Characterization of deformation processes in a Zn-22% Al alloy using atomic force microscopy

The Zn-22% Al eutectoid alloy was subjected to equal-channel angular pressing at a temperature of 473 K to give an as-pressed grain size of 1.3 m. Subsequent tensile testing of the as-pressed alloy at room temperature revealed a transition from deformation by a dislocation mechanism at the higher strain rates to superplastic flow at strain rates below 5 × 10^–3 s^–1: this corresponds to the transition from region III to region II in conventional superplasticity. Samples were pulled to relatively low total strains, of the order of 0.2–0.5, and the surface topography was then examined using an atomic force microscope (AFM). The AFM observations confirm the transition in deformation mechanisms with decreasing strain rate and they provide direct evidence for the occurrence of grain boundary sliding within the superplastic regime.     

Spherical and rod-like zinc oxide microcrystals: morphological characterization and microstructural evolution with temperature

Spherical ZnO microcrystals obtained by spray pyrolysis and thermal decomposition methods as well as rod-like ZnO particles (prismatic and needle shaped) prepared from precipitation in aqueous solutions, have been characterized by electron microscopy, X-ray diffraction and infrared spectroscopy. Very different sizes of ZnO particles were obtained from spray pyrolysis. However, only the larger particles (0.7 m) were found to be slightly deformed by infrared spectroscopy. From thermal decomposition of zinc acetate, fine particles of average size 0.05 m, rather spherical and agglomeration free were obtained. The role of initial size and morphology in the thermal evolution is fundamental: very fine spherical particles (0.01–0.02 m), can be sintered to give particles of 0.1–0.3 m at 875 °C with unchanged morphology. When the temperature induces a change in spherical shape, the first microstructural changes appear to take place through the crystallographic c-axis. However, for rod-like particles, changes begin from the a, b axes, being faster for needle-shaped microcrystals.     

Intergrain necking and the reversal of magnetization of fine, highly acicular ferromagnetic skeleton particles

In this paper the relationship between the reversal of magnetization and the morphology of very fine, iron-based and highly acicular particles used in high-density magnetic recording media is investigated. The iron-based particle referred here is of a skeleton type, which consists of a very fine granular material; the so-called grain. The grain belongs to the bcc phase and its size ranges between about 100 to 300 Å depending on usage. Therefore the grain can be treated as a single domain particle. As the morphology of the skeleton particle prepared for 8 m/m video-recording media is mainly determined by the grain size and the intergrain necking, the effects on the coercivity were studied. The magnetization reversal of a long chain-of-spheres which are in contact with each other over a finite area was investigated to determine the quantitative relationship with intergrain necking of the skeleton particles when a type of exchange anisotropy, which is proportional to the contact area between two adjacent unit spheres, is introduced into the chain. The symmetric fanning mode is preferential in increasing the intergrain necking. The introduction of the exchange anisotropy can result in decrease in the coercivity with increasing intergrain necking, this quantitatively reproduces the experimental behavior observed for very fine, highly acicular skeleton particles of -Fe. On the other hand, no essential change in the behavior of the angular variation of the coercivity is induced even if exchange anisotropy is introduced into the chain. Finally, under the present scheme, it has been discussed as to how to interpret the experimentally-found dependence of coercivity on grain size, where a possibility to introduce an influence of the unit-sphere size on the characteristic constant of the exchange anisotropy is suggested.     

Surface repair of porous and damaged alumina bodies using carboxylate-alumoxane nanoparticles

Aqueous solutions of acetate-functionalized alumina nanoparticles (A-alumoxane), with an average particle size of 28 nm, have been used as alumina precursors for the surface infiltration and repair of porous and/or damaged alumina surfaces. SEM and AFM measurements indicate that treatment with a 1 wt% solution results in a reduction of surface roughness from >0.6 m to 100 nm for surface pores in the 100 nm to 1 m range. The use of 6 wt% solutions gives better infiltration repair for 50 m features, but surface cracking is observed. The surface hardness of the porous alumina substrate is increased upon infiltration. No spallation of the surface infiltration layer is observed after indentation measurements and grain dislodgment is overcome.     

