Aluminum Bicrystals Research Articles

Influence of Grain Misorientation on Material Hardening on Example of Aluminium Bicrystals Deformed in Channel Die at 77K

The influence of grain misorientation on pure aluminium hardening is presented. The study has been focused on plastic strain inhomogeneities developed in high purity aluminium bicrystals deformed in channel die at 77K, with various combinations of grain orientations and with the grain boundary situated parallel to the compression plane. The character of the structure was determined by optical microscopy as well as by systematic local orientation measurements using high resolution EBSD in SEM-FEG scanning electron microscope. The experiment results were compared with crystal plasticity finite element simulations used for prediction of local deformation areas. The behavior of each bicrystal was described by two flow curves obtained from plane strain compression tests of single crystals with different orientations. As a result, clear effect of the grain orientations and grain boundary misorientation on the deformation kinematics within both crystallites forming bicrystals has been observed.

Prediction of the orientation spread in an aluminum bicrystal during plane strain compression using a DIC-based Taylor model

The orientation spread in an aluminum bicrystal was predicted using a DIC-based Taylor model during plane strain compression up to a strain of 5%. The modified Taylor model was integrated with a digital image correlation (DIC) technique, with the aid of which the micro-strain behavior of the bicrystal was investigated, using an experimental approach on one hand and, on the other hand, the strain components determined by DIC were computed for the local orientation rotation together with a Taylor model. It was observed that the higher crystal deforms more than the low crystal. The orientation spread predicted using the modified Taylor model shows a similar trend as that measured by EBSD for the higher crystal, but the spread direction of the low crystal was not correctly predicted using the DIC-based Taylor model.

On Strain-Induced Grain Growth Using Modified Monte Carlo Method and Digital Image Correlation Technique

In this study the strain-induced grain growth was simulated on an aluminum bicrystal by using channel-die compression. After compression of the bicrystal up to 5% deformation the strain mapping were characterized by using digital image correlation (DIC) technique and the 2D strain filed provided data to simulate grain growth using a modified Monte Carlo method. The strain-induced grain growth on grain boundary was simulated and compared with experiment after annealing at 450°C for 4 hours. The relation between the deformation heterogeneity and the grain growth was discussed in this work.

Mechanically Driven Migration of <100> Tilt Grain Boundaries in Al-Bicrystals

The stress induced migration of planar grain boundaries in aluminium bicrystals was measured. Symmetrical <100> tilt grain boundaries with misorientation angles in the range between 5.7° and 17.8° were examined. Boundary migration under a shear stress was observed to be ideally coupled to the lateral translation of grains. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with the respective boundary geometry. The temperature dependence of grain boundary mobility was measured, and the corresponding activation parameters were determined. The activation enthalpy of boundary migration was found to be independent of misorientation angle in the investigated misorientation range and amounts to H=1.44 eV.

Strain hardening and microstructure evolution of channel-die compressed aluminium bicrystals

The different types of strain heterogeneities developed in pure aluminium bicrystals during plane strain compression have been characterized by advanced metallographic techniques over several length scales. Bicrystals, with three grain orientation combinations (one hard/soft, and two stable/unstable) containing grain boundaries perpendicular to the compression axis, were deformed up to strains of 1.5 at 77 K and their deformation substructure investigated by optical microscopy, high resolution EBSD and transmission electron microscopy. At the macroscopic scale the hard/soft combination develops major strain variations as the softer grain flows out over the harder grain. This is consistent with finite element simulations of bicrystal deformation using the appropriate individual grain hardening laws and leads to a large transition zone near the boundary. At the microscopic level, the grains of the other combinations behave essentially as single crystals under the same deformation conditions, in particular with respect to the deformation substructure and microtexture distributions. It is concluded that for most grain pair combinations of deformed bicrystals the individual grain orientations play a dominant role in deformation substructure development as a consequence of their dependence on slip system interactions.

