Diffraction, microstructure and thermal stability analysis in a double phase nanocrystalline Al₂₀Mg₂₀Ni₂₀Cr₂₀Ti₂₀ high entropy alloy

1 Introduction

Alloying is the greatest gift of metallurgy to mankind. The development of new alloy is always based on microstructure modification, alloying addition and/or surface modification. The conventional strategy for developing alloys is to choose one or two elements as the main factors for primary attributes and other minor elements as alloying addition for modifying microstructure and properties [1]. Conventional metallurgical knowledge suggests more than one principal element involved in alloying leads to complex phase formation and poor mechanical properties. But Yeh et al. suggested that HEAs composed of more than five principal elements usually possesses simple solid solutions and amorphous phases rather than intermetallics [2]. The term “high entropy” in HEAs originates from the resultant high entropy of mixing (>1.61 R, where R is a gas constant) when more than five elements are mixed in equiatomic ratio [3]. By designing the composition of the principal elements appropriately, the HEAs can be tailor made to suit the required properties. Since the time of their evolution in 2004 [4], multicomponent high entropy alloys (HEAs) have engrossed the consideration of research groups across the globe due to their excellent properties [5] and because of the fact that they are radically different compared to conventional alloys [6]. On the other hand, if one adheres to the strict definition of HEAs containing at least five principal-elements with 5–35 at.% concentrations for each elements, these two particular chemistries should not be classified as HEAs [7]. Solid solutions with multiprincipal elements have been normally found to be more stable than inter-metallic compounds at high temperatures due to their large entropy of mixing, which lead to the sluggish diffusion of atoms [8]. Slow diffusion in HEAs is thought to be a direct consequence of the complex arrangement of atoms of diverse elemental species [9]. In reality, HEAs have haggard more and more consideration due to their stupendous mechanical characteristics besides other attractive properties like large saturation magnetization [10]. The mechanical properties of HEAs usually depend on the type of crystallographic lattice [11]. HEAs have been reported to exhibit superior properties such as excellent thermal stability, high strength, high temperature resistance, better wear and oxidation resistance [12]. HEAs are being considered for a wide range of functional and structural applications [13].

2 Material selection and processing

3 Experimental procedure

Metal powders of Al, Mg, Ni, Cr, Ti with purity 99–99.8% (Alfa Aesar, Lancashire, UK) and particle size ≤45 μm were used as a starting material. The equi-atomic ratio for the above defined composition is established using analytical relationship for molar and atom conversion by trial and error method. The metal powders were mixed as per the established equi-atomic ratio and then milled in a planetary ball mill (Fritsch Pulverisette – P5 High Energy Ball Mill, Fritsch Asia-Pacific Pte. Ltd., Singapore) for 15 h at 300 rpm. Stainless steel vials and stainless steel balls were used as milling media with ball-to-powder weight ratio of 10:1. Toluene (Spectrum Chemical Mfg Corp., New Jersey, USA) was used as a process controlling agent (PCA) in order to eliminate cold welding as well as a reducing medium to avoid oxidation of the alloy. During milling, powder samples were taken out in every 5 h intervals (5, 10, 15 h) for further characterization and consolidation. The milled powder samples of different intervals (5, 10, 15 h) were also analyzed by XRD using X’Pert Pro panalytical instrument (PANalytical B.V., The Netherlands) with Cu kα radiation for monitoring phase formation and also to estimate the crystallite size and lattice strain. DSC (NETZSCH-DSC, NETZSCH Instruments North America, MA, USA) study was carried out in the 15 h milled sample till 800°C in Argon Atmosphere at a heating rate of 10 k/min to understand the thermal stability of alloy based on endothermic and exothermic curves. Subsequently the 15 h milled powder was consolidated by spark plasma sintering (Dr Sinter SPS-2500, Fuji Electronic Industrial Co., Ltd., Japan) in a 20 mm inner diameter graphite die at 800°C for 10 min with a heating rate of 100 k/min and by applying an axial pressure of 50 Mpa. The sintered sample of ∅ 20 mm×10 mm long was further studied under XRD using X’Pert Pro panalytical instrument with Cu kα radiation to monitor the phase evolution after sintering process. The diamond polished sintered sample was characterized using SEM in both scattered electron (SE) and back scattered electron (BSE) mode to observe the particle size, morphology and phase differentiation. The composition of the alloy was characterized using EDS equipped with SEM to observe the chemical homogeneity. Density of sintered sample was measured using Archimedes Principle. Finally, the hardness was measured on the sintered sample by applying a load of 1 kg with a dwell time of 10 s in Vickers Microhardness Tester (SHIMADZU Corporation, Japan). Hardness values reported are from an average of 10 readings on both sides of the sintered sample.

