On revealing the precipitation sequence in low Nb grade Alloy 718

The low Nb grade of Alloy 718 is a variant of the well-known aerospace grade Ni-based superalloy. While initially developed for the oil and gas industry, it now has found broader engineering applications across various corrosive environments, and seen a recent uptake in additive manufacturing. Besides having excellent strength and corrosion resistance, the low Nb grade exhibits improved resistance against hydrogen embrittlement. To achieve this, precipitation of the δ-phase (Ni 3 Nb), which is known to exacerbate hydrogen embrittlement, is suppressed by lowering the Nb content and modifying the heat treatment schedule. The low Nb grade exhibits the same microstructural constituents as the aerospace grade, however, it undergoes a different, poorly understood precipitation sequence. While precipitation sequence in the aerospace grade is well-researched, a correlation between the nano-scale evolution in the low Nb grade and its mechanical properties remains missing. Hence, this study aims to reveal the hierarchical microstructure evolution in the low Nb grade during various ageing stages, utilising the recently introduced INVIZO 6000 atom probe, which enables larger analysis volumes to be captured. Unlike the aerospace grade, precipitation during the early ageing stage is dominated by γ″ (also Ni 3 Nb), whereas γ′ (Ni 3 (Al,Ti)) experiences discontinuous nucleation bursts preferably on pre-existing γ″/γ-matrix interfaces. The current industrial heat treatment schedule leads to over-ageing of these precipitates, reducing the hardness by 20 HV from the peak-aged condition (354 HV10). Our findings help bridge current knowledge gap in the alloy’s strengthening properties and precipitation sequence, for the optimisation of heat treatments to achieve superior mechanical performance. • During early ageing, γ″ precipitation dominates instead of γ′ albeit high (Al+Ti)/Nb • Majority of strengthening stems from order strengthening of low-aspect ratio γ″ • Agreement between computed and experimentally calculated yield strengths confirmed • Ultrawide field-of-view atom probe enables improved quantification of γ′ and γ″