The influence of deformation texture on nucleation and growth of cube grains in aluminium

The influence of deformation texture on the initial stages of recrystallization of deformed pure Al single crystals and commercial AA1050 alloy were characterized. To precisely quantify the orientation relationship between nuclei and simple deformation structures, single crystalline samples of stable Br(110) [1-1-2] and semi-stable S’(234) [20-28 11] orientations, were plane strain compressed (PSC) to 40% to develop a dominant homogeneous microstructure composed of two sets of microbands. The samples were then lightly annealed to reach the initial stage of primary recrystallization. The results on single crystals were related to the behaviour of the polycrystalline samples in state after hot-rolling then PSC to the same degree and lightly annealed. SEM/EBSD analyses of single crystalline samples demonstrate that the orientations of recrystallized grains are limited to a certain number of orientation groups. However, the orientations of the as-deformed state are not retained in the orientations of new grains. Deformed samples with Br orientation do not show the formation of cube grains during recrystallization, whereas strong nucleation of cube-oriented grains is observed in homogeneously and heterogeneously deformed areas in the semi-stable S’ orientation, despite the cube-oriented nuclei not present in the as-deformed structure. The recrystallization texture components of the AA1050 alloy were related to the standard β-fibre texture formed during the previous cold deformation. Cube grains formed intensively in PSC polycrystalline samples, primarily in association with as-deformed areas with near four variants of S(124) [21-1] and S’ orientations and were characterized by local misorientations around the <111> axes.


Introduction
The texture transformation during annealing of deformed face-centred cubic (fcc) metals has been discussed for several years.It is widely accepted that the orientations of recrystallized grains originate from the orientations of the as-deformed areas from which they grow [1].In the concept proposed in [2,3], the orientations identified in the as-deformed matrix are considered the 'starting point' for the formation of orientations of the nuclei.However, it is not clear whether the orientation of each new grain originates from the same orientation present in the deformed structure, as has been concluded by Humphreys and Hatherly [4], or whether orientations of the nuclei evolve during annealing from the asdeformed state orientations to the nucleus orientations, as originally proposed in [2,5,6].This indicates that there is still no consensus on the description of the nucleation mechanism.
Most of the earlier works widely accept the mechanism of recrystallization nucleation based on the assumption that the initial embryo, with the orientation of the nuclei, pre-exists in the as-deformed matrix.However, this standard description seems to be not fully correct.Detailed recrystallization studies using high-resolution electron microscopy techniques, carried out on single crystals with stable orientations (in that case only slight scattering of the initial orientation is observed) of many fcc metals shown that orientations of new grains are different to that identified in the as-deformed state [7].Moreover, the orientations of new grains are not random and only strictly defined groups of new grains orientation are observed.This indicates that the deformed matrix does not necessarily have orientations corresponding to the recrystallized grains and therefore must operate a mechanism 'transforming' the orientations of the deformed state into orientations of new grains.
The above issue is of immense importance in the context of explaining the reasons for the strong cube texture formation during the annealing of deformed fcc metals.Most of the authors argue that cube{100}<001>-oriented grains evolve from cube bands.They are present in the as-deformed microstructure, often as thin (near)cube-oriented regions between areas of high-orientation gradients created by diverging rotations of unstable orientations or fragmented leftovers of the original cube grains [8][9][10].These approaches assume a priori the presence of the initial cube nucleus in the deformed structure.However, the assumption that cube-oriented nuclei originate from fragmented leftovers of the original cube grains is strongly disputable since the cube-oriented crystallites are very unstable during room temperature plane strain compression (PSC) or rolling.At the initial stages of deformation, the cube orientation splits up by either transverse and elongation/rolling direction rotations and evolves, in the range of very high deformations (> 90%), towards four variants of ~S orientation.To solve this problem, some of the authors assume [9,10] that small fragments of cube-oriented crystallites are subjected to a different stress state and conserve orientations near the original cube orientation.They could be the source of cube-oriented grains during recrystallization.But this description of the phenomenon leads to another difficulty -it is exceedingly difficult to accept the assumption that during annealing these less deformed cube-oriented leftovers are more privileged sites for nucleation of new grains (areas of lower stored energy) as compared to highly deformed ones (areas of higher stored energy).
In this work, we used an annealing experiment where the number of 'free parameters' influencing the (near) cube grains formation during annealing is strongly limited.This can be ensured for single crystals of stable Br(110) [1-1-2] orientation as well as semi-stable and S'(234) [20 -28 11] orientation, i.e. ~90° ND-rotated S(124) [21-1]-orientation, where: ND is normal direction, both subjected to plane strain compression (PSC) in channel-die.The results obtained on single crystals were applicated to the description of the cube grains formation in polycrystalline AA1050 alloy.The crystallite with stable orientation was selected to explain the orientation relationship between the as-deformed and recrystallized state, whereas the S' orientations were to explain the origin of cube-oriented grains in Al.The S and S'oriented single crystals are similar crystallographically since one of the <111> pole overlaps with the <111> pole of cube orientation.This also provokes discussion on the mechanisms of cube grains growth in areas, where no S orientation is present.Additionally, to exclude the uncertainty associated with plastic flow instabilities formation the single crystals were deformed only up to 40%.Moreover, the high stacking fault energy (SFE) of Al practically excludes deformation twinning and strongly limits recrystallization twins [8].This considerably facilitates the analysis of the mechanisms responsible for the texture transformation during the initial stages of annealing of polycrystalline AA1050 alloy.To measure the disorientation angles and lattice orientations a scanning electron microscopy equipped with an electron backscattered diffraction (SEM/EBSD) facility was used.

