Elemental effect on formation of solid solution phase in CoCrFeNiX and CoCuFeNiX (X = Ti,Zn,Si,Al) high entropy alloys

Abstract

The paper aims to investigate the effect of elements addition, its enthalpy of mixing, crystal structure and atomic size difference on the formation of solid solution phase during the synthesis of high entropy alloy (HEA) by mechanical alloying. For this CoCrFeNiX and CoCuFeNiX (where X = Ti, Zn, Si, Al), alloys were prepared by mechanical alloying. The phases formed during mechanical alloying were characterised by X-ray diffraction analysis, transmission electron microscopy and differential scanning calorimetry. Titanium and Aluminium addition facilitate solid solution formation during mechanical alloying. Formation of a BCC and FCC solid solution phase was observed for CoCrFeNiX and CoCuFeNiX system (where X = Ti, Al), respectively. Single solid solution phase was not observed for CoCrFeNiZn, CoCrFeNiSi, CoCuFeNiZn and CoCuFeNiSi HEA up to 20 hours of milling.

Keywords

High entropy alloys mechanical alloying solid solution formation differential scanning calorimetry X-ray diffraction analysis enthalpy of mixing;entropy of mixing atomic size difference

Introduction

Traditional alloy system consists of one principal element and one or two alloying elements (metal or non-metal). The concentration of alloying elements is controlled to produce desired properties [1]. In 2004, Yeh et al. [2] introduced a new concept of alloy design called high entropy alloy (HEA). HEA is defined as an alloy containing at least five or more elements in equiatomic or near equiatomic compositions and forms a solid solution phase with FCC, BCC or FCC+ BCC structure. The formation of simple solid solution phase was attributed to high mixing entropy [2 –4]. However, the composition of the constituent element and method of preparation influences the crystal structure (FCC or BCC) of the solid solution formed in HEA [5]. Up to now, AlCoCrCuFeNi HEA has been extensively investigated. Splat quenching, mechanical alloying and DC magnetron sputtering of AlCoCrCuFeNi HEA led to the formation of BCC solid solution phase. Formation of Cu-rich FCC phase within the interdendritic region and dendrites of BCC structure has been reported for AlCoCrCuFeNi HEA produced by melting and casting route [6 –9]. In most cases, HEA prepared by melting and casting route shows phase segregation [10 –14]. Mechanical alloying helps to produce ultrafine grained nanocrystalline HEA's with homogeneous composition and improved properties [15 –24]. The properties of the HEA mainly depends on the phase formation and its evolution [5].

Vaidya et al. [24] reported that the proportions of the FCC and BCC phase differ upon changing the sequence of the alloying element during sequential alloying of AlCoCrFeNi HEA. Varalakshmi et al. [16] observed that upon increasing the number of elements from binary to hexanary, longer milling hours are required to form a solid solution phase and was attributed to slower diffusivities of atoms. Zhang et al. [25] and Gao et al. [26] predicted the solid solution phase formation rules for multicomponent HEA. Less atomic size difference (δ), near zero value of the absolute enthalpy of mixing (ΔH_mix) and high entropy of mixing (ΔS_mix), facilitates the formation of solid solution phase in equiatomic alloy [25]. The solid solution can only form when the three parameters are in the range of 0 ≤ δ ≤ 8.5, −22 ≤ ΔH_mix≤ 7 kJ mol⁻¹ and 11 ≤ ΔS_mix≤ 19.5 J (K·mol)⁻¹ otherwise, intermetallic compounds or equiatomic amorphous phase will exist in the alloys [26].

The formation of solid solution phase was observed for CuNiCoZnAlTi [16], AlFeTiCrZnCu [15] and AlFeTiCrZn [15] HEA after 10, 20 and 15 hours of mechanical alloying, respectively. The above alloys were milled in a similar condition but it was observed that few alloys need a longer duration of mechanical alloying to form a single solid solution phase. Crystal structure, atomic size difference and electronegativity of elements affect solid solution formation. The present study investigates the effect of element addition, its enthalpy of mixing, crystal structure and atomic size difference on the formation of solid solution phase during mechanical alloying. For this CoCrFeNiX and CoCuFeNiX alloy system where X = (Ti, Zn, Si, Al) were investigated from ternary to quinary.

