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

Abstract

The micro/macro mechanical properties of the metastable β-Ti-29Nb–14Ta-4.5Zr (TNTZ) alloys reinforced by various nano-sized second phases of α'', α and ω 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 α'' second phase. The highest hardness, Young's modulus, and strength have been also achieved through the precipitation of the ω phase in the β matrix. However, the elongation to fracture of ω containing specimens decreases down to ~1%. The formation of the α phase has no significant effect on Young's modulus and the ultimate strength of the β matrix. The detailed microstructural studies reveal that the single β 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 β+α'' specimen cannot operate as an effective obstacle against the dislocation movement. This well justifies the lowers hardness, strength and higher ductility of α'' containing specimen. In the case of β+α microstructure, high the population of the moving dislocation is accumulated behind α precipitates, and then cause the shear displacement of the lamella. Due to the appreciable volume fraction of high hardness ω phase in the β matrix, the β+ω specimen shows a complete brittleness and extremely high ultimate strength (~964 MPa).