Experimental
The single crystal bars of aluminium were grown by directional solidification and then Br(110) [1-1-2] and S'(234) [20 -28 11]-oriented samples were carefully cut from the bars to dimensions of 10×10×10 mm 3 (height × width × length) using a wire saw.Crystal orientations were checked by X-ray diffraction.The initial crystal orientations deviate less than 5° from the ideal positions.The deformation was carried out by PSC at 293 K up to a 40% of thickness reduction, where homogeneous microband structures were observed.The polycrystalline samples of the same dimensions were cut-off from the hot-rolled plate, characterized by the microstructure of strongly flattened grains, and then PSC to the same deformation degree.All the deformed samples were cut perpendicular to transverse direction (TD) into ~ 2.5 mm sheets using a wire saw and then the issue of microstructure and texture transformations is revisited in two groups of experiments: (i) after static recrystallization (in an air furnace), through 1-hour annealing at varying temperatures, and (ii) to the first stages of primary recrystallization during in-situ recrystallization in the SEM on samples deformed up to 40% in a channel-die (PSC).Before annealing all samples were mechanically ground on all faces to 4000 grid SiC paper and electropolished to rule out the effects of strain damage leading to the introduction of random orientations.After static recrystallization specimens were water quenched.All the microstructures were analysed in the longitudinal sections, i.e.ND/ED in the sample mid-thickness, where: ED is the extension direction.The electron backscattered diffraction (EBSD) measurements made using a TSL-EDX orientation imaging microscopy system on FEI QUANTA 3D FEG-SEM.The maps were created with step sizes ranging between 50 nm and 5 µm and presented using the inverse pole figure (IPF) colour code combined with the distribution of boundaries and/or image quality factor.The number of correctly indexed points was always above 98%.

'Homogeneous' microstructure of the as-deformed state vs. new grains orientations
The as-deformed microstructure of Br(110) [1-1-2]-oriented crystal, PSC up to 40% is composed of two sets of microbands.It also observed strong stability of the initial orientation up to moderate strains.Statistical analysis of the disorientation angle distribution concerning the average orientation demonstrates that deviations larger than 8° are not detected in the deformed structures.This can also be seen with typical disorientation line scans for Br(110) [1-1-2]-oriented single crystal, as confirmed many times in the past, e.g.[7].The obtained results confirm that the recrystallized grains do possess no random orientations since only a repeatable and limited number of groups of recrystallized grain orientations were observed.Furthermore, the new grain orientations are clockwise and anti-clockwise rotated around axes grouped near selected the <111> poles of the as-deformed/recovered state (figure 1).The recrystallized grains are related to the matrix by a rotation of 25-45°.Some of the grains are characterized by strongly elongated shapes with longer facets situation along traces of the {111} planes.In the case of S' orientation, a major part of the sample underwent homogeneous deformation with no tendency to high-angle boundaries formation.The orientation maps taken from these areas revealed  a microstructure composed of elongated microbands of one or two families, surrounded by low-angle boundaries (figure 2a).Some heterogeneities are observed as thin layers of localized strain situated at 15-20° to ED.The homogeneously deformed matrix displayed strong orientation stability, while the occurrence of localized shearing leads to the formation of high-angle boundaries.The disorientation across the boundary separating the band and less deformed matrix reaches 40-50°, as shown by pole figures and the disorientation line scan (figures 2b-e).However, the disorientation between particular cells inside the bands of localized strain is low (and similar to that of the homogeneously deformed matrix) and typically does not exceed 15° (figure 2c).The thin bands of the localized deformation exhibit the rotation tendency towards another variant of the ~ S' orientation (figure 2b).However, we did not observe rotations towards cube orientation.This means that in the deformed state, there are no areas with an orientation close to {100}<001> (figures 2b and c).
During annealing, the rapidly growing flattened grains, along specific directions, are observed in stable Br orientation and semi-stable S' orientation.The grains with the {100}<001> orientation nucleate in S' orientation with remarkably high intensity, both in the areas of homogeneous and nonhomogeneous deformation, i.e. inside the deformation bands (figure 3).The recrystallization texture results from the rotation of the deformed state orientations around the axes close to <111> (or coincide with them) by an angle of about 40°.The orientations of the deformed state are not retained in the orientation of the recrystallized grains.A significant part of the recrystallized grains exhibits a strongly elongated shape, which confirms an anisotropic growth rate in the homogeneously deformed microstructure.The cube-oriented grains also show a pronounced tendency for growth along the {111} planes.However, only one set of the elongated grains exhibited an orientation close to {100}<001>.This points to the importance of the mechanism of strictly oriented rotation around the normal of {111} plane, along which we observe a privileged growth.

