Microstructure and phase stability studies on Heusler phase Ni2AlHf and G-phase Ni16Hf6Si7 in directionally solidified NiAl-Cr(Mo) eutectic alloyed with Hf

Small additions of Hf to directionally solidified NiAl-Cr(Mo) eutectic resu lted in precipitation of a high density of Heusler phase Ni2AlHf along with fine G-phase Ni16Hf6Si7. The Heusler phase was mainly located on the grain boundary region. The fine G-phase formed in the presence of Si, which was a contamination resulting from contact with ceramic shell molds during dire ctional solidification of the alloy. These fine G-phases were cuboidal in s hape and coherent with the NiAl matrix. After hot isostatic pressing and ag ing treatment, the fine G-phases completely disappeared. The density of the Heusler phase was partially reduced, and the Heusler particles precipitate d preferentially on the NiAl/Cr(Mo) interfaces and grain boundaries of the NiAl matrix. Some Heusler particles precipitated locally within the NiAl ma trix, and small amounts of them precipitated within the Cr(Mo) phase. The s tructures of the NiAl/Ni2AlHf and NiAl/Ni16Hf6Si7 interfaces were investiga ted by high-resolution electron microscopy. The habit plane of the fine G-p hase was {001}(NiAl). This result was in good agreement with calculation ba sed on the linear elastic theory. The misfit dislocation network on the NiA l/Ni2AlHf ((1) over bar 10) interface was calculated from the O-lattice mod el and compared with the observation, which showed good agreement.