AlNiCrFexMo0.2CoCu High Entropy Alloys Prepared by Powder Metallurgy

Rare Metal Materials and Engineering Volume 42, Issue 6, June 2013 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2013, 42(6): 1127-1129.

ARTICLE

AlNiCrFexMo0.2CoCu High Entropy Alloys Prepared by Powder Metallurgy Fan Yuhu,

Zhang Yunpeng,

Guan Hongyan,

Suo Huimin,

He Li

Xi’an University of Technology, Xi’an 710048, China

Abstract: AlNiCrFexMo0.2CoCu (x=0.5, 1.0, 1.5 2.0) high entropy alloys were prepared by the method of powder metallurgy. Effects of Fe content on microstructure, hardness and comprehensive mechanical properties were investigated. The XRD results show that constituent phases change from bcc+fcc+ σ at x=0.5 to bcc+fcc at x=2.0. The hardness of the alloys varies from HBW3170 MPa at x=0.5 to HBW2290 MPa at x=2.0. The fracture strengths of all the AlNiCrFexMo0.2CoCu alloys are higher than 1100 MPa, and have a good plasticity. Key words: high entropy alloy; powder metallurgy; density; hardness; compression mechanical properties

The research and development of high entropy alloys (HEAs) start from 1990s. HEAs by definition contain five or more major metal elements, each at over 5 at% but less than 35 at%[1-3]. Different from traditional alloys limited to a major metal element with the addition of minor elements to modify properties, HEAs are composed of multiprincipal elements in equimolar or near-equimolar ratios. Because of the effect of high entropy, solid solutions with multiprincipal elements tend to be more stable, so HEAs reportedly exhibit promising properties, such as high strength, good oxidation and corrosion resistance, superior temper-softening resistance, and excellent abrasive wear resistance[2]. At present, HEAs are mostly prepared by the method of arc smelting and casting at home and abroad, but the reports about a powder metallurgy (PM) method are less. Thus AlNiCrFexMo0.2CoCu (x=0.5, 1.0, 1.5 and 2.0) high entropy alloys were prepared by PM, and the effects of Fe content on the microstructure and mechanical properties were investigated. To some extent, this paper provides a certain reference as a new method to prepare HEAs.

1

Experiment

Table 1 presents the size and purity of raw material powders. The Al, Ni, Cr, Fe, Mo, Co and Cu powders for Fe0.5, Fe1.0,

Fe1.5 and Fe2.0 alloys were mixed for 8 h, and then compacted under 310 MPa in a cold uniaxial pressing. Afterwards, the specimens were sintered in argon atmosphere. The specimens were sintered at 500 °C for 30 min firstly, then the temperature was raised to 1300 °C at the speed of 10 °C/min for a sintering time of 120 min, and they were allowed to get cooled to room temperature in the furnace itself. Specimens were ground with 400, 600, 800, 1000 and 1200 SiC papers followed by fine wet wheel polishing with diamond paste, and diluted aqua regia was used as etchant. Specimens were investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM) and X-ray energy dispersive spectrometry (EDS). Rectangular parallelepipeds (5 mm×5 mm×8 mm) for each specimen, obtained by wire-electrode cutting, were used for compression test. Table 1 Size and purity of raw material powders Element

Size/μm

Purity/%

Al

50

≥99.5

Ni

50

≥99.5

Cr

50

≥99.5

Fe

50

≥99.5

Mo

50

≥99.5

Co

50

≥99.4

Cu

50

≥99.9

Received date: June 20, 2012 Corresponding author: Zhang Yunpeng, Ph. D., Professor, School of Material Science and Engineering, Xi’an University of Technology, Xi’an 710048, P. R. China, Tel: 0086-29-82312818, E-mail: [email protected] Copyright © 2013, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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2

Results and Discussion

a

2.1 Constituent phase and microstructure Fig.1 presents the XRD patterns of AlNiCrFexMo0.2CoCu alloys with various Fe contents. The crystal structure is identified as bcc, fcc and σ phase in Fe0.5, Fe1.0 and Fe1.5 alloys, while only bcc and fcc phases are identified in Fe2.0 alloy. This reveals that although AlNiCrFexMo0.2CoCu alloys have seven principal elements, they exhibit a number of simple phases much less than eight phases allowed by Gibbs phase rule. This phenomenon has been generally found in reported HEAs. It has been attributable to the high entropy effect interfering in the intermetallic compounds of a multiprincipal element system to enhance the stability of solid solution phases. According to the relevant reports[4-7] and analysis, σ phase is similar to that of CoCr phase. Similar structures are also observed in σ-FeCr and σ-FeMo phases. σ phase effectively dissolves Fe and Ni. Fig.2 shows the SEM images of AlNiCrFexMo0.2CoCu alloys. In Fe0.5, Fe1.0 and Fe1.5 alloys, there are lots of white stringers(labeled as A) riched Cr and Mo on grains; beside the stringers, the gray part (labeled as B) between stringers riched Al and Ni, and the grain boundaries are credited as C1 With the increase of Fe content, the size of A decreases gradually. Table 2 shows the composition of the A, B and C1 phase in Fe0.5 alloy. Along the grain boundaries in Fe1.5 and Fe2.0 alloys, the new structure are credited as C2, and the EDS analysis of C2 in Fe1.5 alloy is also presented in Table 2. Referring to the phase morphology and distribution, it can be depicted that there are no stringers(A) in Fe2.0 alloy, which is different from the first three alloys. According to the relevant reports[4-7] and analysis, A are σ phase, while B are bcc phase, and white σ phase precipitate out of B during the cooling process after solidification. Because the HEAs can be effectively regarded as a multi-element solid solution with at most a small degree of long-range order, there is no peak at around 31°. The grain boundaries C1 riched copper are fcc phase. As Fig.2e shows, there is sub-micron modulated structure in Fe2.0 alloy C2. --•-- fcc --♦--bcc --∗-- σ

Intensity/a.u.

