Volume 102, Issue 4

In the present work, the processing parameters which govern the evolution of microstructure during rotary swaging and intermediate/subsequent heat treatments in copper rods were studied. Copper ingots with an initial diameter of 40 mm were reduced to a final diameter of 11.7 mm by rotary swaging. Processing sequences were applied with different intermediate anneals and various final heat treatments. The resulting microstructures were characterized using orientation imaging microscopy, optical microscopy and hardness measurements. Special emphasis was placed on the evolution of microstructure with respect to the radial and longitudinal position in the rod. Most importantly, microstructural evidence for torsional loading during swaging was found, and a spiral grain morphology was observed. Moreover, localized deformation events were identified and evidence for abnormal grain growth was found. Finally, a combination of swaging and heat treatment parameters was identified which allowed a homogeneous grain structure to be produced.

By linking thermodynamic and atomic mobility databases with two-dimensional phase-field simulation, the evolution of interdiffusion microstructures in a series of Ni–Al diffusion couples associated with the γ, γ′, and β-phases was studied. The formation and subsequent growth of the γ′-phase layer in β/γ and γ′ + β/γ diffusion couples reproduced the experimental observations well. Moreover, the effect of coherent strain on the γ – γ′ microstructural evolution, as well as that of an external compressive force on the γ + γ′/γ + γ′ diffusion couple, was investigated. The phase-field simulated concentration profiles of some of the Ni–Al diffusion couples were also compared with the corresponding experimental data and the results of one-dimensional DICTRA (DIffusion Controlled TRAnsformations) simulations. A discussion of the rafting direction was also made by comprehensively comparing the phase-field simulations with the predicted results from an elastic model.

The thermal and mechanical properties of Au, Pd, Ag, Pt pure metals and their alloys Au–Pd and Ag–Pt are studied using molecular dynamics simulations. The Finnis–Sinclair potential is used to describe the atomic interactions. Lattice constants and elastic constants of Au 3 Pd, AuPd, AuPd 3 , Ag 3 Pt, AgPt and AgPt 3 at different temperatures are predicted for the first time. The melting temperatures of these alloys are also calculated. Furthermore, the values of lattice constants, elastic constants and melting temperatures of pure metals Au, Pd, Ag, and Pt are calculated for comparison. By comparing the simulation results of pure metals and alloys, it is found that for Au 3 Pd, AuPd, AuPd 3 and Ag 3 Pt, lattice constants, elastic constants and melting temperatures lie between those of their components. For AgPt and AgPt 3 , the values of shear modulus C 44 and melting temperatures are higher than those for both Ag and Pt pure metals.

Pyramidal lattice core sandwich structures with millimeter scale hollow trusses have been assembled from 304 stainless steel tubes and bilayer face sheets, and bonded using a vacuum brazing approach. Rigid, large interfacial area nodes between the trusses and face sheets could be made by this approach. The through-thickness compression and transverse shear stiffness and strengths of these structures have been measured and compared with analytical predictions based upon plastic yielding and the various modes of lattice strut buckling. The compressive and shear strengths of hollow pyramidal lattices with relative densities of 1 to 6 % were 3 to 5 times those of solid pyramidal lattices of equivalent relative density and were accompanied by significant strength retention of the post buckled structures resulting in very high specific energy absorption.

Orthorhombic polycrystalline Ca 2 Si sintered compact bulk crystals were synthesized via a two-step treatment, and the experimental evaluation of the electrical and thermoelectric properties of Ca 2 Si is reported for the first time. First, single-phase Ca 2 Si powders were synthesized by the heat treatment of Mg 2 Si powders under a Ca vapor. Then Ca 2 Si sintered compacts were synthesized by the spark plasma sintering technique using the Ca 2 Si powders. It was found that single-phase Ca 2 Si sintered compacts were obtained when the sintering was carried out at temperatures lower than 673 K with high pressure of 300 MPa. The structural, electrical and thermoelectric properties of the Ca-silicide sintered compacts and their dependence on process conditions are discussed.

