Enhanced strength and ductility in a powder metallurgy Ti material by the oxygen scavenger of CaB6

Abstract

• There exists a continuous oxide layer on the irregular HDH Ti powder surface. • CaB 6 addition promotes the improved sintered density and refines the grain size. • The scavenging of O by CaB 6 addition makes a high strength and superior ductility for α-Ti alloy. The challenge of producing a high-ductility titanium (Ti) material using inexpensive high-oxygen hydride-dehydride (HDH) Ti powder is hereby addressed by the incorporation of CaB 6 oxygen-scavenger. The oxygen-scavenging behaviour, microstructure evolution, mechanical behavior and improvement mechanism were systematically investigated. A continuous TiO 2 oxide layer with a thickness of approximately 9.3 nm is presented on the HDH Ti powder surface. The oxide layer will dissolve into Ti matrix during sintering, making the increase of c / a value and leading to the Ti-Ti bonds developing from plastic metal bonds toward brittle covalent bonds. The CaB 6 addition can scavenge O impurity and make a significant increase in tensile ductility for α-Ti matrix. A small addition of 0.2 wt.% CaB 6 provides a superior tensile elongation of 22.2% for Ti material, almost three times as high as that of pure Ti (7.5%). The increase of deformation twining activity and grain refinement are responsible for the improved ductility. Furthermore, the CaB 6 oxygen-scavenger can react with the surface oxide layer to in-situ form rod-like TiB and granular CaTiO 3 reinforcements, refining the coarse near equiaxed grain of α-Ti matrix into fine equiaxed crystals. The multiple mechanisms of grain-boundary strengthening, load-bearing strengthening of TiB and Orowan strengthening of CaTiO 3 nanoparticles work together to increase the tensile strength of Ti/CaB 6 composites. This work offers an effective method to fabricate high-performance Ti material using inexpensive high-oxygen HDH Ti powder.