Minimum Shared-Power Edge Cut

We introduce a problem called the Minimum Shared-Power Edge Cut (MSPEC). The input to the problem is an undirected edge-weighted graph with distinguished vertices s and t, and the goal is to find an s-t cut by assigning "powers" at the vertices and removing an edge if the sum of the powers at its endpoints is at least its weight. The objective is to minimize the sum of the assigned powers. MSPEC is a graph generalization of a barrier coverage problem in a wireless sensor network: given a set of unit disks with centers in a rectangle, what is the minimum total amount by which we must shrink the disks to permit an intruder to cross the rectangle undetected, i.e. without entering any disc. This is a more sophisticated measure of barrier coverage than the minimum number of disks whose removal breaks the barrier. We develop a fully polynomial time approximation scheme (FPTAS) for MSPEC. We give polynomial time algorithms for the special cases where the edge weights are uniform, or the power values are restricted to a bounded set. Although MSPEC is related to network flow and matching problems, its computational complexity (in P or NP-hard) remains open

Structural relaxation of nanocrystalline PdAu alloy: Mapping pathways through the potential energy landscape

Preparation history and processing have a crucial influence on which configurational state material systems assume. Glasses and nanocrystalline materials usually reside in nonequilibrium states at room temperature, and as a consequence, their thermodynamic, dynamical, and physical properties change with time—even years after manufacture. Such changes, entitled aging or structural relaxation, are all manifestations of paths taken in the underlying potential energy landscape. Since it is highly multidimensional, there is a need to reduce complexity. Here, we demonstrate how to construct a one-dimensional pathway across the energy landscape using strain/volume as an order parameter. On its way to equilibrium, we map the system’s release of energy by calorimetry and the spectrum of barrier heights by dilatometry. The potential energy of the system is reduced by approximately B during relaxation, whereas the crossing of saddle points requires activation energies in the order of 1eV/atom relative to the energy minima. As a consequence, the system behaves as a bad global minimum finder. We also discovered that aging is accompanied by a decrease in the non-ergodicity parameter, suggesting a decline in density fluctuations during aging

Development and research of nanostructured multilayer composite coatings for tungsten-free carbides with extended area of technological applications

This paper discusses aspects of the development of nanostructured multilayer composite coatings (NMCCs) formed using the processes of filtered cathodic vacuum arc deposition (FCVAD) for application to the tungsten-free carbides (cermets) based on TiC-(Ni,Mo) and TiCN-(Ni,Mo) compounds in order to improve cutting properties of tools and to expand the area of their technological application. NMCCs were used not only to improve the physical and mechanical properties of the working surfaces of tools but also to ensure the control over contact processes during cutting. The study has shown that despite their high hardness, thermal stability and resistance to scaling, low tendency to diffusion with the material being machined, and tungsten-free carbides are characterized by relatively low fracture toughness and bending strength, low thermal conductivity. With regard to the above properties, tungsten-free carbides are inferior not only to tungsten WC-Co carbides but also to WC-TiC-Co carbides with binder content of less than 8 % (by weight). Therefore, cutting tools made of tungsten-free carbides have a limited range of technological application in interrupted cutting, machining of hard-to-cut alloys and steels. With respect to this, the paper considers the possibility of directional control over contact processes during cutting with the use of NMCCs to create more balanced properties of tungsten-free carbides with regard to hardness and toughness. This work has developed architecture of three-component nanostructured multilayer composite coatings, the methods for selecting functions and rational component parameters of architecture for tools made of tungsten-free carbides. The developed compositions of NMCCs have improved cutting properties of tungsten-free carbides and expanded the area of their technological application in cutting of heat-treated steels of increased hardness and machining of heat-resistant alloys. © 2016 Springer-Verlag Londo

Melt processing and characterisation of lightweight metal composites reinforced by nanocarbon

