Grain distribution characteristics and effect of diverse size distribution on the Hall–Petch relationship for additively manufactured metal alloys

Additively manufactured (AM) metal alloys normally exhibit very diverse grain size distributions. The size measured by electron backscatter diffraction (EBSD) is generally smaller than that from optical microscope (OM) due to better resolution, but proper scan setup must be chosen to reveal the existing fine grains. In this work, we compared these two widely applied techniques and investigated the effects of different parameters on the resulting average diameter d. Regarding the discrepancies of parameters in the literatures, a standard EBSD setup is suggested, so that >99% of the total grain area should be detected. The grain distribution characteristics for AM fabricated alloys were also illustrated. A simply modified Hall–Petch equation is proposed by considering the grain size distributions using grains covering >99% of the measured area. It gives remarkable agreement between predicted strength and experimental values for almost all the AM alloy systems we tested, which possess very diverse grain size distributions. This revised model can extend to other AM alloys with heterogeneous microstructures and is very practical for engineering approach.