Magnetic properties of spray-formed Fe–6.5%Si and Fe–6.5%Si–1.0%Al after rolling and heat treatment

The maximum silicon content in commercial Fe–Si steels is limited to about 3.5 wt%Si, since the ductility declines sharply as this maximum is exceeded, hindering the production of thin sheets by cold/hot rolling. However, the best magnetic properties are attained at about 6.5 wt%Si, a silicon content that renders magnetostriction practically null and minimizes magnetic losses. Using spray-forming, our research group has successfully produced this type of high silicon alloy in thin sheet form by carefully controlling the many variables of the process and subsequent rolling operations. In the present study, we investigated the magnetic properties and the microstructure of spray-formed Fe–6.5 wt%Si and Fe–6.5 wt%Si–1.0 wt%Al alloys after warm rolling and heat treatment. The main cause for the brittleness of Fe–6.5 wt%Si alloy has been attributed to the B2 phase long-range ordering, which leads to premature fractures. The presence of aluminum could avoid B2 formation and improve the alloy's ductility. The binary Fe–6.5 wt% Si alloy showed the best magnetic properties, which were ascribed to a recrystallized, coarse grain size (∼500 μm; and 340 μm for the Al-containing alloy). TEM analysis showed that a well-developed B2 domain structure (about 50–300 nm in size) was formed in the binary alloy when low cooling rates are prevailing after heat treatment. This structure contributed to improve additionally the magnetic properties, but its effect was not so strong as that of the grain size. The addition of Al to the binary alloy suppressed B2 formation, as indicated by Mossbauer spectroscopy, and apparently hindered excessive grain growth, which may explain the slightly poorer magnetic properties when compared with the binary alloy.