Investigation on compressibility of Al–SiC composite powders

Abstract

The consolidation behaviour of particulate reinforced metal matrix composite powders during cold uniaxial compaction in a rigid die was studied. Al–SiC powder mixtures with varying SiC particle size, ranging from nanoscale (50 nm) to microscale (40 µm), at different volume fractions up to 30% were used. Based on the experimental results, the effect of the reinforcement particles on the densification mechanisms, i.e. particle rearrangement and plastic deformation, was studied using modified Cooper–Eaton equation. It was found that by increasing the reinforcement volume fraction or decreasing its size, the contribution of particle rearrangement on the densification increases while the plastic deformation becomes restricted. In fact, when percolation network of the ultrafine reinforcement particles is formed, the rearrangement could be the dominant mechanism of consolidation. It was also shown that at tap condition and at the early stage of compaction where the particle rearrangement is dominant, the highest density is achieved when the reinforcement particle size is properly lower than the matrix (0˙3<the size ratio<0˙5) and the fraction of hard particles is relatively low (<10%). At high compaction pressures, the reinforcement particles significantly influence the yield pressure of composite powders, thereby retarding the densification.     

A Note on the Misuse of Area Images to Obtain Particle Size Information in Solid-Solid Systems

Abstract

Multiphase solids (slag + matte, metals + inclusions) are frequently sectioned and the areas of the phases visible at the resulting surfaces are obtained using one of a number of microscopic techniques. Number concentrations of particles can be obtained unambiguously from these images. However, misinterpretation of these area distributions as particle size distributions for the dispersed phases arises frequently and in diverse contexts. When particles are binned coarsely and the particles are nearly spherical, the impact of such misinterpretations on mean particle size and interfacial area per unit volume and even relative dispersed phase volume fraction is shown to be small. In other cases the errors can be significant. In all cases, the number fraction of small particles is overstated. Thus, interfacial area per unit volume of dispersed phase tends to be overstated, the mean particle size and relative dispersed phase volume fraction underestimated. In this note, an analytical method for the deconvolution of area distributions for dispersed spherical particles is rehearsed and numerical results for randomly oriented axially-symmetric particles and analogues for disc-like and needle-like particles are presented which illustrate the significance of the impact of such misinterpretations. Other limitations on the interpretation of area distributions are also discussed.     

Viscosity Modifiers in the Mining Industry

Abstract

Viscosity modifiers are chemical reagents which alter the flow properties of fluids in general, and in particular of suspensions of solids in liquids. In principle such reagents act by altering the interaction between the solid particles in suspension, and in doing so either increase or decrease the deviation from Newtonian behaviour. The most effective example is the so-called drag reduction of thick suspensions using dispersants such as sodium tripolyphosphate, where a suspension with the consistency of toothpaste flows like milk after treatment. The dispersant reduces the particle-particle interaction by altering the surface charge, which prevents the particles from aggregating and effectively reduces the yield stress of the suspension. Several examples of viscosity modification in industrial slurries have been described although the cost of modifiers limits their general application. However there are situations where the controlled use of such chemicals can lead to considerable economic advantages. Two cases are described where chemical additives have been used to modify the flow properties of thick slurries.     

Plastic relaxation of thermoelastic stress in aluminum/ceramic composites

The dislocation generation due to a thermoelastic stress in 2024 Al/ceramic (SiC or TiC) composites was studied using transmission electron microscopy Composites containing different ceramic particulates, ceramic volume fraction, and particle size were investigated. Dislocation density profiles were measured as a function of the distance from an Al/ceramic interface and compared with those calculated from an elastoplasticity model which accounts for the volume fraction of the ceramic particles. The intensity of dislocation generation showed a strong particle size dependence: as the ceramic particle size became of the order of a micron, the intensity of dislocation generation increased significantly. With an increase in the volume fraction of the ceramic particles, the dislocation density also increased, and the dislocation structure became a more tangled arrangement. If heat dissipation was taken into account as part of the plastic work, the predicted dislocation densities of the elastoplasticity model were found to be in reasonable agreement with the measured dislocation densities of 10⁹ to 10¹⁰ cm⁻².     

