Rare Metal Materials and Engineering Volume 43, Issue 6, June 2014 Online English edition of the Chinese language journal

Cite this article as: Rare Metal Materials and Engineering, 2014, 43(6): 1286-1290.

Received date: January 02, 2014 Foundation item: Post-doctoral Scientific Research Foundation of China (2013M541986); Doctoral Scientific Foundation of Henan Institute of Engineering (062101); Post-doctoral Scientific Research Foundation of Zhengzhou University Corresponding author: Ren Bo, Ph. D., Lecturer, School of Mechanical Engineering, Henan Institute of Engineering, Zhengzhou 451191, P. R. China, Tel: 0086-371-62508765, E-mail: renbo193513@163.com

Copyright 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

ARTICLE

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Age Hardening of AlCrMoNiTi High Entropy Alloy Prepared by Powder Metallurgy Ren Bo1, 2, Ma Jianhui1, Zhao Ruifeng1, Guan Shaokang2, Zhang Hongsong1 1 Henan Institute of Engineering, Zhengzhou 451191, China; 2 Zhengzhou University, Zhengzhou 450052, China

Abstract: The AlCrMoNiTi high entropy alloy was prepared by powder metallurgy process. Its microstructure and hardness of as-cast and annealed state were studied. The results show that the as-cast alloy exhibits a mixture of the dendrite (Cr, Mo)-rich bcc phase and the interdendrite (Al, Ni, Ti)-rich fcc phase. The aged alloy can obtain a peak hardness HV of 6150 MPa at 900 oC, and then anneal softening occurs at 1000 oC, but its hardness HV still maintained at a high level of 5160 MPa. This shows that the AlCrMoNiTi alloy exhibits a good high-temperature age hardening performance. Age hardening of the alloy is mainly attributed to the grain refinement strengthening at 800 oC and the precipitation hardening of the second phase (bcc2) at 900 oC. The disappeared second phase and grain coarsening are the main reasons for the softening anneal at 1000 oC.

Key words: high entropy alloy; powder metallurgy process; solid solution; microstructure; age hardening

In the 1990s, Yeh and Cantor suggested a novel concept about alloy design, namely multi-principal high entropy alloys (HEAs) composed of at least five principal elements in equimolar or near-equimolar ratios[1,2]. HEAs have many interesting characteristics as a result of their multi-principal- element mixtures, such as high mixing entropy, lattice distortion, sluggish diffusion, and cocktail effect, which are the core factors influencing their microstructures and properties[1]. Compared with conventional alloys, HEAs usually have simple microstructures and do not tend to form complex intermetallic compounds, but to form amorphous and nano-crystalline[3-6]. In terms of performances, they have high hardness and superior resistance to temper softening, oxidation, wear, and corrosion[7-11]. Therefore, HEAs have attracted much attention from the researchers[7-14]. Ranganathan regarded the HEAs, bulk metallic glass and rubber metal as the most three breakthroughs of alloying theory during recent decades[15].

At present, the study on the HEAs is still in the exploratory stage. Some scholars have already studied the HEAs as viewed from the elements and contents, preparation process,

structures and properties, but there is still some knowledge on the HEAs required to be known. In the past, the preparation methods of HEAs were mainly casting process and mechanical alloying[16-22]. The alloys prepared by the traditional cast technique have many structural defects such as voids, porosity, low as-cast hardness. Recently, the AlCrFeNiCoCu and AlNiCrFexMo0.2CoCu high entropy alloys have been successfully prepared by powder metallurgy process[23,24]. The alloys present not only simple solid solution structures, but also high hardness, good plasticity, excellent corrosion resistance and compression performance. In the present work, the powder metallurgy technique was also adopted to fabricate the AlCrMoNiTi high entropy alloy. The microstructure and hardness of the as-cast and the as-annealed were investigated to provide a reference for its further study and application.

