High-oxygen MIM Ti-6Al-7Nb: Microstructure, tensile and fatigue properties

The intention to make cost-effective titanium parts by powder metallurgy (PM) is a challenge. The oxygen content in PM titanium alloys depends on the powder production method used and increases during handling and sintering of powders. This paper contributes to understand the influence of oxygen on tensile and high cycle fatigue (HCF) properties of Ti-6Al-7Nb processed by MIM. It was confirmed that oxygen could purposely be used as a potent strengthener. However, special attention has to be paid if using an excessive amount of oxygen that might cause a dramatic drop of ductility due to deformation mode changes and evolution of non-equilibrium microstructures. It has been determined that such a non-equilibrium microstructure contributes to strength-ening and loss of ductility and it can be controlled using an optimised cooling rate during the sintering cycle. Moreover, it was found that oxygen produces a progressive reduction of HCF properties. In summary, this study validates the tolerance of MIM alpha + beta titanium alloys to certain amounts of oxygen content in relation to the mechanical properties.