On the Sources of Discrepancies Between Grain Size Measurements

Accurate quantification of grain size in polycrystals is extraordinarily important for quality assurance and the development of structure-property relationships, as grain size affects nearly all engineering properties of structural alloys, from strength and fatigue life to corrosion resistance and thermal conductivity. Despite nearly a century of the existence of standards for measuring grain size, issues persist in obtaining consistent grain size measurements across several techniques. Of particular interest is comparing optical microscopy or scanning electron microscopy on an etched surface with mapping of crystal orientations using electron backscatter diffraction (EBSD). In this work, discrepancies between methods in the ASTM E 112 and ASTM E 2627 standards are explored through simulated measurements on synthetic microstructures. It is concluded that a small but systematic discrepancy exists between planimetric and lineal intercept-based approaches, and a new empirical relationship between the ASTM grain size number G and lineal intercepts is proposed.