On the structures formed in eutectic alloys at high growth rates

Abstract

It is well established that dendrite-free structures can be produced in off-eutectic alloys by solidification at high growth rates. It has also been demonstrated that solidification at high growth rate can lead to the formation of a layer of supercooled liquid at the solid-liquid interface. In the present work the relation between the presence of supercooled interface liquid and the formation of dendrite-free structures is examined. Observations on tin-rich lead-tin eutectic alloys have shown that wholly lamellar structures can be produced when a supercooled layer is present, but that such a supercooled layer is not a necessary condition for the formation of dendrite-free structures.

Résumé

Il est bien connu qu'il est possible de produire des structures libres de dendrites dans les alliages quasi-eutectiques à condition d'utiliser des taux de croissance élevés. De plus, on sait que des taux étevés de solidification donnent lieu à la formation d'une zone de liquide en surfusion à l'interface solide-liquide. Les auteurs étudient ici la relation entre la présence de cette zone de liquide en surfusion et la formation des structures libres de dendrites. Ils trouvent que des structures complètement lamellaires peuvent exister en présence d'une zone de liquide en surfusion mais que la présence de cette zone n'est pas une condition nécessaire pour la formation de structures libres de dendrites.     

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.     

Annealing Twin Formation in a Cast and Annealed Cu-4 at.% Sn Alloy

Abstract

No annealing twins were observed in Cu-4 at. % Sn in the as-cast condition. During the initial stages of annealing at 700°C, four types of twins formed in the following chronological order: edge twins, partial twins, complete twins and enclosed twins. These observations provide support for the “grain growth accident” model of twin formation. Under conditions where the dendritic structure still exists after short homogenization anneals, the results suggest that a diffusion-controlled phenomenon may play an important role in twin formation.     

A study of shell growth irregularities in continuously cast 310S stainless steel

Abstract

Growth irregularities in continuous casting are believed to be associated with crack formation and breakouts. Differential thermal analysis on 310S stainless steel samples indicated primary precipitations of both austenite and ferrite during solidification. In tensile tests on solidifying samples, abrupt shrinkages in volume were detected in the peritectic range of temperatures. Micrographic and microsegregation analysis on samples extracted from a breakout shell revealed high ratios of primary-precipitated austenite in the thick sections of the shell, and high ratios of primary-precipitated ferrite in the thin sections. Alternating precipitations of austenite and ferrite are proposed to occur during solidification. Regions of the shell with high ratios of primary austenite remain in contact with the mould and exhibit high growth rates, whereas regions with high ratios of primary ferrite shrink in volume due to the ferrite to austenite transformation, which results in the formation of air gaps between the shell and the mould and reductions in growth rate.     

Present Technology of Hard and Soft Ferrites

Abstract

In order to improve the mechanical properties of cemented carbides, Ti(C,N)–Ni–Mo alloys, in which carbon in the titanium carbide is replaced by nitrogen, are of potential interest from the viewpoint of grain size control. Since grain size control by nitrogen was also observed in Ti(C,N)–Ni alloys containing no molybdenum, the effect of nitrogen on grain growth of Ti(C,N)–Ni alloys was investigated by comparing TiC–Ni and TiN–Ni alloys. The grain growth rate of Ti(C,N)–Ni alloys showed a minimum value at the carbon content C/C+N = 0·5. From the results of chemical and grain size analyses, it is considered that the growth rate depends on the solubility of Ti(C,N) in the nickel solid solution and the degree of coalescence because the solubility decreases with decreasing carbon content. With nitride, the amount of liquid increases by denitrification of TiN, owing to the formation of the nickel solid solution or TiNi₃ phases, and growth of TiN grains by Ostwald ripening is observed. The apparent activation energy for the growth of TiC and TiN grains is calculated to be ～4·4×10⁵ and 1·9 × 10⁵ J mol^?1 respectively. PM/0201     

Coating microstructure development during Zinquench galvanising of sheet steel

Abstract

The influence of an elevated strip entry temperature on the kinetics of the galvanising reaction has been investigated for a titanium stabilised, interstitial free steel. Elevated strip entry temperatures (>550°C) have been shown to accelerate greatly alloying between the substrate and the molten zinc, increasing the growth rate of the δ phase from 1 to ～7 μm s^-1 and preventing the formation of the ζ phase. The substantial Fe–Zn phase layer seen at the coating/substrate interface at high strip entry temperatures (>550°C) is attributed to the dissolution of iron from the substrate into the molten zinc in the first second of immersion, and the limited inhibition provided by the bath aluminium content (<0·15 wt-%effective).     

Ferrite growth from austenite in C–Mn steels during continuous cooling

Abstract

Experiments are carried out to examine the ferrite formation in four lean construction steels of low carbon and low alloy contents during continuous cooling. The variation of the heat capacity during the transformation is measured by means of differential thermal analysis, and the heat capacity is converted to the volume fraction ferrite as a function of temperature. The ferrite growth is modelled in two ways: (i) according to an interface mobility model, and (ii) according to a diffusion control model. Different assumptions are made to model the geometry of the microstructure. It is shown that the interface mobility model, applying the proper driving force and geometry, can account for the experimental ferrite growth at the early stage of transformation, but carbon diffusion has to be taken into account quantitatively when the carbon concentration profile becomes significant. The simulation according to the diffusion control model is performed using the Dictra program based on the local equilibrium hypothesis. The resulting discrepancies with the experiments are partially due to the simple spherical geometry that is assumed.     

FEM technique to study residual stresses developed in continuously cast steel during solid–solid phase transformation

Abstract

Solid–solid phase transformation of as cast steel may generate stress concentration zones in the microstructure due to the accumulation of thermodynamic (cooling and phase transformation processes) stresses in the microstructure at the interface between phases. These stress concentration zones are vulnerable regions to the formation of microcracks or the growth of flaws in these regions. In the present investigation, a finite element model was created to simulate the cooling of ASTM-SAE grade 1010 steel with different cooling rates. The phase transformation simulations were based on the continuous cooling transformation diagram. Therefore, they were quasi-real models. The models predict analytically the generation of stress concentration regions due to thermodynamic strains during the cooling of a sample from the austenite temperature range with different cooling rates.     

WEAR AND FRICTION STUDIES OF NICKEL–TUNGSTEN CARBIDE–GRAPHITE COMPOSITES

Abstract

Wear and friction studies have been carried out on pressed and sintered composites of 75–95% theoretical density. Carbonyl nickel and tungsten carbide, both with a particle size of 5 μm, and natural crystalline flake graphite with an approximate size of 1·1 μm were used. The wear specimens were run against a rotating (100 rev/min) steel cylinder. The wear-resistance correlates well with the ratio of the volume fraction of tungsten carbide to that of graphite (WC/graphite); there is a minimum in weight loss at a ratio near unity. This effect is explained in terms of powder coating with graphite during blending, which affects grain-boundary formation and grain growth during sintering; supporting metallographic evidence is given. The wear-debris particle size produced is related to wear; the wear rate increases with increasing particle size. Residual porosity in the materials reduces wear-resistance. The wear rate does not correlate with hardness or coefficient of friction; the results of wear tests in oil or water are very similar to those for tests run in air.     

Microstructure and precipitation in as cast low carbon Nb–V–Ti microalloyed medium thin slab

Abstract

The solidification structure, austenite and precipitates in a quenched compact strip processing (CSP) medium thin slab (170 mm thick) of Nb–V–Ti microalloyed steel have been studied. It was found that secondary dendrite arm spacing and austenite grain size are slightly larger than that of similar steel produced by CSP thin slab. This is partially attributed to the slower cooling rate caused by the increased slab thickness. On the other hand, the formation of carbonitride during solidification reduces the width of secondary dendrite arm spacing, while TiN particles and alloying elements in solution may inhibit the growth of austenite grain during solidification and subsequent cooling. In addition to the semidendritic, larger cubic and fine cubic precipitates, which can be observed in CSP thin slab, dendritic precipitates were also found in CSP medium thin slab.     

