Full length article Martensitic twin boundary migration as a source of irreversible slip in shape memory alloys Ahmed Sameer Khan Mohammed, Huseyin Sehitoglu* Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206W Green St., Urbana, IL 61801, United States ARTICLE INFO Article History: Received 31 May 2019 Revised 15 November 2019 Accepted 21 December 2019 Available online 31 December 2019 ABSTRACT The mechanistic origin of fatigue in Shape Memory Alloys (SMAs) is addressed using atomistic simulations. A causal explanation is proposed for the known agreement between the fatigue-activated slip system and the martensitic twinning system. As a model system, the Type II twin boundary (TB) in NiTi B19 0 martensite phase is analyzed. TEM-based models have established the presence of disconnections on the TB. Topological models establish the TB migration to depend on the motion of twinning partials on these disconnections. A disconnection is setup within a Molecular Statics (MS) framework. A twinning partial is positioned on it by enforcing continuum displacement fields external to a prescribed core of atoms which is subsequently relaxed under governance of the interatomic potential. The displacement fields are calculated from the aniso- tropic Eshelby-Stroh formalism and enforced in a non-Cauchy-Born adherent manner to obtain the right core structulre. TB migration is simulated as a motion of this disconnection under applied load. In the presence of a barrier to this motion, a dislocation reaction occurs where a stacking fault emits at the TB while returning a residual negated partial. The emissary fault partial is proposed as a precursor to the resulting slip observed in reverse-transformed austenite. © 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Interface structure NiTi Shape Memory Alloys Slip emission Twin boundary migration 1. Introduction SMAs form a special class of materials that can handle large strains (several percent) and exhibit exceptional strain-recoverability owing to a diffusionless martensitic transformation [1]. When their microstructures are engineered to have load orientations favoring transformation over plasticity [2], they offer the exciting solution of handling large reversible strains under fatigue loading, possibly evad- ing plasticity-induced fatigue damage dominant in conventional metallic materials [3,4]. Such reversibility is enhanced when the criti- cal slip stresses of the individual phases (austenite and martensite) are higher than the critical stress for transformation. Considerable research in the field of SMAs is directed at further separating the two stress levels [5,6]. Nevertheless, these materials are not immune to fatigue damage mechanisms, and are subject to irreversible plastic slip activity, typically observed to accumulate in the austenite phase [715]. The manifestations of such mechanisms have two fronts, one is structural fatigue damage as observed in conventional metallic materials while the other is functional fatigue where shape memory performance characteristics such as recoverable strain, stress hyster- esis etc. are significantly diminished [14,1621]. In turn, the preva- lence of such damage mechanisms negatively impacts applications of SMAs in all domains, spanning biomedical (stents, orthodontics), automotive (valves) and aerospace fields [22]. One of the most puzzling aspects of SMA fatigue is the occurrence of plastic slip accumulation at stress levels far lower than the plastic flow stresses of the individual phases [5]. In fact, even stress-free ther- mal cycling of SMAs exhibit increasing dislocation density [13,23]. A continuum micromechanical approach to the problem may state that the higher local stress state at the interface, necessary to accommodate lattice and constitutive mismatch, can drive plastic slip. However, without a dislocation source, the stress levels must approach those of the ideal shear slip strength of the phases and this is unlikely. This is supported by the high unstable stacking fault energy barrier in the Generalized Stacking Fault Energy (GSFE) curves of the austenitic phase, particularly in those slip systems which have been shown to prevail under fatigue loading [24]. Further, several studies over the past few decades have consistently shown that the fatigue-activated slip system emanates from the austenite-martensite interface. Also, the slip system was found to align with the internal twinning system of martensite. In other words, the direction of Burgers vector of the emitted slip dislocation and the corresponding slip plane match the direction of twinning shear and twinning plane within the martensite phase, respectively (Fig. 1)[12,13,23]. These arguments lead us to believe that there is indeed a dislocation mechanism that is active at nominal stress levels (at the transformation stress) and is closely tied to the internal twinning within martensite. * Corresponding author. E-mail address: huseyin@illinois.edu (H. Sehitoglu). https://doi.org/10.1016/j.actamat.2019.12.043 1359-6454/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Acta Materialia 186 (2020) 5067 Contents lists available at ScienceDirect Acta Materialia journal homepage: www.elsevier.com/locate/actamat