Regular Article Nucleation driving force for ω-assisted formation of α and associated ω morphology in β-Ti alloys Tong Li a,b, ⁎, Damon Kent c,d,e , Gang Sha f , Hongwei Liu b , Suzana G. Fries g , Anna V. Ceguerra b , Matthew S. Dargusch d,e , Julie M. Cairney b,e a Institute for Materials & ZGH, Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany b Australian Centre for Microscopy and Microanalysis and School of Aerospace, Mechanical & Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia c School of Science and Engineering, University of the Sunshine Coast, Maroochydore DC, QLD 4558, Australia d Queensland Centre for Advanced Materials Processing and Manufacturing, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia e ARC Research Hub for Advanced Manufacturing of Medical Devices, The University of Queensland, Brisbane, QLD 4072, Australia f School of Materials Science and Engineering, Nanjing University of Science and Technology, Jiangsu, 210094, China g Interdisciplinary Centre for Advanced Materials Simulation (ICAMS), Ruhr-Universität Bochum, 44801 Bochum, Germany abstract article info Article history: Received 15 March 2018 Received in revised form 15 June 2018 Accepted 20 June 2018 Available online xxxx The structural and chemical changes at ω/β interfaces and the evolution of the morphology of ω in a near-β alloy during isothermal ageing at 573 K were investigated by atom probe tomography and aberration-corrected high- resolution transmission electron microscopy. Ledges and local O enrichment at semi-coherent isothermal ω in- terfaces are proposed to provide the key driving force for nucleation of ω-assisted α. Following nucleation of α, the morphology of ω evolves from ellipsoidal to rod-like, induced by rapid consumption of ω by α. © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Word Titanium alloy Phase transformations Atom probe tomography HREM β titanium alloys have attracted considerable attention in aerospace and biomedical applications due to their superior mechanical properties and biocompatibility [1]. The mechanical properties can be improved by precipitation of uniform distributions of fine α phase, nucleated from the metastable ω phase [2]. The ω-assisted formation of α has been widely adopted to optimise mechanical properties via appropriately de- signed heat treatments [2]. Recently, athermal ω [3, 4], i.e. incommen- surate (embryonic) ω [5], was demonstrated to not directly assist α formation; while isothermal ω, i.e. commensurate ω, actively assists α formation [6]. However, the key driving force for nucleation in the ω- to-α phase transformation remains elusive. For large ω/β misfit systems in which cuboidal-shaped ω precipi- tates form (e.g. Ti-V [2, 4]), defects such as ledges at the ω/β interface provide nucleation sites for the α phase [4, 7]. In low misfit systems, such as Ti-Mo [2, 8], ellipsoidal ω precipitates form within β due to elas- tic strain energy considerations. The precise role of coherent ω/β inter- faces in formation of the α phase remains unclear. A recent study asserted that elastic stresses associated with coherent ω/β interfaces, along with compositional variation, potentially trigger formation of the α phase [3]. In previous work [9], we observed ω phase surrounded by O-enriched regions (within α), which may assist nucleation of α. However, it is unclear if either compositional variation or elastic stress, or both are the dominant driving force for nucleation of α. Furthermore, ω can co-exist with α upon isothermal ageing [10, 11]. Changes to the morphology of the ω phase during co-growth with α have not been re- ported. Understanding the morphology evolution of the ω phase, and associated structural and compositional changes at or around the ω/β interface, may clarify the stage at which ω assists α formation. There- fore, further experimental evidence is required to determine the domi- nant nucleation driving force and preferred nucleation sites for ω- assisted α formation. Atom probe tomography (APT) is a powerful tech- nique that provides quantitative three-dimensional information on ele- mental distributions at the atomic scale [12]. In this study, we combine APT with aberration-corrected TEM to investigate structure and chemis- try at the ω/β interface, and the morphology and chemical evolution of the ω phase in an isothermally aged near-β Ti alloy, with the aim to i) ascertain the key nucleation driving force for ω-assisted α, and ii) understand growth and dissolution of ω during the ω-to-α transformation. Scripta Materialia 155 (2018) 149–154 ⁎ Corresponding author at: Universitätsstr.150, Bochum 44801, Germany. E-mail address: tong.li@rub.de (T. Li). https://doi.org/10.1016/j.scriptamat.2018.06.039 1359-6462/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat