Elsevier

Materials Science and Engineering: A

Volume 655, 8 February 2016, Pages 212-220
Materials Science and Engineering: A

Shear strain localization in AA 2219-T8 aluminum alloy at high strain rates

https://doi.org/10.1016/j.msea.2015.12.100Get rights and content

Abstract

AA 2219 aluminum alloy is characterised by high temperature strength, good weldability and excellent suitability as choice material for several components in defense and aerospace structures. In this paper, the dynamic impact response of cylindrical specimens of AA 2219-T8 alloy was investigated using the split Hopkinson pressure bar while strain evolution during the dynamic deformation was monitored in-situ using digital image correlation (DIC) technique. The results of DIC analysis and microstructural evaluation of the impacted specimens indicated occurrence of heterogeneous deformation characterised by intense shear strain localization. The strain localisation became noticeable after 80 μs from the start of deformation. Both transformed and deformed shear bands developed in the specimens and they cracked along the transformed bands at high strain rates. Microstructural analysis of the as-received alloy suggested that it consists of two type of dispersed second phase particles: coarse particles which are 10–45 μm in size and fine precipitates of sizes less than 1 μm. Intense adiabatic heating and strain localization led to the dissolution of the coarse second phase particles inside the transformed bands while the fine second phase particles survived the heat. The dynamic mechanical response of the alloy and its tendency to form adiabatic shear bands are influenced by the length to diameter ratio of the cylindrical test specimens.

Introduction

Localized adiabatic heating often occurs during dynamic impact loading of metallic alloys leading to local thermal softening and intense strain localization along narrow bands called adiabatic shear bands (ASBs). The consequence is low fracture energy [1] leading to crack initiation and propagation along the ASBs [2]. Armstrong and Zerilli [3] suggested that localized heating at dislocation pile-ups in BCC metal promotes formation of adiabatic shear bands at high strain rates. The microstructure of shear bands in metallic alloys can vary from elongated grains in deformed shear bands (DSBs) to ultrafine grains in transformed shear bands (TSBs). The ultrafine grains in transformed shear bands are proposed to form by dynamic recovery or dynamic recrystallization (DRX) [4], both of which are consequences of dislocation dynamics within the ASBs during formation. DRX in shear bands occur by rotation mechanism rather than by grain boundary migration that is typical of static recrystallization [5], [6]. Contrary to the popular opinion that thermal softening promotes strain localization and DRX in adiabatic shear bands, Osoviski et al. [7], [8] opined that DRX promoted by the stored energy of deformation contributes to strain softening leading to the formation of ASBs. They suggested that the role of thermal softening is negligible in materials that exhibit early DRX while thermal softening dominates in materials that exhibit late or no DRX [8].

Both deformed and transformed shear bands have been reported to develop in aluminum alloys during high strain-rate deformation [2], [9]. A minimum critical strain for formation of deformed and transformed bands is reported for AA 8090 Al-Li alloys, while the strain inside the ASBs and the global strain in the impacted specimen were determined to be 8.0 and 0.25 respectively [9]. The critical strain for occurrence of shear bands in metallic alloys depends on the strain rates, temperature and grain size [10]. The propensity of an aluminium alloy to form ASBs and fail at high strain rates is dependent on the initial temper condition [2], [11], [12]. Owolabi et al. [13] reported that ceramic particle reinforcement of AA 6061-T6 alloy affects its susceptibility to formation of ASBs. Shear bands can develop in some aluminium alloys under both quasi-static loading and high strain-rate compressive loading, although the intensity of strain localization is higher at high strain rates [14]. Shear strain localization acting as precursor to failure in aluminium alloys will continue to attract research interest in order to generate mechanical and microstructural data needed for an in-depth understanding of their behavior under dynamic impact loading condition and for mitigation of impact failure.

AA 2219 is a high strength Al–Cu alloy with excellent weldability and it retains its good mechanical properties over a wide range of temperatures from cryogenic up to 250 °C [15]. This makes it a candidate material for propellant storage tank in space vehicles. Surekha et al. [16], [17], [18] identified galvanic coupling between coarse Al2Cu particles and the continuous α-Al phase in its microstructure as a major setback to the use of this alloy in some applications. Surface treatment can be used to refine the structure and make this alloy resistant to galvanic corrosion [16]. Narasayya et al. [15] reported on the tensile behavior of the alloy under different aging condition while Suenger et al. [19] also discussed the influenced of processing parameters on the alloy's hardness. Jha et al. [20] investigated the failure of AA 2219 aluminum alloy used in welded propellant tank for space vehicle and traced its fracture to occurrence of strain localization and formation of ASBs. Evidence of melting, underscoring the intensity of adiabatic heating, was also observed on the fracture surface of the tank. Since information on the dynamic mechanical response of this alloy in relation to shear strain localization is limited, it is imperative to investigate the various parameters that can influence shear strain localization in AA 2219 aluminum alloy at high strain rates. In the current study, the microstructural evolution accompanying shear strain localization in cylindrical specimens of AA 2219-T8 aluminium alloy is investigated. The effects of length to diameter (aspect) ratio of the test specimens on its dynamic mechanical response and susceptibility to strain localization and adiabatic shear failure are discussed.

Section snippets

Materials and methods

The investigated AA 2219-T8 aluminum alloy was supplied by the NASA research center. Its nominal composition is presented in Table 1. The alloy can contain up to 6.8 wt% Cu with minor additions of Mn, Ti, V and Zr. Iron is an impurity that is commonly present in most aluminum alloys and its content in AA 2019 can be up to 0.3 wt%. The high copper content is responsible for the excellent weldability of this alloy [21]. Two groups of cylindrical test specimens having a diameter of 5 mm were machined

Microstructures of the as-received alloy

Results of the microstructural investigation of the as-received alloy using the optical (OM) microscope and scanning electron microscope (SEM) are presented in Fig. 3, Fig. 4. The microstructure consists of dispersed second phase particles within a continuous α-aluminum phase. Based on the sizes, two types of second phase particles can be identified in the micrographs (Fig. 3): coarse and fine particles. The coarse particles are irregular in shape and their dimension varies between 10 and 30 μm.

Conclusion

Plastic deformation of cylindrical specimens of AA 2219-T8 aluminum alloy with length to diameter ratios of 0.8 and 1.0 were investigated at high strain rates in compression. The length to diameter ratio of the test specimens did not have significant influence on the flow stress under dynamic impact loading. However the total amount of deformation for a given strain rate is higher for the specimens with the higher aspect ratio. For deformation at 1500 s−1 as an example, the true strain were

Acknowledgment

The authors would like to acknowledge the financial support from the Department of Defense through the research and educational program HBCU/MSI (contract # W911NF-12-1-061) under the direct supervision of Dr. Asher A. Rubinstein (Program Manager, ARO Solid Mechanics Program).

References (36)

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