Increasing the specific strength of lightweight structural metals such as magnesium, which has two-thirds the density of aluminium, has the potential to significantly widen their application and improve the system energy efficiency and performance across transport and defence industries. Reinforcing magnesium matrices with nano-sized particles to form metal matrix nanocomposites (MMNCs) has shown promise as a way of achieving an increase in specific properties, albeit without the traditionally associated decrease in ductility. This study manufactured and characterised magnesium-based MMNCs to investigate the impact of nanoparticle reinforcements on the mechanical properties of the composites, their microstructure and the theoretical models used for the potential strengthening mechanisms. Multi-walled carbon nanotubes (MWCNTs) can be considered as the ideal reinforcing nanoparticle due to their exceptional mechanical properties, geometry and low density, which can take full advantage of the proposed strengthening mechanisms that occur when nanoparticles are added to a metal matrix. MWCNTs are however notoriously difficult to be homogenously dispersed in a metal matrix and are poorly wetted by molten metal; therefore, nickel coated MWCNTs (NiCNTs) and silicon carbide nanoparticles are also investigated as a potential solution for wettability in this study. Magnesium alloy AZ91D was used as the metal matrix material and the MMNCs were made using metal melt-stirring combined with a nanoparticle pre-dispersion technique. Melt processing and casting techniques are favoured in component manufacturing processes due their scalability and cost-effectiveness. The melt stirring processing parameters such as casting temperature, stirring time and stirring speed were thoroughly examined and modified to successfully produce MMNC samples. The compression mechanical properties of the AZ91D-MWCNT composites showed no change for a range of MWCNT concentrations for the melt stirring processing parameters tested. Theoretical studies of nanoparticle dispersion and wettability combined with electron microscopy of the cross sections of the samples showed that the pre-dispersion and melt stirring processes were insufficient to homogenously disperse the MWCNTs in the metal matrix, therefore NiCNTs were utilised. Whilst no significant differences in compression mechanical properties were seen for the AZ91D-NiCNT samples, a 13% increase in hardness was achieved. An 11% and 20% increase in hardness was achieved for samples of AZ91D composites reinforced with equal concentration of silicon carbide nanoparticles and whiskers, respectively. The poor wettability of MWCNTs by AZ91D melt, apparent from SEM imaging of the composite fracture surfaces, suggested that in order to take advantage of the superior mechanical properties of MWCNTs, a coherent interface between the MWCNT and the Mg matrix is necessary. The nickel coating and silicon carbide nanoparticles, known to wet with molten magnesium, may have provided a coherent interface that resulted in the measured increase in hardness.Open Acces

Atomistic Simulations and Microscopic Experiments to Understand Nanoscale Composition Control

In this dissertation, the possibility of using the quantum mechanics calculation in combination with experimental result is explored, in order to explain experimentally observed phenomena in materials science problems. A series of published works in this theory-experiment combinatory approach will be introduced. The topics include the phase stability of Sb2O4, surface instability of MgB2, the interplay of diffusion and mechanical strengthening effect in multilayer, and latest findings of the syntheses of metallic nano-foams. The theory-experiment combinatory approach has proven to be useful in various materials science problems. The phase transformation trajectory of the Sb2O4 polymorphs, the surface reconstruction pathways of MgB2(0001), and diffusion kinetics of Cr-Cu dilute alloy system are calculated by the density functional theory coupled with the nudged elastic band method. Finally, the syntheses of Cu and Cu-Ni alloy nano-foams are reported, detailing their microstructure and morphology characterized by electron microscopies. The potential application of the theory-experiment combinatory approaches in the nano-foam synthesis is further discussed toward better understanding of the structure-property relations of the metallic nano-foams

Creep Behavior of a Zirconium Diboride-Silicon Carbide Composite and Preliminary ZrB2-WC Quasi-Binary Alloy Development for Long Duty Cycle Aerosurfaces and Structural Propulsion Applications

The mechanical behavior of select ultra-high temperature ceramics were studied for extreme environment aerospace applications. Hot-pressed ZrB2-20 vol% SiC composites and ZrB2-WC quasi-binary alloys were developed for assessing room temperature mechanical properties and creep behavior. A thermochemical model describing alloy phase stability and reaction equilibria, for promoting WC dissolution, is presented. Room temperature structure-property relationships were developed correlating fracture strength and KIC with microstructure constituent size. Flexural creep studies of ZrB2-20 vol% SiC were conducted over the range of 1400°C to 1820°C assessing the macroscopic creep behavior using power-law stress and temperature dependent constants. Inert environment creep experiments were conducted for probing the local grain deformation mechanism in anticipation of bridging the deformation length scales. A two decade increase in creep rate, between 1500 and 1600°C, suggests a clear transition between the low temperature (1400-1500°C) diffusion creep and high temperature (>1600°C) grain boundary sliding creep having stress exponents of unity and 1.7<n<2.2, respectively. A novel indentation deformation mapping experiment clearly defined the local ZrB2 grain boundary sliding event with its components of 80% grain translations and rotations and 20% grain deformation. EBSD and texture theory confirmed the direct observation of ZrB2 grains deforming by dislocation flow, confined to near-grain boundary (mantle) zones, accommodating the grain rotation and translation events. A transition from the grain core to mantle deformation deviated from single crystal behavior as a result of extra geometrically necessary dislocations accommodating the deformation gradient. Microstructure observations shows evidence of <5% and <20% SiC grain deformation, contributing to the macroscopic creep strain, for tension and compression bending fibers, respectively. Cavitation accounts for less than 5% contribution to the accumulated creep strain. Preliminary ZrB2-WC quasi binary alloy creep experiments reveal a decade decrease in the steady state creep rate with a 1.1 mol% increasing WC composition. Improved creep behavior is discussed in the context of solute interactions with accommodation dislocations from grain boundary sliding. Alloy creep rates of 10-7-10-6 s-1 were measured contrasting with 10-5-10-4 s-1 for the ZrB2-SiC composite approaching the design creep rate of 10-8s-1 for long duty cycle aerospace applications.Mechanical Engineering, Department o

