220 research outputs found
The analysis of Al-based alloys by calorimetry: quantitative analysis of reactions and reaction kinetics
No full textDifferential scanning calorimetry (DSC) and isothermal calorimetry have been applied extensively to the analysis of light metals, especially Al based alloys. Isothermal calorimetry and differential scanning calorimetry are used for analysis of solid state reactions, such as precipitation, homogenisation, devitrivication and recrystallisation; and solid–liquid reactions, such as incipient melting and solidification, are studied by differential scanning calorimetry. In producing repeatable calorimetry data on Al alloys, sample preparation, reproducibility and baseline drift need to be considered in detail. Calorimetry can be used effectively to study the different solid state reactions and solid–liquid reactions that occur during the main processing steps of Al based alloys (solidification, homogenisation, precipitation). Also, devitrivication of amorphous and ultrafine grained Al based powders and flakes can be studied effectively. Quantitative analysis of the kinetics of reactions is assessed through reviewing the interrelation between activation energy analysis methods, equivalent time approaches, impingement parameter approaches, mean field models for precipitation, the Johnson-Mehl-Avrami-Kolmogorov model, as well as novel models which have not yet found application in calorimetry. Differential scanning calorimetry has occasionally been used in attempts to measure the volume fractions of phases present in Al based alloys, and attempts at determining volume fractions of intermetallic phases in commercial alloys and amounts of devitrified phase in glasses are reviewed. The requirements for the validity of these quantitative applications are also reviewed. Contents 1. INTRODUCTION 2 EXPERIMENTAL ASPECTS OF CALORIMETRY 2.1 EXPERIMENTAL ASPECTS OF ISOTHERMAL CALORIMETRIC ANALYSIS 2.2 EXPERIMENTAL ASPECTS OF DIFFERENTIAL SCANNING CALORIMETRY 2.3 SAMPLE PREPARATION FOR CALORIMETRY OF LIGHT METAL ALLOYS. 2.4 BASELINE CORRECTION IN CALORIMETRY 2.4.1 Initial transient 2.4.2 Baseline variability and drift 2.4.3 Combined baseline variability and heat capacity effects in DSC 3 APPLICATIONS OF THERMAL ANALYSIS FOR AL BASED ALLOYS. 3.1 IDENTIFICATION OF THERMAL EFFECTS. 3.2 HEAT CAPACITY DETERMINATION. 3.3 SOLID STATE REACTIONS 3.3.1 Homogenising and solution treatment studies 3.3.2 Precipitation studies – Qualitative analysis 3.3.3 Determination of thermal history / Fingerprinting of heat treated alloys 3.3.4 Precipitate coarsening – Qualitative analysis 3.3.5 Defect annihilation, recovery and recrystallisation in wrought alloys 3.3.6 Devitrivication; nanocrystallisation3.3.7 Multi-layers and interfacial reactions3.4 SOLID-LIQUID AND LIQUID-SOLID REACTIONS.3.4.1 Solidification.3.4.2 Melting and incipient melting3.5 CALPHAD; MIXING AND DISSOLUTION OF POWDERS AND LIQUIDS4 MODELLING OF THERMALLY ACTIVATED REACTIONS.4.1 INTRODUCTION; GENERAL OBJECTIVES.4.2 SINGLE STATE VARIABLE APPROACHES (SINGLE ARRHENIUS TERM)4.3 EQUIVALENT TIME, THE STATE VARIABLE AND THE TEMPERATURE INTEGRAL.4.3.1 The state variable approach and equivalent times 4.3.2 The temperature integral and its approximations4.4 ACTIVATION ENERGY DETERMINATION.4.4.1 Activation Energy analysis using isoconversion methods 4.4.2 Accuracies of isoconversion analysis methods4.4.3 Measured activation energies in Al-based alloys 4.5 SINGLE STAGE REACTION MODELS. 4.5.1 JMAK, mean field and other models 4.5.2 Other models for precipitation in Al based alloys. 4.5.3 Determination of the reaction exponent, n 4.6 MULTI STAGE MODELS (MULTIPLE ARRHENIUS TERMS) 5 DETERMINATION OF VOLUME FRACTIONS OF REACTION PRODUCTS AND PHASES 5.1 GENERAL OBJECTIVES 5.2 VOLUME FRACTIONS OF PRECIPITATES MEASURED FROM SOLID-SOLID REACTIONS 5.3 VOLUME FRACTIONS CRYSTALLISED DURING DEVITRIVICATION 5.4 VOLUME FRACTIONS OF INTERMETALLIC PHASES MEASURED FROM SOLID-LIQUID REACTIONS 6 CONCLUDING REMARK
Microstructure and strength modelling of Al-Cu-Mg alloys during non-isothermal treatments: part 1 – controlled heating and cooling
No full textA model is developed to predict the precipitation kinetics and strengthening in Al Cu Mg alloys during non-isothermal treatments consisting of controlled heating and cooling. The prediction of the precipitation kinetics is based on the Kampmann and Wagner model. The precipitation strengthening by the shearable Cu:Mg co clusters is modelled on the basis of the modulus strengthening mechanism and the strengthening by the non-shearable S phase precipitates is based on the Orowan looping mechanism. The model predictions are verified by comparing with hardness, transmission electron microscopy and differential scanning calorimetry data on 2024-T351 aluminium alloys. The microstructural development and strength predictions of the model are generally in close agreement with the experimental data