On sintering of Ti–Ni (–TiB2) alloys to near full density

Abstract

Using a combination of mixed elemental powders and TiB₂, a series of Ti–Ni and Ti–Ni–B alloys were optimised for sintering by varying the nickel and boron contents, the particle size of the elemental powders and the compaction pressure. The sintering temperature was maintained at 1200°C to limit the costs of a potential commercial sintering operation. For Ti–Ni alloys, a density of 99% was attained in Ti–7Ni made using fine Ti and Ni powders sintered in the solid state, and from liquid phase sintering of Ti–8Ni made using coarser powders. Porosity was almost eliminated from Ti–7Ni–xB alloys made by adding 1–3%TiB₂ to the coarser Ti and Ni powders. The action of TiB₂ as a sintering aid is possibly owing to a combination of the formation of a small amount of liquid at the sintering temperature and the restriction of grain growth owing to the presence of TiB particles.     

Determination of optimum blast furnace slag cooling rate for slag recycling in cement manufacture

Abstract

The crystallisation behaviour of molten blast furnace slag was observed in situ using the single hot thermocouple technique. Isothermal and non-isothermal experiments were conducted to construct the diagrams for time temperature transformation and continuous cooling transformation. The molten slag should be cooled at a minimum critical cooling rate of 10°C s^?1. During crystallisation, melilite is the main crystal phase and rankinite is the primary phase. The crystallisation mechanism of the melilite crystal phase involves one-dimensional direction growth with bulk nucleation, whereas the growth mechanism of the rankinite crystal phase is between the surface nucleation mechanism and the one-dimensional direction growth mechanism. The crystallisation activation energies of the melilite and rankinite crystal phases are 238.07±28.81 and 523.52±58.56 kJ mol^?1 respectively.     

Thermodynamic analysis and generalized computer program for hydrocarbon (C m H n ) — air mixtures

Abstract

Elastic and elastic-plastic fracture mechanics solutions are modified to predict the growth of short fatigue cracks by introducing an effective crack length, which is equal to the actual crack length increased by an amount l ₀, into the solutions for intensity factors and the J integral method of analysis. Since the threshold stress approaches the fatigue limit of the material as the crack length tends to zero the value of l ₀ can be obtained from the threshold stress intensity factor and the fatigue limit. Crack growth results for short cracks, in both elastic and plastic strain fields of smooth specimens, when interpreted in terms of the modified solutions, show excellent agreement with elastic long crack data. The accuracy of the term l ₀ in predicting crack growth rates for short cracks is found to be independent of the applied strain level. It varies linearly with grain size for low carbon steels and can be considered at the surface as a measure of the reduced flow resistance of surface grains due to their lack of constraint. It also accounts for the effect of the free boundary on the distribution of stresses around the crack tip.

Résumé

Pour la fracture dans les domaines de déformation élastique et élastique-plastique, les équations permettant de prédire la croissance de fissures de fatigue courtes sont modifiees par l'introduction d'une longueur efficace de fissure—qui est égale à la longueur réelle de fissure, plus un terme l ₀—dans les équations définissant les facteurs d'intensité et pour la méthode d'analyse par intégrale J. Puisque la contrainte seuil tend vers la limite de fatigue du matèriau quand la longueur de fissure tend vers zero, la valeur l ₀ peut être obtenue à partir du facteur d'intensityé à la contrainte seuil et de la limite de fatigue.

Dans les domaines de déformation élastique et plastique d' échantillons ductiles, les résultats de propagation de fissures courtes, interprétés en termes de solutions modifiées, sont en excellent accord avec les données de fissures longues dans le domaine élastique.

Pour la prédiction de la propagation de fissures dans le cas de fissures courtes, la précision du terme l ₀ est trouvée être indépendante de la déformation appliquée.

l ₀ varie linéairement avec la taille des grains pour les aciers à bas carbone et peut être considérée comme une mesure de la résistance réduite au fluage des grains de surface, par suite de leur mangue de contrainte. Elle tient également compte de l'effet de bord libre sur la distribution des contraintes autour du bout de la fissure.     