SiC particle cracking in powder metallurgy processed aluminum matrix composite materials

Particle cracking is one of the key elements in the fracture process of particulate-reinforced metal-matrix composite (MMC) materials. The present study quantitatively examined the amount of new surface area created by particle cracking and the number fraction of cracked particles in a series of SiC-reinforced aluminum-matrix composite materials. These composite materials were fabricated by liquid-phase sintering and contained 9 vol pct of 23, 63, or 142 μm SiC. The matrix properties were varied by heat treating to either an underaged or peak-aged condition. In general, the new surface area created by particle cracking (S _v ) and the number fraction of cracked particles (F_no) were linearly dependent on the local strain along the tensile specimen. Multiple cracks were frequently observed in the composites containing large particles. It was found that the new surface area created by particle cracking per unit strain was higher for the case of high-strength matrices and was not systematically affected by particle size within the range studied. The number fraction of cracked particles was affected by both particle size and matrix strength. A higher number fraction of particles cracked in the composites reinforced with large particles and with high matrix yield strengths. These results are interpreted in terms of the size of the particle defects, which is a function of particle size, and the critical flaw size necessary to crack a given particle, which is a function of the stress on the particle. The new surface area created by cracking and the fraction of cracked particles were related and are in good agreement for the large and medium sized particles.     

Effect of reinforcement volume fraction on mechanical alloying of Al–SiC nanocomposite powders

Abstract

Mixtures of aluminium powder and nanoscaled SiC particles (n-SiC) at various volume fractions of 0, 1, 3, 5, 7 and 10 are comilled in a high energy planetary ball mill under an argon atmosphere to produce nanocrystalline Al–SiC nanocomposites. High resolution scanning electron microscopy (HRSEM), X-ray diffraction (XRD) method, laser particle size analysis and powder density measurement were used to study the morphological changes and microstructural evolution occurred during mechanical alloying. Al–SiC composite powder with microscaled SiC particles (1 m m) was also synthesised and characterised to examine the influence of reinforcement particle size on the milling process. It was found that with increasing volume fraction of n-SiC, a finer composite powder with more uniform particle size distribution is obtained. The morphology of the particles also became more equiaxed at shorter milling times. Furthermore, the analysis of XRD patterns by Williamson–Hall method indicated that the crystallite size of the aluminium matrix decreases with increasing reinforcement volume content while the lattice strain changes marginally. As compared with microscaled SiC particles, it appeared that the effect of n-SiC on the milling stages is more pronounced. The results clearly show that the reinforcement particles influence the work hardening and fracture of the metal matrix upon milling, affecting the structural evolution. With decreasing size of the ceramic particles to nanoscale, this influence becomes more pronounced as the surface to volume fraction increases.     

In situ stress-strain response of small metal particles embedded in a ceramic matrix

The in situ stress-strain response of metal particles embedded in a ceramic matrix was obtained by combining the measurement and the modeling of the crack opening displacement field of a crack in a brittle material bridged by metal particles. The experiments were done on a composite made from platinum particles with a volume fraction of 10% in a magnesium aluminate spinel matrix. The size of the platinum particles was varied from 1 to 12 μm to study the influence of scale on the deformation behavior. Large strain to failure (85%) and ultimate tensile strength of 550 MPa were obtained for the 1 μm particles. But the larger particles failed at a strain of less than 25%; the ultimate tensile strength was also lower. This difference in ductility is explained in terms of debonding at the metal ceramic interface. It is argued that the debonding depends on the length of the dislocation pile up at the interface, and, therefore, on the particle size. The results and the metallographic observations are consistent with a model presented here; in this model the failure condition is given by a combination of the intrinsic yield stress of platinum, and the hydrostatic constraining stress in the metal particle.     

On the particle-size dependence of fatigue-crack propagation thresholds in SiC-particulate-reinforced aluminum-alloy composites: Role of crack closure and crack trapping

A study has been made of ambient-temperature fatigue-crack propagation behavior in P/M Al-Zn-Mg-Cu metal-matrix composites reinforced with either 15 or 20 vol.% silicon-carbide particulate, with specific emphasis on the role of SiC-particle size on the fatigue-crack growth threshold condition. It is found that measured threshold stress-intensity levels ΔK_TH, are a function of both SiC-particle size and volume fraction; however, whereas coarse-particle distribution results in higher ΔK_TH values at low load ratios, fine particles give higher threshold at high load ratios. Such behavior is analyzed in terms of the interaction of SiC particles with the crack path, both in terms of the promotion of (roughness-induced) crack closure at low load ratios and by crack trapping by particles. Consideration of the latter mechanism yields a limiting requirement for the intrinsic threshold condition in these materials that the maximum plastic-zone size must exceed the effective mean particle size; this implies that for near-threshold crack advance, the tensile stress in the matrix must exceed the yield strength of the material beyond the particle.     

