Effect of N₂ on microstructure and mechanical properties of additive manufactured titanium matrix composites

Titanium alloys are extensively used in areas such as aerospace and biomedicine. However, inadequate mechanical qualities (e.g., low hardness and poor wear resistance) and poor machinability limit the scope of their application expansion. To directly manufacture near-net-shape titanium alloy components with complicated architectures and improved performances, titanium matrix composites (TMCs) were fabricated based on the Ti–N reaction by introducing nitrogen gas (N₂) in the process of selective laser melting (SLM) of Ti6Al4V. The formation principle of this novel method is as follows: Laser-induced N₂ decomposition near the melt pool of Ti6Al4V generates N atoms or ions, which react with Ti atoms in the melt pool to in-situ synthesize TiN-reinforcement particles. In turn, TiN-reinforced Ti6Al4V matrix composites are manufactured layer-by-layer. This approach has some important advantages, which are as follows: Above all, in-situ gas-liquid synthesized reinforcements are equally dispersed due to N₂, good diffusivity and dispersibility. Furthermore, extremely small gas molecules have the potential to produce nanoscaled reinforcement. Moreover, the in-situ reaction mode produces a clean interface and strong interfacial bonding between the matrix and reinforcement. In this study, TMCs were prepared by SLM in three different N₂ volume fractions of 3%, 10% and 30%, which were compared to the Ti6Al4V alloy fabricated in an argon atmosphere. The microstructures were observed by SEM. Interstitial solid solutions of N in the Ti lattices were confirmed by XRD patterns. The presence of TiN was verified by EDS. The high-resolution transmission electron microscope (HR-TEM) picture indicated that the matrix and reinforcement were TiN and Ti, respectively. Such in situ synthesized nitride reinforcements were uniformly distributed; in particular, numerous nanoscale reinforcements were uniformly dispersed in the composites manufactured in low volume fraction N₂ atmospheres (3% and 10%). Additionally, the improved strength and plasticity were simultaneously achieved in a diluted N₂ atmosphere (10%). The effect of varying N₂ concentrations on the microstructure and mechanical characteristics of the TMCs was investigated. The content of the TiN particles increased with increasing N₂ concentration due to the increased availability of N atoms and ions for nucleation and growth of the reinforcement. Nevertheless, the TMC produced in a high N₂ atmosphere (30%) demonstrated degradation of the mechanical properties (particularly plasticity and ultimate strength) because of the presence of excessive N solid solutions and brittle TiN particles. The strengthening mechanisms were primarily grain refinement strengthening of the Ti matrix due to the "pinning" effect of TiN particles, the precipitation hardening and dispersion strengthening effects of uniformly distributed reinforcement particles, interstitial solid solution strengthening caused by the results from the portion of N in the Ti lattices, Orowan strengthening caused by the in-situ synthesis of nanoscaled reinforcements, and the load transfer effect from Ti matrix to TiN reinforcements because of the clean interface.