High Strength and Thermal Stability of Multilayered Cu/Al Composites Fabricated Through Accumulative Roll Bonding and Cryorolling

Multilayered Cu/Al composites with high strength and thermal stability were successfully fabricated by combining accumulative roll bonding (ARB) and cryorolling. The microstructure, tensile properties, and thermal stability of the multilayered Cu/Al composites subjected to cold rolling and cryorolling were analysed. Subsequent cryorolling can be used to modify interfacial flatness and local necking, induce the formation of high-density stacking faults in the Cu matrix, and enhance interfacial bonding strength, which improves the mechanical properties of ARB composites. The initial lamellar structure is gradually transformed into serious mixing with an increase in annealing temperature, accompanied by the formation of excessive Cu–Al intermetallic compounds (IMCs). Cryorolled samples exhibited higher thermal stability than cold-rolled samples. At low annealing temperature, high-density stacking faults induced by cryorolling facilitated the transition from low-angle grain boundaries to high-angle grain boundaries, which led to the formation of ultra-fine grains. For the samples annealed at high temperatures, cryorolling led to the effective inhibition of Cu–Al IMC formation and growth due to the genetic effect of less heat input.

Graphical Abstract

     

Principles of metal oxidation

Abstract

The principles governing formation of superficial oxides on metal surfaces are surveyed. Oxide crystals grow on a metal surface by rearrangement and place exchange within metal-oxygen substructures arising from the accumulation of oxygen by adsorption processes. The rate-controlling steps for growth of films are discussed by means of film models describing ion and electron migration under the influence of electrical potential gradients. These condiderations give rise to inverse logarithmic, direct logarithmic, cubic and parabolic time equations for film growth. The most rigorously tested theoretical basis in metal oxidation is that for the parabolic growth of scales based upon ambipolar diffusion of the reactants by point defects in oxides such as interstitials and vacancies. In several instances low resistance diffusion paths in oxides, such as grain boundaries and dislocations, cause leakage paths in films and short-circuit paths in scales. Several cases are discussed, where oxidation is controlled to linear kinetics by phase boundary reaction steps.

Résumé

Les auteurs passent en revue les principes régissant la formation d'oxydes superficiels sur des surfaces métalliques. Les cristaux d'oxyde croissent sur une surface métallique par un réarrangement et un changement de site à l'interieur de la sous-structure métal-oxygène provenant de l'accumulation d'oxygène par adsorption. Ces étapes controllant le taux de croissance des films sont discutées en fonction de modèles decrivant la migration des ions et des électrons sous l'effet de gradients de potentiel électrique. Ces considérations donnent des équations de temps de fonne logarithmique, inverse ou direct, cubique et parabolique pour la croissance des films. La base théorique la plus rigoureuse pour l'oxydation d'un métal est la croissance parabolique de l'oxyde basée sur la diffusion ambipolaire des réactifs par défauts ponctuels dans l'oxyde, interstitiels et lacunes. Dans plusieurs cas, des chemins de diffusion facHes dans les oxydes, joints de grain ou dislocations, fonnent des chemins de fuite dans les films et des courts-circuits dans la calamine. Plusieurs cas pour lesquels l'oxydation est controllée par des réactions en joints de phases amenant une cinétique linéaire sont étudiés.     

Phase Transformations in Zirconium and Its Alloys

Abstract

A critical review is presented of all published data on phase transformation in zirconium and its alloys. Topics discussed include: The α → β transformation; factors affecting the stability of α and β zirconium; transformations in the β-phase including ω-phase formation, eutectoidal decomposition and strain induced transformations; transformations in the α-phase including the tempering of martensite and supersaturated alpha, and peritectoid formation of ordered Zr₃Al. The review concludes with examples of phase transformations in commercial zirconium alloys and the effects of these transformations on the mechanical properties of the alloys.

Résumé

Une revue critique de toutes les donnees publiees sur les transformations de phase du zirconium et de ses alliages est présentée. Les sujets traités sont: la transformation α → β, les facteurs influençant la stabilité des phases α et β, les transformations ayant lieu dans la phase β incluant la formation de la phase ω, la décomposition eutectoîdique et les transformations induites par une déformation, les transformations ayant lieu dans la phase α incluant le revenu de la martensite et de la phase α sursaturée, et la formation péritectoïdique d'une phase ordonnee Zr₃Al. Cette revue se termine sur la présentation de plusieurs exemples de transformation de phases des alliages commerciaux de zirconium et l'effet de ces transformations sur les propriétés mécaniques de ces alliages.     