Modeling of damage to articulating surfaces by third body particles in total joint replacements

Numerous small scratches and some larger scratches have been observed on metallic femoral heads of explanted hip prostheses, with the larger scratches believed to be a major contributor to increased wear of the polyethylene acetabular cups. Previous work in our group has shown that smaller scratches, with a mean lip height up to 0.35 m, can be caused by bone cement and bone particles up to 500 m in size [1]. However, the larger scratches were not readily replicated with these particles. Therefore in this study experimental and theoretical models have been developed to investigate the damage caused by harder metallic and ceramic particles. Small 10 m diameter spherical metallic particles were also found to produce small fine scratches on the metallic counterface. However larger diameter spherical metal particles greater than 100 m in diameter, which were embedded in polyethylene pins, caused severe sharp scratching of the metallic counterface with scratch lips greater than 0.5 m. This level of damage, which was comparable to the severe damage found in vivo, was also simulated by a three body finite element model. Thus the larger metal particles led to the type of damage which was predicted to increase wear dramatically. This technique for simulating severe in vivo third body damage using spherical metal particles was found to be reproducible and reliable and will be used in the future in hip simulator testing to replicate third body damage and wear.     

Microstructural aspects of hypervelocity impact cratering and jetting in copper

Light and transmission electron microscopy techniques have been applied in observations of hypervelocity impact craters in two different copper targets: a 38 m grain size mill-processed target, and a 763 m grain size annealed target, the smaller grained target being impacted with a 1100 aluminium sphere and the larger grained target being impacted with a soda-lime glass sphere, at velocities near 6 km s^–1. Both target craters exhibited dynamic recrystallization near the crater wall. The jetting associated with these two craters was very different. Considerably more plastic flow and a larger rim characterized the larger grained target. No significant melt-related phenomena were observed either near the crater wall or in the jetted rim for either crater. Consequently, the principal features of crater formation involve extreme plastic flow in the solid state. Microbands were observed to occur profusely in a zone below the smaller grained mill-processed target crater while more profuse and extremely long, unidirectional bundles of microbands (which were coincident with traces of {1 1 1} planes) occurred below the annealed larger grained target crater. These observations attest to the dominant and unique role played by deformation microbands in cratering in copper, because essentially no deformation twins were observed in either target.     

Effect of initial microstructure on high velocity and hypervelocity impact cratering and crater-related microstructures in thick copper targets: Part II Stainless steel projectiles

Three different, thick copper targets (an as-received, 98 m grain size containing 1010 dislocations/cm2 (Vickers hardness of 0.89 GPa); an annealed, 124 m grain size containing 109 dislocations/cm2 (Vicker's hardness of 0.69 GPa); and a 763 m grain size containing 109 dislocations/cm2 (Vickers hardness of 0.67 GPa) were impacted with 3.18 mm diameter ferritic stainless steel projectiles at nominal velocities of 0.7, 2 and 5 km s-1. Like companion experiments utilizing soda-lime glass projectiles (Part I), absolute grain size of the target was observed to be less important than the dislocation density in the cratering process. At low impact velocity, depth/diameter ratios were observed to increase dramatically in contrast to less dense soda-lime glass impactors, and the impactor behaviours were also very different. The ferritic stainless steel impactors spalled into small fragments at or above 2 km s-1 impact velocity and a significant fraction of these fragments remained in the craters. No significant melt phenomena were observed either in connection with projectile fragmentation or in the crater-related, residual microstructures. Dynamic recrystallization, dislocation cell structures and microbands were significant microstructural features in the targets. They extended from the crater walls and contributed to hardness profiles within the cratered targets. These hardness profiles and actual hardness zones generally increased in extent from the crater wall with both impact velocity and projectile density.     