Importance of slip mode for dispersion-hardened β-titanium alloys

The mechanical properties of Ti-7 Mo-7 Al and Ti-7 Mo-16 Al (in at. pct) were correlated to the microstructure. The mechanical properties of the alloy with low aluminum content, consisting of α+ β phases, were dependent on the size of the α particles. Although the α phase is softer than the β phase, the small α particles, upon plastic deformation of the alloy, functioned as typical hard agents in a dispersion-hardened system and the volume fraction of the particles controlled the macroscopic ductility. A rapid strain-hardening behavior of the small α particles seemed to be responsible for this effect. Large α particles behaved like soft, incoherent particles, the volume fraction having little effect on the inherent ductility of the alloy. The two phase (β+ Ti₃Al) microstructure of the alloy with high aluminum content resulting from high temperature aging to 900°C exhibited a yield stress of 130 ksi and an elongation to fracture of 5 pct. The ductility of this microstructure was controlled by the volume fraction of the Ti₃Al particles inducing homogeneous slip. The favorable ductility properties of the microstructures with low Ti₃Al volume fraction were lost if the slip mode was changed from homogeneous slip to planar slip. Formerly Staff Member, Materials Research Center, Allied Chemical Corp., Morristown, N. J.     

Strengthening of composites due to microstructural changes in the matrix

The addition of discontinuous silicon carbide (SiC) to aluminum (Al) alloys can result in a five-fold increase in the yield stress. The magnitude of the increase is obviously a function of the volume fraction and the particle size of the SiC. Previously, it was proposed that the strength increase due to SiC addition to Al alloys was the result of change in the matrix strength, i.e. an increase in dislocation density and a reduction of subgrain size. The data obtained from a series of experiments indicate that dislocation density increases with an increase in volume fraction of SiC and decreases with an increase in particle size. The subgrain size decreases as the volume fraction increases and increases as the particle size increases. There is a good correlation between the microstructural changes in the matrix and the changes in the yield stress of the composites.     

Observations on the effect of cementite particles on the fracture toughness of spheroidized carbon steels

The results of experimental studies of the influence of cementite particles on the fracture toughness of a number of spheroidized carbon steels at low temperatures were analyzed in terms of current theories of crack-tip behavior. The fracture toughness parameterK _IC was evaluated by using circumferentially notched and fatigue-cracked cylindrical specimens. The conclusions are summarized as follows: 1) In general,K _IC decreases with increasing volume fraction and increasing size of the carbide particles. 2) Crack initiation occurs at the carbide particles. 3) Crack propagation occurs by cleavage if the stress conditions satisfy the Ritchie, Knott and Rice criterion that a critical cleavage stress is achieved over a minimum microstructural size scale. The critical stress is that required to propagate a crack from a particle and the minimum size scale is of the order of 1 to 2 grain sizes. 4) Crack propagation occurs initially by fibrous rupture if the stress intensification is insufficient to attain the critical cleavage stress. P. Rawal was formerly affiliated.     

Microstructural effects on the tensile and fracture behavior of aluminum casting alloys A356/357

The tensile properties and fracture behavior of cast aluminum alloys A356 and A357 strongly depend on secondary dendrite arm spacing (SDAS), Mg content, and, in particular, the size and shape of eutectic silicon particles and Fe-rich intermetallics. In the unmodified alloys, increasing the cooling rate during solidification refines both the dendrites and eutectic particles and increases ductility. Strontium modification reduces the size and aspect ratio of the eutectic silicon particles, leading to a fairly constant particle size and aspect ratio over the range of SDAS studied. In comparison with the unmodified alloys, the Sr-modified alloys show higher ductility, particularly the A356 alloy, but slightly lower yield strength. In the microstructures with large SDAS (>50 μm), the ductility of the Sr-modified alloys does not continuously decrease with SDAS as it does in the unmodified alloy. Increasing Mg content increases both the matrix strength and eutectic particle size. This decreases ductility in both the Sr-modified and unmodified alloys. The A356/357 alloys with large and elongated particles show higher strain hardening and, thus, have a higher damage accumulation rate by particle cracking. Compared to A356, the increased volume fraction and size of the Fe-rich intermetallics (π phase) in the A357 alloy are responsible for the lower ductility, especially in the Sr-modified alloy. In alloys with large SDAS (>50 μm), final fracture occurs along the cell boundaries, and the fracture mode is transgranular. In the small SDAS (<30 μm) alloys, final fracture tends to concentrate along grain boundaries. The transition from transgranular to intergranular fracture mode is accompanied by an increase in the ductility of the alloys.     

