Microstructure, precipitates and compressive properties of various holmium doped NiAl/Cr(Mo, Hf) eutectic alloys

The microstructure and mechanical properties of NiAl–28Cr–6Mo–0.2Hf eutectic alloys with various holmium additions have been investigated by using of SEM, TEM and compression test. The results reveal that minor Ho addition could optimize the microstructure by refining the lamella inside of eutectic cell and controlling the coarsening of intercellular region. However the minor Ho addition results in the Ni2Al3Ho phase, which has hexagonal crystal structure and an orientation relationship with NiAl phase of [120]Ni2Al3Ho//[111]NiAl and (002¯)Ni2Al3Ho//(11¯0)NiAl. Moreover the Ni17Ho2 phase is found in the Ni2Al3Ho phase, which has twin crystal inside and an orientation relationship with Ni2Al3Ho phase of [123]Al17Ho2//[124]Ni2Al3Ho and (111¯)Al17Ho2//(2¯10)Ni2Al3Ho. In addition, the Ho addition promotes the precipitation of Ni2AlHf phase. More Ho addition coarsens the Cr(Mo) phases along the intercellular and results in more strengthening precipitates inside eutectic cell. When the Ho addition comes to 0.5 at.%, the alloy almost loses the cellular eutectic characteristic and exhibits a microstructure with coarse NiAl, massive Cr(Mo) and Ni2Al3Ho phases. The microstructure optimizations improve the mechanical properties of the alloy significantly, especially the alloy with 0.1 at.% Ho content.

► Trace Ho addition optimizes the microstructure of the NiAl/Cr(Mo, Hf) eutectic alloy.
► An orientation relationship of [120]Ni2A13Ho//[111]NiA1 and (002¯)Ni2A13Ho//(11¯0)NiA1 is found.
► Ho addition promotes the precipitation of Ni2AlHf Heusler phase.
► Minor Ho addition coarsens the microstructure, especially the intercellular region.
► More Ho addition changes the original cellular eutectic microstructure.