FCC to FCC + BCC and finally to fully BCC phases [17-19]. Among the AlxCoCrCuFeNi alloys, the Al0.5CoCrCuFeNi alloy consists of simple FCC solid-solution structure and is ductile, work hardenable, and strong even at temperature up to 800 oC [20]. It is anticipated that this alloy would have potential applications in high-temperature structures and working tools. In traditional metallic materials like steel, the proper addition of non-metallic elements with small atomic radii, such as C and Si, is beneficial to their structures and properties [21, 22]. However, the effects of these non-metallic elements on the multi-principal component alloys are still unclear [23]. In this paper, the Al0.5CoCrCuFeNiSix HEAs with varying Si content are designed and the effect of Si on the microstructure, phase assemblage and properties of Al0.5CoCrCuFeNiSix alloys are investigated systematically in this paper. It is indicated that the addition of Si changes the alloy structure distinctly, and thus leads to great enhancement of the alloy properties. The structure and phases evolution, strengthening mechanism of the alloys are discussed in detail. 2. Experimental Alloy ingots with nominal compositions of the Al0.5CoCrCuFeNiSix (x value in molar ratio, x = 0, 0.4 and 0.8) were prepared by arc melting pure elements with purity higher than 99.9 wt % under high-purity argon atmosphere on a water-cooled Cu hearth. The alloys were re-melted four times to improve the homogeneity of alloy compositions. The crystal structure was characterized by Xray diffraction (XRD) with the 2 theta scan ranging from 20 o to 100 o at a speed of 2 o/min and the typical radiation condition was 30 kV and 20 mA with a copper target. Vickers hardness was measured using a FM-700 tester with a load of 50 g and a duration time of 15 seconds for each measurement. The hardness values were averaged by nine measurements at least for each sample. The wear behavior of the Al0.5CoCrCuFeNiSix alloys was examined using a MT-2000 tribometer
5
under dry sliding at room temperature where the tested samples were a pin 6 mm in diameter and 12 mm in height. The counterpart wear material is Cr12MoV steel with an average hardness about 760 HV. During the test, each pin was fixed in a holder to wear against the rotating C12MoV steel disk which has a diameter of 70 mm. The distance between the pin and the center of the disk was 30 mm. The sliding speed was 0.8 mm/s, with a normal load of 100 N. The anti-wear property was measured by wear resistance, which is defined as Wr = L/∆V, where Wr is the wear resistance and ∆V is the volumetric loss of the specimen after it has slid through a distance L, and is obtained by dividing the weight loss by the sample density. The microstructure and chemical composition of the alloys, the worn surface and wear debris were analyzed using a Super Scan SS-550 scanning electron microscope (SEM) with energy dispersive spectrometry (EDS). Compression test was performed on MTS 800 machine with a strain rate of 1 x 10-4 s-1 at room temperature. 3. Results 3.1 Microstructure and phase assemblages The crystal structures of the as-cast Al0.5CoCrCuFeNiSix alloys were characterized using XRD as shown in Fig. 1. Simple solid solution structures, essentially FCC and BCC, were identified in the Al0.5CoCrCuFeNiSix alloys. Only XRD diffraction peaks corresponding to a FCC crystal structure was identified in the alloy without Si addition. In the alloys with the addition of Si, peaks corresponding to a BCC crystal structure started to appear and the intensity increased with increasing Si content. Fig. 1(b) shows an enlarged picture of the main peak of BCC phase in the alloys with Si addition. It is shown that the peaks of BCC phase shifted rightwards with increasing Si content, indicating that the lattice parameter of the BCC phase decreases slightly. By linearity extrapolation method, the lattice constants of the BCC phases in the alloys were calculated from the X-ray diffraction peaks. The lattice constant of the BCC were 2.867 Å and 2.859 Å for the alloys with x = 0.4 and x = 0.8, respectively. The addition of Si changed the
6
crystal structure from FCC to BCC and the lattice constant of the BCC phase decreased with increasing Si content. Fig. 2 shows the back-scattered images of the as-cast Al0.5CoCrCuFeNiSix alloys with varying Si content. Table 1 lists the chemical compositions of the areas indicated in Fig. 2 (b, d and f). Typical cast dendrite (DR) and interdendrite (ID) structures were observed in the Al0.5CoCrCuFeNiSix alloys. Although copper segregation at the interdendrite regions of small volume fraction was observed, the dendrites of large volume fractions were basically composed of multiprincipal elements. For the alloy without Si addition, both the dendrites and interdendrites were of FCC structure. With the addition of Si, a BCC solid solution phase started to appear. When x = 0.4, the dendrites were a mixture of FCC and BCC phases and the interdendrites were still the copper-rich FCC phase. When x = 0.8, the FCC dendrites disappeared and the microstructure composed of the BCC dendrites and copper-rich FCC interdendrites. Thus, with increasing Si content, the alloys have a dominated BCC dendrite phase and a small amount of FCC interdendrite phase with Cu enrichment. The BCC dendrites had higher Si content than the FCC dendrites and the Si content in the Cu-rich interdendrites was minor. Also, two characters were also noted in the microstructure of the alloys with Si additions, one is the fine needle-like precipitations from the BCC dendrites which were Cu-rich phases according to the XRD and EDS analyses, and the other is the changing of the microstructures from dendritic to network-like for the alloys with Si additions. 3.2 Hardness and wear resistance Fig. 3 shows the hardness and wear resistance of the Al0.5CoCrCuFeNiSix alloys as a function of Si content. The hardness values of the alloys increased as x was varied from 0 to 0.8. The hardness of the alloy with x = 0.8, 653 HV, was 2.48 times that of the alloy with x = 0, 263 HV. The wear resistance of the alloy with x = 0.8, 0.86 km/mm3, is about 8.6 times that of the alloy
7
with x = 0, 0.10 km/mm3. The insert in Fig. 3 presents the wear resistance as a function of hardness of the Al0.5CoCrCuFeNiSix alloys. The wear resistance of the Al0.5CoCrCuFeNiSix alloys was seemingly sensitive to the hardness with varying Si content and exhibited a strong correlation to the hardness. Fig. 4 shows the morphologies of the worn surface of the alloys after test and the debris produced during the test. For the alloy without Si addition, the worn surface was grooved and noticeable plastic deformation features along the grooves were seen, indicating the high ductility of the alloy. Moreover, a few lateral cracks were found on the worn surface indicating the fracture of the alloy resulting in relatively low wear resistance. The debris of the alloy without Si addition had a flakelike shape and was the biggest, around 300-1000 µm in size. The flake-like shape indicates that the debris was worn off from Al0.5CoCrCuFeNi and the adhesive wear mechanism of Al0.5CoCrCuFeNi was predominantly of delamination wear, by which the worn surface underwent a periodic delamination fracture [24]. This is reasonable since delamination wear is a typical mechanism for ductile metals and the alloy without Si addition was composed of ductile FCC phase. The chemical compositions of both the worn surface and wear debris presented in Table 2 show that they all contain significant amount of oxygen indicating oxidation occurs on the worn surface heated by friction and deformation. For the alloys with Si additions, plastic features and grooves on the worn surface were seen again. However, compared with the alloy without Si addition, the plastic features and grooves were inhibited greatly. When x = 0.8, the plastic features disappeared. The debris of the alloys with Si additions still had flake-like shape but their size decreased with increasing Si content, 250-600 µm for the alloy with x = 0.4 and 70200 µm for the alloy with x = 0.8. The oxygen contents of the worn surface and debris were all higher than those of the alloy without Si. This indicated that the wear type of the alloy with Si additions was still predominantly of delamination wear but the oxidative wear became pronounced. 8
