Impact of Hf content on the microstructure and mechanical properties of bulk NiCoCrAlYMoHf alloys

Hafnium (Hf) was added as an alloying element to enhance the strength in the NiCoCrAlYMoHf alloys, and the effects of Hf content on constituent phases, microstructural characteristics, and mechanical properties of the as-cast alloy were investigated. The experimental results showed that microstructure of the alloy with 1.0 wt.% Hf was composed of β phase and γ′ phase. The experimental results showed that microstructure of the alloy with 1.0 wt.% Hf was composed of β phase and γ′ phase. With increase in Hf content, the β phase content increased, while the γ′ phase content decreased, and σ phase gradually appeared and precipitated at grain boundary, which resulted in an increase in the hardness of alloys from 429 to 586 HV. The indentation hardness, Young's modulus, and compressive strength of NiCoCrAlYMoHf alloy with 4.0 wt.% Hf could reach 7.12 GPa, 185.52 GPa, and 2659 MPa, respectively, which means that more addition of Hf is beneficial for improving performances of NiCoCrAlYMoHf alloys. The microhardness value is 1.95 times higher than that of the Hf-free alloy. Furthermore, the nanoindentation hardness and Young's modulus are 1.05 and 1.75 times higher, respectively, than those of the Hf-free alloy. Graphical abstract: [Figure not available: see fulltext.].