On the fracture toughness behavior of in-situ Al-Ti composites produced via mechanical alloying and hot extrusion

Abstract

Mechanical alloying and hot extrusion techniques were employed to produce in-situ fully dens Al-Ti composites without applying any heat treatment. The focus of this study is the investigation of fracture toughness in produced composites in order to improve it and to explain the mechanisms responsible for the fracture. The morphology of milled powders was studied by Scanning Electron Microscopy (SEM). The microstructure of the extruded composites was examined by Optical Microscopy (OM) and Field Emission Scanning Electron Microscopy (FESEM). Moreover, the density and hardness of samples were investigated. Three point bending test of the single edge notched beam (SENB) samples was applied to examine the fracture toughness behavior under the quasi-static condition. In order to find the effective mechanism of fracture, the crack propagation paths and the fracture surfaces were studied using optical microscopy and SEM, respectively. The results showed that the morphology of the milled powders has a significant impact on the final mechanical properties of the extruded samples. The best bonding between particles corresponds to the powders with equiaxial morphology. The high energy vibratory mill was used to fabricate Al-20Ti-4hrV nano composite sample while low energy planetary mill was used to produce Al-20Ti-20hrP and Al-10Ti-60hrP composites. The results indicated the improved distribution of particles with steady state equiaxial morphology and the formation of in-situ nanometric intermetallics in the ultrafine Al matrix in Al-20Ti-4hrV sample. Accordingly, the highest density, the best mechanical properties and fracture toughness were obtained in Al-20Ti-4hrV sample. The use of low energy planetary mill led to the formation of flattened particles, low density and subsequently, poor mechanical properties in Al-20Ti-20hrP and Al-10Ti-60hrP composite samples. The fracture mechanism of Al-20Ti-4hrV sample was distinguished as micro void coalescence (MVC) whereas those of Al-20Ti-20hrP and Al-10Ti-60hrP samples were fracture and debonding of Ti particles, respectively. In addition, the good agreement between the experimental results of fracture toughness and the Rice & Johnson model predictions attained which confirms that this model is applicable for describing the fracture toughness of these composites.