Spark Plasma Sintering Route Design for Sustainable Fabrication of Corrosion-Resistant BCC Al₁₅Cr₁₅Fe₃₀Ni₃₀Mn₁₀ Components

Present Address:

NicholusMalatji1

RudolfKanyane1✉EmailLrkanyane@gmail.com

MxolisiShongwe1

VincentTau2

1Department of Chemical, Metallurgical and Materials EngineeringTshwane University of TechnologyP.M.B. X680PretoriaSouth Africa

2Technology Innovation AgencyTechnology Station in Chemical, Tshwane University of TechnologyGa-RankuwaSouth Africa

Nicholus Malatji¹, Rudolf Kanyane^1*, Mxolisi Shongwe¹ and Vincent Tau²

¹Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, P.M.B. X680, Pretoria, South Africa

²Technology Station in Chemical, Technology Innovation Agency, Tshwane University of Technology, Ga-Rankuwa, South Africa

Corresponding Author: (Rudolf Kanyane) Lrkanyane@gmail.com

Abstract

Material failure due to corrosion has adverse impacts on the economy and, if unattended, usually results in disastrous, unwanted events. Owing to this, the current materials used in aggressive environments depend on protective coatings to prevent degradation. An emerging group of alloys named ‘High entropy alloys’ (HEAs) has been proposed as a suitable material for this application and there is limited research on the performance of these alloys under aggressive conditions. In this work, Al₁₅Cr₁₅Fe₃₀Ni₃₀Mn₁₀ HEA is synthesized via spark plasma sintering (SPS), and the influence of sintering parameters on microhardness, densification, and corrosion behavior is investigated. The attained results showed that the microhardness obtained from the sintered sample was slightly higher than that of stainless steel and Ti64 traditional alloys currently used in corrosive environments. It was also observed that sintering parameters had a huge effect on the evolving microstructure and corrosion attributes and the findings are fully discussed in this paper.

Keywords:

High Entropy Alloy (HEA)

Spark Plasma Sintering (SPS)

Microhardness

Corrosion

INTRODUCTION

High entropy alloys (HEAs) are an emerging metallic alloy that are often regarded as highly capable structural materials in high-temperature applications. They are considered to be solid solution alloys with more than five principal elements in the equal to near equal atomic percentage range [1–4]. HEAs have come to the attention of the science community due to their characteristics, such as sluggish diffusion, high entropy effect, and cocktail effect. They are considered as one of the promising materials for structural applications as they have excellent ductility and they show potential resistance to wear and corrosion [5–7]. The HEA microstructures can be regulated by the optimization of process parameters of the fabrication technique used [8]. Due to the broad range of composition and the numerous alloy systems in HEAs, their mechanical strength significantly varies and the key contributing attributes are relative volume ratio of each composition phase, strength of each composition in the alloy and distribution or morphology of the composing phase [9, 10]. So far, extensive research has been done on HEAs produced through the vacuum arc melting casting technique. But at times, this fabrication route is problematic due to its limitation in the shapes of final products [11]. As a result, various synthesis approaches for HEAs have emerged as optional fabrication techniques. Mechanical alloying (MA) together with Spark plasma sintering is a convenient method that has successfully synthesised nanocrystalline materials, producing a uniform microstructure. The short-time consolidation of SPS is known to promote the inhibition of grain growth as well as control the microstructure, thus retaining the original grain size and consequently, better mechanical attributes for the produced fabrications [12–14]. According to Zhang, Zhou [15], HEAs are made attractive by attributes like superior hardness and strength, and wear and corrosion behavior. Attempts to optimize the properties of HEAs have been made with different alloys formed using different compositions of different elements to optimize different properties. The chosen elements used to fabricate the alloy under investigation have properties with the potential to improve microhardness and corrosion. Maulik, Kumar [15] studied the structural integrity of spark sintered AlFeCuCrMg HEAs, and from the attained results, it was observed that Mg content significantly increased the hardness, to an increased hardness value of 853 HV for AlFeCuCrMg_0.5, while the AlFeCuCrMg1.7 alloy dropped to 533 HV. The focus of this study is to fabricate novel Al₁₅Cr₁₅Fe₃₀Ni₃₀Mn₁₀ HEA by using the SPS technology, for future application in corrosive conditions. The use of this alloy in corrosive environments will reduce the problem faced by traditional alloys and reduce maintenance costs.

