Effect of heat treatment on the modification of microstructure of selective laser melted (SLM) IN718 and its consequence
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ADDITIVE MANUFACTURING OF METALS: COMPLEX MICROSTRUCTURES AND ARCHITECTURE DESIGN
Effect of heat treatment on the modification of microstructure of selective laser melted (SLM) IN718 and its consequences on mechanical behavior R.J. Vikram, Anubhav Singh, Satyam Suwasa) Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India a) Address all correspondence to this author. e-mail: [email protected] Received: 2 January 2020; accepted: 5 May 2020
In this investigation, the superalloy IN718 has been prepared by additive manufacturing (AM) following a selective laser melting technique, and the post-AM heat treatments have been optimized. The microstructure of additively manufactured (AM) IN718 is characterized by the presence of dendritic and cellular features with large spatial heterogeneity along and across the build plane. Along the build direction, the 〈100〉 fiber texture dominates. Heat treatment involving two-step solution treatment, and subsequently, two-step aging treatment was specifically designed to facilitate the precipitation of δ phase at the grain boundaries to make the material resistant to grain boundary sliding (GBS). The AM IN718 showed dynamic strain aging (DSA) at three different temperatures, while the critical strain for the onset of serration was extended to a higher value after the heat treatment.
Introduction IN718 is an austenitic nickel–chromium–iron-based superalloy which is primarily strengthened by γ′′ precipitates. The alloy is highly resistant to oxidation, corrosion and is most widely used in gas turbine disks and other high-temperature applications up to 650 °C [1, 2, 3, 4]. The main strengthening phase in IN718 is the γ′′ -Ni3Nb (Body Centered Tetragonal - BCT) precipitate which is a metastable phase and has the chemical composition same as the high-temperature stable incoherent δ-Ni3Nb (orthorhombic) phase [2]. However, this phase could contribute to strengthening when it preferentially nucleates at the grain boundary [5, 6]. The formation of the δ phase consumes Nb (solid solution strengthener) from the γ matrix. The depletion of Nb results in loss of matrix strength. It is, therefore, desirable to do right optimization of the individual precipitate content in order to achieve good creep and fatigue strengths [3, 7, 8]. The predominant failure mechanism for IN718 at elevated temperature is grain boundary sliding (GBS). Further, it is known that the creep resistance of IN718 improves when the δ phase preferentially precipitates at the grain boundary [9]. The optimized volume fraction of the δ phase at the grain boundaries enhances the strength by
© Materials Research Society 2020
hindering dislocation movement at the interface and improves resistance to grain boundary creep fracture [7, 9, 10]. Additive manufacturing (AM) has drawn the attention because of its intriguing ability to design complex shape, precise control on chemical composition, high buy-to-fly ratio, and possibility to control texture, which, in turn, affects anisotropy in mech
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