- The problem of stability of interfaces in metal-matrix composites is reduced to the investigation of the segregation of copper atoms at the metal/fibre interface. Calculations of excess thermodynamic functions of interstitial solid solutions are performed. The influence of additives on the stability of copper segregations formed on the fibres is discussed. The wettability of the carbon/copper interfaces is regulated by the solubility of the fibre in the matrix with different dopants. It is shown that alloying of the matrix by the additional element, substituting copper in the interstitial metal-metalloid solid solution, changes the value of the mixing energy and influences the volume fraction of two-dimensional segregations of copper. Some additives are predicted as the best alloying elements for preventing the copper segregations on the composite interfaces.