The data on changing cohesion energies (by 1–2%) in b.c.c. and f.c.c. phases of iron and cupper at increasing lattice parameters up to values corresponding to premelting stage are presented. Taking into account the latent heat of melting, these changes compose 4–5% and testify about an absence of the physical causes for atomwise disordering of a melt. The sharp dependence of cohesion energy on lattice parameter causes the appearing of large tensions in comparison with strength characteristics and Peierls tensions. This process during heating of crystals up to temperatures higher the Debye temperature by 1.5–2 times causes the multiple generation of dislocations, and their polygonization— to spontaneous fragmentation of a structure. The reversibility of this phenomenon during a cooling testifies that thermally-activated polygonal processes manifests arising new degrees of freedom of atoms’ motion at thermal activation of solids. The results of structural investigations (in situ) of metals embrace the cryogenic temperatures, stages of premelting, melting, heating of melt, and solidification. The cooling of iron and chromium up to the temperatures $\leq 140$ К causes the spontaneous fragmentation of their grains. The process is provided with an arising of twinnings and formation of growing cellular dislocation structures. According to data of electron microscopy, heating of films of pure metals (Fe, Cu, Ni, Ag, and Au) up to the temperatures higher the Debye temperature leads to beginning of spontaneous fragmentation of grains. At further elevation of temperature up to the premelting stage, the phenomenon of fragmentation is increasing, and dislocation density reaches the values $\sim 10^{11}$ cm$^2$. The transition of metals within the range of latent heat of melting is accompanied with cluster fragmentation on blocks of coherent scattering with a size $\sim 3–5$ nm. This process accompanies an appearing of coalescence and increasing of Brownian mobility of structural fragments of a melt. In compressed state, such a system would be similar to a heavily-deformed one with a dislocation density $\sim 10^{13}$ cm$^2$. Very overheated melt is disordered up to the cluster level. A flow of the melt causes its separation onto structural polyfragments, which are inherited during a solidification. The initial stage of solidification accompanies an increasing (by an order of magnitude and more) of a number of crystalline nanofragments that is compared with a decreasing of dislocation density (by two orders of magnitude and more). The aftercooling leads to the creation of nanostructural fragments.