Inception of macroscopic shear bands during hot working of aluminum alloys

https://doi.org/10.1016/j.ijplas.2023.103632Get rights and content

Highlights

  • Several aluminum alloys were ‘strip’ tested at 298 K and 573 K. However, macroscopic shear bands (MSBs) predominated only in al-6mg deformed at 573 K.

  • The MSB regions contained higher misorientation, grain elongation and fragmentation. MSBs both improved ductility, and were the sites of eventual fracture.

  • Though classical criteria predict flow instability at lower strains, our plasticity modeling clearly established MSB inception only at a strain of 0.20.

  • The ‘delay’ in MSB inception corresponded to the development of a percolating network of deforming grains.

  • Strain percolation, in particular, were triggered by differential dynamic recovery between the soft- and hard-oriented grains.

Abstract

Macroscopic shear bands (MSB) may develop during hot working of metallic materials. They are well-understood as a physical manifestation of flow instability, and the processing regimes wherein they form are well-charted. However, the microstructural transitions that occur between the onset of flow instability and MSB inception are not fully understood. In order to elucidate them, several aluminum alloy specimens were subjected to strip testing in a thermomechanical simulator (Gleeble™) at 298 K and 573 K. Prominent MSB were observed along the diagonals of the strip volume of only Aluminum-6 wt% Magnesium alloy specimen deformed at 573 K. Comparing the experimental grain morphology and crystallographic textures with those from plastic flow models revealed that MSB inception occurred only after ∼0.20 homogeneous plane strain deformation. However, classical flow instability was predicted at much smaller strain. This ‘delay’ was explained experimentally by showing that clusters of neighbouring severely deforming, fragmenting, mostly soft-oriented grains gradually developed due to lattice rotations along the specimen diagonals, and that MSB inception corresponded to the formation of a percolating network of such grains spanning the specimen. Further, clear experimental evidence revealed that differential dynamic recovery between hard- and soft-oriented grains was essential for MSB formation.

Introduction

In the hierarchy of deformation heterogeneities (Dieter and Bacon, 1976; Humphreys and Hatherly, 2012; Verlinden et al., 2007a), macroscopic shear bands (MSBs) are the coarsest. MSBs are transgranular in nature, often span the entire specimen, and are the signature of localized material flow driven by material and/or geometrical softening (Semiatin et al., 1983; Semiatin and Jonas, 1984). Adiabatic shear bands may also be regarded as MSBs formed under dynamic conditions. They are thought to arise from material softening accompanying rapid local temperature increase due to plasticity (Dodd and Bai, 2014). However, this view has been be contested recently (Guo et al., 2020, 2019). An alternate mechanism, via continuous dynamic recrystallization or extended recovery (Verlinden et al., 2007b), has been proposed as the cause of associated softening (Magagnosc et al., 2021).

Flow instability, and the formation of MSBs cannot occur without geometrical and/or material softening. This can be inferred from the calculations of Hill (1950) and Green (1951) in the 1950s. Hill (1950) and Green (1951) determined the slip-line fields describing the plastic flow during plane strain indentation of a rigid-perfectly plastic material. They found that the slip lines emerged like a fan from the singularities at the four edges of contact between the dies and the strip. The plastic flow was not localized. Barring small triangular regions contacting the dies that remained rigid, material flowed throughout the specimen. Further, flow localization has been experimentally found absent in materials exhibiting dominance of work hardening (Raabe et al., 2001). However, in materials with significant work softening, typically realized under hot-working conditions, the flow field has been found to localize within MSBs. MSBs were also observed during different modes of thermomechanical processing, with examples ranging from hot torsion (Semiatin and Lahoti, 1981) to hot forging (Semiatin and Lahoti, 1982) and even plane strain compression in a thermomechanical simulator (Tang et al., 2018). It has been reported (Semiatin and Lahoti, 1982) that MSB inception required considerable additional strain past the point of homogeneous flow instability. This important observation has been pursued further in the present study.

While softening is necessary for MSB formation, the set of microstructural transitions that lead to MSB is not clear. An important milestone in the detailed study of microstructural transitions is due to Korbel et al. (1986) and Korbel and Martin (1986), who studied Aluminum - 4.8 wt% Magnesium. They observed the formation of microbands, or regions of intense localized shear, confined to individual grains. These evolved from a tangle of dislocations after about 20% rolling reduction. With increasing strain, these microbands were able to penetrate grain boundaries, and extend into neighbouring grains. Between 60% and 80% reduction, they reported the formation of MSB extending across the thickness of the specimen. In brief, the dense clusters of microbands transformed into MSB with increasing strain. The microstructural transition from isolated grain-interior microbands to that of the MSB was abrupt. This abrupt transition, however, was not deliberated in depth by Korbel et al. This, arguably, required simultaneous characterization of the microstructure at multiple length scales.

