Individual Powder Particles Research Articles

Direct measurement of the coating thickness on spherical particles

In the course of the development of an coating process of metallic powders, the dependence of the coated layer thickness on the process variables and on the initial particle size had to be determined. The initial, atomized powder particles in the 5-80 μm size range, were perfect spheres to a good approximation.We chose to employ a “particulate” approach according to which individual powder particles were repeatedly cross-sectioned and examined at known intervals, sufficiently close to allow the same spheres to be cross-cut more than once. Micrographs of selected particles were taken at each stage. Simple chord measurements taken from the micrographs, which provide two-dimensional information, and the measured spacing between the corresponding cross-sections, giving the third dimension, allows to calculate both the particle diameter and the coating thickness. The accuracy of the results depends critically on the accuracy of the spacing measurement. The present communication describes a simple, practical, and precise method for this measurement, eliminating the necessity for precision alignment and dedicated tools.

Physical spreading of oxidation in solid polypropylene as studied by chemiluminescence

Highly sensitive chemiluminescence equipment was applied to study the oxidative behaviour at 125–150°C of individual polypropylene powder particles (10–500 μm) and film sections pressed from the powder. The chemiluminescence curves from the oxidation of single isolated powder particles revealed a range of induction periods equivalent to different intrinsic stabilities of the individual particles. Physical contact of the particles, however, resulted in a relatively early oxidation of the combined sample, i.e. a single induction period, as a result of the apparent initiation by the weakest particle and a subsequent spreading of the oxidation through the ensemble of particles. Evidence of this oxidative spreading was also found in polypropylene film samples. The physical spreading of the oxidation from an initial highly reactive centre has therefore been established as being the predominant step in the oxidation of a larger sample, and thus the weakest centre can control the overall oxidation of a sample. This mechanism was found to be consistent with the sigmoidal curve shape of the chemiluminescence curve obtained from the oxidation of both powder and film. The observed spreading is believed to be a highly efficient process, as the oxidation can ‘jump’ from particle to particle over distances of approximately 100 μm. In stabilized film the oxidation at the end of the induction period shows features different from that of an unstabilized film, and it is proposed that the spreading of the oxidation is limited by residual stabilizer in zones of lower reactivity.

Structural characteristics of a nickel-modified Al-20Si-3Cu-1Mg alloy powder

An attempt has been made to characterize a new, complicated Al-20Si-7.5Ni-3Cu-1Mg alloy powder produced by air atomization as a means of rapid solidification and its structural evolutions during continuous heating, in order to provide basic information for further investigations on its deformation behaviour and properties. The characterization consisted of size measurements, morphological observations, structural and thermal analyses of bulk powder, and microstructural examinations of individual powder particles. It was observed that the powder had a wide size distribution and irregular shapes, which were closely related to its varying internal structures. X-ray diffractometry (XRD) showed little shift of the diffraction line from the aluminium matrix of the powder, but a significant broadening, which has been attributed partly to the non-uniformity of supersaturation in the matrix of the powder and partly to the strains caused by the silicon crystals in the material. A differential scanning calorimetry (DSC) analysis revealed complex decomposition behaviour of the meta-stable aluminium matrix and transformations of nickel-bearing intermetallic compounds when heat was applied to the powder. XRD also showed that the meta-stable compounds formed in the powder did not match any known phases, and that they were transformed into Al3Ni, Al3(NiCu)2 and Al7Cu4Ni intermetallic dispersoids upon heating. The analyses also indicated that, due to the addition of nickel, some copper-containing phases, initially desired to create precipitation strengthening effects, no longer existed. This would diminish the ageing response of the alloy and probably change its category to be non-heat treatable — an important modification that has not yet been recognized by the alloy designers and users. Examinations on the powder particle sections showed variations in microstructure with powder particle size. Transitions in solidification mode within powder particles in accordance with local conditions of undercooling and heat extraction were also observed. The significant inhomogeneities in the microstructure of the powder have raised a problem to which special attention should be paid in both powder production and subsequent processing.

Microporosity of powders produced by centrifugal sputtering

The nature of the liquid-metal film flow about the end of the pulverized billet is defined by its rotational velocity. In the case of a turbulent film flow regime gas is captured and pores are formed in individual powder particles.

