Aluminum Bicrystals Research Articles

Simulating the deformation and recrystallization of aluminum bicrystals

To understand the formation of the deformation substructure and its evolution during annealing to produce unique microstructures and textures in aluminum alloys, the development and coupling of physically based models spanning different length scales is necessary. Under hot deformation conditions, the deformation substructure is a collection of cells/subgrains of different sizes and orientations. Recrystallization following hot deformation occurs by the heterogeneous evolution of the subgrain structure, the kinetics of which is controlled by variations in driving forces and boundary properties at the microstructural length scale. This article describes a mesoscale approach for modeling the microstructure and texture evolution during recrystallization following hot deformation. A Monte Carlo simulation technique is used to evolve the substructure and texture during recrystallization. The simulations are applied to the deformation and recrystallization of aluminum bicrystals with specific combinations of crystallographic orientations. The simulation results are compared with experimental results.

Mobility of grain boundaries in stress fields

Abstract The mobility of grain boundaries under certain conditions is a key factor for the evolution of polycrystalline microstructures and material properties. In recent experiments it was shown that grain boundaries can interact with mechanical stress fields and that the motion of planar grain boundaries, low angle as well as high angle grain boundaries, could be induced by an external mechanical stress field [1,2]. Therefore the current study was performed to investigate the details of grain boundary motion and interactions between definite, planar grain boundaries and the mechanical stress field during high temperature low cycle deformation of high purity aluminium bicrystals. The grain boundary planes were in all cases perpendicular to the loading axis. Experiments were made with large as well as small stresses at different temperatures and numbers of cycles. The displacement of grain boundaries was measured by using optical microscopy, and the microstructural features, as well as orientations of individual grains were recorded by the electron backscatter diffraction technique (EBSD) after each deformation experiment.

Grain Boundary Sliding of Ʃ5(001) Twist Grain Boundary in Aluminium Bicrystal from First-Principles Calculations

Grain Boundary Sliding of Ʃ5(001) Twist Grain Boundary in Aluminium Bicrystal from First-Principles Calculations

アルミニウム双結晶の曲げ繰り返し変形挙動に及ぼす結晶粒界の影響

In order to examine the effect of grain boundaries on bending cyclic behavior, aluminum bicrystals and their component crystals (A and B) were investigated under the constant strain amplitude of 0.13% at room temperature. Bending moments of the bicrystals were compared with mixture models which were calculated by joining results of component single crystals. The cyclic saturation moments at 1000 cycles of the bicrystals were higher than those of the calculated ones. It was suggested that the existence of the grain boundaries possibly shortened the fatigue lives in aluminum bicrystals.

On the influence of the grain boundary misorientation on the plastic deformation of aluminum bicrystals

Aluminum bicrystals with symmetric 〈1 1 2〉 tilt boundaries and misorientations of 8.7° (small angle), 15.4° (transition), and 31.5° (large angle) were deformed in a channel die experiment in order to study the influence of misorientation on the deformation at grain boundaries. Samples were characterized by strain measurements and microtexture mappings. The experiments were compared to crystal plasticity finite element simulations. We studied strain heterogeneity at the macroscopic and at the microscopic level. Even macroscopically homogeneous areas showed microscopic heterogeneity in the form of bands of different sets of glide systems. We observed clear effects of the grain boundary misorientation on the deformation kinematics close to the boundaries. The 8.7° grain boundary did not show any orientation change which was interpreted in terms of free dislocation penetration. In contrast, the 15.4° and 31.5° bicrystals showed orientation changes which were attributed to dislocation pile-ups.

