Influence of Ho and Hf on the microstructure and mechanical properties of NiAl and NiAl-Cr(Mo) eutectic alloy

Abstract

In the present paper, the Ho and Hf was synergistically added in the NiAl and NiAl-Cr(Mo) eutectic alloy fabricated by conventional casting technology. Their microstructure, precipitates and mechanical properties were investigated. The results exhibit that the solid solubility of Ho in NiAl is so low that minor addition of Ho in NiAl would lead to the formation of Ni2Al3Ho and Ni3.5Al1.5Ho phases, which both have the hexagonal crystal structure. The synergistic addition of Hf and Ho would lead to the precipitates of Ni2AlHf and Ni2Al3Ho along NiAl grain boundary and forms the skeleton structure. The synergistic addition of Hf and Ho in NiAl-Cr(Mo) eutectic alloy refines the NiAl/Cr(Mo) eutectic lamella but coarsens the NiAl and Cr(Mo) phases in the intercellular region. The Ni2AlHf and Ni2Al3Ho mainly precipitate along the NiAl/Cr(Mo) phase interface in the intercellular region. Moreover, the synergistic addition of Hf and Ho promotes the formation of coarse NiAl particle in the Cr(Mo) phase. The synergistic addition of Hf and Ho increases the microhardness of NiAl and NiAl-Cr(Mo) eutectic alloy. In addition, they also improve the strength of the NiAl-Cr(Mo) eutectic alloy at elevated temperature but the bulk precipitates along the phase interface are apt to be the initiation of the cracks.