Abstract
this study, efforts were made in understanding the effect of reinforcing titanium aluminide alloy with niobium nitride (NbN) and tantalum nitride (TaN) on densification, microstructure, and mechanical properties. A composition consisting of 48Ti-48Al-2Cr-2Nb powdered titanium aluminide was blended with (0, 2, 4, 6, 8, and 10 wt.%) concentrations of TaN and NbN ceramic powders. A tubular mixer was used to blend the powders for 8 hours. Spark plasma sintering technique was used to synthesise the powders contained in a graphite die of 30 mm internal diameter. And the blended powders were sintered for 10 minutes, under a pressure of 50 MPa while heating of 100 °C/min was applied to 1150 °C. Post the sintering process, the densification phenomenon, phase composition, microstructural, and mechanical properties were evaluated. The evaluation encompassed the employment of Archimedes’ principle, x-ray diffraction, SEM/EDS and vickers microhardness tester and a wear universal micro-tribometer. The results obtained from this study shows that keeping the same sintering parameters when reinforcing TiAl alloy with NbN and TaN enhanced the mechanical properties and density of the unreinforced TiAl alloy. The unreinforced TiAl alloy achieved a sintered density of 3.97 g/cm3 which is very close to the theoretical density 4.0 g/cm3. After reinforcing TiAl with NbN and TaN ceramics the density of the resulting composites is greater than that of unreinforced TiAl and it increased with the reinforcement added. TiAl alloy achieved 99.26% relative density, TaN and NbN reinforced composites attained maximum densification of 99.58% and 99.64%, respectively. As a result of good densification, fine grain macrostructures were obtained. The respective introduction of 2wt% of TaN and NbN into TiAl alloy changed the TiAl duplex microstructure to a fully lamellar structure but with more reinforcement added the structure changed back to duplex structure. With the changes in microstructure and phases present a change in hardness among the composites is observed. Unreinforced TiAl recorded hardness of 304 HV, the maximum hardness achieved among the respective composites is 499 HV by TiAl+6TaN and 467 HV by TiAl+10NbN. Post annealing at 750 °C, TiAl’s microhardness decreased and for all NbN reinforced composites the microhardness improved. To further understand the mechanical properties of sintered composites, a dry wear test was conducted at 5 N to 10 N for about 900 seconds and the coefficient of friction decreased with an increase in load. Good wear resistance is achieved at 10 N for all the compositions. For TaN reinforced composites the best wear resistance is attained at 8 wt.% TaN (TiAl+8TaN) and for the NbN reinforced composites it is obtained at 2 wt.% NbN (TiAl+2NbN).