Considerations sur la seluence de precipitation dans les alliages aluminium-linc

le have studied the ageing of aluminium-linc alloys by X-ray scattering and electron microscopy over the temperature range t-80°C to + 150°C. Only tlo precipitated phases have been observed:

• 1.(1) Guinier-Preston lones for lhich the sile effect has a constant value of ϵt- t-0.018. le have also calculated the crystalline lattice deformation inside the lones. These deformations are of the rhombohedral type such that the lones become ellipsoidal and for the greatest sile of lones are of the same order of magnitude as these characterising the phase χt'_R.
• 2.(2) A semi-coherent hexagonal precipitate the parameters of lhich are found to be close to these pure linc.

     

Microstructure of alumina-forming oxidation resistant Al-Ti-Cr alloys

• 1.1) The protective alumina-forming Al-Ti-Cr alloys identified by Perkins et al. [2] are based primarily on the τ (L1₂) and TiCrAl laves phases.
• 2.2) The τ phase in the multiphase protective alumina-forming Al-Ti-Cr alloys is not stable at 1073K and decomposes to r-Al₂Ti and Cr₂Al.
• 3.3) Future coating alloy development in the Al-Ti-Cr system should focus on the γ/TiCrAl laves two phase field.

     

Reversible and irreversible length changes in amorphous Fe40Ni40B20 during structural relaxation

Measurements are presented of the isothermal and isochronal change of length during structural relaxation of amorphous Fe₄₀Ni₄₀B₂₀. The observed changes in length can be separated in three contributions:

• 1.(1) An irreversible part with a well denned activation energy of 250 kJ/mol, which is well described by the free volume model.
• 2.(2) A relatively small reversible part in the activation energy range from 210 to 250kJ/mol and with a magnitude of about 0.4ppm/K. This is ascribed to Chemical Short Range Ordering.
• 3.(3) An irreversible part in the activation energy range from 130 to 250 kJ/mol, with a magnitude of about 800 ppm in as-quenched specimens.

It is shown that the length isothermals of as-quenched specimens, after correction for the free volume contribution, can be scaled to form a “masterplot”.     

Grain boundary ductile fracture in precipitation hardened aluminum alloys

Published microstructural studies of grain boundary (gb) fracture in precipitation hardened aluminum alloys are reviewed with respect to the three main ideas that have been developed to explain the gb fracture surfaces. The ideas are

• 1.(1) microvoid growth at large gb precipitates,
• 2.(2) strain localization in the soft, and sometimes solute-free, gb precipitate free zones (pfz) and
• 3.(3) the influence of matrix precipitate shear giving rise to inhomogeneous “planar” slip that may apply large stress concentrations to the gb at the end of slip bands. Although the last two processes have a supporting role in many cases, the published evidence strongly suggests that the first process is of overwhelming importance. This conclusion has been tested by reversion experiments in model Al-Li alloys in which microstructures with increasing area fractions, A_f, of large stable δ precipitates (Al-Li) were produced, but with equivalent matrix structures and yield strengths. The materials show marked falls of toughness and of fracture strain as A_f was increased. Studies of surface slip markings in the Al-Li alloys suggested that slip was initiated at the large gb δ precipitates. Only very limited evidence for a role of planar slip in the fracture of the Al-Li alloys was found in contrast to observations on high purity Al-Zn-Mg-Cu alloys where planar slip seemed to show more importance. Brief studies on a nickel based alloy, MAR-M200, suggested that even in the absence of a pfz, strong room temperature embrittlement by gb precipitates was produced. The results of this study suggest that the marked problems of gb fracture in Al-Li alloys are associated with large gb δ precipitates. Jensrud and Ryum [Mater. Sci. Engng64, 229 (1984)] have shown how gb precipitate growth is facilitated in this system as the gb δ phase is very much less soluble than the strengthening δ′ (Al₃Li) phase.

     

Overview no. 84: An analysis of diffusion and diffusional creep in stoichiometric and hyperstoichiometric uranium dioxide

Diffusion and diffusional creep mechanisms are analyzed for stoichiometric and hyperstoichiometric UO_2+x. A variety of criteria are used in the evaluation based on a critical review of literature data:

• 1.(1) agreement between measured tracer diffusivities and those calculated from creep rates or reported for sintering;
• 2.(2) self-consistent values of the activation energies for various diffusion controlled processes;
• 3.(3) self-consistent and theoretically reasonable stoichiometry dependencies for self-diffusion, creep, and sintering;
• 4.(4) a satisfactory grain size correlation in the diffusional creep regime.

