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Mechanical stress and time dependence effect on the domain structure of metallic glasses
Materials Science and Engineering, A 110 (1989) L 13-L 15
L 13
Letter
Mechanical stress and time dependence effect on the domain structure of metallic glasses Z. VI~RTESY and E. KISDI-KOSZO
Central Research Institute for Physics of the Hungarian Academy of Sciences, P.O. Box 49 H-1525 Budapest 114 (Hungao') (Received October 14, 1988)
Abstract Changes in the magnenc domains in stressed and released Fe-Cr-B metallic glass ribbons were observed by scanning electron microscopy and in situ deformation. On release from a stress of 350 MPa the as-quenched state was reached in a short time. However, after longer storage the state resembling that under stress returned (the memory effecO.
The magnetic domain structure in iron-based ribbons is sensitive to mechanical stresses because of the high magnetostriction constant. Therefore, with the intention of obtaining information on the role of stresses in surface magnetic properties of metallic glasses the domain structure of stressed and unstressed ribbons was observed using the backscattered electron imaging mode of a JEOL 840 scanning electron microscope [1]. This method does not show contrast between the domains but a white or black line can be seen where 180 ° domain walls occur, depending on the direction change. Investigations on the domain structure of stressed samples have been carried out by Livingston [2] using a Bitter solution and dark field microscopy. Stressed samples were dealt with in ref. 3 but the general aim was to observe magnetic parameters such as the magnetoelastic anisotropy energy, the coercive force and the static hysteresis loss as functions of tensile stress. The investigated material was Fes0CrsB~5 metallic glass in the form of ribbons 1.5 mm in 0921-5093/89/$3.50
width and 38 /~m thick. The material was prepared by the melt-spinning method. The metallic glass samples were investigated by scanning electron microscopy at an accelerating voltage of 40 kV using a 0.1 ~tA electron beam. The ribbon was placed in a special JEOL-made tensile holder which permits the application of a tensile force to the specimen. Using a given magnification (100x), almost the whole width of the sample can be seen. The specimen placed in a holder without any tension was observed first. The domain structure of this sample was typical of the as-quenched state. Figure 1 shows the domain structure in an unstressed sample. Wide domains obliquely oriented in relation to the sample edge as well as small closure domains are visible. Under a mechanical tension of about 350 MPa applied along the length of the ribbon, an easy axis was induced in the same direction. The domain structure is changed considerably. Small closure domains disappear and wide domains with a direction parallel to the ribbon length occur (Fig. 2(a)). The sample was under stress for 1 month. Observations were made directly after the beginning of tension and then 1 h, 1 day, 1 week and 1 month after the beginning of tension. It was observed that the walls of wide domains moved in a direction perpendicular to the ribbon length, i.e. the width of domains changed. As expected, the domain structure of
Fig. 1. Domain structure in the as-quenched ribbon without mechanical stress (the ribbon length axis is always vertical: the arrows show domain walls).
© Elsevier Sequoia/Printed in The Netherlands
El4
Fig. 2. Domain structures in the same sample (a) immediately after the application of 350 MPa stress and (b) after 1 month under stress.
the sample under tension showed considerable changes initially (a 1 h and 1 day), and then the change became less. After 3 weeks, only three domains were formed in the whole width of the ribbon and this state became stable. This is shown in Fig. 2(b). It was also observed that domains in the sample under stress were about four times wider than domains in the unstressed state at the beginning of tension and more than 10 times wider after 3 weeks. After the sample had been under mechanical stress for a period of 1 month, the stress was removed. Figure 3(a) shows the state of the domain structure at this moment. The wide long domains split into much smaller domains but their direction remained nearly parallel to the ribbon length. After 1 day the character of the domains and the direction changed strongly. Wide, obliquely oriented domains and small closure domains were formed again. The domain structure became similar to the structure in the unstressed sample (Fig. 3(b)). For some days the walls of the wide domains wander in a direction more or less parallel to the ribbon length. The most strange modification in domain structure took place after 1 month of being unstressed; the
Fig. 3. Domain structures m the sample without mechanical stress (a) immediately after release, (b) 1 day after release and (c) 1 month after release.
wide domains again became quasi-parallel to the sample length (Fig. 3(c)). One more feature is worth mentioning: there is a difference between the domain structure of an unstressed sample placed in a tensile holder and the domain structure of a sample which was stuck on the generally used sample holder. The average width of the domains in the stuck sample was about twice those in the sample palced in the tensile holder. Moreover, in the sample placed in the tensile holder, many more closure domains exist. The reason for this difference can be explained by the fixing mode of the sample.
L15
The change in the domain structure took place over a long period of time because the material needed a long time to accommodate to the tensile state at room temperature. The same is characteristic of the sample which was unstressed again. The state of the domain structure in such a sample after a long period of change became nearly the same as at the moment it was released from stress. It seems as if the sample remembered the state of induced easy axis of the magnetoelastic anisotropy but is not able to return to this state completely. That is why in all measurements carried out on such samples this feature should be taken into account.
Acknowledgments The authors are grateful to Professor J. Gyulai for his careful reading of the manuscript and valuable suggestions.
References 1 L. Pog~ny, Z. V6rtesy, Sz. S~indor and G. Konczos, Proc. llth Int. Congr. on Electron Microscopy, Kyoto, 1986, Japan Society of Electron Microscopy, Tokyo, 1986, pp. 1737-1738. 2 J. D. Livingston, Phys. Status SolidiA, 56 (1979) 637. 3 L. Potock~, P. Koll~ir, Z. Jur~inek, E. Kisdi-Kosz6, G. V6rtesy, L. Pog~inyand Z. V6rtesy, Acta Phys. Pol. A, 72 (1987) 801.
