Unveiling the effect of Hf-(Mo,W) addition on the coarsening, agglomeration, and dissolution behaviour of gamma'-Ni3Al precipitates in model Ni-based superalloys

In compositional design of Ni-based superalloys, atomic size of alloying elements appears as the primary factor affecting alloy strengthening. Depending on atomic size, the phase partitioning preferences of al-loying elements not only control the coarsening and dissolution characteristics of gamma' precipitates, but also govern their morphological development upon aging. The current study, therefore, investigates the influ-ence of larger atomic radius X = Hf element addition and its co-additions with smaller atomic radii re-fractory X = Mo(W) elements on the microstructural and mechanical properties of model Ni80Al15Hf5 and Ni80Al15Hf2.5Mo(W)2.5 superalloys. Accordingly, it is concluded that the replacement of 2.5 at% X = Hf ad-dition with the same amount of X = Mo(W) additions reduces and increases gamma' participation tendency of X = Hf and X = Mo(W) elements, respectively. Agreeing with classical Lifshitz-Slyozov-Wagner (LSW) ki-netics, gamma'-Ni3Al-Hf-W precipitates coarsen at a slower rate compared to their gamma'-Ni3Al-Hf-Mo counterparts, both begin to agglomerate at longer aging times, i.e., 256 h. On the other hand, readily coarsened, aligned, and agglomerated gamma'-Ni3Al-Hf precipitates start to dissolve in gamma matrix after 16 h, which hence disobey LSW theory. Most probably, higher gamma'/gamma interfacial energy contribution of faster-diffusivity X = Hf element leads to this three-step temporal evolution of gamma'-Ni3Al-Hf precipitates, i.e., (i) coarsening, (ii) alignment/agglom-eration, and (iii) dissolution, to occur much earlier and accelerates the strength degradation. Conversely, X = Hf-W co-addition, which causes better mechanical strength than both X = Hf and X = Hf-Mo additions, reduces the degree of strength worsening possibly due to the beneficial contributions of X = W element at gamma'/gamma interfaces. (c) 2022 Elsevier B.V. All rights reserved.