In the present investigation a finite element study is performed concerning the local contact behavior of spherical powders in unit cell arrangements of composite powder materials. In particular, die compaction of unit cells of regular body-centered cubic lattices have been studied and the numerical results are compared to previous analytical and numerical findings describing, in particular, local contact but also global behavior in composite powder compaction. The foundation for the analytical results rests on previously published results where approximate composite contact relations are derived from self-similarity considerations in a situation where the powder particles are of the same material. Results pertinent to isostatic compaction are also presented for comparative reasons. The finite element calculations are continued up to full density of solid material in the unit cells.

We synthesized a new kind of solid polymer electrolyte with polyethylene oxide (PEO) as a host, lithium triflate (LiCF 3 SO 3 ) as a salt, mixtures of ethylene carbonate (EC) and dibuthyl phthalate (DBP) as a double plasticizer and nanosilica (SiO 2 ) as a filler using the solution casting technique. Nanosilica was produced through mechanical milling with a size range of 60 – 80 nm as shown by transmission electron microscopy. X-ray diffraction patterns confirmed the complex formation of the PEO – LiCF 3 SO 3 – EC – DBP – SiO 2 system. The optimum conductivity value at room temperature was 5.1 · 10 −3 S cm −1 for 4 wt.% of SiO 2 . Differential scanning calorimetry results show that the glass transition temperature of the composite polymer electrolyte was found to increase when SiO 2 was added. Addition of salt and plasticizers decreases the Young's modulus of the polymer electrolyte. However, the addition of 4 wt.% SiO 2 increases the Young's modulus value from 5.1 · 10 − 2 to 1.08 · 10 − 1 MPa.

In this study, AISI 304 (X5CrNi18-10, material identification number 1.4301) austenitic stainless steels, 3.0 mm thick, were joined by friction stir welding by applying different tool rotation speeds and traverse speeds, compressive forces and tool angles. The strength of the welded joints was improved by selecting suitable welding parameters. The maximum notch impact toughness was achieved on samples produced with 950 rpm rotation speed, 60 mm min −1 traverse speed, 9 kN compressive force, and 1.5° tool tilt angle. The maximum tensile strength of the weld zone was obtained on samples welded with 47.5 mm min −1 traverse speed, a rotational speed of 750 rpm, compressive force of 9 kN and tool tilt angle of 1.5°. The traverse speed of 47.5 mm min −1 was found to optimize the results of tensile strength and impact tests. Fine-grained microstructures occurred in the welded area. The weld joints obtained with friction stir welding have lower tensile strength compared to that of the base material. The experimental results indicate that AISI 304 austenitic stainless steels can be successfully joined considering both the strength of the welded joint and the appearance of the welding bead by selecting proper tool material and welding parameters using friction stir welding.

Two different approaches are used in the evaluation of Fe 2 B growth and its kinetics over the surface of AISI 1018 steel. The first approach considers a diffusion model based on the mass balance at the growing Fe 2 B/substrate interface. In the second approach, the method of dimensional analysis was applied to obtain an expression for the boride layer thickness as a function of dependence parameters involved in the boriding process. The Fe 2 B layers were formed by the paste boriding process at temperatures of 1 123 to 1 273 K with 2, 4, 6, and 8 h of exposure using a boron carbide paste thickness of 4 mm over the surface of the steel. Another set of boriding temperatures with 5 h of exposure were used on the surface of the steel to compare the experimental values of Fe 2 B layer thicknesses with the results obtained by both diffusion models. The predicted values showed good agreement with the experimental boride layer thicknesses achieved in this work.

A Zr 50 Cu 40 Al 10 (at.%) alloy was atomized by high pressure Ar gas, and the oxygen content, morphology, microstructure, phase composition, glass-forming ability and thermal stability of the as-atomized powders were investigated. Most of the powders are spherical in shape, but the surface morphologies are different for powders of different sizes. Powders with a size below 38 μm show an amorphous characteristic. The presence of oxygen in the metallic glass powders leads to heterogeneous nucleation of crystalline phases, and the smallest powders have the highest cooling rate, so even with oxygen contamination, it is possible to prevent the crystallization process. Oxygen decreases the thermal stability and the glass-forming ability of the metallic glass powders compared with the Zr 50 Cu 40 Al 10 bulk metallic glass. The metallic glass powders also show a high thermal stability indicated by T rg , γ, and δ parameters.