Influence of particle size and volume percent of flaky mo particles on the mechanical properties of AI2O3/Mo composites

The influence of particle size and volume percent of Mo particles on flake-forming behavior of Mo powders during a ball milling process and three-point flexural strength and fracture toughness of A1₂O₃ composites reinforced with flaky Mo particles have been investigated. The flake-forming behavior of Mo powders mixed with A1₂O₃ powders becomes prominent with increasing Mo particle size, while remaining almost independent of Mo volume percent. The microstructure of the composites reinforced with flaky Mo particles is anisotropic, depending on the arrangement of these Mo particles in the A1₂O₃ matrix. The microdispersion of flaky Mo particles contributes remarkably to an increase in both flexural strength and fracture toughness. The flexural strength increases with decreasing Mo particle size, while the fracture toughness increases with increasing Mo particle size, which corresponds to an increase of the flake-forming tendency of these particles. Furthermore, the flexural strength and fracture toughness can be simultaneously improved by increasing the volume fraction of flaky Mo particles. The microstructural observations indicate that the improvement in strength may be attributed to a grain-refining effect due to inhibition of grain growth of the matrix by the presence of Mo particles. In addition, the improvement in fracture toughness may be due to plastic deformation of Mo particles at a crack tip, which is accelerated more by the flaky rather than the small spherical shape.     

Microstructural design for large superplastic elongations in aluminum-base materials containing particles

The microstructural features (grain size, particle size, and volume fraction of particles) for large superplastic elongations in aluminum-base materials that contain particles were designed from the viewpoint of the superplastic region and the inhibition of cavity nucleation, where the size of the grains is larger than that of the particles and the particles are particulates. These features were derived from the constitutive equations and the local critical stress that inhibits cavity nucleation. The microstructural features derived in this work were compared with those of the various aluminum-base materials containing Si₃N₄ particles. The results indicated these features were comparable. It is concluded that the method for designing microstructural features in this study is effective for large superplastic elongations in aluminum-base materials that contain particles.     

Predicting the stress-strain behavior of polycrystalline α-lron containing hard spherical particles

Recent interest in the work hardening of metal crystals containing a dispersion of hard particles has resulted in analytical expressions relating the work hardening to strain, particle diameter, and volume fraction as well as other material parameters. In this study, these models have been used to calculate the tensile stress-strain behavior of polycrystalline α iron containing dispersions of the intermetallic compound Fe₂Ta. The structural characteristics of the Fe-Ta alloys were thoroughly evaluated. The particle morphology was measured for randomness, mean particle diameter, standard deviation of the particle diameter, volume fraction, and planar interparticle spacing. Also, the matrix flow strength, composition, crystallographic randomness, dislocation morphology and grain size were evaluated. It was found that an Orowan type relationship as modified by Ashby satisfactorily described the yield strength as a function of the interparticle spacing and particle diameter. An experimental slope of 11.1 x 10^-5 kg-cm/mm² and a calculated slope of 9.75 x 10^-5 kg-cm/mm² were found. Both the Hart revised FHP work hardening model and Ashby’s model based on the generation of secondary dislocations were in good agreement with the experimental data. Hart’s revised FHP model required the use of empirically obtained values for the particle volume fraction which differed by a factor of 10 from the measured volume fraction and therefore is not suitable for predictive purposes. At tensile strains greater than 5 pct, the work hardening was characteristic of the matrix without particles; therefore, deviation between the experimental and calculated results based on Ashby’s model occurred at large strains. It is hoped that this study represents a step towards applying work hardening models to more complex polycrystalline alloys.     