Ultrasonic spray pyrolysis for nanoparticles synthesis

This article presents new findings regarding the effects of precursor drop size and precursor concentration on product particle size and morphology in ultrasonic spray pyrolysis. Large precursor drops (diameter > 30 m) generated by ultrasonic atomization at 120 kHz yielded particles with holes due to high solvent evaporation rate, as predicted by the conventional one particle per drop mechanism. Precursor drops 6–9 m in diameter, generated by an ultrasonic nebulizer at 1.65 MHz and 23.5 W electric drive power, yielded uniform spherical particles 90 nm in diameter with proper control of precursor concentration and residence time. Moreover, air-assisted ultrasonic spray pyrolysis at 120 kHz and 2.3 W yielded spherical particles about 70% of which were smaller than those produced by the ultrasonic spray pyrolysis of the 6–9 m precursor drops, despite much larger precursor drop size (28 m peak diameter versus 7 m mean diameter). These particles are much smaller than predicted by the conventional one particle per drop mechanism, suggesting that a gas-to-particle conversion mechanism may also be involved in spray pyrolysis.     

Deformation behaviour of retained β phase inβ-eutectoid Ti-Cr alloys

Plastic deformation mode and its relation to tensile properties were investigated in retained phase of-eutectoid type Ti-Cr alloys. A plate-like single variant phase is induced during deformation of the most unstable phase having a minimum chromium content required to suppress martensitic transformation. A selected area electron diffraction pattern taken from a boundary region of the stress-induced phase plate can be explained by the idea that a single variant of phase is induced in a {3 3 2} 1 1 3 twin produced during deformation. Anisotropy in population of four phase variants decreases with increasing chromium content. On further increasing chromium content, deformation occurs by slip. Enhanced ductility is obtained in as-quenched Ti-Cr alloys accompanied by phase transformation or {332} 1 1 3 twinning during deformation, phase of as-quenched Ti-Cr alloys changes continuously from commensurate structure with sharp reflections to incommensurate structure with diffuse reflections with increasing chromium content. The obtained results in-eutectoid type Ti-Cr alloys are quite similar to those in-isomorphous type Ti-V alloys.     

The role of Coble creep and interface control in superplastic Sn-Pb alloys

The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260m. In Sn-2 wt% Pb with grain sizes larger than 50 m, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses ( 0.1 MPa) and strain rates ( 10^–10 sec^–1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.     

Rapid solidification and decomposition of a hypomonotectic Al-Cd alloy

Rapid solidification of Al-1.5at % Cd alloy can lead to a wide variety of morphologies. The sequence of the morphologies with increasing foil thickness is single phase elongated cell hexagonal cell dendrite. Monotectic reaction ahead of the solid-liquid interface leads to a unique distribution of the cadmium particles. Models are proposed to explain the genesis of the distribution of these particles. The study of the decomposition behaviour confirms the existence of a preprecipitation stage. The G.P. zones are found to be spherical and lead to a precipitate-free zone near grain/cell boundaries. The coarsening of the precipitates at liquid temperatures takes place by the migration and coalescence of the droplets.     

The grain-bed impact process in aeolian saltation

Summary We report the results of impact experiments in which high velocity steel spheres (BBs) were directed against a loose bed of similar particles. The purpose of these experiments is to shed some light on the collision processes which occur when saltating sand grains driven by the wind strike the bed. The scattered particles fall into two categories: a single high energy rebound which scatters quasi-specularly, and a number of low energy recoils. The high energy rebound is identified with the successive saltation particle of Rumpel, and the low energy recoils are interpreted as creeping, or reptating particles. These observations provide information on the splash function of Ungar and Haff, which describes the response of a bed to grain impact and which plays a central role in the theory of saltation.With 1 Figure     

Fabrication of in situ TiC reinforced aluminum matrix composites Part I: Microstructural characterization

