Sharif Digital Repository

We studied the flow stress and microstructural changes of nanostructured Al-6063 alloy produced by mechanical alloying at various temperatures and strain rates. The analysis of flow curves was performed by a constitutive equation, and the stress exponent and activation energy were determined as functions of strain. The deformation mechanisms were elaborated through microstructural observations by electron backscattering diffraction and transmission electron microscopy. Coarsening of the subgrains and grain growth upon deformation was monitored and related to the Zener–Hollomon parameter  

A molecular dynamics simulation study has been performed to investigate the solidification and remelting of aluminum using Sutton - Chen many body potential. Different numbers of atoms from 108 to 2048 atoms were considered to find an adequate size for the system. Three different cooling and heating rates, i.e. 1 0 12 K/s, 10 13 K/s and 10 14 K/s, were used. The structure of the system was examined using radial distribution function. The melting and crystallization temperatures of aluminum were evaluated by calculating the variation of heat capacity during the phase t ransformation. Additionally, Wendt – Abraham parameters were calculated to determine the glass transition temperature. It is... 

Ultrafine-grained (UFG) Al6063 alloy reinforced with 0.8 vol% nanometric alumina particles (25 nm) was prepared by reactive mechanical alloying and direct powder extrusion. Transmission electron microscopy and electron backscatter diffraction analysis showed that the grain structure of the nanocomposite composed of nanosize grains (<0.1 μm), ultrafine grains (0.1–1 μm) and micronsize grains (>1 μm) with random orientations. Mechanical properties of the material were examined at room and high temperatures by compression test. It was found that the yield strength of the UFG composite material is mainly controlled by the Orowan mechanism rather than the grain boundaries. The deformation... 

Nanostructured AA6063 (NS-Al) powder with an average grain size of ∼100 nm was synthesized by high-energy attrition milling of gas-atomized AA6063 powder followed by hot extrusion. The microstructural features of the consolidated specimen were studied by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques and compared with those of coarse-grained AA6063 (CG-Al) produced by hot powder extrusion of gas-atomized powder (without using mechanical milling). The consolidated NS-Al alloy consisted of elongated ultrafine grains (aspect ratio of ∼2.9) and equiaxed nanostructured grains. A high fraction (∼78%) of high-angle grain boundaries with average... 

Cu/SiC nanocomposite powders with homogeneously distributed nanosize SiC particles were produced by high energy mechanical milling (MM). Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and micro-hardness and density measurements were performed to understand the effects of microstructure and hardness on compaction behavior during MM. The effects of SiC nanoparticle content and mechanical milling time on apparent density (AD) and tap density (TD) of the nanocomposite powders were systematically investigated. The Hausner ratio (HR), defined as TD to AD, were estimated to evaluate friction between the particles. Increasing MM duration and SiC content resulted... 

Effect of Al2O3 nanoparticles (80 nm) on the grain structure and phase formation in Ni-50Ti system during high-energy mechanical alloying (MA) was studied. While the formation of NiTi B2 phase occurs progressively during MA, it is shown that the hard inclusions cause abrupt phase formation at short milling times, particularly at higher nano-Al2O3 contents. High-resolution transmission electron microscopy showed significant grain refinement in the presence of alumina nanoparticles to sizes less than 10 nm, which precedes the formation of semicrystalline structure and reduces the diffusion length and thus accelerates the phase formation. The composite powder reached steady-state MA condition... 

Copper based hybrid composites containing nano-sized silicon carbide and carbon nanotubes reinforcements with minimal porosity were fabricated via mechanical milling followed by hot pressing technique. Microstructures of the powders and consolidated materials were studied using scanning electron microscope, X-ray diffraction, Raman spectroscopy, and scanning transmission electron microscope. Microstructural characterization of the materials revealed that the addition of nanosized silicon carbide reinforcement lowered the grain growth rate and enhanced the homogenization during mechanical milling. Microhardness measurements and compression test showed considerable improvements in mechanical...