Grain boundary liquid metal wetting: A synchrotron micro-radiographic investigation

The penetration of liquid metals into the grain boundaries (GB) of solid metals can lead to severe embrittlement of the solid. Grain boundary wetting, which is the precursor process of liquid metal embrittlement, is still far from being well understood. We report in situ observations of the propagation kinetics of liquid metal into the GB of bicrystals and polycrystals. The experiments are performed using synchrotron radiation micro-radiography. This imaging technique is well adapted for the in situ non-destructive bulk characterization of such phenomenon due to suitable spatial and temporal resolutions. Experiments have shown that the Ga penetration front in aluminum bicrystals propagates roughly with linear time dependence, whereas a discontinuous behaviour occurs in polycrystals.

Atomistic simulation of grain boundary sliding in pure and magnesium doped aluminum bicrystals

Molecular dynamics and statics simulations are used to study grain boundary sliding and energy in bicrystals with symmetric tilt grain boundaries of Al and Mg doped Al. There is an increase in grain boundary energy in Al bicrystals with the presence of Mg depending on the position of Mg atom. Simulations of sliding show a clear dependence of magnitude of sliding on grain boundary energy.

Effect of piled-up dislocations on strain induced boundary migration (SIBM) in deformed aluminum bicrystals with originally ∑3 twin boundary

Using aluminum bicrystal specimens with ∑3 twin boundary at the initial orientation, deformation and recrystallization experiments were performed in order to clarify the contribution of piled-up dislocations at the grain boundary (GB) to the occurrence of the strain induced grain boundary migration (SIBM). In the tensile deformation of the specimen along the <011> axis to a strain of 20%, an inhomogeneously deformed structure was developed along the GB. After annealing, the SIBM was observed along the original GB. These screw dislocations passed easily through the GB because of the special orientational and geometric conditions, whose motion acted as the stress relief mechanism against the imposed strain. It was found that the SIBM was caused by the edge component of dislocations piled up against the GB.

X-Ray Diffraction Analysis and Modeling of Strain Induced Thermal Cycling in a Thin Aluminum (011) Bicrystal Film

ABSTRACTHeteroepitaxial films of aluminum bicrystals grown on silicon provide a model system in which to study plasticity in polycrystalline metal thin films. For the bicrystal films, dislocations are confined to move on two different slip plane orientations because of the orientation of the crystals on the substrate. In-situ transmission electron microscopy (TEM) observations during thermal cycling have shown two threshold temperatures for dislocation motion on cooling. A simple model uses the resolved shear stress on the possible slip planes to explain the TEM observations. Mechanisms responsible for the dislocation behavior are studied in-situ during thermal cycling between room temperature and 450°C with x-ray diffraction. The strains are determined using a sin2(Ψ) analysis at each temperature. Direct comparisons are made between the TEM observations, the model and x-ray diffraction results.

Interface Models for GB Sliding and Migration

The predominant strain producing mechanism in superplastic materials is attributed to grain boundary sliding (GBS). Though there have been many attempts to model GBS, most of them are heuristic and not based on the microscopic deformation at the scale of grain boundaries. Lack of such models has hindered the progress of a methodical development of alloying processes. Hence an attempt is made to develop a constitutive model of grain boundary using atomic simulation results. Detailed atomic simulations are carried out on aluminum bicrystals of Σ3 and Σ9 coincident site lattice boundaries to obtain characteristic behavior of grain boundary sliding/migration. Using the results thus obtained a constitutive model based on cohesive zone approach is developed to predict the behavior of grain boundaries under the external loads and hence GBS. This constitutive model is implemented in a finite element method (FEM) code to analyze the GBS/migration at a continuum scale.

Relationship between Deformation and Recrystallization in &lt;I&gt;&amp;Sigma;&lt;/I&gt;&lt;B&gt;3&lt;/B&gt; Isoaxial Aluminum Bicrystal Deformed in Tension along the &lt;&lt;B&gt;011&lt;/B&gt;&gt; Axis

Deformation and recrystallization behaviors were examined using a Σ3 isoaxial aluminum bicrystal specimen deformed in tension along the (011) axis. At a tensile strain of 20%, an inhomogeneously deformed structure was developed near the GB, The recrystallization at the GB was dominated by the strain-induced grain boundary migration (SIBM). The strong edge components of piled-up dislocations contributed the occurrence of the SIBM, whereas the serew dislocations moved into the GB from both component crystals were considered to pass through or canceled each other in the GB, because of the special orientational and geometric conditions in the crystals.

