Acta Metallurgica (Acta Metall )

Publisher: American Society for Metals; American Institute of Mining, Metallurgical, and Petroleum Engineers; American Society for Testing and Materials, Elsevier


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Publications in this journal

  • Acta Metallurgica 01/1998; 46(17).
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    ABSTRACT: ABSTRACT: Abstract The paper presents the structure characteristics of porous cellular aluminium alloys molded into metallic shapes which containing soluble granular material (NaCl). In order to reduce the heat losses of the shape into which the granular material has been introduced is necessary for it to be preheated to a temperature close of alloy casting temperature.
    Acta Metallurgica 01/1998; No. 3(ISSN-1335-1532):. 258-261.
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    ABSTRACT: High resolution transmission electron microscopy (HRTEM) was used to study the structure of annealing twins in α-Al2O3. A basal twin was found where each of the two twin boundaries with the matrix had a different two-dimensional structure. The energies of alternative twin boundary structures were calculated using an ionic model. The relative difference in the energy of the boundary structures is appreciable, and the relative energies of steps at the boundary required for twin formation and termination show large differences for opposite growth directions in the basal plane. This suggests that the different twin boundary structures are the result of anisotropic growth of alumina, which is related to the lack of centro-symmetry in the trigonal corundum structure. The model is important for an understanding of annealing twin formation and growth mechanisms in sapphire, and could be extended to predict anisotropic growth phenomena for other non-centro-symmetric structures.
    Acta Metallurgica 01/1995; 43:835-848.
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    ABSTRACT: The nonequilibrium growth of partially supersaturated ferrite plates in Fe-C alloys is examined as a function of the carbon concentration, using a published theory for coupled diffusional/displacive transformations. It is found that the theory correctly predicts the variation in the martensite-start temperature with carbon concentration, but fails to estimate the corresponding variations in the bainite-start temperatures of the same steels. This is attributed to the fact that the model does not include any variations in stored energy has been calculated by fitting against available data.
    Acta Metallurgica 01/1993; 41(3).
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    ABSTRACT: This paper considers the evolution of equilibrium shape during the coarsening of a system of cubic precipitates in a cubic matrix with positive anisotropy [Delta] = C[sub 11] - C[sub 12] - 2C[sub 44] [gt] 0). The system is assumed to have homogeneous elastic constants and isotropic surface tension. The low-energy shapes include the sphere, and the octahedron, tetrahedron and plate with [l brace]111[r brace] faces. Minimization of the sum of the elastic and surface energies shows that the sphere is preferred at arbitrarily small sizes, but ordinarily transforms into a tetrahedron, and finally in a plate as size increases. When [Delta] [gt] 0.79C[sub 11] the octahedron is preferred for a small range of sizes between sphere and tetrahedron. Analytic expressions are given from the equilibrium shape transitions. The results are compared to experimental observations of shape changes during coarsening in (mg, Y)-ZrO[sub 2]. The experimentally observed absence of octahedral shaped precipitates and the size at which tetrahedral shaped precipitates become stable in this system are consistent with the theoretical predictions.
