Change in the Phase Composition and Lattice Parameters of the Solid Solution Based on α-Ti in the Surface Layers of the Ti–6Al–4V Alloy Subjected to Electron-Beam Treatment

The microstructure, phase composition, and lattice parameters of the α-Ti- based solid solution in the Ti–6Al–4V alloy treated by pulsed and continuous electron beams with the energy density of 18–24 and 450 J/cm², respectively, have been investigated using the methods of X-ray diffraction analysis and transmission and scanning electron microscopy. In the initial state, the two-phase (α + β) alloy had a polycrystalline structure with the equiaxed α-phase grains and β-phase grains located at the junctions or along the boundaries of the α-phase grains. After the electron-beam treatment, α' martensite with a lamellar structure is formed in the molten surface layer, which then experiences an α' → α + α'' + β phase transformation. In the α phase, the lamellar structure inherited from the α' martensite is retained; the β phase is located along the boundaries of lamellar grains of the α phase; the α'' phase is located both in the β phase and inside the lamellar grains of the α phase. It has been revealed that the greater the total volume fraction of the β and α'' phases, the greater the lattice parameters of α-Ti and their axial ratio c/a, and the less the total mean-square displacements of atoms in the 101 direction in the α-Ti phase. The decrease in the total mean-square displacements in the α-Ti phase is due to the diffusion of the vanadium atoms into the β phase. With an increase in the energy density of the electron beam and with a decrease in the rate of cooling of the molten layer, the total volume fraction of the β and α'' phases increases and reaches 6%.