Numerical studies of nonuniform deformation, stress state evolution, and subgrain formation in bicrystals in (110) channel die compression

An elastoplastic numerical model for f.c.c. bicrystals in (110) channel die compression is investigated to a logarithmic strain of 10% and compared with the analytical rigid-plastic solution at the yield point. Three lattice orientations of an aluminum bicrystal are calculated between constraint directions [00 1 ̄ ] and [1 1 ̄ 2 ̄ ] . It is found that the elastoplastic results for each of stress state, velocity field and lattice-rotation rate (indicating subgrain formation) pass very close to the analytical solution at 0.2% strain for two of the orientations. This is in spite of small tangential velocity discontinuities along characteristic directions of that solution which are not permitted in the numerical (finite element) modeling. The results are never as close in the third orientation, for which there are much greater tangential velocity discontinuities in the rigid-plastic model. It is suggested that the elastoplastic comparisons with the rigid-plastic model may provide insight into realistic strain levels at the beginning of fully plastic response. However, it also is concluded that a deeper analytical representation of the tangential velocity discontinuities is needed.

HRTEM Study on Σ7 Grain Boundary in Aluminium Bicrystals with and without Ga Doping

A HRTEM investigation was carried out on the Σ7{213} tilt grain boundary in bicrystals of both high purity aluminium and same aluminium doped with 10 ppm gallium. It was determined by image simulation that the bright contrast in the TEM images show the positions of atom columns. By measuring contrast profiles it was found that in the specimen of aluminium-10 ppm gallium, the atoms which are closest to the grain boundary, are anomalously shifted along the ‹121› direction compared to equivalent atom positions of the pure aluminium sample. This anomalous shift makes the disparity in the grain boundary structure of both samples. It is assumed that this anomalous shift is most likely the reason for the higher mobility of the grain boundary of the aluminium-10 ppm gallium sample.

Observations of lattice curvature near the interface of a deformed aluminium bicrystal

Abstract Reported here is a study of the pattern of lattice curvature near the interface of deformed high-purity aluminium (99.9999%) bicrystals of specified crystallographic character (large-angle random). Curvature data are obtained from electron back-scattering diffraction pattern observations using orientation imaging microscopy. The concept of geometrically necessary dislocations (GNDs) is used as the central tool in the description of the observations. The samples studied were channel-die compressed perpendicular to the interface to plastic strain levels of 0.1 and 0.3. At a strain level of 0.1 the primary observation is the development of a pile-up of GNDs (i.e. lattice curvature) near the interface. At the higher strain level of 0.3, however, a dramatic change in the distribution is observed. The nature of this change suggests that the interface has absorbed (or emitted) some components of the nearby GND field. with an accompanying change in the local character of the interface towards a broader dispersion of misorientation character.

Observations of lattice curvature near the interface of a deformed aluminium bicrystal

Observations of lattice curvature near the interface of a deformed aluminium bicrystal

Recrystallization nucleation mechanism along boundaries in hot deformed Al bicrystals

The orientation dependency of strain induced boundary migration (SIBM) has been studied on 99.995% purity aluminium bicrystals deformed in hot plane strain compression. Four bicrystal combinations were chosen to represent some typical grain boundaries in hot rolled aluminium, i.e. boundaries between a grain of S {142} orientation and adjacent grains with cube, Cu {112} , Bs {011} or another S variant orientation. The bicrystals were deformed in a channel die to true strains of ∼1.5 at 300–400°C and then annealed for short periods to allow grain boundary movement, usually by SIBM. The microstructural changes were followed by channelling contrast SEM and EBSD microtexture measurements. The stored energies of the individual grains were estimated from measurements of the average cell size and misorientation using the Read Shockley expression for the sub-boundary energies; typical values range from ∼10 to 50 kJ m−3. SIBM occurs extensively in bicrystals containing boundaries with the 40° orientation relation. Grain boundaries in other bicrystals either scarcely move or, in the case of the S/Bs combination, become nucleation sites for near-cube germs. The boundary mobilities – as deduced from the local driving forces and the migration distances – are compared with previous data from deformed polycrystals.

