Physical Mesomechanics最新文献

A pattern of macroscopic deformation bands with a characteristic size of ~10 mm was revealed experimentally in a uniaxially deformed sample at the prefracture stage prior to macroscopic necking. The bands moved at different velocities to the place of subsequent necking. The observed banding pattern in the deformed medium was described using two identified dynamic order parameters, which are the amplitudes of unstable plastic and elastic deformation modes. The change in the dynamic order parameters was described by a system of two coupled nonlinear parabolic equations. Analysis of solutions to the equations revealed the conditions for the formation and development of the found pattern at the prefracture stage.

The combination effect of substitutional impurities of group IVB–VIB transition metals on the oxygen vacancy formation energy in rutile titania was studied by the projector augmented wave method within density functional theory. The pair interaction energy of impurity atoms was estimated depending on the interatomic distance. It was shown that the interaction in the Zr + Zr, Hf + Hf and Nb + Ta pairs leads to the energy preference of their orientation along the <100> direction at a distance of the lattice parameter a. The Mo + Mo and Nb + Mo pairs prefer to orient along the [001] direction at a distance of the lattice parameter c. If the impurity atoms are farther than the third neighbors, then their interaction can be neglected. It was found that the change in the oxygen vacancy formation energy due to doping with several impurities can be estimated as the sum of the energy changes due to single impurity divided by a coefficient whose value depends on the mutual arrangement of the impurity atoms.

The paper reports on a transmission electron microscopy analysis of the dislocation structure of normalized 09G2S steel with strain aging. The analysis shows that in a steel specimen deformed to its yield stress, the dislocation density peaks at the center of a strain localization band (in its active zone) and decreases by about an order of magnitude at its sides. The structure of these band zones, which are in different strain hardening stages, is examined and the Burgers vector of dislocations of the primary slip system is determined. The ends of such dislocations form ordered planar dipole walls, leading to polygonization by dislocation slip. Also considered is the specimen structure at the yield and ultimate stresses.

The paper presents experimental studies on laser cladding synthesis of a titanium matrix composite based on Ti64 titanium alloy and TiB₂ ceramic reinforcement. The weight percentage of TiB₂ ceramics in the composite was 5, 10 and 15%. The phase composition of the resulting materials was analyzed by standard X-ray diffraction and synchrotron X-ray diffraction. It was found that the structure of the titanium matrix composite with 5 wt % ceramics consists of TiB nanowhiskers, and that of samples with higher ceramic content exhibits TiB whiskers with a width of several micrometers. The addition of TiB₂ ceramics increases Young’s modulus, nano- and microhardness of composite samples compared to Ti64 alloy. The indentation method was used to study the formation of a phase that is different from TiB₂ ceramics and TiB microwhiskers and has elastic properties exceeding the elastic properties of the original Ti64 matrix phase. Analytical predictions showed an increase in the effective elastic properties of the formed heterogeneous material with the predicted new phase. It was also found that a lower friction coefficient can be achieved by forming a structure with nanowhiskers, while higher Young’s modulus and microhardness can be obtained by forming a structure with microwhiskers.

The paper provides a review of experimental and numerical studies of deformation-induced surface roughening in loaded metals and alloys. Consideration is given to various parameters for assessing strain-induced surface roughness, factors influencing the roughness characteristics, and models describing surface roughening in polycrystalline alloys.

The creep resistance and structure of 10% Cr–3% Сo–2% W–0.29% Cu–0.17% Re steel with 0.1% carbon, low nitrogen content and high boron content were investigated by creep rupture testing at a temperature of 650°C and stresses from 200 to 100 MPa applied in 20-MPa increments. For comparison, 9% Cr steel with 0.1% carbon, 0.05% nitrogen, and 0.005% boron was considered. The steels were subjected to preliminary heat treatment including normalizing at 1050°C for 1 hour, tempering at 750–770°C for 3 hours, and cooling in air. The structures of both heat-treated steels exhibited martensite laths with boundaries pinned by М₂₃С₆ carbides, and the rearrangement of dislocations was retarded by MX particles. A significant difference between 10% Cr and 9% Cr steels was the presence of fine М₂₃С₆ carbide particles characterized by orientational relationships with the ferrite matrix and MX carbonitrides, whose volume fraction was 6 times lower. Short-term tensile tests at room temperature showed no differences between the steels, while the creep rupture strength of 10% Cr steel was 13% higher than for 9% Cr steel. The creep deformation mechanism of the steels was also different. Structural analysis of 10% Cr steel after creep tests revealed no substantial changes in its lath structure: the lath width increased by only 58% and the dislocation density was reduced by a factor of 2. Comparison with 9% Cr steel showed that the good structural stability of 10% Cr steel during creep is caused by the high coarsening resistance of second phase particles, whose coarsening rate is 1-2 orders of magnitude lower than that in 9% Cr steel.

