Production and characterization of mechanically alloyed TiC and Y2O3 particle reinforced AA7075 metal matrix nanocomposites.

This study's main purpose is to utilize the mechanical alloying (ball milling) method to produce AA7075 Al alloy matrix nanomaterials with different reinforcement types (TiC and Y2O3), different reinforcement ratios (0.5%, 1 and 5 wt.%), and different ball milling times (0.25, 1, 1.5, 2 and 10 hours). Also, it aims to perform a detailed characterization of both milled powders and composites by consolidating the produced powders with hot pressing. The same procedures were also applied to the as-received AA7075 matrix powder and the sample produced from this powder for comparison purposes. Effect of reinforcement type, reinforcement ratio, and milling time on the morphology and crystallographic properties of powders was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), particle size analysis, and X-ray diffraction (XRD) analysis. Considering the effects of different parameters, it was determined that the milling time was the most influential parameter on the morphological and crystallographic properties of the powders. In each powder group, which was milled in two different reinforcement types, the initially spherical powder shape was preserved in short milling times, while the milling time increased to a specific value (1 and 1.5 hours) depending on the type of reinforcement, it turned into a flake-like morphology. These flakes were fractured randomly into smaller particles with the increasing milling time. Overall, it was observed that the average particle sizes in both powder groups decreased with increasing milling time, and the smallest particle sizes in each group were achieved after 10 hours of milling. The particle size of the as-received AA7075 powder was measured as 46 µm. However, the 10 h milled powder's particle size varied between 16-20 µm depending on the powder group. This is because increasing small and irregular particles within the ductile matrix phase due to excessive plastic deformation in the last milling stage (10 h). Besides, both nanoparticles' homogeneous dispersion in the matrix was attained in the last stage of milling. This period could be varied between 1.5 and 2 hours based on the reinforcement type and ratio. However, in the early stages of milling (0.25 and 1 hour), it was observed that the nanoparticles mostly agglomerate on the exterior surface of the matrix. According to XRD analysis, it was observed that a monolithic decrease in peak intensity with visible peak broadening attributed to solid solution mechanism, distribution of reinforcement materials or minor elements in the lattice, and stress accumulation on the lattice, triggered by commonly known ball milling phenomena because of the effect of excessive plastic deformation. For each group, a decrement in crystallite size along with an increment in lattice strain and dislocation density were observed according to crystallographic properties evaluated by using the XRD curves of the powders. Among these groups, the lowest crystallite size, the highest lattice strain, and the highest dislocation density was obtained after 10 h of milling. It was determined that the most effective parameter on crystallographic properties was the milling time, which is similar to the change in particle sizes. After the powder characterization, consolidation of the milled powders was performed via hot pressing under 400 MPa, and 430 oC for 30 min and MMCs were successfully produced. The effect of different production parameters (reinforcement type, ratio, and milling time) on microstructure and mechanical properties of MMCs was evaluated by optical microscope, SEM, TEM, EDS, relative density, and hardness results. Results revealed that the 10 h milled composites were almost 3 times harder than the initial AA7075 alloy due to the dispersion hardening resulting from the nanoparticles' homogeneous dispersion within the matrix as a result of excessive plastic deformation with prolonging milling time. The hardness values of initial AA7075 alloy, 10 h milled 5 wt.% TiC reinforced and 5 wt.% Y2O3 reinforced composites were measured as 94, 280, and 260 HB, respectively. However, the same singularity was not matter on the relative density results. It was determined that there is a significant decrease in density values for each MMC group with increasing milling time. Besides, increasing reinforcement ratio caused a decrement in the density of MMCs. In conclusion, the achievement of the best packing density and enhancement of the structural integrity is associated not only with the uniform distribution of nanoparticles within the matrix but also with obtaining the optimal particle size range and particle morphology. According to density results, it was found that the highest density values for TiC and Y2O3 reinforced MMCs are 1 and 1.5 hours of ball milling, respectively. Considering the published studies in the literature about the AA7075 matrix-based composites, it was noticed that produced MMCs in this thesis exhibit superior hardness results than the composite materials produced by the traditional cold-hot press and modern production techniques. As a result, it was concluded that the obtained findings in this thesis had beneficial effects on the mechanical properties of nanoparticle reinforced Al-based composites.