The effect of layer interface on recrystallization behavior of layered aluminum: An in-situ EBSD study

In recent years, layered heterogeneous metallic materials have received considerable attention. In this work, we fabricated an AA3003/AA1060 layered aluminum and introduced heterogeneity by regulating recrystallization behavior via accumulative roll bonding and annealing processes. The annealing process was studied by in-situ electron backscatter diffraction (EBSD) observation. The present work shows that the recrystallization rate in the AA1060 layer is significantly higher than that in the AA3003 layer. This disparity can be attributed to the varying element composition, which generates a layered aluminum structure comprising alternating fine-grained and coarse-grained layers. Moreover, the influence of layer interface on the recrystallization behavior of the AA1060 layer was investigated. The result shows that high energy storage near the interface promotes recrystallization nucleation and grain growth. This study reveals the formation mechanism of layered heterogeneous metallic materials, which can help the design and preparation of high-performance heterogeneous metallic materials.


Introduction
In metallic materials, increasing the strength or ductility invariably compromises the other, a dilemma known as the strength-ductility dilemma [1].Recent studies suggest that the question of achieving both high ductility and strength in materials may be answered through the design of heterogeneous layered structures [2,3].
The regulation of recrystallization is a feasible method to introduce heterogeneity in materials [3,4].The recrystallization kineties of metal materials are highly dependent on the element composition, precipitates and plastic deformation.The element addition could increase the activation energy required for recrystallization and raise the recrystallization to start temperature [5,6].Additionally, nanoprecipitates could act as obstacles, pinning the grain boundaries and impedes their migration, hence resulting in a significant decrease in grain growth rate [7,8].While large non-deformable particles could promote the recrystallization though particles stimulated nucleation (PSN) [9,10].The last but not the least, the plastic deformation is always non-uniform due to microstructural heterogeneity, resulting in uneven dislocation structures [11] and varying distribution of deformation energy storage.The deformation energy storage is recognized as a key factor influencing the recrystallization behavior [10][11][12].For heterogeneous layered metals, the presence of composition differences and microstructural variations between constituent layers introduces additional complexities, which have not been fully understood in terms of their effects on recrystallization behaviors.
In this study, we designed and fabricated a heterogeneous layered aluminum consisting of AA3003 alloy and AA1060 alloy through accumulative rolling bonding (ARB).The element heterogeneity across the layer interface and strain heterogeneity near the interface was introduced in the as-rolled layered aluminum.An in-situ EBSD characterization is performed to monitor the microstructural evolution during the subsequent annealing process for revealing the role of layer interface on recrystallization behaviors.

Experimental methods
Commercial pure aluminum AA1060 and aluminum-manganese alloy AA3003 sheets with a thickness of 1.5 mm were chosen as raw materials.The A3003/AA1060 layered aluminum was fabricated by accumulative rolling bonding (ARB).The schematic illustration of ARB process is shown in figure 1a.The raw AA1060 sheet and AA3003 sheet were annealed at 550 ℃ and 610 ℃, respectively, for 1 hour.During each rolling step of ARB, the surface was thoroughly cleaned and roughed, and then the sheet was rolled to 50 % of its original thickness at 200 ℃, followed by a cut from the center position.This process was repeated 4 times without changing the rolling direction.More details about ARB processing can be found in our previous work [15].
The microstructure evolution of ARBed layered aluminum during annealing was characterized by a TESCAN S8000 scanning electron microscope with an EDAX-EBSD system.The sample for EBSD observation was cut from the rolled plate, and the surfaces in the ND-TD direction were polished electrolytically.The sample was annealed at 320 ℃ in a furnace with a protective atmosphere, and the EBSD scanning was performed on the same region at intervals of 20 minutes for a total 4 scans.The step size of EBSD scanning was 0.35 μm.

