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Consolidation of Fe-Based Metallic Glass Powders by Hot Pressing

Abstract

An alternative route to obtain bulk metallic glasses is by consolidation of metallic glass powders by deforming these materials in the temperature interval between the crystallization temperature and the glass transition temperature, where the material flows with a reduced viscosity. In the present work, bulk parts of the Fe43.2Co28.8B19.2Si4.8Nb4 alloy were produced by hot-pressing gas-atomized powders (GAP) under different uniaxial pressures. Different microstructural analysis revealed that the initial powder as well as the consolidated parts were mostly amorphous, with similar transformation temperatures, showing that bulk samples of this alloy can be produced by conformation in the supercooled liquid region. The sample conformed under the highest pressure (1GPa) exhibited the highest relative density of 96.1±0.5%. These results show that hot pressing of Fe-based gas-atomized powders is a promising route for producing Fe-based bulk metallic glasses.

Keywords:
Iron-based metallic glass alloys; atomization; hot pressing


1. Introduction

Metallic glasses are a relatively new class of materials characterized by the absence of long-range ordering 11 Greer AL. Metallic glasses...on the threshold. Materials Today. 2009;12(1-2):14-22.,22 Suryanarayana C, Inoue A. Bulk Metallic Glass. 1st ed. Boca Raton: CRC Press; 2010. 565 p.. Due to their unique atomic structure, many unusual properties are observed in these alloys, such as high mechanical strength33 Ashby MF, Greer AL. Metallic glasses as structural materials. Scripta Materialia. 2006;54(3):321-326., soft magnetic properties44 Hasegawa R. Applications of amorphous magnetic alloys in electronic devices. Journal of Non-Crystalline Solids. 2001;287(1-3):405-412. and good wear and corrosion resistance 55 Ishida M, Takeda H, Nishiyama N, Kita K, Shimizu Y, Saotome Y, et al. Wear resistivity of super-precision microgear made of Ni-based metallic glass. Materials Science and Engineering: A. 2007;448-451:149-154., among others.

Iron-based metallic glasses often have complex compositions and usually exhibit poor glass forming ability (GFA) compared to other non-ferrous metallic glasses 66 Suryanarayana C, Inoue A. Iron-based bulk metallic glasses. International Materials Reviews. 2013;58(3):131-166.. However, these alloys have attracted considerable attention due to their low cost, high availability, good soft magnetic properties and high mechanical strengths 77 Inoue A, Kong FL, Man QK, Shen BL, Li RW, Al-Marzouki F. Development and applications of Fe- and Co-based bulk glassy alloys and their prospects. Journal of Alloys and Compounds. 2015;615(Supp 1):S2-S8.,88 Stoica M. Fe-Based Bulk Metallic Glasses - Understanding the Influence of Impurities on Glass Formation. Wiesbaden: Springer Fachmedien Wiesbaden; 2017. 126 p.. Among the iron-based alloys, the composition Fe43.2Co28.8B19.2Si4.8Nb4 shows a relatively high GFA, being able to produce amorphous samples of up to 4 mm by copper mold casting 99 Inoue A, Shen BL, Chang CT. Super-high strength of over 4000 MPa for Fe-based bulk glassy alloys in [(Fe1-xCox)0.75B0.2Si0.05]96Nb4 system. Acta Materialia. 2004;52(14):4093-4099.. Furthermore, it has a compressive yield strength above 4000 MPa, making it a promising candidate for applications in micro-engineering components.

Metallic glasses show a Newtonian flow when they are deformed at low strain rates within the supercooled liquid region, i.e., between the crystallization temperature, Tx, and the glass transition temperature, Tg 1010 Kawamura Y, Nakamura T, Kato H, Mano H, Inoue A. Newtonian and non-Newtonian viscosity of supercooled liquid in metallic glasses. Materials Science and Engineering: A. 2001;304-306:674-678.

11 Yavari AR, de Oliveira MF, Kiminami CS, Inoue A, Botta Filho WJ. Electromechanical shaping, assembly and engraving of bulk metallic glasses. Materials Science and Engineering: A. 2004;375-377:227-234.

