Investigation of secondary phases and tensile strength of nitrogen-containing Alloy 718 weldment
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Investigation of secondary phases and tensile strength of nitrogen-containing Alloy 718 weldment Behrooz Nabavi 1), Massoud Goodarzi 1), and Abdul Khaliq Khan 2) 1) School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Tehran 1684613114, Iran 2) Department of Mechanical Engineering, Manitoba Institute for Materials, University of Manitoba, Winnipeg R3T 2N2, Canada (Received: 2 October 2019; revised: 6 February 2020; accepted: 11 February 2020)
Abstract: The influence of nitrogen content on the precipitation of secondary phases and the tensile strength of Alloy 718 during gas tungsten arc welding was investigated. Various types of precipitates were characterized using scanning electron microscopy and transmission electron microscopy. The results showed that in the fusion zone, the volume fraction of Nb-rich phases such as Laves, (Nb,Ti)C, and δ phases, as well as Ti-rich phases such as (Ti,Nb)CN and (Ti,Nb)N, increased with increase in the nitrogen content due to the microsegregation of Nb and Ti within interdendritic areas. Nitrogen was also found to decrease the size of γ′′ particles within γ dendrites. For precipitates in the partially melted zone, constitutional liquation was observed for both (Nb,Ti)C and (Ti,Nb)N particles. Based on the results of tensile tests, the weld containing 0.015wt% nitrogen exhibited the highest ultimate tensile strength (UTS), whereas more addition of nitrogen led to a decrease in both the UTS and yield strength due to the increased content of brittle Laves phases and decreased size of γ′′. Keywords: nitrogen; secondary phases; gas tungsten arc welding; tensile strength; Alloy 718
1. Introduction Alloy 718, a γ′′-strengthened superalloy, has a broad range of applications in aero engines and land-based gas turbine parts owing to its strong resistance to strain-age cracking upon post-weld heat treatment (PWHT) [1]. In this nickelbased alloy, depending on its fabrication and historical heat treatment, two types of precipitates can be formed: 1) intermetallic compounds such as Laves phases (Ni,Fe,Cr)2 (Nb,Mo,Ti) [2–4], γ′′ (Ni3Nb/body-centered tetragonal structure) [1,5–6], and δ (Ni3Nb/orthorhombic structure) [6–8] and 2) solid solution phases such as Nb-rich MC-type carbides [9], MN-type nitrides [10–11], and MCN-type carbonitrides [12–13], whose presence can affect the mechanical properties of the alloy. In this context, the destructive effects of Laves phase, delta phase, and MC-type carbide on ductility and tensile strength [6,9] have been reported. Moreover, the presence of secondary phases causes some weldability issues such as the constitutional liquation of Nbrich carbide and Laves phase in wrought and cast materials, respectively, which may even result in heat-affected zone (HAZ) liquation cracking [6]. On the other hand, the benefits of carbides and nitrides on prevention of grain growth pre
vention [11] and concomitant increas
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