Phase Dispersoids Research Articles

Effects of air atomisation, spray deposition and melt overflow techniques on microstructural features of Al–Fe–V–Si alloy

Microstructural features of atomised powders, spray deposited preforms and melt overflow strips of 8009 series AlFeVSi alloy were investigated to reveal the microstructural evolution associated with the processing condition variations. X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM) techniques were used to identify the crystal structures of dispersoid phases in the specimens. The primary intermetallic phase was characterised as bcc α-Al(Fe,V)Si having a lattice parameter in the range 12·51–12·53 Å, other phases are identified as icosahedral, Al3Fe and Al13(Fe,V)4 by XRD. SEM was used to examine the morphological changes and quantitative element analysis to reveal the chemical composition of these phases in specimens. Various phase morphologies such as starlike, band shape and needlelike intermetallics were observed in these products, whereas the primary phase crystal structure is cubic and stable.

Dispersoid quantification and size distribution in hot and cold processed AA3103

A quantification procedure coupling Field Emission Gun Scanning Electron Microscope (FEGSEM) and image analysis software has been developed to obtain size distributions of secondary phase dispersoids. The procedure is described in detail for aluminium alloys, and each involved parameter is evaluated separately. The first part of the procedure is the acquisition of the micrograph; both microscope and software parameters are important in this stage. The second part is the most critical step, where the micrograph is transformed to a binary image. Here, the choice of the threshold value is of great importance because it corresponds to the maximum loss of information from the original image. The quantification procedure is then presented with considerations and examples of statistical meaning. The quantification method is applied to a series of AA3103 samples taken from different industrial processing steps. The results obtained are consistent with the expected trends and prove the quantification procedure to be valid.

Reactive synthesis of alumina-boron nitride composites

The reactions of aluminum borates (9Al 2O 3 · 2B 2O 3 and 2Al 2O 3 · B 2O 3) with aluminum nitride (AlN) have been used as a new chemical route to synthesize alumina-boron nitride (Al 2O 3–BN) composites. Reaction mechanisms were investigated by TG-DTA and static reaction process. The reactions started at around 1200 °C and completed at around 1500 °C. Soaking at temperatures higher than 1800 °C resulted in the reverse reaction that caused great weight loss. Hot pressing promoted the reactions due to the improved diffusion process. The in situ formed BN phase was in agglomerate shape located at the pockets of Al 2O 3 matrix particles and this distribution was suggested to be beneficial to the strength of materials with weak phase dispersoids. The fracture surface analysis demonstrated that the main fracture mode was transgranular, indicating the existence of a strong Al 2O 3 network in the in situ synthesized composites. The prepared composites exhibited high strength, low Young’s modulus and high strain tolerance.

Transport properties and battery discharge characteristics of the Ag+ ion conducting composite electrolyte system (1−x)[0.75AgI: 0.25AgCl]: xFe2O3

Ion transport and battery discharge characteristic studies are reported for a new Ag+ ion conducting two-phase composite electrolyte system (1−x)[0.75AgI: 0.25AgCl]: xFe2O3, where 0 ≤ x ≤ 0.5 in molar weight fraction. An alternative single-phase host-matrix ‘annealed [0.75AgI: 0.25AgCl] mixed system/ solid solution’ has been used in place of the traditional host, AgI. Submicron size particles (<1 µm) of Fe2O3 has been used as second phase dispersoid. The composition 0.8[0.75AgI: 0.25AgCl]: 0.2Fe2O3, exhibiting the highest room temperature conductivity has been referred to as the optimum conducting composition (OCC). The reason for an enhancement of an order of magnitude in the conductivity from that of the pure host has been identified through direct determination of ionic mobility (μ) and mobile ion concentration (n) using transient ionic current (TIC) technique. The XRD study confirmed the coexistence of the constituent phases. The ionic transference number is found to be very close to unity. This reveals the fact that the silver ions are the sole charge carriers in the system. The results are discussed in the light of space-charge models proposed for the two-phase composite electrolyte systems. Solid state batteries, fabricated using OCC as electrolyte, Ag-metal as anode and mixtures of iodine & graphite, viz. (C+I2), (C+KI3), (C+(CH3)4NI3), (C+(C2H5)4NI3), etc. as cathodes, were discharged under different load conditions. The battery with (C+I2) cathode performed satisfactorily specially under low current drain states.

Grain refinement in as-cast 7475 aluminum alloy under hot deformation

Abstract Grain refinement in an as-cast 0.16%Zr modified 7475 aluminum alloy was studied by means of compression at 490 °C and metallographic observations. The σ versus e behavior shows a significant strain softening just after yielding and an apparent steady-state flow at strains of above 0.3. The structural changes are mainly characterized by development of deformation bands at an early stage of deformation ( e ≤0.3), followed by new grain evolution during steady-state flow due to operation of grain boundary sliding (GBS). GBS can result in local lattice rotation and formation of deformation bands in the as-cast coarse grain interiors. The number and the misorientation angle of deformation bands increase with deformation, followed by evolution of new grains in high strain. It is concluded that grain refinement occurs by a deformation-induced continuous reaction; that is continuous dynamic recrystallization (CDRX). A key role of GBS and second phase dispersoids on CDRX is discussed in detail.