1 Experiment A high entropy alloy (HEA), AlCrMoNiTi, was prepared by

powder metallurgy which was composed of mechanical milling and vacuum hot-pressing sintering. The Al, Cr, Mo, Ni,

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and Ti elemental powders with purity higher than 99.9 wt% and particle size 45 m were mechanically alloyed in equiatomic ratio. The milling was carried out up to 12 h in high energy horizontal type ball mill at 200 r/min with a ball to powder weight ratio of 10:1. High performance stainless steel vials and balls were used as the milling media. Alcohol was used as the process controlling agent. To avoid oxidation of powder, purge argon was filled to the stainless steel vials for about 5 min. A graphite mould coated by CBN coating was put in vacuum induction hot pressing furnace (ZT40-20Y) to sinter the samples under argon atmosphere, the heating rate was 10 oCmin-1, and the pressure with 1.5104 N load was conducted during the whole procedure. The samples were kept at 1200 oC for 2 h, and then down cooled to room temperature slowly. Ultimately, an alloy ingot with size of 62 mm8 mm was acquired. The samples with a size of 7 mm7 mm5 mm were cut from the alloy ingot by wire cut electrical discharge machining (WEDM) and heat treated for 12 h at 700~1000 oC in a chamber electric furnace.

The hardness was measured using a Vickers hardness tester (HVS-50) under a load of 98 N for 20 s. The average value was obtained by taking seven points for each sample. An XPert PROX diffractometer operated at 35 kV and 30 mA with a copper target was used to determine the phase composition of the specimens. The range of the scanning angle (2) was from 30o to 90o for aged alloys at a speed of 6omin-1. The aged specimens were observed under a scanning electron microscope (SEM, JEOL, JSM-6700F). The chemical composition of the aged alloys was analyzed by energy dispersive spectrometry (EDS) assembly in SEM.

2 Results and Discussion

2.1 XRD analysis The crystal structures of AlCrMoNiTi alloys at different

aging temperatures are presented by the XRD patterns, as shown in Fig.1a. Among these alloys, the as-cast alloy is composed of a mixed structure of fcc + bcc solid solution phase rather than intermetallic compounds. Ren and Zhang et al have found that the atomic-size difference, mixed entropy (Smix), and mixed enthalpy (Hmix) of the alloys have an important effect on the formation of solid solution phase[25, 26]. Zhang et al have proposed a criterion for forming the solid-solution phase, namely 1.1103 K, 6.6%. Here describes the effects of Smix and Hmix, while represents the comprehensive effect of the atomic-size difference in the n-element alloy. However, Ren et al considered that the alloy system will form simple solid solution when 2.77% and Hmix 8.8 kJmol-1. The values of Smix and Hmix can be calculated according to Ref. [27]. The parameters and can be expressed as follows:

m mix

mix

T S

H= , m m1

ni ii

T c (T )== (1)

Fig.1 XRD patterns (a) and diffraction peak shift (b) of the AlCrMoNiTi alloy

1

n 2i ii

c (1 - r /r)== , 1n i iir c r== (2) where Tm is the average melting temperature of n-element alloy, ci and (Tm)i are the atomic percentages and melting point of the ith component of the alloy, respectively, and ri is the atomic radius. The values of Smix, Hmix, Tm, , and of AlCrMoNiTi and other HEAs were calculated according to the formula (1) and (2), as shown in Table 1. Based on the inference of Zhang, it is obvious that and values of the alloys, such as AlCrMoNiTi, CuCrFeMnNi, VCuFeCoNi, and TiZrNbMo, are located in the area for forming solid-solution phase that is 1.1103 K, 6.6%. On the contrary, the AlTiVYZr and ZrTiVCuNiBe alloys have higher values than that of other alloys and exceed 6.6%. Therefore, the AlCrMoNiTi alloy is more inclined to form simple solid-solution phase rather than intermetallic compounds. However, only the CuCrFeNiMn alloy is appropriated for the criterion of Ren et al[26]. In comparison, the inferences of Zhang are more applicable[25].

