Papers by Keyword: Disclination

Abstract: Atomic structure of nonequilibrium [112] tilt grain boundaries in nickel containing disclination dipoles is studied by means of molecular dynamics simulations. Initial systems for simulations are constructed by joining together pieces of two bicrystals one of which contains a symmetric tilt GB S=11 / 62.96° and the other a GB S=105 / 57.12°, or S=125 / 55.39°, or S=31 / 52.20°, so disclination dipoles with strengths w = 5.84°, 7.58° and 10.76° are created. Stress maps plotted after relaxation at zero temperature indicate the presence of high long-range stresses induced by disclination dipoles. Excess energy of GBs due to the nonequilibrium structure is calculated. Effect of oscillating tension-compression stresses on the nonequilibrium GB structure is studied at temperature T = 300 K. The simulations show that the oscillating stress results in a generation of partial lattice dislocations by the GB, their glide across grains and sink at appropriate surfaces that results in a compensation of the disclination stress fields and recovery of an equilibrium GB structure and energy.

Abstract: The influence of temperature fields and different reactive media on the structure and morphology of small icosahedral particles (ISPs) are investigated. It is shown experimentally that ISPs of micron size can be transformed into advanced functional materials with a developed surface. The leading role of disclination defects for the transformations occurring in ISPs is established. Potential fields of application of the new functional materials based on transformed ISPs are discussed.

Abstract: The paper deals with a mathematical model able to describe the presence of lattice defects of the crystalline materials, such as dislocation and disclination. Within the constitutive framework of second order plasticity developed by the author, the evolution equations to describe the disclinations that are compatible with the screw dislocations are derived.

Abstract: In order to investigate roles of grain boundaries on the improved fracture tough-ness in ultrafine-grained metals, interactions between crack tips, dislocations, and disclinationdipoles at grain boundaries are performed to aluminium bicrystal models containing a crackand h112i tilt grain boundaries using molecular dynamics simulations. A proposed mechanismto express the improved fracture toughness in ultrafine-grained metals is the disclination shield-ing effect on the crack tip mechanical field. The disclination shielding can be activated whena transition of dislocation sources from crack tips to grain boundaries and a transition of thegrain boundary structure into a neighbouring energetically stable boundary by emitting dis-locations from the grain boundary occur. The disclination shielding effect becomes large asdislocations are continuously emitted from the grain boundary without dislocation emissionsfrom crack tips. This mechanism can further shield the mechanical field around the crack tipand obtain the plastic deformation by dislocation emissions from grain boundaries, hence itcan be expected that the disclination shielding effect can improve the fracture toughness inultrafine-grained metals

Abstract: Deformation mechanisms of polycrystals as a function of the grain size in the 1nm…1cm interval are studied in this paper. The critical grain sizes are identified. Activity of dislocation and diffusion mechanisms is analyzed. The distribution of deformation in grains with different sizes within the same polycrystal is considered.

Abstract: The paper presents the results of the transmission electron microscopic (TEM) investigation of the structure and phase composition of nanocrystalline copper obtained by the severe plastic deformation using the high pressure torsion (HPT) method. Special attention is paid to the triple junctions of grain boundaries. It was established that the triple junctions contained the partial disclinations and particles of the secondary phases. The dependences of such junction fractions on the deformation were measured of the formation at nanocrystalline copper. The phase analyses of the secondary phase structure were carried out, the sizes of the phase particles and their volume fractions were determined. The bending – torsion of the crystal lattice arising near the triple junctions was measured. The problem of the long – range stress field screening was considered.

Abstract: Magnetic shape-memory alloys owe their exceptional properties primarily to the accompanying effects of a martensitic phase transformation. The twinning disconnection as elementary carrier of magnetic-field-induced deformation is the starting point of the present study. A disconnection is a line defect similar to a dislocation but located at an interface and exhibiting a step character besides a dislocation character. The mutual interaction of disconnections is fully tractable by the theory of dislocations. Due to the martensitic transformation, a hierarchical twin microstructure evolves, details of which are controlled through disconnection-disconnection interaction. Depending on the mutual orientation of twin boundaries on different hierarchical levels, twinning disconnections are incorporated in higher hierarchical twin boundaries forming disclination walls, or they stand off individually from those interfaces. Disconnections which stand off from interfaces contribute to magnetoelasticity, i.e. recoverable magnetic-field-induced deformation. Disconnections in disclination walls contribute to magnetoplasticity, i.e. permanent magnetic-field-induced deformation, if the twin thickness is large. In self-accommodated martensite with very thin twins, resulting from a martensitic transformation without training, the deformation is fully magnetoelastic and small. In single-domain crystals, resulting from effective thermo-magnetomechanical training, the deformation is fully magnetoplastic and large. Between these limiting cases, there is a continuous spectrum where, as a rule, the fraction of magnetoplastic strain and the total strain increase with increasing effectiveness of training.

Abstract: Physical interpretations of the incompatibility tensor are extensively discussed and applied to model several practically-important dislocation substructures in metallic materials. This paper firstly performs a tangible decomposition of the incompatibility tensor into the two types of defects by introducing Nye’s contortion tensor, and also clarifies the interrelationship with expressions given based on differential geometry. The effects on the evolutions of intra-granular substructures like cells and geometrically-necessary type bands are examined based on finite element simulations on multi-grain models under tension and simple shear with several representative orientations.

Abstract: Slip deformation phenomena in compatible type multi crystal models subjected to tensile load are analyzed by a finite element crystal plasticity analysis code, and accumulation of geometrically-necessary and statistically-stored dislocations (GNDs and SSDs) are evaluated in detail. Crystal orientations for the grains are chosen so that mutual constraint of deformation through grain boundary planes does not take place. We call these models as compatible type multi crystals, because “compatibility requirements” at grain boundaries are automatically maintained by slip deformation only on the primary systems and uniform deformation is expected to occur in each grain. Results of the analysis, however, show non-uniform deformation with high density of GNDs accumulated in a form of band. Growth of such kind of structure of GNDs caused localized accumulation of SSDs at grain boundary triple junctions. Mechanism for the band-shaped accumulation of GNDs in the compatible type multi crystals are discussed from the viewpoint of multi body interactions which arise from shape change of crystal grains after slip deformation.