4 Results and discussion

The XRD analysis, DSC curves, SEM micrographs, EDS analysis and hardness measurements reported here are the characteristic results of the composition AlMgNiCrTi formulated in the present study. All the results presented here are observed to be consistent on both sides of the samples thus ensuring the homogeneity and uniformity of the dense samples.

4.1 Mechanical alloying

4.1.1 X-ray diffraction analysis

The XRD patterns of equiatomic HEA powders produced under different milling time was shown in Figure 1.

Figure 1:

XRD patterns of AlMgNiCrTi HEA powder during different milling times.

It is observed that after 5 h milling, the small elemental diffraction peaks still appeared and depicts that long milling time would be needed. With increase in milling time, peak broadening is obvious and small elemental diffraction peaks become invisible after 10 h. The disappearance of diffraction peak is attributed to crystal refinement, lattice distortion and solid solution [15]. As the milling time increases to 15 h, only the most intense diffraction peaks can be clearly seen, which indicates the complete formation of solid solution structure. During the milling process, the decrease in intensity and peak broadening may result from the three following factors: diminishing crystallite size, refined crystal structure and high lattice strain [15], [21], [22]. A single BCC (designated as B) phase was observed in AlMgNiCrTi HEA after 15 h of milling as shown in Figure 1. The evolution of BCC phase after 15 h of milling can be attributed to the presence of strong BCC promoting elements such as Al, Cr and Ti and because of the fact that Cr being the highest melting point element, the diffusivity of Cr will be lower than the other elements. From the relative diffusivity and peak intensity, it may be inferred that BCC phase evolves by dissolution of Mg and Al followed by Ni and Ti into Cr lattice. The crystallite size (CS) and lattice strain (LS) of BCC phase after different milling times have been observed from the XRD data (Scherrer’s formula based observation) which are presented in Table 1. It can be inferred that the crystallite size is refined as the milling time increases and reaches 5 nm after 15 h of mechanical alloying.

Table 1:

Crystallite size (CS) and lattice strain (LS) of equiatomic AlMgNiCrTi high entropy alloy powder with milling time.

Milling time (h)	CS (nm)	LS (%)
5	12	0.89
10	7	1.52
15	5	2.13

4.1.2 Thermal stability studies of 15 h milled AlMgNiCrTi powders

Figure 2 illustrates the DSC traces of 15 h milled AlMgNiCrTi powders. DSC is a thermo analytical technique in which the amount of heat required to increase the temperature of the given sample and the reference is measured as a function of temperature. This analysis was carried out under argon atmosphere with a heating rate of 10 k/min. The stability of temperature is based on the endothermic and exothermic reactions. From Figure 2 it is very much evident that there is no endothermic curve and one or two exothermic curves till 800°C, and as a result the sintering of the composition AlMgNiCrTi can be done ≥800°C, even though the melting point of Al and Mg is 660°C and 650°C, respectively. Hence the stated alloy composition is thermally stable until 800°C.

Figure 2:

DSC curve of 15 h milled AlMgNiCrTi HEA powders.

4.2 Consolidation of the composition by spark plasma sintering (SPS)

4.2.1 Phase evolution after SPS

Recent studies on SPS have shown that applying a maximum pressure at higher temperature helps in better bonding between the powder particles and improved ductility [16]. In adding together, the higher sintering temperature (800°C), which was close to the melting point of the HEA and longer dwelling time (10 min) could ensure fully homogenization of alloying elements in the HEA [23]. The 15 h milled AlMgNiCrTi HEA powder was consolidated by SPS at 800°C with a pressure of 50 Mpa in argon atmosphere for 10 min. Figure 3 shows the XRD pattern of AlMgNiCrTi after SPS at 800°C.

