Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea Influence of bonding time on the transient liquid phase bonding behavior of Hastelloy X using Ni-Cr-B-Si-Fe filler alloy A. Malekan a , M. Farvizi a,* , S.E. Mirsalehi b , N. Saito c , K. Nakashima c a Ceramic Division, Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran b Department of Mining and Metallurgical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, 15875-4413, Iran c Department of Materials Science and Engineering, Kyushu University, 744, Motooka, Nishi-ku, Fukuoka, 819-0395, Japan ARTICLE INFO Keywords: Hastelloy X superalloy TLP process Isothermal solidification EPMA Microstructure Mechanical properties ABSTRACT The effects of different transient liquid phase (TLP) bonding times on the microstructure and mechanical properties of Hastelloy X joints made by Ni–Cr–B–Si–Fe filler alloy were investigated. The specimens were TLP bonded at 1070 °C for holding times of 5, 20, 80, 320, and 640 min. The electron probe microanalysis (EPMA) results revealed that the main eutectic phases observed at the joints following incomplete isothermal solidifi- cation were Ni-rich borides, Ni-rich silicides, Ni–Si eutectic, and some Cr-rich borides. A high density of plate- like, blocky, and acicular (Mo and Cr)-rich borides were observed in the diffusion-affected zone (DAZ) of the samples; however, increasing the holding time decreased the contents of these phases. The solid-state diffusion was found to be a more effective transportation phenomenon than base metal dissolution at longer holding times. The increased DAZ thickness and the complete isothermal solidification as a result of the improved solid- state diffusion helped increase the uniformity of the hardness profile of the TLP bond at higher holding times (320 and 640 min). The results showed reverse relationship between the athermally solidified zone (ASZ) width and the bonding strength. The highest tensile strength (∼617 MPa) was achieved for the sample bonded at a holding time of 320 min; this strength was more than 80% of the base metal strength. A fractographic analysis of the tensile failure revealed a cellular fracture surface, exhibiting the characteristics of both brittle and ductile fractures. The sites prone to stress concentration and crack initiation were reduced with the completion of isothermal solidification. 1. Introduction Hastelloy X is a solid-solution strengthened Ni-Cr-Fe-Mo alloy with remarkable mechanical properties, fracture toughness, structural sta- bility, and improved high-temperature corrosion and creep resistance. Hastelloy X is supplied in a solution heat-treated form and needs to be rapidly cooled in air or water. The microstructure of this superalloy comprises a face-centered cubic (FCC) solid-solution matrix with a few Mo-rich M 6 C carbides. During aging at intermediate temperatures, Cr- rich M 23 C 6 carbides are found to be more stable than the Mo-rich M 6 C carbides [1]. This superalloy can be readily wrought into sheet and bars, which are widely used in transition ducts, combustor chambers, and afterburners of gas turbine engines, tail pipes, and furnace parts. It is considered as a candidate material for high-temperature gas-cooled reactor components owing to its good oxidation resistance and high- temperature strength [2–4]. Most of the mentioned components consist of sheet-shaped parts which need to be bonded. Thus, the joining is a crucial part of manufacturing processes for fabrication of more complex shapes from this superalloy. Moreover, it is worthwhile to repair service damaged components rather than complete replacement due to lower costs and shorter delivery time of repairing [5,6]. A wide variety of fusion welding processes (including gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), shielded metal arc welding (SMAW), submerged arc welding (SAW), plasma arc welding (PAW), and electron beam welding (EBW)) can be used for welding Ni-based superalloys such as Hastelloy X. GTAW is the main candidate for welding different shapes, particularly thin parts. In a fu- sion welding process, the microstructure, chemical composition, and mechanical properties in the heat-affected zone (HAZ) can be affected by the heat input and heating/cooling conditions. A common problem in conventional fusion welding processes for Ni-based superalloys, such as Hastelloy X, is the excessive formation of brittle phases, such as M 6 C and M 23 C 6 carbides, in the HAZ, leading to cracking and failure in this zone of the weld [7,8]. https://doi.org/10.1016/j.msea.2019.03.124 Received 10 February 2019; Received in revised form 28 March 2019; Accepted 30 March 2019 * Corresponding author. E-mail addresses: mmfarvizi@yahoo.com, mmfarvizi@merc.ac.ir (M. Farvizi). Materials Science & Engineering A 755 (2019) 37–49 Available online 04 April 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved. T