Improvement of Ductility in an Intermetallic Compound

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Fe-6.5mass%Si has excellent properties as a transformer sheet. Because of its low ductility, however, we cannot make thin sheet by usual cold rolling. This study aimed at improving ductility of the alloy. Results are summarized as follows:1. Order structure : In the Fe-Si alloys, ordered phases obtained by usual heat treatment procedures are different from those reported in the thermally equiribrium phase diagrams; The range of B2 ordered phase extends toward higher Si Content. In the 6.5%Si alloy, the B2 structure is stable up to about 800K and then transforms to the disorder phase above the temperature.2. Temperature dependence of yield stress : The yield stress vs. temperature curve of the 6.5%Si alloy has a peak at 600K. This peak has been attributed to DO_3 B2 transformation. However, the transformation does not actually take place at the peak temperature; A further investigation is needed to elucidate the origin of the peak.3. Ductile-brittle transition : The 6.5%Si alloy fractures in a brittle manner below about 600K (region B), whereas it deforms without cracking above the temperature (region D). In the ductile region, when specimens are stretched at low strain rates, fracture surfaces contain cleavage facets (mixed region of D and B). The boundary between regions B and D moves toward lower temperature as strain rate increases. This suggests that we may successfully roll down the 6.5%Si alloy at a sufficiently high deformation rate at a lower temperature.4. Rolling : The alloy cracked at lower rolling speeds, but it could be rolled down without cracking at higher rolling speeds. This fact proves the expectation mentioned above. The alloy was workable down to 473K if rolling speed is sufficiently high.