The nanostructure of surface layers formed in electroexplosive alloying has recently been demonstrated. The corresponding structure is revealed as a result of layer-by-layer study of zones of electroexplosive alloying—carburization and carboboriding of nickel [1] and iron [2, 3]; boriding [4], copper plating, and cuproboriding [5] of nickel; and copper plating and cuproboriding [6], aluminum plating [7], and alumoboriding [8] of iron—by highly informative diffractional electronmicroscope analysis of thin foil. The results provide the basis for electroexplosive alloying to produce nanocomposite layers on the surface and permit the selection of means of controlling the structure formation of the modified layers. It is found that several layers with different structural and phase states alternate in an orderly fashion over the depth of the electroexplosive-alloying zone. The volume ratio between the layers is different in different types of alloying. The basic layers are characterized by cellular crystallization and a granular structure. Usually, the change in morphology of the crystallization front is described on the basis of the theory of concentrational supercooling [9]. Accordingly, these concepts are used in the present work to analyze the crystallization of practically important systems formed at the surface of iron and nickel by electroexplosive treatment. Plates of technically pure 0.08ZhR iron (thickness 3‐5 mm) and NP1 nickel (thickness 2 mm) are treated at the pressure of the residual atmosphere in a technological chamber (100 Pa). The explosive wires are graphite fiber [1‐3] and nickel [4], aluminum [7, 8], and copper [5, 6] foil of mass 180, 220, 40, and 100 mg, respectively. In boriding nickel and two-component alloying in the explosive region, a weighed portion of amorphous-boron powder (60 mg) is introduced. The effective treatment time is 100 µ s; the absorbed power density at the jet axis is 6.0 GW/m 2 ; the pressure in the impact-compressed layer formed close to the irradiated surface is 14.2 MPa. Foil for electron-microscopic investigation is prepared by electrolytic thinning of plates cut by electrospark erosion parallel to the treated surface. Jet treatment is used for one-sided polishing of the foil. Surface treatment in electroexplosive alloying is by pulsed heterogeneous plasma jets formed in the discharge of a capacitive energy store at a wire, using a coaxial system of current-bearing electrodes. The jets contain a purely plasma high-speed head, with gradual increase in the content of condensed explosion-product particles and powder toward the tail. As a result of thermalization of the plasma on impact at the surface, the metal is heated and, with certain parameter values, melts. The condensed particles in the tail of the jet interact with the melt and form the surface relief. In very intense treatment, as in the present work, the surface relief is also formed on account of radial melt flow under the jet pressure at the surface, from the center to the periphery of the treatment zone. Preliminary optical-microscope study of chemically etched transverse sections shows that central, intermediate, and peripheral regions may be distinguished over the radius of the treatment zone; they differ in the roughness of the external surface and in the depth and degree of alloying. These parameters are greatest in the central region (diameter 20 mm) under the nozzle of the plasma accelerator. On the basis of these results, subsequent structural and phase analysis is based on transmission electron-diffraction microscopy of thin foil in the intermediate region of the alloying zone, at a distance of 10‐15 mm from the center, where its thickness is 20‐25 µ m. The surface of the samples is investigated, as well as layers at different depths. The volume fraction of a particular phase is calculated from the area that it occupies in comparison with the total area of the microphotograph.