Understanding the mechanism of Beraha-I type color etching: Determination of the orientation dependent etch rate, layer refractive index and a method for quantifying the angle between surface normal and the 〈100〉, 〈111〉 directions for individual grains

Abstract

A grey cast iron specimen was investigated by Beraha-I type color etching for the purpose of developing a functional model for the etching kinetics and subsequently a method which would enable the estimation of ferrite grain orientations with respect to specific directions, solely based on optical microscopy. For this, the etching process was observed real-time in a microfluidic cell and a custom image processing software was developed to interpret the color information. Scanning electron microscopy with electron backscatter diffraction (SEM-EBSD) and atomic force microscopy (AFM) were used to determine the grain orientation and the relative thickness of the developed interference layer respectively, which were used for further calculations. It was shown that in the initial phase the etching process experiences a delay, which can be connected to breaking an oxide barrier above the ferrite grains. This delay was found to be influenced by the idle time between sample preparation and the start of etching. However, it was proven that a characteristic cycle etch rate can be defined between the first interference minimum and maximum, which shows correlation with both the 〈100〉 and 〈111〉 directions and which is also independent from the delay during the initiation phase. The exact value and orientation dependence of this cycle etch rate was experimentally determined along with the refractive index of the developed film. Our method was demonstrated to be applicable for the estimation of the angle between the surface normal and the 〈100〉, 〈111〉 directions for individual ferrite grains in the cast iron specimen, with an average absolute error of 3°, by using optical microscopy only. The method is implemented and was made available in the form of a Matlab program.