3.3 Compressive properties Fig. 5 shows the compressive engineering stress-strain curves of room-temperature test for the ascast Al0.5CoCrCuFeNiSix alloys. The insert in Fig. 5 shows the morphologies of the fractographs of the deformed samples. It was found that the alloy strength increased obviously with increasing Si content. However, the alloy ductility was weakened at the same time. The alloy without Si addition exhibited good yield stress, fracture strength, plastic deformation and obvious hardening behavior. The samples with the barrel-like form did not fracture after compressive deformation though some surface cracks were observed. The alloy strength continued to be improved evidently accompanying the sacrifice of ductility as Si content increased. The alloy without Si has the maximum compressive strength of 1.26 GPa and the plastic deformation of > 30%. The alloy with x = 0.4 has the maximum compressive strength of 1.79 GPa and the total plastic deformation of 23%. The alloy with x = 0.8 has the maximum compressive of 1.06 GPa and the total plastic deformation of 3.7 %. The addition of Si enhanced the compressive strength but lowered the plasticity of the alloys. When x = 0.8, the alloy showed a brittle fracture without any plasticity observed. 4. Disscussion Using the physicochemical and thermodynamic parameters for the constituent alloying elements as listed in Table 3, Ω, δ and VEC for the present Al0.5CoCrCuFeNiSix alloys were calculated. The values of Ω and δ of the Al0.5CoCrCuFeNiSix alloys with varying Si content were shown in Fig. 6. With increasing Si contents, the Ω values, reflecting the competition between ∆Smix and ∆Hmix, decreased from 16.26 to 1.73 and the δ values increased from 4.10 to 4.67. It is obvious that the values of both Ω and δ fulfilled the aforementioned criteria for forming HE stabilized solid solution (Ω ≥ 1.1, δ ≤ 6.6 %), and therefore, the Al0.5CoCrCuFeNiSix alloys are inclined to form solid solution phase during solidification. 9
The VEC values of the Al0.5CoCrCuFeNiSix alloys with varying Si content were shown in Fig. 7. It is noted that with increasing Si content, the VEC value decreased linearly, confirming that the Si addition promotes covalency. VEC > 8 was obtained for the alloy without Si addition indicating that FCC structured solid solutions are more stable, while 6.87 < VEC < 8 was obtained for the alloys with Si additions indicating that FCC and BCC solid solutions coexist, which is consistent with the XRD and SEM analysis aforementioned. The structure changes of the Al0.5CoCrCuFeNiSix alloys can be explained by using the atomic packing efficiency of the FCC and the BCC phases. Generally, BCC structure has a lower atomic packing density (68 %) than FCC structure (74 %). When Si, which has a smaller radius than the other components, was initially added into the FCC structured Al0.5CoCrCuFeNi alloy, lattice distortion and subsequently an apparent lattice straining was introduced. From a viewpoint of strain energy, the addition of Si will destabilize the FCC structure and introduce the transition from a close-packed FCC structure to a loose-packed BCC structure to relax the lattice distortion energy [25]. This trend toward the formation of BCC phase is very reasonable since silicon is a well-known BCC former and stabilizer [26]. Cu segregation to the interdendrites could be explained based on the positive mixing enthalpy between copper and the other components. In the alloy without Si addition, the Cu atoms were rejected to the grain boundaries during solidification forming Cu-rich solid solutions due to the positive enthalpy values of Cu-Co, Cu-Cr and Cu-Fe atomic pairs as shown in Table 3. Al and Ni have greater chemical affinity with Cu than the other elements and thus Al and Ni were also detected in the interdendrites. In the alloys with Si additions, though the mixing enthalpy between Si and Cu is -19 kJ⋅mol-1, it is still a relatively positive value among the mixing enthalpies between Si and the other components [27, 28]. Therefore, the addition of Si would further reject the Cu atoms into the grain boundary, and exacerbate the copper segregation at the interdendrites
10
zones. The EDS analysis results that Si mainly existed in the dendrites rather than in the Cu-rich interdendrites support this. The improved mechanical properties of the Al0.5CoCrCuFeNiSix alloys come from the basic structure changes (transition from FCC to BCC phases) and solution hardening mechanism. For the basic structure factor, the BCC phase is much stronger than the FCC phase. Slip along the closest packing planes {110} in the BCC structure is more difficult than that along those {111} in the FCC structure since {110} planes are less dense and more irregular and thus possess a smaller interplanar spacing and higher lattice friction for dislocation motion than {111} planes in the atomic scale [29]. A similar phenomenon was also reported in the study of AlxCoCrCuFeNi alloys with varying Al content which corresponds to a gradual change from FCC to BCC phase [24]. For the solution hardening mechanism factor, the improved properties of alloys are attributed to solid solution of Si. In Al0.5CoCrCuFeNiSix alloys, each atom can be expected as a solute atom and it can randomly occupy the crystal lattice sites. Nevertheless these solute atoms with different sizes and properties interact with each other and elastically distort the crystal lattice, which induces local elastic stress field. The interactions between these local elastic stress fields and the stress field of dislocations in alloy will hinder dislocation movements, and cause the increase of strength. For cubic crystals, the lattice strain can be expressed as ε = ∆α/α0, where ∆α = |α-α0|, α and α0 is the lattice constant of actual alloy crystal and non-deformed perfect crystal, respectively [30]. The strengthening caused by atomic size mismatch will increase with the increase of atomic size difference. In Al0.5CoCrCuFeNiSix alloys, the atomic size difference increases with increasing Si content, as a result, the corresponding strength of the alloys increased. As aforementioned, the lattice constants of the BCC phase decrease as Si content increases from 0 to 0.8, which results in the increase of ε. It is deduced that the lattice distortion energy will increase significantly and the effect of solution strengthening is enhanced. Thus, the alloy strength 11
increases greatly with the decrease of ductility. The large improvement in wear resistance of the alloys with Si additions is attributed to the high hardness and strength, which not only resists plastic deformation and delamination, but also brings about the oxidative wear in which oxide film could assist the wear resistance. 5. Conclusions 1) The microstructure of the Al0.5CoCrCuFeNiSix alloys gradually transited from FCC solid solution to BCC solid solution with increasing Si content from 0 to 0.8. Cu segregation occurred in the interdendrites due to the positive enthalpies between Cu and the elements. The addition of Si further exacerbated the Cu segregation in interdendrites. 2) With the additions of Si, the hardness, wear resistance of the alloys were improved. The wear type of the alloys is predominantly of delamination wear but the oxidative wear became pronounced with increasing Si content. The alloys compressive strength increased greatly with increasing Si content, while the ductility of the alloys decreased with increasing Si content. 3) The improved mechanical properties of the Al0.5CoCrCuFeNiSix alloys are due to the basic structure changes from FCC to BCC phase and the solution hardening mechanism.
