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Preparation of bulk amorphous Fe–Cr–Mo–Ga–P–C–B alloys by copper mold casting
Materials Science and Engineering A 375–377 (2004) 399–402
Preparation of bulk amorphous Fe–Cr–Mo–Ga–P–C–B alloys by copper mold casting M. Stoica∗ , J. Eckert, S. Roth, L. Schultz IFW Dresden, Institute for Metallic Materials, P.O. Box 270016, D-01171 Dresden, Germany
Abstract The Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 bulk glassy alloy was prepared in the shape of rods and ribbons using copper mold casting and melt spinning, respectively. The structure was investigated by X-ray diffraction (XRD) and the thermal stability was checked by differential scanning calorimetry (DSC). It was found that the samples consist of an a single amorphous phase and exhibit a rather wide supercooled liquid region before crystallization. Some characteristic thermal properties of the amorphous samples as a function of the geometrical dimensions are discussed, as well as the dependence of the glass-forming ability on composition. Additionally, some magnetic data like coercivity Hc , saturation polarization Js and Curie temperature Tc are presented. © 2003 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glass; Iron-based alloys; Thermal stability; Supercooled liquid region; Glass-forming ability
1. Introduction The iron-based amorphous alloys found a few years ago by Inoue and Gook [1,2] and Inoue et al. [3] exhibit a large supercooled liquid region between the glass transition temperature Tg and the crystallization temperature Tx visible upon heating at a constant rate to elevated temperatures. They have good soft magnetic properties characterized by a low coercive force and a high permeability [4–6]. Using copper mold casting or water quenching techniques, such alloys can be cast in form of bulk specimens directly suitable for use as magnetic cores [7]. However, the critical cooling rate of about 102 K s−1 required for glass formation is higher than the value of about 1–10 K s−1 characteristic for alloys with very good glass-forming ability [8]. Thus, the maximum achievable diameter of these Fe-based alloys is limited to only a few millimeters [8]. The other hindrance that can influence bulk glass formation is the presence of impurities in the melt [9] that can be removed using fluxing techniques, or of crystalline inclusions that can form upon solidification of the melt. In the case of Fe–Cr–Mo–Ga–P–C–B alloys, Shen and Schwarz [7] used the flux-melting technique to remove the oxide inclusions from the melt, and subsequent water ∗
Corresponding author. E-mail address: [email protected] (M. Stoica).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.190
quenching allows to produce rods with 4 mm diameter. The nominal composition Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 was found to have the best thermal stability. Concerning the magnetic properties, the coercive force is below 5 A m−1 , and the saturation polarization is around 0.8 T. The magnetic characteristics might be improved by increasing the iron content, but this may deteriorate the glass-forming ability. The present work focuses on the possibility to cast Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 alloy directly in bulk form using the copper mold casting technique and to investigate the effect of the composition on the glass-forming ability.
2. Experimental In a first step, Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 with 2 ≤ x ≤ 4, 2 ≤ y ≤ 4, 0 ≤ z ≤ 4, master alloy ingots for different x, y and z content were obtained by induction melting starting from Fe–B, Fe–C, Fe–Ga, Fe–P pre-alloys and Mo (99.4% purity), Cr (99.95% purity), Fe (99.9% purity), or crystalline B (99.99% purity) pure elements. Induction melting of Fe with B, Fe with C (99.9% purity) and Fe with Ga (99.7% purity) allowed to produce the respective pre-alloys. Mechanical alloying of Fe powder (99.9% purity, less than 10 m particle size) with amorphous red P (99% purity, less than 100 m particle size) for 5 h in a
M. Stoica et al. / Materials Science and Engineering A 375–377 (2004) 399–402
