The relationship between TiB2 volume fraction and fatigue crack growth behavior in the in situ TiB2/A356 composites

The relationship between TiB2 volume fraction and fatigue crack growth behavior in the A356 alloy matrix composites reinforced with 3, 5.6, and 7.8 vol% in situ TiB2 particles has been investigated. The mechanisms of crack propagation in the TiB2/A356 composites were also discussed. The results show that the 3 vol% TiB2/A356 composite has nearly the same crack growth behavior as the matrix alloy, while the 5.6 vol% TiB2/A356 composite exhibits a little bit faster crack growth rate. The 7.8 vol% TiB2/A356 composite presents the lowest resistance to crack growth, indicating that the crack growth is accelerated by increasing TiB2 volume fraction. Fractographies reveal that an increase in TiB2 volume fraction results in a change from the formation of striation and slip to the failure of voids nucleation, growth, and coalescence. Cracks tend to propagate within the matrix and avoid eutectic silicon and TiB2 particles in the intermediate ΔK region, while prefer to propagate along interfaces of eutectic silicon and TiB2 particles and link the fractured eutectic silicon particles in the near fractured ΔK region. Furthermore, the propensity for the separation of TiB2 increases with the increase in TiB2 volume fraction. The massive voids caused by fractured eutectic silicon and separated TiB2 particles propagate and coalesce, and then accelerates the crack growth in TiB2/A356 composites.