Development of the titanium structure in explosive collapse of thick-walled cylinders

The dependence of the titanium structure on the total deformation in the explosive collapse of thick-walled cylinders is studied. It is shown that structure evolution as a whole and the critical parameters for the appearance of an unstable plastic flow in titanium are not the functions of the final deformation of the cylinders. This instability, which is governed by the internal structure of the material, is the principal mechanism of the loss of the axial symmetry of collapse in the given geometry under specified load conditions. It is found that the instability of the plastic flow in titanium is manifested in the formation of adiabatic-shear bands. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 5, pp. 122–129, September–October 1998     

Microstructure and crystal structure of an equimolar Mg-Ti alloy processed by Simoloyer high-energy ball mill

The solid solubility of 50-50 at.% Mg-Ti powder mixtures was achieved by means of high energy ball milling in a Simoloyer equipment. XRD and HRTEM analyses revealed the existence of FCC and BCC matrix of Ti solid solution in Mg containing small amounts of an HCP Ti-rich phase formed after milling for 48 and 72 h, respectively at 800 rpm. An intermediate FCC solid solution of Ti in Mg was identified in powders milled for 24 h or less. The chemical composition of the matrix products extended from Ti56:Mg44 to Ti50:Mg50, which is close to the targeted equimolar ratio. XRD analysis of the structure suggested that the release of the lattice strain energy contributed to the driving force for transformation and solid solution between Mg and Ti after ball milling. Twinning was observed in Ti-rich crystallites at intermediate milling time. The twinning observed could be attributed to the deformation of Ti particles. However, in the Mg-Ti system, it might also indicate a strain induced martensitic transformation of the metastable ω-FCC into BCC product. The crystallite boundaries acted as preferential sites for the heterogeneous nucleation of the twins and for the formation of solid solution by release of the lattice strain energy.     

Fabrication of nanocrystalline copper by explosive loading and its dynamic mechanics properties

Nanocrystalline (NC) copper is fabricated by the method of severe plastic deformation of coarse-grained copper under explosive dynamic loading at high strain rates. The dynamic mechanical properties of NC copper are studied by the split Hopkinson pressure bar method. The results show that it is feasible to fabricate nanocrystalline copper by explosive dynamic plastic deformation of coarse-grained copper and the grain size of NC copper can be smaller than 100 nm. Twinning and formation of dislocations are the main mechanisms of grain refining. The dynamic yield strength of NC copper increases with decreasing average grain size and increasing strain rate.     

Effect of size, manufacturing and prestrain on high-rate failure (spall) of PT-3V titanium alloy and 12Kh18N10T steel

Experimental results are presented on the spall strength of PT-3V titanium alloy and 12Kh18N10T steel as the system size is changed by a factor of five. The effects of a 0.5–5% dynamic prestrain and of the direction of the load relative to the manufacturing rolling direction on spall failure were also studied for the titanium alloy. It was established that failure of these metals under high-rate one-dimensional strain shows significant energy-related size effects. Effects of the rolling direction and prestrain were less pronounced for the titanium alloy than for steel. All-Union Research Institute of Experimental Physics, 607200 Sarov. Translated from Fizika Goreniya i Bzryva, Vol. 31, No. 6, pp. 130–139. November–December, 1995.     

Twin structures in copper after shock and Shockless high-rate loading

The structure of copper formed after high-rate loading up to pressures of 20–80 GPa with a strain rate of 10⁵–10⁹ sec^?1 is considered. In situations with pressures above 20 GPa and strain rates above 10⁶ sec^?1, the deformation twins are grouped into packets, which are seen in an optical microscope as parallel bands of localized strains inside individual grains. The number of bands in the structure increases with increasing grain size and strain rate, with decreasing sample temperature, and with increasing period of sample loading. The characteristic time of formation of twin bands in copper is estimated as 0.3 µsec.     

