Sharif Digital Repository

Nanostructured W-20 wt% Cu composite powder was synthesized by mechanical alloying (MA) in an Attritor ball mill. The morphological changes and structural evolution of the composite powder during MA was studied by employing scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), laser particle size analyzer (LPS), inductively coupled plasma (ICP) spectrometry, atomic absorption spectrophotometery (AAS), and the bulk powder density measurement. The results were compared with those obtained from attrition milling of monolithic W and Cu powders processed at the same condition. Whereas the milling mechanism of the monolithic powders follow the ductile (for Cu)... 

The strengthening behavior of particle reinforced metal-matrix composites is primarily attributed to the dislocation strengthening effect and the load transfer effect. To account for these two effects in a unified way, a new multi-scale approach is developed in this paper incorporating the aspect ratio effect into the geometrically necessary dislocation strengthening relationships. By making use of this multi-scale approach, the deformation behavior of metal-matrix composites (MMCs) and metal-matrix nanocomposites (MMNCs) as a function of size, volume fraction, aspect ratio, etc. of the particles has been investigated. Comparison with the previously proposed models and the available... 

The effect of high-energy ball milling on the textural evolution of alumina nanopowders (compaction response, sinter-ability, grain growth and the degree of agglomeration) during post sintering process is studied. The applied pressure required for the breakage of the agglomerates (P y) during milling was estimated and the key elements of compressibility (i.e. critical pressure (P cr) and compressibility (b)) were calculated. Based on the results, the fracture point of the agglomerates decreased from 150 to 75 MPa with prolonged milling time from 3 to 60 min. Furthermore, the powders were formed by different shaping methods such as cold isostatic press (CIP) and uniaxial press (UP) to better... 

The densification response of aluminum powder reinforced with 5 vol.% nanometric alumina particles (35 nm) during uniaxial compaction in a rigid die was studied. The composite powder was prepared by blending and mechanical milling procedures. To determine the effect of the reinforcement nanoparticles on the compressibility of aluminum powder, monolithic Al powder, i.e. without the addition of alumina, was also examined. It was shown that at the early stage of compaction when the rearrangement of particles is the dominant mechanism of the densification, disintegration of the nanoparticle clusters and agglomerates under the applied load contributes in the densification of the composite powder... 

Two different hydroxyapatite based composites reinforced by oxide ceramic (20 wt%) nano crystals were synthesized by high-energy ball milling and sintered by pressure less technique. Alumina and titania nanoparticles as secondary phases improved densification and mechanical behavior of apatite and postponed its decomposition to the tricalcium phosphate (TCP) phases at elevated temperatures. Increasing the relative density of apatite using nano reinforcements leads to enhance the bending strength by more than 40% and 27% (as compared to the pure HA) and increase the hardness from 2.52 to 5.12 (Al2O3 composite) and 4.21 (TiO2 addition) GPa, respectively. Transmission electron microscopy (TEM),... 

Fatigue behavior of quasi-isotropic carbon/epoxy laminates were studied, using six stacking sequences. Experimental results show that there is a significant stacking sequence effect on monotonic and cyclic performance of laminates. Though by changing the stacking sequence the difference between failure strength of the strongest and weakest lay-ups reaches only to ∼ 12 %, the difference between fatigue lives (cycles to failure) increased ∼ 1000% in some loading regions. In addition, both linear and sigmoidal functions were examined to model the fatigue life behavior  

This paper presents experimental results on the synthesis of nanostructured aluminium matrix nanocomposite powders by comilling of nanoscaled SiC and Al2O3 particles and micrometric aluminium powder. The effect of the nanometric reinforcement particles on the mechanical milling (MA) process of the soft matrix was studied by scanning electron microscopy, X-ray diffraction (XRD), transmission electron microscopy, laser particle size analyser and standard metallographic techniques. It was found that at the early stage of milling, the nanoparticles are smeared on the surface of the aluminium powder and thus do not significantly contribute in the MA process. As the milling continues, the hard... 