A study on an atomized Al-Fe-Mo-Zr powder to be processed for high temperature applications

The performance of a powder metallurgy material in processing and service depends very much upon the initial characteristics of the atomized powder. An investigation on the characterization of an Al-8.5Fe-2Mo-1Zr alloy powder, to be further processed for high temperature applications, has, therefore, been carried out. Analyses have been performed of its composition, morphology, size and microstructure by means of chemical and metallographic methods. Results show that although the powder was atomized in nitrogen, its oxide and hydrogen contents are high enough to be comparable to those of other aluminium alloy powders atomized in air. Auger spectroscopy indicates the presence of discrete oxide particles at a depth of 100 nm below the powder particle surface, which corresponds to the topography of the powder particles revealed by SEM. The oxidation during atomization, plus further oxidation and moisture adsorption during subsequent handling, is considered to be mainly responsible for the analysed results, in addition to the contributing factor that the present powder has a relatively high specific surface area. Applying an appropriate degassing process is, therefore, of particular importance for obtaining the desired properties of the material processed from the powder. In order to simplify the process and to minimize the coarsening of microstructure, an on-line degassing technique has been proposed. X-ray diffractometry shows that the lattice parameter of the α-Al matrix in the present powder is altered in a complicated way, and that the diffraction spectrum of intermetallic particles does not match any established phase, presumably due to the involvement of molybdenum in a metastable phase to form an Al-Fe-Mo intermetallic. The microstructure of the powder particles finer than 10 μm is featureless, and larger ones exhibit a distinctive microstructure which is composed of a featureless zone, a transitional zone and a cellular zone. The relative percentage of the three zones is strongly dependent upon the size of individual powder particles and thus their cooling rates. Solidification processes responsible for the formation of the three zones are described in this paper. It is also found that as a result of microstructural inhomogeneity, microhardness within and between the powder particles differs significantly, and this difference can be retained after consolidation and influence the properties of the final engineering material. It is thus thought that creating an extremely fine powder particle size (smaller than 10 μm) with an overwhelming featureless microstructure may not be commercially feasible at present, while producing a fairly homogeneous microstructure by narrowing the scattering range of powder particle size could be more important for obtaining uniform deformation and oxide break-up during consolidation, and desired mechanical properties of the final product.

Microhardness measurement of individual AlMnCr powder particles produced by rapid solidification

Hardness measurement was carried out on powder particles of AlMnCr alloys (Al-5Mn-2.5Cr, Al-5Mn-4Cr-4Cr and Al-5Mn-6Cr), which were produced by ultrasonic helium gas atomization. It has been demonstrated that it is feasible to use conventional Vickers hardness measurement to characterize individual AlMnCr powder particles as small as 10 μm in size by an applied load of 5 g. It has been shown that the hardness is a function of alloy composition, size of powder particles and heat treatment. The hardness increased with the contents of alloying elements, i.e. manganese and chromium, and decreased when the size of powder increased. Decomposition of the supersaturation in the powders was accompanied by a drop in hardness after ageing of the rapidly solidified powders of the Al-5Mn-2.5Cr at 210 °C, 300 °C and 420 °. The results of hardness measurements are consistent with those from the investigations by X-ray diffraction, EDX and TEM.

Dynamic consolidation of rapidly solidified titanium alloy powders by explosives

Consolidation of rapidly solidified titanium alloy powders employing explosively generated shock pressures was carried out successfully. The cylindrical explosive consolidation technique was utilized, and compacts with densities in the range 97 to 100% were produced. Better consolidation (with more interparticle melting regions and less cracking) was achieved by using a double tube design in which the outer tube (flyer tube) was explosively accelerated, impacting the powder container. Optical and transmission electron microscopy observations were carried out to establish microstructural properties of the products. It was observed that consolidation is achieved by interparticle melting occurring during the process. The interior of the particles in Ti-17 alloy exhibited planar arrays of dislocations and twin-like features characteristic of shock loading. Two dominant types of microstructures (lath and equiaxed) were observed both in Ti-662 and Ti-6242 + 1% Er compacts, and very fine erbia (Er2O3) particles were seen in the latter alloy. The micro-indentation hardness of the consolidated products was found to be higher than that of the as-received powder material; and the yield and ultimate tensile strengths were found to be approximately the same as in the as-cast and forged conditions. The ductilities (as measured by the total elongation) of the shockcompacted materials were much lower than those of the cast or forged alloys. Hot isostatic pressing of the shock-consolidated alloys increased their ductility. This enhancement in ductility is thought to be due to the closure of existing cracks. These excellent mechanical properties are a consequence of strong interparticle bonding between individual powder particles. It was also established that scaling up the powder compacts in size is possible and compacts with 50, 75, and 100 mm diameter were successfully produced.

