Nasser Al-Aqeeli

Cyclic hardening of metallic glasses under Hertzian contacts: Experiments and STZ dynamics simulations

A combined program of experiments and simulations is used to study the problem of cyclic indentation loading on metallic glasses. The experiments use a spherical nanoindenter tip to study shear band formation in three glasses (two based on Pd and one on Fe), after subjecting the glass to cycles of load in the nominal elastic range. In all three glasses, such elastic cycles lead to significant increases in the load required to subsequently trigger the first shear band. This cyclic hardening occurs progressively over several cycles, but eventually saturates. The effect requires cycles of sufficient amplitude and is not induced by sustained loading alone. The simulations employed a new shear transformation zone (STZ) dynamics code to reveal the local STZ operations that occur beneath an indenter during cycling. These results reveal a plausible mechanism for the observed cyclic hardening: local regions of confined microplasticity can develop progressively over several cycles, without being detectable in the global load–displacement response. It is inferred that significant structural change must attend such microplasticity, leading to hardening of the glass.

Influence of dust and mud on the optical, chemical, and mechanical properties of a pv protective glass.

Recent developments in climate change have increased the frequency of dust storms in the Middle East. Dust storms significantly influence the performances of solar energy harvesting systems, particularly (photovoltaic) PV systems. The characteristics of the dust and the mud formed from this dust are examined using various analytical tools, including optical, scanning electron, and atomic force microscopies, X-ray diffraction, energy spectroscopy, and Fourier transform infrared spectroscopy. The adhesion, cohesion and frictional forces present during the removal of dry mud from the glass surface are determined using a microtribometer. Alkali and alkaline earth metal compounds in the dust dissolve in water to form a chemically active solution at the glass surface. This solution modifies the texture of the glass surface, thereby increasing the microhardness and decreasing the transmittance of the incident optical radiation. The force required to remove the dry mud from the glass surface is high due to the cohesive forces that result from the dried mud solution at the interface between the mud and the glass. The ability altering the characteristics of the glass surface could address the dust/mud-related limitations of protective surfaces and has implications for efficiency enhancements in solar energy systems.

Structure of Mechanically Milled CNT-Reinforced Al-Alloy Nanocomposites

Mechanical milling of aluminum-based two Al-Si-Mg alloys were reinforced with multiwalled carbon nanotubes (CNTs) using a planetary ball mill, forming a nanocomposite structure. The milling took place for different timings of 1, 3, and 5 h to follow the dispersion and distribution of CNTs. The effects of concentration of CNTs (ranging from 0.5 to 2 wt%) and milling periods on the final microstructure of the composites were investigated. The mechanically milled powders were investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), and particle size analyzer. The crystallite size and the accumulating internal strains increased with milling time, and the smallest grain sizes achieved were about 100 nm. The results showed that milling was able to adequately disperse CNTs in the aluminum alloy matrices at different mechanical milling times and range of CNT concentrations.

Characterization of Nanoreinforcement Dispersion in Inorganic Nanocomposites: A Review

Metal and ceramic matrix composites have been developed to enhance the stiffness and strength of metals and alloys, and improve the toughness of monolithic ceramics, respectively. It is possible to further improve their properties by using nanoreinforcement, which led to the development of metal and ceramic matrix nanocomposites, in which case, the dimension of the reinforcement is on the order of nanometer, typically less than 100 nm. However, in many cases, the properties measured experimentally remain far from those estimated theoretically. This is mainly due to the fact that the properties of nanocomposites depend not only on the properties of the individual constituents, i.e., the matrix and reinforcement as well as the interface between them, but also on the extent of nanoreinforcement dispersion. Therefore, obtaining a uniform dispersion of the nanoreinforcement in the matrix remains a key issue in the development of nanocomposites with the desired properties. The issue of nanoreinforcement dispersion was not fully addressed in review papers dedicated to processing, characterization, and properties of inorganic nanocomposites. In addition, characterization of nanoparticles dispersion, reported in literature, remains largely qualitative. The objective of this review is to provide a comprehensive description of characterization techniques used to evaluate the extent of nanoreinforcement dispersion in inorganic nanocomposites and critically review published work. Moreover, methodologies and techniques used to characterize reinforcement dispersion in conventional composites, which may be used for quantitative characterization of nanoreinforcement dispersion in nanocomposites, is also presented.

Matrix Structure Evolution and Nanoreinforcement Distribution in Mechanically Milled and Spark Plasma Sintered Al-SiC Nanocomposites

Development of homogenous metal matrix nanocomposites with uniform distribution of nanoreinforcement, preserved matrix nanostructure features, and improved properties, was possible by means of innovative processing techniques. In this work, Al-SiC nanocomposites were synthesized by mechanical milling and consolidated through spark plasma sintering. Field Emission Scanning Electron Microscope (FE-SEM) with Energy Dispersive X-ray Spectroscopy (EDS) facility was used for the characterization of the extent of SiC particles’ distribution in the mechanically milled powders and spark plasma sintered samples. The change of the matrix crystallite size and lattice strain during milling and sintering was followed through X-ray diffraction (XRD). The density and hardness of the developed materials were evaluated as function of SiC content at fixed sintering conditions using a densimeter and a digital microhardness tester, respectively. It was found that milling for 24 h led to uniform distribution of SiC nanoreinforcement, reduced particle size and crystallite size of the aluminum matrix, and increased lattice strain. The presence and amount of SiC reinforcement enhanced the milling effect. The uniform distribution of SiC achieved by mechanical milling was maintained in sintered samples. Sintering led to the increase in the crystallite size of the aluminum matrix; however, it remained less than 100 nm in the composite containing 10 wt.% SiC. Density and hardness of sintered nanocomposites were reported and compared with those published in the literature.