Results and Discussion
Figure 1b shows a SEM micrograph of layered aluminum made from AA1060 and AA3003.The interface layer is relatively flat, without noticeable cracks or bending, which implies a solid interface bond.The thickness of constituent layers is very close, ~ 80 μm.The chemical compositions of AA1060 and AA3003 are shown in table 1.The constituent layers with obvious dark grey particles (highlighted by yellow arrows) are AA3003 layers.This is attributed to the presence of Mn element, which leads to the formation of large secondary phases, Al6(Mn, Fe) and Al12(Mn, Fe), as reported by the previous work [16].   2 shows the IPF maps after annealing for 20 minutes, 40 minutes, and 60 minutes, respectively.After 20 minutes of annealing, it shows a low-quality EBSD scanning in the AA3003 layer.The reason behind this might be a lack of recovery, so the highdensity dislocations in the rolled plates reduced the indexing rate of EBSD scanning, consistent with observations found in the literature.[17,18].As the annealing process continued, recovery occurred in the AA3003 layer, gradually improving the quality of EBSD scanning.After 60 minutes of annealing, the average grain size in the AA3003 layer is 2.32 μm.On the other hand, after annealing for 20 minutes, recovered grains with an average size of 1.31 μm were observed in the AA1060 layer.When the annealing time reached 40 minutes, the average grain size significantly increased to 12.65 μm in the AA1060 layer.After annealing for 60 minutes, a few grains continued to grow, and the average grain size in the AA1060 layer eventually reached 14.08 μm, which is significantly higher than that in the AA3003 layer.The addition of Mn element leads to the formation of precipitates in the AA3003 layer, which can impede the grain boundary migration and elevate the recrystallization temperature.The recovery and recrystallization behavior of the AA3003 layer is considerably impeded compared to the AA1060 layer.Considering the relative promising EBSD quality, the investigation on the recrystallization behavior near the layer interface was mainly carried in the AA1060 layer.The microstructural parameters within every 10 μm of AA1060 layers as a function of the distance from the interface was tracked during the annealing.The grain size was measured by transect lines along TD.After 20 minutes of annealing, the grain size is uniformly distributed throughout the layer thickness direction.As shown in figure 3a, with increasing annealing time, the grain size near the interface became larger than within the layer.Figure 3b shows the statistical analysis of average misorientation angles at subregions with different distances from the interface.The results show that after 20 minutes of annealing, the average misorientation angle is greater near the interface than within the layer.Furthermore, the difference between the average misorientation angle near the interface and within the layer becomes more significant as the annealing time increases.The observed phenomena indicate that the annealed microstructures within the AA1060 layers have evident heterogeneity related to layer interface.
The grain orientation spread (GOS) is always used to judge the recrystallized and deformed microstructures.Here, grains in the AA1060 layers with a GOS less than 2° are regarded as recrystallization nucleus, highlighted in green in Figure 4a.It is found that the number of nucleated grains near the interface surpasses that within the layer after 20 minutes of annealing.This is also supported by the recrystallization nucleus numbers as a function of distance to layer interface, shown in figure 4b.It suggests that grain nucleation is more likely to occur near the layer interface while the inner layer retains more deformed structures.The kernel average misorientation (KAM) distribution in the AA1060 layer along the normal direction was calculated, shown in figure 5.It can be observed that as the distance to the layer interface decreases, the value of KAM increases.This could be attributed to the distinct mechanical response of AA1060 and AA3003.During the ARB process, AA3003 and AA1060 layers co-deform mainly through dislocation slides.However, the AA3003 layer is more prone to plasticity instability compared to the AA1060 layer [15].Therefore, two constituent layers must accommodate the strain gradient across the layer interface, resulting in greater deformation stored energy near the layer interface.In the following annealing process, the KAM, which can roughly evaluate the deformation stored energy, show a faster drop near the layer interface than that at the layer interior.On the one hand, this is consistent to a great degree of recrystallization nucleation near the layer interface (figure 4), suggesting that high deformation energy storage is more favorable for recrystallization nucleation.On the other hand, the recrystallization is also significantly influenced by grain boundary migration.As is shown in figure 6, we tracked the evolution of some single grains during annealing and analyzed the KAM around these grains.It was found that the mobility of the grain boundary is related to the strain gradient near the grain boundary.The grain boundaries of recrystallized grains always migrate towards the area with higher KAM preliminarily.However, the grain boundaries between recrystallized grains with low KAM barely move.This result suggested that the strain energy stored in the deformed structure provides the impetus for grain boundary migration during annealing.As mentioned above, there is a higher KAM near the layer interface in the AA1060 layer.Higher deformation energy storage near the layer interface could promote the migration of grain boundaries toward the interface.Hence, the effects of initial microstructures on recrystallization nucleation and grain growth can explain the microstructural variations within AA1060 layers during the annealing process.

Conclusions
In this study, we prepared the AA3003/AA1060 layered aluminum by ARB, and investigated the recrystallization behavior through the in-situ EBSD experiment.The grain size difference between constituent layers is attributed to the element heterogeneity across layer interfaces.Meanwhile, the strain heterogeneity near the layer interface plays a key role in nucleation and growth of recrystallized grain, leading to the microstructural gradient within AA1060 layers.This research can aid in tailoring the microstructures of such heterostructured metals, and guide the design of their composition and processing.

Figure 1 .
Figure 1.(a) Schematic illustration of ARB process of layered aluminum; (b) Microstructure of AA1060/AA3003 layered sample in ND-TD section.

Figure 3 .
Figure 3. Statistical results of (a) grains size, (b) average disorientation angle as a function of the distance from the interface in AA1060 layer annealed at 320 ℃ for 20 min, 40 min and 60 min.

Figure 4 .
Figure 4. (a) Distribution of nucleus and (b) Counts of nucleated grains as a function of the distance from the interface in AA1060 layer annealed at 320 ℃ for 20 min.

Figure 5 .
Figure 5. KAM distribution of AA1060 layer annealed at 320 ℃ for (a) 20 min, (b) 40 min, (c) 60 min.(d) Statistical results of KAM as a function of the distance from the interface in AA1060 layer.The kernel average misorientation (KAM) distribution in the AA1060 layer along the normal direction was calculated, shown in figure 5.It can be observed that as the distance to the layer interface decreases, the value of KAM increases.This could be attributed to the distinct mechanical response of

Figure 6 .
Figure 6.KAM distribution near a single grain in layer AA1060 after annealing for (a) (b)40 min and (c) (d) 60 min.

Table 1
The chemical composition of AA1060 and AA3003 alloy (wt.%).The element heterogeneity across layer interface in the present case could result in distinct recrystallization behaviors in different layers.Figure