Analysis of a Partially Recrystallized Steel Specimen Using Image Quality Measurements

Matt Nowell, EDAX

The image quality (IQ) value is a measure of the intensity and sharpness of the observed bands in EBSD patterns. This value is strongly influenced by the quality of the crystal lattice at the measurement position. Localized plastic strain, grain boundaries, and poor sample preparation, among other factors, can all reduce the measured IQ value.

Analysis of Partially Recrystallized Steel Specimen Using Image Quality Measurements

Figure 1: The image quality map from a partially recrystallized steel sample. In this image, the lower the measured IQ value at a given point, the darker the grey-scale shading. It is clear that there are some bright regions of defect-free grains and other darker regions with retained lattice distortions.

Analysis of Partially Recrystallized Steel Specimen Using Image Quality Measurements

Figure 2: A histogram chart of the image quality distribution. A bi-modal distribution is observed, and each distribution has been shaded.

The distributions in Figure 2 can easily be highlighted on the IQ map. The corresponding map is shown in Figure 3. From the distribution, the fraction of high quality data, shown in red, can also be determined.

Analysis of Partially Recrystallized Steel Specimen Using Image Quality Measurements
Figure 3

Analysis of Partially Recrystallized Steel Specimen Using Image Quality Measurements

The correlation between high IQ values and the recrystallized portion of the microstructure can be further confirmed by evaluating misorientations within each grain. To do this, for each point the average misorientation between that point and all of its neighbors is calculated, with the proviso that points having a misorientation greater than 5 degrees are excluded. A misorientation greater than 5 degrees was the criterion used to determine grain boundaries, so the proviso effectively confines the calculations to individual grains.

Analysis of Partially Recrystallized Steel Specimen Using Image Quality Measurements

Analysis of Partially Recrystallized Steel Specimen Using Image Quality Measurements

Figure 4: The resulting map. In this map, points with less than 1 degree average misorientation are shaded red, while points with greater than 1 degree misorientation are shaded blue. Upon examination, there is reasonable agreement of the two types of measurements for distinguishing recrystallized and un-recrystallized components of the microstructure.

Analysis of Partially Recrystallized Steel Specimen Using Image Quality Measurements