Prominent role of multi-scale microstructural heterogeneities on superplastic deformation of a high solid solution Al–7Mg alloy

Highlights

• The superplastic response is studied in an Al–7Mg alloy with multi-scale microstructural heterogeneities.

• The Al–7Mg exhibits enhanced thermal stability and superplasticity of ∼523% at 573 K and 1 × 10⁻³ s⁻¹.

• Enhanced thermal stability owes to cooperative effects of bimodal grain structure and mg segregation along GBs.

• The high superplasticity is due to a cooperated mechanism involving dislocation slip accommodated by CDRX and GBS.

Abstract

Achieving high superplasticity in single-phase Al alloys remains a challenge, since the fine-grained structure required for superplastic deformation coarsens rapidly in the absence of dispersed second-phase particles during tensile deformation at elevated temperatures. This paper concentrates on the superplastic response of a high solid solution Al–7Mg alloy processed by equal-channel angular pressing (ECAP) under uniaxial tension. The ECAP-processed Al–7Mg alloy features multi-scale microstructural heterogeneities including a bimodal grain structure and Mg solute segregation along grain boundaries (GBs) of nano/ultrafine grains. To identify effects of multi-scale microstructural heterogeneities on superplastic deformation behavior of the high solid solution Al–7Mg alloy, microstructural evolutions are studied systematically by combing electron backscatter diffraction (EBSD), ASTAR-transmission electron microscopy (TEM) orientation imaging and atom probe tomography (APT). During deformation at the optimal tensile condition of 573 K and 1 × 10⁻³ s⁻¹, the heterogeneous microstructure evolves to a stable uniform fine grain structure via continuous dynamic recrystallization (CDRX), and impressive superplasticity of ∼523% elongation is achieved. The high superplasticity is discussed in terms of the cooperated mechanism by dislocation slip accommodated by CDRX at the early tensile deformation stage and grain boundary sliding (GBS) at the late deformation stage. Our findings show that the evolution of microstructural heterogeneities in high solid solution Al–Mg alloys can be regulated, favoring for superplastic deformation, which offers an alternative strategy for developing low-cost Al alloys for enhanced mechanical properties.

Introduction

Al–Mg alloys are increasingly being applied in light-weight structural applications in aerospace and automotive industries to improve their fuel economy and reduce greenhouse gas emissions (Jobba et al., 2015; Kabirian et al., 2014; Koju and Mishin, 2020; Reyne et al., 2020). However, their usage is being limited mainly arising from their lower formability at room temperature and undesirable surface finish during sheet metal forming (Jobba et al., 2015; Kabirian et al., 2014). Due to their interactions with crystalline defects including dislocation and grain boundary (GB), solute Mg atoms play a critical role on the microstructural evolution and deformation behaviours of high solid solution Al–Mg alloys. Upon elevating temperatures, the Portevin-Le Chatelier (PLC) effect, i.e. flow instability, prevailing in Al–Mg alloys would disappear and ductility increases due to increased atom mobility and facilitated dislocation climb, accompanying with positive strain rate sensitivity, m (Pandey et al., 2013). In particular, superplasticity at high temperatures, i.e. huge plastic strains beyond hundreds of percent, has been extensively studied in high-specific strength Al–Mg based alloys from scientific and engineering perspectives, as the superplastic forming (SPF) technique enables near-net-shape manufacturing (Masuda and Sato, 2020).

It is well known that superplasticity is mainly caused by grain boundary sliding (GBS) mechanism. The enhanced ductility associated with GBS is attributed to the considerable increase in m values. The m index increases to 0.3–0.7 with GBS in a certain range of temperature, strain rate and grain size (Masuda and Sato, 2020). The increasing m index represents the enhanced ability to resist neck deformation at macroscopic scales, which leads to considerable ductility (Jia et al., 2019; Liu et al., 2020; Masuda and Sato, 2020). Thereby, superplastic alloys generally have been designed by the following principles: (i) achieving a uniform fine equiaxed grain structure (Avtokratova et al., 2012), to increase GB density and enhance GB plasticity, and (ii) enhancing the thermal stability of fine grain structure at elevated temperatures by pinning GBs, employing multiphase structures and dispersed fine second-phase particles (Masuda and Sato, 2020).

Superplasticity in Al–Mg based alloys processed by severe plastic deformation (SPD) techniques, e.g. equal-channel angular pressing (ECAP), friction stir processing (FSP) and high-pressure torsion (HPT), has been studied widely in the last two decades (Avtokratova et al., 2012; Chuvil et al., 2021; Malopheyev et al., 2016). To obtain stable ultrafine-grained structures and high superplasticity with large elongations, Sc and/or Zr are often added into Al–Mg alloys prepared by SPD, as the coherent nano-sized Al₃Sc or Al₃(Sc,Zr) precipitates generated upon SPD can effectively restrain grain growth during hot tensile deformation (Avtokratova et al., 2012; Chuvil et al., 2021; Duan et al., 2017; Malopheyev et al., 2016; Yang et al., 2016). However, the high cost of alloying elements, especially Sc, restricts their widespread commercial applications. In contrast, little attention has been paid to hot tensile deformation of single-phase Al–Mg alloys because of their relatively poor thermal stability (Meng et al., 2019). Especially, for SPD-processed Al–Mg alloys, the GBs of nano/ultrafine grains in general have high-energy non-equilibrium configuration, which are extremely unstable (Sauvage et al., 2012).

