Grain Boundary Sliding in Fatigue of Pure Aluminum

The distributions of plastic strain near grain boundaries induced by fatigue loading were investigatedby the fiducial grid method in pure aluminum specimens, and the resulted grain boundary sliding(GBS) was systematically analysed. The results show that the strain field near a grain boundary isnonuniform. GBS is restricted by the junction of grain boundaries and causes discontinuities of bothdisplacement and strain. A peak value of shear strain was created in short-range area across the grainboundary. GBS plays an important role in cyclic softening and secondary hardening. The control fac-tor of GBS is the relative orientation between two grains and the macro orientation of the grainboundary rather than the ∑ value of the boundary.     

Detection of grain-boundary resistance to slip transfer using nanoindentation

Nanoindentation measurements near a high-angle grain boundary in a Fe-14%Si bicrystal showed dislocation pile-up and transmission across the boundary. The latter is observed as a characteristic displacement jump, from which the Hall–Petch slope can be calculated as a measure for the slip transmission properties of the boundary.     

Molecular dynamics simulation of copper bicrystal response to shear loading

The behavior of a large-angle grain boundary of the Σ = 5 (210)[001] special type in a copper bicrystal under shear loading conditions has been computer simulated. It is established that, simultaneously with the relative slippage of grains in the direction of applied load, the grain boundary shifts in the direction perpendicular to that of shear straining. This motion of the grain boundary exhibits a discrete character and leads to a growth of one grain at the expense of another. The mechanism of this displacement is analyzed and the influence of the loading rate and direction on the character of grain boundary motion is studied. The obtained results provide better understanding of the atomic mechanisms of plastic strain development in polycrystalline materials.     

Simulation of the behavior of a Σ5 grain boundary under combined thermal and external shear loading

The behavior of the grain boundary of a copper bicrystal under shear loading is studied by molecular dynamics simulation. The subject of the research is a high-angle boundary of the Σ = 5(210)[001] special type. The effect of the temperature of the sample on the characteristics of the behavior of the grain boundary under shear deformation is analyzed. It is shown that, in the sample heated above a certain temperature, the previously found mechanism of grain-boundary sliding in fcc crystals, which is accompanied by a simultaneous motion of the boundary in the direction perpendicular to the applied load, gives way to the conventional mechanism of grain-boundary sliding associated with the displacement of the grains relative to each other along the plane of the defect. The features of the change in the structure of the grain boundary upon a change in its response to shear deformation with increasing temperature are studied.     

A method of analyzing elastic constraints due to grain or interphase boundaries

Elastic constraint due to grain or interphase boundaries is considered by taking a bicrystal under a general boundary condition as an example. The concept of reference deformation is introduced on which the effect of elastic constraints is superposed to get the actual state of deformation. Reference deformation accompanies, in general, incompatibilities of both traction and displacement at the boundary. Tractional incompatibility is resolvable by solving an elasticity problem in which requisite body force is distributed along the boundary. Incompatibility of displacement can be nullified in two steps. First, an arbitrarily chosen point within the boundary is joined by a rigid translation of either one of the boundary faces. Then, the incompatibility relative to that point is nullified by introducing the surface dislocations within the boundary, the density of which is proportional to an external force and vanishes as the external force ceases to act.     

Direct observation of microscopic plastic deformation of stainless steel using lattice drawn by focused ion beam of Ga+

Abstract

Visualisation of the microscopic deformation of a stainless steel was attempted. A mirror polished, flat surface specimen was subjected to a simple tension test, and the deformation of a fine lattice drawn by a focused ion beam (FIB) of Ga⁺ was observed. The depth of the lattice was of the order of a few tens of nanometres. The penetration depth of Ga⁺ was estimated using SRIM software, and the result indicated that the use of a FIB might not cause serious detriment to the mechanical properties of the lattice surface. After testing, lattices were examined by field emission scanning electron microscopy (FESEM). The results showed that displacement was continuous in the grain as well as across the grain boundary, and the microscopic deformation was categorised into three patterns: (a) a clear thin layer of shear deformation which was discontinuous across the grain boundary, (b) an area of uniform deformation inside this thin layer and (c) microscopic shear bands appearing sporadically in the grains.     

Interaction of lattice dislocations with a grain boundary during nanoindentation simulation

Interaction of dislocations with a Σ = 5 (210) [001] grain boundary was investigated using molecular dynamics simulation with EAM potentials. The results showed that the dislocation transmitted across the grain boundary during nanoindentation and left a step in the boundary plane. Burgers vector analysis suggested that a partial dislocation in grain I merged into the grain boundary and it was dissociated into another partial dislocation in grain II and a grain boundary dislocation, introducing a step in the grain boundary. Simulation also indicated that, after the transmission, the leading partial dislocation in the grain across the boundary was not followed by the trailing partials, expanding the width of the stacking fault. The results suggested that the creation of the step that accompanied grain boundary motion and expansion of the stacking fault caused resistance to nanoindentation.     

