Assessment of Maximum Sensitivity of Acoustic Emission Technique

Knowledge of the ultimate sensitivity of the acoustic emission (AE) method is useful in studies of the mechanisms of plastic deformation of the structure, the formation and development of micro-, meso- and macro-cracks, as well as continuous processes, such as the outflow of liquids and gases, friction and a number of others. Analysis of literary data on assessment of limit sensitivity at identification of AE sources was carried out. It has been shown that when using standard resonant piezoelectric transducers with a frequency bandwidth of 30 ± 10 kHz, the ultimate sensitivity is the fraction of the nanometer in the displacement of the surface of the object and the unit micron of the size of the microcrack during its formation and hopping development. When testing industrial facilities, in many cases the level of external noise is significantly higher, the detection of defects decreases and amounts to a fraction of a millimeter. However, an increase in the energy and amplitude of AE signals as the defect develops in the vast majority of cases leads to the fact that when a crack reaches dimensions that begin to threaten the strength of the monitored object, AE signals are reliably detected by the equipment. Knowledge of the maximum sensitivity of the AE method makes it possible to compare it with other NDT methods by this parameter.

The Effects of Grain Boundary Misorientation on the Mechanical Properties and Mechanism of Plastic Deformation of Ni/Ni3Al: A Molecular Dynamics Study

The effects of grain boundary misorientation angle (θ) on mechanical properties and the mechanism of plastic deformation of the Ni/Ni3Al interface under tensile loading were investigated using molecular dynamics simulations. The results show that the space lattice arrangement at the interface is dependent on grain boundary misorientations, while the interfacial energy is dependent on the arrangement. The interfacial energy varies in a W pattern as the grain boundary misorientation increases from 0° to 90°. Specifically, the interfacial energy first decreases and then increases in both segments of 0–60° and 60–90°. The yield strength, elastic modulus, and mean flow stress decrease as the interfacial energy increases. The mechanism of plastic deformation varies as the grain boundary misorientation angle (θ) increases from 0° to 90°. When θ = 0°, the microscopic plastic deformation mechanisms of the Ni and Ni3Al layers are both dominated by stacking faults induced by Shockley dislocations. When θ = 30°, 60°, and 80°, the mechanisms of plastic deformation of the Ni and Ni3Al layers are the decomposition of stacking faults into twin grain boundaries caused by extended dislocations and the proliferation of stacking faults, respectively. When θ = 90°, the mechanisms of plastic deformation of both the Ni and Ni3Al layers are dominated by twinning area growth resulting from extended dislocations.

Influence of grain size on mechanisms of plastic deformation and yield stress

The influence of grain size on the physical yield strength of the polycrystal is considered by the method of cellular automata. The physical yield strength of the polycrystal in this model is defined as the stress at which, the plastic deformation covers the entire cross section of the sample from one edge to another. Three mechanisms of plastic deformation are considered. The first one is an initiation of plastic flow from grain to grain by dislocation pile-ups. The second one is plastic flow in different grains independently of each other under the action of external stress and the third one is intergranular slippage. Computer simulations have shown that at large grain sizes (d > 200 nm) deformation propagates from grain to grain by initiating dislocations pile-ups, since in this case pile-ups are quite powerful and have a large effect on neighboring grains. At average values of grain size (20 nm <d <200 nm) plastic deformation occurs in the grains independently of each other, and the external strain give a major influence on plastic deformation. With further reduction of the grain sizes (d <20 nm) the main mechanism of deformation is intergranular slippage. because in grains of this size are quite large image stresses that do not allow large dislocation clusters. In small grains the image forces are quite large to prevent large dislocation pile-ups formation, but the mass and volume of grain are quite small to turn or slip its under the action of external stresses. In accordance with these mechanisms, on the calculated dependence of the physical yield strength vs grain size, there are three areas with different angles of inclination in logarithmic coordinates. Keywords: yield point, grain size, Hall―Petch low.

Nucleation and development of plasticity in nanocrystalline BCC iron under shear loading

The features of the nucleation and development of plasticity in nanocrystalline iron with BCC lattice under shear were studied. The mechanisms of plastic deformation playing the main role in the development of structural rearrangements during loading were revealed. It was shown that the development of plasticity can be conditionally divided into several stages. The first stage of plasticity development is associated with the formation and propagation of dislocations and twins. At the second stage, intraganular slip and intergranular sliding begin to make the main contribution to plastic deformation. These processes initiate a change in the shape of the grains. At large shear, the deformation behavior of the sample is governed by the migration of the interfaces. Not only grain boundaries migrate but also twin ones do. As a result of migration processes, the grain sizes of the nanocrystalline sample are enlarged.

Кинетика структуры титанового сплава при высокоскоростном проникании ударника

The article is devoted to the study of the behavior of a titanium alloy under conditions of high-speed penetration at a speed of approximately 2.0 km / s. It is shown that in the target during penetration, three penetration zones are observed that differ in the mechanisms of plastic deformation and fracture.

Твердофазная низкотемпературная эволюция бинарной системы Pb-Au: через контактные взаимодействия к интерметаллидам, через деструктуризацию к новому многоэлементному состоянию

The article is devoted to the study of the behavior of a titanium alloy under conditions of high-speed penetration at a speed of approximately 2.0 km / s. It is shown that in the target during penetration, three penetration zones are observed that differ in the mechanisms of plastic deformation and fracture.

Joining of Titanium and its Alloys with Aluminum Alloys by Friction Stir Welding

This article is devoted to the study of the mechanism of formation of dissimilar welded joints Ti-2Al-1.5Mn, pure titanium (Ti35A) and aluminum (Аl (pure), Аl-6Mg-0.5Mn) alloys obtained by friction stir welding (FSW). The investigated microstructure of the weld joint nugget (WN), zones of thermo mechanically affected zone (TMAZ) and heat affected zone (HAZ) formed at FSW between Ti-2Al-1.5Mn, Ti35A and aluminum (Аl (pure), Аl-6Mg-0.5Mn) alloys. Zones of welded joints at FSW are formed in the mode of structural superplasticity (SP) with a specific shear-band layered structure with alternating layers. The achievement of superplastic state (SPS) in the formation of WN, TMAZ, HAZ is provided by the step–by–step transformation of various mechanisms of plastic deformation in the mode of simple, collective, abnormal dynamic recrystallization, prepared by the processes of dynamic return, polygonization with the transition to post-dynamic recrystallization by the mechanisms of Bailey–Hirsch, Kahn-Burgers-Taylor. At FSW aluminum and titanium alloys with polymorphism, SPS is supported additionally due to recrystallization by twinning and as a result of phase transformations of alpha-gamma or alpha-beta phases.