12 Schuh CA, Hufnagel TC, Ramamurty U. Mechanical behavior of amorphous alloys. Acta Materialia. 2007;55(12):4067-4109.
-1313 Kawamura Y, Shibata T, Inoue A, Masumoto T. Deformation behavior of Zr65Al10Ni10Cu15 glassy alloy with wide supercooled liquid region. Applied Physics Letters. 1996;69(9):1208-1210.. They exhibit a significant decrease in their viscosity in this temperature interval during heating and they are able to get wet and consequently adhere another metallic surface during this configurational rearrangement process. Some works have already used these properties to braze 1414 Suh JY, Lohwongwatana B, Garland CM, Conner RD, Johnson WL, Suh D. Novel thermoplastic bonding using a bulk metallic glass solder. Scripta Materialia. 2008;59(8):905-908., weld 1515 de Oliveira M, Botta Filho WJ, Yavari AR. Connecting, Assemblage and Electromechanical Shaping of Bulk Metallic Glasses. Materials Transactions, JIM. 2000;41(11):1501-1504. and consolidate metallic glasses 1616 Sordelet DJ, Rozhkova E, Huang P, Wheelock PB, Besser MF, Kramer MJ, et al. Synthesis of Cu47Ti34Zr11Ni8 Bulk Metallic Glass by Warm Extrusion of Gas Atomized Powders. Journal of Materials Research. 2002;17(1):186-198..

Only a very few works used these supercooled liquid properties to consolidate Fe-based metallic glasses 1717 Ishihara S, Zhang W, Inoue A. Hot pressing of Fe-Co-Nd-Dy-B glassy powders in supercooled liquid state and hard magnetic properties of the consolidated alloys. Scripta Materialia. 2002;47(4):231-235.

18 Kang EY, Kim YB, Kim KY, Chung YH, Baik HK. Vacuum hot pressing of Fe-Si-B-Nb-based amorphous powder cores and their high-frequency magnetic properties. Journal of Applied Physics. 2006;99(8):08F111.
-1919 Yodoshi N, Yamada R, Kawasaki A, Watanabe R. Consolidation of [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 Metallic Glassy Powder by SPS Method. Materials Transactions. 2009;50(9):2264-2269..

In this work, bulk parts of the Fe43.2Co28.8B19.2Si4.8Nb4 alloy were produced by hot-pressing gas-atomized powders (GAP) at a temperature within their supercooled liquid interval.

2. Experimental Procedure

An Fe43.2Co28.8B19.2Si4.8Nb4 ingot was prepared by vacuum induction melting under an inert argon atmosphere using high purity elements, namely, iron (99.98 wt.%), cobalt (99.9+ wt.%), boron (99,5 wt.%), silicon (99,9995 wt.%) and niobium (99.8 wt.%). The ingot chemical analysis was performed by inductively coupled plasma optical emission spectrometry (ICP-OES), using a spectrometer Varian model Vista. The ingot was remelted in an induction furnace and the molten alloy was poured into a tundish with a quartz nozzle with a bore of 4 mm diameter and atomized using nitrogen (N2) with a 1.0 MPa pressure. The microstructure of the GAP was investigated by Optical Microscopy (OM) after chemical etching (1,5g de picric acid + 100mL H2O + 20 mL neutral detergent) at 70 °C and their morphology were characterized by scanning electron microscopy (SEM) using a FEI Inspect S 50 operating under 25 kV. The powders with particle size in the 32-75 µm range were encapsulated in copper cans with an outer diameter of 14 mm and inner diameter of 12.7 mm. Subsequently, they were consolidated by uniaxial hot pressing at 565 °C during 300s and at 325, 700 and 1000 MPa, using an EMIC model DL-60.000 machine. These samples were named C1, C2 and C3, respectively. The microstructures of the hot pressed samples were also characterized by SEM. The amorphous state of the powders and consolidated samples were verified by X-Ray Diffraction in a Bruker D8 ADVANCE diffractometer operating with Cu-Kα radiation. The Tg and Tx temperatures were determined by differential scanning calorimetry (DSC) using a Netzsch 404 instrument at a heating rate of 40 °C/min. The density of the powders was determined using a helium pycnometer Micromeritics AccuPyc 1330 equipment and density of the consolidated samples was measured by Archimedes’ method.

3. Results and Discussion

The results of the ingot chemical analysis show that a very close composition to the target one was achieved, namely Fe42.4Co30.3B18.0Si5.2Nb4.0 (nominal composition = Fe43.2Co28.8B19.2Si4.8Nb4.0).

The gas-atomized powders with particle size in the range 32-75 µm can be seen in Figure 1 (a). Most of particles had a spherical shape and a clean surface, typical of amorphous powders 2020 German RM. Powder Metallurgy & Particulate Materials Processing. Princeton: Metal Powder Industry Federation; 2005. 528 p.. Some irregularly shaped particles can be also observed in Figure 1 (a). These particles most likely hit the atomization chamber without being completely solidified, and were deformed by the impact. Crystalline regions were detected by optical microscopy after chemical etching, as indicated by red arrows in Figure 1 (b).