Acknowledgements The authors would like to thank the Fundamental Research Funds for the Central Universities (N120409003) and University students’ Innovation Plan of China (No. 130066 and No. 140219) for their financial supports on this research.
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Fig. 1 XRD patterns of the as-cast Al0.5CoCrCuFeNiSix (x=0, 0.4 and 0.8) alloys Fig. 2 Back scattered SEM images of the as-cast Al0.5CoCrCuFeNiSix alloys (a) and (b) x=0; (c) and (d) x=0.4; (e) and (f) x=0.8 Fig. 3 Hardness and wear resistance of the as-cast Al0.5CoCrCuFeNiSix alloys Fig. 4 SEM images of the wear surface (a, c, e) of the Al0.5CoCrCuFeNiSix alloys and the debris (b, d, f) produced during the test (a) and (b) x=0; (c) and (d) x=0.4; (e) and (f) x=0.8 Fig. 5 Room temperature compressive engineering stress-strain curves of the as-castAl0.5CoCrCuFeNiSix alloys (the inserts are the morphologies of the fractographs of the samples after test) Fig. 6 The curves of Ω and δ as a function of Si contents for Al0.5CoCrCuFeNiSix alloys Fig. 7 The relation between VEC and Si contents of the alloys Table 1 Chemical compositions of the Al0.5CoCrCuFeNiSix alloys (at.%) Table 2 Chemical compositions of the worn surface and wear debris produced of the Al0.5CoCrCuFeNiSix alloys (at.%) Table 3 the physicochemical and thermodynamic parameters for the constituent alloying elements
15
On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high entropy alloys
Intensity (arb. units)
Si0.8
FCC BCC
(b) Si0.8
Intensity (arb. units)
(a)
Si0.4
Si0.4
Si0
40
50
60 70 2θ (Degrees)
80
90
42
43
44
45
46
47
2θ (Degrees)
Fig. 1 XRD patterns of the as-cast Al0.5CoCrCuFeNiSix (x=0, 0.4 and 0.8) alloys
48
(a)
(b) ID(FCC) DR(FCC)
(c)
(d)
ID(FCC)
DR(FCC) DR(BCC)
(e)
(f) DR(BCC) ID(FCC)
Fig. 2 Back scattered SEM images of the as-cast Al0.5CoCrCuFeNiSix alloys (a) and (b) x=0; (c) and (d) x=0.4; (e) and (f) x=0.8
Table 1 Chemical compositions of the Al0.5CoCrCuFeNiSix alloys (at. %) Alloys Si0.0
Si0.4
Si0.8
Areas in Fig. 2 Norminal ID (Cu-rich) DR(FCC) Norminal ID(Cu-rich) DR(FCC) DR(BCC) Norminal ID(Cu-rich) DR(BCC)
Al 9.09 14.98 7.38 8.47 9.25 4.44 8.65 7.94 10.70 9.11
Co 18.18 7.51 23.73 16.95 4.69 22.66 19.60 15.87 2.63 19.56
Cr 18.18 5.05 21.31 16.95 2.91 23.07 20.47 15.87 1.37 18.70
Cu 18.18 51.26 9.73 16.95 64.52 7.24 5.22 15.87 73.55 4.42
Fe 18.18 6.76 20.83 16.95 4.39 24.44 19.63 15.87 2.38 20.24
Ni 18.18 14.44 17.02 16.95 13.28 13.02 18.16 15.87 8.48 16.67
Si 6.78 0.96 5.14 8.26 12.70 0.89 11.71
1000 )
0.8
3
1.0
Wear resistance (km/mm
Hardness (HV)
600
Hardness Wear resistance 1.4 1.2
0.6 0.4
1.0
0.2 0.0 200
300
400
500
600
800
900
Hardness (HV)
400 200
0.0
0.2
0.8 0.6
0.4 For tribometric test: Normal load: 100 N 0.2 Sliding speed: 0.8 mm/s 0.0 0.4 0.6 0.8
X in Al0.5CoCrCuFeNiSix Fig. 3 Hardness and wear resistance of the as-cast Al0.5CoCrCuFeNiSix alloys
3
0
700
Wear resistance (km/mm )
800
1.6
1.2
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4 SEM images of the wear surface (a, c, e) of the Al0.5CoCrCuFeNiSix alloys and the debris (b, d, f) produced during the test (a) and (b) x=0; (c) and (d) x=0.4; (e) and (f) x=0.8
Table 2 Chemical compositions of the worn surface and wear debris produced of the AlCoCrCuFeNiSix alloys (at.%) Alloys Areas in Fig. 2 Norminal
Al 9.09
Co 18.18
Cr 18.18
Cu 18.18
Fe 18.18
Ni 18.18
Worn surface
9.74
16.80
15.22
13.98
17.24
14.87
12.16
Debris Norminal Worn surface Debris Norminal Worn surface Debris
8.12 8.47 7.70 9.38 7.94 3.78 3.31
15.67 16.95 11.22 6.73 15.87 7.53 5.82
14.14 16.95 11.33 6.66 15.87 9.64 9.93
13.59 16.95 10.70 5.86 15.87 5.11 2.76
13.39 16.95 11.83 6.09 15.87 12.98 12.11
14.16 16.95 12.53 5.41 15.87 6.91 6.45
20.94
Si0
Si0.4 Si0.8
Si
O
6.78 5.52 7.38 12.70 5.83
29.18 52.49
5.72
53.91
49.98
2000
Stress (MPa)
1500 Si0.8
1000
Si0.4
500 0
Si0
0
-5
-10
-15
Strain (%)
-20
-25
-30
Fig. 5 Room temperature compressive engineering stress-strain curves of the ascastAl0.5CoCrCuFeNiSix alloys (the inserts are the morphologies of the fractographs of the samples after test)
Table 3 the physicochemical and thermodynamic parameters for the constituent alloying elements Elements Radius (Å) Tm (K) VEC Al (FCC) Co (HCP) Cr (BCC) Cu (FCC) Fe (BCC) Ni (FCC) Si (Diamond)
Al 1.432 933 3 Al
Co 1.251 1768 9 -19 Co
Cr 1.249 2163 6 -10 -4 Cr
Cu 1.278 1356 11 -1 6 12 Cu
Fe 1.241 1808 8 -11 -1 -1 13 Fe
Ni 1.246 1726 10 -22 0 -7 4 -2 Ni
Si 1.150 1693 4 -19 -38 -37 -19 -35 -40 Si
Ω
5.5
15
5.0
10
4.5
5
4.0
0
3.5
0.0
0.2
0.4
0.6
0.8
δ (%)
20
X value in Al0.5CoCrCuFeNiSix Fig. 6 The curves of Ωand δas a function of Si contents for Al0.5CoCrCuFeNiSix alloys
9.0 8.5
FCC
VEC
8.0 7.5
FCC+BCC
7.0 6.5 6.0
BCC
0.0
0.2
0.4
0.6
0.8
