Experimental and crystal plasticity study on deformation bands in single crystal and multi-crystal pure aluminium

Abstract

Deformation bands (DBs) formed in metals even in single crystals are known to give rise to the microstructural heterogeneities, thus contributing to some long-standing microstructure formation problems, such as the occurrence of recrystallization on the basis of deformed microstructure. Previous experimental transmission electron microscope (TEM) work has identified two types of DBs in the microscopic scale, i.e. kink bands and bands of secondary slips, showing the importance of understanding the slip activation for DBs. To extend the theory in mesoscale, single crystal and multi-crystal pure aluminium, as well as their corresponding crystal plasticity finite element (CPFE) models, are used in this paper to explore the effect of grain orientation, strain level and neighbouring grains on the formation of DBs. It is demonstrated that slip band intersection of primary and secondary slips is predicted to constrain the lattice sliding but facilitate the lattice rotation for the formation of DBs regarding the wall of DBs and its orientation. It is found that the impact of the above factors on the formation of DBs is caused by the slip field of primary slips. A sufficient amount of primary slips activated inside grains would be the key to the formation of distinct DBs with high area fraction and aspect ratio.

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.