In this work, solid-state mechanical alloying (MA) and cold spraying (CS) processes were applied to fabricate powder AlNiCoFeCrTi high-entropy alloy (HEA) and then to produce HE coatings on steel substrate. Shot-time MA for 3 h has been employed to synthesize nanostructured equiatomic AlNiCoFeCrTi HEA of metastable supersaturated substitutional solid solution with bcc crystal structure. Although alloying is not complete at this shot milling time, it goes to completion during thermal annealing to achieve the alloy formation. XRD study on mechanically alloyed high-entropy AlNiCoFeCrTi alloy after thermal annealing at 1200 °C for 1 h revealed the formation of a three-phase structure consisting of ordered bcc phase with fine precipitates of intermetallic σ-phase (FeCr) and titanium carbide TiC. The powder agglomerates resulted from annealing were grinded in a ball mill for 1 h. Nanostructured disordered bcc solid solution, TiC and σ-phases are noticed after milling. Coatings of 450 μm in mean thickness were deposited by the CS process using an air like a working gas, temperature and pressure of 450 °C and 0.9 MPa, respectively. The experimental results confirm that CS process can be used to produce HE coatings with low porosity. As a low-temperature deposition process, CS completely retained the HEA phase composition and nanostructure in the coating without any phase transformation. The AlFeNiCoCrTi HE coatings exhibit 10.3 ± 0.3 GPa in Vickers hardness.
This study reports the structural evolution of multicomponent AlNiCoFeCrTi high-entropy alloy from elemental materials to nanocrystalline metastable solid solution during mechanical alloying (MA), and further, to equilibrium phases during subsequent thermal annealing. It was justified experimentally that shot-time mechanical milling of Al-Ni-Co-Fe-Cr-Ti powder mixture during 3 hours resulted in a single-phase nanocrystalline high-entropy alloy (HEA) with a structure of bcc solid solution. During thermal annealing of the bcc solid solution phase transformation take place, and grain growth of equilibrium phases occur. The phase composition of as-MA alloy transforms to fcc and bcc solid solutions with TiC precipitations' when the MA powder was annealed at 1200 °C for 1 h. X-ray diffraction and electron microscopy data show that the nanocrystalline powder microstructure is retained in the alloy with no grain growth. The AlNiCoFeCrTi HEA exhibit 7.1 GPa and 9.2±0.3 GPa in Vickers hardness after mechanical alloying and after thermal annealing, respectively.
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