X value in Al0.5CoCrCuFeNiSix Fig. 7 The relation between VEC and Si contents of the alloys
The highlights of this manuscript were summarized as followed:
The Microstructure, phase assemblage and mechanical properties of the Al0.5CoCrCuFeNiSix alloys were investigated.
The addition of Si induced a microstructure change from FCC solid solution to BCC solid solution.
With the additions of Si, the hardness, wear resistance and strength were greatly enhanced.
The excellent properties came from the changes from FCC to BCC structure and the solution strengthening of Si.
16
S0925-8388(14)02733-9 http://dx.doi.org/10.1016/j.jallcom.2014.11.085 JALCOM 32639
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
8 July 2014 9 November 2014 12 November 2014
Please cite this article as: X. Liu, W. Lei, L. Ma, J. Liu, J. Liu, J. Cui, On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high-entropy alloys, Journal of Alloys and Compounds (2014), doi: http:// dx.doi.org/10.1016/j.jallcom.2014.11.085
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On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high-entropy alloys
Xiaotao Liu1, Wenbin Lei1, Lijuan Ma2, Jing Liu2, Jinling Liu1, Jianzhong Cui1 1.
The Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang, 110819, China 2.
The School of Science of Northeastern University, Shenyang, 110004, China
Abstract: The microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix (x = 0, 0.4, 0.8) high-entropy alloys are studied in this paper. Alloy with x = 0 is a simple FCC solid solution. As Si content x increases, alloys are gradually changed from FCC to BCC. As Si content x increases, compressive strength of the alloys increases, while ductility decreases. The parameters representing simple solid-solution formation ability in Al0.5CoCrCuFeNiSix alloys are estimated. The addition of Si destabilizes the FCC structure and introduces the transition from a closed-packed FCC structure to a loose-packed BCC structure to relax the lattice distortion energy due to the smaller radius of Si. The improved mechanical properties of the alloys are attributed to the basic structure changes from FCC to BCC phases and solution hardening mechanism. Keywords: High-entropy alloys; Microstructures; Mechanical properties; Solution strengthening 1. Introduction The high-entropy alloys (HEAs) with multiple principal elements in equiatomic or close-toequiatomic ratios in one alloy system were developed base on a novel alloy design concept beyond traditional metallurgy by Yeh et al. in 1995 [1, 2]. HEAs exhibits superiority in the formation of simple solid solutions over that of intermetallic compounds and terminal solid solutions as expected in conventional alloys system. Due to their simple solid solution structures 1
and promising properties, including high strength/hardness, outstanding wear resistance, exceptional high-temperature strength, good structural stability, good corrosion and oxidation resistance and combinations of aforementioned properties, HEAs have received more and more attentions in the past decades and become an attractive field of metallic materials [3]. The phase selection in multicomponental alloy systems is related to fundamental properties of the constituent elements [4]. Thus, from the alloy design point of view, phase selection and competition are important topics, which is critical for developing multicomponent HEAs with solid-solution phases rather than amorphous phase or intermetallic compounds. To design appropriate alloy compositions with required properties and optimize the properties for the existing HEAs can widen their potential applications. Even though the high mixing entropy in HEAs facilitates the formation of simple solid-solution phases, as felt from the name, it is not the sole parameter in this regard. The mutual interaction between constituent elements is a key issue for the prediction of phase formation in HEAs. So far, several criteria for predicting phase formation in HEAs were proposed as a guideline for alloy design based on the standpoint of enthalpy of mixing (∆Hmix), entropy of mixing (∆Smix), atomic size difference (δ), and valence electron concentration (VEC) [5, 6]. Zhang et al. related the formation of simple phases (i.e. FCC, BCC or their mixtures) to the mixing enthalpy (∆Hmix), the mixing entropy (∆Smix) and atomic size difference (δ) and proposed a solid-solution formation criteria for HEAs where two parameters, Ω, the entropy of mixing timing the average melting temperature of the elements over the enthalpy of mixing, and δ, the mean square deviation of the atomic size of elements, were used to estimate the phase formation behavior of HEAs [7, 8]. These two parameters were defined by the following equations, respectively.