Experimental Procedure

To investigate the influence of sintering holding time and temperature, the spark plasma sintering technique was used to sinter the composition Al₁₅Cr₁₅Fe₃₀Ni₃₀Mn₁₀. The elemental powders, which were mixed for a period of 8 hours, were poured into a die that was covered with a graphite paper to avoid contact between the powders and the die. The die was pressed to eliminate the spaces between the particles to avoid porosity in the sintered part. Six samples were used for sintering, where for the first three, the following sintering conditions were used: a temperature of 800^°C, and holding times of 4, 8 and 12 minutes. The other three samples were sintered at the same holding times using a temperature of 900^°C. A heating rate of 100^°C and a pressure of 30 MPa were adopted for all samples to achieve a homogeneous distribution of different constituents within the powder. Powders were mixed in a cylindrical ball mill without grinding media for 8 hours to ensure homogeneous distribution of the input powders. This is also important so that the sintered sample achieves uniform density. Removal of the graphite layer that covered the sample after sintering was done by sandblasting the sintered HEA with silica sand. Removal of the graphite layer in the sintered samples is essential as this will affect the density and measured mass. Vickers hardness measurements were taken using Micro Met Scientific Vickers micro hardness testing equipment. The polished surface of each sample was randomly indented on the surface using a diamond indenter. The hardness test was conducted using an indentation load of 200 gf at a dwell time of 20 seconds. An average value of the five indentations was used as the hardness value of the material. The Autolab Potentiostat was used for corrosion testing to establish the linear polarisation of the sintered samples, which were tested in 5mol sulphuric acid. A scan rate of 0.001, with a start and stop potential of -1.5V and 1.5V respectively were used for the corrosion test.

Results and Discussion

Scanning electron microscope (SEM) results

Figure 1 presents the SEM micrographs of Al₁₅Cr₁₅Fe₃₀Ni₃₀Mn₁₀ HEA sintered at 800^°C for 4, 8 and 12 minutes. It was evident that the micrographs and attributes of the fabricated alloy are influenced by the processing parameters. The fusion of particles and the grain size of the alloy were found to be dependent on the holding time. According to Shongwe, Makena [16], holding time plays a critical role in the fusion of powders during sintering. Longer holding times increase heat flux and allow sufficient fusion to occur between the particles. The sample sintered at 800^°C for 4 minutes (Fig. 1A), displayed limited particle-to-particle necking, which suggests that there is more porosity between the grain boundaries. As the sintering time increased to 8 and 12 minutes, it can be observed that the particle-to-particle fusion was improved, as presented in Fig. 1B and C. This can be attributed to the fact that there is enough time for particle fusion at high temperature [17]. It was also noticed that the grain boundary area and the concentration of pores were reduced because of increased sintering time. During sintering, localized melting of particles at the point of contact occurs and uniaxial pressing of the compact causes the molten regions to flow and fill the pores. The presence of aluminium in the alloy composition may also enhance this phenomenon, leading to higher metallurgical bonding between the particles and improved densification. Figure 2 (A-C) presents HEA sintered at holding time of 4, 8 and 12 minutes at 900^°C. A similar behavior in relation to holding time was observed when compared to samples sintered at 800^oC. However, more particle necking and fusion were evident at the higher sintering temperature. The samples obtained at 900^oC were also characterized by grain coarsening and lower porosity when the holding time was increased. The combined increase in temperature and holding time promotes interdiffusion and grain growth. Two distinct phases (represented as bright and dark phases) were also evident in all the samples. According to the calculated VEC value, a dual phase structure with high content of BCC solid solution is favorable.

Fig. 1

SEM image of sample sintered at a temperature of 800^°C with holding times of a) 4 b) 8 and c) 12 minutes.

Fig. 2

SEM image of sample sintered at a temperature of 900^°C with holding times of a) 4 b) 8 and c) 12 minutes.

Fig. 3

Elemental constituents of the developed samples.