Figure 3:

XRD patterns of AlMgNiCrTi HEA after SPS at 800°C.

A detailed analysis of the XRD pattern of AlMgNiCrTi composition after sintering illustrates that a BCC phase along with Titanium carbide (FCC) phase was formed. It is remarkable to note that there is not much increase in the crystallite size after sintering. It is also important to note that BCC phase evolves by dissolution of Mg and Al followed by Ni and Ti into Cr lattice, while Titanium carbide (FCC) phase were not observed in the milled condition. The FCC phase (111), (200), (311) in the sample was only observed after sintering. It is quite possible that carbon impurities from milling medium would have combined with Titanium leading to formation of Titanium carbide. In principle, in HEAs there is a possibility of formation of σ phase when Cr along with Ni is present in the alloy [2]. Only when two BCC structures coincide, σ phase arises but in the present study it is observed that only one BCC structure evolved and as a result there is no formation of σ phase. The crystallite size and lattice strain of AlMgNiCrTi HEA after sintering were 6 nm and 1.559%, respectively.

4.2.2 Microstructure analysis

Figure 4 shows the BSE image of the sintered sample of AlMgNiCrTi. Two phase contrast was observed clearly in the BSE image of AlMgNiCrTi, which agree with the two phases (BCC and FCC) observation in the XRD pattern (Figure 3). The homogeneity of chemical composition of all high entropy alloys has been confirmed by SEM equipped EDS analysis [24].

Figure 4:

SEM-BSE image of AlMgNiCrTi HEA after SPS at 800°C.

Figure 5 shows the EDS spectrum obtained from the sintered sample of AlMgNiCrTi after sintering. The nominal composition of each element in this alloy is 20 at %, and the quantitative elemental analysis results from EDS spectrum in Figure 5 clearly indicates that the homogeneity and the equiatomic composition is maintained in each particle of the alloy after sintering.

Figure 5:

EDS spectrum and quantitative analysis of AlMgNiCrTi HEA after SPS at 800°C.

The relative density of sintered sample is 98.23% which is obtained through Archimedes principle. Low density range of 4.3312 g/cm³ was achieved when compared to lightweight Al₂NbTi₃V₂Zr high entropy alloy (5.25 g/cm³) [25]. The micro hardness of sintered sample is 710±20 HV which is measured using Vickers’s Micro Hardness Tester. The hardness value suggests that this nano-crystalline multi-component solid solution have higher strength when compared to Al alloy grade series which ranges from 72 HV to 189 HV. It is also having higher strength than the commercial alloy Ti6Al4V hardness of which is 349 HV. According to Kruschov’s Conclusion [22] the wear resistance of the material is in general proportional to their Vickers’ Hardness. Therefore according to this conclusion, the nanocrystalline AlMgNiCrTi equiatomic HEA will have very good wear resistance.

5 Conclusion

• The nanocrystalline equiatomic AlMgNiCrTi high entropy alloy has been successfully synthesized by mechanical alloying and consolidated by spark plasma sintering at 800°C with 50 Mpa pressure.

• Simple structured BCC solid solution with grain size <10 nm was obtained after milling which maintained its crystallite size after sintering also.

• The nanocrystalline high entropy alloy AlMgNiCrTi is stable even after sintering at 800°C. The BCC solid solution phase started to evolve along with the FCC phase above 700°C.

• The formation of two phases (BCC and FCC) in this AlMgNiCrTi alloy produces a synergy which makes it suitable for both high strength and high temperature applications.

• Nanocrystalline AlMgNiCrTi equiatomic HEA due to it is high hardness will have very good wear resistance.

• The very high hardness of 710±20 HV is due to solid solution strengthening and ultrafine grains of BCC structures that demonstrates the potential future of the alloy which will find its’ use in wide applications requiring high strength, high temperature, good wear resistance etc.