Abstract
Guided by CALPHAD (Calculation of Phase Diagrams) modeling, the refractory medium-entropy alloy MoNbTaV was synthesized by vacuum arc melting under a high-purity argon atmosphere. A body-centered cubic solid solution phase was experimentally confirmed in the as-cast ingot using X-ray diffraction and scanning electron microscopy. The measured lattice parameter of the alloy (3.208 Å) obeys the rule of mixtures (ROM), but the Vickers microhardness (4.95 GPa) and the yield strength (1.5 GPa) are about 4.5 and 4.6 times those estimated from the ROM, respectively. Using a simple model on solid solution strengthening predicts a yield strength of approximately 1.5 GPa. Thermodynamic analysis shows that the total entropy of the alloy is more than three times the configurational entropy at room temperature, and the entropy of mixing exhibits a small negative departure from ideal mixing.
Highlights
Materials development is closely related to the civilization of human society
The present study focuses on a medium-entropy alloy MoNbTaV with the objective of achieving a single body-centered cubic (BCC) structure with lower density and higher ductility than MoNbTaW
The Nb content in both regions deviates little from the bulk concentration and composition of the dendritic arms (Cdr ), and the average composition of the interdendritic regions falls within uncertainty of EDXofmeasurements
Summary
Materials development is closely related to the civilization of human society. Most transportation and energy applications call for structural materials that have high strength, good fracture toughness, and great thermal stability. Other advanced structural materials need to be discovered with superior temperature capability to Ni-based alloys for ultra-high temperature applications. Since 2010, a series of HEAs with refractory elements were investigated for high temperature applications, including alloys based on the following element combinations: MoNbTaW, HfNbTaTiZr, MoNbTiZrVx , and NbTaTiVAlx [7,17,18,19,20,21]. Other properties are still problematical, including higher than desired density and lower than expected room-temperature ductility. These issues still need to be resolved [7,22]. The present study focuses on a medium-entropy alloy MoNbTaV with the objective of achieving a single BCC structure with lower density and higher ductility than MoNbTaW. Diagrams (CALPHAD) method is used to predict phase formation during solidification, and solid solution strengthening is analyzed using a simple model based on traditional elastic theory
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