Mechanical behavior of microcrystalline aluminum-lithium alloy under superplasticity conditions

Aluminum-lithium alloy 1420, which, after equal-channel angular pressing, has a grain size of about 3 μm, is shown to possess superplasticity in a temperature range of T=320–395°C upon tension at a constant relative strain rate of 10⁻²–10⁻³ s⁻¹. The axial deformation at fracture can exceed 1800%. The data processing at such large deformations should be carried out using true strains ɛ_t and stresses σ_t. In the flow curve, a short stage of hardening is followed by a long softening stage. They can be described by the relation with a constant exponent n≈2 and activation energies U≈1 eV for the softening stage and U≈1.4 eV for the hardening stage. The deformation is supposed to be controlled by grain-boundary sliding at the stage of softening and by self-diffusion in the bulk of grains at the hardening stage. __________ Translated from Fizika Tverdogo Tela, Vol. 43, No. 5, 2001, pp. 833–838. Original Russian Text Copyright ? 2001 by Myshlyaev, Prokunin, Shpeizman.     

High-speed superplasticity of microcrystalline alloys under conditions of local grain boundary melting

A model is proposed for the high-speed superplasticity of materials under conditions of local grain boundary melting at temperatures close to solidus. It is shown that the local melting of grain boundaries containing segregations of impurity atoms, results in the formation of a structure consisting of liquid-phase regions and solid intergranular bridges which provide cohesion of the grains during the deformation process. The equilibrium concentration, dimensions, and activation energy for the formation of solid bridges are determined as a function of the temperature, initial impurity concentration in the boundary, and the boundary thickness. A mechanism is proposed for grain-boundary slip under conditions of local grain boundary at anomalously high strain rates. Zh. Tekh. Fiz. 68, 38–42 (December 1998)     

Structure and phase composition of an Al-Mg-Li-Zr alloy under high-rate superplasticity conditions

Uniaxial-tensile tests are performed on samples of a commercial aluminum-lithium alloy subjected to equal-channel angular extrusion. It is found that the material under study has a highly fine-grain structure and exhibits superplasticity under tension. The microstructure of the samples is studied during their plastic deformation.     

On the multistage nature of deformation of the microcrystalline aluminum-lithium alloy 1420 under superplasticity conditions

The constant-rate tensile deformation and creep of aluminum-lithium alloy 1420 with a grain size of 3 µm (obtained by equal-channel angular extrusion) exhibiting superplasticity at temperatures of 600–670 K and relative-deformation rates of 10^?2–10^?3 s^?1 are considered. It is shown that, upon tension at a constant rate V _m, a steady-state segment appears in the true stress σ_t-true strain ?_t dependence, which is described by the expression $\dot \varepsilon _t \sim \sigma _t^n \exp ( - U/kT)$ with constant coefficients, and that the rate of deformation $\dot \varepsilon _t$ is close to the creep rate at comparable stresses and strains. The conclusion is made that, upon deformation under superplasticity conditions, an equilibrium structure is formed, which remains unaltered in the process of further deformation until the sample goes over (because of geometrical conditions) to a prefracture state.     

Strain hardening of alloys with face-centered cubic lattice

A brief review is given of the peculiarities of the strain hardening of face-centered cubic (fee) alloys as compared with pure metals. The fundamental equations connecting the strain hardening characteristics to the quantitative characteristics of the dislocation structure and the slip trace picture are considered in application to alloys with a high friction stress of a nondislocation nature _F. The shape of the strain hardening curves of alloy mono- and polycrystals is analyzed; it is shown that it depends substantially on the stress level F.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 132–150, August, 1976.     

Magnetic structural properties in Alnico alloys under equilibrium conditions and under magnetization

Magnetic structure is studied as a function of the orientation of the easy magnetic axis with respect to ac (4 Hz) and dc magnetizing fields in specimens with various coercivities. Alternating boundary polarity was observed, and a system of magnetization distribution in the equilibrium state is given. Domain width in equilibrium states is measured for various angles of the demagnetizing field. Upon magnetization at various angles, various structural change processes were observed, which have been conditionally divided into four distinct types.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 68–73, August, 1975.     

Strain hardening in polymer glasses: limitations of network models

Simulations are used to examine the microscopic origins of strain hardening in polymer glasses. While traditional entropic network models can be fit to the total stress, their underlying assumptions are inconsistent with simulation results. There is a substantial energetic contribution to the stress that rises rapidly as segments between entanglements are pulled taut. The thermal component of stress is less sensitive to entanglements, mostly irreversible, and directly related to the rate of local plastic rearrangements. Entangled and unentangled chains show the same strain hardening when plotted against the microscopic chain orientation rather than the macroscopic strain.     

