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Scanning electron microscope and ferromagnetic resonance investigations of fractured metglas 2826 alloy
Journal of Non-Crystalline Solids 55 (1983) 301-305 North-Holland Publishing Company
301
L E T T E R TO T H E E D I T O R SCANNING E L E C T R O N M I C R O S C O P E AND F E R R O M A G N E T I C R E S O N A N C E I N V E S T I G A T I O N S OF FRACTURED M E T G L A S 2826 ALLOY R.S. PARASHAR, C.S. S U N A N D A N A and Anil K. B H A T N A G A R School of Physics, University of Hyderabad, Hyderabad 500134, India
Received 21 July 1982 Revised manuscript received4 November 1982
Amorphous magnetic alloys are distinguished by their exceptional strength and distinctive deformation mechanisms. Fracture studies of N i - F e metallic glasses have shown the formation of vein patterns in the cross-sectional area and generation of shear bands at the adjacent ribbon surface [1]. It has been suggested [2] that these changes are associated with a change in the chemical potential at the deformed zones. It is, therefore, reasonable to believe that these deformation zones may influence the magnetic and mechanical properties useful in practical applications of these new materials. Indeed, it has been shown that thermal treatment of the deformed region results in accelerated crystallization [3]. Hence, it would be worthwhile to carry out a systematic study of various modes of deformation. So far, investigations seem to have been limited only to optical and electron-microscopy. In magnetic metallic glasses, the Ferromagnetic Resonance (FMR) technique could be used as a sensitive method to study deformation induced effects. Different kinds of fracture treatments produce different types of sample boundaries and associated magnetization changes. These effects as well as surface pits, grain boundaries and disordered scattering would lead to variations in the intrinsic F M R line shape and width [4]. Therefore, a correlation between the direct observation of the surface through a Scanning Electron Microscope (SEM) and associated effects in F M R spectra could give more detailed information. The F M R technique has already been found useful in observing the effect of thermal aging, surface effects, structural and long-range magnetic ordering in metallic glasses [4,5]. In this paper, we have, for the first time, investigated the effect of various fracture treatments of metallic glass ribbons on the F M R spectra and have tried to evolve a method which results in minimum damage. The commercially available amorphous magnetic alloy Fe40Ni40P14B6 (metglas 2826, Allied Chemicals, U.S.A.) ribbon 1.8 mm wide and 0.06 mm thick was used in the present study. Ribbons approximately 5-6 mm in length were cut using different treatments. In the first case, the fractured specimens were obtained by bending on a sharp surgical knife edge through 180 ° (method 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland
302
R.S. Parashar et aL / Fractured rnetglas 2826 alloy
'A'). Other methods included the use of scissors (B) and rotating diamond wheel (C). The speed of the diamond wheel was carefully adjusted to ensure minimal damage. All the specimens were cut perpendicular to the long ribbon axis. Surface damage near the fractured specimens was examined through a Scanning Electron Microscope JEOL model JSM 25. For any individual specimen, the nature of deformation was found to be similar on either side of the ribbon surface. For F M R investigations (JEOL ESR Spectrometer model FE-3X) specimens were mounted on a Teflon sample holder with a flat tip so that the magnetic field could easily be arranged parallel to the plane of the sample. The long axis of the specimen was always perpendicular to the horizontal magnetic field (parallel-vertical configuration). While recording F M R spectra the input microwave power level was kept low enough (! mW) to avoid nonlinear power effects and line shape distortions. For comparison, the specimens were studied after electropolishing using an 8 : 1 : 1 mixture of acetic acid, perchloric acid and methanol. Deformation at temperatures lower than the glass transition temperature occurs in the form of highly inhomogeneous shear bands. Such bands are readily observed on the surface upon failure by tearing or bending [1]. At the yield stress failure occurs