Scanning electron microscopy and transmission electron microscopy in situ studies of grain boundary migration in cold‐deformed aluminium bicrystals

The crystallography of recrystallization has been investigated in channel-die deformed pure aluminium bicrystals with {100}<011>/{110}<001> orientations. The microstructural and microtextural changes during the early stages of recrystallization were followed by systematic local orientation measurements using scanning and transmission electron microscopes. In particular, orientation mapping combined with in situ sample heating was used to investigate the formation and growth of new grains at very early stages of recrystallization. Grain boundary migration and 'consumption' of the as-deformed areas was always favoured along directions parallel to the traces of the {111} slip planes that had been most active during deformation.

Cyclic motion of [1 0 0] twist grain boundaries in aluminium bicrystals

Aluminium bicrystals with [1 0 0] twist grain boundaries were fabricated and deformed under high temperature low cycle fatigue conditions to investigate the cyclic grain boundary motion. The observed results show that the [1 0 0] planar twist grain boundaries were mobile under the stress field of cyclic deformation at different temperatures irrespective of misorientation angle and mode of deformation. The presented results are discussed with regard to the measured activation energies as well as possible migration mechanisms.

New Orientation Formation During Recrystallization of Cold Deformed, High Symmetry Aluminium Bicrystals

The crystallography of recrystallization nucleation has been investigated in channel-die deformed pure Al bicrystals with {100}〈011〉/{110}〈001〉, {100}〈001〉/{110}〈011〉 and {100}〈001〉/{110}〈001〉 orientations. The new grain orientations and misorientations were followed by systematic local orientation measurements using SEM and semi-automatic measurements in TEM. The bicrystals were cold deformed by channel-die compression up to true strains of 1.5. During recrystallization annealing, the deformation bands of unstable orientations are the privileged sites for the formation of new grains. These nuclei were misoriented with respect to the orientations identified within the neighbouring deformed areas by α(〈111〉, 〈112〉 or 〈100〉) relations. Grain boundary migration and ‘consumption’ of the as-deformed areas was always favoured along directions parallel to the traces of the {111} slip planes that had been most active during deformation as shown in the stable and structurally homogeneous Goss {110}〈001〉 oriented grains. At logarithmic strains below 1–1.5 the grain boundary does not seem to be a specific nucleation site of new grains.

Relations between the parameters of grain-boundary diffusion of zinc in aluminum

Experimental results on the grain-boundary diffusion of zinc in aluminum bicrystals containing 〈100〉 and 〈111〉 tilt boundaries at normal and high hydrostatic pressures have been considered. A comparison of the diffusion activation energies and the activation volumes for grain boundaries having the same crystallographic parameters has revealed a linear relation between these kinetic constants. The diffusion activation energy is found to increase with the activation volume. This relation is shown to correspond to the compensation effect well-known in diffusion theory.

Surface superconductivity and twinning-plane superconductivity in aluminum

The critical supercooling field Hsc is measured in aluminum single crystals and twinned bicrystals in a temperature range slightly below Tc0 (Tc0 − 0.055 K < T < Tc0), where Tc0 is the critical superconducting transition temperature. It is found that, even in this small temperature range, the Hsc(Hc) dependence, which is considered to be identical to the Hc3(Hc) dependence for single crystals, is substantially nonlinear. The Hsc(Hc) dependences of the twinned bicrystals and single crystals are shown to be significantly different. The qualitative features of the phase diagram of the twinned aluminum bicrystals coincide with those of the phase diagram of twinning-plane superconductivity obtained earlier for tin in [1]. These findings allow the conclusion that the phenomenon of twinning-plane superconductivity also exists in face-centered cubic crystal lattices.

Deformation and Recrystallization of Aluminum Bicrystals Having Asymmetric Tilt Grain Boundary

Two types of aluminum bicrystals (Bicrystal 1 and 2) having an asymmetric tilt grain boundary were grown. In Bicrystal I, the orientations of component grains were related to each other by 45° rotation about the normal of the wide surface of the tensile specimen. In Bicrystal 2, although the primary slip planes were symmetric about the grain boundary plane, the component grains were related to each other by 180° rotation about the tensile axis of the specimen. The specimens were deformed to a tensile strain of 0.3 and subsequently annealed. In both bicrystals, deformation bands were not formed symmetrically about the grain boundary. After annealing, no strain induced boundary migration was found. Recrystallization occurred through (111) rotation mechanism. These results were compared with the deformation and recrystallization in aluminum bicrystals having a symmetric tilt grain boundary.

