Elsevier

Acta Materialia

Volume 193, July 2020, Pages 99-114
Acta Materialia

Full length article
Mixed-mode (I and II) fracture behavior of a basal-textured magnesium alloy

https://doi.org/10.1016/j.actamat.2020.03.023Get rights and content

Abstract

Mixed-mode (I/II) fracture experiments on rolled AZ31 Mg alloy are conducted using notched four point bend specimens along with in-situ imaging. Digital Image Correlation (DIC) technique is used to analyze the images and map out the deformation and strain fields. With increase in mode II component of loading, a monotonic reduction in the notched fracture toughness (Jc) is observed. Fractographs reveal that the fracture mechanism transitions first from (ductile) micro-void growth and coalescence to twin induced quasi-brittle failure and then to shear cracking as the loading changes from mode I to II. Detailed optical micrographs and EBSD maps show profuse tensile twinning in the ligament, especially near the far-edge of the specimen leading to pronounced texture changes. However, the micro void size near the notch tip as well as the twin density in the ligament reduce with higher mode II component. The pronounced drop in fracture toughness and the changes in the associated mechanism with increase in mode-mixity are rationalized from the combined effects of hydrostatic stress, plastic strain localization and tensile twinning.

Introduction

Magnesium alloys are being considered in automobile and aerospace applications owing to their low density, high specific strength, good machinability and damping characteristics [1], [2]. However, their moderate ductility and fracture toughness as well as low formability at room temperate pose challenges that need to be addressed. The room temperature deformation of Mg is accommodated by various slip systems, viz., basal, prismatic, pyramidal  < a > , pyramidal <c+a> [3], [4] and twin systems of the type {101¯2}  and {101¯1} [5], [6]. The {101¯2} - type twins cause extension, whereas {101¯1} - type twins result in contraction along the c-axis, and, hence they are referred as tensile twin (TT) and contraction twin (CT), respectively. These twins are also accompanied by abrupt lattice reorientation of the grains resulting in significant texture hardening [7]. Among the above deformation modes, basal slip and TTs get easily activated, owing to their low critical resolved shear stress (CRSS) [3], [8]. The limited number of slip systems with vast variation in their CRSS and the low symmetry hexagonal close packed (HCP) crystal structure result in high plastic anisotropy [9]. Further, the initial texture, loading direction and twin assisted hardening mechanisms [7] play a vital role in influencing the mechanical and fracture response of Mg alloys [7], [9], [10], [11], [12], [13], [14], [15], [16], [17].

A series of experiments was conducted in [18], [19], [20] to understand the role of TTs, CTs and double twins on the deformation response of Mg alloys. During uniaxial tension tests of basal textured AZ31 Mg alloy, Koike [18] noted that TTs, which form at an early stage, accommodate strain incompatibility caused by variation in basal slip between neighboring grains, whereas CTs are accompanied by severe shear localization and usually form at later stage of deformation. Ando et al. [19], [20] showed that the formation of {101¯1}-{101¯2} double twins are detrimental to ductility as they become crack initiation sites and lead to premature failure.

In [13], [14], [21], [22], [23], [24], the effects of texture and grain size on the fracture toughness were examined by performing experiments on rolled/extruded Mg alloys using fatigue pre-cracked three point bend (3-PB) specimens. In all these tests, a low values of KIc in the range 15–29 MPa m1/2 were reported, based on the stretch zone width (SZW). Further, Somekawa et al. [23] observed many twin induced brittle cracks on the fracture surface and concluded that twins are detrimental to the toughness. The mode I fracture behavior of Mg single crystal for different lattice orientations was studied by conducting experiments using notched 3-PB specimens by Kaushik et al. [15]. Although crack propagation along twin boundaries was observed, they noted that the energy release rate history correlates with evolution of twin volume fraction in the specimen and concluded that dissipation due to twinning makes a significant contribution to the toughness. Prasad et al. [16], [17] also showed from experiments with compact tension and four point bend (4-PB) specimens of rolled AZ31 Mg alloy that TTs constitute an important mode of plastic dissipation and impart toughness. They observed ductile fracture features in the case of notched samples and quasi-brittle characteristics in the fatigue pre-cracked specimens [17]. The notched fracture toughness corresponding to notch root radius of about 45 µm was determined in [16] as Jc  ∼  46 N/mm, out of which the contribution from TT-induced plastic dissipation was estimated as 70%.

The effects of stress triaxiality on ductility, plastic anisotropy and damage mechanism in the context of AZ31 Mg alloy was studied by Kondori and Benzerga [12] by conducting experiments using smooth and notched cylindrical specimens. In particular, they observed that with decrease in stress triaxiality (i.e., by moderate increase in notch root radius), ductility enhances, plastic anisotropy becomes less pronounced and the fracture mechanism changes from quasi-brittle (twinning induced micro-cracking) to ductile (micro-void growth and coalescence). A similar experimental study on magnesium rare earth alloy WE43 was performed along with some simulations in [25]. The above findings were further reinforced by the polycrystal finite element simulations of Selvarajou et al. [26], [27].

In [28], [29], [30], [31], [32], in-situ observations during tensile tests on thin sheet specimens of Mg alloys with pre-drilled micro holes were made to gain insights on ductile damage mechanisms. Nemcko and Wilkinson [29] observed that void fraction and inter-void distance play a vital role in determining void growth and coalescence. They reported that with decrease in void fraction, the linkage mechanism changes from internal ligament necking to twin/grain boundary failure.