instantaneously by shear rupture through an intense shear band. Propagation of the shear band on the excess volume front gives rise to the observed pattern depending upon the method used. Recent measurements on Fez9Ni49P14B6Si2 amorphous alloy ribbon (2826B) show that bending of the ribbon results in the initiation of cracks along with the shear bands. Further bending results in rapid growth and propagation of the crack front followed by an instant failure [6]. Figs. l(a)-(c) show the SEM patterns of Fe40Ni40Pl4B6 specimens fractured using different methods. It is clear that the specimen fractured by bending receives the maximum damage while that fractured using the diamond wheel is least affected. In type 'A' specimens, fracturing results in the formation of an intense crack front at the edge which dies out exponentially with increasing distance. (Besides the crack pattern some shear lines parallel to the ribbon axis are apparent which we believe are formed during the process of fracturing and may sometimes result in bending of the ribbon along its long axis.) The widths of the crack lines are found to vary between 0.8/~m and 4/~m. For scissor-cut specimens the propagation of crack front is somewhat limited and width of the lines varies in the range 0.8 /~m to 2/~m. Table 1 shows various parameters of all the specimens investigated. It is apparent that the fractured boundaries of an individual specimen differ appreciably from others and, therefore, the corresponding F M R spectra are likely to be affected by the magnetization variation at the boundaries. Indeed, the F M R linewidth seems strongly related to the damage produced near the fractured edge. Figs. 2(a), (b) represent the F M R spectra of typical type 'A' and 'B' specimens. The difference in the line shape of the spectra could easily be attributed to the degree of deformation produced near the fractured edge. Baianu et al. [5] have estimated the effect of spin-wave scattering due to surface effects. They observed that surface pits 1-5 /~m in
R.S. Parashar et al. / Fractured metglas 2826 alloy
303
Fig. 1. SEM patterns of the fractured metglas specimens using methods A, B and C. Marker bar = 20/~m.
diameter form the major contribution to the linewidth. It therefore appears that the formation and propagation of the crack front is responsible for the observed linewidth (table 1). Removal of these deformation patterns would result in a smooth surface which should in turn reduce the linewidth. It was achieved by electrolytic polishing of the specimen. Fig. 2(c) shows the FMR spectra of a typical electropolished, scissor-cut specimen. Comparison of the F M R spectra shows that there is a definite change in the line shape and width after the specimen is electropolished. The observed line shape is smoother and the peak to peak width is narrowed down. Thus, present results seem to be in qualitative agreement with those reported earlier [5]. Further, the linewidth observed in case of the type 'C' specimen and associated changes before and after electropolishing are minimum indicating that the diamond wheel cutting induces least deformation into the specimen and hence could be considered a suitable technique.
304
R.S. Parashar et al. / Fractured metglas 2826 alloy
1300 Oe
c
Fig. 2. FMR spectra of the 'metglas' specimens: (a) cut by bending; (b) cut using scissors; (c) scissor-cut electropolished.
Table 1 Typical FMR and SEM data at 300 K S1. No.
Method of cutting
No. of FMR peaks
FMR linewidth, H (Oe)
Before After polishing
Before After polishing
Length of damage produced (SEM) (/~m)
A
Bending
4
9
245
173
80
B C
Scissors Diamond wheel
3 3
2 2
205 185
163 170
45 8
R.S. Parashar et al. / Fractured metglas 2826 alloy
305
References [1] L.A. Davis, 1978 Metallic glasses (ASM, Metals Park, Ohio, 1978) pp. 190-223. [2] C.A. Pampillo and H.S. Chen, Mat. Sci. Eng. 13 (1974) 181; H.S. Chen, H.J. Leamy and M.J. O'Brien, Scr. Metall. 7 (1973) 115. [3] U. K/3ster and U. Herald, in: Glassy metals 1, ed. H.J. Guntherodt and H. Beck (Springer Verlag, 1981) p. 255; R.S. Parashar and A.K. Bhatnagar, to be published. [4] I.C. Baianu, K.A. Rubinson and J. Patterson, Phys. Stat. So !. (a) 53 (1979) K133; J. Phys. Chem. Sol. 40 (1979) 941. [5] I.C. Baianu, J. Patterson and K.A. Rubinson, Mat. Sci. Eng. 40 (1979) 273. [6] R.S. Parashar and A.K. Bhatnagar, to be published.