From Fig.1a, it can also be observed that the aged alloys at 700 and 800 oC have a mixed structure of fcc + bcc. When the aging temperature is 900 oC, an additional phase of Mo-rich bcc2 phase (M phase) is found in the X-ray patterns. However, the intensity of diffraction peak of M phase decreases as the aging temperature is increased to 1000 oC. In these aged alloys, the phase structure has not obvious change. This indicates that the AlCrMoNiTi alloy exhibits an excellent thermostability.

In previous works[17,28], diffraction peak shift was observed

30 40 50 60 70 80 90

a

Inte

nsity

/a.u

.

F: fcc B: bcc1 M: bcc2

BF

FB

F B F 1000 C

900 C

800 C

700 C

As-cast

MM M

40 41 42 43 44 452/()

Inte

nsity

/a.u

.

M

B(110)

F(220)

F: fcc B: bcc1 M: bcc2

b

1000 C

900 C

800 C

700 C

As-cast

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Table 1 Structure and parameters of Smix, Hmix , Tm , , and for the HEAs

Alloy Smix/kJmol-1 Hmix/ kJmol-1 Tm/K /103K /% Phase ReferenceAlCrMoNiTi 13.38 20.32 1928 1.27 6.30 fcc+bcc This workCuCrFeMnNi 13.38 1.60 1716 14.35 0.92 fcc+bcc [17] VCuFeCoNi 13.38 2.24 1767 10.56 2.88 fcc [27] TiZrNbMo 11.53 2.50 2429 11.20 5.20 bcc [26] AlTiVYZr 13.38 14.88 1796 1.62 10.35 Compound [27]

ZrTiVCuNiBe 14.90 24.89 2176 1.30 11.27 Compound [27]

in the as-cast alloys, while a similar phenomenon was also observed in the aged alloys, as shown in Fig.1b. The peak position (2) and calculated lattice constants of the bcc1 and fcc phases are listed in Table 2. Compared with the diffraction peaks of the as-cast alloy, we can find that the diffraction peaks of bcc1 shift to the left in the aged alloys at 700~900 oC, and then shift to the right at 1000 oC. The diffraction peaks of fcc shift to the left at 800 oC and to the right at 1000 oC. As the ageing temperature increases, the lattice constants of bcc1 and fcc phase have a tendency of increase firstly and then decrease. Therefore, the diffraction peak shift results from the differences of lattice constants. 2.2 Microstructure analysis

Fig.2 shows the microstructures of AlCrMoNiTi high entropy alloy of the as-cast and 900 oC annealing conditions. It can be seen that the typical dendrite and interdendrite structures are presented in the as-cast and aged alloys. From Fig.2b, some particles are observed in the interdendrite regions. Three typical regions marked as 1, 2, and 3 are shown in the magnified pictures (Fig.2b and 2d). The chemical composition of these three regions has been analyzed and the result is listed in Table 3. From Table 3, more Cr and Mo elements are found in the region 1, compared with the regions 2 and 3. In region 2, there is a higher content of Al element. However, more contents of Al, Ti and Ni distribute in region 3. Combined with the XRD results, it can be inferred that the dendrite structure is the (Cr, Mo)-rich bcc phase, while the interdendrite structure is the (Al, Ni, Ti)-rich fcc phase. The formation of (Cr, Mo)-rich bcc phase may be related to the high melting point of Cr and Mo elements. During the solidification process of as-cast alloy, Cr-Mo solid solution will precipitate prior to other phases and is distributed in the dendrite structure. This phenomenon is similar to the AlCoCrFexMo0.5Ni alloy[29]. The formation of (Al, Ni, Ti)-rich

fcc phase is mainly attributed to the mixing enthalpy between these elements. From Table 4, the mixing enthalpy of Ti-Al and Ti-Ni are 30 and 35 kJ/mol, respectively and far below that of others. As a result, the (Al, Ni, Ti)-rich fcc phase is formed in the interdendrite region. Compared with the as-cast alloy, there is no significant change in the microstructure of the annealed alloy at 900 oC (Fig.2c and 2d). The XRD results also show that the aged alloys have a stable structure. 2.3 Age hardening