Strain hardening behaviour and the Taylor factor of pure magnesium

Taylor orientation factors for strain hardening in textured and random polycrystals of magnesium were derived from the ratio of the strain hardening rates of polycrystals to that of single crystals deforming by equivalent polyslip. For polycrystals with textures that inhibit basal and prismatic slip while favouring pyramidal polyslip, the Taylor factor is estimated to be between 2.1 and 2.5, increasing to about 4.5 for randomly textured polycrystals. The micromechanics of strain hardening in polycrystals are discussed.     

Melt structural self-organization in the transition layer during crystal growth under microgravity conditions

A brief review is given of the results obtained and published in 2003–2007 on the study and simulation of Ge:Ga, Ge:Sb, GaSb:Te, and InP:S single crystal growth from stoichiometric and nonstoichiometric melts aboard Foton satellites. The promising use of microgravity conditions in research of structural self-organization processes (cluster formation) in the crystal-melt transition layer during solidification is shown and substantiated. A mathematical model of heat and mass transfer that takes into account the two-phase character of the medium in boundary layers near the interface is created for the first time as an independent tool for studying these processes. Prospects for the development of this new field of space material science are discussed.     

Modifying the structural phase state of fine-grained titanium under conditions of ion irradiation

The results from quantitative investigations into the structural phase state of finely dispersed titanium before and after implantation with aluminum ions are presented. Two types of ??-Ti grains differing by phase composition, defect structure, and size are distinguished in the structure: fine grains in the range of 0.1?C0.5 ??m and coarse grains in the range of 0.5?C5 ??m. The presence of two types of TiO₂ particles in the material, i.e., rounded particles found at dislocations in the bulk of the ??-Ti grains and lamellar particles found only inside coarse ??-Ti grains, is established. The formation of the Ti₃Al phase is observed in the form of lamellar inclusions along the grain boundaries and rounded particles in triple joints. It is found that the particles of the TiAl₃ phase are isolated with a smaller volume fraction than the Ti₃Al phase; they are localized along the boundaries of coarse grains of the titanium matrix. It is established that the granular state and defect structure of the material change substantially after ion irradiation; i.e., the dislocation density and the fields of internal stresses in fine grains grow considerably, relative to the initial state of titanium.     

The structural and magnetic properties of Co-doped titanate nanotubes synthesized under hydrothermal conditions

Co-doped titanate nanotubes were prepared from hydrothermal treatment on Co-doped anatase TiO₂ powders in a concentrated NaOH aqueous solution and characterized by a variety of techniques. We mainly investigated the structural and magnetic properties of these nanotubes. It was found that the obtained nanotubes might possibly be constructed from H₂Ti₃O₇, their undoped counterpart. The result from Raman spectra showed that Ti–OH bonds existed in the tubular structure. The electron paramagnetic resonance and X-ray photoelectron spectroscopy studies clearly showed that the Co was incorporated into the titanate lattice as Co²⁺ and substituted for Ti cations sites. Also, from the photoluminescence measurement, the presence of oxygen vacancies was detected in the nanotubes. The vibrating sample magnetometer measurement revealed a clear paramagnetic behavior with weakly ferromagnetic ordering at room temperature. The results suggested that ferromagnetism of the nanotubes was related to the oxygen vacancies and could be suppressed by their quantum size. PACS 81.07.De; 75.75.+a; 75.50.Pp     

Current problems of superplasticity

The possibility of conversion of intermetallic materials (Ti-Al, Fe-Cr-Co and Mn-Al-C) into the superplastic state is shown. The microstructural changes occurring during the superplastic flow in the Ti-Al intermetallics are analysed. A very important role of grain boundary structure in the superplastic deformation of intermetallics is shown. The experimental results obtained are interpreted on the basis of a model taking into account interactions between lattice dislocations and grain boundaries during the superplastic deformation.     

Strain localization under dynamic loading

A spallation model of strain localization is suggested according to which localization bands under pulsed loading result from unloading wave interference such that negative stresses in the extension zone are lower than the ultimate strength of the material. The temperature in the localization band is estimated to be close to the environmental temperature.     

Specific features of structural defects in twisted nematic liquid crystals under conditions of electrohydrodynamic instability

The influence of inhomogeneous boundary conditions (director orientation) on the specific features of the formation and evolution of structural defects in 90·-twisted nematic liquid crystals (twisted structures) is investigated in the regime of electrohydrodynamic instability. It is found that, unlike the domain structure of nematic liquid crystals with a planar orientation, in which defects with topological indices of ±1 are formed under conditions of electrohydrodynamic instability, the domain structure of twisted nematic liquid crystals contains both the above defects and defects with a topological index of 0. It is shown that structural defects with a topological index of 0 are stable and that the existence of these defects is associated with the axial velocity u_a of nematic liquid-crystal flow in the domains.