SPEX 8000 ball mill using hardened steel balls and vial and a ball-to-powder weight ratio of 3:1 and consolidation of the resulting powders by cold pressing and subsequent induction melting was employed to obtain the Fe–P pre-alloy. All materials were handled in a glove box under purified argon atmosphere (less than 1 ppm O2 and H2 O). Amorphous rods with various diameters between 1.5 and 3 mm and a length of 70 mm, as well as ribbons (10 mm × 0.03 mm) were prepared from the master alloy. The rods were obtained by induction melting under argon atmosphere at a pressure of 80 × 103 Pa and injection of the melt into a copper mold under an applied pressure of 3 × 105 Pa. The ribbons were prepared by single-roller melt spinning under argon flow on a copper wheel at 24 m s−1 tangential velocity of the wheel. The oxygen content of the master alloys and pre-alloys was checked by hot extraction using a C436 LECO analyzer. We found values of around 50 ppm for master alloys, 180 ppm for Fe–P and 130 ppm for the other pre-alloys, respectively. The amorphous structure of the samples was checked by X-ray diffraction (XRD) using a Phillips PW 3020 diffractometer with Co K␣ radiation (λ = 0.17889 nm). For the XRD measurements, the cast rods were crushed to small pieces and bonded into amorphous resin in order to have good resolution. The thermal stability was examined by differential scanning calorimetry (DSC), using a Netzsch DSC 404 under a flow of purified argon. The glass transition temperature Tg and the crystallization temperature Tx were determined as the onset temperatures of the glass transition and the crystallization events, respectively, during heating with a constant rate of 40 K min−1 . In addition, the melting temperature Tm defined by the liquidus temperature Tliq at the onset of solidification upon cooling with a constant rate of 10 K min−1 was measured using the same Netsch DSC 404 calorimeter. The magnetic properties of the samples were also investigated. The coercive force, Hc , was measured using a Foerster Koerzimat with premagnetizing field pulses of 2000 A cm−1 , the saturation polarization, Js , was measured by a vibrating sample magnetometer (VSM) under a maximum dc magnetic field of 15 000 A cm−1 and the variation of the saturation polarization versus temperature by a Faraday magnetometer at a heating rate of 10 K min−1 .
3. Results and discussion Fig. 1 shows the XRD scans for Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 glassy alloys as a function of composition and shape. The patterns display mainly just a broad maximum around 2θ = 51◦ which is characteristic for an amorphous phase. For the ribbons, there is no doubt concerning complete amorphicity. At least for the rods with 3 mm diameter in the case of Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 , 1.5 mm diameter in the case of Fe67.5 Cr4 Mo4 Ga2 P12 C5 B5.5 , and certainly, 1.5 mm diameter in the case of Fe69.5 Cr4 Mo4 P12 C5 B5.5 ,
x = y =z = 2
Intensity [a.u.]
400
x =y =4, z =0
ribbon φ1. 5 mm
x =y =4, z =0
ribbon
x =y =4, z =2
φ 2 mm
x =y =4, z =2
φ1. 5 mm
x =y =4, z =2
ribbon
x = y =z = 4
φ 3 mm
x =y =z =4
φ 2. 5 mm
x = y =z = 4
φ 2 mm
x =y =z =4
φ 1.5 mm ribbon
x = y =z = 4
2 0˚ 3 0˚ 4 0˚ 5 0˚ 6 0˚ 7 0˚ 80˚ 90˚ Diffraction Angle, 2 Fig. 1. The XRD patterns for as-cast Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 glassy alloy samples at room temperature.
there are additional weak diffraction peaks superimposed on the broad diffraction maximum of the amorphous phase. This points to the formation of crystalline inclusions coexisting with the glass. Their influence on the magnetic properties will be discussed later. However, this type of investigation cannot rule out the possible existence of a small volume fraction of nanoscale crystalline inclusions. The presence of such crystalline phase(s) as well as their formation can be clearly revealed by in situ X-ray diffraction measurements in transmission configuration using a high intensity high-energy monochromatic synchrotron beam. In previous work [10], we presented in detail the XRD patterns taken at ESRF Grenoble for the Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 glassy alloy. The influence of the composition on the glass-forming ability and thermal stability can be evaluated from thermal measurements. For all the samples, the DSC scans show a glass transition followed by a supercooled liquid region and crystallization. The values for Tg , Tx and the liquidus temperatures Tliq as a function of the composition and the geometrical dimensions for as-cast samples are given in Table 1. Using these values the extension of the supercooled liquid region Tx and the reduced glass transition temperatures Tg /Tliq were calculated. Their variation with the composition for melt-spun ribbons is plotted in Fig. 2. The glass transition temperatures and the crystallization temperatures slowly increase with increasing rod diameter in the case of Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 and remain almost constant for Fe67.5 Cr4 Mo4 Ga2 P12 C5 B5.5 and
M. Stoica et al. / Materials Science and Engineering A 375–377 (2004) 399–402
x, y, z
Sample
Tg (K)
Tx (K)
Tliq (K)
4, 4, 4
Ribbon Rod 1.5 Rod 2 Rod 2.5 Rod 3
743 750 753 755 757
809 811 814 815 817
1256 1251 1248 1250 1255
4, 4, 2
Ribbon Rod 1.5 Rod 2
752 755 753
811 811 811
1299 1299 1299
4, 4, 0
Ribbon Rod 1.5
747 747
794 794
1314 1311
