Magnetic domains around annealed shear bands and embrittlement of FeBSi metallic glass
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I. INTRODUCTION
Metallic glasses, especially the Fe-B type, in transformer core applications need to be annealed below their crystallization temperatures in order to obtain optimum soft-magnetic properties. For low frequency applications, one of the contributors to the hysteresis losses is the presence of fine domains due to off-axis or perpendicular anisotropies. Fine domains in as-quenched metallic glasses of positive magnetostriction are known to be present in regions of compressive stresses1 and were attributed to inhomogeneous arrangements of 'quasidislocation dipoles' arising out of inhomogeneous quenching rates. Any improvement of hysteresis losses by the reduction in the domains of perpendicular anisotropy or these dipoles would necessarily be related to the behavior of internal stress relaxation of the particular metallic glass. Stress relaxation has been studied extensively in metallic glasses in the past to understand the structural rearrangement below the crystallization temperature Tx by several different methods. A commonly used method involves coiling the ribbon into a circle of radius rs and annealing for a given time at some temperature and measuring the new relaxed radius ra afterward. The ratio rs/ra gives a measure of the fraction of stress relieved and has been reported by several authors2"6 for different alloys. Luborsky2 found that the normalized magnetization ratio (MT — M,)/{MF — Mt) where MT is the magnetization after annealing at T, Mt the initial magnetization, and MF is that after the final anneal, when plotted against the annealing temperature, showed a curve that coincided almost exactly with rs/ra, the fraction of stress relieved versus temperature up to about 300 °C in a Ni-Fe-P-B metallic glass. This seems to suggest a relationship between internal stress and magnetization. When fine domains due to perpendicular anisotropies possess a standard 'closure' domain structure, the following relation exists1: J. Mater. Res., Vol. 6, No. 2, Feb 1991 http://journals.cambridge.org
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Ko = 64AT0/D$ (1) where Ko is the anisotropy energy per unit volume, A is the exchange parameter, Do is the domain width, and To is the domain thickness. If the anisotropy energy Ko is due predominantly to an average internal stress a, it is equal in magnitude to -(3/2)Ao- where A is the magnetostriction constant. Hence, the magnitude of the stress is inversely proportional to the fourth power of the domain wall spacing, and the changes in them should reflect the change in the magnitude of the stress present provided that the magnetostriction constant A and the exchange parameter A do not change significantly. Previously,7 the magnetic domain structures in the vicinity of shear bands produced by bending were reported for metallic glasses 2605-SC and 2605-S2.8 The domain structures on the tensile side of bending of this positive A metallic glass have a fine structure with zigzag domain walls and domain widths of roughly 1 ixm. Though the details of the exact magnetization directions are no
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