Precipitation of NiHfSi Phase in NiAl Single Crystals Containing Hf

摘要

Small additions of Hf to NiAl produce a significant increase in the high-temperature strength of single crystals. Hf has a very limited solubility in NiAl and in the presence of Si, results in a high density of G-phase (Ni16Hf6Si7) cuboidal precipitates and some G-platelets in a NiAl matrix. These precipitate have a F.C.C structure and nucleate on (100)(sub NiAl) planes with almost perfect coherency and a cube-on-cube orientation-relationship (O.R.). However, G-phase is metastable and after prolonged aging at high temperature dissolves at the expense of a more stable Heusler (rho'-Ni2AlHf) phase. In addition to these two phases, a third phase was shown to be present in a NiAl-0.3at.% Hf alloy, but was not previously identified. In this work, we report the morphology, crystal-structure, O.R., and stability of this unknown phase, which were determined using conventional and analytic transmission electron microscopy (TEM).