Full length article High-resolution electron microscopy characterization of modulated structure in high Nb-containing lamellar g-TiAl alloy Guo-dong Ren, Jian Sun * Shanghai Key Laboratoryof Advanced High-Temperature Materials and Precision Forming, School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, People's Republic of China article info Article history: Received 18 August 2017 Received in revised form 3 November 2017 Accepted 5 November 2017 Available online 8 November 2017 Keywords: Titanium aluminides Modulated structure Orthorhombic phase Phase transformation HRTEM abstract The microstructure in a lamellar g-based Ti-45Al-8.5Nb alloy has been investigated by transmission electron microscopy (TEM). The results show that there exists a nano-scale modulated structure with tweed contrast within a 2 laths and the orthorhombic phase as a constituent of the modulated structure has the O1 structure with a random occupancy of Ti and Nb atoms on the Wyckoff sites 8 g and 4c2. The orthorhombic phase has a thin plate-like morphology with orientation relationships of {001} O //{0001}a 2 and <110> O //<11 20>a 2 . The habit plane for orthorhombic phase variants lies at {350} O //{13 40}a 2 , and the O1/a 2 interface is not atomic flat. The results further exhibit an enrichment of Nb in the orthorhombic phase in comparison with the a 2 parent phase. Based on the TEM evidences, the formation of the orthorhombic phase arises from a phase separation reaction in the a 2 parent phase, which is dominated by a diffusion mechanism. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction A ternary intermetallic alloy based on the stoichiometry Ti 2 AlNb was first reported by Banerjee et al. in the ternary Ti-Al-Nb system [1], which is designated as the O phase on the basis of its ortho- rhombic structure with the Cmcm symmetry. It has been shown that Nb-rich a 2 -based Ti 3 Al alloys containing primarily the O phase have relatively higher combinations of strength, toughness and creep resistance than Nb-lean Ti 3 Al alloys [2e4]. Studies of the crystallography, microstructure and formation mechanism of the O phase have been widely reported in a 2 -based Ti 3 Al-Nb alloys since 1990's. Muraleedharan et al. investigated morphologies and spatial arrangements of the O phase in a a 2 - based Ti-28.5Al-13Nb alloy (all compositions are given in at.%) [5e7]. The results showed that the O phase forms with plate-like morphologies or mosaic patterns intervened with small amount of the retained a 2 , which is significantly dependent on the volume fraction of the O phase generating from different annealing tem- peratures [5]. Moreover, they identified the O phase existing in two ordered forms, i.e. the O1 structure with a random occupation of Ti and Nb atoms on the Wyckoff sites 8 g and 4c2, and the O2 structure where Ti atoms mainly occupy the Wyckoff sites 8 g and Nb atoms preferentially the 4c2 sites, respectively [6,7]. Based on experimental results of orientation relationships, habit plane and the same site occupancy for Ti, Al and Nb atoms in the O1 structure as in the parent a 2 phase, Muraleedharan et al. suggested that the transformation from a 2 to O1 can be accompanied purely by a mechanical displacement of atoms described by a simple shear and an additional small shuffle [6,7]. According to them, the trans- formation from the O1 to O2 structure can be accompanied by a replacive ordering wave in the alloys [7]. The orthorhombic phase as a constituent of the modulated structure at nano-scale in lamellar microstructure in a g-based Ti- (40e44)Al-8.5Nb alloy was first reported by Appel et al. [8e10]. This alloy with the modulated structure exhibits a high yield stress in combination with improved ductility at room temperature compared with the conventional lamellar g-based TiAl alloys. The orthorhombic phase was identified as the B19 structure by Appel et al. using high-resolution TEM, which forms by decomposition of the B2 phase according to a transformation mechanism suggested by Nguyen-Manh et al. [11]. Meanwhile, Schmoelzer et al. found an orthorhombic phase as a transitional phase occurring in the su- persaturated a 2 phase by in-situ synchrotron radiation during reheating process in a quenched Tie45Ale3Moe0.1B alloy [12]. The orthorhombic phase was thought to have the B19 structure in their studies. In a lamellar g-based Ti-45Al-8.5Nb-0.2W-0.2B-0.2Y alloy, * Corresponding author. E-mail address: jsun@sjtu.edu.cn (J. Sun). Contents lists available at ScienceDirect Acta Materialia journal homepage: www.elsevier.com/locate/actamat https://doi.org/10.1016/j.actamat.2017.11.016 1359-6454/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Acta Materialia 144 (2018) 516e523