Precipitation behavior of Heusler phase (Ni2AlHf) in multiphase NiAl alloy.

Precipitation behavior of Heusler phase (Ni2AlHf) in a directionally solidified (DS) NiAl- 28Cr-5Mo-1Hf (at.%) alloy was examined using scanning electron microscope (SEM) and transmission electron microscope (TEM). In the as-cast alloy, the Ni2AlHf phase generally appeared on the NiAl/Cr(Mo) interface, which degraded the NiAl/Cr(Mo) eutectic structure. In the heat-treated alloy, the density of the intercellular Ni2AlHf phase was slightly reduced. In addition, the spherical Ni2AlHf phase precipitated heterogeneously in the NiAl matrix, but the Ni2AlHf phase did not precipitate in the lamellar Cr(Mo) phase. The precipitation behavior of the Ni2AlHf phase could be explained in terms of the interfacial energy. A lattice model was also proposed to explain the NiAl↔Ni2AlHf phase transformation. [ABSTRACT FROM AUTHOR]