Citation: Babaei-Dehkordi, A.; Soltanieh, M.; Mirjalili, M.; Asherloo, M.; Mostafaei, A. Understanding Interfacial Reactions in Ti–Ni Diffusion Couple. Materials 2023, 16, 2267. https://doi.org/10.3390/ ma16062267 Academic Editors: Renhai Shi, Lijun Zhang and Ying Tang Received: 12 February 2023 Revised: 5 March 2023 Accepted: 7 March 2023 Published: 11 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Understanding Interfacial Reactions in Ti–Ni Diffusion Couple Amin Babaei-Dehkordi 1 , Mansour Soltanieh 1, *, Mostafa Mirjalili 2 , Mohammadreza Asherloo 3 and Amir Mostafaei 3, * 1 School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Tehran 16846-13114, Iran; a.babaeidehkordi@gmail.com 2 Department of Materials and Metallurgical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad 91775-1111, Iran 3 Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, 10 W 32nd Street, Chicago, IL 60616, USA * Correspondence: mansour_soltanieh@iust.ac.ir (M.S.); mostafaei@iit.edu (A.M.) Abstract: The diffusion phenomenon in the Ti–Ni binary system was investigated at a temperature of 1173 K. Microstructure and texture analysis revealed the formation of three stable intermetallic compounds, namely Ti 2 Ni, TiNi, and TiNi 3 , as well as two metastable intermetallic compounds, including Ti 3 Ni 4 and Ti 2 Ni 3 , at the interfacial diffusion zone. The nucleation surface energy increase was analytically estimated, and marker experiments were conducted using thoria particles, both of which showed that Ti 2 Ni was the first compound to form at the Ti–Ni diffusion interface. At a temperature of 1173 K, using the Wagner method, the integrated diffusion coefficients for the Ti 2 Ni, TiNi, and TiNi 3 phases were calculated to be 3.53 × 10 −12 , 18.1 × 10 −15 , and 6.2 × 10 −15 m 2 /s, for, respectively. Keywords: Ti–Ni system; intermetallic compounds; integrated diffusion coefficient; scanning electron microscopy 1. Introduction Ti–Ni alloys, known for their exceptional properties such as shape memory effect, corrosion resistance, shock absorption, superelasticity, and biocompatibility, have found applications in diverse fields ranging from aerospace, automotive, and oil to biomedical sec- tors [1–4]. Given their importance, ongoing research in the field of the Ti–Ni binary system is focused on exploring its potential applications [5–8]. Depending on the heat treatment conditions, the Ti–Ni binary system can produce three stable intermetallic compounds, namely Ti 2 Ni, TiNi, and TiNi 3 , as well as two metastable intermetallic compounds, Ti 3 Ni 4 and Ti 2 Ni 3 [9]. However, limited attention has been given to the diffusion coefficients of alloying elements, which significantly affect the kinetics of intermetallic compound formation [1,10–15]. In the Ti–Ni binary system, TiNi is the most important compound, and its formation is influenced by the two other stable phases, Ti 2 Ni and TiNi 3 . Therefore, understanding the sequence and formation mechanism of TiNi is crucial. A diffusion couple study is a practical approach to investigate diffusion phenomena in solid-state conditions. The formation of different intermetallic compounds in the Ti–Ni binary system is based on diffusional transformation, including the metastable phases of Ti 3 Ni 4 → Ti 2 Ni 3 → TiNi 3 (stable), where temperature and holding duration are critical factors that determine the kinetics of this process [16]. The presence of metastable phases such as Ti 3 Ni 4 and Ti 2 Ni 3 in Ti–Ni alloys can affect their shape memory behavior [17–20]. However, in earlier studies investigating the formation of different intermetallic compounds in the Ti–Ni binary system using diffusion coupling, the formation of these metastable phases was disregarded [21–25]. The diffusion Materials 2023, 16, 2267. https://doi.org/10.3390/ma16062267 https://www.mdpi.com/journal/materials