In the present work traditional ingot metallurgy plus rapid solidification techniques were used to in situ produce Al-TiC composites with refined microstructures and enhanced dispersion hardening of the reinforcing phases. Microstructural characterization of the experimental materials were comprehensively done by optical, electron microscopy and X-ray diffraction. The results show that the in situ synthesized TiC particles possess a metastable fcc crystal structure with an atomic composition of TiC08 and a lattice parameter of 0.431 nm. The typical ingot metallurgy microstructures exhibit aggregates of TiC particle phase segregated generally at the -Al subgrain or grain boundaries and consisted of fine particles of 0.2–1.0 m. After re-melting of the ingots and hence rapid solidification, the microstructures formed under certain thermal history conditions contained uniform fine-scale dispersion of TiC phase particles with a size range of 40–80 nm in an Al supersaturated matrix of 0.30–0.85 m grain size. In the most case these dispersed TiC particles have a semi-coherent relationship with the -Al matrix.     

Kinetics of alpha precipitation in Ti-6 wt % Cr and Ti-11 wt % Mo

The morphology and kinetics of the precipitation of the alpha phase produced by two different heat treatment routes, namely, (a) direct isothermal decomposition and (b)-quenching and subsequent ageing, were studied. In isothermally decomposed samples the (supersaturated) + transformation was seen to occur mainly through the discontinuous growth of the transformed zone consisting of groups of parallel side plates from the grain boundary regions towards the interior of the grain. Unlike for the case of a regular discontinuous precipitation, here the transformed regions are not separated from the untransformed by an incoherent interface and the growing-plates do obey a fixed orientation relationship with the grain from which they are evolved. The theory of cellular reaction has been applied to explain the growth rate of the duplex ( + ) region. The overall reaction kinetics were analysed on the basis of the Johnson-Mehl formulation and were found to be consistent with that of a discontinuous precipitation reaction, where grain boundary nucleation sites were saturated at an early stage of the transformation. The structure of the-quenched samples showed a uniform distribution of athermal omega particles which acted as precursors to the-precipitates. As a consequence, the reaction rate was greatly enhanced and-precipitation in the quenched and aged samples was seen to occur continuously in the entire body of the grain.     

Ceramic joining IV. effects of processing conditions on the properties of alumina joined via Cu/Nb/Cu interlayers

Multilayer copper/niobium/copper interlayers consisting of 3 m thick cladding layers of copper on a 125 m thick niobium core layer were used to join aluminum oxides at 1150°C or 1400°C, or both. Three microstructurally distinct aluminum oxides were joined—a 25 m grain size 99.5% pure alumina, a submicron grain size 99.9% pure alumina, and single crystal sapphire. Two-phase interlayer microstructures containing both copper-rich and niobium-rich phases developed during bonding. In some cases, the initially continuous copper film evolved via Rayleigh instabilities into an array of discrete copper-rich particles along the interlayer/alumina interface with concurrent increases in the niobium/alumina contact area. Processing conditions (temperature and applied load) and the alumina microstructure (grain size) impacted the extent of film breakup, the morphologies of the copper-rich and niobium-rich phases, the interlayer/alumina interfacial microstructure, and thereby the strength characteristics. Joints possessing a large copper/alumina interfacial area fraction were comparatively weak. Increases in bonding pressure and especially bonding temperature yielded interfaces with higher fractional niobium/alumina contact area. For joined polycrystals, such microstructures resulted in higher and more consistent room temperature fracture strengths. Joined 99.9% alumina polycrystals retained strengths >200 MPa to 1200°C. Relationships between processing conditions, interlayer and ceramic microstructure, and joint strength are discussed.     

