Kinetic study of rapidly quenched Ni81P19 amorphous alloys
Introduction
The structure of melted alloys has been investigated for more than 20 years [1]. Amorphous alloys can be prepared by rapid quenching from melt. The structure of the solidified amorphous alloys is determined by the liquid structure and by the relaxation processes taking place during the quenching. If there are no significant relaxation processes during the rapid quenching, then the obtained amorphous alloy keeps the internal structure of the original melt. The investigation of amorphous alloys provides valuable information on the internal structure of melting alloy, including the short- and long-range ordering. Previous Mössbauer investigations showed that the effect of relaxation on the structure of the Ni81P19 alloys is much smaller then that observed in other systems [2], [9].
The crystallization kinetics of the Ni81P19 amorphous alloy has been extensively studied [10]. The crystallization of the Ni81P19 alloy is an exothermal process and the differential scanning calorimetry proved to be a suitable method for investigation of the crystallization process.
In thermal analysis, the usual way of the kinetic evaluation is the application of some sort of linearization. In our opinion, these linearization techniques do not take sufficient care on the effect of experimental errors, and frequently lead to wrong results and conclusions. In other fields of chemistry the general means of kinetic evaluation is the non-linear method of least squares. Regardless of its statistical background, it is usually suitable to get an optimal or near-to-optimal fit between the measured and the theoretical data.
Many other evaluation techniques are proposed and used in the literature of the thermal analysis, which do not involve a true, least squares curve fitting process [4], [5]. In our opinion, these methods have the following drawbacks:
- 1.
They are based on logarithmic linearization, which leads to a non-uniform sensitivity on the experimental errors.
- 2.
Sometimes the parameter determination is based only on a few points of the experimental curves.
Therefore, the evaluations of the present work were based on the non-linear method of least squares.
Section snippets
Samples and preparation
The amorphous alloys of composition Ni81P19 were prepared by rapid quenching from melts which were treated at different temperatures (920–1600°C) for 20 min to achieve an equilibrium state. For rapid quenching we used the so-called single roller melt-spinning technique in an argon atmosphere. We had three different sample groups:
Group 1: Samples produced by different quenching rate from same temperature (950°C).
Group 2: Samples cooled down from different temperature at same quenching rate (1750
Applied mathematical model and evaluation method
We tried two different models in the evaluation:
- 1.
According to the usual assumptions of the Avrami–Mampel–Erofeev model, we assumed that the nuclei form during the DSC experiments. The nuclei grow and later, in the ‘decay’ period of the experiments, overlap each other.
- 2.
As an alternative description of the experiments, we assumed that the nuclei were formed before the DSC experiments, during the preparation and pretreatment of the samples. When the samples are heated in a DSC experiments, the
Results and discussion
First we evaluated the experiments by the well-known Avrami–Mampel–Erofeev model. In this way we obtained extremely high activation energy, above 2000 kJ/mol. For these reason we preferred the alternative model discussed above.
This mathematical model gave better curve fit and more reasonable parameter values than the Avrami–Erofeev equation.
Conclusions
Differential scanning calorimetry (DSC) measurements were used to study the crystallization kinetics of the Ni81P19 amorphous alloy. A mathematical model was proposed which gives better curve fit and more reasonable parameter values than the Avrami–Erofeev equation. This model assumes that the nuclei are formed before the DSC experiments, during the preparation and pretreatment of the samples. Kinetic parameters were presented for amorphous ribbons quenched from 920 to 1400°C at different
Acknowledgements
The participation of G. Várhegyi in this work was due to the help of the Hungarian National Research Fund (OTKA T 025347).
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