Sintering and heat treatment of Al15Ti5Co35Ni25Fe20 high-entropy alloy

The new Al15Ti5Co35Ni25Fe20 high entropy alloy was obtained by the powder metallurgy from the elemental powders (-325 mesh). The obtained alloy should be characterized by the dual BCC + FCC phase microstructure according to the valence electron concentration calculations (VEC=7.9). The main aim of the research was to determine the optimal conditions for sintering and heat treatment of Al15Ti5Co35Ni25Fe20 high entropy alloy. The alloy was sintered for 40 minutes at 600 °C for the preliminary consolidation of the powders and initial diffusion of the material. Then samples were annealed at various temperatures: 700, 800, 900, 1000 °C for 1 hour. Samples after annealing were characterized by the XRD and SEM. For the chosen temperature the annealing time was elongated to 2, 3, 5, 10 and 20 hours. The chemical homogeneity was determined.


Introduction
The rapid development of modern technology includes high-entropy alloys (HEAs) which could be used instead of conventional metallic materials, which are mostly composed from one or two principal elements. HEAs are composed of at least five principal elements with concentrations of each between 5-35 at.%. Number of elements in alloys could be explained by the dependence between mixing entropy and enthalpy. It is considered that formation of solid solution phases in HEAs is possible when the mixing entropy is high enough to counterbalance the mixing enthalpy, what is observed when HEAs consist at least five principal elements [1][2][3][4][5]. HEAs usually form simple solid solution phases with FCC and/or BCC structures. The number of obtained phases is much lower than calculated by Gibbs phase rule, which directly results from effects of high entropy [6,7]. As atoms of each principal element can be located in all lattice positions with equal probability, the multielement lattices are highly disordered. Furthermore as metallic diameters of the elements are different, this intensifies the disorder effect. At least HEAs could be considered like an atomic-scale composite, where interactions between principal  [8][9][10].
As an alternative for mostly used methods of synthesis HEAs such like arc melting [11][12][13] and mechanical alloying or spark plasma sintering [14][15][16][17] investigated in this paper Al15Ti5Co35Ni25Fe20 alloy was synthesized by the use powder metallurgy.
2 Experimental Al15Ti5Co35Ni25Fe20 alloy (indexes represents atomic percent of elements) have been prepared from the metal base fine powders (~325 mesh; purity 99.9 at. pct.). Metal base powders have been mixed by 10 hours in the Turbula Shaker-Mixer under argon atmosphere. In order to intensify the mixing of powders the metal balls were used (ball to powder ratio 1:12). Mixed powders were pressed hydrostatically under pressure 400 MPa and sintered in vacuum furnace CLASSIC 5015T under pressure 10 -3 -10 -5 Pa (temperature 600 °C for 40 minutes). Sintering parameters have been chosen to avoid liquid phase (aluminum melting point 660 °C) and formation of intermetallic phases. However, these parameters were enough for the preliminary diffusion of elements.
Due to finding proper heat treatment temperature samples obtained after sintering have been annealed at varied temperatures: 700, 800, 900 and 1000 °C for 1 hour. After treatment samples have been investigated by Scanning Electron Microscope JEOL JSM 7600F (back-scattered electrons imaging;15 kV) and by the XRD measurements (X'Pert PRO MPD in Bragg-Brentano geometry using cobalt X-ray tube 45kV, 30 mA with pinhole 4x1 mm2). Analysis of obtained data allows to choose one temperature of annealing (1000 °C) for further investigation where annealing time was elongated to 2, 3, 5, 10 and 20 hours.

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Results and discussion Parameter VEC and thermodynamic parameters calculated for obtained samples are presented in Table 1. Parameter VEC (Valence Electron Concentration) corresponds to the phase composition of alloy. For investigated sample calculated VEC is 7.9 what represents dual phase structure of obtained sample -BCC+FCC. ΔHmix allows to predict the formation of solid solutions, calculated value fulfils criteria of the High-Entrophy Alloys (-14.58 kJ/mol), even if the value of mixing entropy is high while the ΔHmix is to low or to high the solid solution would not be formed. Rest of parameters δ (ratio between atomic radius of single alloy element to average atomic radius of alloy) and Ω (Ω=TmΔSmix/|ΔHmix| where = ∑ ( ) , (Tm)i -melting point of each element) combined with ΔHmix indicates formation of solid state. Calculated values fulfil the criteria -δ is 6.01 and Ω is 1.21.

