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7/27/2019 Synthesis and Characterization of Nanocrystalline AlFeTiCrZnCu High Entropy Solid Solution by Mechanical Alloying

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Journal of Alloys and Compounds 460 (2008) 253257

Synthesis and characterization of nanocrystalline AlFeTiCrZnCuhigh entropy solid solution by mechanical alloying

S. Varalakshmi, M. Kamaraj, B.S. Murty

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India

Received 5 May 2007; received in revised form 28 May 2007; accepted 29 May 2007

Available online 2 June 2007

Abstract

Traditional alloys are based on one or two major alloying elements. High entropy alloys are equiatomic multicomponent alloys, whereinconfigurational entropy is maximized to obtain single phase solid solutions. The present paper reports synthesis of nanostructured equiatomic high

entropy solid solutions from binary to hexanary compositions in AlFeTiCrZnCu system by mechanical alloying. These alloys have BCC

structure with crystallite size less than 10 nm. The high entropy solid solution in these alloys is stable even after annealing at 800 C for 1 h. The

hardness of AlFeTiCrZnCu solid solution is 2 GPa in the sintered condition with a density of 99%. The similar nanostructured solid solutions have

also been synthesized in CuNiCoZnAlTi and NiFeCrCoMnW alloys.

2007 Elsevier B.V. All rights reserved.

Keywords: Mechanical alloying; Entropy; Enthalpy; Transmission electron microscopy

1. Introduction

High entropy alloys are a new generation alloys and are quitedifferent from traditional alloys, which are based on one or two

elements. These multicomponent alloys are solid solutions with

equiatomic or near equiatomic compositions [1]. The principle

behind the high entropy alloys is that the configurational entropy

is maximum at equiatomic composition and it increases with

increase in the number of elements in the system, thus making

it difficult for the formation of intermetallics. The configura-

tional entropy at equiatomic compositions for binary, ternary,

quaternary, quinary and hexanary alloys is i.e. 5.8, 9.2, 11.6,

13.5, 15.0 respectively which is higher than even the entropy

of fusion (711 J mol1 K1) of most of the common metals.

The formation of amorphous phase, which can be another com-

peting phase in these multicomponent systems, can be avoided

by choosing elements carefully, which have small size factors.

Table 1 gives the size factor and the enthalpy of mixing for the

binary equiatomic alloys in the AlFeTiCrZnCu hexanary

system [2]. Even if the enthalpy of mixing is higher (either pos-

itive or negative) in some binary combinations, due to the high

Corresponding author. Tel.: +91 44 22574754; fax: +91 44 22574752.

E-mail address: [email protected] (B.S. Murty).

mixing entropy these multicomponent equiatomic alloys tend to

form only solid solutions. Their structure being simple, these

alloys were easy to analyze.Yeh and co-workers [36] is the only group that has so far

reported the synthesis of these advanced alloys by conventional

casting and thin film deposition techniques. These alloys can

be used for various applications that demand high temperature

strength, oxidation, corrosion and wear resistance. However,

mechanical alloying (MA) has not been reported so far as a pro-

cessing route for the synthesis of these alloys. MA is a widely

used solid state processing route for the synthesis of advanced

materials [7,8]. High entropy alloys reported by Yeh and co-

workers [36] are microcrystalline and their properties can be

significantly enhanced if they can be synthesized in nanocrys-

talline form. MA can easily lead to the formation of high entropy

alloys in the nanocrystalline and hence the present study was

taken up to demonstrate this in AlFeTiCrZnCu hexanary

system.

2. Experimental details

Al, Fe, Ti, Cr, Zn, Cu powders with purity higher than 99.5% and particle

size of45m (325mesh) were mechanically alloyed from binary AlFe

to hexanary AlFeTiCrZnCu systems in equiatomic ratio. The milling

was carried out up to 20 h in high energy planetary ball mill (Fritsch Pul-

verisette P-5) at 300 rpm with a ball to powder weight ratio of 10:1. Tungsten

0925-8388/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jallcom.2007.05.104
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254 S. Varalakshmi et al. / Journal of Alloys and Compounds 460 (2008) 253257

Table 1

The size factor and the enthalpy mixing for various binary equiatomic alloys in the AlFeTiCrZnCu hexanary system calculated using Miedemas approach [6]

S. no. Binary system Size factor Hchem (kJ/mol)

AB BA

1 AlFe 11 13 11

2 AlTi 1 1 30

3 AlCr 9 10 104 AlZn 3 3 1

5 AlCu 10 11 1

6 FeTi 15 13 17

7 FeCr 3 3 1

8 FeZn 9 8 4

9 FeCu 1 1 13

10 TiCr 10 11 31

11 TiCu 11 13 9

12 ZnCu 7 8 1

carbide vials and balls were used as a milling media and toluene was used

as a process controlling agent. The powder samples were taken at regular

intervals of 5 h of milling. The milled powders were analyzed by Shimadzu

XD-D1 X-ray diffractometer (XRD) with a Cu K radiation. The chemical

composition of the milled powders was determined by energy dispersive X-ray

(EDX) microanalysis system equipped with FEI-Quanta 200 scanning electron

microscope (SEM). The nanocrystalline nature and the crystal structure of the

milled powders were analyzed by Philips CM12 transmission electron micro-

scope (TEM). The multicomponent highentropyalloy powderswere compacted

at 1.8 GPa and sintered at 800 C for 1 h in an argon atmosphere. The den-

sity of the samples was measured by Archimedess principle. The hardness

of the sintered samples was measured using Vickers hardness tester at 3 kg

load.

