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The paper studies how microalloying with amorphous boron (Bam) influences the structure and properties of boride coatings. Two factors are found to strongly affect the structure and phase formation in composite coatings produced by electron beam surfacing, especially when using thermoreactive powders: (i) high reactive diffusion between the composite and substrate components, which causes the coating structure degradation, and (ii) slight deviations from phase concentrations and equilibrium crystallization, which give rise to such nonequilibrium phases in the coating structure under the electron beam as ferroboron, ferrotitanium and binary eutectics. The above findings suggest the possibility of controlling the structure formation and phase composition by varying the electron beam surfacing parameters. The parameters, among others, include the crystallization rate varied by the introduction of different modifiers, the proportion of components and granulometric composition of the fused powder mixture.