The Effect of Boron Content on the Structure and Mechanical Properties of Electron-Beam High-Entropy AlNiCoFeCrTiB Coatings

High-entropy coatings produced by electron-beam deposition of multicomponent Al‒Ni‒Co‒Fe‒Cr‒Ti‒B_x (x = 0, 0.25, 0.5, and 1 mol) powder mixtures onto steel substrates in vacuum were examined. The effect of boron content on the phase composition, structure, and strength properties of the AlNiCoFeCrTiB_x coatings was studied employing X-ray diffraction, microstructural analysis, and micromechanical tests. The AlNiCoFeCrTi and AlNiCoFeCrTiB_0.25 coatings showed a typical dendritic and interdendritic structure and consisted of two substitutional solid solutions with a body-centered cubic (bcc) structure, differing in lattice parameters. An increase in the boron content to 0.5 mol changed the phase composition and led to the formation of in-situ titanium diboride TiB₂ as fine inclusions and chromium boride Cr₂B as elongated inclusions in the coatings besides the two bcc solid solutions (bcc1 and bcc2). When 1 mol of boron was added, the coatings remained four-phase, while the amount and sizes of TiB₂ and Cr₂B inclusions increased. Moreover, with 1 mol of boron, the ratio between the bcc1 and bcc2 phases increased toward bcc2 because of the removal of chromium and titanium atoms. Mechanical tests showed that the microhardness and yield stress of the AlNiCoFeCrTiB_x coatings produced by electron-beam deposition increased by 1.6 times when boron content raised to 1 mol: from 8.8 and 2.4 GPa for the AlNiCoFeCrTi coatings to 14.2 and 4 GPa for the AlNiCoFeCrTiB coatings. The significant enhancement in the strength indicators (hardness and yield stress) of the high-entropy coatings with greater boron content could be attributed to the solid-solution strengthening effect of interstitial boron atoms and to the strengthening effect of boride phase inclusions.