Anisotropy of grain growth
In the initial stages of recrystallization of Br and S'-oriented single crystals, a considerable number of grains show growth anisotropy and the directions of fast growth well-coincide with traces of the {111} planes.As the deformed microstructure of Br and S' orientations crystals are free of significant orientation gradients (even if small orientation gradients occur, they do not coincide with directions of privileged growth), the stored strain energy cannot be the only driving force for boundary migration.This indicates that the growth velocity of recrystallized grains is not reduced by the disorientation effect The morphology of these flat grains was in all cases such that the broad faces of the plates were perpendicular to the rotation axis, as observed earlier in [7,[11][12][13].Generally, the crystal lattice rotation of elongated grains takes place around axes near the <111> pole, corresponding to the {111} plane along which preferred grain growth occurs (figure 4).Moreover, the longest grain direction of flattened grains runs mostly along the traces of the most active planes during strain.

Nucleation of cube-oriented grains in polycrystalline AA1050 alloy
A large amount of experimental data can be found in the literature confirming the critical role, of the socalled 'cube-oriented bands', in the nucleation of new grains with 'cube' orientation in Al and Al-based alloys.However, a detailed analysis of these data leads to an ambiguous conclusion since it is not clear whether these areas are elements of the deformed or recrystallized state structure.In this work, the issue was analysed in two types of experiments: during static recrystallisation and in-situ recrystallisation in the SEM.
PSC up to 40% of hot-rolled material conserves the structure of strongly flattened grains characterized by orientations from the surroundings of the S, C and Br components (for more details of the initial microstructure/texture see in [14]).The structures do not show any tendency to form plastic flow instabilities.Analysis of the microstructure changes after static recrystallization was carried out in the central areas of the sample cross-section, after annealing in the temperature range of 100°C -325°C for 1h.In most observed cases the recrystallized grains were grouped into chains reflecting the 'directionality' of the as-deformed structure.This 'directionality' was observed in the range of lower annealing temperatures.In the range of higher temperatures, recrystallized grains gradually replaced the structure of elongated dislocation cells and sub-grains in individual layers and then, in advanced stages of recrystallization, showed a tendency to broaden, i.e. the new grains reached dimensions greater than the thickness of the layers.This process was initiated at ~ 300°C.
The intensity and position of the main texture components for samples annealed below 300°C were like those observed for the deformed state.At 300°C, a radical change in the texture image is associated with the appearance of a strong cube {100}<001> texture component and the restoration of 4 variants of the S component in the orientations of the new grains was observed, but with lower intensity than in the samples after the deformation.(A 15° scattering from the ideal position was used).In addition, somewhat surprisingly, areas of 'approximately cube{100}<001>' orientation (described here as areas of orientation deviated by 15-20°C from the ideal position) observed in the deformed state are preserved after annealing in the temperature range up to 325°C, i.e., grains with 'cube' orientation and deviated from 'cube' orientation by rotation around the ND coexist each other in advanced stages of recrystallization (up to 310°C).This clearly shows that the nucleation of grains with a 'cube' orientation from areas with the same orientation (or close to it) is strongly debatable.

