On the isothermal kinetics analysis of transformations in metastable systems: combined use of isothermal and non-isothermal calorimetry

Abstract

1On the isothermal kinetics analysis of transformations in metastable systems. A combined use of isothermal and non-isothermal calorimetry. A procedure to optimize the isothermal calorimetric data of very slow transformation processes of metastable systems is proposed. The method uses an experimental baseline to identify the transitory effects due to the equipment. Moreover, the combined use of isothermal and non-isothermal results is shown to be effective to overcome the intrinsic problems of low signal and signal drift for such processes. The procedure has been applied to the analysis of the nanocrystallization kinetics of the Fe 60 Co 18 Nb 6 B 16 alloy at different devitrification stages. Based on microstructural observations, an instantaneous growth approach was assumed and a phenomenological expression of the dependence of the nucleation frequency with both the transformed fraction and the temperature was obtained.

Correlation between microstructure and temperature dependence of magnetic properties in Fe60Co18(Nb,Zr)6B15Cu1 alloy series

“…These small changes are in agreement with the instantaneous growth approximation describing the nanocrystallization kinetics of this kind of alloy. 23,24 Under this approach, the time required for a nanocrystal to achieve its final size is negligible in comparison with the time of nanocrystallization process.…”

“…This can be understood in the frame of a recently proposed crystallization mechanism for these nanocrystalline systems based on continuous nucleation and instantaneous growth processes. 23,24 This mechanism predicts faster crystallization kinetics ͑al-though slow in comparison with conventional nucleation and three dimensional growth mechanisms͒ at the very beginning of nanocrystallization process, characterized by an Avrami exponent close to 1, which decreases as nanocrystallization progresses. The slowing down of the kinetics after the early stages of nanocrystallization can be correlated with a larger enthalpy per Fe atom incorporated to crystalline sites, i.e., with a more difficult mechanism of crystallization.…”

Development of nanostructured materials by mechanical alloying and/or rapid solidification

“…The kinetics of transformation gives information relative to the stability and applicability of these materials. One of the common experimental methods of obtaining the kinetic behaviour is through the use of differential scanning calorimetry (DSC) [7,8]. In this work, we apply the Johnson-Mehl-AvramiErofe'ev (JMAE) equation to perform the kinetic analysis.…”

On the use of classical JMAK crystallization kinetic theory to describe simultaneous processes leading to the formation of different phases in metals

“…in order to use the material as a sensor), however, in other cases, phase transformations must be avoided to keep the specific microstructure responsible for the desired properties. This is particularly important in the case of metastable systems (such as crystallization of amorphous alloys [1,2,3,4,5], polymers [6,7,8] and other glasses [9,10]), for which the metastable state should not be easily recovered once the transformation occurs. Therefore, understanding the mechanisms affecting the transformation kinetics is a key point in order to optimize the systems or to establish a comparison on the performance and perspectives of lifetime of a certain device.…”