It is conclusively shown that Coble creep is the dominant mechanism for any practical grain size where viscous creep processes are diffusion limited. This conclusion applies at stoichiometric and all oxygen-rich compositions. At low stress interface controlled diffusional creep is an important mechanism. A compilation of the “best” set of uranium grain boundary and lattice diffusivities is presented for the entire range of UO_2+x stoichiometries.     

Literature Review

• Powder production
• Pressing
• Sintering
• Fundamentals
• Aluminium
• Copper
• Iron and steel
• Magnesium
• Molybdenum
• Nickel
• Titanium
• Tungsten
• Ceramic materials
• Composite materials
• Electrical and magnetic materials
• Hard materials and tool steels
• Mechanical alloying
• Porous materials
• Powder forging
• Powder injection moulding
• PM parts
• Hard materials
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• Microstructure property relationships
• Powder compaction
• Metal matrix composites
• Surface treatment and coatings
• Process modelling

     

Correlations between stress,strain and dislocation arrangement in weakly deformed copper single crystals

The development of dislocation structure along the stress strain curve in stage I and the beginning of stage II of Cu single-crystals was followed up by means of etch pits. The following experimental results were found:

• 1.(i) The critical resolved shear stress is linearly (not proportional) dependent on the square root of the initial dislocation density.
• 2.(ii) The transition from stage I to II shows notably an unusual weak dependence of the etch pit density at the cross glide plane (primarydislocation density) on the shear strain.
• 3.(iii) A linear dependence arises between the flow stress in a certain slip plane and the square root of the etch pit density in this plane.
• 4.(iv) The latent hardening rises in stage I to a value of three and falls in stage II to a constant value of approx. 1.2.
• 5.(v) At unloading, the dislocations glide only slightly backwards.

They arrange themselves strongly in subboundaries.     

Computer simulation of grain growth—V. Abnormal grain growth

Monte Carlo computer simulation techniques have been utilized to investigate abnormal grain growth in a two dimensional matrix. The growth of abnormally large grains is modelled under two conditions:

• 1.(a) where the driving force is provided solely by curvature and
• 2.(b) where the driving force is provided by the difference in the gas-metal surface energy between grains of different crystallographic orientation.

For curvature driven growth three cases are considered:

• 1.(a) the growth of abnormally large grains in microstructures without grain growth restraints,
• 2.(b) the growth of abnormally large grains in microstructures with particle dispersions, and
• 3.(c) grain growth in a particle pinned microstructure in which a sudden decrease in the number of particles occurs.

In all these cases, the initiation of abnormal grain growth/secondary recrystallization is not found to occur. In systems free from grain growth restraints the normal grain size distribution is very robust and strongly resistant to perturbations. For systems which contain particle dispersions strong pinning of the grain boundaries is always observed. However, when a preferred surface energy orientation is introduced, abnormal grain growth/secondary recrystallization does take place. The microstructural evolution observed during secondary recrystallization is in good correspondence with experiment. The area fraction of secondary grains exhibits sigmoidal behavior as a function of time, and is characterized by an Avrami exponent of 1.8 ± 0.3 when fit to a modified Avrami equation.     

Computer modelling of Ostwald ripening

On the basis of the multiparticle diffusion equation in the Ostwald ripening, we construct a new effective model by using three simplifications:

• 1.(1) explicit consideration of the screening effect of the diffusion field,
• 2.(2) the dimensional reduction,
• 3.(3) the expansion in the volume fraction.

We simulate this model for various values of the volume fraction, and obtain the droplet size distribution functions, coarsening rates, the standard deviations and the skewness of the distribution function, which are compared with those of the earlier theories and the recent direct computer simulations of the multiparticle diffusion equation. The present results are in good agreement with those of the theory by Tokuyama and Kawasaki, which have pointed out the importance of the soft-collision processes.     

A continuous constraints model for large strain grain deformations

A relatively simple model is proposed to describe the large strain plastic deformation modes of individual grains in a polycrystalline aggregate undergoing constrained flow. Based upon the Taylor or full constraints theory, the transition to a relaxed constraints mode of grain deformation is modelled continuously by allowing the grains to select their current deformation mode according to a minimum second order plastic work criterion. This second order work term is shown to depend upon the following work hardening mechanisms:

• 1.(i) slip system hardening,
• 2.(ii) rotational hardening, and
• 3.(iii) grain interaction hardening.

This continuous constraints model is illustrated by a semi-analytical two-dimensional example and the influence of the different hardening mechanisms discussed.     