A new model for diffusion-controlled precipitation reactions using the extended volume concept
No full textIn this work a new model for diffusion-controlled precipitation reactions is derived, analysed and tested against a wide range of data. The model incorporates elements of the extended volume concept and combines this with a new treatment of soft impingement of diffusion fields. The model derivation involves an integration over iso-concentration regions in the parent phase in the extended volume, which leads to a single analytical equation describing the relation the fraction transformed, ?, and the extended volume fraction, ?ext, as: ? = {exp(-2?ext)-1}/(2?ext) + 1. The model is compared to a range of new and old data on diffusion-controlled reactions including precipitation reactions and exsolution reactions, showing a very good performance, outperforming classical and recent models. The model allows new interpretation of existing data which, for the first time, show a consistent analysis, in which Avrami constants, n, equal values that are always consistent with transformation theory
Analysis of nucleation and growth with the model for diffusion-controlled precipitation reactions based on the extended volume concept
No full textRecently (M.J. Starink, Thermochim Acta 596, 2014, 109-119) a new model for diffusion-controlled precipitation reactions based on the extended volume concept was derived. The model leads to an analytical equation describing the relation between the fraction transformed, alfa, the reaction time, t, and the reaction exponent, n, as:alfa = {exp(-2(kt)^n)-1}/(2(kt)^n) + 1In the present work, new analysis methods are derived which allow determination of the reaction exponent n. The new methods are applied to analysis of nucleation and it is shown that generally during a reaction with growth in 3 dimensions there are only 2 modes: either the nucleation rate in the extended volume is constant or it is negligibly small. A new approach to the interaction of diffusion-controlled growth and nucleation is proposed to rationalise these findings. The exponential decay of the average solute content predicted by the new model is further analysed and compared with a range of experimental data and contrasted with other models. The new model is found to correspond excellently to these solute decay data.<br/
Microstructure and strength modelling in Al-Cu-Mg alloys during non-isothermal treatments: Part 2 - Welds
No full textThe present work applies a model for microstructural evolution in the solid state and Al-Cu-Mg alloys and expands it in a computationally efficient way to include solid-liquid reactions in fusion welds. The model is used to predict local strength and hardness of the welds, using a formulation that incorporates hardening due to two types of precipitates, i.e. Cu-Mg co-clusters and the S phase precipitates. The model predictions are compared with hardness, differential scanning calorimetry and transmission electron microscopy data for a fusion welded 2024-T351 aluminium alloy. The model predicts solid state reactions and solid-liquid reactions including co-cluster dissolution, S phase formation, growth, coarsening and dissolution, co-cluster reformation on cooling, and solute partitioning on resolidification. The model predictions are in good agreement with the experimental results and illustrate the dominant role that (sub-)nanoscale co-clusters play in strengthening of welds. The yield strength of as welded material tested normal to the weld is mainly due to the co-clusters
Comments on: ‘Calorimetric study of 6061-Al–15vol.% SiCw PM composites extruded at different temperatures’ by A. Borrego, G. González-Doncel, Mater. Sci. Eng. A 245 (1998) 10
No full textIn this contribution, the ?? and ?? phase precipitation rates in a 6061 (Al–Si–Mg) alloy, as measured by differential scanning calorimetry (DSC), are fitted employing the Starink–Zahra (SZ) model. Similar to precipitation reactions in a large number of other alloys, the SZ model fits the DSC data on 6061 close to perfectly. The distinctive elements of the SZ model, i.e. its treatment of nucleation rates, impingement and temperature-dependent solubility, are discussed and contrasted with other models
Analysis of hydrogen desorption from linear heating experiments: Accuracy of activation energy determinations