Materials and method

The elemental powders of Cobalt (Co), Chromium (Cr), Copper (Cu), Iron (Fe), Nickel (Ni), Zinc (Zn), Titanium (Ti), Silicon (Si) and Aluminium (Al) from Alfa Aesar with a purity of 99.5% and particle size ≤45 µm were used to synthesise CoCrFe, CoCrFeNi, CoCrFeNiTi, CoCrFeNiZn, CoCrFeNiSi CoCrFeNiAl, CoCuFe, CoCuFeNi, CoCuFeNiTi, CoCuFeNiZn, CoCuFeNiSi and CoCuFeNiAl alloy. The alloys were synthesised by mechanical alloying in a Fritsch Pulverisette-P5 high energy ball mill at 300 rpm with a ball to powder weight ratio of 10:1 and toluene as a process control agent. Tungsten carbide vials and balls were used as the milling media. In order to understand the phase evolution during mechanical alloying, powder samples after every 5 hours of milling were collected and characterised by X-ray diffractometer (Xpert Pro Panalytical instrument with Cu-kα radiation). The crystallite size was calculated using Scherrer's formula (using the Lorentz function after removing instrumental broadening). The melting point of HEA's was investigated using differential scanning calorimetry (DSC; LABSYS evo STA 1600, SETARAM). The HEA powder was placed in carbon-coated copper grid and was analysed by HR-TEM (JOEL, JAPAN, JEM-2100 plus) operated at 300 kV for crystal structure determination. The enthalpy of mixing (ΔH_mix), entropy of mixing (ΔS_mix) and atomic size difference (δ) of the alloy were calculated based on the method reported by Guo et al. [26]. The densities of the sintered sample were calculated by Archimedes principle. The hardness of sintered sample was measured using Vickers hardness tester (MATUSZAWA) at a load of 3 kg for 10 seconds.

Results and discussion

Phase evolution during mechanical alloying

Figure 1(a–f) shows the X-ray diffraction (XRD) pattern of CoCrFe, CoCrFeNi, CoCrFeNiTi, CoCrFeNiZn, CoCrFeNiSi and CoCrFeNiAl alloys with varying milling time. XRD peak corresponding to Co, Cr and Fe were observed in 0 hours milled powder mixture of CoCrFe (Figure 1 (a)). The 5 hours milled sample shows a decrease in intensity of XRD peaks of these elements and after 10 hours of milling a less intense XRD peak of cobalt was noticed along with BCC phase. After 15 hours of milling, the XRD peaks of individual element disappear and the formation of a BCC solid solution phase was observed. Further milling up to 20 hours led to broadening of XRD peak of BCC phase. The formation of BCC solid solution in ternary CoCrFe system is attributed to Cr, which is a strong BCC former [24]. The overall enthalpy of mixing and size factor (δ) of CoCrFe system is −2.66 kJ mol⁻¹ and 0.38, respectively (Table 1). CoCrFeNi powder mixture milled for 0 hours shows XRD peaks of Co, Cr, Fe and Ni (Figure 1(b)). Upon further milling (5, 10 and 15 hours), the most intense peak at 2θ = 44.5° shift towards the lower angle and the intensity of XRD peaks of the individual element decreases (Figure 1(b)). FCC solid solution phase was observed after 20 hours of mechanical alloying. Praveen et al. [22] reported the formation of major FCC and minor BCC phase in case of CoCrFeNi system milled for 15 hours. The formation of the FCC phase was due to the presence of Ni which is a FCC stabiliser [19]. The overall enthalpy of mixing and size factor (δ) for CoCrFeNi system is −3.75 kJ mol⁻¹ and 0.30, respectively (Table 1). Formation of BCC solid solution phase was observed in case of CoCrFeNiTi powder mixture after 5 hours of mechanical alloying (Figure 1(c)). Increase in milling period (10–20 hours) led to an increase in broadening of XRD peak of the bcc phase. The increase in strain and crystallite refinement during milling can be attributed to large mechanical deformation [23]. The formation of solid solution phase is observed after 5 hours of milling in CoCrFeNiTi system, whereas it is observed after 20 and 15 hours of mechanical alloying in CoCrFeNi (Figure 1(b)) and CoCrFe system (Figure 1(a)), respectively. The entropy of mixing (ΔS_mix = 1.61R), negative enthalpy of mixing (ΔH_mix = −16.32 kJ mol⁻¹) and atomic size difference (δ = 6.68) of CoCrFeNiTi system is large as compared to that of quarternary CoCrFeNi (ΔS_mix= 1.34, ΔH_mix= −3.75 kJ mol⁻¹ and δ = 0.3) and ternary CoCrFe (ΔS_mix= 1.10 R, ΔH_mix= −2.66 kJ mol⁻¹ and δ = 0.38) system as shown in Table 1. The high entropy of mixing facilitates the formation of simple solid solution phase with FCC and BCC structure in multicomponent HEA [2 –4]. However, it was observed that CoCrFe and CoCrFeNiTi form solid solution phase with BCC structure, whereas CoCrFeNi forms solid solution with FCC structure. In order to investigate the effect of enthalpy of mixing on solid solution formation quinary CoCrFeNiZn system was investigated. CoCrFeNiZn system has similar entropy of mixing as CoCrFeNiTi system but have a lower overall enthalpy of mixing (ΔH_mix = −3.2 kJ mol⁻¹, Table 1). Zn has a large atomic size (1.395 A°) and HCP structure similar to Ti (atomic size = 1.462 A°). The XRD pattern of CoCrFeNiZn HEA is shown in Figure 1(d). After 20 hours of milling, a less intense XRD peak of Zn and Co was observed along with an FCC and BCC phase. No single solid solution phase was observed up to 20 hours of milling. The reason for the incomplete dissolution of Zn could be due to the less negative overall enthalpy of mixing for CoCrFeNiZn system as compared to that of CoCrFeNiTi. To study the effect of crystal structure, another CoCrFeNiSi system was investigated. In CoCrFeNiTi HEA, Ti (crystal structure HCP) was replaced with Si (crystal structure FCDC). CoCrFeNiSi system has a large negative enthalpy of mixing as compared CoCrFeNiTi (ΔH_mix = −26.56 kJ mol⁻¹, Table 1). Si also has a negative enthalpy of mixing with all the elements (Table 2). The XRD pattern of CoCrFeNiSi system shows a less intense XRD peak of individual elements (Si, Co) along with FCC and BCC phase (as shown in Figure 1(e)) after 20 hours of milling. For CoCrFeNiSi system ΔH_mix does not lie in the suitable range predicted by Guo et al. [26]. The formation of a single solid solution phase was not obtained for CoCrFeNiSi system up to 20 hours of mechanical alloying. The formation of a BCC solid solution phase was observed for CoCrFeNiAl system milled for 10 hours (shown in Figure 1(f)). Ji et al. [28] also reported the formation of BCC solid solution phase in quinary CoCrFeNiAl HEA. The formation of BCC solid solution phase was observed for CoCrFe, CoCrFeNiTi, CoCrFeNiAl alloy system and FCC for CoCrFeNi. The formation of single solid solution was not observed for CoCrFeNiZn and CoCrFeNiSi system up to 20 hours of milling.