3. Results and discussion

The XRD patterns of binary AlFe, ternary AlFeTi, qua-ternary AlFeTiCr, quinary AlFeTiCrZn, and hexanary

AlFeTiCrZnCu equiatomic elemental blend at different

intervalsof milling are shown in Fig.1(a)(e). Thecomplete dis-

appearance of all theelemental peaks andthe formation of single

phase solid solution are evident within 10 h of milling in the case

of binary AlFe to quaternary AlFeTiCr system. In case of

quinary AlFeTiCrZn and hexanary AlFeTiCrZnCu

compositions, single phase solid solution was observed within

15 and 20 h of milling, respectively. The longer milling time

needed for the solid solution formation in systems with larger

number of elements could be attributed to slower diffusivities. In

general, in allthe cases,significant broadening of theXRD peaks

and disappearance of all the peaks of the solid solution except

the most intense one has been observed. This can be attributed

to the nanocrystallite formation and the lattice strain. The crys-

tallite size of the alloy has been calculated from the X-ray peak

broadening using Viogt peak profile analysis after eliminating

the instrumental and strain contributions. The crystallite size in

most of the cases is below 10 nm after 10 h of milling as shown

in Table 2. Similar multicomponent equiatomic nanostructured

high entropy solid solutions have also been synthesized in the

present work in CuNiCoZnAlTi and NiFeCrCoMnW alloys as

shown in Fig. 2(a) and (b), respectively.

The homogeneity of the chemical composition of all the high

entropy alloys has been confirmed by EDXmicroanalysis. Fig. 3

shows the EDX spectrum [9] obtained from one of the pow-der particle in the hexanary AlFeTiCrZnCu high entropy alloy

milled for 10 h. The nominal composition of each element in

this alloy is 16.6 at.%, and the quantitative elemental analysis

results from the EDX spectrum in Fig. 3 clearly indicate that the

homogeneity and the equiatomic composition is maintained in

each particle of the alloy after 20h. The nanocrystalline nature

of the high entropy alloy has been confirmed from the TEM

bright field image and the corresponding selected area diffrac-

tion (SAD) pattern shown in Fig. 4 for the multicomponent

AlFeTiCrZnCu high entropy alloy. The particle size obtained

from the TEM study (10 nm) is very close to the crystallite size

Table 2

The crystallite size and lattice strain of binary to hexanary equiatomic alloys in AlFeTiCrZnCu system

Milling time (h) AlFe AlFeTi AlFeTiCr AlFeTiCrZn AlFeTiCrZnCu

CS (nm) Lattice

strain (%)

CS (nm) Lattice

strain (%)

CS (nm) Lattice

strain (%)

CS (nm) Lattice

strain (%)

CS (nm) Lattice

strain (%)

0

5 14 0.89 18 0.71 20 0.63 25 0.49 10 1.31

10 10 1.34 9 1.47 10 1.23 14 0.50 9 1.42

15 14 0.94 9 1.39

20 9 1.52

cs: crystallite size.


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S. Varalakshmi et al. / Journal of Alloys and Compounds 460 (2008) 253257 255

Fig. 1. XRD patterns of equiatomic (a) binary AlFe, (b) ternary AlFeTi, (c) quaternary AlFeTiCr, (d) quinary AlFeTiCrZn, and (e) hexanary AlFeTiCrZnCu high

entropy alloys as a function of milling.

calculated from the X-ray peak broadening. The analysis of the

rings in the SAD pattern ofFig. 4 has clearly indicated that the

phase has BCC structure. Similarly equiatomic binary AlFe [9],

ternary AlFeTi, quaternary AlFeTiCr and the quinary AlFeTi-

CrZn nanocrystalline high entropy alloys have also been found

to have BCC structure.

The compacted AlFeTiCrZnCu alloy sintered at 800 C for

1 h in an argon atmosphere has shown 99% density. The XRD

pattern of AlFeTiCrZnCu high entropy alloy sintered at 800 C

for 1 h is shown in Fig. 5, which reveals the alloy is a single

phase solid solution with BCC structure and is nanocrystalline

(crystallite size of 20 nm) even after sintering, which proves the

high stability of the high entropy solid solution. The hardness

of the sintered hexanary high entropy alloy is 2 GPa suggesting

that these nanocrystalline multicomponent solid solutions have

high strength.


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256 S. Varalakshmi et al. / Journal of Alloys and Compounds 460 (2008) 253257

Fig. 2. The XRD patterns of equiatomic hexanary (a) CuNiZnCoAlTi and (b)

NiFeCrCoMnW high entropy alloys as a function of milling time.

Fig. 3. The EDX spectrum and quantitative analysis of equiatomic AlFeTi-

CrZnCu hexanary alloy after 20 h of milling.

Fig.4. TheTEM bright fieldimageand SADpatternof AlFeTiCrZnCu hexanary

alloy after 20 h of MA.

Fig. 5. The XRD patterns of 20 h milled and 20h milled + sintered equiatomic

hexanary AlFeTiCrZnCu high entropy alloy.

4. Conclusions

The binary to hexanary equiatomic high entropy alloys in

AlFeTiCrZnCu system have been successfully synthe-

sized by MA. The formation of nanostructured solid solutions

with BCC crystal structure has been observed in all the compo-sitions. The high entropy alloys prepared by MA have excellent

homogeneity in composition and have a crystallite size of

about 10 nm. The nanocrystalline high entropy alloy is stable

even after sintering at 800 C for 1 h and has a high hard-

ness of 2 GPa. The similar nanostructured single phase solid

solutions have also been obtained in CuNiCoZnAlTi and

NiFeCrCoMnW systems.

References

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