Rotational components of deformation in metal bodies under dynamic loading

The plane problem of an initially quiescent cylinder subjected to a gradient flow of a highly viscous fluid is solved. It is shown that the cylinder is brought into rotation with angular velocity ω=−εé, where εé in the shear-strain rate of the medium. The solution is used to analyze flows under high-rate loading of metallic bodies that occurs at the level of the microstructure. The appearance of rotation is associated with the presence of fragments in the structure of the substance (grains, fragments, cells, and inclusions) that are unable to change their shape under the given loading conditions and, consequently, begin to rotate in the process of shear strain. Rotations that occur at the microlevel during joint deformation of solid bodies lead to transfer of oxide, hydroxide, and other surface films into the depth of the material, and this contributes to formation of a bond. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 2, pp. 129–133, March–April, 1998.     

Effect of the degree of defectiveness of an original material on the deformation structure formed in the explosive collapse of thick-walled hollow cylinders

The effect of shock-wave preloading on the formation of a deformation structure in copper and tantalum upon subsequent high-velocity plastic deformation performed by the method of explosive collapse of a hollow thick-walled cylinder has been studied. Prehardening is shown to favor the structural uniformity of deformation up to large values owing to creation of a highly disperse intragranular structure. Lavrent’ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 6, pp. 108–120, November–December, 1997.     

Localization of plastic deformation on contacts, determining the formation of a strong joint

It is shown that a strong joint is formed on the contact surface in explosive welding. A band of material on which plastic deformation is localized is formed along the boundary of the surface, this band representing a qualitatively new structure. Use of the laws governing strain localization, obtained from a study of the collapse of thick-walled cylinders that were subjected to explosive shock loading, makes it possible to predict the collision parameters in explosive welding in accordance with the grain size of the materials that are used. M. A. Lavrent'ev Institute of Hydrodynamics, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk. Translated from Fizika Goreniya i Vzryva, No. 5, pp. 122–128, September–October, 1995.     

Onset critical strains as an effective parameter for compatibilizer efficiency in a polypropylene-poly(ethylene terephthalate) blend

The effectiveness of a compatibilizer is responsible for improved mechanical properties of immiscible blends. The enhanced interactions between the phases are assessed via critical plastic strain (onset of fine slip caused by local intra-/interlamellar slip) and critical elastic strain (onset of fibrillation of crystalline skeleton). Polypropylene-poly(ethylene terephthalate) (PP-PET) blends were compatibilized with three maleic anhydride grafted compatibilizers with different backbones: PP, SEBS, and POE. Using this critical onset strain method, via free shrinkage experiments, allowed to identify the effect of PET on the matrix's deformation and also the different contributions of the compatibilizers to the deformation mechanisms. Results showed that PPgMAH promoted best PET's plastic deformation to the matrix, relative to the other compatibilizers. However, SEBSgMAH's elastomeric backbone provide better stress dissipation before onset of fibrillation compared to the binary blend and matrix. This method can be used as a way to assess the effectiveness of a compatibilizer in an immiscible blend.     

Granularity-induced plastic deformation mechanism of pure polycrystalline cubic boron nitride

Combined X-ray diffraction(XRD) profile analysis and HRTEM observation, a method for exploration of plastic deformation in pure polycrystalline cubic boron nitride (PcBN) samples with sizes of primary cBN powders was developed. XRD profile results showed that the coarse-grained PcBN exhibited a larger micro-strain ε, a greater deformation stacking faults probability fD, which was an order of magnitude larger than that of fine-grained PcBN, but a smaller twin stacking faults probability fT. It was deduced that the plastic deformation of the coarse-grained PcBN was dominated by stacking faults and mechanical twins mode, which would result in strain strengthening and then recrystallization. While the manner of growth twins was the mainly modes of fine-grained PcBN by phase transform, especially utilized the curled SP2 structure as a basis for a cubic structure nucleation. Fundamental plastic deformation principles of the ultrafine polycrystalline cBN was crucial for the field of high-precision cutting tools.     