Table 1

Atomic size difference (δ), enthalpy of mixing (ΔH mix), entropy of mixing (ΔS mix) and crystallite size (nm) for the investigated alloy.

Alloy composition	Atomic size difference (δ)	Enthalpy of mixing (ΔH mix) (kJ mol⁻¹)	Entropy of mixing (ΔS mix) (R = 8.314 J mol K⁻¹)	Crystallite size after 10 hours of milling (nm)
CoCrFe	0.38	−2.66	1.10 R	20.9
CoCrFeNi	0.30	−3.75	1.34 R	21.2
CoCrFeNiTi	6.68	−16.32	1.61 R	9.0
CoCrFeNiZn	4.65	−3.20	1.61 R	–
CoCrFeNiSi	3.06	−26.56	1.61 R	–
CoCrFeNiAl	5.78	−12.48	1.61 R	6.0
CoCuFe	1.62	7.98	1.10 R	–
CoCuFeNi	1.14	5.00	1.34 R	–
CoCuFeNiTi	6.50	−11.05	1.61 R	6.6
CoCuFeNiZn	4.51	1.76	1.61 R
CoCuFeNiSi	3.43	−17.92	1.61 R
CoCuFeNiAl	5.61	−5.28	1.61R	5.7

Table 2

Mixing enthalpy (kJ mol⁻¹) of binary elements based on Miedema's model [27].

	Co	Fe	Cu	Ni	Cr	Ti	Si	Zn	Al
Co	0	−1	6	0	−4	−28	−38	−5	−19
Fe	−1	0	13	−2	−1	−17	−35	4	−11
Cu	6	13	0	4	12	−9	−19	1	−1
Ni	0	−2	4	0	−7	−35	−40	−9	−22
Cr	−4	−2	12	−7	0	−7	−37	5	−10
Ti	−28	−17	−9	−35	−7	0	−66	−15	−30
Si	−38	−35	−19	−40	−37	−66	0	−18	−19
Zn	−5	4	1	−9	5	−15	−18	0	1
Al	−19	−11	−1	−22	−10	−30	−19	1	0