Nanocrystalline Al-5vol%Al2O3 nanocomposite was synthesised by mechanical milling of a mixture containing nanometric alumina with an average particle size of 35 nm. Morphology of as-synthesised powder was investigated by SEM while crystallite size of Al matrix was determined by XRD analysis. The results confirmed formation of nanocrystalline Al matrix induced by severe plastic deformation during mechanical milling. Nanocomposite bars were produced by hot powder extrusion route. TEM investigation of as-extruded nanocomposite revealed formation of elongated grains along the extrusion direction decorated by alumina nanoparticles. Tensile and compressive properties of as-extruded nanocomposite... 

Two-step sintering (TSS) was applied to suppress the accelerated grain growth of sub-micron (∼150 nm) alumina powder. The application of an optimum TSS regime led to a remarkable decrease of grain size down to ∼500 nm; while the grain size of the full-dense structures produced by conventional sintering ranged between 1 and 2 μm. To find how important the temperatures at sintering steps might be, several TSS regimes were conducted. The results showed that the temperatures at both sintering steps play vital roles in densification and grain growth of the alumina compacts. Based on the results, the optimum regime consisted of heating the green bodies up to 1250 °C (first step) and then holding... 

Hot deformation behaviour of as-extruded Al-5vol% Al 2O 3 nanocomposite was investigated at temperatures range 350 to 450°C and initial strain rates of 5.5 × 10 -4 to 10 -1 s -1 and compared with those of monolithic (unreinforced) aluminium. Both extruded materials exhibited work-softening during hot deformation. The results showed that with the addition of 5 vol% alumina nanoparticles with an average particle size of 35 nm, a significant increase in compressive strength of aluminium was obtained. For instance, at 350°C an abrupt rise of ∼340% in hot strength of the nanocomposite relative to monolithic aluminium was achieved. TEM investigation of microstructure of the nanocomposite after hot... 

The morphological and microstructural changes during mechanical milling of Al powder mixed with 5 vol% nanoscaled alumina particles (35 nm) were studied. The milling was performed in a planetary ball mill under argon atmosphere for various times up to 24 h. The process was also conducted for Al and Al-5 vol% Al2O3 (1 μm) powders to explore the role of reinforcement nanoparticles on the mechanical milling stages. The results showed that the addition of hard particles accelerate the milling process, leading to faster work hardening rate and fracture of the aluminum matrix. Meanwhile, the structural evolution during mechanical milling of the microcomposite powder occurred faster than that of... 

Aluminum powder and various volume fractions of SiC particles with an average diameter of 50 nm were milled by a high-energy planetary ball mill to produce nanocrystalline Al-SiC nanocomposite powders. Double pressing/sintering process was used to consolidate powders to cylindrical specimens. It was shown that a double cycle of cold pressing and sintering can be utilized to obtain high density Al-SiC nanocomposite parts without using a hot-working step. High resolution scanning electron microscopy (HRSEM), X-ray diffraction (XRD) and laser particle size analyzer (PSA) were used to study the morphological and microstructural evolution of nanocomposite powders and bulk samples. The role of... 

Iranian water authority has recently announced that one of the effective ways to avoid unprecedented high water consumption in Iran's agriculture sector is to increase water price. This paper analyzes the feasibility of this policy by using a hydro-economic approach with the aim to consider the role of water pricing in agricultural water management. Such an analysis was conducted through comparing price of water consumed for producing selected agricultural crops (i.e. wheat, sugar beets, onion, tomato, barley, potato, corn, alfalfa hay and watermelon) in a case study on two provinces (East Azarbaijan and West Azarbaijan) in Iran to that in the state of California (CA) in the USA. According... 

In the present work, ultrafine-grained Al-5vol.% Al2O3 nanocomposite was synthesized through mechanical milling followed by direct powder extrusion method. The characteristics of the processed nanocomposite were examined by electron microscopy (SEM and TEM), Xray diffraction (XRD), tensile test and Vickers hardness measurement. It was shown that the addition of the reinforcement nanoparticles accelerates the milling process of the aluminum matrix and enhances the grain refinement of the aluminum matrix. An improved mechanical strength as compared with Al-Al2O3 microcompoiste was obtained. A dimple-type fracture mode was observed, which is a clear evidence of micro-deformation. In addition,...