In-situ recrystallization
To identify areas of preferential nucleation of grains with a 'cube' orientation, an 'in-situ' recrystallization experiment was performed in the SEM using the EBSD system.The experiments were carried out on samples cut off from hot-rolled plates.The samples were PSC to 40% in a channel die and then subjected to several annealing steps; at each step, the annealing temperature was gradually increased: 275°C, 300°C, 325°C and 350°C, using 15-minute heating intervals.After each annealing step, an orientation map was taken from the same area.The results are shown in figure 5 in the form of the {111} and {100} pole figures.In the initial state, i.e. before the annealing, the analysed area was characterized by the absence of (sub)grains with 'cube' orientations.After annealing (at each stage of the process) it was observed that the vast majority of the orientations of the new grains belonged to the neighbourhood of the 'cube' orientation with some minor quantity of the orientations belonging to the surroundings of the different variants of the S orientation and those resulting from the rotation of the 'cube' orientation around the ED direction.In other sample regions, cube-oriented areas identified in the deformed state, remain stable until the advanced stages of recrystallization and grains with 'cube' orientations 'grow' out of areas with S or S' orientations.
Figure 5.The {111} and {100} pole figures show texture changes during insitu annealing in the same area of the sample PSC up to 40%.The selected asdeformed area did not contain areas with {100}<001> orientation.SEM/EBSD measurements with a step of 100 nm on the ND/ED section.The results were transformed to the standard 'rolling' coordinate system, i.e., to the ED/TD plane.

Summary
The present experiments provide detailed information about the development of deformation and recrystallization microstructures along with new grain orientations in Al of stable Br(110) [1-1-2] orientation as well as semi-stable S'(234) [20 -28 11] orientation.Based on SEM/EBSD measurements the orientation relations which appear at the initial stages of recrystallization between recrystallized grains and the as-deformed areas have been analysed and possible mechanisms operating during grain growth are discussed.The following detailed conclusions can be drawn: (i) The orientations of recrystallized grains during primary recrystallization of Al single crystals of stable and semi-stable orientations are not random and at the initial stages of recrystallization, only a limited number of new grain orientation groups are developed.Nevertheless, the orientations of the as-deformed state have never been reproduced in the new grain orientations.(ii) In the homogeneous structure of Br and S'-oriented crystals a large fraction of the new grains develop a strongly elongated shape with the longer axes of grains situated along the traces of the {111} planes, despite the isotropic distribution of disorientations in the as-deformed states.This indicates that in the initial stages of primary recrystallization, the migration rate of the recrystallization front is not dependent only on the disorientation between the deformed and recrystallized phases.(iii) The (near)cube-oriented grains were intensively formed during the annealing of semi-stable S'(234) [20 -28 11] single crystals, despite no (near) cube-oriented areas observed in the asdeformed microstructure.However, only one set of elongated grains in an S'-oriented crystal shows the (near)cube orientation.This confirms the validity of strictly directed rotation mechanism(s) around normal of the {111} plane along which the privileged growth occurs.(iv) In AA1050 alloy a spontaneous nucleation of cube-oriented grains is observed from the initial stages of annealing and these grains reach 'significant' sizes.The appearance of grains with a cube orientation is inextricably linked to the presence of an S and/or S' component.This convergence may be correlated with the proximity of one of the <111> poles in all three orientations (cube and S or S'), around which rotation occurs during the formation of the cube-oriented embryo.New grains appear in areas of the deformed state with the S or S' orientations and grow into their interior preferentially reaching significant sizes.Another interesting observation is those components close to the 'cube' orientation, observed after hot deformation, are retained until the advanced stages of primary recrystallization.

Figure 1 .
Figure 1.(a) Disorientation line scan in Br(110) [1-1-2]-oriented single crystal showing strong stability of the initial orientation in sample PSC up to 40%.(b) The {111} pole figure showing new grains orientations and validity near the (+/-)α <111>-type rotation around selected axes of the as-deformed state orientations in the sample annealed at 618 K for 25 s.Blue circles indicate the scattering of <111> poles of the as-deformed state, whereas the black and red dashed linesare the main components of the recrystallization texture.SEM/EBSD measurements with the step size of 80 nm.ND/ED section.

Figure 3 .Figure 2 .
Figure 3. (a) Orientation map showing the final stage of primary recrystallization in the sample after annealing for the 30 s at 420°C and corresponding (b) the {100}

Figure 4 .
Figure 4. Directionality of new grains growth in the partly recrystallized structure of S'-oriented single crystal.(a) SEM/BSE image, (b) orientation map and (c) corresponding {111} pole figure.Sample annealed for the 30 s at 380°C.