HEA synthesised with elements of large difference in enthalpy of mixing is prone to segregation [6]. In order to understand the criteria behind the formation of FCC and BCC phase in the multicomponent alloy, enthalpy of mixing of binary system in CoCrFe, CoCrFeNi, CoCrFeNiTi, CoCrFeNiAl, CoCrFeNiZn and CoCrFeNiSi alloys were evaluated based on Miedema's Model (Table 2). In quinary CoCrFeNiTi HEA, the enthalpy of mixing for binary system Co–Cr, Co–Fe, Co–Ti, Cr–Fe, Cr–Ni, Cr–Ti, Fe–Ni, Fe–Ti and Ni–Ti is negative and is zero for Co–Ni (Table 2) [27]. Similarly, in CoCrFeNiAl HEA, the enthalpy of mixing for binary system Co–Cr, Co–Fe, Co–Al, Cr–Fe, Cr–Ni, Cr–Al, Fe–Ni, Fe–Al and Ni–Al is negative and is zero for Co–Ni (Table 2). It indicates that the multicomponent alloy system for which the enthalpy of mixing of binary systems are mostly negative facilitates the formation of BCC solid solution phase.

The presence of FCC and BCC phase up to 20 hours of milling in CoCrFeNiZn HEA is observed. This could be attributed to large difference in the enthalpy of mixing of binary system. In CoCrFeNiZn HEA Co–Ni, Cr–Zn and Fe–Zn (Table 2) binary system possess positive enthalpy of mixing, while it is negative for Co–Cr, Co–Fe, Co–Zn, Cr–Fe, Cr–Ni, Fe–Ni and Ni–Zn. The overall enthalpy of mixing for this system becomes less negative and hence the formation of a single solid solution during mechanical alloying is delayed.

In order to investigate the effect of Ti, Zn, Si, Al element addition on the formation of a solid solution phase another set of CoCuFeNiX system was investigated from ternary to quinary. Figure 2(a) shows the XRD pattern of CoCuFe system with varying milling time. The 0-hour milled powders show XRD peaks corresponding to Co, Cu and Fe. XRD peaks of cobalt disappear after 10 hours milling and phases corresponding to Fe and Cu was observed. FCC and BCC phases were observed after 20 hours of milling. No single solid solution phase is observed and this could be due to positive overall enthalpy of mixing. The atomic size difference and overall enthalpy of mixing of CoCuFe system are 1.62 and 7.98 kJ mol⁻¹. XRD peaks corresponding to two FCC phases were observed for CoCuFeNi system milled for 20 hours (Figure 2(b)). No single solid solution phase is observed up to 20 hours of milling. The formation of FCC solid solution phase was reported for CoCuFeNi system upon mechanical alloying [22]. Figure 2(c) shows the XRD pattern of CoCuFeNiTi HEA with varying milling time. The 0-hour milled powders show XRD peaks corresponding to Co, Cu, Fe, Ni and Ti. After 10 hours of milling formation of FCC solid solution phase with a crystallite size of about 6.6 nm is observed. The mixing enthalpy of CoCuFeNiTi system is −11.05 kJ mol⁻¹. The mechanical alloying of CoCuFeNiZn for 20 hours shows two FCC phase along with XRD peak of Co (shown in Figure 2(d)). Complete disappearance of elemental peaks was not observed up to 20 hours of milling. This could be attributed to the positive overall enthalpy of mixing of elements. The XRD pattern of CoCuFeNiSi milled for 20 hours shows two FCC phases and XRD peaks of Si and Co (Figure 2(e)). Solid solution phase formation is not observed for CoCuFeNiZn and CoCuFeNiSi HEA system after 20 hours of milling as shown in Figure 2(d,e), respectively, whereas it was observed for CoCuFeNiTi HEA after 10 hours of milling. In order to study the effect of Al, an alloy system of CoCuFeNiAl was milled under similar condition. The formation of FCC solid solution phase was observed for CoCuFeNiAl system milled for 10 hours (Figure 2(f)). The overall enthalpy of mixing of CoCuFeNiAl system is negative which facilitates solid solution formation. The formation of FCC phase was observed for CoCuFeNi, CoCuFeNiTi and CoCuFeNiAl. The mixing enthalpy of binary element calculated by Miedema's model is positive for binary system Co–Cu, Co–Ni, Cu–Fe and Cu–Ni for both CoCuFeNiTi and CoCuFeNiAl HEA. The mixing enthalpy of binary system is negative for Co–Fe, Co–Ti, Cu–Ti, Fe–Ni, Fe–Ti, Ni–Ti in case of CoCuFeNiTi and Co–Fe, Co–Al, Cu–Al, Fe–Ni, Fe–Al, Ni–Al in case of CoCuFeNiAl (Table 2) [27]. In a multicomponent system in which mixing enthalpy of binary system composed of both positive and negative values form FCC solid solution phases during mechanical alloying.