L 13
Letter
Mechanical stress and time dependence effect on the domain structure of metallic glasses Z. VI~RTESY and E. KISDI-KOSZO
Central Research Institute for Physics of the Hungarian Academy of Sciences, P.O. Box 49 H-1525 Budapest 114 (Hungao') (Received October 14, 1988)
Abstract Changes in the magnenc domains in stressed and released Fe-Cr-B metallic glass ribbons were observed by scanning electron microscopy and in situ deformation. On release from a stress of 350 MPa the as-quenched state was reached in a short time. However, after longer storage the state resembling that under stress returned (the memory effecO.
The magnetic domain structure in iron-based ribbons is sensitive to mechanical stresses because of the high magnetostriction constant. Therefore, with the intention of obtaining information on the role of stresses in surface magnetic properties of metallic glasses the domain structure of stressed and unstressed ribbons was observed using the backscattered electron imaging mode of a JEOL 840 scanning electron microscope [1]. This method does not show contrast between the domains but a white or black line can be seen where 180 ° domain walls occur, depending on the direction change. Investigations on the domain structure of stressed samples have been carried out by Livingston [2] using a Bitter solution and dark field microscopy. Stressed samples were dealt with in ref. 3 but the general aim was to observe magnetic parameters such as the magnetoelastic anisotropy energy, the coercive force and the static hysteresis loss as functions of tensile stress. The investigated material was Fes0CrsB~5 metallic glass in the form of ribbons 1.5 mm in 0921-5093/89/$3.50
width and 38 /~m thick. The material was prepared by the melt-spinning method. The metallic glass samples were investigated by scanning electron microscopy at an accelerating voltage of 40 kV using a 0.1 ~tA electron beam. The ribbon was placed in a special JEOL-made tensile holder which permits the application of a tensile force to the specimen. Using a given magnification (100x), almost the whole width of the sample can be seen. The specimen placed in a holder without any tension was observed first. The domain structure of this sample was typical of the as-quenched state. Figure 1 shows the domain structure in an unstressed sample. Wide domains obliquely oriented in relation to the sample edge as well as small closure domains are visible. Under a mechanical tension of about 350 MPa applied along the length of the ribbon, an easy axis was induced in the same direction. The domain structure is changed considerably. Small closure domains disappear and wide domains with a direction parallel to the ribbon length occur (Fig. 2(a)). The sample was under stress for 1 month. Observations were made directly after the beginning of tension and then 1 h, 1 day, 1 week and 1 month after the beginning of tension. It was observed that the walls of wide domains moved in a direction perpendicular to the ribbon length, i.e. the width of domains changed. As expected, the domain structure of
Fig. 1. Domain structure in the as-quenched ribbon without mechanical stress (the ribbon length axis is always vertical: the arrows show domain walls).
© Elsevier Sequoia/Printed in The Netherlands
El4
Fig. 2. Domain structures in the same sample (a) immediately after the application of 350 MPa stress and (b) after 1 month under stress.
the sample under tension showed considerable changes initially (a 1 h and 1 day), and then the change became less. After 3 weeks, only three domains were formed in the whole width of the ribbon and this state became stable. This is shown in Fig. 2(b). It was also observed that domains in the sample under stress were about four times wider than domains in the unstressed state at the beginning of tension and more than 10 times wider after 3 weeks. After the sample had been under mechanical stress for a period of 1 month, the stress was removed. Figure 3(a) shows the state of the domain structure at this moment. The wide long domains split into much smaller domains but their direction remained nearly parallel to the ribbon length. After 1 day the character of the domains and the direction changed strongly. Wide, obliquely oriented domains and small closure domains were formed again. The domain structure became similar to the structure in the unstressed sample (Fig. 3(b)). For some days the walls of the wide domains wander in a direction more or less parallel to the ribbon length. The most strange modification in domain structure took place after 1 month of being unstressed; the
Fig. 3. Domain structures m the sample without mechanical stress (a) immediately after release, (b) 1 day after release and (c) 1 month after release.
wide domains again became quasi-parallel to the sample length (Fig. 3(c)). One more feature is worth mentioning: there is a difference between the domain structure of an unstressed sample placed in a tensile holder and the domain structure of a sample which was stuck on the generally used sample holder. The average width of the domains in the stuck sample was about twice those in the sample palced in the tensile holder. Moreover, in the sample placed in the tensile holder, many more closure domains exist. The reason for this difference can be explained by the fixing mode of the sample.
L15
The change in the domain structure took place over a long period of time because the material needed a long time to accommodate to the tensile state at room temperature. The same is characteristic of the sample which was unstressed again. The state of the domain structure in such a sample after a long period of change became nearly the same as at the moment it was released from stress. It seems as if the sample remembered the state of induced easy axis of the magnetoelastic anisotropy but is not able to return to this state completely. That is why in all measurements carried out on such samples this feature should be taken into account.
Acknowledgments The authors are grateful to Professor J. Gyulai for his careful reading of the manuscript and valuable suggestions.
References 1 L. Pog~ny, Z. V6rtesy, Sz. S~indor and G. Konczos, Proc. llth Int. Congr. on Electron Microscopy, Kyoto, 1986, Japan Society of Electron Microscopy, Tokyo, 1986, pp. 1737-1738. 2 J. D. Livingston, Phys. Status SolidiA, 56 (1979) 637. 3 L. Potock~, P. Koll~ir, Z. Jur~inek, E. Kisdi-Kosz6, G. V6rtesy, L. Pog~inyand Z. V6rtesy, Acta Phys. Pol. A, 72 (1987) 801.