Acoustic precursor of unstable plastic deformation in the aluminum-magnesium alloy AMg6

A relation between the acoustic signal and the dynamics of the first deformation bands at the initial stage of development of the plastic deformation jump in the artificially aged AMg6 alloy has been established using the high-speed video recording and simultaneous measurement of the acoustic emission signal. It has been demonstrated that a sharp increase in the potential of a piezoelectric transducer in the first approximately 10 ms of the development of the deformation jump is the most informative characteristic of the acoustic signal. The mechanisms of propagation of deformation bands and generation of acoustic signals, as well as the possible use of acoustic precursors of plastic deformation jumps, have been discussed.     

Deformation chaos and self-organization at the prefracture stage of 5556 alloy

Based on the analysis of the data of high-speed video recording of the surface of the deformed aluminum-magnesium alloy 5556, it has been found that the unstable plastic deformation associated with the deformation band dynamics tends to deformation chaos with increasing deformation. It has been shown that the ductile fracture of the 5556 alloy with a recrystallized structure should be considered as a global self-organization in a nonlinear nonequilibrium system being in the state of deterministic chaos.     

Mechanisms of deformation and fracture of the Al-Zn-Mg alloy

Laws of dislocation substructure evolution in the Al-Zn-Mg alloy subjected to compression and tension in different structural states are compared with laws of forming a deformation relief. It is established that long aging of the alloy changes the deformation localization mechanisms compared to its evolution in the alloy subjected to short-term aging. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 62–67, November, 2007.     

Plastic deformation macrolocalization during serrated creep of an aluminum-magnesium Al-6 wt % Mg alloy

The nonlinear dynamics of the space-time structure of macrolocalized deformation is studied by a set of high-speed in situ methods under the conditions of serrated creep in an aluminum-magnesium Al-6 wt % Mg alloy at room temperature. Macroscopic deformation jumps with an amplitude of several percent are detected in the creep curve of this alloy. It is found that a complex space-time structure of macrolocalized deformation bands moving in a correlated manner forms spontaneously in the material during the development of a deformation jump. The difference between the observed picture of deformation bands and the well-known Portevin-Le Chatelier classification of deformation bands is discussed.     

On the deformation and fracture behaviour of a Zr-based glassy alloy

The deformation behaviour of Zr₆₀Cu₁₆Ni₁₄Al₁₀ glassy alloy was found to exhibit cleavage-like fracture relief paired with typical vein patterns. The mirror-like cleavage area fraction is found to be comparable with that of vein areas. These two types of patterns alternate on the fracture surface in a direction normal to shear deformation. The structure of the studied alloy was characterised by X-ray diffractometry and scanning and transmission electron microscopy. A small volume fraction of crystals in the form of eutectic colonies were found here and there in the specimen. No distinct difference in the glassy structure of the cleavage-like fracture areas and the areas corresponding to vein patterns was found within the resolution of experimental techniques employed.     

Combustion of aluminum-magnesium alloy particles under microgravity conditions

The combustion of 200- to 600-μm single particles of 0.85 Al/0.15 Mg and 0.15 Al/0.85 Mg alloys in the CO₂ and O₂-N₂ (20: 80) media at pressures of 0.1 to 4.0 MPa is studied. The combustion occurred in a free-falling combustion chamber after ignition with a ruby laser. The specificity of the mechanism of the combustion of particles of these alloys is identified, and the combustion times of particles are determined. The character of this process (luminescence pulsations, fragmentation, etc.) is examined, and the solid combustion products are analyzed.     

Spectral and dynamic analysis of plastic instabilities during serrated creep of the aluminum-magnesium alloy

The force response to the development of a macroscopic plastic deformation jump under the conditions of serrated creep of the aluminummagnesium alloy 5456 has been studied using spectral and dynamic analysis methods. The flicker-noise structure of the force response indicating the self-organized criticality state has been revealed. It has been found that a short-term state of plastic instability flatter spontaneously appears during the development of the macroscopic deformation step.     