Regions of existence of special and non-special grain boundaries

The whole complex of experimental results on the structure and properties of grain boundaries in different materials has been analysed on the basis of the original works available by the present time. This has made it possible to show that

• 1.(a) special boundaries exist in a certain, finite interval of misorientation angles, the magnitude of this interval decreasing with increasing Σ, that is, reciprocal density of coincident sites;
• 2.(b) special boundaries exist in a finite temperature interval, and the “special boundary-non-special boundary” transition can occur below the melting temperature, the temperature of this transition decreasing with an increase of I;
• 3.(c) there is a threshold value of Σ = Σ_max that is temperature dependent and decreases with rising temperature.

Above Σ_max for a given temperature the boundaries with special misorientation angles are not different in properties from those of the general type. The analogy has been formulated between special and non-special grain boundaries with commensurate and incommensurate phases in adsorbed layers on the external surface.     

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Formation of recrystallization textures in rolled aluminum single crystals

Rolling and recrystallization textures of Al-single crystals have been measured with great accuracy. In all cases a clear 40° 〈111〉-orientation relationship was found. An analyses of these results and also of those of the literature led to the conclusion

• 1.(i) that this relationship is definitely a result of oriented growth,
• 2.(ii) that the nuclei effective in the present experiments were part of the deformed matrix with orientations near the edge of the spread of the deformation texture and
• 3.(iii) that the selection of the observed ± 40° 〈111〉-orientations out of the eight possible ones is determined by the availability of nuclei.

Strong arguments were found that, in contrast to recent opinion [11]. also in polycrystalline A1 oriented growth is largely responsible for the observed recrystallization textures.     

Atomic mechanisms of precipitate plate growth in the AlAg system—II. High-resolution transmission electron microscopy

High-resolution electron microscopy was used to study the interfacial structure of γ′ precipitates in an Al-15 wt% Ag alloy aged at 350°C. The results of these studies show that:

• 1.(1) all ledges are multiples of two {111} planes high, supporting the theory and conventional transmission electron microscopy observations that plate thickening occurs by passage of Shockley partial dislocations on alternate {111} planes
• 2.(2) most ledges are more than just two planes high, indicating a strong tendency toward diffusional and/or elastic interactions
• 3.(3) the terraces between ledges are atomically flat and ledges are uniformly stepped-down from the centers to the edges of isolated precipitates as predicted by the general theory of precipitate morphology
• 4.(4) the {111} planes are continuous across the edges of ledges, indicating that they are largely coherent and not disordered as treated in most kinetic analyses, and
• 5.(5) the edges of precipitate plates appear to be composed of similar two-plane ledges arranged vertically above one another and hence, may grow by the same mechanism of atomic attachment as ledges on the broad faces.

Examination of γ′ plates during early stages of growth indicates that their aspect ratio may deviate from the equilibrium value almost immediately, probably due to the ledge mechanism of growth. Lastly, an atomic model of a γ′ precipitate was used to test the high-resolution images obtained, and illustrate possible atomic growth mechanisms of the ledges.     

Rapid solidification of Al-Cu eutectic alloy by laser remelting

Rapid surface resolidification using a high powered CO₂-laser has been performed on eutectic Al-32.7 wt% Cu at speeds between 0.2 and 8 m/s. By means of longitudinal cuts through the centre of the laser trace, the local growth rate has been measured by observation of the orientation of the microstructure using transmission electron microscopy. The various microstructures as a function of growth rate, are:

• 1.(a) regular lamellar eutectic α-Al/θ-Al₂Cu structure for growth rates below 20 cm/s with interlamellar spacing as fine as 17 nm;
• 2.(b) a new wavy eutectic α-Al/θ'-Al₂Cu morphology for growth rates of between 20 and 50 cm/s;
• 3.(c) a banded structure formed by alternating supersaturated α-Al solid solution and the wavy eutectic for growth rates greater than 50 cm/s.

A recent analytical model for eutectic growth under rapid solidification condition is compared to the experimental results. Contrary to the classical λ²V_s = const. relationship, which predicts a continuous decrease in spacing as the growth rate increases, this new theoretical model clearly predicts a limit for the coupled eutectic growth which finds its analogy in single phase solidification in the limit of absolute stability.     

The formation of polytwinned structures in FePt and FePd alloys

• 1.1. The polytwinned structures in the FePt and FePd alloys derived from a nucleation and growth process in which the coherent ordered regions form aligned particle arrangements under the influence of the elastic strain energy. The impingement and coalescence of the ordered particles to form twin related structural domains give rise to a high density of APB's within the twin plates.
• 2.2. The nuclei form as disks along the {110} planes of the cubic matrix and these nuclei may merge with an order parameter or c/a ratio less than the equilibrium value but as the strain energy of the system relaxes the c/a ratio approaches equilibrium.
• 3.3. The polytwinned structure undergo a coarsening process under the mutual influence of the strain and surface energies analogous to “discontinuous coarsening” of lamellar two-phase aggregates resulting from cellular phase separation.