TEM observations on the microstructure of an atomized high-purity aluminium powder

The purpose of this study was to explore the possible intrinsic microstructural stability of individual powder particles, in order to avoid ambiguities introduced during consolidation or by surface films. High-purity aluminium was chosen as a model system because an extensive literature exists concerning quenching effects in this material

Thinning of aluminum powder particles for transmission electron microscopy

This article outlines a relatively simple and quick method for producing thin-foil samples of aluminum powder particles for transmission electron microscopy (TEM) examination. Details of solution preparation, experimental procedure, and results achieved are discussed. The procedure involves deposition of a nickel substrate around the individual aluminum powder particles to facilitate thinning to a thickness compatible with TEM examination. Photomicrographs of successful depositions and examples of microstructures of elevated temperature and high-strength aluminum powder metallurgy alloys are included.

Microshear banding in shock-consolidated microcrystalline alloy powders

Microshear bands (<0.02–0.03 μm thick), formed as a result of inhomogeneous plastic deformation within heavily deformed powder particle interiors, are observed during shock consolidation of microcrystalline Markomet 1064 (Ni 55.8 Mo 25.7 Cr 9.7 B 8.8), and nickel based superalloys Pyromet 718 and Rene 95. Although part of the shock energy is utilized in melting of particle surfaces due to preferential plastic deformation near interfaces, a nonnegligible amount of energy is dissipated in plastic deformation within particle interiors. This involves localized regions of shear which are manifested as fine microshear bands of heavily twinned material, confined within individual powder particles. The thickness and length of the microshear bands is more than several orders of magnitude smaller than that in the macroscopic bands observed in materials subjected to unconstrained high-strain-rate deformation. The small size of the shear bands is attributed to the fact that the particle strain is limited, as individual powder particles are deformed to conform to their neighbors in the shock consolidation process.

An analysis of the microstructure of rapidly solidified Al-8 wt pct Fe powder

Rapidly solidified powders of Al-8 wt pct Fe exhibit four distinct microstructures with increasing particle diameter in the size range of 5 μm to 45 μm: microcellular α-Al; cellular α-Al; a-Al + Al6Fe eutectic; and Al3Fe primary intermetallic structure. Small powder particles (~10 μm or less) undercool significantly prior to solidification and typically exhibit a two-zone microcellular-cellular structure in individual powder particles. In the two-zone microstructure, there is a transition from solidification dominated by internal heat flow during recalescence with high growth rates (microcellular) to solidification dominated by external heat flow and slower growth rates (cellular). The origin of the two-zone microstructure from an initially cellular or dendritic structure is interpreted on the basis of growth controlled primarily by solute redistribution. Larger particles experience little or no initial undercooling prior to solidification and do not exhibit the two-zone structure. The larger particles contain cellular, eutectic, or primary intermetallic structures that are consistent with growth rates controlled by heat extraction through the particle surface (external heat flow).

Rapid solidification of a droplet-processed stainless steel

Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed.

Production Structure and Properties of Al–Fe–Ni–Co Alloy Prepared from Atomized Powder

Abstract The preparation of an Al–3·8Fe–3·6Ni–3·8Co engineering material from atomized powders is described. The production route involves cold compaction followed by extrusion at 400–600°C. It is shown that products having acceptable surface finishes may be produced and that the properties of the products are related to the extrusion parameters. The structure is considerably modified during the deformation processing and is much coarser than that existing in the individual powder particles. The properties of the product are superior at room temperature to those of existing non-heat treatable Al alloys and the material maintains its considerable strength at elevated temperatures. PM/0284

Microstructural aspects of extrusion of rapidly solidified Al–10Mg alloy powder

AbstractThe microstructures of rapidly solidified Al–10Mg alloy powders have been observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. A technique is described which enabled individual powder particles to be made electron transparent. The alloy powders were then consolidated by cold compaction followed by hot extrusion using a wide range of temperature compensated strain rates. The microstructures evolved are reported and it is shown that there is a correlation between the microstructures and the process parameters. It is also shown that the initial powder structure is an important variable. Magnesium is retained in solid solution in the extrudate but is metastable at room temperature leading to decomposition over long periods of time. However, the very fine grain size observed (1–3 μm) is thermally stable up to 300°C and aging processes may be utilized to obtain acceptable structures.