The Effect of a Grain Boundary on Deformation in an Aluminum Bicrystal with a Common Tensile Axis of [100

The effect of a grain boundary on deformation has been examined using an aluminum bicrystal specimen composed of crystals having a common tensile axis of [100] by the measurement of orientation change with the electron backscatter diffraction technique. After deformation to a strain of 20%, the crystal rotation axis (CRA) map, which provides the orientation distribution of crystal rotation axes relative to the initial crystal orientations, reveals areas affected by the grain boundary. The thickness of the affected zone was 100 μm in one crystal and 150 μm in the other. In both crystals, the crystal rotation axis relative to the initial crystal orientation differed between the affected zone and the crystal interior. In the crystal interior, two slip systems showed a higher activity than the other slip systems, whereas in the affected zone, three or four slip systems were more active than the other slip systems. The CRA map showed that the width and shape of the affected zones across the grain boundary were not symmetrical with respect to the grain boundary.

Internal friction peak in pure Al bicrystals with ⟨100⟩ tilt boundaries

A substantial internal friction peak associated with grain boundary (GB) has been observed in pure aluminum bicrystals with ⟨100⟩ tilt GBs, which is similar to that observed for ⟨112⟩ tilt GBs in the previous work [Phys. Rev. B 71, 060101(R) (2005)]. The peak is suggested to be induced by GB sliding, but with different mechanisms in microscopic scale for different types of GBs by considering their difference in relaxation parameters. The relaxation parameters for low-angle GBs with a ⟨100⟩ tilt axis are at the same level as those with a ⟨112⟩ tilt axis, and can be interpreted by the glide and climb of dislocations composing the GB, but controlled by climb. The studied high-angle high $\ensuremath{\Sigma}\phantom{\rule{0.3em}{0ex}}(\ensuremath{\Sigma}=29)$ ⟨100⟩ tilt GB behaves similarly to high-angle general and high $\ensuremath{\Sigma}$ ⟨112⟩ GBs; their mechanism is possibly associated with a local rearrangement of disordered atom groups. The studied low $\ensuremath{\Sigma}\phantom{\rule{0.3em}{0ex}}(\ensuremath{\Sigma}=5)$ ⟨100⟩ tilt GB exhibits a special behavior (i.e., high activation energy and low relaxation strength), which is attributed to its low free volume. The ⟨100⟩ tilt GBs with angular deviations from exact $\ensuremath{\Sigma}5$ but within the near-CSL (coincidence site lattice) limit also exhibit some special behavior, but to a lesser extent.

Cyclic deformation of pure aluminum bicrystals

The dynamic behavior of 〈112〉-tilt grain boundaries in aluminium bicrystals under the influence of cyclic stresses at elevated temperatures is reviewed. Bicrystals, containing low- and high-angle grain boundaries within a wide range of misorientation angles, were deformed at several combinations of stress, temperature, and number of cycles. The grain-boundary (GB) displacement and the deformed structure of bicrystals were framed using standard optical microscopy. The grain orientations were measured using the electron backscatter diffraction (EBSD) technique with a scanning electron microscope (SEM), before and after the deformation. There is distinct evidence of a sharp transition angle between low- and high-angle grain boundaries, with respect to the ability of the boundaries to move under the given parameters. The experimental observations lead to the conclusion that a difference in the dislocation structure in two grains causes the driving force for GB migration.

High-pressure effect on grain boundary wetting in aluminium bicrystals

Abstract The effect of pressure and misorientation on grain boundary wetting in aluminium bicrystals has been investigated. The grain boundaries were of [100] symmetrical tilt type. The wetting liquid was an Sn–Zn alloy. It is shown that the wetting angle is a function of misorientation but not of pressure. The reasons of the above results are discussed, assuming a linear dependence between the interface energy and pressure. It is shown that the difference of energy of the liquid/solid and solid/solid interface as well as the misorientation dependence of energy is simply proportional to the free volume of the interfaces.

