<i>In-situ</i> Observation of Growth Behavior of Fe-Zn Intermetallic Compounds at Initial Stage of Galvannealing Process

Abstract

In-situ observation was performed with X-ray diffraction technique using synchrotron radiation to reveal growth behavior of the Fe-Zn intermetallic compounds, theand � 1 phases, at the initial stage of galvannealing process. The galvanized sample and electroplated sample were used in the observation. The diffraction peak profiles were successfully obtained at intervals of 1 second with heating the sample, and the growth of the Fe-Zn intermetallic compounds was observed dynamically. In the galvanized sample including a small amount of aluminum in the coating, there was an incubation period of 7 seconds before the � 1 phase started to grow. The thickness estimated with the peak intensity of the � 1 phase increased in proportion to the square root of heating time when the incubation period was taken into account. In the electroplated sample including no aluminum in the coating, the thickness ofphase increased in proportion to the square root of heating time. The � 1 phase started to grow as soon as thephase occupied the entire coating. The thickness of the � 1 phase also increased in proportion to the square root of heating time. These results suggest that that the growth behavior of the � 1 phase at the initial stage of galvannealing is dominated by the interdiffusion between Fe and Zn, neither by interfacial reaction nor by autocatalytic reaction whether the coating contains aluminum or not. However, the growth behavior of the Fe-Zn during galvanizing or galvannealing process has been observed by static analyses such as a cross-sectional observation of the coating with an optical or electron microscope, and a measurement of iron contents in the coating using the samples quenched after annealing. In these analyses, it is very difficult to understand the growth behavior in detail because those reactions occur in a short period. Therefore, a rapid detection, i.e. ,' 'in-situ observation'' system is required to follow those reactions dynamically. The in-situ observation using X-ray diffraction method is a very useful technique to identify the Fe-Zn intermetallic compounds and to quantify their growth in the coating. In order to perform the in-situ observation of the growth behavior of the Fe-Zn in the coating, penetration length of X-ray and time definition of detector are important factors because it is necessary to observe the whole of coating having 10� 20 mm thickness, and to observe the rapid reaction. Although it is an easy way for achieving good time definition of detector to increase the intensity of source, it is very difficult for a conventional X- ray source to obtain enough intensity for the in-situ observation. Therefore, synchrotron radiation, which has higher energy and higher intensity than those of the conven- tional X-ray source, is necessary for the in-situ observation as an X-ray source. Recently, some synchrotron radiation experiments applied to an observation of the Fe-Zn reaction has been reported. 9-11) In this study, the in-situ observation of the growth of the Fe-Zn intermetallic compounds in the zinc coating during galvannealing process was performed with X-ray diffraction method using synchrotron radiation. In the experiment, a time dependency of diffraction profiles was detected quanti- tatively with galvanized samples including aluminum in the coating and electroplated samples including no aluminum in the coating. The growth behavior of the � 1 andphase in the coating at the initial stage of the galvannealing process was discussed with the quantitative results of their growth. Furthermore, the effect of aluminum in the coating on the growth behavior was also discussed.