On the competition between the stress-induced formation of martensite and dislocation plasticity during crack propagatio

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an Frenzel and Susanne Gollerthan Institut für Werkstoffe, Ruhr-Universität Bochum, 44801 Bochum, Germany

Gábor Ribárik Department of Materials Physics, Eötvös University Budapest, Budapest H-1518, Hungary

Levente Balogh Department of Materials Physics, Eötvös University Budapest, Budapest H-1518, Hungary; and Department of Mechanical and Materials Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada

Gunther Eggeler Institut für Werkstoffe, Ruhr-Universität Bochum, 44801 Bochum, Germany (Received 16 May 2017; accepted 13 June 2017)

The present work addresses the competition between dislocation plasticity and stress-induced martensitic transformations in crack affected regions of a pseudoelastic NiTi miniature compact tension specimen. For this purpose X-ray line proﬁle analysis was performed after fracture to identify dislocation densities and remnant martensite volume fractions in regions along the crack path. Special emphasis was placed on characterizing sub fracture surface zones to obtain depth proﬁles. The stress affected zone in front of the crack-tip is interpreted in terms of a true plastic zone associated with dislocation plasticity and a pseudoelastic zone where stress-induced martensite can form. On unloading, most of the stress-induced martensite transforms back to austenite but a fraction of it is stabilized by dislocations in both, the irreversible martensite and the surrounding austenite phase. The largest volume fraction of the irreversible or remnant martensite along with the highest density of dislocations in this phase was found close to the primary crack-tip. With increasing distance from the primary crack-tip both, the dislocation density and the volume fraction of irreversible martensite decrease to lower values.

I. INTRODUCTION

NiTi shape memory alloys (SMAs) are functional materials which undergo martensitic transformations.1,2 Depending on the Ni-content, they can show the one way effect (1WE) and pseudo-elasticity (PE).1–4 SMAs are used in engineering and medical technology,5,6 with good static7–9 and cyclic toughness.10–13 Fracture mechanisms in PE SMAs are of recent interest.14–24 Gollerthan et al.15–17 used compact tension (CT) specimens to study the behavior of cracks in NiTi SMAs. Their experiments focused on the formation of the pseudoelastic zone around the crack-tip and showed that cracks always propagate into microstructures consisting of favorably

Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: [email protected] Dedicated to the 80th Birthday of Professor Haël Mughrabi. DOI: 10.1557/jmr.2017.267

oriented martensite variants. Synchrotron experiments revealed that stress-induced martensite forms in a region extending almost 2 mm at the front of the crack tip.16 Thermography experiments showed the heat effects associated with the forward and reverse martensitic transformations.16,17 The synchrotron experiments revealed the butterﬂy shaped elastic strain distribution around the crack-tip.19 Finite element calculations of

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