Corrosion and thermal behaviour of AlCr1.5CuFeNi2Tix high-entropy alloys

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ScienceDirect Materials Today: Proceedings 5 (2018) 17073–17079

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AMPCO-2017

Corrosion and thermal behaviour of AlCr1.5CuFeNi2Tix high-entropy alloys Vikas Kukshala,*, Amar Patnaikb, I. K. Bhatc a

Mechanical Engineering Department, NIT, Uttarakhand-246174, India b Mechanical Engineering Department, MNIT Jaipur-302017, India c Applied Mechanics Department, MNNIT Allahabad-211004, India

Abstract The high entropy alloys (HEAs) are capable to form simple solid solution and hence are the most favourable choice for high temperature applications. Till now, the research on HEAs is based on exploring the various properties of equimolar or near equimolar alloys consisting of almost similar elements. However, the corrosion behaviour of most of the novel high–entropy alloys is yet to be studied. The purpose of this study is to investigate the effect of Ti on the corrosion and thermal behaviour of AlCr1.5CuFeNi2Tix (x = 0, 0.25, 0.5, 0.75,1 in molar ratio) alloys. The AlCr1.5CuFeNi2TixHEAs samples were cast by high temperature vacuum induction furnace under inert gas environment. The electrochemical-corrosion behaviour of the alloys immersed in 3.5 wt.% NaCl solution was studied by potentiodynamic-polarization method. The results show that the addition of Titanium improves the corrosion resistance of as-cast alloys. The minimum and maximum value of corrosion rate is found to be 0.068 and 0.300 mmpy for Ti0 and Ti0.75 respectively. The thermal conductivity of the alloy was determined by hot disk thermal constant analyser at various temperatures varying from 300 K to 500 K. The thermal conductivity of the alloy decreases with the increase in Titanium content but on the contrary the thermal conductivity increases with increasing temperature. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Advances in Materials & Processing: Challenges & Opportunities (AMPCO-2017). Keywords:High-entropy alloy; Titanium; Corrosion; Thermal conductivity

1. Introduction Prevailing metals and alloys are susceptible to their working environment affecting the life of the component to large extent. While Ni-based superalloys and variety of steels are extensively used for high temperature applications

* Corresponding author. Tel.: +91-963-470-6332 E-mail address:[email protected] 2214-7853© 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Advances in Materials & Processing: Challenges & Opportunities (AMPCO-2017).

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in the different industries, still they suffer from the limitations of temperature and corrosion. The prevention of existing metals and alloys from high temperature corrosive environment is still a massive challenge in the field of materials. There is an urgent need of such materials that can provide an alternative for the replacement of existing materials used in various components in the field of oil, mining industry and power plants industry. A novel class of materials known as high-entropy alloys (HEAs) or multi-component equimolar or nearequimolar alloys has emerged as a potential source for providing a number of solutions in various fields [1,2]. HEAs exhibit superior properties in comparison to the conventional metals and alloys such as high hardness [3-4], enhanced tensile strength [5-6], high compressive strength [7-8], excellent wear [9-10] and improved corrosion resistance [11-14].There have been few studies based on the equimolar or near equimolar high-entropy alloy system consisting of body-centered cubic (BCC) structures (Fe and Cr) and face-centered cubic (FCC) structures (Al, Cu and Ni) [15-18]. Although, the elements exhibits different structures but their alloys in the equimolar ratio form simple solid solutions consisting of mono-phase structures and hence improved properties due to high entropy. Therefore, corrosion behavior study of novel HEAs is crucial in order to understand its utility in the high temperature applications. Previous studies have reported the improved corrosion property of developed HEAs [1920] and similarly better thermal conductivity at high temperatures [21-22]. It is imperative to comprehend the thermodynamic behavior of the alloys in order to develop equimolar or near equimolar single phase solid solution alloys. The thermodynamic parameters assist in correlating the different properties of the alloys and prediction of behaviour in the adverse conditions. Zhang et al. [23] proposed three parameters to depict the behaviour of the various elements in a multicomponent alloy i.e. entropy of mixing (ΔSmix), mixing enthalpy (ΔHmix) and atomic size difference (δ) as defined by equation 1-3.

