Kyushu University · Engineering
Professor Yasuhito Kawahara's research lab specializes in the atomic-scale understanding of deformation mechanisms and strengthening mechanisms in advanced steels, particularly austenitic stainless steels. The lab focuses on the roles of interstitial (e.g., nitrogen) and substitutional (e.g., chromium, silicon) elements in modifying stacking-fault energy, dislocation structures, and defect interactions. Using advanced characterization techniques such as weak-beam TEM, high-resolution TEM, and atom-probe tomography, the lab investigates how microstructural features—such as dislocation configurations, carbide clustering, and short-range order—affect mechanical properties at both room and elevated temperatures. Their work bridges fundamental materials science with practical steel design for high-strength, high-performance applications.
Figures are computed from collected data and may differ slightly.
Austenitic stainless steels have superior room temperature and high temperature strengths, strongly influenced by stacking-faults in the steel microstructure. Nitrogen addition makes substantial contribution to room temperature and high temperature strengths, so it is essential to consider the effect of nitrogen on the stacking-fault energies (SFE) to understand strength mechanism of the steel and to enhance the strength. In this study, SFE were measured by weak-beam TEM method, and deformation
Combined addition of interstitial-substitutional elements has been acknowledged to contribute to the increase in the strengths of steels. For further improvements in mechanical properties, their atomic-scale interaction mechanisms with dislocations are required to be examined. In this study, both high-resolution transmission electron microscopy and atom-probe tomography were used to correlate interstitial-substitutional elements with dislocation characteristics in austenitic stainless steels. Th
The structures of carbon clusters and carbides in a low-carbon ferritic steel were investigated at atomic-scale by annular dark-field scanning transmission electron microscopy. In the low-carbon ferritic steel aged at 473 K for 1 h, some homogeneously dispersed ε-carbides were formed within the matrix as closely spaced granules aligned to <001> of the ferrite matrix, and others were heterogeneously formed on AlN precipitates. The ε-carbides formed on AlN precipitates were coexisted with carbon c
The stacking-fault energies (SFE) were measured by weak-beam TEM method, and deformation mechanisms in room and high temperature were characterized in terms of the effect of nitrogen addition on SFE in Si-added austenitic stainless steel (Fe-19%Cr-13%Ni-0.05%C-3%Si-x%N). Nitrogen addition resulted in decrease of SFE and changing the dislocation configuration from tangled to planar in room temperature. In high temperature, nitrogen addition resulted in the increase of dislocation density in the s
Abstract Interstitial-substitutional pairs have been acknowledged to contribute to the increase in the strengths of steels. For further improvements in mechanical properties, their atomic-scale structures and interaction mechanisms with defects are required to be examined. In this study, both high-resolution transmission electron microscopy and atom-probe tomography were used to correlate interstitial-substitutional pairs with defect characteristics in austenitic stainless steels. Three types of
Planar slips have been frequently recognized in nitrogen-added austenitic stainless steels, and often discussed in terms of stacking-fault energy (SFE). On the other hand, nitrogen addition promotes the formation of N-Cr short-range order (SRO), which has been proposed to cause the planar slips. In this study, a chromium-free Fe-N binary austenite with 2.4 mass% of fully solid-solutioned N was fabricated to simplify the relationship among deformation structures, SFE and N-Cr SRO. The Fe-N alloy
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