Visual assessment of special rod-like α-Ti precipitates within the in situ TiC crystals and the mechanical responses of titanium matrix composites

TiC crystals are an ideal and irreplaceable reinforcement in metal matrix composites (MMCs). The C, Ti vacancies and dislocations provide elemental diffusion channels, and thermal processing provides sufficient activation energy to induce a special rod-like alpha-Ti phase, even the phase transformation of beta-Ti ->alpha-Ti within the in situ TiC crystals. The ultimate tensile strength of the 8 vol % TiC/Ti6Al4V at 723K increase by approximately 12.6%, and the fatigue crack at 723K grows much faster than the Ti6Al4V matrix, which is nearly 2 times. The rod-like alpha-Ti imposes an additive effect on the stress concentration within the TiC crystals during deformation, and results in the formation of microcracks and microvoids within the TiC crystals, ultimately, the premature fracture of TiC crystals, thus, reducing the strengthening effect on the TMCs and accelerating the fatigue crack propagation. To prevent effectively rod-like alpha-Ti precipitates within the TiC crystals and improve the tensile properties and fatigue performances, the TiC content should be reduced as much as possible (lower than 2.5 vol %), and its size should be as small as possible (smaller than 10 mu m). Our work provides strong evidence for atomic diffusion within the in situ TiC crystals and paves the way to make full use of the strengthening effect of TiC crystals on MMCs.

Automated summary

TiC crystals are important reinforcements in metal matrix composites, but they can accelerate fatigue crack propagation and lead to premature fracture. To improve performance, reducing TiC content and minimizing crystal size are recommended.