Analysis of aluminium based alloys by calorimetry: quantitative analysis of reactions and reaction kinetics

Abstract

Differential 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.Keywords: Differential scanning calorimetry, Precipitation, Aluminium, Modelling, Transformation IMR/419 IntroductionCalorimetry is an analysis technique that is part of a group of techniques collectively known as thermal analysis methods. In its broadest sense, thermal analysis refers to the measurement of changes in properties of substances under a controlled temperature program. Thermal analysis techniques can be classified according to the type of temperature program that the sample is subjected to and the measured (output) signal. The most commonly used temperature programs are either isothermal hold or heating (scanning) at constant rate, while more recently, temperature modulated scanning and reaction controlled heating have also found application. The signals measured in thermal analysis can include heat flows, temperature changes, mass, evolved gasses, length changes, elastic modulus, and many other properties that characterise properties or reactions of interest. Calorimetry refers to thermal analysis methods that measure the heat evolution from a sample under a controlled temperature program. The two most often applied calorimetry techniques are isothermal calorimetry and differential scanning calorimetry (DSC), which, as the name suggests, is by definition non-isothermal (i.e. a temperature scan). Apart from the more common applications to polymers,...

Quench-induced precipitates in Al–Si alloys: Calorimetric determination of solute content and characterisation of microstructure

“…This would apply for heterogeneous nucleation e.g. at dispersoids or coarse intermetallic phases when the number of such sites is limited [19,37]. However, those phases are not present in the pure binary alloys investigated in this paper, and hence, n should be 2.5, which is valid for continuous 360 nucleation during the course of the reaction [19,37].…”

“…at dispersoids or coarse intermetallic phases when the number of such sites is limited [19,37]. However, those phases are not present in the pure binary alloys investigated in this paper, and hence, n should be 2.5, which is valid for continuous 360 nucleation during the course of the reaction [19,37]. The parameter ηi is the impingement factor and its values found in literature typically range between about 1 and 2.2 when a single process occurs [19,32,33].…”

“…However, those phases are not present in the pure binary alloys investigated in this paper, and hence, n should be 2.5, which is valid for continuous 360 nucleation during the course of the reaction [19,37]. The parameter ηi is the impingement factor and its values found in literature typically range between about 1 and 2.2 when a single process occurs [19,32,33]. However, in the present case clearly multiple processes in a wide range of temperatures are involved, and we will here fit ηi.…”

“…As the enthalpy change is in direct proportion to the mass and volume fraction of precipitated particles [18,19] …”

“…the microchemical composition of the matrix and the corresponding precipitation state, differential scanning calorimetry (DSC) measurements are of great importance. It was shown in [18,19] that the precipitation heat (enthalpy change) is directly proportional to the volume fraction of precipitated particles, which allows in principle a calculation of remaining solute amount in dependence of cooling rate and temperature [20].…”