Twinning-assisted environmental cracking: A new fracture mechanism for the crash-resistant twinning-induced plasticity steels

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Scripta Materialia 67 (2012) 943–946 www.elsevier.com/locate/scriptamat

Twinning-assisted environmental cracking: A new fracture mechanism for the crash-resistant twinning-induced plasticity steels R.K. Singh Raman,a,b,⇑ Muhammed Khalissia and Shahin Khoddama a

Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Victoria 3800, Australia b Department of Chemical Engineering, Bldg 31, Monash University, Melbourne, Victoria 3800, Australia Received 17 July 2012; revised 16 August 2012; accepted 17 August 2012 Available online 23 August 2012

This article presents a novel finding of the role of twinning-induced plasticity (TWIP) in environment-assisted cracking (EAC). TWIP alloys are becoming increasingly attractive for remarkable combination of strength and ductility. EAC of a high-manganese TWIP steel was investigated in a passivating environment, to understand the combined role of the frequent localized deformation due to dynamic twinning and quick establishment of surface oxide layer. Here, we show a new mechanism where localized formation of twin bands facilitates EAC. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Environment-assisted cracking (EAC); Twinning induced plasticity (TWIP) steel; Fractography; Stress corrosion cracking (SCC)

Metals and alloys with low stacking fault energy (SFE) undergo twinning within their grains (shown in Fig. 1). Some high-manganese ferrous alloys with a low SFE (620 mJ m–2) readily undergo twinning, thus triggering deformation at very low stresses (300 MPa). This phenomenon is called twinninginduced plasticity (TWIP) [1]. TWIP steels possess an unprecedented combination of strength and ductility, and therefore offer enormous potential for applications where impact resistance is critically important, such as in automobiles, which can experience sudden impact in accidents. However, alloys with attractive mechanical properties when exposed to corrosive environment must also possess a certain degree of resistance to environment-assisted cracking (EAC), such as stress corrosion cracking (SCC), a phenomenon caused by the synergy of in-service tensile stress and corrosion. Corrosion and its mitigation costs developed economies 4% of their gross national product (GNP). EAC is believed to be responsible for 30% of this cost. On the basis of their GNPs [2], EAC accounts for an annual loss of $75b to the USA and 3b to Australia. Corrosion-resistant alloys such as stainless steels develop a protective/passive surface film when exposed

⇑ Corresponding author at: Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Victoria 3800, Australia. Tel.: +61 3 9905 3671; e-mail: [email protected]

to an oxidizing environment. Disruption of this film is essential for the initiation and propagation of stress corrosion cracks [3]. A balance between the rate of formation and disruption of the film (i.e. a synergistic effect of the chemistry of the environment and mechanical straining) is critical for the propagation of a crack. It is imperative, therefore, that any phenomenon of localized deformation, such as twinning of the underlying alloy, will crucially influence the mechanical disruption of the surface corrosion film. For example, as schematically shown in Figure 1(a and b), dissolution of slip steps (slip step formation is a form of local deformation) is the most common mechanism for SCC crack initiation and propagation of ductile metals/alloys. The slip step formation rate is a function of the straining rate, whereas the dissolution rate is governed by the aggressiveness of the environment [4–6]. Because of the favorable energetics for twinning, localized plastic deformation occurs much more readily due to twinning than due to the phenomenon of slip step formation. Therefore, it is hypothesized that the films developed over/along twins will be more likely to suffer disruption/dissolution, as schematically shown in Figure 1(c and d). The validation of this hypothesis will open an altogether novel mechanistic domain for SCC. Stress corrosion cracks propagate rapidly because, under the synergy of stress and a corrosive environment, a ductile material turns brittle. Therefore, it is valid to consider whether the inherent characteristic of

1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.08.022

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Figure 2. Stress–strain curves for TWIP steel tested at a strain rate of 4  10 7 s 1 in air and alkaline solution (pH 12.4). The plot showing a maximum strain of 0.65 is for caustic solution.