Recrystallization texture of aluminum bicrystals with S orientations deformed by channel die compression

Abstract The texture changes during recrystallization in high-purity aluminum bicrystals with S orientations, e.g. (123)[41 2 ] / (123)[ 4 1 2] and (123)[41 2 ] / ( 1 2 3 )[41 2 ] , deformed by channel die compression were investigated by Blicharski et al. There was no 〈111〉 rotational relationship between the recrystallized and the deformed matrix phases. This result could be explained by the strain energy release maximization model, in which the direction of absolute maximum internal stress due to dislocations in the deformed matrix becomes parallel to the minimum Young’s modulus direction in recrystallized grains, whereby the energy release during recrystallization can be maximized.

Integranular Fracture: The Effect of Grain Boundary Orientation and Crack Growth Directions

ABSTRACTIntergranular fracture is a common failure mechanism for which many issues remain to be resolved. In this study we investigate intergranular fracture behavior of specially oriented symmetric tilt bicrystals of aluminum as well as the fracture behavior of a crack along the interface of a copper-sapphire bicrystal. We begin by describing briefly the structure of a symmetric tilt grain boundary which leads to a discussion of the types of issues related to intergranular fracture that can be addressed with symmetric tilt grain boundaries. We then discuss in detail one of these issues, that of the directional dependence of fracture, and present results of finite element simulations of a copper-sapphire bicrystal specimen that exhibits the directional dependence of fracture. The simulations account for the single crystal nature of the constituents and use a cohesive-zone model, for which the grain boundary energy can be varied, to simulate the fracture process along the interface. The directional dependence of fracture emerges from the simulations for a broad range of parameters in the constitutive models of both the single crystal constituents as well as the interfacial cohesive-zone.

The Effect of Solubility of the Diffusant on its Grain-Boundary Penetration in Aluminium Bicrystals

Diffusion of various elements differing in the solubility in aluminium was carried out in aluminium bicrystals with a tilt grain boundary (GB) and common direction Correlation of GB penetration has been looked for with various properties of the diffusing atoms (solubility, atomic size, reactive diffusion) The strongest correlation has been stated with the limit of solubility of the elements in aluminium. GB diffusion of oxygen atoms has been stated in aluminium bicrystals and oxide wedge was formed at the grain boundary.

Grain boundary motion during high temperature cyclic deformation of high purity aluminium bicrystals

During cyclic deformation of polycrystals at elevated temperatures conspicuous microstructural changes, in particular grain boundary motion and grain boundary alignment have been reported for a variety of metallic materials. The current study addressed this issue by investigating in detail the motion of grain boundaries during cyclic deformation of bicrystals of very high purity aluminium. Bicrystals containing different types of grain boundaries were investigated to probe the influence of grain boundary character on their behaviour under cyclic loading. The displacement of the grain boundaries was recorded using optical microscopy without additional preparation. The evolution of microtexture was measured by the electron backscatter diffraction technique (EBSD) using a scanning electron microscope. During cyclic deformation a deformation structure occurred within each grain. The character of deformation structure, in particular the difference in dislocation density on both sides of the grain boundary was found to be responsible for the displacement of the boundaries. A model is proposed to explain the grain boundary motion and the grain boundary alignment during cyclic deformation of aluminium bicrystals. The relevance of these results for grain boundary behaviour in polycrystals is discussed.