2, 2, 2
Ribbon
736
781
1301
Fe69.5 Cr4 Mo4 P12 C5 B5.5 . The differences in thermal stability between rods with different diameters and melt-spun ribbons prepared of same composition are caused by a different degree of relaxation stemming from the different cooling rates achieved during solidification. Another reason can be a slightly different composition of the glasses obtained by the different synthesis routes and the decreasing cooling rate of the melt with increasing rod diameter. Such compositional variations may arise if at least the rods with larger diameters are not fully amorphous but contain some crystalline phase(s) which may form due to an insufficient cooling rate for complete glass formation upon casting. This agrees well with the conclusions drawn from the XRD patterns. Some crystalline inclusions can form even from the molten state. In a previous paper [11] the formation of such crystalline phase(s) was presented in detail, as well as the entire mechanism of crystallization and phase transformation for the Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 glassy alloy. The extension of the supercooled liquid region decreases with the increasing iron content (Fig. 2). For
∆ Tx [K]
70
Tx Tg/Tliq
60 50
ribbons
40
Tg/Tliq
0.60 ribbons
0.55 65
66
67
68
69
Fe content [at.%]
70
71
72
Fig. 2. Extension of the supercooled liquid region Tx = Tx − Tg and the reduced glass transition temperature Tg /Tliq of Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 amorphous ribbons as a function of iron content.
520
Curie temperature, Tc [K]
Table 1 Glass transition temperatures Tg , crystallization temperatures Tx and liquidus temperatures Tliq for as-cast Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 glassy alloy
401
x=y=z=2
500 480 460 440
x=y=z=4 ribbons
x = y = 4, z = 2
420 400
x = y = 4, z = 0
0,0 0,5 1,0 1,5 2,0
Diameter,
[mm]
66
68
70
72
74
Fe content [at.%]
Fig. 3. Curie temperature for as-cast Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 glassy alloys as a function of sample diameter at different Cr, Mo and Ga content and in the case of ribbons as a function of iron content.
the 65.6 at.% iron content Tx is around 66 K. For the Fe67.5 Cr4 Mo4 Ga2 P12 C5 B5.5 amorphous alloy it decrease to 59 K, for Fe69.5 Cr4 Mo4 P12 C5 B5.5 to 47 K and for Fe71.5.5 Cr2 Mo2 Ga2 P12 C5 B5.5 Tx reaches 45 K. The reduced glass transition temperature Tg /Tliq shows a similar decreasing trend with increasing Fe content, i.e. Tg /Tliq drops from 0.59 for x = y = z = 4 to 0.56 for x = y = z = 2. This indicates that the glass-forming ability decreases with increasing iron content, and this is reflected in the possibility to cast such alloys directly in bulk shape. All the completely amorphous samples exhibit a very low coercive force Hc , below 10 A m−1 . The lowest coercivity value of about 1 A m−1 is found for the ribbons and the coercivity increases to 5–9 A m−1 for the cast rods. However, the coercivity does not increase monotonically with increasing diameter of the rods, i.e. does not depend on the geometry of the sample, but is probably most sensitive to residual stresses induced during the casting and to crystalline inclusions. For the as-cast rod with 3 mm diameter in the case of Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 , with 1.5 mm diameter in the case of Fe67.5 Cr4 Mo4 Ga2 P12 C5 B5.5 and with 1.5 mm diameter in the case of Fe69.5 Cr4 Mo4 P12 C5 B5.5 the samples are not fully amorphous. The coercivity increases by one order of magnitude, reaching values between 17 and 75 A m−1 , and it is expected to depend on the volume fraction of crystalline inclusions. The values of the Curie temperature, Tc , as a function of composition and sample diameter are plotted in Fig. 3. As a general trend it is observed that the Curie temperature is larger for rod samples in comparison to the ribbon samples, as well as it increases with increasing rod diameter. Such a variation of the Curie temperature is due to a more relaxed amorphous structure of the rods with increasing diameter and in comparison with the ribbon. Concerning the composition, the Curie temperature decrease with the increasing Fe content when the Ga content decreases, from 435 K for 65.5 at.% Fe to 402 K for 69.5 at.% Fe, but increases
402
M. Stoica et al. / Materials Science and Engineering A 375–377 (2004) 399–402
to 515 K for 71.5 at.% Fe when the content of Cr, Mo and Ga are all 2 at.%. The values of the saturation polarization are in the range of 0.77–0.82 T for the as-cast samples with 65.5 at.% Fe content and increase to 1.16 T for the samples with 71.5 at.% Fe. The main errors that can appear during the magnetic characterization are due to the difficulties to estimate precisely the rather small cross-section of the ribbons.