Regularities in thermodynamic properties of interoxide polymeric compounds

Abstract

It is shown that in terms of the number N of elements in solid interoxide polymeric compounds and minerals, the molar heat capacity ratio C _/N is a _p single function of temperature. The molar heat capacity of a solid polymeric compound is found to be equal to the sum of the molar heat capacities of its constituent solid oxides at all temperatures. The standard entropies of formation of the alkali and alkaline earth polymeric compounds per atom of oxygen are both-95±5 J [atom O]- 1 K-1, which is similar to all other inorganic compounds. The standard free energies and enthalpies of formation of polymeric interoxide compounds at a given temperature T are directly proportional to the number N of elements in the compound.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5–FeAl2 powders Part 2 – Densification mechanism analysis

Abstract

The use of Fe₂Al₅–FeAl₂ prealloyed powders and heating rates >150 K min^?1 overcomes the formation of density restricting Kirkendall porosity in the Fe–Al system. X-ray diffraction, electron probe micro analysis and differential thermal analysis suggest that the absence of a persistent liquid, experienced when liquid phase sintering with elemental powders, is overcome. Homogenisation is greater during heating at a rate of 20 K min^?1 than for 150, 250 or 400 K min^?1 and homogenous Fe₃Al forms across the compact at temperatures below the melting point of the liquid forming constituent, indicating that a liquid will not form under such processing conditions. The maximum density achieved under the processing conditions in the present study is 92% of theoretical density. The presence of large pores shortly after liquid formation suggests that the remaining porosity is largely due to powder agglomeration during mixing.     

The Effect of Solute on Grain Growth in A Pure Metal: Where the Distribution Coefficient of the Solute in the Solvent is Greater than Unity

Abstract

The effect of antimony on the grain growth of zone-refined tin is markedly different from the effects of other solutes. It is suggested that, since antimony has a distribution coefficient greater than unity in tin, the antimony will not segregate positively to the grain boundaries and will thus be pushed ahead of an advancing boundary during grain growth. The effect of simultaneous diffusion of antimony in the matrix ahead of the boundary during the grain growth is also discussed.     

The annealing of bainite while being concurrently deformed

Abstract

Annealing with oncurrent deformation has been carried out on a bainitic high-carbon steel in a temperature range just below the lower transformation temperature, i.e. ～850 – 975K.

The enhancement of carbide coarsening was measured during hot torsion under conditions of (a) constant strain rate and (b) constant experiment duration. The carbide coarsening rates are compared with results obtained from earlier experiments employing isothermal annealing without deformation, and the enhancement of coarsening due to concurrent deformation is quantitatively assessed from measurements of the mean particle radius and mean free ferrite path.

From kinetic data, the rate limiting step in coarsening during concurrent deformation is identified as the self-diffusion of iron and two dominant mechanisms are proposed which result in a discontinuity of coarsening rate at critical strain levels. The effective diffusion coefficient for coarsening is shown to be a linear function of strain rate and the strain rate coefficient is evaluated from the present data for bainitic 1070 steel.

Résumé

Un traitement de revenu associé à une déformation simultanée a été appliqué à un acier bainitique à haut carbone dans une gamme de températures juste inférieures à la température de transformation (850–975K).

L'accélération de la coalescence des carbures pendant la torsion à chaud a été mesurée soit àvitesse de déformation constante, soit à temps d'expérience constant.

Les lois de coalescence des carbures sont comparées àcelles obtenues au cours d'un traitement isotherme sans déformation et l'accélération de la croissance due àune déformation simultanée est évaluée quantitativement grâce aux mesures du rayon moyen des particules et de la longueur moyenne de ferrite libre.

A partir des résultats de cinétique, le facteur limitant la coalescence sous déformation est identifie comme étant l'auto-diffusion du fer et les auteurs proposent deux mécanismes prédominants qui conduisent à une discontinuité de la vitesse de coalescence pour des niveaux critiques de la déformation. Le coefficient effectif de diffusion pour la coalescence est une fonction linéaire de la vitesse de déformation et le coefficient de vitesse de déformation est évalué à partir de ces résultats expérimentaux pour un acier bainitique 1070.