     

Morphological changes due to diffusion induced grain boundary migration

The evolution of the morphological changes that are caused by diffusion induced grain boundary migration (DIGM) has been studied on Fe exposed to Zn vapor at 600°C. The changes can be divided into four types; the first three involve pre-existing grain boundaries while the fourth is the growth of zinc-rich hillocks on the surfaces of grains. Those changes that occur at grain boundaries comprise:

• 1.(a) normal DIGM in which boundaries merely migrate,
• 2.(b) a combination of normal DIGM and the formation of new grains which grow in the other direction
• 3.(c) the nucleation, along a boundary, of two sets of new grains which grow in opposite directions and then coalesce giving rise to four orientations where there was initially two. Types b and c are interpreted as arising from diffusion induced boundary dissociation. Dezincification experiments showed that the boundaries return to their former positions without reversing topographic and compositional changes produced by their earlier migration.

     

The response of carbo-nitride particles in hsla steels to weld thermal cycles

The behaviour of carbo-nitride particles in HSLA steels when subjected to a weld thermal cycle corresponding to heat inputs of 2, 5, 10 or 20 kJ/mm has been studied by analytical electron microscopy. Three types of response were observed:

• 1.(a) complete dissolution followed by reprecipitation on cooling (in a Nb-V containing steel),
• 2.(b) partial dissolution accompanied by precipitate coarsening (in two Ti bearing steels)
• 3.(c) dissolution followed by reprecipitation at the peak temperature of the thermal cycle (in Ti-Nb bearing steels).

The latter behaviour was shown to be dependent upon the reheat temperature used during manufacture and was only found in a steel reheated to 950°C; this treatment promoted the formation of Nb-rich particles having a narrow size range in the base metal. The rates of dissolution observed in the experiments were supported by numerical calculations, which gave dissolution times of the order of 1 s or less at the peak temperature (1350°C) of the weld thermal cycle. The changes in the particle size distributions in the Ti and Ti-Nb bearing steels and the composition changes observed for (Ti, Nb) carbo-nitride particles were explained by modelling the precipitation behaviour in the austenite phase field.     

Grain reorientations in rolled aluminium sheet: comparison with predictions of continuous constraints model

The crystal orientations of 24 grains that were, initially, roughly equiaxed have been followed during the rolling of an aluminium sheet for thickness reductions up to 75%. The experimental results for the individual grains are compared with the theoretical lattice rotations calculated for three different simple grain deformation modes. These are

• 1.(i) the classical full constraints (FC) mode,
• 2.(ii) a relaxed constraints (RC) mode in which both in-compression plane shears are free, and
• 3.(iii) a recent continuous constraints (CC) mode in which all the shears in the grain are partially relaxed by using a minimum work hardening rate criterion. The grains were classified in four groups according to their final lattice orientation. For the 19 grains in the first two groups, the CC model gives a good quantitative agreement with the experimental results. In particular, the theoretically predicted continuous transition from the FC to the RC deformation mode of the grains is experimentally confirmed. The remaining 5 grains either rotate towards a Brass orientation or deform inhomogeneously.

     

Strain-hardening of spheroidized high carbon steels

The strain-hardening behavior of spheroidized steels is examined in terms of continuum and quasi-continuum models based on the requirement that continuity be maintained at various boundaries in the two-phase polycrystals. For plastic strains up to 3.5%, the increase in the flow stress appears to by composed of contributions arising from

• 1.(i) dislocations stored in the material for
  • 1.1.a) statistical and
  • 1.2.b) geometrical reasons
• 2.(ii) an average back stress predominantly due to the presence of the unrelaxed plastic strain discontinuity between the ferrite matrix and the cementite particles.

The latter contribution, which is a large fraction of the total strain-hardening increment, increases rapidly with strain up to a plastic strain of 3.5% and remains approximately constant thereafter. The attainment of the maximum value of the back stress corresponds to the observed transition in the overall strain-hardening rate (“double-n” behavior). The experimentally determined components of the hardening increment due to the geometrically necessary dislocations agree well with the predictions of Ashby's theory.At strains greater than 3.5%. the average back stress remains approximately constant and the geometrically necessary dislocation density increases only slightly with strain. This indicates a deviation from Ashby's theory which predicts a continuous increase of this dislocation population with strain. The strain-hardening increments at these strain levels therefore appear to be controlled mainh by the increase in the statistical dislocation population.