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Scripta Materialia 67 (2012) 943–946

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

R.K. Singh Raman,a,b,⇑ Muhammed Khalissia and Shahin Khoddama

aDepartment of Mechanical and Aerospace Engineering, Monash University, Melbourne, Victoria 3800, AustraliabDepartment of Chemical Engineering, Bldg 31, Monash University, Melbourne, Victoria 3800, Australia

Received 17 July 2012; revised 16 August 2012; accepted 17 August 2012Available 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-manganeseTWIP steel was investigated in a passivating environment, to understand the combined role of the frequent localized deformationdue to dynamic twinning and quick establishment of surface oxide layer. Here, we show a new mechanism where localized formationof twin bands facilitates EAC.� 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 (shownin Fig. 1). Some high-manganese ferrous alloys with alow SFE (620 mJ m–2) readily undergo twinning,thus triggering deformation at very low stresses(�300 MPa). This phenomenon is called twinning-induced plasticity (TWIP) [1]. TWIP steels possess anunprecedented combination of strength and ductility,and therefore offer enormous potential for applicationswhere impact resistance is critically important, such asin automobiles, which can experience sudden impact inaccidents. However, alloys with attractive mechanicalproperties when exposed to corrosive environment mustalso possess a certain degree of resistance to environ-ment-assisted cracking (EAC), such as stress corrosioncracking (SCC), a phenomenon caused by the synergyof in-service tensile stress and corrosion. Corrosionand its mitigation costs developed economies �4% oftheir gross national product (GNP). EAC is believedto be responsible for �30% of this cost. On the basisof their GNPs [2], EAC accounts for an annual loss of�$75b to the USA and �3b to Australia.

Corrosion-resistant alloys such as stainless steelsdevelop a protective/passive surface film when exposed

1359-6462/$ - see front matter � 2012 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2012.08.022

⇑Corresponding author at: Department of Mechanical and AerospaceEngineering, Monash University, Melbourne, Victoria 3800,Australia. Tel.: +61 3 9905 3671; e-mail: raman.singh@monash.edu

to an oxidizing environment. Disruption of this film isessential for the initiation and propagation of stress cor-rosion cracks [3]. A balance between the rate of forma-tion and disruption of the film (i.e. a synergistic effect ofthe chemistry of the environment and mechanical strain-ing) is critical for the propagation of a crack. It is imper-ative, therefore, that any phenomenon of localizeddeformation, such as twinning of the underlying alloy,will crucially influence the mechanical disruption ofthe surface corrosion film. For example, as schemati-cally shown in Figure 1(a and b), dissolution of slip steps(slip step formation is a form of local deformation) is themost common mechanism for SCC crack initiation andpropagation of ductile metals/alloys. The slip step for-mation rate is a function of the straining rate, whereasthe dissolution rate is governed by the aggressivenessof the environment [4–6].

Because of the favorable energetics for twinning,localized plastic deformation occurs much more readilydue to twinning than due to the phenomenon of slip stepformation. Therefore, it is hypothesized that the filmsdeveloped over/along twins will be more likely to sufferdisruption/dissolution, as schematically shown in Fig-ure 1(c and d). The validation of this hypothesis willopen 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 toconsider whether the inherent characteristic of

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

Figure 2. Stress–strain curves for TWIP steel tested at a strain rate of4 � 10�7 s�1 in air and alkaline solution (pH 12.4). The plot showing amaximum strain of 0.65 is for caustic solution.

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exceptionally high localized deformation (i.e. twinning)will resist the brittle SCC crack propagation. The re-ported literature on SCC of TWIP steels is limited tojust one publication [7]. However, that study was carriedout in an entirely different environment (3.5% NaCl),and its findings were explained on the basis of the estab-lished mechanisms.

The current article presents a novel SCC mechanismas a result of the synergy of the dynamic developmentof twins in a TWIP alloy under tensile loading and theformation of a robust passive film in the highly oxidizingenvironment of a caustic solution.

The investigated TWIP steel (with a major alloyingcomposition (wt.%) of Mn: 26.34, C: 0.018C, Al:4.84Al, Si: 3.56) develops extensive twin bands duringcold deformation.

Transgranular or intergranular cracking are the onlyknown fractographic features of SCC of ferrous alloys.By the following description, we identify the uniquemechanism of stress corrosion cracking, i.e. crack prop-agation along twin bands, leading to a featureless frac-ture surface that operates exclusively for TWIP steels.

For evaluation of SCC, the steel was subjected toslow strain rate (SSR) tests [8], by straining at a veryslow rate (4 � 10�7 s�1) in both air (“inert”) and thehighly oxidizing environment of a caustic solution (pH12.4). Stress–strain plots in the two environments weresimilar until a strain of �0.65 was attained (Fig. 2),when the steel tested in the alkaline solution suddenlyfailed; the steel tested in air attained a strain of 0.9(Fig. 2). The steel tested in air had little environmentalinfluence, as confirmed by the presence of typical ductiledimples over of the fracture surface (as seen over themajority of the area shown in Fig. 3a). The cracks prop-agating along twin bands produced the featureless bands

that were present in the midst of ductile dimples(Fig. 3a). However, the twin bands were frequently ob-served in the gauge length of the tested specimens(Fig. 3b).