Finite-element method simulation of effects of microstructure, stress state, and interface strength on flow localization and constraint development in Nb/Cr2Nb in situ composites

The effects of volume fraction of particles, stress state, and interface strength on the yield strength, flow localization, plastic constraint, and damage development in Nb/Cr₂Nb in situ composites were investigated by the finite-element method (FEM). The microstructure of the in situ composite was represented in terms of a unit rectangular or square cell containing Cr₂Nb particles embedded within a solid-solution-alloy matrix. The hard particles were considered to be elastic and isotropic, while the matrix was elastic-plastic, obeying the Ramberg-Osgood constitutive relation. The FEM model was utilized to compute the composite strength, local hydrostatic stress, and plastic strain distributions as functions of volume fraction of particles, stress state, and interface strength. The results were used to elucidate the influence of volume fracture of particles, stress state, and interface property on the development of plastic constraint and damage in Nb/Cr₂Nb composites.     

Control of γ′ particle size and volume fraction in the high temperature superalloy Udimet 700

Experiments were conducted to determine the chemical composition, volume fraction and particle size of the γ′ precipitate in Udimet 700 as a function of temperature and time. Growth of the γ′ particles was found to followt ^1/3 diffusion controlled coarsening kinetics. The composition of γ′ varied only slightly with temperature and was independent of time. From this information, a method was developed to estimate the volume fraction and average particle size of the γ′ precipitate for any given heat treatment. It is suggested that this approach could be applied to other γ′ strengthened superalloys.     

Determination of Yield Stress from Initiation of Motion on an Inclined Plane

Abstract

The yield stress of liquid-solid suspensions was evaluated experimentally on a static inclined plane apparatus in terms of the stability criterion.

This equation was tested for 1000 ≤ρ≤ 1831 kg/m³, 5.5 ≤ H ≤ 38mm, 3.0 ≤ σ^yx ≤ 118 Pa. The results are compared with those obtained for the suspensions by vane torsion and by extrapolation of the flow-curve. Reasonable agreement was observed for 16 fluids with deviations in the range 0.2 ? 48% (mean 15%). By comparison, the deviations between vane torsion and extrapolation of the flow-curve were comparable and in the range 2.2 ? 90% (mean 19%).

It was found necessary to roughen the base of the plane in order to avoid erratic behaviour, presumably due to slip. It was also found that the use of a shallow depth of test suspension gave more accurate results owing to less creeping and better applicability of the proposed criterion.

The inclined plane technique holds promise for yield stress determination especially for application to processes in which concentrated suspensions flow down inclined surfaces. The technique is simple and cheap.     

Correlation of yield strength with internal coherency strains for age-hardened Cu−Ni−Fe alloys

The maximum yield strengths for a given aging temperature were measured for age-hardened Cu−Ni−Fe alloys. The yield strengths were found to be proportional to the difference in cubic lattice parameters of unstressed precipitating phases and independent of other factors such as precipitate particle size and precipitate volume fraction. The yield strength dependence on lattice parameter differences alone indicated coherency stresses controlled the yield strengths. An analysis of the yield strength based only on internal coherency strains and stresses subsequently led to the derivation of an equation for the yield strength,i.e., where is the Taylor factor for converting from single crystal shear stress to polycrystalline tensile stress results,C _ijare single crystal elastic stiffness constants and Δa is the difference in, anda ₀ the average of the cubic lattice parameters of the precipitating phases. The equation indicates the yield strength is dependent only on the internal coherency strains and independent of particle size and precipitate volume fraction, as observed. The correlation of the experimentally measured yield strengths with the equation was quite good.     

Synthesis and characterisation of Ti6Al4V/xTa alloy processed by solid state sintering

ABSTRACT

This work investigates the effect of Ta particle addition into a Ti6Al4V alloy processed by solid state sintering. The volume fraction of Ta ranged between 0 and 30?vol.-%. The sintering kinetics of powder mixes are evaluated by dilatometry. Sintered materials are characterised by SEM and XRD, and their mechanical properties are obtained from microhardness and compression tests. Sintering behaviour and final microstructure are affected by Ta particles, which slow down the densification, lower the temperature of α-to-β phase transition and stabilise the β phase. Mechanical properties, as microhardness, Young’s modulus and yield stress, depend on the microstructure reached after sintering and on the residual porosity. An equation expressing the Young’s modulus of Ti6Al4V/xTa alloy as function of x and porosity is proposed and validated. The materials with at least 20?vol.-% of Ta exhibited a high strength to modulus ratio, which is suitable for orthopaedic implants.