Full length articleExperimental and crystal plasticity study on deformation bands in single crystal and multi-crystal pure aluminium
Graphical abstract
Introduction
The term deformation band (DB) defined by Barrett et al. [1] in 1939 describes a volume of approximately constant orientation that is significantly different from the orientation elsewhere in that grain. DBs are found in various alloys no matter in single crystal or polycrystalline structure, such as steel [2], zinc [3], aluminium [4], copper [5], nickel [6], magnesium [7], titanium [8] and etc. The formation of DBs is able to effectively accommodate the plastic strain and give rise to the heterogeneities in microstructure by forming noticeable orientation gradients and eventual fragmentation of grains [9]. Therefore, understanding the formation of DBs by examining its crystallographic features might be the key to understanding the grain nucleation in the recrystallization process regarding nucleation criteria and nucleated grain orientations on the basis of heterogeneous deformed microstructure.
During the plastic deformation, the lattice sliding occurs which is subject to the simple shear exhibiting the dislocation glide of single slips without creating the lattice rotation [10]. By contrast, the lattice tilting could also occur which is subject to the pure shear [10,11] exhibiting the rotation of a local lattice [12] due to the constraint of lattice sliding (also known as lattice rotation).
The formation process of two kinds of DBs, i.e. kink bands and bands of secondary slip, was identified by etch pit tests and Laue back-reflection method with an X-ray beam in face centred cubic crystals (FCC) in tension in the 1990s by Higashida et al. [5]. These dislocation structures of two kinds of DBs observed by etch pit technique have an excellent agreement with the observations by transmission electron microscope (TEM) [13,14] and X-ray topography. The kink band is found to be a wall perpendicular to the primary slip direction and the band of secondary slips is a zone approximately parallel to the primary slip plane in which secondary slips are operating more actively than the primary slip [5]. Kink bands and bands of secondary slip are transmuted from the local bend-gliding regions and the coplanar slip zones of primary slips respectively. However, the orientations formed in DBs in mesoscale and factors affecting the shape and the amount of DBs are not clearly understood.
In some previous papers, the effect of grain orientation on the formation of DBs is studied for rolling as it is relevant to the generation of deformation textures [9,15]. For instance, the {110}<001> Goss orientation in FCC crystals is stable under rolling in single crystal and polycrystals with large grains, i.e. the Goss orientated grains can undergo extensive deformation without the development of large-scale heterogeneities [9]. By contrast, the Cube orientated FCC crystals are metastable during room temperature compression by virtue of the highly symmetrical nature of its slip systems. The Cube orientation often exhibits very heterogeneous deformation and the splitting of the lattice into strongly misorientated deformation bands [15]. The orientation in DBs is found by rotations about the transverse direction. However, the rotation direction and its reasons are not clearly addressed. Therefore, apart from the orientation formed in DBs, how the strain level and neighbouring grains affect the formation of DBs have not been systematically discussed. This long-standing problem retards the understanding of the subsequent microstructural evolution, such as recrystallization [16], [17], [18].
The development of recrystallized microstructure is affected by the formation and growth of nuclei in the deformed structure, while the grain nucleation is dominated by the rate of dislocation accumulation and heterogeneities of deformed structure [9,19]. In order to understand and predict the microstructural evolution during recrystallization and grain growth [20,21], it is essential to examine the development of the heterogeneities in the deformed state [9,16].
It is extremely important to thoroughly understand the formation of DBs for the study of recrystallization since it provides both the heterogeneous strain distribution and high orientation gradients regarding the crystallographic features. The amount of stored energy and orientation gradients after deformation would be important conditions for the formation of nucleation sites [22,23], affecting the recrystallization rate, recrystallized grain size and recrystallized orientation. For instance, single crystals that are deformed in single glide and recovered during annealing may not recrystallize. Because the dislocation structure which does not contain the heterogeneities and orientation gradients cannot provide nucleation sites during annealing [9].
Furthermore, the formation of DBs is a crucial source of new orientations acquired for the deformed structure. In order to accurately predict the recrystallised texture [24] developed after grain nucleation and grain growth based on the deformed texture, understanding the deformed texture, i.e. orientation formed in DBs, could shed some lights on the orientations in the nucleus during recrystallization. For instance, in {100}<001> orientated silicon-iron single crystals, the recrystallized orientation kept the same with the orientation in the deformed structure which was {001}<210> [25].
In this paper, single crystals pure aluminium with four different orientations and multi-crystals with different grain morphology were compressed to 0.3 and 0.2 engineering strain. Crystal plasticity finite element (CPFE) models are developed to assess the experimental observations on DBs. Strain distributions were evaluated using the digital image correlation (DIC) technique [26], [27], [28] and applied to validate the CPFE model. Deformation activities were evaluated using optical microscopy and electron back-scattered diffraction (EBSD) for the slip lines and DBs, as well as the CPFE modelling for the effective plastic strain and primary slip field calculation. These results provide explanations for the observation in as a unified manner.
Section snippets
Materials preparation
This work used two types of pure aluminium obtained by the Czochralski process [29] from Shanghai Jiaotong University (China) to study the formation of DBs: one is cubic single crystals with >99.999% purity (hereafter referred as 5 N); the other one is cuboid columnar multi-crystals with >99.9% purity (hereafter referred as 3 N). The chemical composition of 5 N and 3 N are listed in Table 1.
Face XY and ZY of each sample for EBSD analysis were prepared metallographically with SiC papers (up to
Validation of CPFE model
The simulated stress-strain curves shown in Fig. 1 give reasonably good agreement with experimental data for both single crystals and multi-crystals. Note that there is a very slight stress drop in the stress-strain curve for MC 1 at ∼0.06 strain. By checking the deformed shape of MC 1, the slight shift of stress could be attributed to the generation of obvious slip lines (i.e. shifts of lattice structure to sample surface).
In addition, the DIC experimental and CPFE numerical strain yy maps,
Discussions
In 1979, the homogeneous distribution of primary and secondary slip lines cross all over the sample was already observed by Franciosi et al. [34] in copper and aluminium single crystals under optical microscopes. These crystallographic slip lines were also found by Asaro [37] in 1983 in polycrystalline alloys. Gioacchino and Fonseca [2] in 2012 firstly related the local strain saturation with slip bands (i.e. with respect to slip lines) by using the digital image correlation technique. However,
Conclusions
In this work, integrated experimental study and CPFE modelling in mesoscale are utilized to provide new insights in understanding the mechanism of DBs formation in pure aluminium and its affecting factors. It has been shown that:
- 1.
The DBs are produced aligning parallel to the secondary slip system due to the constraints from the slip band intersection of primary and secondary slips. It is rational to speculate that the lattice sliding is prohibited at the intersection area while the lattice
CRediT authorship contribution statement
Qinmeng Luan: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Validation, Visualization, Writing - original draft, Writing - review & editing. Hui Xing: . Jiao Zhang: . Jun Jiang: Conceptualization, Project administration, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Qinmeng Luan is grateful for the strong support from the Aviation Industry Corporation of China (AVIC), the First Aircraft Institute (FAI), China Scholarship Council (CSC), the Henry Lester Trust and the Great Britain-China Educational Trust for this funded research. The research was performed at the AVIC Centre for Structural Design and Manufacture at Imperial College London. Qinmeng Luan would also like to thank Dr Ruth Brooker for training and support of instrument usage, gratefully thank Dr
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