Multi-phase nanocrystalline MoNb (Fe) and MoNbTi (Fe) based multi-principal element alloys with superior “density-normalized” hardness

MoNb (Fe) and MoNbTi (Fe) based refractory multi-principal element alloys are processed using high-energy ball milling and spark plasma sintering (SPS). Multiple phases are observed after sintering the single-phase MoNb (Fe) and MoNbTi (Fe) milled powders. Fe (from milling media) is involved in the phase formations in both MoNb (Fe) and MoNbTi (Fe) alloys. The phases after SPS match well with the Calphad (Calculation of Phase Diagram) studies. The density of the MoNbTi (Fe) alloy (7.67 g/cc) is very low compared to the various commercial Niobium alloys like C-103 (8.85 g/cc), C-129Y (9.5 g/cc), and C3009 (10.1 g/cc). The combination of high hardness and low density in the present work is exceptional and it surpasses many commercial Nb-based alloys, indicating their potential for high-temperature aerospace applications. The inference from the present study is that the strengthening of the alloy depends not only on the number of elements but also on the elements selected. Thus, binary and ternary alloys can also offer more strengthening advantages compared to the systems containing 5 or 6 elements in high concentrations which in turn will lead to cost reduction. • Novel nanocrystalline MoNb (Fe) and MoNbTi (Fe) alloys are developed. • Thermo-Calc predictions and experimental observations are in well agreement. • A high hardness of 11 GPa was observed for both MoNb (Fe) and MoNbTi (Fe) alloys. • Depth-independent hardness is estimated using the classic Nix-Gao model. • A high “density-normalized’ hardness of 145 × 10 4 N-m/kg for the MoNbTi (Fe) alloy. • The “density-normalized” hardness of MoNbTi (Fe) alloy is superior to several commercial Nb-based alloys.