Multiscale characterization techniques, especially various forms of digital image correlation (DIC), have now been extensively developed (Efstathiou et al., 2010). Aydıner and Telemez (2014), for example, established that macroscopic shear bands emerged abruptly, by the explosive propagation of microscopic twins extending several grains in an Mg alloy. On the other hand, Üçel et al. (2019) reported a considerably lower degree of strain localization in the same alloy under uniaxial tension. However, DIC has not been used to describe initation of MSB in hot worked microstructures.

Technologically, MSB formation limits the range of possible thermomechanical processing operations (Verlinden et al., 2007b), and also affects such diverse areas as metalcutting, ballistics and geology (Semiatin and Jonas, 1984). MSB accommodate large plastic strains and thus introduce deformability or ductility (Needleman et al., 1988). However, the final failure also initiates in the MSB (Deve and Asaro, 1989). MSBs also introduce significant deformation and shear strain gradients (Paul et al., 2015). Any post-processing of a deformed material with MSB (for example, strip production from rolled coils) might result in warpage (Mukherjee et al., 2020; Hidveghy et al. 2003). This has been attributed to residual stress accomodation from MSB. In summary, MSB formation in metallic materials is an important aspect of industrial thermomechanical processing. Understanding the mechanisms of MSB formation may enable the development of techniques for their avoidance, or control, and thereby extend the capabilities of metal forming, and other operations.

The present work aims to uncover the microstructural transitions leading up from flow localizations to the inception of MSB. To this end, specimens from several aluminum alloys were subjected to plane strain indentation (strip testing) in a thermomechanical simulator (Gleeble™). These tests were conducted at both ambient (298 K) and elevated (573 K) temperatures. MSB formation was prominently observed in coarse-grained Aluminum - 6 wt% Magnesium (Al-6Mg) alloy deformed at 573 K. These conditions fall outside the extensively studied Portevin-LeChatelier regime (Reyne et al., 2020; Xu et al., 2022; Yuzbekova et al., 2017), and outside the superplastic regime (Zha et al., 2021). Nominally identical specimens of Al-6Mg were subjected to progressive plastic deformation and their ex-situ microstructures were extensively characterized. Comparing these microstructures, and local crystallographic textures with predictions from a polycrystal plasticity model yielded physical insights into the transition from global to localised shear deformation mode within the MSB.

Section snippets

Experimental procedure

This study used four different aluminum alloys: AA1050, AA2219, Al-3Mg, and Al-6Mg. The chemical compositions are given in Table 1. All grades were obtained from the Indian Space Research Organization (ISRO) as hot rolled plates. These were then processed by cold rolling, recrystallization, grain coarsening anneals, and finally solution treatment to obtain fully recrystallized solutionized material. The alloys were processed to have similar average grain sizes (∼120 μm) and near-random

Experimental results

Fig. 1a shows the physical appearance of a deformed specimen and the formation of severe flow localization on the LT plane within macroscopic shear bands or MSB. The final fracture also took place within the MSB (see Fig. 1a). The stress-strain responses from strip tests are collated in Fig. 1b (for Al-6Mg) and Fig. 1c (for different aluminum alloys). The anisotropy in crystallographic texture can be represented as texture index or f(g)2dg (Vadavadagi et al., 2016; Yerra et al., 2005). It is

Discussion: combining microstructure and plasticity modeling

MSB formation has been reported in both precipitation-hardenable (Chang and Asaro, 1981; Harren et al., 1988a; Wagner et al., 1995) and strain-hardenable (Korbel et al., 1986; Korbel and Martin, 1986) aluminum alloys. Hence, we have considered both alloys, bound by a common thread of application in ISRO's (Indian Space Research Organization) launch vehicles. It is shown in this section that MSB formation is determined by its mechanism, and not by the nature of the aluminum alloy.

Conclusions

  • 1

    Different aluminum alloys were subjected to plane strain indentations or strip tests at 298 K and 573 K in a Gleeble™ 3800. Only Al-6Mg deformed at 573 K developed a pair of macroscopic shear bands or MSBs and showed an associated softening in the stress-strain response. The MSBs provided significantly higher ductility to Al-6Mg, although the final fracture also originated from within the MSB.

  • 2

    The MSB was associated with marginally higher misorientation and hardness; but exhibited noticeably

CRediT authorship contribution statement

Aditya Prakash: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Writing – original draft. Tawqeer Nasir Tak: Investigation, Data curation, Formal analysis, Writing – original draft. Namit N. Pai: Investigation, Data curation, Visualization, Writing – review & editing. Harita Seekala: Investigation, Data curation, Writing – review & editing. S.V.S. Narayana Murty: Resources, Supervision, Writing – review & editing. P.S. Phani: Investigation, Resources, Writing –

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.

Acknowledgement

The authors acknowledge support from the Indian Space Research Organization (ISRO). Support from the National Facility of Texture and OIM. CoEST (center for excellence in steel technology) Gleeble™ and Themis-300 TEM laboratories are also acknowledged.

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