Modelling of small fatigue crack growth interacting with grain boundary

The slip band at the tip of a small fatigue crack interacting with grain boundaries is modelled for four cases: a slip band not reaching the grain boundary, a slip band blocked by the grain boundary, a slip band propagated into an adjacent grain, and a slip band propagated through one and then blocked by a second grain boundary. The theory for continuously distributed dislocations is used to calculate the crack-tip sliding or opening displacement and the microscopic stress intensity factor under tensile and shear loading. Assuming that the range of the tip displacement directly determines the propagation rate of both Stage I and II cracks, prediction of the propagation behavior of a small crack is made as a function of the distance between the crack tip and the grain boundary, and of the difficulty to propagate slip into adjacent grains, as well as a function of crack length and stress level. The directions for further development of modelling are discussed.     

Effect of dopants on alumina grain boundary sliding: implications for creep inhibition

We investigate by means of periodic density functional theory the mechanism of grain boundary sliding along the α-alumina Σ11 tilt grain boundary. We identify minimum and maximum energy structures along a preferential sliding pathway for the pure grain boundary, as well as for grain boundaries doped with a series of early transition metals, as well as barium, gadolinium, and neodymium. We predict that the segregation of those dopants results in a considerable increase in the grain boundary sliding barrier. Grain boundary sliding occurs by a series of bond breaking and forming across the grain boundary. Our results suggest that the presence of large cations inhibits the regeneration of bonds during sliding, which results in a decrease in total number of bonds across the grain boundary interface, thereby raising the barrier to sliding. Trends in predicted grain boundary sliding energies are in good agreement with recently measured creep activation energies in polycrystalline alumina, lending further credence to the notion that grain boundary sliding plays a dominant role in alumina creep.     

Nonuniform cleavage cracking across persistent grain boundary

The nonuniform characteristics of cleavage cracking across high-angle grain boundaries are analyzed in considerable detail. To break through a grain boundary, a cleavage front would first penetrate across the boundary at its central part, with the side sections being locally arrested. Such a front behavior causes a strong crack trapping effect and a large increase in required crack growth driving force. Eventually, as the persistent grain boundary areas are separated apart, the crack front bypasses the grain boundary. The critical condition of the unstable crack propagation is determined by both the local fracture resistance and its increase rate with respect to the expansion of the break-through window. The grain boundary toughness is dominated by the effective grain boundary ductility.     

Oxygen diffusion blocking of single grain boundary in yttria-doped zirconia bicrystals

Yttrium-doped zirconia bicrystals with [001] symmetric tilt Σ 5 grain boundaries were fabricated by a diffusion bonding technique, and oxygen diffusion behavior across the grain boundary was measured by secondary ion mass spectrometry (SIMS), tracing ¹⁸O isotope. It was found that the ¹⁸O fraction across the boundary exhibited explicit decrease around the boundary plane, indicating that the oxygen-diffusion is blocked by the presence of the Σ 5 grain boundary. This is the first experimental detection of oxygen diffusion blocking at a single grain boundary in zirconia ceramics. From high-resolution transmission electron microscopy observations and energy dispersive X-ray spectroscopy analysis, neither amorphous layers nor Si impurity segregation were found at the grain boundary. The grain boundary blocking effect of the Σ 5 boundary must be an intrinsic feature arising from its core structure and yttrium solute segregation of the grain boundary.     

Molecular Dynamics Simulation for Grain BoundaryDeformation under Tensile Loading Condition

［1］H.George, J.H.Bishop, Ralph, T.Kwon and S.Yip: J. of Appl. Phys., 1982, 53. ［2］A.Nakatani: Ph.D Thesis, Osaka University, 1993. ［3］R.A.Johnson: Phys. Rev., 1964, 134. ［4］Y.Kim, D.Choi and J.Park: Metal and Materials,1999, 5.     