Figure 1. Mechanisms for SCC crack initiation and propagation: (a and b) the well-established slip mechanism (e.g. SCC of austenitic stainless steels); (c and d) the proposed novel TWIP mechanism.

exceptionally high localized deformation (i.e. twinning) will resist the brittle SCC crack propagation. The reported literature on SCC of TWIP steels is limited to just one publication [7]. However, that study was carried out in an entirely different environment (3.5% NaCl), and its findings were explained on the basis of the established mechanisms. The current article presents a novel SCC mechanism as a result of the synergy of the dynamic development of twins in a TWIP alloy under tensile loading and the formation of a robust passive film in the highly oxidizing environment of a caustic solution. The investigated TWIP steel (with a major alloying composition (wt.%) of Mn: 26.34, C: 0.018C, Al: 4.84Al, Si: 3.56) develops extensive twin bands during cold deformation. Transgranular or intergranular cracking are the only known fractographic features of SCC of ferrous alloys. By the following description, we identify the unique mechanism of stress corrosion cracking, i.e. crack propagation along twin bands, leading to a featureless fracture surface that operates exclusively for TWIP steels. For evaluation of SCC, the steel was subjected to slow strain rate (SSR) tests [8], by straining at a very slow rate (4  10 7 s 1) in both air (“inert”) and the highly oxidizing environment of a caustic solution (pH 12.4). Stress–strain plots in the two environments were similar until a strain of 0.65 was attained (Fig. 2), when the steel tested in the alkaline solution suddenly failed; the steel tested in air attained a strain of 0.9 (Fig. 2). The steel tested in air had little environmental influence, as confirmed by the presence of typical ductile dimples over of the fracture surface (as seen over the majority of the area shown in Fig. 3a). The cracks propagating along twin bands produced the featureless bands

that were present in the midst of ductile dimples (Fig. 3a). However, the twin bands were frequently observed in the gauge length of the tested specimens (Fig. 3b). The steel develops a robust passive film in the caustic solution, and as a result there was little difference in the mechanical properties in the two environments until the strain of 0.65 was achieved (Fig. 2). However, as will be evidenced from electron backscattered diffusion (EBSD) results extensive twinning occurred, in reaching this strain. When twinning attained the rate required for the sustained recurrence of the cycle of disruption in passive film, dissolution and repassivation at the crack tip (i.e. the dissolution–repassivation mechanism for SCC [9]), the crack advanced rapidly, leading to fracture. This explanation is duly supported by: (i) the evidence of passivation of the steel in caustic solution (which was established through electrochemical testing and surface microscopy) and (ii) the fractographic feature evidencing the role of twin bands in predominant environment-assisted crack propagation in the caustic solution. At higher magnifications, the fracture surface of the TWIP steel tested in the alkaline solution (pH 12.4) possessed different fractographic features than the air-tested steel. The featureless appearance (similar to that in Fig. 3a) that occupies a considerable fraction of the fracture surface is attributed to the crack propagation as a result of the repetition of localized deformation due to twinning (which caused disruption of the passive film) and rapid localized dissolution along the twin band. The phenomenon of corrosion-film disruption and dissolution along twin bands is duly evidenced by the features of extensive generation of twin bands and cracked corrosion films in the specimen gauge length (Fig. 3(c)–(e)). However, when environment-assisted crack propagation reduced the cross-section of the specimen to a critical area, the overload took over and the specimen failed mechanically, which was evidenced by the ductile dimples over the rest of the fracture surface of the specimen tested in the caustic solution. EBSD results of the bulk of the undeformed and deformed steel (Fig. 4) provide evidence that the extensive twinning seen at the surface (shown in Fig. 3(c)–(e)) are

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Figure 3. SEM fractographs of TWIP steel. (a and b) tested in air: though the common feature was ductile dimples, there are (a) featureless areas in the midst of the ductile dimples on the fracture surface and (b) in the gauge length there is evidence of the development of twin bands as the common feature. (c–e) Tested in NaOH: the gauge length evidences the development of twin bands, the cracking of corrosion films and the propagation of stress corrosion cracks along the twin bands.

Figure 4. EBSD results: (a) KPQ map of the undeformed TWIP steel, (b) IPF map of the undeformed steel, (c) the misorientation profile across the twin (b), (d) KPQ map of the deformed steel (SSR tested), (e) IPF map of the deformed steel and (f) the misorientation profile along the line in (e). The red lines/patches in (a) and (d) represent the twin boundaries.

actually also prevalent in the bulk of TWIP steel during the SSR testing. The Kikuchi pattern quality (KPQ) was excellent and the indexing rate was high (98%) for the undeformed steel (Fig. 4a), whereas the indexing of the Kikuchi patterns got difficult in the case of the deformed TWIP steel due to the formation of narrower twins and twin boundaries, as well as highly misoriented sub-boundaries

