PSI - Issue 68

Ritsuki Morohoshi et al. / Procedia Structural Integrity 68 (2025) 701–707 R. Morohoshi et al. / Structural Integrity Procedia 00 (2024) 000–000

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Fig. 8: Assembled ′ ratio - Mises strain curves

previous steps, there is no need to retroactively reconstruct search , but this correction is applied to all subsequent data processing. After applying this correction, the coordinate systems of all EBSD images align almost perfectly. Using this alignment, a more accurate crystal tracking graph, refined , is created. The only difference between refined and search lies in the method for generating edges. The edge generation process is as follows: For a given grain at load (hereafter referred to as the reference grain), a set of grains is searched for in the EBSD image at that occupies the same coordinate region as the reference grain, and these grains form the candidate set. To narrow down the most probable candidates, only those grains in the candidate set with a crystallographic orientation deviation of 5° or less from the reference grain are selected and registered in refined . If the reference grain is ′ and the candidate grains are , no edges are generated. Next, the focus turns to the effect of stress triaxiality on the ease of ′ transformation. The relationship between the ′ transformation ratio and Mises strain is shown in fig. 8. As can be seen, there is no significant difference between uniaxial and approximately 0.5 triaxiality, as shown by Polatidis. However, it is evident that shear significantly inhibits the ′ transformation. The result represents the first direct consequence of our study. The findings support prior research indicating a significant difference in the ′ transformation behavior between shear specimens and others. Now, turning to the core question: why does the ease of ′ transformation depend on stress triaxiality? This study focuses on examining this dependency through the lens of the Schmid factor and grain size. The rationale is that stress triaxiality dependence suggests that the observed ′ transformation is strain induced, meaning that plastic deformation plays a critical role. Since the Schmid factor, which influences the ease of plastic deformation, is expected to be affected by stress triaxiality, it is reasonable to investigate this relationship. Additionally, there is prior research showing that grain size influences the ease of ′ transformation, making it another important factor to consider in this analysis. Let’s begin by examining the Schmid factor. Specifically, when an ′ transformation occurs due to strain, the tendency of the Schmid factor can be analyzed by extracting the grains just before transformation from refined at the corresponding load. However, not all grains transform into ′ , and it is difficult to spatially determine which parent grain gave rise to the ′ transformation, especially when the transforma tion occurs at grain boundaries that satisfy a double K-S relationship. This limitation necessitated an enhancement of refined . The following is the procedure used: Since the coordinates occupied by a single grain remain constant in the affine-transformed coordinate system, regardless of load, the grains present at the coordinates occupied by a specific ′ grain just after transformation are examined in the previous load step. Ideally, a part of the corresponding grain just before the ′ transformation will be found there. Using the restricted EBSD image from the previous load step, grains within the 5° crystallographic orientation threshold are reconstructed. These reconstructed grains are then matched with the grains in Affined EBSD ,ℓ , and edges are established to link the newly formed ′ grain with its corresponding grain. The results is presented in fig. 9. It could be said that all the trend is the same no matter how different specimens they are. Therefore, Schmid factor is not the cause of stress field dependency of martensitic stability. This is the additional evaluation to investigate the stability of martensite (fig. 10). X axis shows the initial grain diameter of the austenite. Y axis shows the amount of the newly-made martensite. Generally,