- The reason for an award -

In ensuring the toughness and ductility of high-carbon martensitic steels, low-temperature tempering is one of the most important heat treatment processes. Since this treatment induces a wide variety of microstructural changes, achieving a comprehensive understanding of them has been extremely challenging.

This paper makes a significant contribution to the comprehensive understanding of low-temperature tempering phenomena by combining multidimensional and multifaceted characterization and analysis techniques including calorimetry, dilatometry, in-situ neutron diffraction, transmission electron microscopy, and three-dimensional atom probe analysis. Through these approaches, the authors successfully obtained new insights related to the formation of carbon clusters, the reduction of solute carbon content due to the precipitation of metastable carbides, the accompanying changes in tetragonality, and the stress states acting on retained austenite. Notably, careful experimental strategies were employed to avoid the influence of self-tempering, thereby enabling the acquisition of high-quality and quantitative data that isolate only the effects of low-temperature tempering. This achievement provided a solid basis for essential and substantive discussions.

In conclusion, this paper systematically elucidates the complex phenomena associated with low-temperature tempering, demonstrating high academic value and a high level of completeness without any logical inconsistencies, even in fine details. It is therefore highly commendable and can be judged as fully deserving of the Sawamura Award.

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Hydrogen embrittlement is more pronounced in high-strength steels, material design to improve hydrogen embrittlement properties is necessary for the widespread application of advanced high-strength steels. Since failure caused by the occurrence and growth of microcracks is governed by microstructural factors and strength and deformation characteristics, it is necessary to understand the relationship between crack initiation and propagation and hydrogen and microstructural factors.

In this study, hydrogen embrittlement failure in martensitic steel is quantitatively examined based on crack propagation patterns obtained from three-dimensional fracture surface information visualized by X-ray CT, focusing on macroscopic mechanical responses (J-integral and tearing modulus: parameters corresponding to crack propagation resistance). Even when the hydrogen content was high up to 4.00 wt ppm, unstable premature fracture did not immediately occur, and a certain crack-growth resistance could be confirmed. This study also clarifies that cracks propagate continuously in three dimensions, and the crack opening-displacement is small. A new parameter, “crack-propagated thickness”, was defined to represent crack meandering. Since the tearing modulus increases with the increase in crack-propagated thickness, it was concluded that crack meandering increases crack growth resistance.

By advancing this technology, it becomes possible to control macroscopic mechanical properties by controlling the microstructural factors, and is of high academic and engineering significance, making it suitable for the Sawamura Award.

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Control of solidification microstructure is essential to suppress segregation and cracking in the steel solidification, and an understanding of the various solidification processes is required. Solidification phenomena involving the nucleation of metastable phases from undercooled liquid and subsequent phase transformation into equilibrium solid phases have been studied in the Fe-Ni and Fe-Cr-Ni systems, but the discussion has been limited to such systems.

In this paper, the solidification phenomenon of an Fe-Mn-C alloy with austenite as the equilibrium primary phase was investigated using time-resolved X-ray tomography, X-ray diffraction measurements, and thermal analysis. In-situ observation and diffraction analysis revealed that a metastable ferrite phase nucleates and grows from the undercooled liquid phase, followed by a massive transformation from ferrite to austenite and a crystallization of the residual liquid to austenite. The respective phase transformation temperatures were also confirmed by thermal analysis, and then the solidification mechanism through metastable ferrite phase formation was determined. This research not only achieved scientific progress by elucidating the solidification phenomenon in Fe-Mn-C alloys which occurs through the formation of metastable phase, but also presented the possibility for a new solidification microstructure control technology to improve steel properties, indicating a great significance in terms of industrial application.

As described above, this paper clarified the solidification phenomenon of Fe alloys through the nucleation and growth of metastable phases and the subsequent microstructure formation using the advanced observation and measurement techniques. It is highly evaluated both academically and industrially, and is therefore appropriate for the Sawamura Award.

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Understanding the thermophysical properties of refining slags or mold fluxes is crucial for manufacturing processes. Phonons have been known to be the heat carriers in silicate melt and glass, and there has been an increasing number of studies on the phonon mean free path. However, a general understanding of the composition-dependent mechanism of the mean free path remains unclear.

In this paper, the authors evaluate the effect of metal oxide addition on the phonon mean free path of silicate glasses, systematically varying the amount of metallic oxides added. The thermal diffusivity and sound velocity in silicate glasses are measured. It is shown that the probability of phonon scattering in PbO-added specimen is lower than that in alkali and alkaline earth silicates. The approximate equation based on the mean free path model for gas molecules is proposed, and it is found that phonons are scattered near non-bridging oxygen atoms. In the case that the metal oxide added to the glass is a network-forming oxide, the decrease of mean free path is small, indicating that the mean free path is a useful parameter for understanding the structural role of metal cations. The phonon mean free path prediction model is capable of predicting the thermal conductivity of silicate materials with high accuracy, and is therefore expected to contribute to a universal understanding of composition dependence and its applications.

As described above, this paper is recognized as valuable from both academic and industrial perspective, and is therefore to be worthy of the Sawamura Award.

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The paper presents an exceptionally original contribution by experimentally demonstrating, for the first time in the world, the direct measurement of the agglomeration force between alumina particles in molten steel and clarifying its underlying mechanism. Whereas previous studies on inclusion agglomeration relied largely on fluid dynamics models or indirect observations, this work established an innovative experimental method that eliminated the influence of molten steel flow. The authors identified the cavity bridge force, rather than van der Waals forces, as the dominant cause of particle agglomeration, and validated this conclusion both theoretically and experimentally.

The results are highly significant academically, as they construct a new scientific framework for understanding inclusion behavior consistent with analytical models. Industrially, the findings offer valuable insights into the removal of non-metallic inclusions during steelmaking, contributing directly to quality control in continuous casting and the production of clean steel.

The proposed models and techniques have already found applications in subsequent studies and are becoming standard tools in the field. Moreover, the underlying principles are extendable to inclusions beyond alumina, suggesting broad applicability to materials development and process simulation. For these reasons, this paper is regarded as one of the most influential and impactful contributions in the past decade and is deemed fully deserving of the Distinguished Article Award.