Cost-effective, high strength and ductility titanium materials by powder metallurgy process

Ubiquitous light elements such as oxygen, nitrogen, hydrogen, carbon and silicon are used as solid-solution strengthening factors of powder metallurgy (PM) titanium and its alloys. In this study, the conventional rare metals never employed as strengthening elements because they generally cause the increment of materials cost and the decrease of biocompatibility. In case of oxygen and nitrogen, two processing routes for PM titanium materials with their solid-solution strengthening effects are effective; [a] direct reaction between titanium powder and each gas, and [b] resolution of TiO2 or TiN in the elemental mixture of titanium powder and their particles. For example, the sintered and extruded Ti powder materials with oxygen solid-solution were prepared by using the elemental mixture of pure Ti powder and TiO2 particles. With increase in TiO2 content, XRD patterns indicated that the lattice constant in c-axis, not a-axis, of hcp-Ti(α) materials proportionally increased due to oxygen atoms solid solution. TEM-EDS analysis also obviously showed a lot of dislocations around oxygen atoms. Yield stress of wrought pure Ti materials was proportional to the oxygen content, but a decrease of elongation was very small. YS increment calculated by Labusch model showed a good agreement with the experimental values. For example, extruded pure Ti powder material with 1.5wt% TiO₂ revealed 1025 MPa YS and 24.8% elongation. The additive heat treatment after the above sintering process was much effective to improve the balance of strength and ductility because of the complete diffusion and solution of oxygen atoms into hcp-Ti, and resulted in the further increment of elongation of PM pure hcp-Ti materials with oxygen soluted atoms. These experimental data corresponded well to the theoretical values calculated by using Labusch model.