Nucleation and growth effects in thin ferromagnetic sheets: A review focusing on surface energy-induced secondary recrys
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I.
BRIEF HISTORY OF MAGNETIC SHEETS
B E C A U S E anisotropy usually exists in their magnetic properties, principally in the permeability and energy loss during cyclic magnetization, most soft ferromagnetic alloys are processed in a way to yield material with a high degree of preferred crystal texture. This is particularly true for the sheets or laminations used as cores in power and distribution transformers. In these applications, an alloy of - 3 pct Si in Fe (Si-Fe) with a (110)[001] sheet texture is usually used. For this iron-base alloy, the (100) direction is a low loss and easy magnetization direction. To keep the eddy currents low during cyclic magnetization, laminations are usually less than 0.3-mm thick. The (110)[001] texture (Goss texture or Hipersil) in this alloy can be traced back to the 19308. [~] Over the years, significant improvements have been made in the quality or perfection of this texture, resulting in higher permeability and reduced magnetic losses. However, texture alone is not the only factor important in minimizing losses, which are usually separated into a hysteresis and an eddy current component. Hysteresis loss depends on crystal orientation, alloy purity, internal stress, and the smoothness of the sheet surface, t2] Although these factors may also influence the eddy current losses, the resistivity of the laminations, grain size, sheet thickness, and stress effects, from the coating used to electrically separate the sheets, are particularly important here. t2] While transformers today can be greater than 95 pct efficient, even small fractional improvements in efficiency are very significant because of the widespread use of such devices. JOHN J. KRAMER, Professor of Electrical Engineering, Material Science Faculty, is with the Department of Electrical Engineering, University of Delaware, Newark, DE 19716. This paper is based on a presentation made in the "G. Marshall Pound Memorial Symposium on the Kinetics of Phase Transformations" presented as part of the 1990 fall meeting of TMS, October 8-12, 1990, in Detroit, MI, under the auspices of the ASM/MSD Phase Transformations Committee. METALLURGICAL TRANSACTIONS A
In highly textured materials with the preferred orientation (110)[001], the magnetization process occurs almost exclusively by domain wall motion. The local magnetization change at the moving domain wall generates eddy currents within the conducting sheets, contributing to the magnetic losses. A detailed analysis of this phenomenon, using Maxwell's equations, suggests that for a material of given thickness and resistivity, an infinitely fine, favorably oriented domain structure with highly mobile domain walls would be the ultimate in low losses, t3] However, a highly oriented single crystal sheet is not the optimum, since the domain structure in such material is usually coarse. The development and control of the (110)[001] orientation have involved research into all facets of a multistage deformation and recrystallization process, culminating in a secondary recrystallization
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