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

Abstract

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 α-Ti phase, even the phase transformation of β-Ti→α-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 α-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 α-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 μm). 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. • C vacancies, Ti vacancies, and dislocations within the TiC crystals serve as atomic diffusion channels, and the diffusion of Al, V and C atoms result in special rod-like α-Ti precipitates within in situ TiC crystals. • Phase transformation of β.→α occurs in the TiC crystals, and they obey ORs <11 2 ‾ 0> α //<110> TiC //<111> β and {0001} α //{111} TiC //{110} β . • The rod-like α-Ti precipitates impose an additive effect on the stress concentration within the TiC crystals, reducing the strengthening effect on the TMCs. • Different deformation abilities, coefficients of thermal expansion, and hardness resulted in the formation of microvoids and microcracks within the TiC crystals, even the cleavage fracture along {100}, {110} and {111} planes.