Fig.3 shows the effect of aging temperature on the hardness of the AlCrMoNiTi alloy. It can be seen that the hardness HV of as-cast alloy is 4470 MPa. As the aging temperature increases, the hardness increases linearly from room temperature to 800 oC and then increase to peak hardness of 6150 MPa at 900 oC. When the aging temperature is 1000 oC, the hardness HV decreases to 5160 MPa, indicating that anneal softening of the alloy takes place, but the hardness is still maintained at a high level. From Fig.1 and Table 2, it can be seen that the structure of the aged alloy has no change below 900 oC but the crystalline sizes of the bcc1 and fcc phases gradually decrease. This indicates that the improved hardness should be ascribed to the grain refinement strengthening. When the aging temperature is 900 oC, the crystalline size of bcc1 phase increases to 28.3 nm, which would result in the decreased hardness according to Hall-Petch relationship[30,31]. Fig.1b shows that the second phase (bcc2) precipitates in the annealed alloy and will result in the precipitation hardening. However, the hardness HV of the aged alloy still increases to 6150 MPa. This demonstrates that the precipitation hardening plays a leading role in the increased hardness. When the aging temperature is 1000 oC, the second phase disappears and grain coarsening occurs, resulting in the decreased hardness. Therefore, the optimal age hardening occurs at 900 oC.

Table 2 Peak position (2), lattice constant, and crystalline size of the bcc1 and fcc phases of the aged alloys

bcc1 phase fcc phase Alloy state

2/() Lattice constant/nm Crystalline size/nm 2/(o ) Lattice constant/nm Crystalline size/nm As-cast 41.902 0.347 18.1 43.351 0.5900 73.0

Aged-700 oC 41.875 0.349 15.3 43.352 0.5900 28.4 Aged-800 oC 41.750 0.358 10.0 43.209 0.5919 27.2 Aged-900 oC 41.821 0.353 28.3 43.361 0.5899 12.8

Aged-1000 oC 42.017 0.339 23.1 43.533 0.5877 51.1

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Fig.2 Microstructure of AlCrMoNiTi alloy: (a, b) as-cast and

(c, d) annealed at 900 oC

Table 3 EDS results of the AlCrMoNiTi alloy in Fig.2 (at%) Region No. Al Ti Cr Ni Mo

1 2.4 7.6 40.1 2.2 47.7 2 39.3 18.9 7.3 26.6 7.9 Fig.2b 3 17.9 25.5 2.4 50.8 3.3 1 2.0 9.8 39.9 1.7 46.7 2 23.0 26.6 2.0 47.6 0.8 Fig.2d 3 42.8 35.3 3.7 14.8 3.4

Table 4 Mixing enthalpy of binary liquid alloy of the alloying

elements in HEAs (kJ/mol)

Element Al Cr Mo Ni Ti

Al 0 10 5 22 30

Cr 0 0 7 7

Mo 0 7 4

Ni 0 35

Ti 0

Fig.3 Hardness of the aged alloy

3 Conclusions 1) The as-cast AlCrMoNiTi alloy is composed of a mixture

of the dendrite (Cr, Mo)-rich bcc phase and the interdendrite (Al, Ni, Ti)-rich fcc phase.

2) The aged alloy can obtain a peak hardness HV of 6150 MPa at 900 oC, and then anneal softening occurs at 1000 oC. The AlCrMoNiTi alloy exhibits a good high-temperature age hardening performance.

3) Age hardening of the alloy is attributed to the grain refinement strengthening at 800 oC and the precipitation hardening of the second phase (bcc2) at 900 oC. The disappeared second phase and grain coarsening are the main reasons for the anneal softening.

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