Phase Evolution Results

$\:{\varDelta\:H}_{mix}=\:4\sum\:_{i=1,\:j\ne\:i}^{n}{\varDelta\:H}_{ij}^{mix}{c}_{i}{c}_{j}$

$\:\delta\:=\sqrt{\sum\:_{i=1}^{n}{c}_{i}{\left(1-\frac{{r}_{i}}{\sum\:_{i=1}^{n}{c}_{i}{r}_{j}}\right)}^{2}}$

$\:{\Omega\:}=\frac{{T}_{m}{\varDelta\:S}_{mix}}{\left|{\varDelta\:H}_{mix}\right|}$

Where

$\:{T}_{m}=\:\sum\:_{i=1}^{n}{c}_{i}{\left({T}_{m}\right)}_{i}$

is the mean melting temperature and

$\:{\left({T}_{m}\right)}_{i}$

is the melting temperature of the component

$\:i$

. This parameter considers both

$\:\varDelta\:{H}_{mix\:}$

and

$\:{T}_{m}{\varDelta\:S}_{mix}$

. A phase formation rule for the formation of a solid solution is

$\:{\Omega\:}\ge\:1.1$

and

$\:\delta\:\le\:6.6\:\%$

. The calculated thermodynamic values of the mentioned empirical formulas are summarized in Table 1. Based on the obtained XRD patterns and analysis, the results are in harmony with the formation of the predicted phases of Al₁₅Cr₁₅Fe₃₀Mn₃₀Ni₁₀ HEA alloy.

Fig. 4

XRD patterns of HEA sintered at 900^°C and different holding times.

Table 1
Summary of prediction data.
𝛥S_mix (J/K⋅ mol)	𝛥H_mix (KJ/mol)	𝛿 (%)	T_m (K)	Ω	VEC
12.7	-9.06	4.65	1638.8	2.9	6.9

Densification Results

Figure 5 shows the density of HEA sintered at temperatures of 800^oC and 900^oC with holding time 4, 8 and 12 minutes.

Fig. 5

Effect of holding time on densification of the developed HEA.

From the densification results, sintering temperature of 800^oC shows density rise as the holding time increased. At a temperature 800^oC and holding time of 4 minutes, the density was found to be 86% which was lower compared to the sample sintered at 8 minutes and 12 minutes. The low-density temperature shows that the material has more porosity. These results can be confirmed using SEM macrographs of sample presented in Fig. 1 and Fig. 2. High sintering temperature presented high densification as a result of high fusion rate, which resulted in better particle-to-particle necking [16, 24, 25]. A large amount of porosity was shown in the sample sintered at a lower temperature and a small holding time.

Microhardness Results

Figure 6 presents the microhardness results of high entropy alloys sintered at temperatures of 800 and 900^oC with the holding time of 4, 8 and 12 minutes. The results revealed that holding time and sintering temperature play a significant role in the evolution of the mechanical characteristics of the alloy. Holding time and sintering temperature showed a proportional relationship with the hardness of the alloy. The lowest hardness value of 190.5 HV was obtained for a sample sintered at 800^oC and 4 minutes, while the one fabricated at 900^oC and 12 minutes possessed the highest hardness value of 399 HV. Mechanical behavior of SPSed samples is influenced by the microstructure, particle bonding and densification. According to [27], a high diffusion rate in the alloying system is favored at high temperatures, leading to improved bonding among the particles and yielding samples with low porosity and better mechanical performance. The presence of a high proportion of the BCC phase in the alloy also contributes to the higher hardness values obtained. This phase is associated with high hardness and its stability improved with increasing temperature as revealed from the SEM and XRD results.

Fig. 6

Microhardness - the influence of holding time and sintering temperature.

Electrochemical Results

Figure 7 represents the potentiodynamic polarization curves of the sintered high entropy alloy fabricated at 800^oC while being held at 4, 8 and 12 minutes. The linear polarization behavior was studied using 0.5M H₂SO₄. From the plot, it is evident that there is a negative shift in polarization potential and an increase in current density when the sintering holding time is increased. The sample fabricated at a holding time of 4 minutes exhibited a potential of -0.464 V and current density of 2.967 x 10^− 4A/cm², while the sample obtained at a holding time of 12 minutes had a potential of -0.483 V and current density of 8.881 x 10^− 3A/cm². Polarization resistance was also noticed to decrease with increasing holding time, as shown in Taffel data in Table 2. These results suggest that the corrosion resistance of the samples deteriorated with increasing holding time. The results deviate from what other authors have reported since better polarization characteristics were synergistic to improved densification [17, 18]. However, it should be noted that the passivation characteristics of the samples were different when the holding time was increased. Fluctuation of current density in the passive region of the polarization curve was noticed at a lower potential (around 0.2 V) for the sample sintered at 4 minutes, as compared to the other samples (fluctuation of current started around 0.5 V). The variation in current density suggests that the formed passive film of the alloy sintered at a holding time of 4 minutes is not stable and tends to expose fresh surfaces to further corrosion. The coupling of this characteristic with the low densification of the sample further indicates that more surface area of the sample is available to interact with the corrosive medium [28–30]. Hence, the corrosion rate (see in Table 2) of the samples fabricated at higher holding times may be ascribed to this phenomenon. The presence of Cr, Ni and Al in the alloy is believed to be responsible for the formation of the protective film. These elements form oxide scales that are stable in harsh acidic environments. However, the localized degradation of the protective layer causes pitting to occur on the surface of the alloy.