It is generally believed that the ductile fracture toughness enhances with increase in mode II component of loading [33], [34]. However, mixed-mode (I/II) fracture experiments on Al alloys revealed that there is a decrease in the toughness with increase in the mode II component [35], [36]. In order to understand these contrasting trends, numerical simulations of ductile mixed-mode fracture using the Gurson (porous plasticity) model were performed by Ghosal and Narasimhan [37], [38], [39]. They observed that the fracture mechanism is governed by the complex interplay between void growth and shear cracking, which in turn are controlled by hydrostatic stress and plastic strain levels. They showed that plastic strain localization in the ligament between a notch and a void ahead of it happens early with increase in mode II component leading to premature void coalescence and consequent drop in toughness Jc. Ghosal and Narasimhan [39] examined the additional effect of micro-void nucleation under mixed-mode loading by considering different (low and high) strengths for the matrix-inclusion interface. For low interfacial strength, micro-voids nucleate near a notch tip at early stage of loading and Jc decreases when mode II component is increased. The debonding between the matrix and inclusion happens at a later stage and, therefore, Jc increases rapidly with mode II component, in the case of high interfacial strength. Arun Roy and Narasimhan [40], [41] studied the effect of constraint level on ductile fracture process under mixed-mode loading condition by modeling a pre-nucleated hole ahead of the notch.

In the context of Mg alloys, Sharma et al. [42] conducted mixed-mode (I/III) fracture experiments of extruded pure Mg, binary Mg–Zn and Mg–Al alloys with different Al content, using a special bend specimen geometry. Based on measurement of critical SZW, they reported that the fracture toughness is higher for mixed-mode than mode I loading corresponding to pure Mg and Mg–Zn alloy, whereas it decreases with increase in mode III component in the case of Mg–Al alloys.

From the above review, it is clear that a comprehensive study on the mixed-mode (especially, combined I and II modes) fracture behavior of Mg alloys has not yet been performed. As mentioned earlier, Mg alloys are proposed to be used in automotive body components like door panels [1], [2]. Such components are susceptible to complex loading conditions resulting in a biaxial/triaxial state of stress. Thus, a micro-crack that is inclined with respect to the principal stress axes, formed due to manufacturing process or during service, will be subjected to mixed-mode loading. It is possible that Jc may decrease with increase in mode II component of loading for Mg alloys in view of the contrasting trends reported for other metallic alloys [33], [34], [35], [36]. Further, the hydrostatic stress level near a notch tip, which will change with mode mixity, is expected to affect the propensity for twinning [12], [26], [27]. This, in turn, will influence the fracture mechanisms for Mg alloys [12], [25]. Hence, understanding mixed-mode fracture behavior is imperative to facilitate development of fracture-resistant Mg alloys as well as to aid in conservative fracture-based design of structural components. Thus, the following questions need to be addressed through careful experiments:

  • How does the fracture toughness change with mixed-mode (I/II) loading for Mg alloys?

  • How does the strain distribution near the notch tip and in the uncracked ligament vary with mode-mixity?

  • What is the dependence of mode-mixity on the operative fracture mechanism? Are there any transitions in this mechanism either with increase in mode II component or with crack extension?

  • What is the role of twinning on mixed-mode (I/II) fracture response? Specifically, how does the twin volume fraction and pattern of twin development near the crack tip change with mode-mixity? What is its effect on texture evolution and plastic strain distribution as well as the fracture toughness?

To clarify the above issues, mode I and mixed-mode fracture experiments are performed on rolled AZ31 Mg alloy using notched four point bend specimens. It is found that Jc reduces drastically and two transitions in the operative fracture mechanism take place with increase in the mode II component of loading. Tensile twinning is found to influence the deformation and fracture response in multiple ways: from imparting hardening to causing significant dissipation and texture evolution in the specimen.

Section snippets

Materials

The material used in this experimental study is an as-received hot rolled plate of AZ31 Mg alloy of thickness 12.7 mm. The chemical composition of the alloy is presented in Table 1. Fig. 1 depicts an SEM image of the undeformed sample, showing a large number of second-phase particles embedded in the matrix. The composition of these particles has been determined using JEOL JXA8530F Electron Probe Micro Analyzer (EPMA) and they are traced as Al-Mn, Mg17Al12 intermetallics and oxides (MgO), which

Macroscopic response

The macroscopic responses of the tested fracture specimens are presented in Fig. 5. The crack initiation stage is marked by ‘•’ symbols on all curves displayed in this figure. The load (P) versus load-line displacement (Δ) curves are shown in Fig. 5(a). From this figure, it is clear that irrespective of mode-mixity, the P - Δ curves show a bi-linear response with two distinct slopes. These pertain to the initial elastic behavior and the elastic-plastic strain hardening characteristics of the

Origin and role of twinning on mixed-mode fracture response

The optical micrographs and EBSD maps presented in Sections 3.4.2 and 3.4.3 show that twinning occurs both near the crack tip as well as at the far-edge of the specimen along the uncracked ligament for all the mixed-mode specimens. However, twin density and their orientation, especially near the crack tip, depend on the mode-mixity. A clear understanding of their origin and influence on the fracture response is crucial for rationalizing the observed transitions in fracture mechanism and

Summary and conclusions

In this work, experiments have been conducted using notched four point bend specimens to understand the mixed-mode (I/II) fracture behavior of rolled AZ31 Mg alloy. In-situ imaging combined with DIC has been employed to obtain deformation and strain maps as well as J versus load histories and the near-tip plastic mode-mixity parameter based on notch tip displacements. Detailed SEM fractography, optical metallography and EBSD analysis have been performed to gain insights on the deformation and

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.

Acknowledgment

R. Narasimhan would like to gratefully acknowledge the Science and Engineering Research Board (SERB) of Government of India for financial support under the J.C. Bose Fellowship scheme.

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