THREE-DIMENSIONAL CHARACTERIZATION OF MORPHOLOGY AND CRYSTALLOGRAPHY OF LATH MARTENSITE IN MARTENSITIC STAINLESS STEELS

Abstract

Lath martensite, a key microstructural feature in low-carbon martensitic stainless steels, plays a crucial role in determining the mechanical performance of these materials. Despite extensive research, the three-dimensional (3D) morphology, crystallographic characteristics, and hierarchical organization of lath martensite remain inadequately understood due to the limitations of conventional two-dimensional (2D) imaging techniques. This study employs a large-volume 3D electron backscatter diffraction (3D EBSD) approach combined with plasma-focused ion beam (PFIB) serial sectioning tomography to provide a comprehensive investigation of lath martensite in a 13Cr-4Ni martensitic stainless steel. The research presents novel insights into the morphology, habit plane variations, and boundary networks of lath martensite, contributing to a refined understanding of its formation mechanisms and mechanical implications. The microstructural analysis reveals the hierarchical subdivision of martensite into prior austenite grains (PAGs), packets, blocks, and sub-blocks. Using 3D EBSD and Kurdjumov-Sachs (K-S) orientation relationship analysis, intervariant boundary networks were identified and classified, allowing a quantitative assessment of their role in the microstructure. The study highlights the presence of delta-ferrite particles and non-metallic inclusions, reconstructed in 3D to determine their spatial distributions and interactions with the martensitic matrix. Two distinct delta-ferrite morphologies were observed: elongated particles at PAG boundaries and smaller, spherical particles within PAG interiors. Furthermore, block and packet interactions were analyzed, revealing three primary types: hard impingement, mutual intersection, and interpenetration. These findings illustrate how the hierarchical arrangement of laths, blocks, and packets influences the overall boundary network complexity of the steel. A detailed investigation into habit planes was conducted using 3D morphological and crystallographic reconstructions. The dominant habit plane, derived from the normal directions of high-angle block boundaries, was identified between {111}γ and {557}γ, with an orientation of (0.51,0.52,0.66)γ. However, local habit plane deviations were detected in specific regions, primarily due to block bending and interactions between adjacent growing blocks. Spatial interference and growth competition within a single packet and hard impingement mechanisms were found to affect growth paths, leading to macroscopic deflections in interface planes. These observations highlight the importance of considering local habit plane variations in martensitic transformation studies to avoid oversimplified interpretations. Further analysis of the 3D spatial arrangement of packets within PAGs established a tetrahedral pattern governing their distribution. Through geometrical calculations, a direct correlation between this pattern and the dominant habit plane ({557}γ) was demonstrated, providing new insights into the 3D organization of lath martensite. The study also compared 3D-EBSD results with traditional 2D characterizations, revealing potential misinterpretations in habit plane orientation and block morphology when relying solely on 2D analyses. These findings emphasize the necessity of 3D approaches to accurately capture the complex nature of martensitic structures. Overall, this research advances the understanding of lath martensite by integrating large-scale 3D reconstructions with crystallographic and morphological analyses. The results have significant implications for improving predictive models of microstructural evolution and mechanical behavior, aiding in the optimization of martensitic stainless steels for industrial applications such as hydroelectric turbine components and structural engineering materials. By correlating habit plane characteristics with the 3D morphology of martensitic structures, this study provides a foundation for future efforts in refining the processing and performance of these alloys.