    Acta Metallurgica 10/1992;
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    ABSTRACT: Neutron diffraction experiments were conducted on 15vol.% whisker and 20vol.% particulate reinforced aluminum/silicon carbide composites subjected to a rapid quench followed by various deformation histories. Corresponding numerical simulations were carried out using an axisymmetric unit cell model, with a phenomenological, isotropic hardening descriotion of matrix plasticity. Thermal expansion and the temperature dependence of material properties were accounted for. For the whisker reinforced matrix, quantitative agreement was generally found between the measured and calculated residual elastic strains. For the particulate reinforced matrix, the calculations tended to overestimate the magnitude of the residual strains parallel to the deformation axis, but very good agreement was obtained transverse to the deformation axis. For the silicon carbide reinforcement, both whisker and particulate, the variation of predicted residual elastic strains along the deformation axis was qualitatively consistent with the measurements, although quantitative agreement was often lacking. Measured and predicted residual strains perpendicular to the deformation axis for the silicon carbide typically were not in agreement. Parametric studies were carried out to ascertain the dependence of calculated flow strengths and residual strains on cell and reinforcement aspect ratio, and on reinforcement spacing and shape.RésuméDes expériences de diffraction de neutrons ont été réalisées sur des composites aluminium/carbure de silicium (renforcés l'un par 15% en volume de poils (whiskers), l'autre par 20% en volume de particules) soumis à une trempe rapide suivie de différentes séquences de déformation. Les simulations numériques correspondantes ont été effectuées en utilisant un modéle de maille simple comportant un axe de symétrie, à l'aide d'une description phénoménologique de la plasticité de la matrice par durcissement isotrope. On a tenu compte dans ces calculs de la dilatation thermique et de l'effet de la température sur les propriétés du matériau. Dans le cas de la matrice renforcée par des poils, il y a en général accord quantitatif entre les déformations élastiques résiduelles mesurées et calculées. Dans le cas de la matrice renforcée par des particules, les calculs ont tendance à surestimer la valeur des déformations résiduelles paralléles à l'axe de déformation, mais l'accord est trés bon dans la direction perpendiculaire. Dans le cas du renfort SiC, sous forme de poils ou de particules, la variation des déformations élastiques résiduelles prédites le long de l'axe de déformation est en accord qualitatif avec les mesures, l'accord quantitatif faisant souvent défaut. Les déformations résiduelles mesurées et prédites perpendiculairement à l'axe de déformation pour le carbure de silicium ne correspondent pas en général. Des études paramétriques ont été menées pour préciser l'effet des rapports d'aspect de la maille et du renfort, ainsi que de l'espacement et de la forme des renforts, sur les contraintes d'écoulement et sur les déformations résiduelles calculées.ZusammenfassungProben des Verbundwerkstoffs Aluminium/Siliziumkarbid, verstas̈rkt entweder mit 15 Vol.% Whisker oder 20 Volo.% Teilchen, welche rasch abgeschreckt und danach unterschiedlich verformt worden sind, werden mittels Neutronenbeugung untersucht. Entsprechende numerische Simulationen werden mittels eines Modelles einer axisymmetrischen Einheitszelle und einer Beschreibung der Matrizplastizität mit einer phänomenologischen isotropen Härtung durchgeführt. Thermische Ausdehnung und Temperaturvehalten der Materialeigenschaften werden berücksichtigt. Bei der whisker-verstärkten Matrix ergab sich quantitative Übereinstimmung von gemessenen und berechneten elastischen Restverzerrungen. Bei der teilchenverstärkten Matrix neigten die Berechnungen dazu, die Höhe der Restverzerrungen parallel zur Verformungsachse zu überschätzen; sehr gute Übereinstimmung ergab sich quer zur Verformungsachse. Bei Verstärkung mit Siliziumkarbid, sowohl mit Whiskler als auch Teilchen, war die Variation der vorausgesagten elastischen Restdehnungen entlang der Verformungsachse qualitativ verträglich mit den Messungen, quantitative Übereinstimmung wurde oft nicht erreicht. Gemessene und vorausgesagte Restverzerrungen senkrecht zur Verformungsachse stimmten bei Siliziumkarbid typisch-werwise nicht überein. Um die Abhängigkeit der berechneten Fließspannungen und Restverzerrungen vom Aspektverhältnis von Zelle und Verstärkung und von Abstand und Form der Verstärkung abzusichern, werden Parameterstudien durchgeführt.
    Acta Metallurgica 09/1992;
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    ABSTRACT: An approximate computational scheme that accounts for boundary movement duringdiffusion of a surface-deposited tracer into a polycrystalline solid is presented. The method is compared to exact solutions for stationary boundaries and for rapidly-moving boundaries and found to be acceptably accurate. The proposed method bridges the transition between these two limiting cases. When applied to existing data for tracer diffusion into polycrystalline lead, the tracer grain boundary diffusivity is found to be ∼5 times larger than the value deduced for stationary-boundary theories. The importance of measuring grain growth during the diffusion experiments is noted.RésuméOn présente une méthode de calcul approché wui tient compte du mouvement des joints pendant la diffusion d'un traceur déposé en surface dans un solide polycristallin. La méthode est comparée aux solutions exactes pour les joints stationnaires et pour les joints en déplacement rapide et on trouve qu'elle a une précision acceptable. La méthoe proposée fait le pont entre ces deux limites. Lorsqu'on l'applique aux données disponibles pour la diffusion des traceurs dans le plomb polycristallin, on trouve que la diffusivité du traceur aux joints de grains est ∼5 fois plus grande que la valeur déduite des théories du joint stationnaire. On remarque l'importance de la mesure de la croissance des grains pendant les expériences de diffusion.ZusammenfassungEin numerisches Näherungsverfahren wird vorgelegt, mit dem die Korngrenzbewegung während der Diffusion eines an der Oberfläche abgeschiedenen Tracers in einen Polyckristall hineiin berücksichtigt wird. Dieses Verfahren wird mit exakten Lösungen für stationäre und sich rasch bewegende Korngrenzen verglichen: es ist annehmbar genau und kann damit die Lücke zwischen diesen beiden Grenzfällen überbrücken. Anwendung des Verfahrens auf die Tracerdiffution in polykristallines Blei ergibt für die Korngrenzdiffusivität einen Wert, der etwa fünfmal gröβer ist als der Wert, der aus Theorien für stationäre Korngrenzen folgt. Auf die Wichtigkeit, das Kornwachstum während der Diffusionexperimente zu messen, wird hingewiesen.