Mechanisms of New Orientation Formation during Recrystallization of Old Deformed Aluminium Bicrystals

The crystallography of recrystallization nucleation has been investigated in channel-die deformed pure aluminium bicrystals with {100}&lt;011&gt;/{110}&lt;001&gt; and {100}&lt;001&gt;/{110}&lt;001&gt; orientations. The new grain orientations and misorientations were followed by systematic local orientation measurements using SEM and semi-automatic measurements in a TEM. In particular, orientation mapping combined with in-situ sample heating was used to investigate the formation and growth of new grains and their crystallographic orientation changes at very early stages of recrystallization. Grain boundary migration and ‘consumption’ of the as-deformed areas was always favoured along directions parallel to the traces of the {111} slip planes that had been most active during deformation. The orientations of the first formed nuclei were misoriented with respect to the orientations identified within the neighbouring deformed areas by α(&lt;111&gt;, &lt;112&gt;, or &lt;100&gt;)relations.

Internal friction peak in bicrystals with different misorientations

The internal friction in pure aluminum bicrystals with misorientation angles ranging from 5.8\ifmmode^\circ\else\textdegree\fi{} to 44.0\ifmmode^\circ\else\textdegree\fi{} has been investigated. A substantial internal friction peak associated with a grain boundary (GB) appears at medium temperature in each of the bicrystals. The activation parameters of the peak reveal a distinct transition from low- to high-angle boundaries, indicating a structural transition. The difference in activation parameters between low- and high-angle GBs is interpreted by different mechanisms of internal friction.

Recrystallization Nucleation in Some Channel Die Deformed, High Symmetry Aluminium Bicrystals

The crystallography of recrystallization nucleation in high purity aluminium bicrystals, with boundary planes situated parallel to the compression plane, has been characterized in detail by microtexture analyses. The experiments were carried out on samples with 2 types of bicrystal orientations composed of the stable Goss{110}&lt;001&gt; with an unstable cube{100}&lt;001&gt; or shear{100}&lt;011&gt; orientation. The bicrystals were cold deformed in channel-die compression up to true strains of 1.5. During recrystallization annealing, the deformation bands of the unstable orientations are the privileged sites for the formation of new grains. These nuclei were misoriented with respect to the orientations identified within the neighbouring deformed areas by α(&lt;111&gt;, &lt;112&gt; or &lt;100&gt;) relations. Growth of the new grains is always favoured along directions parallel to the traces of the {111} slip planes as shown in the stable and structurally homogeneous Goss {110}&lt;001&gt; oriented grains. The grain boundary can play a role in recrystallization similar to that of the deformation bands.

On the Effect of “Surface Triple Junction” on Grain Boundary Motion

The motion of a curved grain boundary with a “surface triple junction” (“free surface – boundary - free surface”) in aluminum bicrystals is studied. The effect of the “surface triple junction” on grain boundary motion is discussed in the terms of the equilibrium of boundary and junction velocity. Boundary motion in samples with different boundary curvature revealed a strict proportionality of boundary velocity and driving force. This result corroborates the fact that in the entire investigated temperature range the “surface” triple junction does not affect the boundary motion.

Impact of Boundary Orientation on the Motion of Curved Grain Boundaries in Aluminum Bicrystals

The motion of &lt;111&gt; tilt and mixed tilt-twist grain boundaries with misorientation angles in the range between 34° and 42° in pure Al bicrystals was measured over the temperature range between 310 and 610°C. The experiments revealed that the change of the set of boundary planes in the curved moving tilt boundary does not affect its motion. The shape of the curved moving part of the mixed tilt-twist boundary was measured and compared with analytically calculated boundary shape. The results have shown that an increase of the twist component along the curved mixed boundary in studied geometrical configuration does not affect its steady-state motion. Similar to the behaviour of &lt;111&gt; pure tilt boundaries, the mobility of &lt;111&gt; mixed tilt-twist grain boundaries in the vicinity of special misorientation å7 depends on the misorientation angle in a non-monotonic fashion.