Room-temperature micro and macro mechanical properties of the metastable Ti-29Nb-14Ta-4.5Zr alloy holding nano-sized precipitates

The micro/macro mechanical properties of the metastable beta-Ti-29Nb-14Ta-4.5Zr (TNTZ) alloys reinforced by various nano-sized second phases of alpha '', alpha and omega have been studied. The variation in Young modulus, nano-hardness, ultimate strength, and the ductility have been assessed through conducting nano-indentation and uniaxial tensile tests. The lowest Young modulus has been obtained for the specimen which contains alpha '' second phase. The highest hardness, Young's modulus, and strength have been also achieved through the precipitation of the omega phase in the beta matrix. However, the elongation to fracture of omega containing specimens decreases down to similar to 1%. The formation of the alpha phase has no significant effect on Young's modulus and the ultimate strength of the beta matrix. The detailed microstructural studies reveal that the single beta phase accommodates the applied strain through the formation of zigzag-shaped {112} <111> deformation nano-twins, martensite/omega phase transformation, and dislocation slip. These are introduced as the main reason for high hardenability and elongation to fracture of this structure. The initial/secondary martensite laths in the beta+alpha '' specimen cannot operate as an effective obstacle against the dislocation movement. This well Justifies the lowers hardness, strength and higher ductility of alpha '' containing specimen. In the case of beta+alpha microstructure, high the population of the moving dislocation is accumulated behind alpha precipitates, and then cause the shear displacement of the lamella. Due to the appreciable volume fraction of high hardness omega phase in the beta matrix, the beta+omega specimen shows a complete brittleness and extremely high ultimate strength (similar to 964 MPa).