Interfacial free energies, nucleation, and precipitate morphologies in Ni-Al-Cr alloys: Calculations and atom-probe tomographic experiments

Abstract

The effects of Cr additions on the morphologies οf γ′(L12 ordered) precipitates are studied in three Ni-Al-Cr alloys utilizing atom-probe tomography (APT) and first-principles calculations. We find that Cr and Al share the same sublattice sites in the L12-structure: Chromium partitions from the L12 structure into the disordered f.c.c. matrix for a driving force of 0.436 eV atom−1. Mixing between the Ni3Al(L12) and Ni3Cr(L12) structures is energetically unfavorable based on first-principles quasi-random structure calculations. Interfacial Gibbs free energies of Ni/Ni3Al(L12), Ni/Ni3Cr(L12), Ni/Ni3Cr(DO22), and Nin(AlyCr1-y)/Ni3(AlxCr1-x)(L12) interfaces are calculated for the {100}, {110}, and {111} planes. The temperature dependencies of the interfacial Gibbs free energies are determined by including phonon vibrational entropies. The equilibrium interfacial Gibbs free energies are determined using concentration profiles as a function of aging time, obtained using APT. At early aging times, the initial values of the interfacial Gibbs free energies are higher with small Cr concentrations in the γ’ (L12)-precipitates. With increasing aging time, the interfacial Gibbs free energies decrease with increasing Cr concentration and achieve equilibrium values after 16-h of aging at 873 K (600 °C) for the three Ni-Al-Cr alloys. From classical nucleation theory, the nucleation of metastable Ni3Cr(L12) is easier than Ni3Al(L12) at 873 K (600 °C). Ni3Cr(DO22) is more difficult to nucleate than Ni3Cr(L12) and Ni3Al(L12) at 873 K (600 °C). The Cr additions to Ni-Al alloys have significant effects on the critical radius and critical net reversible work of nucleation. With increasing Cr concentration, Ni3(Al,Cr)(L12) requires the smallest critical net reversible work to form but has a larger critical size. An embryo of Ni3Cr(L12) only acts as a nucleant for mixed L12 Ni3(AlxCr1-x). The morphology of Ni3(Al,Cr)(L12) precipitates transforms from cuboidal-to-spheroidal with increasing Cr concentration, in agreement with APT observations.