Image Quality Analysis: A New Method of Characterizing Microstructures

Polycrystalline aggregates are comprised of three microstructural features: grain centers, grain boundaries, and regions affected by grain boundaries. It is these features that determine the mechanical properties, and any advanced understanding of microstructure-property relations requires their quantitative description. Traditionally, descriptions of microstructures have been based on visualization, i.e., how grains appear in the optical or scanning electron microscope (SEM). While this may lead to classification systems that permit differentiation, it does not allow for quantification, especially in complex microstructures, and does not lend itself to either developing or applying structure-property relationships. The goal of this paper is to present a new approach to the characterization of complex microstructures, especially those found in advanced modern high strength steels. For such steels, the new approach employs the fact that different types of ferrite formed at different transformation temperatures have different dislocation or sub-grain boundary densities. Hence, measuring the degree of lattice imperfection of the grain centers of the ferrite is one way of first identifying, then grouping, and finally quantifying, the different types or forms of ferrite. The index chosen in this study to distinguish the degree of lattice imperfection is the image quality (IQ). Finally, as part of the new approach a procedure has been developed to improve the accuracy of applying IQ measurements.