Microstructure and compressive mechanical properties of BCC structured (NbTi)100-x(CrAl)x medium-entropy alloys

To obtain a medium-entropy alloy (MEA) having a body-centered cubic (BCC) structure with relatively high specific strength and room-temperature fracture strain, a NbTiCrAl alloy is manufactured by the vacuum arc melting method. The NbTiCrAl alloy is developed by adding Ti to improve elongation and reduce weight, Al to enhance strength and reduce weight, and Cr to increase oxidation resistance. However, Al and Cr can form intermetallic compounds, which may reduce the mechanical properties of the alloy. To optimize strength and fracture strain, the contents of Al and Cr are varied. All the (NbTi)100-x(CrAl)x alloys, except those with more than 40 at.% Al and Cr, exhibit a BCC solid solution phase as a main phase. As the Al and Cr contents increase to 50 at. %, the short-range ordered B2 phase and the intermetallic compound Laves C14 phase are coexisted, as confirmed by microstructural analysis. The room temperature compression test results show that the yield strength improves without compromising fracture strain when the Cr and Al contents increase. However, when the total Cr and Al contents exceed 40 at.%, the short-range ordered B2 and Laves C14 phases coexist within the BCC matrix, resulting in catastrophic failure. Finally, in the (NbTi)100-x(CrAl)x alloys, (NbTi)70(CrAl)30 MEA exhibits the best properties in terms of specific yield strength and fracture strain.