2
Ω=
δ=
Tm ∆S mix ∆H mix
− ci 1 − ri / r i =1
∑
n
2
Where ci is the atomic percentage of the ith component, −
∑
n
c = 1 , and ri is the atomic radius of
i =1 i
−
the ith component and r is the average atomic radius, r =
∑
n
cr i =1 i i
. ∆Hmix, ∆Smix and Tm are
calculated as follows: n
∆H mix = 4
∑ ∆H
mix ij ci c j
i =1,i ≠ j
∆H ijmix is the enthalpy of mixing of binary liquid alloys, ci and cj are the atomic percentage of the
ith and jth component, respectively. n
∆S mix = − R∑ (ci ln ci ) i =1
Where R ( = 8.314 J⋅K-1mol-1) is gas constant. n
Tm = ∑ ci (Tm )i i =1
Where (Tm)i is the melting point of the ith component. The mixing entropy indicates the tendency for the formation of random solid solution while the enthalpy of mixing indicates the tendency for ordering and clustering. The value of Ω indicates the relative predominance of Tm∆Smix and ∆Hmix. The value of δ represents the atomic size difference between components which causes lattice distortion and subsequently the
3
corresponding strain energy which lower the stability of solid solution. Therefore, Ω and δ are the key determinants for the selective formation of a random solution or an intermetallic compound. Simple crystal structures tend to form in HEAs when Ω ≥ 1.1 and δ ≤ 6.6%. Guo et al. further confirmed that the valence electron concentration (VEC) is the key parameter to control the phase stability for solid solutions and thus can be used to determine whether an alloy crystallizes into FCC or BCC structure. FCC phases are found to be stable at higher VEC ( ≥ 8 ) and BCC phases are stable at lower VEC ( < 6.87 ). At VEC values between 6.87 and 8, FCC and BCC phase coexist [9]. The valence electron concentration, VEC, which has been proven useful in determining the phase stability of intermetallic compounds, is defined by: n
VEC = ∑ ci (VEC )i i =1
Where ci is the atomic percentage and (VEC)i is the VEC for the ith element which counts the total electrons including the d-electrons accommodated in the valence band [10, 11]. Elemental composition affects the alloys behaviors, phase assemblages and properties of HEAs [12, 13]. Up to now, more than 30 elements have been employed to prepare more than 300 reported HEAs [14]. The most commonly used elements in HEAs that have been investigated are Al and transition metal elements of the fourth period, such as Co, Cr, Cu, Fe, Ni, and Ti, etc. Addition of the minor alloying elements with a large atom size difference with the other constituent elements into HEAs can result in the change in the lattice distortion and even lattice types, thereby altering their mechanical properties, such as strength, hardness and plasticity. If a multicomponent alloy contains atomic pairs that are prone to form phase with each other, it would be generally reflected in the microstructure of the resultant multiple component alloy as well [15, 16]. For example, Al with the largest atomic radius has obvious influence on phase transformation and mechanical properties of AlxCoCrCuFeNi alloys. With increasing Al content, 4
the microstructure of the alloys showed gradual changes from FCC to FCC + BCC and finally to fully BCC phases [17-19]. Among the AlxCoCrCuFeNi alloys, the Al0.5CoCrCuFeNi alloy consists of simple FCC solid-solution structure and is ductile, work hardenable, and strong even at temperature up to 800 oC [20]. It is anticipated that this alloy would have potential applications in high-temperature structures and working tools. In traditional metallic materials like steel, the proper addition of non-metallic elements with small atomic radii, such as C and Si, is beneficial to their structures and properties [21, 22]. However, the effects of these non-metallic elements on the multi-principal component alloys are still unclear [23]. In this paper, the Al0.5CoCrCuFeNiSix HEAs with varying Si content are designed and the effect of Si on the microstructure, phase assemblage and properties of Al0.5CoCrCuFeNiSix alloys are investigated systematically in this paper. It is indicated that the addition of Si changes the alloy structure distinctly, and thus leads to great enhancement of the alloy properties. The structure and phases evolution, strengthening mechanism of the alloys are discussed in detail. 2. Experimental Alloy ingots with nominal compositions of the Al0.5CoCrCuFeNiSix (x value in molar ratio, x = 0, 0.4 and 0.8) were prepared by arc melting pure elements with purity higher than 99.9 wt % under high-purity argon atmosphere on a water-cooled Cu hearth. The alloys were re-melted four times to improve the homogeneity of alloy compositions. The crystal structure was characterized by Xray diffraction (XRD) with the 2 theta scan ranging from 20 o to 100 o at a speed of 2 o/min and the typical radiation condition was 30 kV and 20 mA with a copper target. Vickers hardness was measured using a FM-700 tester with a load of 50 g and a duration time of 15 seconds for each measurement. The hardness values were averaged by nine measurements at least for each sample. The wear behavior of the Al0.5CoCrCuFeNiSix alloys was examined using a MT-2000 tribometer
5
under dry sliding at room temperature where the tested samples were a pin 6 mm in diameter and 12 mm in height. The counterpart wear material is Cr12MoV steel with an average hardness about 760 HV. During the test, each pin was fixed in a holder to wear against the rotating C12MoV steel disk which has a diameter of 70 mm. The distance between the pin and the center of the disk was 30 mm. The sliding speed was 0.8 mm/s, with a normal load of 100 N. The anti-wear property was measured by wear resistance, which is defined as Wr = L/∆V, where Wr is the wear resistance and ∆V is the volumetric loss of the specimen after it has slid through a distance L, and is obtained by dividing the weight loss by the sample density. The microstructure and chemical composition of the alloys, the worn surface and wear debris were analyzed using a Super Scan SS-550 scanning electron microscope (SEM) with energy dispersive spectrometry (EDS). Compression test was performed on MTS 800 machine with a strain rate of 1 x 10-4 s-1 at room temperature. 3. Results 3.1 Microstructure and phase assemblages The crystal structures of the as-cast Al0.5CoCrCuFeNiSix alloys were characterized using XRD as shown in Fig. 1. Simple solid solution structures, essentially FCC and BCC, were identified in the Al0.5CoCrCuFeNiSix alloys. Only XRD diffraction peaks corresponding to a FCC crystal structure was identified in the alloy without Si addition. In the alloys with the addition of Si, peaks corresponding to a BCC crystal structure started to appear and the intensity increased with increasing Si content. Fig. 1(b) shows an enlarged picture of the main peak of BCC phase in the alloys with Si addition. It is shown that the peaks of BCC phase shifted rightwards with increasing Si content, indicating that the lattice parameter of the BCC phase decreases slightly. By linearity extrapolation method, the lattice constants of the BCC phases in the alloys were calculated from the X-ray diffraction peaks. The lattice constant of the BCC were 2.867 Å and 2.859 Å for the alloys with x = 0.4 and x = 0.8, respectively. The addition of Si changed the
