Microstructure and mechanical properties of Ni nanoparticle-bonded Inconel 718

Ni nanoparticles were successfully used as a melting point depressant-free filler metal to join Inconel 718 via transient liquid phase (TLP) bonding in a vacuum environment. Ni nanopartides of 22, 29, and 42 nm in diameters were synthesized by controlling the reducing agent injection rates and the TLP bonding was carried out at up to 1050 degrees C with heating rates of 2-15 degrees C/min. Based on the Gibbs-Thomson equation and surface melting models, bonding using Ni nanopartides occurs due to competing solid-state sintering and surface melting processes. It was found that larger particle size, faster heating rate, and higher maximum temperature resulted in higher bonding strength due to less grain boundary contamination, smaller crystallite size, and more robust metallurgical bonding, respectively. Using a faster heating rate limits the amount of solid-state nanoparticle-nanoparticle sintering that occurs at lower temperatures. The suppression of nanopartide-nanopartide sintering as a function of nanopartide diameter is also discussed. The maximum bonding strength achieved is 243 MPa. According to digital image correlation, the strain is mostly sustained in the Ni-Inconel interfacial region. The fractography of Ni nanopartide-bonded joints is also discussed in detail. (C) 2019 The Authors. Published by Elsevier Ltd.