Consolidation of Ti6Al4V alloy and refractory nitride nanoparticles by spark plasma sintering method: Microstructure, mechanical, corrosion and oxidation characteristics

Ti6Al4V alloy combines excellent mechanical and chemical characteristics attractive in some important applications in the automotive and aerospace industries but its insufficient hardness and high-temperature oxidation subdue its extensive use. This work was undertaken to modify the microstructure and improve the current limitation of Ti6Al4V alloy with the intention of not affecting its desirable properties. Therefore, the effects of different refractory nitrides: aluminium nitride (AlN), titanium nitride (TiN) and hexagonal boron nitride (h-BN) reinforcements on the microstructure, mechanical, chemical and oxidation properties of spark plasma sintered Ti6Al4V-based binary composites were investigated. Spark plasma sintering technique was effectively utilized to consolidate the Ti6Al4V powder and 3 wt% nanoparticles of AlN, TiN and h-BN respectively. The microstructure and phase composition of the sintered composites were examined by scanning electron microscopy, optical microscopy, and X-ray diffractometry; densification was evaluated according to Archimedes' principle and microhardness was measured by Vickers' microhardness test The corrosion and oxidation behaviour of the samples were studied by linear polarization experiment and thermal gravimetric analysis respectively. It was found that the binary composites produced attained almost full theoretical relative densification (98.23-99.54%) due to adequate diffusional mass transport in solidly bonded particles at the matrix-reinforcement interfaces. Ti6Al4V-3h-BN composite gave the optimal combination of relative densification (99.54%) and microhardness (7030 MPa) which exceeds 200% of the monolithic alloy and about 48% superior to both composites with AlN and TiN reinforcements. The yield and ultimate tensile strengths of the matrix were improved by approximately 47% via the AlN and TiN nanoparticle additions and significantly by 116% through the nano-h-BN addition. Ti6Al4V-3AlN demonstrated most superior electrochemical corrosion resistance with a current density of 3.66 mu A/cm(2) and a polarization resistance of 8760.2 Omega in the sulphuric acid medium while Ti6Al4V-3TiN with the least normalized weight gain of 0.85 mg/cm(2) showed the greatest resistance against oxidation in the high-temperature oxidizing environment.