6
crystal structure from FCC to BCC and the lattice constant of the BCC phase decreased with increasing Si content. Fig. 2 shows the back-scattered images of the as-cast Al0.5CoCrCuFeNiSix alloys with varying Si content. Table 1 lists the chemical compositions of the areas indicated in Fig. 2 (b, d and f). Typical cast dendrite (DR) and interdendrite (ID) structures were observed in the Al0.5CoCrCuFeNiSix alloys. Although copper segregation at the interdendrite regions of small volume fraction was observed, the dendrites of large volume fractions were basically composed of multiprincipal elements. For the alloy without Si addition, both the dendrites and interdendrites were of FCC structure. With the addition of Si, a BCC solid solution phase started to appear. When x = 0.4, the dendrites were a mixture of FCC and BCC phases and the interdendrites were still the copper-rich FCC phase. When x = 0.8, the FCC dendrites disappeared and the microstructure composed of the BCC dendrites and copper-rich FCC interdendrites. Thus, with increasing Si content, the alloys have a dominated BCC dendrite phase and a small amount of FCC interdendrite phase with Cu enrichment. The BCC dendrites had higher Si content than the FCC dendrites and the Si content in the Cu-rich interdendrites was minor. Also, two characters were also noted in the microstructure of the alloys with Si additions, one is the fine needle-like precipitations from the BCC dendrites which were Cu-rich phases according to the XRD and EDS analyses, and the other is the changing of the microstructures from dendritic to network-like for the alloys with Si additions. 3.2 Hardness and wear resistance Fig. 3 shows the hardness and wear resistance of the Al0.5CoCrCuFeNiSix alloys as a function of Si content. The hardness values of the alloys increased as x was varied from 0 to 0.8. The hardness of the alloy with x = 0.8, 653 HV, was 2.48 times that of the alloy with x = 0, 263 HV. The wear resistance of the alloy with x = 0.8, 0.86 km/mm3, is about 8.6 times that of the alloy
7
with x = 0, 0.10 km/mm3. The insert in Fig. 3 presents the wear resistance as a function of hardness of the Al0.5CoCrCuFeNiSix alloys. The wear resistance of the Al0.5CoCrCuFeNiSix alloys was seemingly sensitive to the hardness with varying Si content and exhibited a strong correlation to the hardness. Fig. 4 shows the morphologies of the worn surface of the alloys after test and the debris produced during the test. For the alloy without Si addition, the worn surface was grooved and noticeable plastic deformation features along the grooves were seen, indicating the high ductility of the alloy. Moreover, a few lateral cracks were found on the worn surface indicating the fracture of the alloy resulting in relatively low wear resistance. The debris of the alloy without Si addition had a flakelike shape and was the biggest, around 300-1000 µm in size. The flake-like shape indicates that the debris was worn off from Al0.5CoCrCuFeNi and the adhesive wear mechanism of Al0.5CoCrCuFeNi was predominantly of delamination wear, by which the worn surface underwent a periodic delamination fracture [24]. This is reasonable since delamination wear is a typical mechanism for ductile metals and the alloy without Si addition was composed of ductile FCC phase. The chemical compositions of both the worn surface and wear debris presented in Table 2 show that they all contain significant amount of oxygen indicating oxidation occurs on the worn surface heated by friction and deformation. For the alloys with Si additions, plastic features and grooves on the worn surface were seen again. However, compared with the alloy without Si addition, the plastic features and grooves were inhibited greatly. When x = 0.8, the plastic features disappeared. The debris of the alloys with Si additions still had flake-like shape but their size decreased with increasing Si content, 250-600 µm for the alloy with x = 0.4 and 70200 µm for the alloy with x = 0.8. The oxygen contents of the worn surface and debris were all higher than those of the alloy without Si. This indicated that the wear type of the alloy with Si additions was still predominantly of delamination wear but the oxidative wear became pronounced. 8
3.3 Compressive properties Fig. 5 shows the compressive engineering stress-strain curves of room-temperature test for the ascast Al0.5CoCrCuFeNiSix alloys. The insert in Fig. 5 shows the morphologies of the fractographs of the deformed samples. It was found that the alloy strength increased obviously with increasing Si content. However, the alloy ductility was weakened at the same time. The alloy without Si addition exhibited good yield stress, fracture strength, plastic deformation and obvious hardening behavior. The samples with the barrel-like form did not fracture after compressive deformation though some surface cracks were observed. The alloy strength continued to be improved evidently accompanying the sacrifice of ductility as Si content increased. The alloy without Si has the maximum compressive strength of 1.26 GPa and the plastic deformation of > 30%. The alloy with x = 0.4 has the maximum compressive strength of 1.79 GPa and the total plastic deformation of 23%. The alloy with x = 0.8 has the maximum compressive of 1.06 GPa and the total plastic deformation of 3.7 %. The addition of Si enhanced the compressive strength but lowered the plasticity of the alloys. When x = 0.8, the alloy showed a brittle fracture without any plasticity observed. 4. Disscussion Using the physicochemical and thermodynamic parameters for the constituent alloying elements as listed in Table 3, Ω, δ and VEC for the present Al0.5CoCrCuFeNiSix alloys were calculated. The values of Ω and δ of the Al0.5CoCrCuFeNiSix alloys with varying Si content were shown in Fig. 6. With increasing Si contents, the Ω values, reflecting the competition between ∆Smix and ∆Hmix, decreased from 16.26 to 1.73 and the δ values increased from 4.10 to 4.67. It is obvious that the values of both Ω and δ fulfilled the aforementioned criteria for forming HE stabilized solid solution (Ω ≥ 1.1, δ ≤ 6.6 %), and therefore, the Al0.5CoCrCuFeNiSix alloys are inclined to form solid solution phase during solidification. 9
