Microstructure and dislocation hardening mechanism of VT8 alloy

Specimens of titanium alloy VT8, which is used for the manufacture of gas turbine engine elements, were investigated in the initial state and after fracture toughness testing by methods of transmission electron microscopy and diffraction analysis. The features of the microstructure, structure morphology, the nature of phase distribution and structural components were established. Defects in the crystal structure, the formations of dislocation inhomogeneities and local concentrators of internal stresses were identified using JEM-200CX transmission electron microscope. The scalar dislocation density is determined by the secant method. The study of VT8 titanium alloy samples before and after destruction, which is used for the manufacture of GTE elements, using the methods of transmission electron microscopy and diffraction analysis was made. Microstructural investigations for a detailed analysis of the structure features, morphology and phase formations distribution, as well as their components establishment, the nature of crystal lattice defects, the formation of dislocation inhomogeneities and local concentrators of internal stresses were performed on a JEM-200CX transmission electron microscope. The scalar dislocation density was measured by the secant method. It is shown that the studied samples of VT8 titanium alloy are characterized by a two-phase (α + β) microstructure in the form of large -phase plates, 0.15 ... 0.76 μm in size, interspersed with an insignificant amount of thin-plate β-phase, with a size of 0.04 ... 0.21 μm. Based on scalar dislocation densities, the level of local internal stresses in the places of dislocation accumulations, which are sources of crack formation, was analytically estimated. Dispersed particles of secondary phases characterized by different sizes and different structure morphologies were identified. The calculated dislocation densities and an estimate of the average distance over which they move in the process of deformation are used as the basis for creating a statistical map of localized deformation level indicators in the alloy structural components and on the fracture surface. It is shown that as a result of fracture after testing for low-cycle fatigue, the dislocation density increases, the level of local internal stresses increases, and the formation of a cellular structure in the α- and β-phases and deformation grain-boundary defects occurs. Keywords: VT8 alloy, dislocation structure, microstructure, transmission electron microscopy, local internal stresses.