Structural states and specific features of the deformation behavior of metals and alloys with mixed nano-and micrograin structure

A classification of the structural states of materials with a mixed nano-and microcrystalline structure is proposed. Theoretical analysis of the structural mechanisms and peculiarities of plastic flow of singlephase and two-phase nanostructured metals and alloys with a bimodal size distribution of grains and phases is performed. The effect of grain-boundary and dislocation mechanisms of plastic flow on the specific features of the deformation behavior and plasticity of nanocrystalline materials is analyzed. A microstructural model of strain hardening of a material with two-scale nano-and micrograin structure is proposed and the condition for the loss of plastic flow stability of such a material is investigated. The dependence of the yield strength and uniform strain of nanocrystalline materials with a two-scale structure on the grain size and the ratio of the volume fractions of the nano-and microstructural components is calculated.     

Relaxation cascade in solids: microscopic theory

The relaxation of an electron excited to the high energy region, is accompanied by the creation of various excitations (plasmons, quasi-particles, phonons). The stages of this many-body, non-stationary phenomenon (cascade) are described microscopically. The electron distribution function n(ɛ, t) and characteristic times for the whole energy range, are calculated. Received 7 February 2003 Published online 11 April 2003     

Characteristics of the relaxation to steady-state deformation in solids

The characteristics of the transient process of relaxation to steady-state deformation in solids are investigated theoretically. Various loading regimes are modeled by the method of mobile cellular automata. It is shown that the stressed state in a material is highly inhomogeneous in the relaxation stage; this property, in turn, can produce stable structures in the velocity field of the material particles and influence the evolution of deformation in later stages. Zh. Tekh. Fiz. 67, 34–37 (September 1997)     

Atomistic deformation modes in strong covalent solids

We report on a first-principles study of the structural deformation modes in diamond, cubic boron nitride (c-BN), and cubic BC2N. We show that (i) the diamond C-C bonds remain strong up to the breaking point, leading to the large and nearly identical shear and tensile strength, (ii) c-BN exhibits a shear failure mode different from that in diamond and a significant softening in the B-N bonds at large tensile strains long before the bond breaking, and (iii) cubic BC2N displays a large disparity between the shear and tensile strength, contrary to the expectation for the hybrid of diamond and c-BN. We examine the microscopic bond-breaking processes to elucidate the atomistic mechanisms for the deformation modes and the implications for material strength.     

Dynamics of plastic deformation: Waves of localized plastic deformation in solids

Conclusions It has been shown here that a localized plastic deformation in structurally inhomogeneous media can be of a wave nature and can propagate in the form of nonlinear plastic waves, not only at the microscopic level but also at the mesoscopic level. It has been established that there is an interrelationship between this new effect and grain-boundary slippage (an effect which has been under study for a long time) and also with certain types of quasiviscous fracture in plastically deformable materials.We have discussed certain specific practical problems in the mechanics of plastic deformation, and for certain types of fracture. In the future, these problems will be discussed at a more profound level and in greater detail, because of experimental studies which are presently being carried out on the dynamics of deformation for various types of loading and fracture [17, 18, 31]. We hope that the approach proposed here for a theoretical study of the localization of deformation and fracture can be taken to study such effects as splitting off [31], the influence of defect fluxes on grain-boundary slippage [22], superplasticity [23], the behavior of tectonic faults and boundaries of various types [32], electroplastic and magnetoplastic effects, and high-temperature localization of deformation [25].The general nature of the approach proposed here results from the circumstance that a localization of deformation is present explicitly or implicitly during plastic deformation, and the behavior of this deformation plays a role of fundamental importance in the propagation of plastic deformation through a material.The author wishes to thank V. E. Panin for a constant discussion of this problem and I. O. Nedavnii for carrying out the numerical calculations.V. V. Kuibyshev Tomsk State University. Institute of Strength Physics and Materials Science. Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 19–41, April, 1992.     