Characterization of basic transport properties in a new fast Ag + ion conducting composite electrolyte system: (1− x)[0.75AgI:0.25AgCl]: xZrO 2

Studies of some basic ionic transport properties of a new fast Ag + ion conducting two-phase composite electrolyte system: (1− x)[0.75AgI:0.25AgCl]: xZrO 2, where 0≤ x≤0.5 (in molar weight fraction), are reported. A ‘quenched/annealed [0.75AgI:0.25AgCl] mixed system/solid solution’ has been used as a first phase host matrix salt as an alternative to the traditional host AgI, while particles (≤5 μm) of the insulating and inert ZrO 2 were dispersed as second phase dispersoid. In order to find the ‘optimum conducting composition’ (OCC), different compositions of the two phases were mixed homogeneously adopting various routes of preparation. The phase identification studies revealed the coexistence of separate phases. The temperature-dependent transport property studies were carried out on OCC employing various techniques. The mechanism of ion transport has been explained on the basis of models proposed for two two-phase composite electrolyte systems.

Advances in liquid phase coating: R.G.Iacocca. (Pennsylvania State University, USA.)

The goal of this study is to fabricate perovskite type ferroelectric particles-dispersed ceramic nanocomposites though conventional hot-pressing or pulse electric current sintering (PECS). This type of nanocomposite is expected to show ferroelectricity or piezoelectricity with retaining mechanical properties. Magnesia (MgO) and barium titanate (BaTiO3) were selected as a matrix and secondary phase dispersoid. From X-ray diffraction analysis, the BaTiO3 was the phase compatible with the MgO matrix, and there were no reaction phases between the matrix and BaTiO3. It was found that the BaTiO3 enhanced the sinterability of the MgO ceramics. Relative density of pure MgO was lower than 80%, while dense MgO/10 vol% BaTiO3 nanocomposites could be successfully prepared by sintering at 1200°C for 10 min through PECS method. Fine BaTiO3 particles were homogeneously dispersed within the MgO matrix grain as well as at grain boundaries. Sintering behavior and microstructure development of the MgO/BaTiO3 nanocomposites were discussed in terms of BaTiO3 content and sintering temperatures.

Fabrication of piezoelectric particle-dispersed ceramic nanocomposite

The goal of this study is to fabricate perovskite type ferroelectric particles-dispersed ceramic nanocomposites though conventional hot-pressing or pulse electric current sintering (PECS). This type of nanocomposite is expected to show ferroelectricity or piezoelectricity with retaining mechanical properties. Magnesia (MgO) and barium titanate (BaTiO 3) were selected as a matrix and secondary phase dispersoid. From X-ray diffraction analysis, the BaTiO 3 was the phase compatible with the MgO matrix, and there were no reaction phases between the matrix and BaTiO 3. It was found that the BaTiO 3 enhanced the sinterability of the MgO ceramics. Relative density of pure MgO was lower than 80%, while dense MgO/10 vol% BaTiO 3 nanocomposites could be successfully prepared by sintering at 1200°C for 10 min through PECS method. Fine BaTiO 3 particles were homogeneously dispersed within the MgO matrix grain as well as at grain boundaries. Sintering behavior and microstructure development of the MgO/BaTiO 3 nanocomposites were discussed in terms of BaTiO 3 content and sintering temperatures.

Cast microstructure and dispersoid formation in spray deposited Al–Li alloys

Spray deposition has been used to increase the concentration of dispersoid forming elements in model Al–Li–TM alloys (where TM stands for transition metals). The cast grain structure and the composition and stability of the dispersoids formed after a range of precipitation treatments was investigated. Very fine, nearly uniform, grain structures were obtained in the sprayed billets. The smallest grain size produced was 13 μm. The substantial grain refinement achievable by spray casting is partly owing to the high cooling rates, but was also found to be strongly dependent on the content and combination of the peritectic dispersoid forming elements, which increase the survival of dendrite fragments formed on impact of the semisolid metal droplets. It was found that materials with the highest peritectic forming element concentrations showed the finest grain size. Combinations of Zr and Ti gave the best grain refinement. The influence of Ti, V, and Sc on the composition and coarsening rates of the β′ Al3 (Zr,X) L12 dispersoids formed during post-spraying precipitation treatments was also investigated. The elements Ti and Sc were found to be soluble in the Al3 (Zr,X) L12 phase substituting for Zr in proportion to the average alloy composition. Titanium containing Al3 (Zr,X) L12 β′ phase dispersoids had the slowest coarsening rates, but their stability was reduced by the presence of Ti and they transformed to the equilibrium D023 phase after prolonged heat treatment. On aging, the precipitation behaviour of the δ′ phase was also affected by the Ti additions.