The steel develops a robust passive film in the causticsolution, and as a result there was little difference in themechanical properties in the two environments until thestrain of 0.65 was achieved (Fig. 2). However, as will beevidenced from electron backscattered diffusion (EBSD)results extensive twinning occurred, in reaching thisstrain. When twinning attained the rate required forthe sustained recurrence of the cycle of disruption inpassive film, dissolution and repassivation at the cracktip (i.e. the dissolution–repassivation mechanism forSCC [9]), the crack advanced rapidly, leading to frac-ture. This explanation is duly supported by: (i) the evi-dence of passivation of the steel in caustic solution(which was established through electrochemical testingand surface microscopy) and (ii) the fractographic fea-ture evidencing the role of twin bands in predominantenvironment-assisted crack propagation in the causticsolution. At higher magnifications, the fracture surfaceof the TWIP steel tested in the alkaline solution (pH12.4) possessed different fractographic features thanthe air-tested steel. The featureless appearance (similarto that in Fig. 3a) that occupies a considerable fractionof the fracture surface is attributed to the crackpropagation as a result of the repetition of localizeddeformation due to twinning (which caused disruptionof the passive film) and rapid localized dissolution alongthe twin band. The phenomenon of corrosion-film dis-ruption and dissolution along twin bands is duly evi-denced by the features of extensive generation of twinbands and cracked corrosion films in the specimen gaugelength (Fig. 3(c)–(e)). However, when environment-as-sisted crack propagation reduced the cross-section ofthe specimen to a critical area, the overload took overand the specimen failed mechanically, which was evi-denced by the ductile dimples over the rest of the frac-ture surface of the specimen tested in the causticsolution.

EBSD results of the bulk of the undeformed and de-formed steel (Fig. 4) provide evidence that the extensivetwinning seen at the surface (shown in Fig. 3(c)–(e)) are

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 inthe 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 commonfeature. (c–e) Tested in NaOH: the gauge length evidences the development of twin bands, the cracking of corrosion films and the propagation ofstress 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 thetwin (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.

R. K. Singh Raman et al. / Scripta Materialia 67 (2012) 943–946 945

actually also prevalent in the bulk of TWIP steel duringthe SSR testing.

The Kikuchi pattern quality (KPQ) was excellent andthe indexing rate was high (98%) for the undeformedsteel (Fig. 4a), whereas the indexing of the Kikuchi pat-terns got difficult in the case of the deformed TWIP steeldue to the formation of narrower twins and twin bound-aries, as well as highly misoriented sub-boundaries

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

946 R. K. Singh Raman et al. / Scripta Materialia 67 (2012) 943–946

twins are much narrower (0.5–2 lm), as shown inFigure 4(d).

The widespread development of mechanical twinsduring deformation in SSR testing (as evidenced inFig. 4) suggests that the cracks that preferentially andextensively develop along the twins at the gauge surfaceof SSR specimens (Fig. 3(c)–(e)) will easily find a twin-assisted path in the bulk, resulting in areas with a fea-tureless appearance over the fracture surface (similarto the one seen in Fig. 3(a)). This crack propagationmechanism and the featureless appearance, which areboth unique for SCC of TWIP steel, are attributed tothe disruption of the robust passive film due essentiallyto the localized deformation/twinning of the TWIP steelsubstrate.

The above description may suggest that, as a result ofthe profound role of twinning in facilitating stress corro-sion crack propagation, a TWIP steel may quickly bedestroyed when encountering conditions conducive forSCC. However, a comparison of stress–strain plotsunambiguously shows that TWIP steel is considerablymore resistant to SCC than common mild steel. Theinferior SCC resistance of mild steel is consistent withthe much greater strain-hardenability of the alloy, whichis known to play a predominant role in the crack tipembrittlement and crack propagation. Nevertheless,the much superior resistance of TWIP steel to SCCcan be confidently attributed to the mechanism basedon the predominant role of the toughness of a materialin its resistance to an advancing SCC crack. It is sug-gested, therefore, that, even though the synergistic ac-tion of twinning and passivating in a causticenvironment may facilitate the advancement of a crack,the zone ahead of the crack tip will deform (as a result ofthe inherent ability of TWIP steel to deform instead ofsuffering decohesion), making it more difficult for thestress corrosion crack to propagate. However, in envi-ronments where the steel cannot passivate, the twins willsimply suffer dissolution, as any metal does at the loca-tion of deformation. The inability of TWIP steels to pas-sivate in such environment has also been reportedelsewhere recently [10]. As a result of the ease and themuch greater intensity of twinning, as well as the contin-uum of the dissolving twinning bands, the steel can bedestroyed very quickly. Indeed, when exposed to more

corrosive solutions, the TWIP steel showed such dissolu-tion and considerable loss in mechanical properties as aresult of twin band dissolution.

To summarize, TWIP steel undergoes stress corrosioncracking in the passivating environment of a causticsolution, following a novel mechanism of cracking ofthe surface corrosion film along the twin bands that de-velop due to extensive occurrence of localized deforma-tion (i.e. twinning) during mechanical loading.However, the steel is still considerably resistant toSCC as a result of its inherent toughness, which resistsSCC crack propagation. In more aggressive environ-ments, where the TWIP steel fails to passivate, it maysuffer remarkable loss in mechanical property as a resultof extensive dissolution along the continuum of thelocalized deformation of twin bands.

M.K. acknowledges the support of a grantfrom the Iraqi Government to pursue a PhD at MonashUniversity. The authors acknowledge the assistance ofthe Monash Electron Microscopy Centre (MCEM) forthe fractography and EBSD results reported in thisarticle.

[1] T. Schroder, Cooking Steel for the Cars of Tomorrow.Max Planck Research, 2004.

[2] GNP 1999, Atlas Method, <http://www.Worldbank.org/data/databytopic/GNP.pdf>, 2000.

[3] S.C. Tjong, Mater. Corros./Werkst. Korros. 37 (1986)444.

[4] H.L. Craig, Stress Corrosion Cracking of Metals – A Stateof 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 theMechanical 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.