Qualification of the Cementitious Material Rock-based Geopolymer in Permanent Plug & Abandonment

When an oil & gas well proves not to be economically favourable anymore, or is technically inviable, the well has served its life and are due to be shut-down and sealed, also referred to as permanent plug & abandonment. Depending on the country one operates in, different local regulatory requirements have to be followed. Various guidelines, set by the regulatory authorities, refer to recognized industry standards. Within permanent plug & abandonment, four recognized industry standards concerning well integrity and barriers are covered; NORSOK D-010 well integrity in drilling and well operations, Oil & Gas United Kingdom well decommissioning guidelines, API wellbore plugging and abandonment and DNV risk based abandonment of wells. Portland cements are the most common used barrier material in today’s permanent plug & abandonment operations. The use of Portland cements have faced excessive well integrity problems since it was first introduced, where common issues has been development of micro-annuli over time due to shrinkage, mechanical failure, and degradation at elevated temperatures. Use of Portland cement has historically been inexpensive and further satisfying fundamental barrier material requirements, despite its weaknesses. The oil & gas industry has researched substitutes in the last decade, but few alternatives have had commercial success. Through a technical qualification process based on review and comparison of industry accepted standards, the rock-based geopolymer cement is proved to be acceptable as a barrier material in the oil & gas industry. Geopolymers naturally expanding properties together with a permeability similar to shale is advantageous to seal the wellbore against leakage to the external environment. Combined with an environmental footprint that is less than half of Portland cements, geopolymer-based cement shows many benefits compared to Portland cement and can thus be a viable barrier substitution material

Investigation on the stress corrosion cracking susceptibility of an alloy 718 prepared by laser powder bed fusion assessed by microcapillary method

openThe study focused on evaluating the stress corrosion cracking (SCC) behaviour of alloy 718 processed by laser powder bed fusion (L-PBF), also called selective laser melting (SLM), with different laser powers, in a solution containing chloride, using the microcapillary method under constant tensile load. An analysis of the surface defects, hardness, and microstructure of the samples under examination was carried out and a correlation was sought between these results and those obtained by electrochemical investigations. The polished surface of the samples was observed under the optical microscope, as was the electro-etched surface. Vickers hardness measurements were carried out. Electrochemical polarization tests were used to evaluate the resistance of the passive layer on the surface of the material. Potentiodynamic and galvanostatic polarization tests were performed. The tests were carried out on as-printed samples and samples under tensile load. The results were compared with conventionally produced counterparts. The microstructure and stress corrosion cracks were observed under the scanning electron microscope (SEM). The 115W laser power produced samples with a lower defect density; therefore, the highest resistance to localized corrosion was found. L-PBF samples under tensile load showed corrosion and SCC resistance superior to that of conventional material. In the L-PBF samples, submicronic cracks were detected adjacent to the boundaries of the subgrain, and the mechanisms that led to their appearance were explained as a synergistic effect of various microstructural factors, specifically, the greater corrosion resistance of the subgrain boundary and the high concentration of dislocations in the adjacent area. The fine microstructure of the L-PBF samples generated much smaller cracks than those observed on the conventional material, which explains the increased resistance to SCC observed in electrochemical tests.The study focused on evaluating the stress corrosion cracking (SCC) behaviour of alloy 718 processed by laser powder bed fusion (L-PBF), also called selective laser melting (SLM), with different laser powers, in a solution containing chloride, using the microcapillary method under constant tensile load. An analysis of the surface defects, hardness, and microstructure of the samples under examination was carried out and a correlation was sought between these results and those obtained by electrochemical investigations. The polished surface of the samples was observed under the optical microscope, as was the electro-etched surface. Vickers hardness measurements were carried out. Electrochemical polarization tests were used to evaluate the resistance of the passive layer on the surface of the material. Potentiodynamic and galvanostatic polarization tests were performed. The tests were carried out on as-printed samples and samples under tensile load. The results were compared with conventionally produced counterparts. The microstructure and stress corrosion cracks were observed under the scanning electron microscope (SEM). The 115W laser power produced samples with a lower defect density; therefore, the highest resistance to localized corrosion was found. L-PBF samples under tensile load showed corrosion and SCC resistance superior to that of conventional material. In the L-PBF samples, submicronic cracks were detected adjacent to the boundaries of the subgrain, and the mechanisms that led to their appearance were explained as a synergistic effect of various microstructural factors, specifically, the greater corrosion resistance of the subgrain boundary and the high concentration of dislocations in the adjacent area. The fine microstructure of the L-PBF samples generated much smaller cracks than those observed on the conventional material, which explains the increased resistance to SCC observed in electrochemical tests