Oxide Film Formation on a Microcrystalline Al Alloy in Sulfuric Acid

The differences in the electrochemical behavior of a rapidly solidified, two‐phase (matrix and dispersoid) Al‐based alloy containing Fe V and Si (FVS0812 alloy) and the bulk form of its matrix and dispersoid phases were investigated in sulfuric acid FVS0812 exhibited generally higher electrochemical activity than the matrix material due to the presence of the very active dispersoid phase in the alloy. Impedance studies indicated that the properties of thin films formed on the FVS0812 and matrix substrates during only 5 min of anodization are similar, whereas the film formed on the alloy during 2.5 h of anodization was substantially less resistive and contained a thinner and/or damaged underlying barrier oxide compared to a comparable film formed on the matrix. Compared to the classical structure of porous Al oxide films with underlying barrier oxide, the oxide film on FVS0812, as seen by TEM, was thinner, with an intermittent banner oxide underlying a porous oxide of contorted morphology. Evidence is seen for the loss of dispersoids from the oxide film, leaving voids throughout its structure, perhaps the reason for its lack of physical adherence and its limiting thickness.

The effect of retrogression and re-ageing on the ductile fracture toughness of Al-Zn-Mg alloys containing different dispersoid phases

The effects of an incoherent and of a semi-coherent dispersoid phase upon the fracture toughness of a 7000-series AlZnMg alloys has been investigated. Standard crack opening displacement tests were successfully applied, the crack propagation being shown to be predominantly intergranular. The presence of manganese bearing incoherent dispersoids lowers the fracture toughness due to their effect of promoting microvoid formation. Retrogression and re-ageing led to a coarsening of the grain-boundary phases, but because the surface-to-surface separation of these particles remained approximately constant, there was no effect of this treatment, at constant yield stress, upon the fracture toughness. The toughness was only enhanced in the system (CZH) where retrogression and re-ageing led to a reduced yield stress.

Effect of V or Zr addition on the mechanical properties of the mechanically alloyed Al-8wt.% Ti alloys

The thermal stability of the MA Al-Ti alloy was improved by the addition V or Zr. Such improvement is attributed to the retarded coarsening of the dispersoid phases due to the improved coherency between dispersoid phases and the Al-matrix in the presence of alloying elements V and Zr. The compressive yield strength of ternary MA Al-8Ti-2.8V and Al-8Ti-5.1Zr alloys were superior to binary MA Al-STi alloy at the temperature of above 400 °C, but the MA Al-8Ti-5.1Zr alloy showed some brittleness at 450 °C.

Discussion of “the distribution of dispersoid phases in the extruded ODS Superalloy MA-957”

Discussion of “the distribution of dispersoid phases in the extruded ODS Superalloy MA-957”

A comparison of the microstructures of As-cast and laser surface melted Ti-8Al-4Y

A Ti-8 at. pct Al-4 at. pct Y alloy has been cast in small quantities and subjected to laser surface melting treatments in an effort to produce a uniform distribution of fine-scale dispersoids by internal oxidation. The microstructures of both as-cast and laser-modified material have been characterized using transmission electron microscopy, energy dispersive X-ray spectroscopy, and convergent beam electron diffraction, and the identity and distribution of second phase dispersoids established. An ability to refine significantly the dendritic structure of the as-cast alloy by laser surface treatment is demonstrated. The matrix phase of the modified alloy is martensitic α′, in contrast to the α phase of the as-cast material. The interdendritic phase observed in both as-cast and laser-modified material and the fine-scale dispersed particles (50 to 70 nm) in the as-cast alloy are shown to be rich in Y, with a structure consistent with that of the oxide Y2O3. Dispersoids within the laser modified zone are typically 8 to 20 nm in diameter and show a tendency to form preferentially at interlath boundaries of the martensite. The origins of these structures and their potential effects on the mechanical properties of the alloy are discussed briefly.

Identification of dispersoid phases created in aluminum during mechanical alloying

Dispersion strengthened aluminum has been produced by mechanical alloying (high energy ball milling) and subsequent extrusion. The production process was modified in several ways in order to determine the changes of the dispersoid structure during annealing. As such, extrusion was performed cold and milling was conducted in part without lubrication. The high strength of the mechanically alloyed aluminum is due to a dispersion of both γ-Al2O3 and Al4C3-particles with sizes of the order of 10 nm. The dispersoids are amorphous after milling and crystallize during heating in the temperature range of 400 to 500 °C (673 to 773 K). The crystallization coincides with an increase of the room temperature hardness of the material. Possible techniques for improving dispersion strengthened materials have been considered and are discussed.