Figure 1
Micrographs of the Fe43.2Co28.8B19.2Si4.8Nb4 gas atomized powders in the 32-75 µm range. (a) SEM - Secondary Electron (SE) image showing the morphology of the particles. (b) Mounted and etched OM micrograph showing that some particles are crystalline. The red arrows indicate the crystalline regions.

The yield strength of Fe43.2Co28.8B19.2Si4.8Nb4 alloy is reported to be 4100 MPa at room temperature 99 Inoue A, Shen BL, Chang CT. Super-high strength of over 4000 MPa for Fe-based bulk glassy alloys in [(Fe1-xCox)0.75B0.2Si0.05]96Nb4 system. Acta Materialia. 2004;52(14):4093-4099.. Assuming that alloy shows a Newtonian flow within its supercooled interval and its viscosity (η) is in the range 1088 Stoica M. Fe-Based Bulk Metallic Glasses - Understanding the Influence of Impurities on Glass Formation. Wiesbaden: Springer Fachmedien Wiesbaden; 2017. 126 p.-109 Pa.s, similar to Fe36.0Co36.0B19.2Si4.8Nb4 glassy alloy 2121 Parthiban R, Stoica M, Kaban I, Kumar R, Eckert J. 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. 2015;66:48-55., the flow stress in the supercooled liquid region under a uniaxial compression test and constant temperature can be estimated by equation 1 2222 Trouton FT. On the Coefficient of Viscous Traction and Its Relation to that of Viscosity. Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences. 1906;77(519):426-440.:

(1) σ = 3 η ε ˙

where ε˙ is the uniaxial strain rate. Considering a typical value of 0.1 s-1 for ε˙ , the uniaxial flow stress within the supercooled interval would be in the 30-300 MPa range. This low value suggests that consolidation of glassy powders in the supercooled liquid region is a promising route to produce bulk parts of Fe43.2Co28.8B19.2Si4.8Nb4 alloy.

Figure 2 shows the DSC curves of the GAP and the hot-pressed samples. The transformation temperatures Tg, Tx and ∆Tx (= Tx - Tg), are summarized in Table 1 together with the data reported by Inoue et al. 99 Inoue A, Shen BL, Chang CT. Super-high strength of over 4000 MPa for Fe-based bulk glassy alloys in [(Fe1-xCox)0.75B0.2Si0.05]96Nb4 system. Acta Materialia. 2004;52(14):4093-4099.. The powders glass transition temperature is 550±2 °C. After a supercooled liquid of about 40±3 °C, an exothermic event can be observed, characteristic of crystallization. Tg is similar to the reported data in the literature, but Tx is about 8 °C lower. Although the GAP and casted cylinders of the literature were probably formed under different cooling rates, Tg is often determined experimentally during heating. When heated, the glassy alloy is structurally relaxed and therefore Tg is expected to be similar when the same heating rate is applied. The decrease of Tx reinforce that the GAP presents some degree of crystallinity, as crystalline phases serve as nucleation points, reducing ΔTx.

Figure 2
DSC curves of the GAP and the consolidated samples.

Table 1
The transformation temperatures Tg, Tx and ΔTx (= Tx – Tg), measured density and mean hardness value of the GAP and consolidated samples.

All the consolidation experiments were carried out at 565 °C (which corresponds to Tg plus 15 °C). C1, the sample consolidated with the smallest pressure, i.e. 325MPa, disintegrated upon cutting. Figure 3 shows a micrograph of the C1 sample that remained bonded. As it can be noticed, the reason for the weak bonding among the powders can be explained by their low deformation, resulting in a low contact area between the particles.

Figure 3
SEM-SE micrograph showing the surface of the C1 sample.

By almost doubling the applied pressure in the C2 sample, the particles were highly deformed, which enabled the sample to be cut, gridded and polished. Figure 4 (a) shows the cross section of C2. The increase in pressure lead to a better bonding between the particles, i.e. a much larger contact area can be observed between the powder particles. However, it is clear that the powders were still not completely packed. The density of the atomized powders measured by helium pycnometer was 7.64± 0.02 g/cm³. This value was used to calculate the relative density (RD) of the samples. A RD of 90.6±0.5 % was obtained for C2.

Figure 4
SEM - Backscattered Electron (BSE) micrographs of the cross section of the consolidated samples (a) C2 and (b) C3.

