High temperature EBSD experiment versus crystallographic reconstruction to study phase transformation induced microstructures - Application examples for steels

This study explores the capabilities and limitations of high-temperature electron backscatter diffraction (HT-EBSD) and crystallographic reconstruction methods for analyzing microstructure evolution induced by phase transformations in steels. Using a state-of-the-art CMOS detector with infrared filtering enabled EBSD map acquisition of austenite in real time at temperatures up to 1000 °C with acquisition speed over 1000 points per second (pps) and high indexing rates. These direct observations revealed critical features of the austenite microstructure, including annealing twins and grain size heterogeneities, that reconstruction methods typically overlook. Moreover, it was possible to confirm austenite grain refinement through austenitization cycles in a medium carbon steel and to reveal for the first-time ultrafine parent austenite grains in 10 % chromium oxide dispersion strengthened (ODS) steel, a case where reconstruction methods failed. Furthermore, in-situ HT-EBSD provides ground truth validation for reconstruction methods. By monitoring in-situ the martensitic transformation on 10 %Cr steel, we confirmed both the high accuracy of reconstructed parent austenite orientations and validated the average orientation relationship (OR) established through reconstruction methods. Finally, in-situ HT-EBSD could also provide valuable data for refining reconstruction algorithms to overcome their limitations in capturing precise grain morphology and annealing twins. Despite its advantages, HT-EBSD faces challenges including temperature measurement uncertainties and potential surface artifacts. This study ultimately advocates for judicious combination of both methodologies to comprehensively understand microstructure evolution during steel phase transformations, including the “memory effect” during re-austenitization and identification of austenite nucleation sites. • State of the art HT-EBSD systems enables capturing real-time austenite microstructure up to 1000 °C • HT- EBSD gives new insights in analyzing microstructure evolution induced by phase transformations in steels • Direct observation offers valuable date to validate and refine reconstruction methods. • HT-EBSD faces limitations including temperature uncertainty and potential surface artifacts. • Complementary use of both methodologies provides comprehensive understanding of phase transformation mechanisms in steels.