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′(L1 2 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 L1 2 -structure: Chromium partitions from the L1 2 structure into the disordered f.c.c. matrix for a driving force of 0.436 eV atom −1 . Mixing between the Ni 3 Al(L1 2 ) and Ni 3 Cr(L1 2 ) structures is energetically unfavorable based on first-principles quasi-random structure calculations. Interfacial Gibbs free energies of Ni/Ni 3 Al(L1 2 ), Ni/Ni 3 Cr(L1 2 ), Ni/Ni 3 Cr(DO 22 ), and Ni n (Al y Cr 1-y )/Ni 3 (Al x Cr 1-x )(L1 2 ) 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’ (L1 2 )-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 Ni 3 Cr(L1 2 ) is easier than Ni 3 Al(L1 2 ) at 873 K (600 °C). Ni 3 Cr(DO 22 ) is more difficult to nucleate than Ni 3 Cr(L1 2 ) and Ni 3 Al(L1 2 ) 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, Ni 3 (Al,Cr)(L1 2 ) requires the smallest critical net reversible work to form but has a larger critical size. An embryo of Ni 3 Cr(L1 2 ) only acts as a nucleant for mixed L1 2 Ni 3 (Al x Cr 1-x ). The morphology of Ni 3 (Al,Cr)(L1 2 ) precipitates transforms from cuboidal-to-spheroidal with increasing Cr concentration, in agreement with APT observations.