The enthalpy of mixing, entropy of mixing and atomic size difference of investigated quinary CoCrFeNiTi, CoCrFeNiZn, CoCrFeNiAl, CoCuFeNiTi, CoCuFeNiZn, CoCuFeNiAl and CoCuFeNiSi system is in accordance with the criteria suggested by Guo et al. [26], whereas for CoCrFeNiSi system enthalpy of mixing (i.e. ΔH_mix = −26.56 kJ mol⁻¹) does not follow the solid solution formation criteria (shown in Table 1). BCC solid solution formation was observed for CoCrFeNiTi and CoCrFeNiAl HEA after 5 and 10 hours of mechanical alloying, respectively (Figure 1(c,f)). FCC solid solution phase was observed for CoCuFeNiTi and CoCuFeNiAl system milled for 10 hours, respectively (shown in Figure 2(c,f)). No single solid solution phase is observed in case of CoCrFeNiZn, CoCrFeNiSi, CoCuFeNiZn and CoCuFeNiSi even after 20 hours of mechanical alloying. The obtained results were compared with the existing report for AlCrFeTi, CuNiCoZnAlTi, NiCoCrCuFe and AlCoCrCuFe HEA milled in similar condition. Figure 3 shows the superimposed effect of ΔH_mix and δ for HEA. For all those alloys (AlCrFeTi, CuNiCoZnAlTi, CoCrFeNiAl, CoCrFeNiTi, AlFeTiCrZn and CoCuFeNiTi) for which solid solution formation was observed within 10 hours of milling is encircled. It was observed that the addition of elements like Al and Ti leads to the early formation of solid solution [13,14,16]. This elucidates that the enthalpy of mixing of constituent element affects the solid solution formation in HEA. Multicomponent HEA with atomic size difference (δ) between 5.6 and 7.6% and enthalpy of mixing (ΔH_mix) between −5.2 and −20 kJ mol⁻¹ accelerates the solid solution formation during mechanical alloying (Figure 3). CoCrFeNiAl, CoCuFeNiAl HEA were sintered to 96% theoretical density and the hardness was observed to be 520 ± 30 and 480 ± 30 HV, respectively.

DSC and TEM analysis

The DSC studies of CoCrFeNiTi and CoCuFeNiTi HEA after 20 hours of milling shows a single melting point at 1379.6 and 1338.5°C, respectively (Figure 4). It confirms the formation of solid solution in both the alloy system. The selected area electron diffraction (SAED) pattern and bright field image of 20 hours ball milled CoCrFeNiTi and CoCuFeNiTi HEA is shown in Figure 5(a,b). The SAED pattern of CoCrFeNiTi HEA shows a BCC structure with a lattice parameter of 0.291 ±0.002 nm, whereas for CoCuFeNiTi HEA an FCC structure with a lattice parameter of 0.358 ± 0.002 nm is observed. The crystal structure of HEA observed in TEM investigation is in accordance with the XRD results.

Conclusions

The solid solution formation in equiatomic ternary CoCrFe, CoCuFe quaternary CoCrFeNi, CoCuFeNi and quinary CoCrFeNiTi, CoCrFeNiZn, CoCrFeNiTi, CoCrFeNiAl, CoCuFeNiTi, CoCuFeNiZn, CoCuFeNiSi and CoCuFeNiAl alloy system is investigated. The addition of elements, its crystal structure and enthalpy of mixing affects the solid solution formation in HEA. The presence of Titanium and Aluminium in the quinary system facilitates the solid solution formation during mechanical alloying. The formation of a BCC solid solution phase is observed for CoCrFeNiX system, whereas an FCC solid solution phase is observed for CoCuFeNiX system (where X = Ti or Al). The lattice parameter of BCC (CoCrFeNiTi HEA) and FCC (CoCuFeNiTi HEA) phase is observed to be 0.291 ± 0.002 and 0.358 ± 0.002 nm, respectively. The DSC analysis further confirms the formation of a single solid solution in CoCrFeNiTi and CoCuFeNiTi HEA. Multicomponent HEA containing Al and Ti for which atomic size difference (δ) lies between 5.6 and 7.6% and enthalpy of mixing (ΔH_mix) between −5.2 and −20 kJ mol⁻¹ shows accelerated solid solution formation during mechanical alloying.

Footnotes

Acknowledgements

The authors would also like to thank SRM Institute of Science and Technology for providing ‘HR-TEM Facility’. The authors acknowledge the help rendered by Mr S. Saravanan, Technical Officer, in carrying out X-ray diffraction measurement.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Mahesh Jadhav

Sheela Singh

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