Sound velocities, elasticity, hardness, and fracture toughness of novel <i>hcp</i> and <i>bcc</i> structured Ti-based medium-entropy alloys

Sound velocities, bulk modulus (B), shear rigidity (G), Young's modulus (E), Vickers hardness (HV), and fracture toughness (KIC) of Ti-based medium-entropy alloys (MEAs) of TiZrHf, TiZrNb, and TiHfNb are investigated using ultrasonic interferometry combined with Vickers hardness measurements. It is found that the Vickers hardness of hexagonal TiZrHf MEA is achieved up to ∼6.0 GPa, which is ∼1.3 times higher than the bcc-structured TiZrNb and TiHfNb ones. Meanwhile, the fracture toughness of cubic TiZrNb and TiHfNb MEAs is ranging from 6.0 to 7.8 MPa m1/2, which is about 1.5–2 times stronger than the hcp-TiZrHf counterpart. Additionally, the hcp-TiZrHf MEA exhibits a higher Young's modulus of around 130 GPa, approximately 20 GPa higher that of the bcc-TiZrNb MEA. The underlying mechanism for improving B, G, E, KIC, and/or HV, as well as the composition and structure dependent mechanical properties are clarified by the variations in bond strength, electronic structure, and lattice distortion of Ti-based MEAs. These findings explore how microstructural characteristics affect properties such as the bulk modulus and shear rigidity of M/HEAs and provide new insights into tuning mechanical properties through the modulation of alloy compositions.