Elemental composition of different steel alloys may vary from traditional 5 elements Cr, Ni, Mo, Mn, Si (apart from Fe, C, P, S) with different wt. % to even 10 or more, when the steel alloy contains also V, W, Ti, Nb, Al, Co, Ta, Zr, Re. There is a similar variety of copper alloys, where presence of Pb, Ni, Fe, Co, Ti, Zr, Cr and other elements apart from conventional alloy elements Cu, Zn, Sn is significant in whole with the aim to ensure them high mechanical, heat and corrosion resistance. As a result of our studies, using optical microscopy, metal elemental and X-ray diffraction analysis, it was found that a number of intermetallic compounds, e.g. FeC, Fe3C, Fe2W3C, Fe4W2C, form under exploitation conditions, in the case of inadequate elemental composition, initial non-uniform alloy structure and under mechanical and thermal loads. The intermetallic compounds embrittle the metal alloy and are the reason of breaking of steel articles. In another case, insufficient alloying of the steel with Cr, Ni, V and exploitation mechanical and thermal loads have caused a complete metal recrystallization with formation of Mn4.6Fe0.4Si3 compound, decomposition of Fe3C and Fe4C and carbon release as X-ray amorphous graphite. Similarly in copper alloys with a reduced content of alloying metals, brittle intermetallic CuZn and Cu5Zn8 compounds form already in the metallurgical process, causing breaking of the articles. Both hot-dip and electrodeposited metal coatings are embrittled on their obtaining by formation of intermetallic compounds. In hot-dip galvanizing, formation of vertical and horizontal cracks in the zinc coating and scaling of the zinc coating off a substratum are caused by a very brittle Fe3Zn10, which forms apart from a usually formed intermetallic FeZn13. Formation of cracks in Sn-Co alloy layers may be caused by formation of CoSn2 compound in a thick layer. At the total amount of 4.53 wt.% of the alloying elements Pb, Fe, Sn, Al, Ni, Mn, Si, Cr, formation of phase Cu5Zn8 is prevented, the amount of phase is reduced, the alloy is formed by the solid solutions Cu0,64Zn0,36 and Cu1,05Zn0,95.