301
L E T T E R TO T H E E D I T O R SCANNING E L E C T R O N M I C R O S C O P E AND F E R R O M A G N E T I C R E S O N A N C E I N V E S T I G A T I O N S OF FRACTURED M E T G L A S 2826 ALLOY R.S. PARASHAR, C.S. S U N A N D A N A and Anil K. B H A T N A G A R School of Physics, University of Hyderabad, Hyderabad 500134, India
Received 21 July 1982 Revised manuscript received4 November 1982
Amorphous magnetic alloys are distinguished by their exceptional strength and distinctive deformation mechanisms. Fracture studies of N i - F e metallic glasses have shown the formation of vein patterns in the cross-sectional area and generation of shear bands at the adjacent ribbon surface [1]. It has been suggested [2] that these changes are associated with a change in the chemical potential at the deformed zones. It is, therefore, reasonable to believe that these deformation zones may influence the magnetic and mechanical properties useful in practical applications of these new materials. Indeed, it has been shown that thermal treatment of the deformed region results in accelerated crystallization [3]. Hence, it would be worthwhile to carry out a systematic study of various modes of deformation. So far, investigations seem to have been limited only to optical and electron-microscopy. In magnetic metallic glasses, the Ferromagnetic Resonance (FMR) technique could be used as a sensitive method to study deformation induced effects. Different kinds of fracture treatments produce different types of sample boundaries and associated magnetization changes. These effects as well as surface pits, grain boundaries and disordered scattering would lead to variations in the intrinsic F M R line shape and width [4]. Therefore, a correlation between the direct observation of the surface through a Scanning Electron Microscope (SEM) and associated effects in F M R spectra could give more detailed information. The F M R technique has already been found useful in observing the effect of thermal aging, surface effects, structural and long-range magnetic ordering in metallic glasses [4,5]. In this paper, we have, for the first time, investigated the effect of various fracture treatments of metallic glass ribbons on the F M R spectra and have tried to evolve a method which results in minimum damage. The commercially available amorphous magnetic alloy Fe40Ni40P14B6 (metglas 2826, Allied Chemicals, U.S.A.) ribbon 1.8 mm wide and 0.06 mm thick was used in the present study. Ribbons approximately 5-6 mm in length were cut using different treatments. In the first case, the fractured specimens were obtained by bending on a sharp surgical knife edge through 180 ° (method 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland
302
R.S. Parashar et aL / Fractured rnetglas 2826 alloy
'A'). Other methods included the use of scissors (B) and rotating diamond wheel (C). The speed of the diamond wheel was carefully adjusted to ensure minimal damage. All the specimens were cut perpendicular to the long ribbon axis. Surface damage near the fractured specimens was examined through a Scanning Electron Microscope JEOL model JSM 25. For any individual specimen, the nature of deformation was found to be similar on either side of the ribbon surface. For F M R investigations (JEOL ESR Spectrometer model FE-3X) specimens were mounted on a Teflon sample holder with a flat tip so that the magnetic field could easily be arranged parallel to the plane of the sample. The long axis of the specimen was always perpendicular to the horizontal magnetic field (parallel-vertical configuration). While recording F M R spectra the input microwave power level was kept low enough (! mW) to avoid nonlinear power effects and line shape distortions. For comparison, the specimens were studied after electropolishing using an 8 : 1 : 1 mixture of acetic acid, perchloric acid and methanol. Deformation at temperatures lower than the glass transition temperature occurs in the form of highly inhomogeneous shear bands. Such bands are readily observed on the surface upon failure by tearing or bending [1]. At the yield stress failure occurs instantaneously by shear rupture through an intense shear band. Propagation of the shear band on the excess volume front