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I.

INTRODUCTION

TWINNING within the grains of metals and alloys (as shown in Figures 1 and 2) is attributed to their low stacking fault energy (SFE). For example, some highmanganese steels (e.g., Fe-25Mn-3Si-3Al alloy) having very low SFE (£20 mJ m 2) readily undergo twinning. As a result, they deform at very low stresses (~300 MPa), and the phenomenon is called twinning induced plasticity (TWIP).[1] With their exceptional combination of tensile strength and ductility, TWIP steels are highly attractive for applications where impact resistance is a critical criterion, such as those components of automotives that experience sudden impact in accidents. Because of manganese as the major alloying content for toughness purposes, the TWIP steels are also a less expensive alternative to other alloys with high toughness (such as the common Cr-Ni austenitic stainless steels that contain considerable amounts of the expensive metal, Ni, for the purpose of toughness[2]). However, even alloys with such attractive mechanical properties when exposed to corrosive environment can suffer premature, sudden, and catastrophic fracture as a result of the synergistic action of tensile stress and corrosion, e.g., stress corrosion cracking (SCC), a phenomenon that is profoundly influenced by the nature of the corrosive environment. There are extremely limited studies on environment-assisted cracking (EAC) of TWIP steels.[3,4]

R.K. SINGH RAMAN, Professor, is with the Department of Mechanical & Aerospace Engineering, Monash University, Melbourne, VIC 3800, Australia, and also with the Department of Chemical Engineering, Monash University. Contact e-mail: raman.singh@monash. edu MUHAMMED KHALISSI, formerly Ph.D. Student, and SHAHIN KHODDAM, formerly Senior Lecturer, with the Department of Mechanical & Aerospace Engineering, Monash University are now Independent Researchers. Manuscript submitted April 3, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A

A. SCC of TWIP Steel in Passivating Environment Metals and alloys can develop a protective/passive surface film when exposed to an oxidizing environment. This film suffers localized disruption under mechanical stress which is a critical process for the initiation and propagation of stress corrosion cracks.[2,5] However, sustenance of the crack propagation requires a delicate balance between the rate of formation and disruption of the film. This balance is a result of the synergistic effect of the chemistry/electrochemistry due to the environment and the characteristics of mechanical straining.[6,7] Therefore, any localized deformation of the underlying alloy will influence the mechanical disruption of the corrosion film. In fact, the profound role of such localized disruption of corrosion film is well-established in the literature. The dissolution of slip steps, as schematically shown in Figures 1(a) and (b) is one of the most common mechanisms for SCC crack initiation and propagation in ductile metals/alloys such as austenitic stainless steels.[3] The slip step formation is a localized deformation. Slip