Acknowledgements The authors thank K. Biswas for his help with some measurements and H. Schulze and M. Peschel for technical assistance. This work was supported by the EU within the framework of the RTN-Network on bulk metallic glasses (contract HPRN-CT-2000-00033).
References 4. Conclusions Amorphous Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 alloys with different content of transition metals were obtained in bulk form using copper mold casting. Since the glass transition temperature exceeds 736 K and the Curie temperature is above 402 K, these ferromagnetic glasses are sufficiently stable for applications that require a continuous operation at elevated temperatures. The critical cooling rate is lower compared to classical Fe–B alloys, but higher than for Zr-based or Pd-based multicomponent easy glass-forming alloys [8]. By increasing the iron content, the saturation polarization can be increased, but the glass-forming ability become worse. Thus, the thickness at which the samples are still fully amorphous is limited.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
A. Inoue, J.S. Gook, Mater. Trans., JIM 37 (1996) 32. A. Inoue, J.S. Gook, Mater. Trans., JIM 36 (1995) 1282. A. Inoue, Mater. Sci. Eng. A 226–228 (1997) 357. A. Inoue, A. Murakami, T. Zhang, A. Takeuchi, Mater. Trans., JIM 38 (1997) 189. A. Inoue, H. Koshiba, T. Zhang, A. Makino, Mater. Trans., JIM 38 (1997) 577. T.D. Shen, U. Harms, R.B. Schwarz, Mater. Sci. Forum 386–388 (2002) 441. T.D. Shen, R.B. Schwarz, Appl. Phys. Lett. 75 (1999) 49. A. Inoue, Acta Mater. 48 (2000) 279. A. Gebert, J. Eckert, L. Schultz, Acta Mater. 46 (1998) 5475. M. Stoica, J. Degmova, S. Roth, J. Eckert, H. Grahl, L. Schultz, A.R. Yavari, A. Kvick, G. Heunen, Mater. Trans., JIM 43 (2002) 1966. M. Stoica, J. Eckert, S. Roth, L. Schultz, A.R. Yavari, A. Kvick, J. Metastable Nanocryst. Mater. 12 (2002) 77.
Preparation of bulk amorphous Fe–Cr–Mo–Ga–P–C–B alloys by copper mold casting M. Stoica∗ , J. Eckert, S. Roth, L. Schultz IFW Dresden, Institute for Metallic Materials, P.O. Box 270016, D-01171 Dresden, Germany
Abstract The Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 bulk glassy alloy was prepared in the shape of rods and ribbons using copper mold casting and melt spinning, respectively. The structure was investigated by X-ray diffraction (XRD) and the thermal stability was checked by differential scanning calorimetry (DSC). It was found that the samples consist of an a single amorphous phase and exhibit a rather wide supercooled liquid region before crystallization. Some characteristic thermal properties of the amorphous samples as a function of the geometrical dimensions are discussed, as well as the dependence of the glass-forming ability on composition. Additionally, some magnetic data like coercivity Hc , saturation polarization Js and Curie temperature Tc are presented. © 2003 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glass; Iron-based alloys; Thermal stability; Supercooled liquid region; Glass-forming ability
1. Introduction The iron-based amorphous alloys found a few years ago by Inoue and Gook [1,2] and Inoue et al. [3] exhibit a large supercooled liquid region between the glass transition temperature Tg and the crystallization temperature Tx visible upon heating at a constant rate to elevated temperatures. They have good soft magnetic properties characterized by a low coercive force and a high permeability [4–6]. Using copper mold casting or water quenching techniques, such alloys can be cast in form of bulk specimens directly suitable for use as magnetic cores [7]. However, the critical cooling rate of about 102 K s−1 required for glass formation is higher than the value of about 1–10 K s−1 characteristic for alloys with very good glass-forming ability [8]. Thus, the maximum achievable diameter of these Fe-based alloys is limited to only a few millimeters [8]. The other hindrance that can influence bulk glass formation is the presence of impurities in the melt [9] that can be removed using fluxing techniques, or of crystalline inclusions that can form upon solidification of the melt. In the case of Fe–Cr–Mo–Ga–P–C–B alloys, Shen and Schwarz [7] used the flux-melting technique to remove the oxide inclusions from the melt, and subsequent water ∗
Corresponding author. E-mail address: [email protected] (M. Stoica).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.190
quenching allows to produce rods with 4 mm diameter. The nominal composition Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 was found to have the best thermal stability. Concerning the magnetic properties, the coercive force is below 5 A m−1 , and the saturation polarization is around 0.8 T. The magnetic characteristics might be improved by increasing the iron content, but this may deteriorate the glass-forming ability. The present work focuses on the possibility to cast Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 alloy directly in bulk form using the copper mold casting technique and to investigate the effect of the composition on the glass-forming ability.
