Microstructure and Phase Stability Studies on Heusler Phase Ni₂AlHf and G-phase Ni₁₆Hf₆Si₇ in Directionally Solidified NiAl–Cr(Mo) Eutectic Alloyed with Hf

Abstract

Small additions of Hf to directionally solidified NiAl–Cr(Mo) eutectic resulted 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 directional solidification of the alloy. These fine G-phases were cuboidal in shape and coherent with the NiAl matrix. After hot isostatic pressing and aging treatment, the fine G-phases completely disappeared. The density of the Heusler phase was partially reduced, and the Heusler particles precipitated preferentially on the NiAl/Cr(Mo) interfaces and grain boundaries of the NiAl matrix. Some Heusler particles precipitated locally within the NiAl matrix, and small amounts of them precipitated within the Cr(Mo) phase. The structures of the NiAl/Ni₂AlHf and NiAl/Ni₁₆Hf₆Si₇ interfaces were investigated by high-resolution electron microscopy. The habit plane of the fine G-phase was {001}_NiAl. This result was in good agreement with calculation based on the linear elastic theory. The misfit dislocation network on the NiAl/Ni₂AlHf (110) interface was calculated from the O-lattice model and compared with the observation, which showed good agreement.