The highest compression load used in this work was 1 GPa, for the C3 sample, in which a relative density of 96.1±0.5 % was obtained. In contrast to C2, only small pores still remain on the microstructure of C3, as can be seen in Figure 4 (b).

Differently from what was initially expected, a high pressure is necessary to produce consolidated glassy parts with a high RD. Considering that glassy powders do not show work hardening behavior 1212 Schuh CA, Hufnagel TC, Ramamurty U. Mechanical behavior of amorphous alloys. Acta Materialia. 2007;55(12):4067-4109., the high pressure is most likely due to one of the following factors: friction effects between the particles, some degree of crystallinity in the powders and/or low pressing temperature, which leads to a higher flow stress, since the viscosity decreases with temperature up to Tx 2121 Parthiban R, Stoica M, Kaban I, Kumar R, Eckert J. 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. 2015;66:48-55..

It has been reported that an oxide layer present in the powder surface can avoid a strong bonding between the glassy powders particles when no pressure is applied 2323 Yan M, Yu P, Kim KB, Lee JK, Schaffer GB, Qian M. The surface structure of gas-atomized metallic glass powders. Scripta Materialia. 2010;62(5):266-269.,2424 Kawamura Y, Inoue A, Masumoto T. Mechanical properties of amorphous alloy compacts prepared by a closed processing system. Scripta Metallurgica et Materialia. 1993;29(1):25-30.. High pressures will break the oxide layer, however, they are usually limited by the used tools. As seen in this work, this necessary pressure to consolidate amorphous parts are much higher than the theoretical stress to flow fully amorphous powder particles. This poses a processing issue since to consolidate bigger parts, very high loads are necessary.

No appreciable difference in the transformation temperatures Tg, Tx and thus ΔTx is noticed between the as-atomized powders and hot-pressed samples. This reveals that the consolidation processes were successfully carried out within the supercooled liquid interval avoiding further crystallization. Furthermore, the exothermic peak in the DSC curve proves the presence of an amorphous phase in the consolidated samples, since only a crystallization event could explain the exothermic event at that temperature range in the Fe43.2Co28.8B19.2Si4.8Nb4 alloy.

Figure 5 shows XRD patterns of the powders and hot-pressed samples. A halo around 2θ = 44 °, typical of an amorphous structure, can be seen in the XRD patterns. The presence of some wide Bragg peaks in the patterns can also be noted, corroborating with the OM images and DSC results.

Figure 5
XRD patterns of the GAP and the consolidated samples.

Overall, it was shown that bulk glassy samples of the Fe43.2Co28.8B19.2Si4.8Nb4 alloy can be produced by hot pressing gas atomized powders in the supercooled liquid region. The obtained relative density was relatively high but the pressure necessary for consolidating these samples was much higher than expected, considering typical viscosity values of these materials. In the future, a better understating of the flow of atomized powders in the supercooled liquid region should be pursed in order to optimize the conformation process of these alloys.

4. Summary and Conclusions

Bulk amorphous parts of the Fe43,2Co28,8B19,2Si4,8Nb4 alloy were produced via consolidation of gas-atomized powders. Gas atomized powders below 75 µm were mostly spherical and had a clean surface. Although the consolidation process was carried out within the powders supercooled liquid interval, a high pressure (1GPa) was necessary to produce parts with high relative density (96.1%). The compacts present glass transition and crystallization temperatures as well diffraction patterns similar to the atomized powders indicating that the same amorphous structure from the powders was present in the bulk samples. The proposed consolidation method is a promising route for producing bulk metallic glasses.

5. Acknowledgements

The authors thank the grant #2015/26705-6, São Paulo Research Foundation (FAPESP) and National Council for Scientific and Technological Development (CNPq - Brazil) for financial support.