•♦

• •♦ Fe1.5 ∗ ∗∗ • •♦∗ Fe1.0 ∗ ∗ • •♦ Fe0.5 ∗ ∗∗ •

Fe2.0

30

40

50

♦

•

♦

•

♦

•

♦

•

♦

•

♦

•

♦

•

♦

•

60

70

80

90

2θ/(°) Fig.1 XRD patterns of AlNiCrFexMo0.2CoCu alloys with various Fe contents

B A C1

b C2

C1

c

A

B A

B C1

d

e

C2 C2

Fig.2 SEM images of AlNiCrFexMo0.2CoCu alloys: (a) x=0.5, (b) x=1.0, (c) x = 1.5, and (d, e) x = 2.0 Table 2 Chemical compositions of AlNiCrFexMo0.2CoCu alloys (at%) Element Al Ni Cr Fe Mo Co Cu

A 6.13 7.51 39.26 12.58 9.29 22.28 2.95

B 31.76 28.91 5.50 7.25 0.31 16.62 9.65

C1 11.51 13.64 9.55 16.39 1.99 13.4 34.18

C2 10.98 13.90 15.24 24.82 3.39 15.56 16.10

According to XRD, C2 in Fe1.5 and Fe2.0 alloys are fcc phase. Different from C1 riched Cu element, there are all elements in C2, which proves that C2 is a solid solution of seven elements. 2.2 Density and hardness Density is an important physical quantity of powder metallurgy materials. According to the principle of Archimedes drainage method: M1 （1） ρ Ture density = M1 − M 2 where M1: quality in air; M2: quality in water. M ρSolid density = （2） M Al M Ni M Cr M Fe M Mo M Co M Cu + + + + + + ρAl ρ Ni ρCr ρFe ρMo ρCo ρCu

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ρ Relative density =

ρ Ture density ρSolid density

Table 4 Compression mechanical properties of

（3）

AlNiCrFexMo0.2CoCu alloys

Table 3 shows the density and hardness of AlNiCrFexMo0.2CoCu alloys. It can be seen that the hardness and relative density of alloys decrease with the increase of Fe content. 2.3 Comprehensive mechanical properties Fig.3 shows the stress-strain curves of AlNiCrFexMo0.2CoCu alloys, and Table 4 presents compression mechanical properties of those alloys. The fracture strengths of all the four HEAs are higher than 1100 MPa, the compressive fracture strength and yield strength of Fe0.5 alloy are as high as 1258 MPa and 1132 MPa, while Fe2.0 alloy has a good plastic strain as 30.2%. According to the XRD analysis, it is the decreased volume fraction of hard σ phase and increased volume fraction of soft FCC phase that leads to the improvement of HEAs’ plasticity. Table 3 Density and hardness of AlNiCrFexMo0.2CoCu alloys Hardness, Fe content/ True density/ Solid density/ Relative g·cm-3 density/% HBW/MPa at% g·cm-3 0.5 6.887 7.157 96.2 3170 1.0 6.845 7.210 94.9 2780 1.5 6.629 7.256 91.4 2550 2.0 6.662 7.296 91.3 2290

3

Fe0.5

Fe1.0

Fe1.5

Fe2.0

Fracture strength /MPa

1258

1222

1124

1221

Yield strength /MPa

1132

818

536

436

Strain/%

18.3

16.9

19.8

30.2

Conclusions

1) The microstructure of AlNiCrFexMo0.2CoCu alloys changes from bcc+fcc+σ at x=0.5, 1.0 and 1.5 to bcc+fcc at x=2.0. With the increase of Fe content, the size of σ phase decreases gradually, but fcc phase improves. 2) The relative density of those alloys are over 91%, while the relative density of Fe0.5 alloy is as high as 96.2%, and the hardness of those alloys decreases gradually due to the decreased amount of σ phase and the decrease of relative density. 3) All AlNiCrFexMo0.2CoCu alloys have a good plasticity. The strain of Fe2.0 alloy is 30.2%, while Fe0.5 alloy’s compressive fracture strength and yield strength are as high as 1258 and 1132 MPa, respectively.

References 1 Hsuan-ping Chou, Yee-Shyi Chang, Swe-Kai Chen et al.

1400 1200 1000

Stress/MPa

Materials Science and Engineering B[J], 2009, 163: 184

Fe0.5 Fe1.0 Fe1.5 Fe2.0

2 Wang Yanping. Thesis for Doctorate[D]. Harbin: Harbin Institute of Technology, 2009, 163: 184 3 Varalakshmi S, Kamaraj M, Murty B S. Journal of Alloys and

800

Compounds[J], 2008, 460: 253

600

4 Chin-You Hsu, Tsing-Shien Sheu, Jien-Wei Yeh et al. Wear[J],

400

2010, 268: 652

200 0

5 Chin-You Hsu, Woei-Ren Wang, Wei-Yeh Tang et al. Advanced

0

4

8

12 16 20 24 28 32 Strain/%

Fig.3 Compression stress-strain curves of AlNiCrFexMo0.2CoCu alloys

Engineering Materials[J], 2010, 12(1-2): 44 6 Chin-You Hsua, Chien-Chang Juana, Woei-Ren Wangb et al. Materials Science and Engineering A[J], 2011, 528: 3581 7 Zhu J M, Fu H M, Zhang H F et al. Materials Science and Engineering A[J], 2010, 527: 6975

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