Inhomogeneity of deformation of solids at the nanometer level

A new method for processing interferometrically recorded deformation data has been implemented for studying an inhomogeneity in the rate and parameters of deformation jumps at the nanostructure level, which provides detection of deformation jumps of less than 300 nm. It is shown that the lower limit for deformation jumps lies in the range 10–30 nm for aluminum and is 130 nm for amorphous polymer (poly(methyl methacrylate)). It is assumed that the sizes of jumps correspond to scales of ordered structures, as was previously established for higher level structures. The results obtained make it possible to investigate more thoroughly the multilevel character of deformation and to evaluate the sizes of the nanostructural units, their evolution during deformation and under the effect of external fields, as well as their relation to the microscopic and macroscopic inhomogeneities of deformation.     

Activation volumes for formation and motion of defects in crystalline solids

By consideration of the factors that affect the activation volume for formation, ΔV_F, and for motion, ΔV_M, of a diffusing species, it is suggested that the sign dependence of the activation volume for self-diffusion, ΔV_D, can be predicted. Our predictions are based on the assumption that (1) interstitial diffusion is associated with an abnormally low activation energy for diffusion, (2) vacancy diffusion is present where normal activation energies are observed, and (3) a disordered, liquid-like region is produced when the diffusing species moves into the activated state. Several examples are presented to illustrate our proposed model.     

Kinetic ion-induced electron emission from the surface of random solids

This is a review of the published papers devoted to kinetic ion-electron emission from the surface of polycrystalline and amorphous solids. It contains the analysis of energy, angular, temperature, and other dependences of the ion-electron emission coefficient as well as of the energy and angular distributions and the statistics of electron emission events. We also give a brief account of the theoretical studies on the subject.     

A new type of plastic deformation waves in solids

The plastic flow of metals and alloys in a single-crystal and a polycrystalline state involves processes in which different types of wave are generated. In the easy-glide and linear work-hardening stages of flow, waves of new type are found to propagate. The motion velocity of these waves is found to be inversely proportional to the work-hardening coefficient. An attempt is made to relate the new wave type to the self-organisation phenomena observable in deforming crystals. The propagation rate of these waves is shown to depend on the energy flow through the specimen tested. The space period of the local strains region is proportional to the logarithm of the specimen length.     

Physics of megaplastic (Severe) deformation in solids

The main regularities of structural and phase transformations occurring in solids have been analyzed experimentally and theoretically within the framework of the concept of manifestation of additional channels providing the dissipation of an elastic energy introduced into a solid under megaplastic deformation. It has been demonstrated that an active participation of low-temperature dynamical recrystallization processes, phase transitions of the type crystal ai amorphous state, and thermal effects under the conditions of an insufficient efficiency of the dislocation and disclination relaxation modes can consistently explain almost all the experimental results obtained for very severe plastic deformations.     

Evolution of microscopic cracks and pores in solids under loading

Experimental data on the development and partial healing of microscopic cracks and pores in loaded crystalline materials are considered. An analysis of the data indicates that fracture development has a number of specific features depending on the state of the materials and the testing conditions and is a kinetic thermal fluctuation process occurring virtually throughout the entire time of loading.     

Energy balance in deformation mechanics of solids at low temperatures

The model of a solid in the form of an ensemble of independent anharmonic oscillators arranged in a uniform stress field has been considered to analyze the energy balance during adiabatic mechanical loading of a solid at low temperatures. Oscillator elongation is determined as the average over the ensemble, and a part of its energy is matched to this quantity. This part has the physical meaning of the mechanical energy of sample deformation and becomes a part of the energy balance upon deformation. After averaging, the uniform force field is replaced by the resultant force associated with the average deformation. Another component of the balance at low temperatures is the energy of zero-point vibrations of oscillators. Thus, upon mechanical deformation of a solid, the energy exchange occurs between two scale levels: the atomic vibration energy at a microlevel and the macroscopic deformation energy of the sample as a whole.