Smix  R  n Ci ln Ci i 1 n H mix    CC i 1,i  j ij i j 2    n C i 1 r / r i 1 i





(1) (2) (3)

Where R is the gas constant, n implies the number of elements in multi-component alloy, Ci and ri is the atomic % and atomic radius of ith component and the average of the atomic radius of all elements existing in the alloy is given as r  in1ci ri . There is a vast scope of study of various properties of the novel HEAs and specifically the corrosion and thermal behavior of the alloys. In the present work, AlCr1.5CuFeNi2Tix HEAs were developed with variation in the Ti content from 0 to 1 in molar ratio using high temperature vacuum induction casting. Further, the influence of Ti addition on corrosion behavior and the thermal conductivity of the alloys were studied. 2. Materials and methods The AlCr1.5CuFeNi2Tix (x = 0, 0.25, 0.5, 0.75,1 in molar ratio) HEAs samples were cast using high temperature vacuum induction furnace under inert gas environment. Solid granules of elements Al, Cr, Cu, Fe, Ni and Ti with purity (>99%) was used for the casting. The property of the elements is listed in Table 1. Approximately 800 g of raw materials were melted in a water-cooled chamber containing ceramic crucible under controlled argon atmosphere. The alloy was melted five times and simultaneously electromagnetically stirred in order to improve the chemical homogeneity. The composition of the alloys as found experimentally by Energy Dispersive Spectroscopy (EDS) as listed in Table 2.The AlCr1.5CuFeNi2Tix alloys are abbreviated as Ti0, Ti 0.25, Ti 0.5, Ti 0.75 and Ti where x = 0, 0.25, 0.5, 0.75 and 1.0, respectively for convenience.

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Table 1: Properties of elements used as raw material in the present study [24].

Element

Atomic radius (pm)

Pauling electronegativity

Valence electron concentration (VEC)

Al

143

1.61

3

Cr

125

1.66

6

Cu

128

1.90

11

Fe

124

1.83

8

Ni

125

1.91

10

Ti

146

1.54

4

Table 2: Energy Dispersive X-ray Spectroscopy (EDS) analysis of AlCr1.5CuFeNi2 high- entropy alloy.

Composition at.% wt.%

Al 19.58 10.23

Cr 16.93 17.04

Cu 14.70 18.09

Fe 15.07 16.30

Ni 33.73 38.34

The electrochemical corrosion test was performed at room temperature on sample of size 10 mm × 10 mm by potentiodynamic-polarization measurement using 3.5 wt.% NaCl solution. The specimens were finished using various grit of SiC paper and then polished with 1µm diamond suspension. In the corrosion test technique (GAMRY Reference 600TM), the measurements was recorded at a scan rate of 0.5 mVs-1 in the range of -1.5 V to 0.5V. The experiment is performed with three electrodes i.e. Specimen, saturated Calomel and platinum electrode. The electrochemical impedance spectra was attained at the open circuit potential in the frequency range of 105 – 10-2 Hz with amplitude of 5mV.All the samples were dipped in NaCl solution (pH ≈ 6) for one and half hour before the test, allowing the system to reach equilibrium with the electrolyte exposing the sample of an area of 1 cm2 to the solution. The corrosion rates for each alloy can be calculated using the following expression (4). K

=

K. EW ρ

(4)

where: icorr = Corrosion current density (A cm-2 ) ρ = mass density (g cm-3), EW= Equivalent weight of the electrode (g), k (constant) = 3272 mm/(A-cm-year)), Kcorr= Corrosion rate in millimetres per year (mmpy). The thermal conductivity of alloys was determined by hot disk method using thermal constant analyzer (Hot Disk TPS 500, Gothenburg, Sweden). A sample of dimension 25mm × 25 mm with 10 mm thickness was used for conducting the experiment. The thermal conductivity of the samples was measure at various temperatures ranging from 300 K to 500 K at a step size of 50 K.A hot disc was attached to the experimental setup in order to heat the sample. A sensor was placed between the two samples and thermal conductivity is calculated as the function of electrical power.

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3. Result and discussion 3.1 Phase evolution of AlCr1.5CuFeNi2Tix high-entropy alloy Fig.1 shows the XRD pattern of as cast AlCr1.5CuFeNi2Tix (x = 0, 0.25, 0.5, 0.75, 1) alloys. It is observed that the alloys are mainly composed of body-centered cubic (BCC) and face-centered cubic (FCC) structures. The AlCr1.5CuFeNi2 base alloy show more of FCC structure as compared to AlCr1.5CuFeNi2Tix which corresponds to the Cu matrix (FCC) solid solution consisting of other elements Al, Cr, Fe and Ni. At x = 0, the sharp peaks of FCC structure can be seen depicting the higher amount of Al, Cu and Ni while very small peaks of BCC structure are seen due to the presence of elements namely Fe and Cr. Peak shift from FCC to BCC structure is found from x = 0 to x = 1 in addition to the diminishing FCC peaks. The AlCr1.5CuFeNi2Ti shows almost equal distribution of BCC and FCC phase due to the presence of Titanium in the alloy. 3.2 Electrochemical corrosion properties Polarization curves of the AlCr1.5CuFeNi2Tix high-entropy alloys are shown in Fig. 2 and their electrochemical parameters are listed in Table 2. The polarization curves of Ti 0, Ti 0.25, Ti 0.5, Ti 0.75 and Ti almost overlap each other signifying similar behaviour with respect to the corrosion resistance. The elements present in the base alloy such as Cr already provide the corrosion resistant and hence addition of Ti further improves the corrosion properties of the HEAs. Conversely, Al present in the alloy depreciates the corrosion resistance by forming the unstable layer at the surface leaving it exposed to the surrounding environment. The elements (Cr and Ti) form a protective layer at the external surface of the alloys thus enhancing the corrosion resistance. Since the developed HEAs consists of duplex phase i.e. BCC+FCC [25], there is an improvement in the corrosion property of the alloys on addition of Ti. It can be found from the Table 3 that the corrosion resistance continuously increases on addition of Ti content till 0.75 molar ratio and there is a slight decrease in the value of corrosion resistance on further increasing the Ti content. This may be due to the formation of susceptible BCC phase in the alloy as compared to the FCC phase. The minimum and maximum value of corrosion rate is found to be 0.068 and 0.300 mmpy for Ti0 and Ti0.75 respectively. As compared to the previous studies [26, 27], the AlCr1.5CuFeNi2Tix shows a remarkable enhancement in the corrosion resistance properties.