Evolution of the metal microstructure and conditions of strain localization under high-strain-rate loading

Specific features of evolution of the microstructure of coarse-grained and fine-grained copper are examined on the basis of an available dependence of the critical parameters of strain localization on the grain size, which was derived in an explosive collapse of hollow thick-walled cylinders. Similarity of the properties and mechanisms of deformation of fine-grained and ultrafine-grained materials is established. Based on the analysis of the results obtained, a scenario of the change in deformation mechanisms of fine-grained copper in the course of the development of deformation structures under high-strain-rate loading is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 121–131, May–June, 2006.     

Multiscale Simulation of Indentation,Retraction and Fracture Processes of Nanocontact

The process of nanocontact including indentation and retraction between a large Ni tip and a Cu substrate is investigated using quasicontinuum (QC) method. The multiscale model reveals that significant plastic deformation occurs during the process of nanocontact between Ni tip and Cu substrate. Plastic deformation is observed in an area as large as 20 nm wide and 10 nm thick beneath Ni tip during the indentation and retraction. Also, plastic deformation at a deep position in the Cu substrate does not disappear after the neck failure. The analysis of generalized planar fault energy curve shows that there is a strong tendency for deformation twinning in Cu substrate. However, deformation twinning will be retarded during indentation due to the high stress intensity caused by stepped surface of Ni tip. The abrupt drop of load curve during tip retraction is attributed to the two different fracture mechanisms. One is atomic rearrangement near the interface of Ni tip and Cu substrate at the initial stage of neck fracture, the other is shear behavior of adjacent {111} planes at the necking point. A comparison of the critical load and critical contact radius for neck fracture is also made between theoretical values and our numerical results.     

Dynamic strength of cylindrical fiberglass shells under multiple explosive loading

We have shown experimentally that, for cylindrical shells made of oriented fiberglass plastic, there exists a critical level of deformations at which the structure sustains a given number of explosions from the inside. The magnitude of the critical deformation depends linearly on the logarithm of the number of loads that cause failure. For a given type of fiberglass, there is a limiting level of explosive action at which the number of loads that do not lead to failure can be rather large (over ∼10²). This level is attained under loads that are an order of magnitude lower than the limiting loads under a single explosive action. Institute of Experimental Physics, Sarov 607200. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 6, pp. 102–107, November–December, 1997.     

Mechanism of metal ignition in an oxygen flow

A mechanism that explains the spontaneous ignition of metal (titanium, zirconium, and their alloys) specimens in a high-pressure sound flow of oxygen at room temperature is proposed on the basis of experimental data. Central to this mechanism is the assumption that the oxide film and the surface layer of metallic structural materials can be fractured by a gas flow. An abrupt cooling at the moment of onset of flow-around contributes to fracture. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 1, pp. 50–56, January–February, 1998.     

Effect of preliminary loading on the deformation and strength properties of high-strength fibres

High-strength, high-modulus polyethylene fibres fabricated with gel technology is similar to low-modulus fibres of the olefin and amide series (Capron, polypropylene) with respect to the character of the correlation of the stress—strain diagrams and curve of accumulation of the residual component of deformation. The residual deformation component is relatively large both for high-strength PE fibre and for p-polyamide fibres. The differences in the character of accumulation of the plastic component in these fibres are due to the fact that the residual strains arising in high-strength PE fibre, as in other flexible-chain polymer fibres (polypropylene, Capron) is initiated by breaking of bonds in the main chain. In p-polyamide fibres (Armos, SVM, Terlon, Kevlar), plastic strains arise due to highly elastic deformation “frozen≓ by hydrogen bonds and orientation of molecular chains. Preliminary deformation affects the strength properties of high-modulus fibres differently: in PE fibres, the strength decreases, it increases for Armos and SVM fibres, and remains unchanged for Terlon fibre. This difference is to a great degree due to the difference in the types of intermolecular interaction in fibres of the olefin and amide series. For all fibres investigated, the character of accumulation of the residual deformation component can be correlated with the type of stress—strain diagram, which will allow creating simpler methods of evaluating residual strains. Translated from Khimicheskie Volokna, No. 3, pp. 30–33, May–June, 1998.     