2. Experimental In a first step, Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 with 2 ≤ x ≤ 4, 2 ≤ y ≤ 4, 0 ≤ z ≤ 4, master alloy ingots for different x, y and z content were obtained by induction melting starting from Fe–B, Fe–C, Fe–Ga, Fe–P pre-alloys and Mo (99.4% purity), Cr (99.95% purity), Fe (99.9% purity), or crystalline B (99.99% purity) pure elements. Induction melting of Fe with B, Fe with C (99.9% purity) and Fe with Ga (99.7% purity) allowed to produce the respective pre-alloys. Mechanical alloying of Fe powder (99.9% purity, less than 10 m particle size) with amorphous red P (99% purity, less than 100 m particle size) for 5 h in a
M. Stoica et al. / Materials Science and Engineering A 375–377 (2004) 399–402
SPEX 8000 ball mill using hardened steel balls and vial and a ball-to-powder weight ratio of 3:1 and consolidation of the resulting powders by cold pressing and subsequent induction melting was employed to obtain the Fe–P pre-alloy. All materials were handled in a glove box under purified argon atmosphere (less than 1 ppm O2 and H2 O). Amorphous rods with various diameters between 1.5 and 3 mm and a length of 70 mm, as well as ribbons (10 mm × 0.03 mm) were prepared from the master alloy. The rods were obtained by induction melting under argon atmosphere at a pressure of 80 × 103 Pa and injection of the melt into a copper mold under an applied pressure of 3 × 105 Pa. The ribbons were prepared by single-roller melt spinning under argon flow on a copper wheel at 24 m s−1 tangential velocity of the wheel. The oxygen content of the master alloys and pre-alloys was checked by hot extraction using a C436 LECO analyzer. We found values of around 50 ppm for master alloys, 180 ppm for Fe–P and 130 ppm for the other pre-alloys, respectively. The amorphous structure of the samples was checked by X-ray diffraction (XRD) using a Phillips PW 3020 diffractometer with Co K␣ radiation (λ = 0.17889 nm). For the XRD measurements, the cast rods were crushed to small pieces and bonded into amorphous resin in order to have good resolution. The thermal stability was examined by differential scanning calorimetry (DSC), using a Netzsch DSC 404 under a flow of purified argon. The glass transition temperature Tg and the crystallization temperature Tx were determined as the onset temperatures of the glass transition and the crystallization events, respectively, during heating with a constant rate of 40 K min−1 . In addition, the melting temperature Tm defined by the liquidus temperature Tliq at the onset of solidification upon cooling with a constant rate of 10 K min−1 was measured using the same Netsch DSC 404 calorimeter. The magnetic properties of the samples were also investigated. The coercive force, Hc , was measured using a Foerster Koerzimat with premagnetizing field pulses of 2000 A cm−1 , the saturation polarization, Js , was measured by a vibrating sample magnetometer (VSM) under a maximum dc magnetic field of 15 000 A cm−1 and the variation of the saturation polarization versus temperature by a Faraday magnetometer at a heating rate of 10 K min−1 .