The synthesis of nanostructured WC-based hardmetals using mechanical alloying and their direct consolidation

Tungsten carbide- (WC-) based hardmetals or cemented carbides represent an important class of materials used in a wide range of industrial applications which primarily include cutting/drilling tools and wear resistant components. The introduction and processing of nanostructured WC-based cemented carbides and their subsequent consolidation to produce dense components have been the subject of several investigations. One of the attractive means of producing this class of materials is bymechanical alloying technique. However, one of the challenging issues in obtaining the right end-product is the possible loss of the nanocrystallite sizes due to the undesirable grain growth during powder sintering step. Many research groups have engaged in multiple projects aiming at exploring the right path of consolidating the nanostructured WC-based powders without substantially loosing the attained nanostructure. The present paper highlights some key issues related to powder synthesis and sintering of WCbased nanostructured materials using mechanical alloying. The path of directly consolidating the powders using nonconventional consolidation techniques will be addressed and some light will be shed on the advantageous use of such techniques. Cobaltbonded hardmetals will be principally covered in this work along with an additional exposure of the use of other binders in the WCbased hardmetals.

Processing of CNTs reinforced al-based nanocomposites using different consolidation techniques

In this work, the development of two types of Al-based alloys with different concentrations of Si reinforced withMWCNTs at 0.5- 2.0wt% is presented. Sonication of the CNTs in ethyl alcohol was carried out for dispersion, and the mixtures were ball milled for 1, 3, and 5 hrs. SEM/EDS were used to study the morphology and the effects of changing milling parameters in addition to changes caused due to increasing concentration of the CNTs. Furthermore, three sintering techniques, namely, Spark Plasma Sintering (SPS), Microwave Sintering (µWS), and Hot Isostatic Press Sintering (HIP) were employed to consolidate the ball milled powders at varying temperatures of 400, 450, and 500°C. It was found that SPS consolidated samples showed the most promising results amongst the three with the highest hardness values; around 100% densification, as well as the finest microstructure. On the other hand, microwave sintered samples showed the least appealing results, this could be attributed to the poor temperature distribution and the pressureless nature of the technique. A sintering temperature of 500°C was found to be the most suitable for these types of alloys.

Formation of an amorphous phase and its crystallization in the immiscible Nb–Zr system by mechanical alloying

Mechanical alloying of binary Nb-Zr powder mixtures was carried out to evaluate the formation of metastable phases in this immiscible system. The milled powders were characterized for their constitution and structure by X-ray diffraction and transmission electron microscopy methods. It was shown that an amorphous phase had formed on milling the binary powder mixture for about 10 h and that it had crystallized on subsequent milling up to 50–70 h, referred to as mechanical crystallization. Thermodynamic and structural arguments have been presented to explain the formation of the amorphous phase and its subsequent crystallization.

Synthesis, characterisation and mechanical properties of SiC reinforced Al based nanocomposites processed by MA and SPS

Abstract Al based alloys reinforced with different amounts (5, 12 and 20 wt-%) of nanosized SiC particulates were synthesised by mechanical alloying and consolidated by the spark plasma sintering (SPS) technique. The distribution of the reinforcement phase in the composite was evaluated as a function of the milling time and the amount of SiC. The processed materials were characterised by scanning electron microscopy and energy dispersive spectroscopy for the morphology and composition and X-ray diffraction. Continuous reduction in crystallite size was observed as milling progressed and after milling for 20 h the resulting powders reached a grain size of <100 nm. These Al–SiC composites were successfully consolidated by the SPS method at different sintering temperatures of 400, 450 and 500°C. It is suggested that a higher hardness can be achieved even at 20 wt-%SiC when a higher sintering temperature, for example, above 500°C, is used.

Environmental dust effects on aluminum surfaces in humid air ambient

Environmental dusts settle on surfaces and influence the performance of concentrated solar energy harvesting devices, such as aluminum troughs. The characteristics of environmental dust and the effects of mud formed from the dust particles as a result of water condensing in humid air conditions on an aluminum wafer surface are examined. The dissolution of alkaline and alkaline earth compounds in water condensate form a chemically active mud liquid with pH 8.2. Due to gravity, the mud liquid settles at the interface of the mud and the aluminum surface while forming locally scattered patches of liquid films. Once the mud liquid dries, adhesion work to remove the dry mud increases significantly. The mud liquid gives rise to the formation of pinholes and local pit sites on the aluminum surface. Morphological changes due to pit sites and residues of the dry mud on the aluminum surface lower the surface reflection after the removal of the dry mud from the surface. The characteristics of the aluminum surface can address the dust/mud-related limitations of reflective surfaces and may have implications for the reductions in the efficiencies of solar concentrated power systems.