Molecular dynamics simulation for grain boundary deformation under tensile loading condition

In this study, MD simulations have been performed to observe the behavior of a grain boundary in an alpha -Fe plate under 2-dimensional loading. In MD simulation, the acceleration of every molecule can be achieved from the potential energy and the force interacting between each molecule, and the integration of the motion equation by using Verlet method gives the displacement of each molecule. Initially, four α-Fe rectangular plates having different misorientation angles of grain boundary were modeled by using the Johnson potential and Morse potential. We compared the potential energy of the grain boundary system with that of the perfect structure model. Also, we could obtain the width of the grain boundary by investigating the local potential energy distribution. The tensile loading for each grain boundary models was applied and the behavior of grain boundary was studied. From this study it was clarified that in the case using Johnson potential, the obvious fracture mechanism occurs along the grain boundary, in the case of Morse potential, the diffusion of the grain boundary appears instead of the grain boundary fracture.     

Simulation of stage I-crack growth using a hybrid boundary element technique

Short fatigue crack propagation often determines the service life of cyclically loaded components and is highly influenced by microstructural features such as grain boundaries. A two-dimensional model to simulate the growth of these stage I-cracks is presented. Cracks are discretised by displacement discontinuity boundary elements and the direct boundary element method is used to mesh the grain boundaries. A superposition procedure couples these different boundary element methods to employ them in one model. Varying elastic properties of the grains are considered and their influence on short crack propagation is studied. A change in crack tip slide displacement determining short crack propagation is observed as well as an influence on the crack path.     

First-principles investigation of the effect of alloying elements Ti, V on grain boundary cohesion of FCC Fe

The discrete variational and Dmol methods within the framework of density functional theory are used to study the effects of Ti and V on the electronic structure of grain boundary in FCC Fe. The results show that both Ti and V prefer to segregate at grain boundary. The differences of segregation energies between the grain boundary and the corresponding free surface are −0.12 and −0.36 eV for solute Ti and V, respectively. According to Rice–Wang model, our results imply that both Ti and V enhance the grain boundary cohesion in FCC Fe. This work also shows that the effects of Ti and V on the bonding behavior are different. When Ti segregates at grain boundary, the bonds across grain boundary are strengthened, while the interactions between Ti and its neighboring atoms are weakened. But for V-doped grain boundary, V interacts with its neighboring atoms stronger than that of clean grain boundary and V also makes the bonds across the grain boundary stronger.     

Dislocation interactions with tilt walls

The flow of plastic deformation in polycrystalline materials can be due to activation of sources in adjacent grains due to the effect of pile up dislocations against the grain boundary and also through the transmission of dislocations across the grain boundary. In this paper, we focus on these two issues by studying the evolution of resolved shear stress as a result of pile up dislocations against the boundary and understanding the basic phenomena of dislocation transmission through grain boundary. We also investigated the relaxed structures a grain boundary acquires after the process of dislocation transmission.     

Nucleation and migration of high angle grain boundaries in bilayer foils II: Migration

Specimens similar to those studied in Part I were heated to study further the migration mechanisms of high angle grain boundaries through misfit dislocation arrays. The change in misorientation across the migrating grain boundaries was found to depend on the efficiency with which the misfit dislocations are absorbed by the boundary. Further observations show that this change in misorientation can result in a decrease in boundary mobility, followed by the nucleation of a twin-related grain from the original grain boundary. The twinning process will continue until a mobile grain boundary is produced; a simple mechanism for this twinning process is proposed.     

Engineering grain boundary sliding and cavitation effects in superplastic alloys employing thermodynamics

Plastic deformation by grain boundary sliding in superplastic alloys is described by a novel thermostatistical approach. The Gibbs free energy for cavity formation at moving grain boundaries is obtained. It equals the competition between the stored energy at the boundaries and the energy dissipated by grain boundary sliding. The latter is approximated by an entropy term induced by moving dislocations to facilitate boundary displacement. Strength loss evolution is estimated from the cavity evolution rate. The theory describes superplastic behaviour of Zn22Al, Zn21Al2Cu and Mg3Al1Zn for various temperatures, strain rates, grain sizes, and specimen geometries. Transition maps are defined for finding the optimal conditions for achieving superplastic behaviour in terms of composition, temperature, geometry and strain rate.     

Structures of a Σ = 9, [110]/{221} symmetrical tilt grain boundary in SrTiO<Subscript>3</Subscript>

In this study, we fabricated a bicrystal of SrTiO₃ containing a Σ = 9, [110]/{221} symmetric tilt grain boundary (GB) and its atomistic structure was directly observed by transmission electron microscopy (TEM) and scanning TEM (STEM). We theoretically estimated the most stable structure by first principles calculations, and by combining this with TEM images determined the atomistic structure of the Σ = 9 grain boundary. We found that when the grain boundary is slightly tilted from the coincident site lattice (CSL) orientation, displacement shift complete (DSC) dislocations are introduced at the grain boundary to accommodate the misorientation between the two adjacent crystals while the most stable atomic structure remains unchanged.