Kinetics of localized plastic flow during deformation and fracture of a D1 alloy

The stress-strain curve of a polycrystalline duralumine (D1) is studied to find three basic deformation stages: linear hardening, parabolic hardening (n = 1/2), and prefracture (n < 1/2). The results obtained show special features of macrolocalization of the plastic flow of the alloy under review. The distribution patterns of localized plastic flow domains develop according to deformation stages. The prefracture stage is characterized by self-correlated motion of the domains to the point of subsequent fracture. It follows from an analysis of the plastic flow localization kinetics that both hardening and softening domains coexist in the specimen in the prefracture stage. The domains move with a constant velocity inherent to each of them and linearly dependent on the position of their nucleation point. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 68–73, November, 2007.     

Quasi-static deformation and final fracture behaviour of aluminium alloy 5083: influence of cryomilling

The commercial aluminium alloy 5083 was processed via cryomilling to produce nanocrystalline (NC) powders with an average grain size of ～25–50?nm. The powders were subsequently degassed at 723 K (450°C), pre-heated and immediately quasi-isostatic (QI)-forged to produce a thermally stable bulk ultrafine grain (UFG) material having average grain size values ranging from 190 to 350?nm, depending on the processing conditions used. In this paper, the tensile properties and fracture behaviour of the bulk UFG material are presented and compared with the tensile properties of its conventionally processed counterpart. The specific influence of preheat temperature on strength and ductility of the alloy is briefly discussed. Three different pre-heat temperatures of 523, 623 and 723?K (250, 350 and 450°C) were chosen and used with the primary objective of controlling grain growth during forging. The influence of preheat temperature on tensile deformation and final fracture behaviour is highlighted. The macroscopic fracture modes of the bulk nanostructured material (BNM) prepared following three pre-heat temperatures are investigated. The microscopic mechanisms controlling tensile deformation and final fracture behaviour are discussed with regards to the intrinsic microstructural effects in the UFG alloy, nature of loading, and the kinetics and mechanisms of deformation.     

Nucleation mechanisms of macrolocalized deformation bands

For the first time, a phenomenological classification of the nucleation mechanisms of bands of macrolocalized deformation is formulated that is based on data from the high speed optical monitoring of a surface of aluminum-magnezium alloy deformed with a uniformly growing rate of applied stress, [(s)\dot] ₀ = const\dot \sigma _0 = const. The classification includes eight nucleation mechanisms that depend on the applied stress: from the initial stage of formation of the Lüders band to the periodical nucleation of the conjugative bands in the structure of a neck prior to sample rupturing. The role of different mechanisms of band nucleation in the general picture of the jerky deformation of a metal is discussed.     

High strain rate deformation and fracture of the magnesium alloy Ma2-1 under shock wave loading

This paper presents the results of measurements of the dynamic elastic limit and spall strength under shock wave loading of specimens of the magnesium alloy Ma2-1 with a thickness ranging from 0.25 to 10 mm at normal and elevated (to 550°C) temperatures. From the results of measurements of the decay of the elastic precursor of a shock compression wave, it has been found that the plastic strain rate behind the front of the elastic precursor decreases from 2 × 10⁵ s^?1 at a distance of 0.25 mm to 10³ s^?1 at a distance of 10 mm. The plastic strain rate in a shock wave is one order of magnitude higher than that in the elastic precursor at the same value of the shear stress. The spall strength of the alloy decreases as the solidus temperature is approached.     

Triaxial deformation in 104Rh of the chiral doublet bands

Triaxial relativistic mean field (RMF) approaches are used to study the properties of 104Rh. The existence of multiple chiral doublets is suggested for 104Rh based on the triaxial deformations and their corresponding proton and neutron configurations.     

Finely striped structure at the edges of localized deformation bands

Under conditions of high-rate loading, plastic strain localization is a result of tension in the zone of interference of unloading waves rather than of thermal softening. At stresses close to the dynamic strength of the material, the microstructure of localized strain bands consists of strongly deformed material, with a large number of incipient microdiscontinuities. At stresses below the Hugoniot elastic limit, the microstructure looks as a set of barely visible stripes. The finely striped structure at the edges of the bands of a spall damage arises as a result of the stretching of initially rounded damage centers attached to the matrix material during dynamic deformation.     