Precrack hysteresis energy in determining polycarbonate ductile–brittle transition. IV. Effect of strain rate

Effect of deformation rate on the ductile–brittle transition behavior for polycarbonate (PC) with different molar mass, notch radius, and rubber content has been investigated. PC with higher molar mass, notch radius, or rubber-modification possesses a higher critical strain rate when the ductile–brittle transition occurs. Whether a notched specimen will fail in a ductile mode or a brittle mode is already decided before the onset of the crack initiation. If size of the precrack plastic zone exceeds a critical level prior to onset of crack initiation, the crack extension developed later will propagate within the plastic zone and result in a ductile mode fracture. The precrack elastic storage energy, the input energy subtracting the hysteresis energy, is the main driving force to strain the crack tip for crack initiation. The precrack hysteresis energy (directly related to the precrack plasticity) increases with the decrease of the applied strain rate. Therefore, the strain rate is also closely related to the size of the precrack plastic zone. If the strain rate is lower than the critical strain rate, the specimen is able to grow a precrack plastic zone exceeding the critical plastic zone and results in a ductile mode fracture. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 655–665, 1997     

Structural explanation on natural draw ratio of metallocene-catalyzed high density polyethylene

The natural draw ratio of metallocen catalyzed high density polyethylenes was investigated with different crosshead speeds, molecular weights, and the cross-section shapes of sample specimens where the elastic components included in the conventional natural draw ratio such as residual strain and elastic aftereffect were eliminated. The perfect plastic deformation took place below a critical crosshead speed, whereas void formation occurred above the critical one. The natural draw ratio without elastic components and void formation was found to be dependent on the molecular weight of samples and the dimension of their specimens. On the basis of the SEM images, we proposed a simple structural model for explaining the necking formation in addition to the molecular weight and the cross-sectional shape dependences of the natural draw ratio.     

Plastic Deformation of Aluminum Oxide by Indentation and Abrasion

Transmission electron microscopy provided direct evidence that plastic deformation occurs during the room-temperature indentation and abrasion of Al₂O₃. Examination of single-crystal and polycrystalline specimens showed that high densities of dislocations are produced within the near-surface regions by mechanical polishing with a fine diamond compound (0.25 μm) and that plastic deformation by both slip and mechanical twinning occurs during the placement of Vickers microhardness indentations. The occurrence of plastic deformation in this normally brittle material is considered to be a consequence of the nature and magnitude of the local stresses developed under pointed indenters and irregularly shaped abrasive particles. Preliminary results on the effect of annealing on the retained substructure are also presented. Annealing at 900°C and higher resulted in the reduction of residual stresses through the motion of dislocations and their rearrangement into lower-energy configurations.     

Effect of deformation on the scintillation properties of barium fluoride

Data on the scintillation characteristics of new scintillation materials developed on the basis of barium fluoride by the methods of intensive plastic deformation are presented. Deformation was performed with the use of high-energy mills and quasi-static pressing within a compaction pressure interval of 250–4400 MPa. The structure of the obtained specimens was studied by the methods of scanning electron microscopy and X-ray diffraction. The study of the scintillation properties included the determination of the scintillation decay time and light output of the deformed specimens. The obtained specimens demonstrated a significant decrease in the scintillation decay time in comparison with single crystals. Specimens obtained at a compaction pressure of more than 2500 MPa were characterized by the lowest scintillation decay time.     

Scale effect in high-rate (spall) failure

Experimental results on the spall strength of copper in which the scale of the system was changed by a factor of ten show that the scale effect for high-rate one-dimensional strain depends on energy. The spall energy per unit surface area for failure increases with time. Arzamas-16. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 6, pp. 88–93, November–December, 1993.