- A study is made of the influence of alloying on the solubility of boron fibers in the composite material Mg-B. Three models of solutions are considered on the assumption that the boron atoms either occupy interstices or replace the matrix atoms. A calculation is made of the parameters of the interaction between the fiber material with the magnesium matrix and with alloying elements of different valence. A correlation is established between the valence of an alloying element and its influence on the solubility of a fiber in the matrix. Silicon and aluminum have positive parameters of the interaction with the boron atoms and prevent dissolution of the fibers in the matrix.