Fig. 1 shows the XRD patterns of the FeCoCrAlCu-xTiC powders and its high entropy alloy coatings. As shown in Fig. 1(a) and (c), the powders of FeCoCrAlCu remained elemental after ball milling, while the coating of FeCoCrAlCu without TiC addition only had a body-centered cubic (BCC) single-phase solid solution crystal structure after laser surface alloying and no intermetallic compounds appeared. Fig. 1(b) shows the XRD patterns of the FeCoCrAlCu-10 wt%TiC powders. Different from Fig. 1(a), TiC phase was detected in XRD patterns. In addition, the coatings of the FeCoCrAlNiTi-xTiC (x = 10, 30, 50) exhibited two simple phases, body-centered cubic (BCC) structure phase and TiC as shown in Fig. 1(c). It was also worth noticing that no other complex brittle phases formed after the addition of TiC particles, indicating that the addition of TiC did not change the solid solution phase in the high entropy alloy coating. Obviously, the diffraction peaks of TiC phase intensified with the addition of TiC, which was consistent with the increase of TiC volume fraction.
Fig. 1. XRD patterns of FeCoCrAlCu-xTiC powders and high entropy alloy coatings: (a) FeCoCrAlCu powders, (b)FeCoCrAlCu-10 wt%TiC powders, (c) FeCoCrAlCu-xTiC high entropy alloy coatings (x = 0, 10, 30, 50 wt%).
According to the Gibbs phase rule: the freedom degree f = C – ϕ + 1, where C and ϕ were components number and phase number, respectively. For the alloy system of FeCoCrAlCu (C = 5), the phase number of non-equilibrium solidification was tend to ϕ > 6 during the LSA process. However, the phase number of the FeCoCrAlCu HEA coating was much less than 6. This phenomenon can be due to the fact that the effect of high mixing entropy inhibited the complex intermetallic compounds formation among the principal elements. Furthermore, the change of Gibbs free energy of HEA systems can be obtained as follows:
where ΔHmix, ΔSmix, ai (Σn i=1ai = 1), R, and T was the enthalpy of mix, the entropy of mix, molar percent of component, gas constant, and absolute temperature, respectively. When a1 = a2 = ⋯ = an, ΔSmix can reach the maximum. Table 2 shows the atomic size and mixing enthalpy of different atom-pair. As can be seen obviously, the mixing enthalpy of different atom-pairs of Fe, Co, Cr were near to zero, and the atomic size of Fe, Co, Cr were nearly in same. Therefore, the ΔHmix of (FeCoCr) was near to zero. According to formula (1), when ΔHmix was in low level, the ΔGmix of (FeCoCr) was mainly decided by ΔSmix. In addition, according to formula (2), when (FeCoCr) matrix was in solid solution state, ΔSmix can reach the maximum, and then the ΔGmix would reach the negative maximum. Therefore, the solid solution state of phase which formed in the (FeCoCr) matrix was more stable than other state. It can be seen from Table 1 that the mixing enthalpy of Al element and other elements was far lower than zero which was more inclined to form intermetallic compounds. However, the high entropy effect in this system inhibited the formation intermetallics. In addition, the proper amount of Cu addition can increase the entropy and lattice distortion of the FeCoCrAlCu HEA system due to the fact that the mixing enthalpy of Cu and other elements was higher.
Table 1 Laser parameters for laser surface alloying of FeCoCrAlCu-xTiC HEA coatings.
Table 2 Mixing enthalpy of different atom-pair.
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