Mechanochemical synthesis of lead selenide PbSe nanoparticles was performed by high-energy milling of lead and selenium powder in a laboratory planetary ball mill and in an industrial eccentric vibratory mill. Structural properties of the synthesized lead selenide were characterized using X-ray diffraction that confirmed crystalline nature of PbSe nanoparticles. The average size of PbSe crystallites of 37 nm was calculated from X-ray diffraction data using the Williamson – Hall method. The methods of particle size distribution analysis, specific surface area measurement, scanning electron microscopy and transmission electron microscopy were used for characterization of surface, mean particle size, and morphology of PbSe. An application of industrial mill verified a possibility of the synthesis of a narrow bandgap semiconductor PbSe at ambient temperature and in a relatively short reaction time.

A duality in the behaviour of small particles, i. e. their ability to migrate along with some grain boundaries and, simultaneously, inability to move along with the others, has been shown to affect the character of the grain growth process. It is found by numerical simulations that a decrease in the particle mobility favours the evolution of abnormal grain growth and that the particle volume fraction, at low particle mobility, also affects its development. It has been supposed that these results explain the evolution of abnormal grain growth in compacted nanocrystalline materials where small voids could play the part of particles.

A mechanism of high-temperature plasticity has been studied in terms of stress exponent and activation energy for deformation. A Ti 3 Al-based intermetallic was processed by hot-rolling in the two-phase field to accomplish a fine-grained microstructure. Compression stress – strain tests were conducted in the temperature interval ranging from 900 to 1 050 °C and strain rates between 6.9 · 10 − 5 and 2.7 · 10 − 2 s − 1 . Results indicate that viscous-drag dislocation glide and grain boundary sliding are the main mechanisms of deformation. Activation energy for deformation suggests dependency on the solute content in the -phase.

The mechanical properties of concrete containing ZnO 2 nanoparticles which was cured in saturated limewater have been optimized. ZnO 2 nanoparticles were added to concrete as nano-fillers. The specimens were cured in water and saturated limewater for 7, 28 and 90 days after casting and then their strength was evaluated by mechanical testing (compressive, split tensile and flexural strength tests). The results showed that replacement of Portland cement with ZnO 2 nanoparticles up to 1.0 wt.% for the specimens cured in water and 2.0 wt.% for the specimens cured in saturated limewater produces concrete with the best strength. It has been shown that curing the specimens in saturated limewater for 28 days and then in water until 90 days, produces a stronger concrete than those cured only in saturated limewater for 90 days. Excess Ca(OH) 2 crystals which form after 28 days, when the specimens are cured in limewater, do not contribute to strength enhancement as much as strengthening gel and hence, the mechanical properties of the specimens do not reach the highest level. On the other hand, curing the specimens in water after 28 days produces more strengthening gel and results in a concrete with higher strength. The addition of nanoparticles improves the pore structure of concretes, thus, the refined extent of the pore structure increases with decreasing nanoparticle content. The pore structure of concrete cured in saturated limewater is better than that of concrete cured in water.

The paper presents results of investigations on texture, microstructure and mechanical properties of magnesium alloy AZ31 and pure zinc after severe plastic deformation using the KoBo extrusion method at room temperature. The KoBo method combines monotonic metal deformation with cyclic changes of deformation path. It allows realization of an extrusion ratio equal to 100 (true strain ∊ = 4.6) at room temperature using a force of less than 1 MN. At the same time the grain structure is refined to the level of a few micrometers and the material attains unusually high mechanical properties not otherwise available regardless of the thermo-mechanical features. Transmission electron microscopy observations in bright and dark field (weak beam technique) contrast conditions allowed identification of clusters of point defects in the extruded wires. In the light of these observations they are the only elements (nano-size elements) of the metals' structure which can be held responsible for the unusual properties of the wires.