6. References

  • 1
    Greer AL. Metallic glasses...on the threshold. Materials Today 2009;12(1-2):14-22.
  • 2
    Suryanarayana C, Inoue A. Bulk Metallic Glass 1st ed. Boca Raton: CRC Press; 2010. 565 p.
  • 3
    Ashby MF, Greer AL. Metallic glasses as structural materials. Scripta Materialia 2006;54(3):321-326.
  • 4
    Hasegawa R. Applications of amorphous magnetic alloys in electronic devices. Journal of Non-Crystalline Solids 2001;287(1-3):405-412.
  • 5
    Ishida M, Takeda H, Nishiyama N, Kita K, Shimizu Y, Saotome Y, et al. Wear resistivity of super-precision microgear made of Ni-based metallic glass. Materials Science and Engineering: A 2007;448-451:149-154.
  • 6
    Suryanarayana C, Inoue A. Iron-based bulk metallic glasses. International Materials Reviews 2013;58(3):131-166.
  • 7
    Inoue A, Kong FL, Man QK, Shen BL, Li RW, Al-Marzouki F. Development and applications of Fe- and Co-based bulk glassy alloys and their prospects. Journal of Alloys and Compounds 2015;615(Supp 1):S2-S8.
  • 8
    Stoica M. Fe-Based Bulk Metallic Glasses - Understanding the Influence of Impurities on Glass Formation Wiesbaden: Springer Fachmedien Wiesbaden; 2017. 126 p.
  • 9
    Inoue A, Shen BL, Chang CT. Super-high strength of over 4000 MPa for Fe-based bulk glassy alloys in [(Fe1-xCox)0.75B0.2Si0.05]96Nb4 system. Acta Materialia 2004;52(14):4093-4099.
  • 10
    Kawamura Y, Nakamura T, Kato H, Mano H, Inoue A. Newtonian and non-Newtonian viscosity of supercooled liquid in metallic glasses. Materials Science and Engineering: A 2001;304-306:674-678.
  • 11
    Yavari AR, de Oliveira MF, Kiminami CS, Inoue A, Botta Filho WJ. Electromechanical shaping, assembly and engraving of bulk metallic glasses. Materials Science and Engineering: A 2004;375-377:227-234.
  • 12
    Schuh CA, Hufnagel TC, Ramamurty U. Mechanical behavior of amorphous alloys. Acta Materialia 2007;55(12):4067-4109.
  • 13
    Kawamura Y, Shibata T, Inoue A, Masumoto T. Deformation behavior of Zr65Al10Ni10Cu15 glassy alloy with wide supercooled liquid region. Applied Physics Letters 1996;69(9):1208-1210.
  • 14
    Suh JY, Lohwongwatana B, Garland CM, Conner RD, Johnson WL, Suh D. Novel thermoplastic bonding using a bulk metallic glass solder. Scripta Materialia 2008;59(8):905-908.
  • 15
    de Oliveira M, Botta Filho WJ, Yavari AR. Connecting, Assemblage and Electromechanical Shaping of Bulk Metallic Glasses. Materials Transactions, JIM 2000;41(11):1501-1504.
  • 16
    Sordelet DJ, Rozhkova E, Huang P, Wheelock PB, Besser MF, Kramer MJ, et al. Synthesis of Cu47Ti34Zr11Ni8 Bulk Metallic Glass by Warm Extrusion of Gas Atomized Powders. Journal of Materials Research 2002;17(1):186-198.
  • 17
    Ishihara S, Zhang W, Inoue A. Hot pressing of Fe-Co-Nd-Dy-B glassy powders in supercooled liquid state and hard magnetic properties of the consolidated alloys. Scripta Materialia 2002;47(4):231-235.
  • 18
    Kang EY, Kim YB, Kim KY, Chung YH, Baik HK. Vacuum hot pressing of Fe-Si-B-Nb-based amorphous powder cores and their high-frequency magnetic properties. Journal of Applied Physics 2006;99(8):08F111.
  • 19
    Yodoshi N, Yamada R, Kawasaki A, Watanabe R. Consolidation of [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 Metallic Glassy Powder by SPS Method. Materials Transactions 2009;50(9):2264-2269.
  • 20
    German RM. Powder Metallurgy & Particulate Materials Processing Princeton: Metal Powder Industry Federation; 2005. 528 p.
  • 21
    Parthiban R, Stoica M, Kaban I, Kumar R, Eckert J. 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 2015;66:48-55.
  • 22
    Trouton FT. On the Coefficient of Viscous Traction and Its Relation to that of Viscosity. Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences 1906;77(519):426-440.
  • 23
    Yan M, Yu P, Kim KB, Lee JK, Schaffer GB, Qian M. The surface structure of gas-atomized metallic glass powders. Scripta Materialia 2010;62(5):266-269.
  • 24
    Kawamura Y, Inoue A, Masumoto T. Mechanical properties of amorphous alloy compacts prepared by a closed processing system. Scripta Metallurgica et Materialia 1993;29(1):25-30.

Publication Dates

  • Publication in this collection
    17 Jan 2019
  • Date of issue
    2019

History

  • Received
    21 Aug 2018
  • Reviewed
    10 Nov 2018
  • Accepted
    05 Dec 2018
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