3. Results and discussion Fig. 1 shows the XRD scans for Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 glassy alloys as a function of composition and shape. The patterns display mainly just a broad maximum around 2θ = 51◦ which is characteristic for an amorphous phase. For the ribbons, there is no doubt concerning complete amorphicity. At least for the rods with 3 mm diameter in the case of Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 , 1.5 mm diameter in the case of Fe67.5 Cr4 Mo4 Ga2 P12 C5 B5.5 , and certainly, 1.5 mm diameter in the case of Fe69.5 Cr4 Mo4 P12 C5 B5.5 ,
x = y =z = 2
Intensity [a.u.]
400
x =y =4, z =0
ribbon φ1. 5 mm
x =y =4, z =0
ribbon
x =y =4, z =2
φ 2 mm
x =y =4, z =2
φ1. 5 mm
x =y =4, z =2
ribbon
x = y =z = 4
φ 3 mm
x =y =z =4
φ 2. 5 mm
x = y =z = 4
φ 2 mm
x =y =z =4
φ 1.5 mm ribbon
x = y =z = 4
2 0˚ 3 0˚ 4 0˚ 5 0˚ 6 0˚ 7 0˚ 80˚ 90˚ Diffraction Angle, 2 Fig. 1. The XRD patterns for as-cast Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 glassy alloy samples at room temperature.
there are additional weak diffraction peaks superimposed on the broad diffraction maximum of the amorphous phase. This points to the formation of crystalline inclusions coexisting with the glass. Their influence on the magnetic properties will be discussed later. However, this type of investigation cannot rule out the possible existence of a small volume fraction of nanoscale crystalline inclusions. The presence of such crystalline phase(s) as well as their formation can be clearly revealed by in situ X-ray diffraction measurements in transmission configuration using a high intensity high-energy monochromatic synchrotron beam. In previous work [10], we presented in detail the XRD patterns taken at ESRF Grenoble for the Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 glassy alloy. The influence of the composition on the glass-forming ability and thermal stability can be evaluated from thermal measurements. For all the samples, the DSC scans show a glass transition followed by a supercooled liquid region and crystallization. The values for Tg , Tx and the liquidus temperatures Tliq as a function of the composition and the geometrical dimensions for as-cast samples are given in Table 1. Using these values the extension of the supercooled liquid region Tx and the reduced glass transition temperatures Tg /Tliq were calculated. Their variation with the composition for melt-spun ribbons is plotted in Fig. 2. The glass transition temperatures and the crystallization temperatures slowly increase with increasing rod diameter in the case of Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 and remain almost constant for Fe67.5 Cr4 Mo4 Ga2 P12 C5 B5.5 and
M. Stoica et al. / Materials Science and Engineering A 375–377 (2004) 399–402
x, y, z
Sample
Tg (K)
Tx (K)
Tliq (K)
4, 4, 4
Ribbon Rod 1.5 Rod 2 Rod 2.5 Rod 3
743 750 753 755 757
809 811 814 815 817
1256 1251 1248 1250 1255
4, 4, 2
Ribbon Rod 1.5 Rod 2
752 755 753
811 811 811
1299 1299 1299
4, 4, 0
Ribbon Rod 1.5
747 747
794 794
1314 1311
2, 2, 2
Ribbon
736
781
1301
Fe69.5 Cr4 Mo4 P12 C5 B5.5 . The differences in thermal stability between rods with different diameters and melt-spun ribbons prepared of same composition are caused by a different degree of relaxation stemming from the different cooling rates achieved during solidification. Another reason can be a slightly different composition of the glasses obtained by the different synthesis routes and the decreasing cooling rate of the melt with increasing rod diameter. Such compositional variations may arise if at least the rods with larger diameters are not fully amorphous but contain some crystalline phase(s) which may form due to an insufficient cooling rate for complete glass formation upon casting. This agrees well with the conclusions drawn from the XRD patterns. Some crystalline inclusions can form even from the molten state. In a previous paper [11] the formation of such crystalline phase(s) was presented in detail, as well as the entire mechanism of crystallization and phase transformation for the Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 glassy alloy. The extension of the supercooled liquid region decreases with the increasing iron content (Fig. 2). For
∆ Tx [K]
70
Tx Tg/Tliq
60 50
ribbons
40
Tg/Tliq
0.60 ribbons
0.55 65
66
67
68
69
Fe content [at.%]
70
71
72
Fig. 2. Extension of the supercooled liquid region Tx = Tx − Tg and the reduced glass transition temperature Tg /Tliq of Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 amorphous ribbons as a function of iron content.