Non-equilibrium GBs in ultrafine-grained alloys processed by SPD are preferential sites for solute segregation and atomic clustering (Sauvage et al., 2014; Tugcu et al., 2012). Mg segregation along GBs and GB diffusion can impact the processing and mechanical properties of Al–Mg alloys (Koju and Mishin, 2020). As demonstrated by simulation studies and experimental results (Garner et al., 2021; Koju and Mishin, 2020; Zhang et al., 2019; Zhao et al., 2018; Sauvage et al., 2014; Liu et al., 2019), Mg atoms easily segregate along GBs and at triple junctions under severely deformed state. Using atom probe tomography (APT) technique, Mg-rich clusters (concentration in the range of 10–20 at.%) with sizes of 5–10 nm were revealed near GBs in a severely deformed Al–Mg alloy prepared by HPT (Sauvage et al., 2014). GB segregation was also detected in a HPT-processed Al–Mg alloy containing significantly less Mg in solid solution (0.5 wt.%) (Liu et al., 2019). In a recent study, concurrent Mg segregation along GBs and precipitation of nanoscale Al₃Mg₂ phase have been reported, which is responsible for enhancing grain coarsening resistance in nanocrystalline Al–Mg alloys during annealing (Devaraj et al., 2019). It is thus expected that either the solute Mg segregation along GBs and/or the fine nanoscale Al₃Mg₂ precipitates could stabilize the ultrafine/nano grains and thus promote superplastic deformation of Al–Mg alloys during hot tensile deformation.

Tailored microstructural heterogeneities, including solute segregation along GBs (Bobylev et al., 2019; Liu et al., 2019) and heterogeneous structures, such as bimodal grain structure (Zhu and Lu, 2012), heterogeneous lamellar (Geng et al., 2020) and harmonic structure (Wang et al., 2020), have been demonstrated as effective strategies to improve mechanical properties of metallic materials. Thereinto, the heterogeneous microstructure offers the opportunity to reap benefits of the strong hardening capability from micron-sized coarse grains and high strength from nano/ultrafine grains, achieving satisfying strength-ductility synergy (Zhu and Lu, 2012). In particular, the high mechanical incompatibility between grains featuring different size scales during deformation would lead to hetero-deformation-induced hardening and contribute to extra strength (Ovid'ko et al., 2018). More recently, a new micromechanical model for heterogeneous nanograined metals has been developed, where the hetero-deformation in plastically deformable inclusions with different shapes is taken into consideration (Li et al., 2021). Nevertheless, according to conventional viewpoints, the bimodal grain structure seems not to benefit the activation of homogeneous GBS for superplastic deformation, since deformation mismatch easily exists in such heterogeneous microstructures (Bussiba et al., 2001).

In our previous studies, it was already demonstrated that by inducing bimodal grain structure together with fully utilizing the strong work-hardening effect of high-content solute Mg, both high strength and high ductility could be gained in the Al–7Mg alloy processed by ECAP, i.e., tensile strength of ∼573–600 MPa and uniform ductility of ∼14% (Zha et al., 2014, 2015a, 2015b). Recently, an Al–13Mg alloy processed by cold rolling was reported to feature an impressive yield stress of ∼653 MPa and a maximum tensile strength of ∼733 MPa but a much lower tensile elongation of ∼5% (Jang et al., 2019). The high strength in Al–Mg alloys containing high solute Mg levels has been attributed to the intensified interactions of Mg atoms and dislocations, facilitating stronger accumulation of dislocations and a heterogeneous/bimodal microstructure containing nano-sized substructures and ultrafine grains (Zha et al., 2014, 2015a, 2015b; Jang et al., 2019). Whereas, the high uniform elongation in Al–7Mg is due primarily to the enhanced work hardening resulting from the high solute Mg content and the bimodal grain structure; meanwhile, the dynamic strain aging (DSA) effect originating from the solute-dislocation interaction could also contribute to the high ductility (Zha et al., 2014, 2015a, 2015b).

Nevertheless, our previous studies have mainly focused on the role of bimodal grain structure on improved strength and ductility at room temperature of binary Al–Mg alloys processed by ECAP (Zha et al., 2014, 2015a, 2015b). Their mechanical behaviours at elevated temperatures remain unknown. Furthermore, the Mg GB segregation can affect deformation behaviours at both room and elevated temperatures. To the best of the authors’ knowledge, however, it is still yet unknown to what extent the deformation conditions (e.g. the deformation temperature and strain rate) and inhomogeneous microstructural characteristics (e.g. the bimodal grain structure and Mg GB segregation) influence the mechanisms of superplastic deformation. It is thus of great interest to explore the mechanisms that govern superplasticity of high solid solution Al–Mg alloys featuring inhomogeneous microstructure at elevated temperatures.

For this purpose, the high solid solution Al–7Mg alloy processed by ECAP was selected and systematically investigated as to microstructural evolution and mechanical behaviours with different deformation parameters at elevated temperatures. In the present study, we found that the Al–7Mg alloy processed by ECAP to 4 passes exhibited a multi-scale microstructural heterogeneities and impressive superplasticity of ∼523% elongation at the optimal tensile condition of 573 K and 1 × 10⁻³ s⁻¹, which is not accessible to previously reported uniform-grained dilute binary Al–Mg alloys. Thereby, systematic studies were carried out to explore microstructural evolution and superplastic deformation behaviors of such a material featuring multi-scale microstructural heterogeneities, by utilizing advanced electron microscopy techniques and APT techniques in conjunction with detailed analysis of grain growth kinetics.

The present article addresses the evolution of multi-scale heterogeneous microstructure during hot tensile deformation and focuses on understanding the roles of the bimodal grain structure and Mg segregation along GBs on superplastic response of the high solid solution Al–7Mg alloy under uniaxial tension. The work provides data that should help in better understanding of the influence of various microstructural heterogeneities on mechanical properties at elevated temperatures of binary high solid solution Al–Mg alloys, which thus may offer an alternative strategy to design low-cost superplastic metallic materials.