Mössbauer study on the magnetic state of iron particles in Fe-N and Fe-Zr-N soft magnetic thin films

The magnetic state of -Fe particles and the behaviour of nitrogen and zirconium during annealing in Fe₉₆N₄ and Fe_85.6Zr_7.6N_6.8 magnetic thin films have been studied by conversion electron Mössbauer spectroscopy for ⁵⁷Fe. The crystalline phases present in the Fe-N annealed films were -Fe and -Fe₄N, and those in the Fe-Zr-N annealed films were -Fe and ZrN. In the Fe-N films annealed below 300°C, about 60% nitrogen is incorporated interstitially into -Fe and the rest is used for the formation of -Fe₄N. In the Fe-N film annealed at 500°C, almost all nitrogen participates in the formation of -Fe₄N, leading to the grain growth of -Fe particles and an increase in coercive force. The values (291–325 kOe) of internal magnetic field of iron sites in -Fe in the Fe-Zr-N films are much smaller than that (333 kOe) of the iron site in pure -Fe. Even if the Fe-Zr-N films were annealed at 500–700°C, some zirconium and nitrogen is still incorporated substitutionally and interstitially into -Fe, respectively. In particular, the substitutional zirconium depresses the grain growth of -Fe particles, perhaps due to a chemical interaction between zirconium and iron.     

Characterization of ultrafine γ-Fe(C), α-Fe(C) and Fe3C particles synthesized by arc-discharge in methane

Ultrafine -Fe(C), -Fe(C) and Fe3C particles were prepared by arc-discharge synthesis in a methane atmosphere. The phases, morphology, structure and surface layer of the particles were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) techniques and X-ray photoelectron spectroscopy (XPS). It was found that the mean particle size ranged from 9.8 to 12.8 nm. The surface of particles mostly consisted of a carbon layer and a little oxide. Phase transformation from -Fe(C) to -Fe(C) was studied by annealing in vacuum and by differential thermal analysis and thermogravimetry (DTA–TGA) measurement. The abundance of -Fe(C) was determined by a magnetization measurement to be approximately 30%. Phase transformation occurred between 300 and 500 °C in a flowing argon atmosphere. The Fe3C particles oxidized to -Fe2O3 and carbon dioxide at 610 °C or so. © 1998 Chapman & Hall     

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

Formation of metastable t′ -phase in solute-redistributed Y-PSZ

Owing to solute redistribution by a liquid phase analogue of diffusion-induced grain-boundary migration metastable and anti-phase domain boundary-bearing tetragonal (t) phase was seen to occur in the cubic (c) matrix grain of sintered (1400 °C, 2 h) and furnace-cooled yttriapartially stabilized zirconia (Y-PSZ) specimens (TZ3Y + 12Y-PSZ in molar ratios of 4:1, 1:1 and 14). In contrast to that formed by rapid cooling, t-phase formed by slow cooling shows no deformation accommodation twins. Subsequent ageing at 1100 °C for up to 240 h caused the formation of finely tweed (c) and tetragonal (t) assemblages at the expense of coarse tweed t-phase in the 12Y-PSZ grains. The absence of secondary deformation twins in the t-phase formed by slow cooling is discussed.     

Distribution of α-AlFeSi and β-AlFeSi particles in surface layer of AA6063 alloy billets after heat treatment

We developed the EPMA mapping method of small -AlFeSi(Al_8.3Fe₂Si) and -AlFeSi(Al_8.9Fe₂Si₂) particles in the billets of Al-Mg-Si alloys such as AA6063 alloys. To discriminate between -AlFeSi and -AlFeSi particles we used the relative X-ray intensities of Fe/Si ratio, the I _Fe/I _Si ratio, instead of the Fe/Si mass ratio. To obtain the I _Fe/I _Si ratio, we used a Monte Carlo method. In this study, using this method the mapping of -AlFeSi and -AlFeSi particles in the surface layer of AA6063 billets after the heat treatment (for 2 h at 580°C) was done. Namely, the distribution of -AlFeSi and -AlFeSi particles of zones from the billet surface to a depth of 800 m was measured. Results showed the zone from the surface to a depth of 200 m was occupied mainly by -AlFeSi particles and the zone from a depth of 200 m toward the center was occupied mainly by -AlFeSi particles. From these results, it was found that if we remove zones from the surface to a depth of 200 m, we can remove the majority of the -AlFeSi particles, and thus improve the quality of anodizing performance of Al-Mg-Si alloys extrusions.