Processamento por metalurgia do pó e caracterização de compósitos da liga Ti-13Nb-13Zr reforçados com TiC

Abstract

This research aims at the development of composites of the metallic matrix of titanium alloy Ti-13Nb-13Zr via powder metallurgy with titanium carbide reinforcement. The titanium alloys are applied in situations which require low density and high mechanical resistance (150MPa to 1200MPa yield point). This combination of properties is called specific resistance and is an important factor to be considered in projects in the aerospace industry. The oxidation/corrosion resistance of these alloys allow them to operate at temperatures up to 500°C. Some titanium alloys can be employed in implants since they do not usually react with body fluids and their modulus of elasticity is closer to that of the bone when compared to other alloys. One of the few disadvantages of titanium alloys is their low resistance to wear. Therefore, to minimize tribological problems and increase mechanical resistance, composites of the titanium metallic matrix with ceramic reinforcement have been produced. In this study, composites of the titanium alloy Ti-13Nb-13Zr with TiC (0 to 20% mass fractions) addition were produced. Initially, the raw material (metallic hydrates and titanium carbide) powders were characterized, the D(50) particle sizes found were smaller than 10 m; the powder morphologies were angular, faceted and rod-like shaped (NbH); the X-ray diffraction results showed only peaks of the powders analyzed. The densifications of the materials characterized at 1400°C ranged between 93% and 95.5%. The chemical composition of the monolithic sample presented coherent results, as follows: 13.3% niobium, 12.2% zircon, and 0.3% iron; the composition was used to obtain the thermodynamic calculation. Regarding the sintered samples, the quantification of phases using X-ray diffraction showed coherent results, namely, monolithic sample: 53.4% alpha phase (HC) and 46.6% beta phase (CCC); with the TiC reinforcement additions, the beta phase kept values close to 41-43%, while the alpha phase reduced from 46.5% to 35.7% with the reinforcement additions in the 20% TiC sample. The characterizations using optical microscopy and scanning electron microscopy showed the matrix microstructure, with morphology of the Widmanstätten type presenting carbides dispersed throughout the matrix (in lower amounts), while in larger amounts, a reinforcement network tended to form around the Widmanstätten cells. Another finding was that a microstructure refinement occurs when the TiC amount is increased, a reduction in the alpha phase regions was observed, which agrees with the X-ray diffraction results. The EDS analysis results showed niobium preferentially partitioned in the beta phase, while zirconium tended to be the neutral element, but in the presence of niobium, the partitioning was favorable to the beta phase. The Vickers hardness results pointed out a significant improvement from 350HV-2 in the monolithic sample and an increase of around 475-500HV-2 in the composites. The composites with intermediate amounts of TiC (5% and 10%) showed the best tribological properties.