The VEC values of the Al0.5CoCrCuFeNiSix alloys with varying Si content were shown in Fig. 7. It is noted that with increasing Si content, the VEC value decreased linearly, confirming that the Si addition promotes covalency. VEC > 8 was obtained for the alloy without Si addition indicating that FCC structured solid solutions are more stable, while 6.87 < VEC < 8 was obtained for the alloys with Si additions indicating that FCC and BCC solid solutions coexist, which is consistent with the XRD and SEM analysis aforementioned. The structure changes of the Al0.5CoCrCuFeNiSix alloys can be explained by using the atomic packing efficiency of the FCC and the BCC phases. Generally, BCC structure has a lower atomic packing density (68 %) than FCC structure (74 %). When Si, which has a smaller radius than the other components, was initially added into the FCC structured Al0.5CoCrCuFeNi alloy, lattice distortion and subsequently an apparent lattice straining was introduced. From a viewpoint of strain energy, the addition of Si will destabilize the FCC structure and introduce the transition from a close-packed FCC structure to a loose-packed BCC structure to relax the lattice distortion energy [25]. This trend toward the formation of BCC phase is very reasonable since silicon is a well-known BCC former and stabilizer [26]. Cu segregation to the interdendrites could be explained based on the positive mixing enthalpy between copper and the other components. In the alloy without Si addition, the Cu atoms were rejected to the grain boundaries during solidification forming Cu-rich solid solutions due to the positive enthalpy values of Cu-Co, Cu-Cr and Cu-Fe atomic pairs as shown in Table 3. Al and Ni have greater chemical affinity with Cu than the other elements and thus Al and Ni were also detected in the interdendrites. In the alloys with Si additions, though the mixing enthalpy between Si and Cu is -19 kJ⋅mol-1, it is still a relatively positive value among the mixing enthalpies between Si and the other components [27, 28]. Therefore, the addition of Si would further reject the Cu atoms into the grain boundary, and exacerbate the copper segregation at the interdendrites
10
zones. The EDS analysis results that Si mainly existed in the dendrites rather than in the Cu-rich interdendrites support this. The improved mechanical properties of the Al0.5CoCrCuFeNiSix alloys come from the basic structure changes (transition from FCC to BCC phases) and solution hardening mechanism. For the basic structure factor, the BCC phase is much stronger than the FCC phase. Slip along the closest packing planes {110} in the BCC structure is more difficult than that along those {111} in the FCC structure since {110} planes are less dense and more irregular and thus possess a smaller interplanar spacing and higher lattice friction for dislocation motion than {111} planes in the atomic scale [29]. A similar phenomenon was also reported in the study of AlxCoCrCuFeNi alloys with varying Al content which corresponds to a gradual change from FCC to BCC phase [24]. For the solution hardening mechanism factor, the improved properties of alloys are attributed to solid solution of Si. In Al0.5CoCrCuFeNiSix alloys, each atom can be expected as a solute atom and it can randomly occupy the crystal lattice sites. Nevertheless these solute atoms with different sizes and properties interact with each other and elastically distort the crystal lattice, which induces local elastic stress field. The interactions between these local elastic stress fields and the stress field of dislocations in alloy will hinder dislocation movements, and cause the increase of strength. For cubic crystals, the lattice strain can be expressed as ε = ∆α/α0, where ∆α = |α-α0|, α and α0 is the lattice constant of actual alloy crystal and non-deformed perfect crystal, respectively [30]. The strengthening caused by atomic size mismatch will increase with the increase of atomic size difference. In Al0.5CoCrCuFeNiSix alloys, the atomic size difference increases with increasing Si content, as a result, the corresponding strength of the alloys increased. As aforementioned, the lattice constants of the BCC phase decrease as Si content increases from 0 to 0.8, which results in the increase of ε. It is deduced that the lattice distortion energy will increase significantly and the effect of solution strengthening is enhanced. Thus, the alloy strength 11
increases greatly with the decrease of ductility. The large improvement in wear resistance of the alloys with Si additions is attributed to the high hardness and strength, which not only resists plastic deformation and delamination, but also brings about the oxidative wear in which oxide film could assist the wear resistance. 5. Conclusions 1) The microstructure of the Al0.5CoCrCuFeNiSix alloys gradually transited from FCC solid solution to BCC solid solution with increasing Si content from 0 to 0.8. Cu segregation occurred in the interdendrites due to the positive enthalpies between Cu and the elements. The addition of Si further exacerbated the Cu segregation in interdendrites. 2) With the additions of Si, the hardness, wear resistance of the alloys were improved. The wear type of the alloys is predominantly of delamination wear but the oxidative wear became pronounced with increasing Si content. The alloys compressive strength increased greatly with increasing Si content, while the ductility of the alloys decreased with increasing Si content. 3) The improved mechanical properties of the Al0.5CoCrCuFeNiSix alloys are due to the basic structure changes from FCC to BCC phase and the solution hardening mechanism.
Acknowledgements The authors would like to thank the Fundamental Research Funds for the Central Universities (N120409003) and University students’ Innovation Plan of China (No. 130066 and No. 140219) for their financial supports on this research.
