Influence of titanium diboride reinforcements on the microstructure, mechanical properties and fracture behavior of cast zinc-aluminum composites

The objective of the present study was to examine the relationships between the microstructure, mechanical properties and failure mechanisms which control the performance of TiB₂ particle reinforced Zn-8 wt.% Al (ZA-8) composites. Unreinforced ZA-8 and four composite heats with nominal particle contents of 5, 10, 20 and 30 vol.% were prepared using stir casting and permanent mold casting techniques. Tensile, compression, Charpy impact and short bar chevron-notch fracture toughness tests were conducted at room temperature. The microstructure and fracture surfaces of the specimens were characterized using optical, scanning electron, transmission electron and focused ion beam microscopy. These experimental results were compared with relevant MMC models and with published data on Zn-based composites.; The TiB₂/ZA-8 composites had a homogeneous distribution of particles in a matrix similar to unreinforced ZA-8. The matrix grain size decreased as the TiB₂ content increased. No interfacial reaction products were detected. A good correlation was obtained between the measured composite stiffness and the values predicted using an Eshelby model. The yield strength and work hardening rate of TiB₂/ZA-8 were only marginally higher than those of unreinforced ZA-8. The strength properties did not increase with the particle content as predicted by the Eshelby model. The premature activation of relaxation processes limited the extent to which particle reinforcement improved the strength of the ZA-8 matrix. Temperature-activated relaxation processes such as diffusion and dislocation motion can occur readily in ZA-8 at room temperature, which corresponds to 0.44 T_M.; Particle reinforcement of the ZA-8 alloy produced a degradation of the toughness which was sensitive to the loading rate. The Charpy energy of TiB ₂/ZA-8 was up to 85% smaller than that of ZA-8, while the corresponding loss in fracture toughness was only 15%. The moderate fracture toughness decrease was explained by the similar yield and work hardening characteristics of ZA-8 and TiB₂/ZA-8 and by the important contribution of matrix failure to the crack propagation process.; Failure of the TiB₂/ZA-8 composites was dominated by cleavage of the ZA-8 matrix. Cleavage cracks were initiated by plastic deformation within the grains and were not associated with cracked interfaces or particles. These observations confirm that ZA-8 and TiB₂ form strong interfaces. The facet size was commensurate with the matrix grain size and interparticle spacing. The TiB₂ content had a significant influence on the cleavage facet size and on the extent of particle cracking.