Vol 22, No 5 (2019)

Ultrasonic and electron beam treatment of commercial titanium VT1-0 and its alloy VT6 (Ti-6Al-4V) produces a nonequilibrium grain-subgrain hierarchical substructure in the surface layer, which causes a multiscale fragmentation of the material and reveals a damping effect. When cooled in the gradient temperature field (during electron beam treatment) and when the β phase of the initial alloy is destroyed by ultrasound, the high-temperature bcc structure of the surface layer undergoes a nonequilibrium phase transition into an hcp α-phase structure. The excess specific volume of the β phase is hierarchically distributed in the a phase through the growth of nonequilibrium α′ and α″ martensite, and in the form of local ω-phase precipitation along the grain boundaries of the α phase. The specific volume of the nonequilibrium phases exceeds the specific volume of the α phase. This eliminates the formation of micropores and causes material fragmentation at the micro- and nanoscale structural levels during the nonequilibrium β → α phase transition. The growing α′ laths cause the fragmentation of the α phase at the microscale level. The α″ laths grow within the nonequilibrium α′ laths; they have a thickness of ∼1.5 nm and fragment the material at the nanoscale level. This process is controlled by the electronic subsystem that creates nanoscale mesoscopic structural states for the formation of nonequilibrium martensite phases. The reversible elastoplastic deformation of the nonequilibrium martensite phases at the nanoscale level governs the damping effect of the surface layer subjected to ultrasonic or electron beam treatment. The generation of nanoscale mesoscopic structural states and the related new mechanism of reversible deformation in the conditions of broken translational invariance of the lattice in a deformable solid has been confirmed experimentally.

The paper reports on a molecular dynamics simulation of plastic deformation in polycrystalline titanium under scratch testing with explicit account of crystallographic orientations determined by electron backscatter diffraction for individual Ti grains. The simulation shows that the presence of a grain boundary breaks the lattice translation invariance and induces a constrained strain zone in which the deformation changes its dislocation mechanism for rotations such that misoriented local regions appear near the grain boundary. The pattern of consistent dynamic rotations of atoms near the grain boundary is governed by the crystallographic orientation of grains. If the indenter sliding direction coincides with one of the easy slip directions of a loaded grain, the material in the grain boundary region is fragmented and atomic clusters move along the grain boundary plane from the surface deep into the material. The simulation results allow us to explain why the profile of scratches differs depending on the scratching direction.

The paper explores the effect of high-temperature cross rolling followed by cold rolling on the internal structure of metastable Fe-Cr-Mn austenitic stainless steel, formation of nonequilibrium martensite ε and α′ phases in it, dynamic rotations on fracture surfaces, fatigue life under alternating bending, and wear resistance. Scratch testing revealed a strong increase in the damping effect in the formed hierarchical mesoscopic substructure that promotes the formation of a nanocrystalline grain structure, formation of hcp ε martensite and bcc α′ martensite in grains, formation of a vortical filamentary substructure on the fracture surface, and an increase in the high-cycle fatigue properties and wear resistance. These processes are associated with a high density of nanoscale mesoscopic structural states that arise in lattice curvature zones during high-temperature cross rolling followed by cold rolling with smooth rolls. The described effects are explained by the self-consistent mechanical behavior of hcp ε martensite laths in fcc austenite grains and bcc α′ martensite laths formed in cold rolling of the steel after high-temperature cross rolling.

Superplastic forming is promising for the manufacturing of complex-shaped metal parts and components. Superplastic conditions allow one to produce unwelded parts, to reduce the number of technological operations and forming forces, and thus to reduce tool wear. Optimal processing modes can be determined using mathematical models based on constitutive equations. However, there are serious difficulties in describing even the simplest uniaxial tests with superplastic behavior because of a very complex deformation scenario, which involves several interacting physical mechanisms with changing roles and significant structure evolution of the material. A similar situation is observed in technological processes involving superplasticity. The search for better technologies requires the mathematical models of material deformation that are able to describe changes in the structure and structure-dependent physicomechanical properties. The most promising in this respect is a multilevel crystal plasticity approach based on the introduction of internal variables and an explicit description of the material structure and physical deformation mechanisms. Here we propose a multilevel model for describing the behavior of polycrystalline metals and alloys with account for key plastic and superplastic deformation mechanisms, such as intragranular dislocation slip, crystal lattice rotation, and grain structure evolution. Special attention is paid to the description of grain boundary sliding, which is the leading mechanism in superplastic deformation, and accompanying accommodation mechanisms, such as grain boundary diffusion and dynamic recrystallization. When describing grain boundary sliding, viscous-to-plastic transitions along crystallite boundaries are considered explicitly, and the submodel is attributed to a separate structural level. The model takes into account the interaction between grain boundary sliding and intragranular slip. The influx of intragranular dislocations into the boundary increases the amount of defects in it (making it nonequilibrium), increases the boundary energy, and promotes grain boundary sliding. On the other hand, grain boundary sliding reduces the number of grain boundary defects and hence the resistance to intragranular slip. The obtained numerical results agree well with experimental data, showing that the proposed multilevel model is suitable for describing various inelastic deformation modes and transitions between them.

The advances in new smart materials are linked with attosecond subatomic technologies which can create entangled subatomic electron pairs by attosecond hard ultraviolet and soft X rays in addition to one-electron excitations by femtosecond optical pulses. The quantum infrastructure of nonequilibrium physical media in smart materials is provided by the Fermi and Bose gas of quasi-electron excitations, and the quantum infrastructure of their space-time scales is specified by the quantum mechanisms of two-electron attophysics and one-electron femtochemistry. The primary scale is subatomic (1.0 pm to 0.1 nm), and the next scale is supra-atomic (0.1 to 10.0 nm), being the scale of nanoelectromechanical systems of sensors and actuators that provide self-organization in the space-time hierarchy of nano-, micro-, meso-, and macroscopic dissipative structures of nonequilibrium physical media in smart materials. Here we show that any quantum nanoelectromechanical system can alternately be a sensor and an actuator of dissipative structures with a two-clock cycle: an attosecond sensor of entangled two-electron excitations and a femtosecond actuator of electromechanical motion modes in a nonequilibrium physical medium. For such nanoelectromechanical sensors-actuators, the motion rhythm is three orders of magnitude faster than for femtosecond nanomolecular sensors of one-electron excitations and nanomolecular actuators of vibrationrotation modes in nonequilibrium physical media.