Research group

Solidification Processes and Complex Structures


Contact person

Assoc. Prof. Dr. Mihai Stoica

Phone: +49 351 4659 644
Email:  m.stoica(at)ifw-dresden.de


Team

Projects

Publications

Devices and techniques

Deformable soft-magnetic bulk glassy alloys


FeCoSiBNbCu bulk metallic glass with large compressive deformability

Ferromagnetic Fe- and (Fe,Co)-based amorphous alloys are regarded as attractive industrial alloys due to relative low price and simple routes for fabrication. Within several new bulk metallic glasses (BMGs) developed in the last decade, (Fe-Co)-Si-B-Nb glassy alloys play an important role because they combine a high glass-forming ability (GFA) with good soft-magnetic properties and very high compressive strength. A major drawback, which hinders the application potential, is the absence of any plastic deformation. Here we considered to add 0.5 at. % Cu to the [(Fe0.5Co0.5)0.75Si0.05B0.20]96Nb4 base alloy. We found that the Cu-added BMG can undergo distinct plastic deformation (Fig. 1). We have not observed any sign of nanocrystallization during the mechanical loading. However, upon facture the samples shatter apart into many small fragments (Fig. 2 (a) and (b), as it is typical for very brittle materials. Interesting is that one can find signs of plastic deformation in some areas — or at least some indications of viscous flow behavior (Fig. 2 (c) and (d)). Therefore, the overall plastic deformation and the fracture mechanism seem to be governed by the interaction between plastic and brittle regions. Taking into account the very high stress level of 4000 MPa, this suggests that most probably the flowed layers have been melted, as it is commonly observed in the case of deformable BMGs, but in the actual case only in very localized areas. Assuming that the shear band operates as a zero-thickness planar heat source of constant flux, we estimated the temperature rise ΔT as a function of the distance d from the center of shear band formation and propagation, for different propagation speeds. It was found that, when the shear propagation speed approaches the speed of sound, a thin layer of 120 nm may be melted (Fig. 3). Therefore the overall behavior and the macroscopic plastic strain depend on the interaction between cleavage-like and viscous flow-like interactions, and, most probably, they are governed by the sample geometry.

M. Stoica, S. Scudino, J. Bednarčik, I. Kaban, J. Eckert: “FeCoSiBNbCu bulk metallic glass with large compressive deformability studied by time-resolved synchrotron X-ray diffraction”, Journal of Applied Physics 115 (2014) 053520 URL 

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Thermal stability, structure evolution and soft magnetic properties of FeCoSiBNbCu bulk metallic glass

Fully amorphous rods with diameters up to 2 mm diameter were obtained upon 0.5 at.% Cu addition to the Fe36Co36B19.2Si4.8Nb4 (i.e. named BA100) bulk metallic glass. The Cu-added glass shows a very good thermal stability but, in comparison with the Cu-free base alloy, the entire crystallization behavior is drastically changed (Fig. 4). Upon heating, the glassy (Fe36Co36B19.2Si4.8Nb4)99.5Cu0.5 (i.e. named BA99.5Cu0.5) samples show two glass transitions-like events, separated by an interval of more than 100 K, in between which a bcc-(Fe,Co) solid solution is formed (Fig. 5 and Fig. 6). The soft magnetic properties are preserved upon Cu-addition and the samples show a saturation magnetization of 1.1 T combined with less than 2 A/m coercivity. The variation of the saturation magnetization with temperature (Fig. 7) is in perfect agreement with the crystallization behavior as observed by calorimetric studies.

M. Stoica, P. Ramasamy, I. Kaban, S. Scudino, M. Nicoara, G. B.M. Vaughan, J. Wright, R. Kumar, J. Eckert: “Structure evolution of soft magnetic (Fe36Co36B19.2Si4.8Nb4)100-xCux (x = 0 and 0.5) bulk glassy alloys”, Acta Materialia 95 (2015) 335-342 URL

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Viscosity and fragility of soft ferromagnetic bulk metallic glasses

The dynamic viscosity of four Fe-based bulk metallic glass-forming alloys, [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4, (alloy A), {[(Fe0.5Co0.5)0.75B0.2Si0.05]0.96Nb0.04}99.5Cu0.5 (alloy B), Fe74Mo4P10C7.5B2.5Si2 (alloy C) and (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 (alloy D), was investigated as a function of temperature in the supercooled liquid region (Fig. 8), as well as above the melting point (Fig. 9). The alloy B is Cu-added alloy A, while the alloy D was obtained upon fine-tuning the alloy C composition. All these alloys may form bulk metallic glasses upon copper mold casting. The viscosities in the supercooled liquid region were calculated using the data obtained upon parallel plate rheometry measurements, as well as upon differential scanning calorimetry (DSC). The values of the supercooled fragility parameter m, 61, 66, 52 and 60 for the alloys A, B, C and D respectively, indicate that these alloys are intermediate glass formers. The behavior of the same alloys, in the molten state, was studied using a high temperature torsional oscillation cup viscometer. The values of the corresponding fragility parameter M was calculated as 5.03, 5.91, 4.25, 4.93 for the alloys A, B, C and D, respectively. They confirm the supercooled liquid behavior and predict that the alloys A and C may form glasses easier than the fine-tuned compositions B and D. Angell plot is constructed for the entire range of viscosities (Fig. 10) and the values from both regions, i.e. above melting point and supercooled liquid region, fit well with the model.

R. Parthiban, M. Stoica, I Kaban, Ravi Kumar,J. Eckert: “Viscosity and fragility of the supercooled liquids and melts from the Fe-Co-B-Si-Nb and Fe-Mo-P-C-B-Si glass-forming alloy systems”, Intermetallics 66 (2015) 48-55 URL

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FeNiMoPCB with 8.5% compressive plastic strain

In the last decade, the improvement of the room temperature plasticity of BMGs has become a hot topic in the development of advanced structural materials. The synthesis of ductile Fe-based BMGs with enhanced glass-forming ability (GFA) has also been pursued in recent years, which is important for the further extension of the application fields as structural and functional materials. One of the goals of our group is to synthesize deformable Fe-based BMGs, combining the best casting methods with compositional fine-tuning, proper thermal annealing treatments. Very recently we synthesized a new Fe-based with superior GFA, produced by addition of a certain amount of Ni to an FeMoPCB composite alloy. The monolithic 1 mm diameter (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 glassy alloy samples exhibit the highest ever reported compressive deformability (8.5%), together with a yield strength of 2940 MPa and an ultimate strength of 3260 MPa (Fig. 11). However, the rods with larger diameter show as well enhanced compressive plastic deformation. Multiple shear bands were observed on the surface of the deformed specimens. The appropriate addition of Ni shifts the composition closer to the eutectic, lowers the liquidus temperature, and significantly enhances the GFA and the plastic strain of the alloys. Upon compression, the samples showed a ductile–brittle type fracture behavior. In Fig. 12 it is shown the mixture between ductile and brittle regions (a), and a detail of the vein patterns at higher magnification (b). The arrows indicate the direction of the applied load.

A. Seifoddini, M. Stoica, M. Nili-Ahmadabadi, S. Heshmati-Manesh, U. Kühn, J. Eckert: “New (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 glassy alloys with enhanced glass-forming ability and large compressive strain”, Materials Science & Engineering A 560 (2013) 575–582 URL

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