Inconel 718 (IN718) is primarily precipitation strengthened by coherent L1₂ structured γ’ (Ni₃ (Al, Ti)) phase and DO₂₂ structured (Ni₃Nb) γ’’ phase; it is an important engineering alloy widely used in oil-gas and aerospace industry. Previous study has shown that minor carbide addition in IN718 powder bed could render higher tensile strength and improved creep resistance after selective laser melting (SLM) process [1]. This study aims to further investigate phase transformation pathways of niobium carbide (NbC) bearing IN718 processed by SLM and different post heat treatments; this research also aims to determine the associated creep deformation mechanisms. With NbC addition, it has been found that direct ageing (DA) heat treatment after SLM process could induce nano-carbide precipitation along cell walls, causing cell size refinement and additional strengthening [2]. In addition, the solution heat treatment plus aging (STA) was applied, i.e. 1100 °C for 2 hours followed by 720 °C for 8 hours then 620 °C with further 8 hours treatment, although both STA and DA promoted the precipitations of γ’ and γ’’, the cellular structure would be eliminated and significant grain growth was observed in STA samples.

Creep tests were conducted under the condition of 650 °C and 650 MPa. Steady state creep strain rate and creep life of STA sample were 6.39*10^-9 s^-1 and 159.6 hours, respectively. Interestingly, DA sample showed a slightly higher steady state creep strain rate of 7.99*10^-9 s^-1 and a longer creep life of 268.5 hours. With NbC addition, the steady state creep strain rate of SLM DA IN718 could be effectively reduced to 5.19*10^-9 s^-1 and the creep rupture life was significantly improved to 374.5 hours. It is possible that the refined cellular dendritic structure and nano-carbides induced from NbC addition could pin dislocation during creep, resulting lower creep strain rate. Furthermore, the refined cellular structure might assist to suppress stress concentrations at grain boundaries, hence the creep rupture life could be prolonged.

References
[1] T. H. Hsu, K. C. Chang, Y. J. Chang, I. T. Ho, S. Tin, C. W. Li, K. Kakehi, C. P. Chen, K. K. Jen, H. Y. Hsieh, A.C. Yeh, Superalloys 2020, 2020, 982-989.
[2] K. C. Chang, M. Y. Lee, T. H. Hsu, Y. J. Chang, K. C. Lo, H. S. Kim, K. K. Jen, A. C. Yeh, Metals, 2021, Volume 11 Issue 11, 1691.