    Acta Metallurgica 06/1992;
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    ABSTRACT: An NbAl3 + Nb2Al composite reinforced with continuous zirconia-toughened alumina, PRD-166 fibers, was produced by pressure casting and was examined by optical and transmission electron microscopy and energy dispersive spectroscopy. Exposure of the fiber to the molten metal resulted in ZrO2 and Al2O3 grain growth, formation of a thin layer of an amorphous phase on the grain boundaries of Al2O3 and transformation of ZrO2. Preferential Al2O3 grain growth near the surface of the fiber led to the rejection of ZrO2 from this region into the molten metal. In NbAl3 slip occurred by the glide of a<110] superdislocations and tolesser extent by the glide of a pair of dislocations on the (112) planes and a/2<110] superpartial dislocations on the (001) plane. The operating slip system in Nb2Al was identified as {010)<100]. a<100] dislocations were dissocaited into ax<100] partial dislocations joined together by a stacking fault.RésuméUn composite NbAl3 + Nb2Al renforcé par de l'alumine durcie par la zircone continue (fibres PRD-166) est produit par coulée sous pression et examiné par microscopie optique, microscopie électronique en transmission, et par spectroscopie par dispersion d'énergie. L'exposition de la fibre au métal fondu provuque la croissance due grain de ZrO2 et de Al2O3, la formation d'une fine couche d'une phase amorphe sur les joints de grains de Al2O3 et la transformation de ZrO2. La croissance préférentielle de grains de Al2O3 près de la surface de la fibre conduit au rejet de ZrO2 hors de cette région vers le métal fondu. Dans le NbAl3, le glissement se produit par mouvement de superdislocations a<110> et, à un degré moindre, par le glissement d'une paire de dislocations dans les plans (112) et de dislocations superpartielles a/2<100] dans le plan (001). Le système de glissement actif dans Nb2Al est identifié comme étant {010}<100]. Les dislocations a<100] sont dissociées en imparfaites a/x<100] reliées entre elles par un défaut d'empilement.ZusammenfassungDas Verbundmaterial NbAl3 + Nb2Al, verstärkt mit kontinuierlichen Fasern aus Zirkonoxid-verstärktem Aluminiumoxid, wird im Druckguβ hergestellt und mittels Lichtund Durchstrahlungselektronenmikroskopie und energiedispersiver Spektroskopie untersucht. Der Kontakt der Faser mit der Metallschmelze führt zu Wachstum von ZrO2- und AlO3-Körnern, zur Bildung einer dünnen Schicht einer amorphen Phase an den Korngrenzen des Al2O3 und zur Transformation des Zro2. Bevortzugtes Wachstum der Al2O3-Körner in der Nähe der Faseroberfläche führt in diesem Bereich zur Zurückweisung des ZrO2 in die Metallschmelze. Im NbAl3 tritt Gelitung durch Superversetzungen a<110] und zu einem geringeren Grade durch ein Superversetzungspaar von -Versetzungen auf (112)-Ebenen und von a/2<110]-Superteilverstzungen auf der (001)-Ebene auf. In Nb2Al ist das Gleitsystem {010}<100] aktiv. Die Versetzungen a<100] sind in Teilversetzungen a/x<100] aufgespalten, die über einen Stapelfehler miteinander verbunden sind.