No full textThrough performing hydrogen desorption experiments at different heating rates, β, the (effective) activation energy, E, of the desorption process can be determined from the shift of a characteristic temperature, T f, of the hydrogen evolution effect with heating rate. In the literature various methods have been employed, and in the present work the accuracy of these methods is investigated. The Kissinger-Akahira-Sunose, Flynn-Wall-Ozawa, Starink, Kissinger and Choo-Lee methods all employ approximations which cause deviations in the activation energy determination, which increase drastically as E/RT (R is the gas constant) becomes smaller. It is shown that for various hydrogen desorption reactions reported in the literature, deviations in reported E between ~1 and ~20% can occur due to inappropriate use of methods. It is shown that the Ozawa and Flynn-Wall-Ozawa methods are highly inaccurate and particularly for hydrogen evolution, where E/RT is often smaller than 15, they are in most cases inappropriate. The Kissinger peak method is accurate for first order reactions, but as hydrogen evolution reactions generally are not first order reactions, application of this method will result in inaccuracies which increase for decreasing E/RT. In general the magnitude of the deviations of such a peak method are not predictable, as this depends on the reaction mechanism. In many cases the Kissinger peak method is inappropriate for high accuracy determination of activation energy for hydrogen evolution reactions. Amongst the methods that provide an activation energy directly from a slope (i.e. without iterative procedures) the Starink method provides the best accuracy of activation energy analysis methods studied in the literature. It provides an accuracy that is better than 2% for E/RT > 6, which covers all known hydrogen desorption reactions, whilst correction for residual errors are possible.</p
The thermodynamics of and strengthening due to co-clusters: general theory and application to the case of Al-Cu-Mg alloys
No full textCo-clusters in ternary or higher order metallic alloys are metastable structures involving two or more distinct alloying atoms that retain the structure of the host lattice. A thermodynamic model based on a single interaction energy of dissimilar nearest neighbour alloying elements is presented, and a model for the strengthening due to these co-cluster dimers is derived. The model includes a new treatment of (short-) order strengthening relevant to these co-clusters and further encompasses modulus hardening and chemical hardening. The models are tested against data on a wide range of Al-Cu-Mg alloys treated at temperatures between 20 and 220ºC. Both quantitative calorimetry data on the enthalpy change due to co-cluster formation and strengthening due to co-clusters is predicted well. It is shown that in general (short-range) order strengthening will be the main strengthening mechanism
A quantitative interpretation of DSC experiments on quenched and aged SiCp reinforced 8090 alloys
No full textDSC curves of the quenched monolithic 8090 alloy and the 8090 MMC have been used to obtain values for the heats of formation of GPB zones, delta' and S phase. Using these ?H values and a correction for overlap of effects, the DSC curves of the aged alloys have been interpreted in terms of amounts of precipitates present. The presented interpretations are consistent with previous microstructural investigations. The solvus of GPB zones and of S phase in 8090 alloys has been obtained. Significant amounts of GPB zones are formed in the monolithic alloy, whilst the addition of SiC particles greatly reduces the amount of GPB zones formed
Age hardening and softening in cold-rolled Al-Mg-Mn alloys with up to 0.4wt%Cu
No full textThe age hardening and age softening of nine solution treated and subsequently cold-rolled Al-(1-3)Mg-(0-0.4)Cu-0.15Si-0.25Mn (in wt%) alloys with potential applications in both packaging and automotive industries have been investigated. Cold work levels were 10, 40 and 90% reduction. The proof strengths of the aged alloys range from 130 to 370MPa. A physically based model for yield strength has been developed which includes a one parameter dislocation evolution model to describe work hardening and recovery and a two precipitate precipitation hardening model. The model is based on analytical equations, avoiding computing time intensive iterative schemes. An exceptionally high model accuracy has been demonstrated. The model parameters are verified by transmission electron microscopy and calorimetry analysis of the materials
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