520
Curie temperature, Tc [K]
Table 1 Glass transition temperatures Tg , crystallization temperatures Tx and liquidus temperatures Tliq for as-cast Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 glassy alloy
401
x=y=z=2
500 480 460 440
x=y=z=4 ribbons
x = y = 4, z = 2
420 400
x = y = 4, z = 0
0,0 0,5 1,0 1,5 2,0
Diameter,
[mm]
66
68
70
72
74
Fe content [at.%]
Fig. 3. Curie temperature for as-cast Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 glassy alloys as a function of sample diameter at different Cr, Mo and Ga content and in the case of ribbons as a function of iron content.
the 65.6 at.% iron content Tx is around 66 K. For the Fe67.5 Cr4 Mo4 Ga2 P12 C5 B5.5 amorphous alloy it decrease to 59 K, for Fe69.5 Cr4 Mo4 P12 C5 B5.5 to 47 K and for Fe71.5.5 Cr2 Mo2 Ga2 P12 C5 B5.5 Tx reaches 45 K. The reduced glass transition temperature Tg /Tliq shows a similar decreasing trend with increasing Fe content, i.e. Tg /Tliq drops from 0.59 for x = y = z = 4 to 0.56 for x = y = z = 2. This indicates that the glass-forming ability decreases with increasing iron content, and this is reflected in the possibility to cast such alloys directly in bulk shape. All the completely amorphous samples exhibit a very low coercive force Hc , below 10 A m−1 . The lowest coercivity value of about 1 A m−1 is found for the ribbons and the coercivity increases to 5–9 A m−1 for the cast rods. However, the coercivity does not increase monotonically with increasing diameter of the rods, i.e. does not depend on the geometry of the sample, but is probably most sensitive to residual stresses induced during the casting and to crystalline inclusions. For the as-cast rod with 3 mm diameter in the case of Fe65.5 Cr4 Mo4 Ga4 P12 C5 B5.5 , with 1.5 mm diameter in the case of Fe67.5 Cr4 Mo4 Ga2 P12 C5 B5.5 and with 1.5 mm diameter in the case of Fe69.5 Cr4 Mo4 P12 C5 B5.5 the samples are not fully amorphous. The coercivity increases by one order of magnitude, reaching values between 17 and 75 A m−1 , and it is expected to depend on the volume fraction of crystalline inclusions. The values of the Curie temperature, Tc , as a function of composition and sample diameter are plotted in Fig. 3. As a general trend it is observed that the Curie temperature is larger for rod samples in comparison to the ribbon samples, as well as it increases with increasing rod diameter. Such a variation of the Curie temperature is due to a more relaxed amorphous structure of the rods with increasing diameter and in comparison with the ribbon. Concerning the composition, the Curie temperature decrease with the increasing Fe content when the Ga content decreases, from 435 K for 65.5 at.% Fe to 402 K for 69.5 at.% Fe, but increases
402
M. Stoica et al. / Materials Science and Engineering A 375–377 (2004) 399–402
to 515 K for 71.5 at.% Fe when the content of Cr, Mo and Ga are all 2 at.%. The values of the saturation polarization are in the range of 0.77–0.82 T for the as-cast samples with 65.5 at.% Fe content and increase to 1.16 T for the samples with 71.5 at.% Fe. The main errors that can appear during the magnetic characterization are due to the difficulties to estimate precisely the rather small cross-section of the ribbons.
Acknowledgements The authors thank K. Biswas for his help with some measurements and H. Schulze and M. Peschel for technical assistance. This work was supported by the EU within the framework of the RTN-Network on bulk metallic glasses (contract HPRN-CT-2000-00033).
References 4. Conclusions Amorphous Fe77.5−x−y−z Crx Moy Gaz P12 C5 B5.5 alloys with different content of transition metals were obtained in bulk form using copper mold casting. Since the glass transition temperature exceeds 736 K and the Curie temperature is above 402 K, these ferromagnetic glasses are sufficiently stable for applications that require a continuous operation at elevated temperatures. The critical cooling rate is lower compared to classical Fe–B alloys, but higher than for Zr-based or Pd-based multicomponent easy glass-forming alloys [8]. By increasing the iron content, the saturation polarization can be increased, but the glass-forming ability become worse. Thus, the thickness at which the samples are still fully amorphous is limited.
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