    Acta Metallurgica 02/1992;
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    ABSTRACT: The morphological evolution of coherent inclusions in elastic media is studied in two-dimensions. The inclusions are simple dilations with isotropic surface energy in a system with homogeneous elastic constants of negative anisotropy. The equilibrium sizes at which a circular inclusion transforms to a rectangle or square, and at which a square splits into a doublet or quartet of separated inclusions are computed analytically. A finite-element model is then constructed to simulate the evolution of an arbitrary distribution of inclusions along the minimum-energy path. In the model, the circle evolves into a square, which splits into a doublet by hollowing from its center, or, if this is forbidden, by drawing in a perturbation on its surface. The sizes at which shapes spontaneously transform are compared to the equilibrium values. Finally, the simulation is used to study the evolution of a random distribution of inclusions. The first metastable state assumed by the distribution depends on the elastic interaction, surface energy and areal fraction of the inclusion phase through a single dimensionless parameter that groups these three effects. The results are compared to prior theoretical and experimental work on coarsening patterns in three dimensions.RésuméOn étudie l'évolution de la morphologie d'inclusions cohérentes dans de milieux élastiques, à deux dimensions. Les inclusions sont de simples dilatations avec une énergie superficielle isotrope dans un système qui présente des constantes élastiques homogènes d'anisotropie négative. Les tailles d'équilibre pour lesquelles une inclusion circulaire se transforme en un rectangle ou en un carré, et pour lesquelles un carré se scinde en un doublet ou en un quartet d'inclusions distinctes sont calculées sur ordinateur par une méthode analytique. Un modèle d'élements finis est alors construit pour simuler l'évolution d'une répartition arbitraire d'inclusions suivant le trajet d'énergie minimale. Dans ce modéle, le cercle se transforme en un carré qui se dédouble en se creusant à partir du centre ou, si ceci est interdit, en tirant sur une perturbation à sa surface. Les tailles pour lesquelles les formes se transforment spontanément sont comparées aléatoire d'inclusions. Le premier état métastable pris par la distribution dépend de répartition aléatoire d'inclusions. Le premier état métastable pris par la distribution dépend de l'interaction élastique, de l'énergie superficielle et de la fraction d'aire occupée par la phase d'inclusion, à travers un seul paramètre sans dimension qui regroupe ces trois effets. Ces résultats sont comparés à des travaux théoriques et expérimentaux antérieurs sur des modèles de grossissement à trois dimensions.ZusammenfassungDie Entwicklung der Morphologie von kohärenten Einschlüssen in elastischen Medien wird in zwei Dimensionen studiert. Die Einschlüsse bestehen in einfachen Dilatationen mit isotroper Oberflächenenergie, die in einem System mit homogenen elastischen Konstanten und negativer Anisotropie verteilt sind. Die Gleichgewichtsgröβe, bei der sich ein kreisförmiger Einschluβ in einen rechteckigen oder quadratischen umwandelt, oder bei der ein Quadrat sich in ein Dublett oder Quadruplett aus getrennten Einschlüssen aufspaltet, werden analytisch berechnet. Danach wird ein Finit-Element-Modell aufgestellt, mit dem die Entwicklung einer beliebigen Verteilung von Einschlüssen entlang des Weges minimaler Energie simuliert wird. Im Modell entwickelt sich der Kreis in ein Qaudrat, welches durch Hohlraumbildung vom Zentrum her, oder—wenn das verboten ist— durch Hereinziehen einer Störung vom Rand her in ein Dublett aufspaltet. Die Gröβen, bei denen sich die Formen spontan unwandeln, werden mit den Gleichgewichtsgröβen verglichen. Schlieβlich mit der Formen spontan Entwicklung einer beliebigen Verteilung von Einschlüssen studiert. Der erste von der Verteilung angenommene metastabile Zustand hängt von der elastischen Wechselwirkung, der Oberflächenenergie und dem Flächenanteil der Einschluβphase über einen einzigen dimansionslosen Parameter ab, der diese drei Einflüsse zusammenfaβt. Die Ergebnisse werden mit früheren theoretischen und experimentellen Arbeiten zu Vergröberungsanordnungen in drei Dimensionen verglichen.
