Room-temperature deformation behavior of directionally solidified multiphase Ni−Fe−Al alloys

Directionally solidified (DS) β+(γ+γ′) Ni−Fe−Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior of a brittle 〈001〉-oriented β (B2) phase. The ductile phase in the composite consisted of a fine distribution of ordered γ′ precipitates in a γ (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar+proeutectic β, and discontinuous γ. The β matrix in the latter two microstructures contained fine-scale bcc precipitates formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (K _Q) of 30.4 MPa were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility and fracture toughness, 〈100〉 slip was seen in the β phase, whereas 〈111〉 slip was seen in the β phase in the microstructure which showed limited ductility. The high ductility and toughness are explained in terms of increased mobile dislocation density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is secondary to that of slip transfer. A. MISRA, formerly Graduate Student, Department of Materials Science and Engineering, University of Michigan is Research Associate     

Phase equilibria in Ti−Fe−Nb alloys

Isothermal sections for 750 and 600°C have been constructed for the Ti?Fe?Nb ternary phase diagram in the region of titanium-rich alloys by the use of metallography, x-ray diffraction, and electron-probe microanalysis. At 750–600°C, 3.0–3.5 mass % niobium raises the solubility of iron in α-titanium from 0.05–0.06 mass % to 1.0–1.5 mass %.     

Effects of Al content on porous Fe–Al alloys

Abstract

Porous Fe–Al alloys with the nominal composition ranging from Fe–20 wt-%Al to Fe–60 wt-%Al have been fabricated by Fe and Al elemental powder reactive synthesis. The effects of the Al content on the pore properties of resultant porous Fe–Al alloys were systematically studied. It has been found that the volume expansion, the open porosity and the permeability can be manipulated by varying the Al content and that their maximum values are reached at Fe–45 wt-%Al. Their mechanical properties suggest that they are strong enough for the filtration applications.     

Microstructural characteristics of quasicrystalline phases in alloys of AlCuFeCo

An investigation of the structural characteristics of quasicrystalline phases obtained in quaternary alloys is carried out. The coexistence between the quasicrystalline phases phases is analyzed. HREM images and diffraction patterns which correspond to the decagonal and icosahedral phases show pronouned deviations from the perfect decagonal and icosahedral symmetries. These effects are more pronounced when the specimen is annealed. Both kind of quasicrystalline phases show planar faults and dislocation-type of defects. Planar faults show the same image contrast features as in the crystalline case. Evidence is presented based on image contrast characteristics of two different types of planar faults. Dislocations in these phases show no evidence of spliting into partials.     

Multiphase diffusion in Fe−Ni−Al system at 1000°C: II. Interdiffusion coefficients for β and γ alloys

Ternary interdiffusion coefficients were determined at 1000°C at several Fe−Ni−Al alloy compositions with multiphase β(bcc)vs γ (fcc) diffusion couples which developed planar β/γ-interfaces. The coefficients, (i,j=Al or Ni) were calculated at compositions corresponding to points of intersections of diffusion paths with Fe taken as the dependent component. These coefficients varied with composition by 1 to 2 orders of magnitude in the β-phase but relatively little in the γ-phase. Empirical relations were derived to describe the composition dependence of the main coefficients. and . Interdiffusion coefficients with either Al or Ni as the dependent component were also evaluated. The relative diffusivities of the elements increase in the order, Fe, Ni, Al for both β- and γ-alloys. The ternary diffusion data were consistent with binary interdiffusion coefficients for Fe−Al and Fe−Ni alloys. G. H. CHENG, formerly a Graduate Student at Purdue University     

Direct observation of coarsening in AlCu alloys

Observations have been made on coarsening of θ-CuAl₂ precipitates at the surface of AlCu alloys. One specific area was examined over extended periods so that dissolution and growth of individual precipitates could be followed. It was found that nearly all precipitates shrank on ageing, even those larger than the average particle size. Only the largest 8% increased in dimension. A simple theory was used which gave good agreement with the observed dissolution kinetics of all precipitates except the very largest. This theory assumes a solute flux from the precipitate to a larger precipitate located a significant distance away. The theory gave a mean value for interfacial energy of 0.81 J/m². Particles were observed to spheroidise during ageing, and by the time their dimension had decreased 50%, they were completely spherical. Particle coalescence took place in a few cases but was generally of minor importance in the overall coarsening process.     

