PSI - Issue 68

Yuki Tampa et al. / Procedia Structural Integrity 68 (2025) 681–686

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Y. Tampa et al. / Structural Integrity Procedia 00 (2025) 000–000

FeCo-2V alloys are subjected to uniaxial tensile loading, the fracture surface is entirely covered with cleavages, Jordan and Stoloff (1969), Sourmail (2005). Although the addition of small amounts of alloying elements to Permendur alloys makes cold rolling available, the brittleness of the ordered phase still remains a problem in terms of handling and workability during manufacturing processes. As for the slip behaviour of the B2 ordered-phase Permendur alloy, Yamaguchi et al. (1982) conducted compression tests using Fe - 54 at.% Co single crystals in the temperature range of 4.2 - 600 K, and reported that the {110} <1 1 1> slip system was primarily activated. In addition, microtensile testing by Kishi et al. (2018) using single- and bi-crystalline specimens of Fe - 49.3 at.% Co revealed that the single crystals exhibited moderate ductility and ruptured after necking, whereas the bi-crystals exhibited slight strain hardening after yielding and then led to brittle fracture due to grain boundary separation. In other words, although the Permendur alloy is essentially capable of slip deformation in single crystals, it behaves brittle in polycrystals. As mentioned above, the deformation behaviour of the B2 ordered-phase Permendur alloy is sensitive to the degree of ordering [9]. Therefore, in order to directly compare the deformation and fracture behaviours in polycrystals and single crystals, it is helpful to fabricate and evaluate multi-scale specimens from the material that has experienced the same processing and thermal history. In this study, we employed multi-scale tensile testing to correlate the brittle fracture behaviour of polycrystals with the slip behaviour of single crystals in Permendur alloys with focus on the effects of V addition and disordering. 2. Materials and experimental methods The materials used in this study were Fe - 49Co - 1.9V and Fe - 50Co (mass%) alloys, which are hereafter denoted as FCV and FC, respectively. For FCV and FC, ordered phases were obtained through heat treatment at a temperature of 1123 K for 3.6 ks followed by furnace cooling. To investigate the effects of disordering, a disordered-phase state was obtained by quenching into iced brine after the same heat treatment as the ordered-phase samples. The ordered- and disordered-phase specimens are distinguished by the suffixes ‘-O’ and ‘-D’, respectively. Figure 1 shows the scanning transmission electron microscopy bright field image around a grain boundary and corresponding energy dispersive X-ray spectroscopy elemental maps in a commercial FeCo - 2V alloy. It is confirmed that V is segregated at the grain boundary. A micrometre-scale tensile specimen with gauge section dimensions of 20 μm × 20 μm × 50 μm were fabricated using focused ion beam. For single-crystalline specimens, the loading axis was set nearly parallel to the [111] direction. A millimetre-scale specimen with gauge section dimensions of 0.5 mm × 0.5 mm × 1.25 mm was prepared via electro discharge machining. Figure 2 shows the electron backscatter diffraction (EBSD) inverse pole figure (IPF) maps of millimetre-scale polycrystalline tensile specimens. The EBSD analysis was conducted using crystallographic orientation analysis software OIM 8.6.0. For each specimen, the crystallographic orientations were randomly distributed. Tensile

Fig. 1. Scanning transmission electron microscopy bright-field image and energy dispersive X-ray spectroscopy elemental maps in a commercial FeCo - 2V alloy.

Fig. 2. EBSD maps of millimetre-scale polycrystalline tensile specimens. The unit triangle shows the IPF colour-coded according to crystallographic orientation corresponding to the loading direction (LD). Black lines denote the grain boundaries with misorientation angles greater than 15 ° .