Influence of boron and oxygen on the microstructure and mechanical properties of high-strength Ti66Nb13Cu8Ni6.8Al6.2 alloys

Abstract

The influence of boron additions and different oxygen contamination levels on the microstructure and the mechanical properties in the Ti66−xNb13Cu8Ni6.8Al6.2Bx (0⩽x⩽1) system were investigated. The alloys were prepared by levitation copper mould casting as rods with a diameter of 5mm using different grades of starting elements. The alloy without boron exhibits a maximum compressive stress of more than 2500MPa, associated with a compressive strain of more than 30%. The ultimate tensile stress is ∼1075MPa with a maximum elongation of 1.6%. Increased oxygen content leads to a rise of yield strength due to solid solution hardening. Boron additions promote grain refinement and reinforce the interdendritic phase compound by forming needle-like TiB precipitates. This change in microstructure increases the yield stress and the Young’s modulus and lowers the plastic strain. The microstructure was analysed in terms of the boron content by means of scanning electron microscopy, Auger electron spectroscopy and transmission electron microscopy. The presented mechanical properties are compared with the compression and tensile properties of the commercially available Ti6Al4V ELI (ELI=extra low interstitial) alloy.