Mesoscopic-and macroscopic-level plastic deformation and fracture of ion-modified Ni50Ti40Zr10 alloy with the shape-memory effect

The influence of the ion-modified surface layer on the ductility, shape-memory effect, and mesoscopic structure of the surface of fracture was studied for Ni₅₀Ti₄₀Zr₁₀ alloy. It was found that ion implantation increases the microhardness of the surface layer about 1 μm thick but makes the bulk of the alloy more ductile. The mesoscopic structures of the fracture surfaces of the initial and implanted specimens suggest the formation of a specific layer immediately under the irradiated surface. Its thickness is a multiple of the grain size of the B2 phase. The mesorelief of this layer and its ductile properties differ from those of the nonirradiated specimens. After such a surface treatment, the shape memory effect parameters do not degrade, and the temperature cycling resistance is even improved.     

Computer simulation of the deformation and fracture of crystals

Methods for computer simulation of strength testing of crystals are proposed. The methods employed are similar to usual static methods, and they are used to investigate deformation and fracture of perfect fcc crystals having different orientations with respect to the tensile force. A strain-induced phase transition from the fcc to the hcp structure is detected, and the formation and displacement of crystal twins are observed. Plastoelastic deformation and fracture of crystals are investigated.     

数字剪切散斑干涉法研究铝合金中Portevin-Le Chatelier带的离面变形行为

作为Portevin-Le Chatelier(PLC)带的重要特征之一,其离面变形仍缺乏实验研究.本文提出使用数字剪切散斑干涉法研究GB6061铝合金中PLC带的离面变形.通过图像相减得到的条纹图,实时观察了PLC带的几何形貌和传播过程,并获得了PLC带离面位移分布.在1/15 s内,PLC带的最大离面位移为245 nm,位置偏向于PLC带传播的前沿.在条纹图中,PLC带传播前沿的亮条纹始终较窄.此外,实验还观察到PLC带位置变更和倾角转向的演化过程.实验表明,数字剪切散斑干涉法具有高灵敏度和防震性,是研究PLC带离面变形简便有效的方法.     

An analysis of the change in the Al3Mg2 microparticle size distribution upon heating of an aluminum-magnesium alloy

Ostwald coarsening of dispersed Al₃Mg₂ particles distributed in an aluminum-magnesium alloy (80% Al+20% Mg) is used to illustrate a method of determining the similarities and differences between an experimental histogram (from an image) and a corresponding theoretical distribution, chosen from a collection of different solutions. The determination of a link between the indicators in experimental histograms and the processes generating the transformation is demonstrated. In particular, solubility and growth of Al₃Mg₂ microparticles upon heating of an alloy strongly depend on the variations in structure and properties of material near interphase boundaries. Zaporozhian State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 8–13, February, 1993.     

Morphological transition from the Euclidean to the fractal shape of the Lüders band in the aluminum-magnesium alloy AMg6

The spatio-temporal structure of the Lüders band in artificially aged and recrystallized AMg6 alloys deformed under uniaxial tension at a constant stress rate = const has been investigated using high-speed video recording. A kinetic morphological transition has been revealed between the Euclidean and fractal shapes of the Lüders band due to a transformation of the initial microstructure of the alloy from the precipitation structure formed as a result of the artificial aging to the collective recrystallization structure. The mechanisms of formation of the dendrite-like fractal structure of the Lüders front have been discussed.     

Emission processes accompanying deformation and fracture of metals

Present-day physical methods of investigation reveal that the fracture and plastic deformation of metals is accompanied by emission processes, in particular, by luminescence and emission of electrons. All the metals studied thus far exhibit a capability of luminescence. The intensity, duration, and spectrum of mechanoluminescence are different for different metals. The intensity is determined by the mechanical and thermal characteristics. For a given metal, the intensity depends on dislocation density in the structure and the sample loading rate. The spectrum of noble metals is governed by the electronic structure of surface states. The dynamics of mechanoluminescence and electron emission (exoemission) depends on the rate of stress variation in the sample under study. This permits one to consider the mechanoluminescence and exoemission not only as physical characteristics but also as a potential tool for probing surface states in metals and the kinetics of emergence of mobile dislocations on the surface with a high time resolution. Fiz. Tverd. Tela (St. Petersburg) 41, 841–843 (May 1999)