2 GeV Electron Beam Irradiation Effects in Advanced Metallic Glasses

Six amorphous alloys (Alloy 1: Fe 56 Co 24 Nb 4 B 13 Si 2 Cu 1 , Alloy 2: Fe 68.5 Co 5 Nb 3 Cu 1 Si 15.5 B 7 , Alloy 3: Fe 75.3 Ni 0.8 Cr 0.9 Si 8.7 B 14.3 , Alloy 4: Fe 56 Co 24 Cr 10 Nb 4 B 3 Si 1 Cu 2 , Alloy 5: Fe 72.9 Nb 3 Cu 1 Si 16.2 B 6.9 , Alloy 6: Fe 83.3 Si 8.6 Nb 5.5 B 1.4 Cu 1.2 ) were selected in terms of their composition and magnetostriction constants and uniformly irradiated in a high radiation environment in Hall A of the Thomas Jefferson National Accelerator Facility. The 2 GeV electron beam irradiation-induced effects were characterized by M ӧ ssbauer spectroscopy. The microstructural changes were related to the evolution of the hyperfine magnetic field distributions and isomer shifts. In particular, the occurrence of stress centers in the amorphous materials was evidenced.


Introduction
To date, irradiation-induced effects have been considered in order to understand the relationship between magnetic behavior and select variation in the structural characteristics of materials [1]- [10]. Mostly, crystalline materials were subject to irradiation with electron beams [11]- [20] and discussed in terms of phenomenological models in which the underlying microstructure played an essential role.
In particular, electron-beam irradiation of amorphous alloys attracts nowadays considerable interest due to the emphasis on the structure-property relationships relevant to rational construction of new magnetic materials. However, this irradiation study [21] was limited to 7 MeV electrons and it makes sense to explore the changes induced by GeV electrons. In these current investigations we study the irradiation-driven modifications induced by 2 GeV electrons in several advanced metallic glasses. 2 ) were obtained from Spang Co. in ribbon form (compositions following the Finemet generation). The foils were exposed to the prompt high radiation environment created by a 2 GeV electron beam incident on a foil of Pb as part of the PREX-II experiment [22]. The foils all received a uniform dose of 0.1 Mrad of ionizing radiation as determined using special dosimeters located with the foils.

Materials and Methods
Room temperature Mӧssbauer spectroscopy measurements were performed with a SeeCo constant acceleration spectrometer using a 57 Co gamma ray source diffused in an Rh matrix. The spectra were analyzed with the WINormos-DIST package of programs, which was able to detail the hyperfine magnetic field distributions extracted from the spectra.  The formation of stress centers in the irradiated materials can tentatively be related to magnetostriction (alloy 1 has λ s = 20 ppm), but it should be observed that the defects are also formed in alloy 2, which is a zero-magnetostriction composition.   3 and 4, respectively, which is consistent with changes in the electronic charge distribution at the nuclei, caused by the modifications in the chemical short-range order induced by electron beam irradiation. Figure 3 displays the room-temperature Mӧssbauer spectra of alloys 5 and 6 after 2 GeV electron irradiation. The hyperfine magnetic field distributions had the average values of <BHF> = 20.55 and 21.08 T for alloys 5 and 6, respectively, with an increased width due to a small low-field component of the distribution. This is consistent with sizeable migration of atoms in the amorphous matrix, but it should be noted that bulk crystallization of the specimens did not occur.

Results
The magnetic texture parameter takes values R 21 = 0.72 and 0.67, with average canting angles of 44.5˚ and 41.9˚, which are close to a random orientation of magnetic moment directions. Alloy 5 has a negative magnetostriction constant, fact which demonstrates that stress centers are also formed in samples of negative λ s due to magnetostriction. The average isomer shift had the values <ISO> = 0.0035 and 0.006 mm/s for alloys 5 and 6, which is consistent with the occurrence of variations in the chemical short-range order in the irradiated specimens.

Discussion and Conclusions
The present study sheds light on the fundamental effects underlying the interaction of high-energy electron beams with advanced metallic glasses. The formation of stress centers around defects induced by irradiation was demonstrated. Since the specimens remain in the amorphous phase, the development of radiation-resistant materials is encouraged.
Due to the thickness of the ribbon foils, the use of transmission electron microscopy is not possible. Scanning electron microscopy does not provide helpful information either, since irradiation is not a surface effect. Unlike low energy electrons, which leave their energy in the sample and induce bulk crystallization [21] [23], high energy electrons just pass through the sample and leave defects behind. Therefore, Mössbauer spectroscopy proves to be a powerful, high-resolution technique when used to investigate minute changes in the structure and properties of magnetic materials.