Fig. 1. XRD pattern for AlCr1.5CuFeNi2Tix alloys.

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Fig. 2. Potentiodynamic-polarization curves of AlCr1.5CuFeNi2Tix HEAs in 3.5 wt. % NaCl at ambient temperature.

Table 3. Electrochemical parameters of AlCr1.5CuFeNi2Tix derived from the potentiodynamic polarization curves by linear fitting

High-entropy alloy

Corrosion rate (mmpy)

Designation

Ecorr(mV)

Icorr(μA/cm2)

AlCr1.5CuFeNi2Ti 0

Ti0

-231.9

7.974

0.068

AlCr1.5CuFeNi2Ti 0.25

Ti 0.25

-227

14.7

0.111

AlCr1.5CuFeNi2Ti 0.5

Ti 0.5

-243.6

28.14

0.214

AlCr1.5CuFeNi2Ti 0.75

Ti 0.75

-220.6

39.13

0.300

AlCr1.5CuFeNi2Ti

Ti

-372

33.64

0.259

3.3 Thermal conductivity This part of the study investigates the thermal behaviour of AlCr1.5CuFeNi2Tix HEAs. Thermal conductivity k(T) is a measure of thermal diffusion coefficient α(T), density ρ(T) and specific heats (T) of the material at temperature T [27]. It determines the capability of the alloys to conduct heat. Fig. 3(a) shows the graph between thermal conductivity and molar ratio x in AlCr1.5CuFeNi2Tix HEAs. It is found that the thermal conductivity of the alloys decreases with increase in the value of x. This is due to the dual phase FCC+BCC present in the HEAs. The phenomenon can be explained with respect to the larger scattering effect due to the addition of the Ti content and hence decrease in the thermal conductivity. As the temperature is increased, the mean free path of the electron increases and consequently the thermal conductivity of the alloy increases. The duplex phase is characterized by the more interface boundary as compared to the single phase resulting in the lowering of the heat transfer in the HEAs. Chou et al. found the similar observation while studying the thermal conductivity of AlxCoCrFeNi [28].

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26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10

24

300 K 350 K 400 K 450 K 500 K

0.0

0.2

0.4

0.6

0.8

1.0

Molar ratio of Ti

Fig. 3(a). Variation of thermal conductivity as a function of molar ration of Ti in AlCr1.5CuFeNi2Tix.

Ti 0 22

Thermal conductivity (W/mK)

Thermal conductivity (W/mK)

17078

Ti 0.25 Ti 0.5

20

Ti 0.75 Ti

18 16 14 12

300

350

400

450

500

Temperature (K)

Fig. 3(b). Variation of thermal conductivity of AlCr1.5CuFeNi2Tix as a function of temperature.

Conclusion Based on the above investigations, the following concluding remarks are listed from the study: (a) The AlCr1.5CuFeNi2Tix HEAs were successfully fabricated by the high vacuum induction casting machine resulting in the dual phase (FCC+BCC). (b) The corrosion resistance ofAlCr1.5CuFeNi2Tix HEAs is found to improve with the addition of Ti content at room temperature in 3.5 wt.%NaCl solution. The formation of the protective layer and the BCC structure of the system results in the improvement in the corrosion resistance.The minimum and maximum value of corrosion rate are found to be 0.068 and 0.300 mmpy for Ti0 and Ti0.75 respectively. (c) The thermal conductivity of AlCr1.5CuFeNi2Tix decreases with the increase in the molar ratio of Titanium whereas the thermal conductivity of the HEAs is found to increase with the increase in the temperature. References [1] [2] [3] [4] [5] [6] [7] [8]

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