Banding phenomena in AlFe alloys subjected to laser surface treatment

Using beam rates of between 0.005 and 18.0 m/s, laser-melted tracks were produced on AlFe alloy samples containing between 0.25 and 8.0 wt% Fe. The local solidification rates were measured by taking a longitudinal section through the centre of the laser trace, and the corresponding microstructures were studied quantitatively using transmission electron microscopy. Two different banded structures were observed: one at slow scanning rates (low-velocity bands) and another at high growth rates (high-velocity bands). The low-velocity bands were shown to depend essentially upon conditions prevailing at the surface, and were attributed to convection (Marangoni) effects. For all of the alloys there was a critical growth rate, at which the cellular-dendritic structure was replaced by a high-velocity banded structure which consisted of a succession of light and dark bands which lay approximately parallel to the solid-liquid interface. The structure of the dark bands was similar to that of the cellular-dendritic structure prior to the onset of the banded structure. Diffraction patterns from the light bands exhibited spots only of the f.c.c. α-Al solid solution and microanalyses showed that, within a light band, the concentration was uniform and equal to the nominal concentration of the alloy. With increasing growth rate, the widths of the dark bands decreased continuously and, since the overall spacing of the bands remained constant, this finally led to the disappearance of the dark bands. A completely precipitation-free was then observed at very high growth rates in the more dilute alloys. A phenomenological model, based upon periodic instabilities of the growth rate, was proposed in order to explain the origin of the high-velocity banded structure.     

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.     

Morphology and growth of alumina inclusions in Fe–Al alloys at low oxygen partial pressure

Abstract

The degree of supersaturation is a factor that influences the Al₂O₃ inclusion characteristics in steel. The influence of the addition of a large amount of Al in the molten steel on the formation, growth and morphology of Al₂O₃ inclusions was investigated by laboratory scale experiments. Consecutive steel samples were taken during the deoxidation process and subjected to chemical analysis (ICP-AES), automated image analysis (AIA) and scanning electron microscopy (SEM) assessment with respect to the extracted inclusions. The characterisation and quantification of Al₂O₃ particles show different growth processes, leading to variations in particle size distribution as well as in the morphology.     

Corrosion of Al—Fe in hydrochloric acid

Abstract

Nanoprocessing can be considered a distinct form of grain boundary engineering by which property enhancements are achieved by deliberately increasing the volume fractions of grain boundaries and triple junctions in a material. Electrodeposition has been shown to be a technologically viable production method to synthesize such materials both in bulk form and as thin films. The mechanical, magnetic, electrical and corrosion properties exhibited by nanocrystalline metals produced by this method make them strong contenders for a number of advanced materials applications.

Résumé

Nous pouvons considérer le nanotraitement comme une forme distincte de génie des joints de grains par lequel nous obtenons des améliorations de propriétés en augmentant délibérément le volume des fractions des joints de grains et des jonctions triples dans les matériaux. On a montré que l'électrodéposition est une méthode technologiquement viable pour synthétiser de tels matériaux, aussi bien en films minces que sous forme massive. Les propriétés mécaniques, magnétiques, électriques et corrosives des métaux nanocristallins obtenus par cette méthode, font d'eux des concurrents sérieux pour nombreuses applications de matériaux avancés.     

Microstructural aspects of strain localization in AlMg alloys

The literature contains a number of continuum plasticity models describing the onset of strain localization but little definitive work which describes the microstructural transitions which accompany localization. In the present study a range of metallographic methods have been used in order to observe the progression of localization from events within single grains to the spatial organization of these events across the entire sample. The deformation mode used was cold rolling and observations were made using a variety of orientations relative to the rolling direction. It is concluded that in the AlMg system, localization begins by a structural instability in the accumulated dislocation substructure and later becomes organized into macroscopic bands due to local stress concentrations. The structure of the macroscopic bands is complex. They contain some high angle boundaries suggesting that they form due to the rapid and cooperative action of a number of slip systems over distances of the order of 0.2 μm. The bands cross grain boundaries and are organized in a cooperative sense because they represent local softening events. Thus, the shear bands in AlMg appear to form without texture softening or the need for the precursor of a lamellar structure. They involve a dramatic local change in the process of dislocation accumulation which is essentially a form a local dynamic recovery. The events become spatially organized to form macroscopic bands inclined at approx. 35° to the rolling plane as required by continuum plasticity.