XRD analysis
Obtained XRD patterns were presented in Figure 1.

Varied temperatures of annealing.
For the sample that was only sintered and have not been annealing, the BCC and FCC phases were observed. However, the patterns assigned as BCC1 and BCC2 phases were combined together. During the heat treatment with the increase of the temperature of annealing the aforementioned patterns became separated. Perhaps the additional patterns (CoFe phase) could be formed by the diffusion of cobalt in to Fe base solid solutions. In the case of FCC phase it should be noted that the intensity of patterns near 50, 60 and 70 °2 increase with the temperature of annealing. Moreover, for the XRD patterns obtained for 1000 °C the intensity of patterns assigned to FCC phase is much higher than for the BCC phase patterns. It could be assumed that with the increase of temperature the samples are more homogenized. This fact determined choosing temperature 1000 °C for further investigations.

Varied time of thermal treatment.
Extend of the time of heat treatment also affects the homogenization of material. For the 20 h of annealing the intensity of FCC patterns is almost two times higher than for the samples annealed by 1 h. Obtained patterns are sharp and well separated. It could be assumed that increase of the time of heat treatment causes more advanced homogenization. As it was aforementioned before in the case of the varied temperatures of annealing also in this case patterns combined of BCC phases separates.

Scanning Electron Microscopy
Microstructures of samples obtained after heat treatment are presented in Figure 2. At the presented figures the bright and dark areas could be noted, that represents phases described by the XRD measurements. Obtained materials are characterized by the huge porosity. The mechanical pore (MP) and diffusion pore (DP) could be observed. First type of pores is created during the hydrostatic pressing and second type during the diffusion of elements atoms. It should be noted that with the increase of the temperature the amount of darker areas and amount of diffusions pores increase. Both phenomena could be explained by the easier diffusion activated by increase of temperature. Not only temperature influences the diffusion activation, the time of annealing also indicate better chemical homogeneity of sintered materials. The EDS map of elements distribution for sintered material (Figure 3) shows that the obtained material is highly heterogeneous. It should be noted that the areas enriched by nickel are also enriched by aluminium and partially by titanium. The EDS map of element distribution obtained for the sample annealed at temperature 1000 °C for 1 hour is presented in Figure 4. Analysis of obtained data allows observing much better homogenization of chemical composition than for sample after only sintering. The brighter area is enriched by iron and cobalt while the darker area is enriched by aluminium, titanium and partially by nickel.  On the basis of data obtained by the SEM and XRD it could be said that the darker areas of examined samples are enriched by aluminium and nickel that constitute the FCC phase (nickel-based solid state solution), while brighter areas represents the BCC phase (iron-based solid state solution).

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Conclusions According to obtained data and calculated values of thermodynamic parameters it is possible to obtain the High-Entropy Alloys from the elemental powders by the powder metallurgy techniques. Sintering of powders in the temperature 600 °C by 40 minutes it is not enough for fully homogenization of alloy, however in those conditions the diffusion of elements is activated. XRD measurements allow to determine dual phase structure composed from BCC and FCC phases. BCC phase corresponds to the iron-based solid solution and to the FeCo phase while the FCC corresponds to the nickel-based solid solution. XRD data confirms theoretical assumptions. It should be noted that increase of temperature and time of annealing leads to the homogenization of chemical and phase composition -separation of BCC phases with decreasing of BCC2 phase; increasing of FCC phase amount; nickel, titanium and aluminium tend to enrich the darker areas of microstructure while iron and cobalt enrich the brighter areas of microstructure. With the increase of both parameters the amount of diffusion pores increase, what also confirm the homogenization of the material.