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.

Mechanisms of Solid-Solution Hardening of Single-Phase Cu-Al and Cu-Mn Alloys with a Mesh Dislocation Substructure

The dislocation structure and dislocation accumulation during deformation of polycrystalline FCC solid solutions of Cu-Al and Cu-Mn systems are studied by transmission diffraction electron microscopy. The Al content in Cu-Al alloys varies from 0.5 to 14 at.%. The Mn content in Cu-Mn alloys varies in the range of 0.4 ÷ 25 at.%. Alloys with a grain size in the range of 20 ÷ 240 µm are studied. The alloy samples are deformed by stretching at a rate of 2×10-2c-1 to failure at 293 K. The structure of samples deformed to various degrees of deformation is studied on foils using electron microscopes at an accelerating voltage of 125 kV. For each degree of deformation, the scalar dislocation density and its components are measured: statistically stored dislocations ρS and geometrically necessary dislocations ρG and some other parameters of the defective structure. The mechanisms and their contributions due to mesh and mesh-mesh dislocation substructures (DSS) are determined using the example of substructural and solid-solution hardening in polycrystalline Cu-Al and Cu-Mn alloys. The relative role of various mechanisms in the formation of the resistance to deformation of alloys at different grain sizes is determined. The role of the packaging defect energy on the value of solid-solution hardening for different grain sizes is revealed. The average scalar dislocation density is considered and determined along with its components: statistically stored dislocations ρS and geometrically necessary dislocations ρG. The dependences of the flow stress on the square root of the densities of geometrically necessary dislocations and the densities of statistically stored dislocations are found.

Моделирование одноосной деформации нанокристаллов магния "жесткой " и "мягкой" ориентировок

Atomistic modeling of high-speed deformation by compression of ideal, defective (5% vacancies, 5% impurity hydrogen atoms) magnesium nanocrystals with "hard" and "soft" orientations at T = 300 – 375 K was carried out. Three interatomic interaction potentials were used. The evolution of the microrelief of the free surface of magnesium nanocrystals in the process of plastic flow is shown. Diagrams "stress σ – deformation ε" are constructed, deformation dependences of the scalar dislocation density are determined, and the strain rate dependences on the degree of deformation ε are constructed. The influence of vacancies and hydrogen atoms on the shape of deformation curves, dislocation structure and scalar dislocation density is shown. Evolution of typical dislocation structures is shown, some typical dislocation reactions are given. Conclusions are drawn on the influence of the type of interatomic interaction potential on the calculated characteristics.

Phase Composition and Defect Substructure of Strengthening Layer Surfaced on Low Alloyed Steel

The microstructure and microhardness distribution in surface of low carbon Hardox 450 steel coated with alloyed powder wires of different chemical compositions are studied. It is shown that the microhardness of 6-8 mm thickness surfaced layer exceeds that of base metal by more than 2 times. The increased mechanical properties of surfaced layer are caused by the submicro and nanoscale dispersed martensite, containing the niobium carbides Nb2C, NbC and iron borides Fe2B. In the bulk plates a dislocation substructure of the net-like type with scalar dislocation density of 1011 cm-2 is observed. The layer surfaced with the wire containing B possesses the highest hardness. The possible mechanisms of niobium and boron carbides formation in surfacing are discussed.

Effect of Grain Size on Defects Density and Internal Stresses in Sub-Microcrystals

The paper is devoted to research of an influence of average grains size on scalar dislocation density, fraction of geometrically necessary dislocations, internal stresses and bending- torsion of crystal lattice. Polycrystals of submicrocrystalline copper produced by torsion under hydrostatic pressure were investigated by TEM method.

Parameters of Dislocation Structure and Work Hardening of Ni3Ge

ABSTRACTOrientation dependence of the yield stress temperature anomaly in Ni3Ge single crystals with the L12 structure was investigated during compression tests. The measurements were carried out in the 4.2 K-1000 K temperature interval for two orientations of single crystals, [001] and [234]. The dislocation structure was studied by TEM. Quantitative measurements of different parameters of dislocation structure were carried out. The values of the scalar dislocation density, ρ, were determined for different temperatures in the deformation interval from the yield stress up to fracture. Temperature dependence of the friction stress τF (T) and the interdislocation interaction parameter α(T) were also obtained. The change in the fraction of straight dislocations as a function of temperature was analyzed.