gives rise to the observed pattern depending upon the method used. Recent measurements on Fez9Ni49P14B6Si2 amorphous alloy ribbon (2826B) show that bending of the ribbon results in the initiation of cracks along with the shear bands. Further bending results in rapid growth and propagation of the crack front followed by an instant failure [6]. Figs. l(a)-(c) show the SEM patterns of Fe40Ni40Pl4B6 specimens fractured using different methods. It is clear that the specimen fractured by bending receives the maximum damage while that fractured using the diamond wheel is least affected. In type 'A' specimens, fracturing results in the formation of an intense crack front at the edge which dies out exponentially with increasing distance. (Besides the crack pattern some shear lines parallel to the ribbon axis are apparent which we believe are formed during the process of fracturing and may sometimes result in bending of the ribbon along its long axis.) The widths of the crack lines are found to vary between 0.8/~m and 4/~m. For scissor-cut specimens the propagation of crack front is somewhat limited and width of the lines varies in the range 0.8 /~m to 2/~m. Table 1 shows various parameters of all the specimens investigated. It is apparent that the fractured boundaries of an individual specimen differ appreciably from others and, therefore, the corresponding F M R spectra are likely to be affected by the magnetization variation at the boundaries. Indeed, the F M R linewidth seems strongly related to the damage produced near the fractured edge. Figs. 2(a), (b) represent the F M R spectra of typical type 'A' and 'B' specimens. The difference in the line shape of the spectra could easily be attributed to the degree of deformation produced near the fractured edge. Baianu et al. [5] have estimated the effect of spin-wave scattering due to surface effects. They observed that surface pits 1-5 /~m in
R.S. Parashar et al. / Fractured metglas 2826 alloy
303
Fig. 1. SEM patterns of the fractured metglas specimens using methods A, B and C. Marker bar = 20/~m.
diameter form the major contribution to the linewidth. It therefore appears that the formation and propagation of the crack front is responsible for the observed linewidth (table 1). Removal of these deformation patterns would result in a smooth surface which should in turn reduce the linewidth. It was achieved by electrolytic polishing of the specimen. Fig. 2(c) shows the FMR spectra of a typical electropolished, scissor-cut specimen. Comparison of the F M R spectra shows that there is a definite change in the line shape and width after the specimen is electropolished. The observed line shape is smoother and the peak to peak width is narrowed down. Thus, present results seem to be in qualitative agreement with those reported earlier [5]. Further, the linewidth observed in case of the type 'C' specimen and associated changes before and after electropolishing are minimum indicating that the diamond wheel cutting induces least deformation into the specimen and hence could be considered a suitable technique.
304
R.S. Parashar et al. / Fractured metglas 2826 alloy
1300 Oe
c
Fig. 2. FMR spectra of the 'metglas' specimens: (a) cut by bending; (b) cut using scissors; (c) scissor-cut electropolished.
Table 1 Typical FMR and SEM data at 300 K S1. No.
Method of cutting
No. of FMR peaks
FMR linewidth, H (Oe)
Before After polishing
Before After polishing
Length of damage produced (SEM) (/~m)
A
Bending
4
9
245
173
80
B C
Scissors Diamond wheel
3 3
2 2
205 185
163 170
45 8
R.S. Parashar et al. / Fractured metglas 2826 alloy
305
References [1] L.A. Davis, 1978 Metallic glasses (ASM, Metals Park, Ohio, 1978) pp. 190-223. [2] C.A. Pampillo and H.S. Chen, Mat. Sci. Eng. 13 (1974) 181; H.S. Chen, H.J. Leamy and M.J. O'Brien, Scr. Metall. 7 (1973) 115. [3] U. K/3ster and U. Herald, in: Glassy metals 1, ed. H.J. Guntherodt and H. Beck (Springer Verlag, 1981) p. 255; R.S. Parashar and A.K. Bhatnagar, to be published. [4] I.C. Baianu, K.A. Rubinson and J. Patterson, Phys. Stat. So !. (a) 53 (1979) K133; J. Phys. Chem. Sol. 40 (1979) 941. [5] I.C. Baianu, J. Patterson and K.A. Rubinson, Mat. Sci. Eng. 40 (1979) 273. [6] R.S. Parashar and A.K. Bhatnagar, to be published.