    Acta Metallurgica 02/1992;
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    ABSTRACT: This paper reports on an experimental investigation that has been carried out to study the effects of controlled microcracking on the fracture resistance of brittle solids. The material chosen as a model system for the experimental study is an aluminum oxide reinforce with 33 vol.% SiC whiskers. The experimental program involves the determination of fracture toughness at room temperature on four-point flexure specimens continuing sharp, through-thickness precracks where different amounts of microcrack damage are introduced a priori at different elevated temperatures and tensile load levels. The room temperature fracture initiation toughness of the pre-damaged material with a controlled amount of small-scale microcracking ahead of the stationary macrocrack is compared and contrasted with that of the undamaged material and the enhancement or reduction in fracture initiation toughness is estimated. Detailed transmission election microscopy of the crack-tip damage zone has been conducted in an attempt to examine the mechanisms of permanent degradation and of microcrack formation.
    Acta Metallurgica 01/1992; 40(2):259-274.
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    ABSTRACT: In this paper, the pressure dependence of the crystallization temperature of amorphous Zr[sub 70]Cu[sub 30] alloy and the variation of its crystallization phases under high pressure annealing is investigated by electrical resistivity measurement and X-ray diffraction method. The results show that pressurizing generally raises the crystallization temperature, but slightly decreases it at the early stage of compression. Evident changes in crystallization phase in the alloy were also observed under high pressure annealing. At atmosphere pressure, the crystallization products are CuZr[sub 2] with a modicum of [alpha]-Zr, while at a pressure of 2 GPa, the alloy crystallizes into a mixture consisting of CuZr[sub 2], Cu[sub 10]Zr[sub 7] and an appreciable amount of [alpha]-Zr. The origin of these observations is discussed.
    Acta Metallurgica 01/1992; 40(9):2185-2190.
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    ABSTRACT: In this paper, the nucleation of [delta][prime] at dislocations at small undercooling is investigated. The primary site for heterogeneous nucleation is matrix dislocations with strong edge components. The edge dislocation half-plane and the development of a [delta][prime] anti-phase boundary is considered. The morphological development of the [delta][prime] precipitate is that of a kidney shape with a consistent relationship with the edge dislocation half-plane. A model is developed based on the energetics of loss of coherency of a spherical precipitate and is employed to predict coherency loss of the [delta][prime] precipitate and to compare it to the experimentally determined diameter of 400 nm. The loss of coherency is shown to be assisted by the growth of the equilibrium precipitate [delta].
    Acta Metallurgica 01/1992; 40(9):2101-2112.
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    ABSTRACT: This paper discusses the dislocation structure of small angle tilt and twist boundaries in ordered NiâAl, with and without boron, investigated using transmission electron microscopy. Dislocation with Burgers vectors that correspond to anti-phase boundary (APB)-coupled superpartials were found in small angle twist boundaries in both boron-free and boron-doped NiâAl, and a small angle tilt boundary in boron-doped NiâAl. The boundary structures are in agreement with theoretical models proposed by Marcinkowski and co-workers. The APB energy determined from the dissociation of the grain boundary dislocations was lower than values reported for isolated APBs in NiâAl. For small angle twist boundaries the presence of boron reduced the APB energy at the interface until it approached zero. This is consistent with the structure of these boundaries containing small regions of increased compositional disorder in the first atomic plane next to the interface.
    Acta Metallurgica 01/1992; 40(1):99-106.
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    ABSTRACT: In this paper, [omega] phase in Al-Cu-Mg and Al-Cu-Mg-Ag is examined to determine the influence of silver addition on the [omega] precipitate structure and the effect of the large misfit normal to the [omega] plate habit plane on the interfacila structure. Using electron diffraction and HREM, no difference in [omega] structure could be discerned between the two alloys with and without Ag additions.
    Acta Metallurgica 01/1992; 40(10):2539-2546.
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    ABSTRACT: In this paper the high-temperature plastic deformation of 6 mol% Y[sub 2])[sub 3]-stabilized ZrO[sub 2] polycrystals with grain sizes of 1.8, 3.4 and 6.3 [mu]m are studied in compression between 1350 and 1450[degrees]C in air at constant strain rate (between 1 [times] 10[sup [minus] 5] and 2 [times] 10[sup [minus]4]S[sup [minus]1]) and under constant load (between 5 and 90 MPa). Two mechanical behaviors were observed depending on strain rate or stress levels: grain boundary sliding controlled by cation bulk diffusion, with an activation energy of 560 kJ/mol, and intergranular cavitation without plastic deformation of the grains.
    Acta Metallurgica 01/1992; 40(10):2717-2726.