Fig. 7

Linear polarization curves of the sintered HEA fabricated at 800^°C while being held at 4, 8 and 12 minutes.

Table 2
Tafel data of sintered HEA at 800 degrees Celsius
Effect of Holding Time (Min)	E_corr, (V)	j_corr (A/cm²)	Corrosion rate (mm/year)	Polarization resistance (Ω)
4	-0.4604	0.0002967	0.036128	1437.31
8	-0.48208	0.0079274	0.036701	1213.86
12	-0.48192	0.0088811	0.022905	1191.19

Potentio-dynamic polarization behavior of sintered high entropy alloy samples produced at a temperature of 900^oC and holding time of 4, 8 and 12 minutes in 0.5M H₂SO₄ solution is shown in Fig. 8. The alloy exhibited similar electrochemical characteristics to those produced at 800^oC. Even though there was a slight positive shift in polarization potential, the current density was lower than that of the samples fabricated at 800^oC. The passive region of the alloy fabricated at 900^oC and holding time of 4 minutes was characterized by fluctuation of current from a potential of -0.1 V, signaling instability of the protective film. The use of higher holding times at this temperature yielded samples with more stable protective films since no current fluctuations were recorded in the cathodic region. The formation of stable oxide layers can be attributed to the low corrosion rates and high polarization resistances associated with the samples, as shown in Table 3. The passive layer formed creates a barrier between the corrosive environment and the surface of the alloy to prevent interaction. Low porosity also contributes to this behavior since the penetration of corrosive into the alloy is minimized [18]. Maximum densification of 96% was achieved at a sintering temperature of 900^oC as compared to 92% obtained for the alloy sintered at 800^oC.

Fig. 8

Linear polarization curves of the sintered HEA developed at 900^°C while being held at 4, 8 and 12 minutes.

Table 3
Tafel data of sintered HEA at 900 degrees Celsius
Effect of Holding Time (Min)	Ecorr, (V)	jcorr (A/cm²)	Corrosion rate (mm/year)	Polarization resistance (Ω)
4	-0.44114	0.004264	0.0020532	1993.55
8	-0.49174	0.0095323	0.0039506	1630.9
12	-0.4768	0.008912	0.00350719	1885.51

CONCLUSIONS

Al₁₅Cr₁₅Fe₃₀Ni₃₀Mn₁₀ HEA was successfully synthesized by means of park plasma sintering technology, from which the researchers drew the following conclusions:

EDS results showed elemental constituents of all the powders used in developing the HEA.

The sintering tempering temperature and holding time were not enough to create a good fusion between the powder, as presented by the SEM images.

The density results show that the material was not dense enough since it has a lot of porosity. Sintering done at 900^oC and 12 minutes had the highest densification of 96%.

A maximum microhardness value of 399 HV was evident at a holding time of 12 minutes and sintering temperature of 900^oC.

The sample sintered at a holding time of 4 minutes and a temperature 900^oC presented better corrosion properties with a minimal corrosion rate of 0.0020532 mm/year.

Acknowledgements

The authors gratefully acknowledge Surface Engineering Research Centre (SERC), the Tshwane University of Technology, Department of Chemical Metallurgical and Materials Engineering, Pretoria, South Africa.

Conflict of Interest Disclosure

The authors have no conflict of interest to declare for this manuscript.

Consent for publication

Not applicable.

Ethics approval

statement

Not applicable.

Funding statement

Not applicable.

Data availability statement

The datasets used during the present study are available from the corresponding author on reasonable request.

Author contributions

N.M was responsible for materials synthesis and processing. R.K was responsible for data analysis and interpretation. M.S was responsible for conceptualization and experimental design. V.T was responsible for manuscript preparation and review.

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