Impact tests of low-alloy martensitic-bainitic steel AB2 showed that the scale of dynamic deformation and the fracture mechanism change in a threshold manner. The change in the mechanism and scale of fracture is triggered by the resonant excitation of large-scale structural elements of the material (grain conglomerates) due to plastic flow oscillations. In this case, the grain-boundary mechanism of dynamic fracture is replaced by a transcrystalline one. Beyond the strain rate threshold, mesoscopic elementary carriers of dynamic deformation are divided into two groups: low-velocity and high-velocity. Accordingly, the velocity distribution of mesoparticles shows two humps. The velocity spread of mesoparticles sharply increases under these conditions, while the mass velocity defect (change in the shock wave amplitude) becomes negative. The latter fact indicates the local acceleration of mesoparticles in discrete regions of the target (the so-called shooting of mesoparticles in the shock wave direction). Transcrystalline cracks are randomly distributed throughout the specimen and have a random orientation.

This paper presents an experimental study of energy dissipation caused by fatigue crack growth in Grade 2 titanium and titanium alloys Ti-1.1Al-0.9Mn and Ti-4.6Al-1.77V using the original heat flux method. It is shown that significant structural changes occur in the material under plastic deformation, leading to internal energy evolution. As is known, a large part of the deformation energy is dissipated as heat. The developed method allows high-accuracy measurements of the heat flux caused by plastic zone development at the crack tip directly in the fatigue experiment. Simultaneous measurements of the crack length and displacements in the stress concentration zone allow estimating the energy balance of the tested specimens. Analysis of the obtained data confirms that the stored strain energy reflecting the structural state of the material can be used as a fracture criterion. Based on the heat flux data, a kinetic equation is derived for predicting the rate of fatigue crack growth under Paris’s law by the energy dissipation rate.

The paper analyzes the features of plastic flow in compression in sintered Al–Sn–Fe alloys, some of which were exposed to compaction in a closed die at a pressure of 300 MPa and temperature of 250°C, and some to equal-channel angular pressing by route A (ECAP-A) at the same temperature. The analysis shows that the sintered composites comprise agglomerates of Sn-cemented Al₃Fe particles formed in place of Fe powder particles due to the interaction of Al and Fe in sintering. The agglomerates are strong but sufficiently ductile, due to Sn, to survive under deformation and to efficiently impede the propagation of strain localization bands and microcracks. In compression, such agglomerates in Al–20Sn–17Al₃Fe hold their form, moving as solid units, while the composite displays good ductility. In ECAP-A, they extend in the direction of plastic flow, and this adversely affects their ductility in further compression.

This study examines how preliminary mechanical milling of a modifying TiN-based powder mixture affects the morphology of the CO₂ laser-treated surface, the weld pool morphology, and the cross-sectional structure of the material. Ultrafine titanium nitride particles used as nanomodifier have low wettability by liquid metal, are not entrained by its convective flows, and tend to accumulate in the subsurface layer, which makes it difficult to effectively modify the structure within the treated material. Ball milling of the modifying Ti + TiN mixture for 9 min leads to the formation of composite particles (5–7 µm) with ultrafine TiN particles uniformly distributed over their surface and volume. When the composite particles are melted by the laser beam, they turn to ultrafine TiN particles of nanomodifier coated with a thin titanium layer, which have a smaller contact angle. As a result, the particles are more evenly distributed over the weld pool and the number of crystallization centers increases, leading to the formation of a fine homogeneous structure of the material. The microhardness increases by 32%, and its standard deviation decreases by a factor of 1.5–3.0.