Hot Isostatic Consolidation of P/M Superalloys

ABSTRACTThe kinetics of powder consolidation, or densification, and the powder morphological changes ocurring during hot isostatic pressing (HIP) are studied as a function of particle size distribution and hold time at HIP temperature for the nickel base superalloy RENE-95. In order to understand the extent of individual powder particle deformation during consolidation and its effect on subsequent prior particle boundaries (PPB), particle size distribution was studied as a variable. Particle size distributions studied include monosized (75–90 um), bimodal ( 75–90 um and 33–35 um) and commercial (&lt;104 um) size distributions. The experimental results of HIP densification kinetics are compared with a newly developed analytical deformation mechanism model for HIP consolidaiton which takes into account the effect of a distribution of particle sizes on the kinetics of densification.

Penetration Phosphors: Preparation and Cathodoluminescence Properties

Two types of phosphors have been prepared; (1) red-emitting (Zn, Cd)S: Ag phosphor layer deposited on non-luminescent SiO2 particles, and (2) green-emitting Zn2SiO4: Mn phosphor particles coated with undored ZnS or (Zn, Cd)S layer. Both layers are deposited on individual powder particles by chemical deposition using thioacetamide as the source of sulfide ions. Various factors affecting the deposition process as well as the effect of post-baking have been studied to establish the optimum preparative conditions and to obtain efficient phosphor layers. The cathodoluminescence of the two-component phosphor screens prepared by mixing these two types of phosphors have been measured as a function of the accelerating voltage of the incident electron beam.

Mathematical description of diffusion on the peripheries of spherical particles

Expressions are given for calculating the concentration and absorption of a diffusing phase in a region constituting a spherical layer on the surface of an individual powder particle or around an inclusion in a sintered material. Errors contained in published solutions of similar problems are pointed out.

THE INFLUENCE OF SILICON AND OTHER ELEMENTS IN CONTROLLING INTERPARTICLE FRICTION AND SINTERING OF BERYLLIUM POWDER

AbstractThe effect of small additions of activating elements such as silicon on the consolidation behaviour of beryllium powder has been investigated. Evidence is given that compacts of activated powder have more uniform high density than those produced from non-activated material. Studies carried out on prepared beryllium discs show that silicon modifies the micro-structure of the surface layer of beryllium oxide and, in consequence, affects its sliding behaviour and bonding characteristics.From these results a model is proposed to account for the observations made on both sintered and hot-pressed beryllium which leads to the conclusion that, in addition to interparticle bonding, some measure of metal particle rearrangement is necessary for maximum densification. Activating elements may, in modifying the surface characteristics of the individual powder particles, assist in achieving an improved balance between particle sliding on the one hand and interparticle bonding on the other. In taking into account...

Correlation of microhardness and microbrittleness of refractory compounds with their electronic structure

The microhardness and microbrittleness of refractory metal carbides were measured on individual powder particles. The authors introduce a microhardness index defined as the product of the reciprocal of the optimum load by the variation of over-all brittleness index with increase in load. It is assumed that microhardness and microbrittleness of the investigated refractory materials are determined essentially by the statistical weight of the stable electronic configurations of metal and nonmetal atoms, and by the proportion of electrons transferred to the collectivized state.

An Interpretation of the Magnetic Properties of some Iron-Oxide Powders

The observed magnetic properties of fine dispersed powders of the ferromagnetic iron oxides are discussed in the light of modern theories of the magnetization of ferrites and of ferromagnetics containing non-magnetic inclusions. The individual powder particles are shown to be probably of single-domain size, and the relative coercivities of γ-ferric oxide powders of nearly spherical and of markedly acicular grains are qualitatively in agreement with theory, on the grounds of shape anisotropy. After allowance is made for volume concentration and small departures from stoichiometric composition, the ratio of the coercivities of apparently identical acicular powders of γ-ferric oxide and of magnetite is found to be approximately that of the saturation magnetization per ferric ion rather than that of the value per unit volume, and both coercivities are lower than theory would predict It is concluded that magneto-crystalline anisotropy is also involved. In contrast, relative and actual values of intrinsic induction at the beginning of the approach to saturation are in satisfactory agreement with theory, as are the relative values at remanence although the actual values here are rather low. It is hoped to extend the study to larger departures from stoichiometric composition involving partial transformation to the antiferromagnetic iron oxides α-Fe2O3 and FeO.