12
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Fig. 1 XRD patterns of the as-cast Al0.5CoCrCuFeNiSix (x=0, 0.4 and 0.8) alloys Fig. 2 Back scattered SEM images of the as-cast Al0.5CoCrCuFeNiSix alloys (a) and (b) x=0; (c) and (d) x=0.4; (e) and (f) x=0.8 Fig. 3 Hardness and wear resistance of the as-cast Al0.5CoCrCuFeNiSix alloys Fig. 4 SEM images of the wear surface (a, c, e) of the Al0.5CoCrCuFeNiSix alloys and the debris (b, d, f) produced during the test (a) and (b) x=0; (c) and (d) x=0.4; (e) and (f) x=0.8 Fig. 5 Room temperature compressive engineering stress-strain curves of the as-castAl0.5CoCrCuFeNiSix alloys (the inserts are the morphologies of the fractographs of the samples after test) Fig. 6 The curves of Ω and δ as a function of Si contents for Al0.5CoCrCuFeNiSix alloys Fig. 7 The relation between VEC and Si contents of the alloys Table 1 Chemical compositions of the Al0.5CoCrCuFeNiSix alloys (at.%) Table 2 Chemical compositions of the worn surface and wear debris produced of the Al0.5CoCrCuFeNiSix alloys (at.%) Table 3 the physicochemical and thermodynamic parameters for the constituent alloying elements
15
On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high entropy alloys
Intensity (arb. units)
Si0.8
FCC BCC
(b) Si0.8
Intensity (arb. units)
(a)
Si0.4
Si0.4
Si0
40
50
60 70 2θ (Degrees)
80
90
42
43
44
45
46
47
2θ (Degrees)
Fig. 1 XRD patterns of the as-cast Al0.5CoCrCuFeNiSix (x=0, 0.4 and 0.8) alloys
48
(a)
(b) ID(FCC) DR(FCC)
(c)
(d)
ID(FCC)
DR(FCC) DR(BCC)
(e)
(f) DR(BCC) ID(FCC)
Fig. 2 Back scattered SEM images of the as-cast Al0.5CoCrCuFeNiSix alloys (a) and (b) x=0; (c) and (d) x=0.4; (e) and (f) x=0.8
Table 1 Chemical compositions of the Al0.5CoCrCuFeNiSix alloys (at. %) Alloys Si0.0
Si0.4
Si0.8
Areas in Fig. 2 Norminal ID (Cu-rich) DR(FCC) Norminal ID(Cu-rich) DR(FCC) DR(BCC) Norminal ID(Cu-rich) DR(BCC)
Al 9.09 14.98 7.38 8.47 9.25 4.44 8.65 7.94 10.70 9.11
Co 18.18 7.51 23.73 16.95 4.69 22.66 19.60 15.87 2.63 19.56
Cr 18.18 5.05 21.31 16.95 2.91 23.07 20.47 15.87 1.37 18.70
Cu 18.18 51.26 9.73 16.95 64.52 7.24 5.22 15.87 73.55 4.42
Fe 18.18 6.76 20.83 16.95 4.39 24.44 19.63 15.87 2.38 20.24
Ni 18.18 14.44 17.02 16.95 13.28 13.02 18.16 15.87 8.48 16.67
Si 6.78 0.96 5.14 8.26 12.70 0.89 11.71
1000 )
0.8
3
1.0
Wear resistance (km/mm
Hardness (HV)
600
Hardness Wear resistance 1.4 1.2
0.6 0.4
1.0
0.2 0.0 200
300
400
500
600
800
900
Hardness (HV)
400 200
0.0
0.2
0.8 0.6
0.4 For tribometric test: Normal load: 100 N 0.2 Sliding speed: 0.8 mm/s 0.0 0.4 0.6 0.8
X in Al0.5CoCrCuFeNiSix Fig. 3 Hardness and wear resistance of the as-cast Al0.5CoCrCuFeNiSix alloys
3
0
700
Wear resistance (km/mm )
800
1.6
1.2
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4 SEM images of the wear surface (a, c, e) of the Al0.5CoCrCuFeNiSix alloys and the debris (b, d, f) produced during the test (a) and (b) x=0; (c) and (d) x=0.4; (e) and (f) x=0.8
Table 2 Chemical compositions of the worn surface and wear debris produced of the AlCoCrCuFeNiSix alloys (at.%) Alloys Areas in Fig. 2 Norminal
Al 9.09
Co 18.18
Cr 18.18
Cu 18.18
Fe 18.18
Ni 18.18
Worn surface
9.74
16.80
15.22
13.98
17.24
14.87
12.16
Debris Norminal Worn surface Debris Norminal Worn surface Debris
8.12 8.47 7.70 9.38 7.94 3.78 3.31
15.67 16.95 11.22 6.73 15.87 7.53 5.82
14.14 16.95 11.33 6.66 15.87 9.64 9.93
13.59 16.95 10.70 5.86 15.87 5.11 2.76
13.39 16.95 11.83 6.09 15.87 12.98 12.11
14.16 16.95 12.53 5.41 15.87 6.91 6.45
20.94
Si0
Si0.4 Si0.8
Si
O
6.78 5.52 7.38 12.70 5.83
29.18 52.49
5.72
53.91
49.98
2000
Stress (MPa)
1500 Si0.8
1000
Si0.4
500 0
Si0
0
-5
-10
-15
Strain (%)
-20
-25
-30
Fig. 5 Room temperature compressive engineering stress-strain curves of the ascastAl0.5CoCrCuFeNiSix alloys (the inserts are the morphologies of the fractographs of the samples after test)
Table 3 the physicochemical and thermodynamic parameters for the constituent alloying elements Elements Radius (Å) Tm (K) VEC Al (FCC) Co (HCP) Cr (BCC) Cu (FCC) Fe (BCC) Ni (FCC) Si (Diamond)
Al 1.432 933 3 Al
Co 1.251 1768 9 -19 Co
Cr 1.249 2163 6 -10 -4 Cr
Cu 1.278 1356 11 -1 6 12 Cu
Fe 1.241 1808 8 -11 -1 -1 13 Fe
Ni 1.246 1726 10 -22 0 -7 4 -2 Ni
Si 1.150 1693 4 -19 -38 -37 -19 -35 -40 Si
Ω
5.5
15
5.0
10
4.5
5
4.0
0
3.5
0.0
0.2
0.4
0.6
0.8
δ (%)
20
X value in Al0.5CoCrCuFeNiSix Fig. 6 The curves of Ωand δas a function of Si contents for Al0.5CoCrCuFeNiSix alloys
9.0 8.5
FCC
VEC
8.0 7.5
FCC+BCC
7.0 6.5 6.0
BCC
0.0
0.2
0.4
0.6
0.8
X value in Al0.5CoCrCuFeNiSix Fig. 7 The relation between VEC and Si contents of the alloys
The highlights of this manuscript were summarized as followed:
The Microstructure, phase assemblage and mechanical properties of the Al0.5CoCrCuFeNiSix alloys were investigated.
The addition of Si induced a microstructure change from FCC solid solution to BCC solid solution.
With the additions of Si, the hardness, wear resistance and strength were greatly enhanced.
The excellent properties came from the changes from FCC to BCC structure and the solution strengthening of Si.
16