(Fig. 4d). The inverse pole figure (IPF) maps (Fig. 4) of the undeformed and deformed TWIP steel (deformed during SSR test) provided a quantitative comparison of the misorientation profiles of twins. For the undeformed steel (Fig. 4(a)), the misorientaion is measured across an annealing twin within a single grain. The width of this twin is about 10 lm, as shown in Figure 4(b). For the strained steel (Fig. 4(c)), the widths of the mechanical

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twins are much narrower (0.5–2 lm), as shown in Figure 4(d). The widespread development of mechanical twins during deformation in SSR testing (as evidenced in Fig. 4) suggests that the cracks that preferentially and extensively develop along the twins at the gauge surface of SSR specimens (Fig. 3(c)–(e)) will easily find a twinassisted path in the bulk, resulting in areas with a featureless appearance over the fracture surface (similar to the one seen in Fig. 3(a)). This crack propagation mechanism and the featureless appearance, which are both unique for SCC of TWIP steel, are attributed to the disruption of the robust passive film due essentially to the localized deformation/twinning of the TWIP steel substrate. The above description may suggest that, as a result of the profound role of twinning in facilitating stress corrosion crack propagation, a TWIP steel may quickly be destroyed when encountering conditions conducive for SCC. However, a comparison of stress–strain plots unambiguously shows that TWIP steel is considerably more resistant to SCC than common mild steel. The inferior SCC resistance of mild steel is consistent with the much greater strain-hardenability of the alloy, which is known to play a predominant role in the crack tip embrittlement and crack propagation. Nevertheless, the much superior resistance of TWIP steel to SCC can be confidently attributed to the mechanism based on the predominant role of the toughness of a material in its resistance to an advancing SCC crack. It is suggested, therefore, that, even though the synergistic action of twinning and passivating in a caustic environment may facilitate the advancement of a crack, the zone ahead of the crack tip will deform (as a result of the inherent ability of TWIP steel to deform instead of suffering decohesion), making it more difficult for the stress corrosion crack to propagate. However, in environments where the steel cannot passivate, the twins will simply suffer dissolution, as any metal does at the location of deformation. The inability of TWIP steels to passivate in such environment has also been reported elsewhere recently [10]. As a result of the ease and the much greater intensity of twinning, as well as the continuum of the dissolving twinning bands, the steel can be destroyed very quickly. Indeed, when exposed to more

corrosive solutions, the TWIP steel showed such dissolution and considerable loss in mechanical properties as a result of twin band dissolution. To summarize, TWIP steel undergoes stress corrosion cracking in the passivating environment of a caustic solution, following a novel mechanism of cracking of the surface corrosion film along the twin bands that develop due to extensive occurrence of localized deformation (i.e. twinning) during mechanical loading. However, the steel is still considerably resistant to SCC as a result of its inherent toughness, which resists SCC crack propagation. In more aggressive environments, where the TWIP steel fails to passivate, it may suffer remarkable loss in mechanical property as a result of extensive dissolution along the continuum of the localized deformation of twin bands. M.K. acknowledges the support of a grant from the Iraqi Government to pursue a PhD at Monash University. The authors acknowledge the assistance of the Monash Electron Microscopy Centre (MCEM) for the fractography and EBSD results reported in this article. [1] T. Schroder, Cooking Steel for the Cars of Tomorrow. Max Planck Research, 2004. [2] GNP 1999, Atlas Method, , 2000. [3] S.C. Tjong, Mater. Corros./Werkst. Korros. 37 (1986) 444. [4] H.L. Craig, Stress Corrosion Cracking of Metals – A State of the Art, ASTM International, West Conshohocken, PA, 1972. [5] M.O. Speidel, Metall. Mater. Trans. A 12 (1981) 779. [6] T.V. Vinoy, H. Shaikh, H.S. Khatak, N. Sivaibharasi, J.B. Gnanamoorthy, J. Nucl. Mater. 238 (1996) 278. [7] M. Khalissi, R.K. Singh Raman, S. Khoddam, in: Proceedings of the 11th International Conference on the Mechanical Behavior of Materials (ICM11), Lake Como, 2011. [8] R.K. Singh Raman, Metall. Mater. Trans. A 36A (2005) 1817–1819. [9] R.K. Singh Raman, Mater. Sci. Eng. A 441 (2006) 342. [10] L.S. Roncery, S. Weber, W. Theisen, Metall. Mater. Trans. A 41A (2010) 2471.