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Thermal stability and microhardness of new Co-based bulk metallic glasses
Materials Science and Engineering A 449–451 (2007) 538–540
Thermal stability and microhardness of new Co-based bulk metallic glasses H. Men ∗ , S.J. Pang, T. Zhang Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China Received 22 August 2005; received in revised form 2 October 2005; accepted 5 December 2005
Abstract In this paper, thermal stability and microhardness of Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 (numbers indicate at.%) glassy alloys were investigated. The two glassy alloys have high thermal stability. Glass transition temperature, supercooled liquid temperature range and reduced glass transition temperature are 819 K, 76 K and 0.58, respectively, for the Er-free alloy and 848 K, 85 K and 0.61, respectively, for the Er-containing alloy. The two glassy alloys exhibit high Vickers hardness of about 1.56 GPa and 1.58 GPa, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glass; Co-based alloys; Thermal stability; Microhardness
1. Introduction In the past decades, a great number of bulk metallic glasses (BMGs) had been synthesized in multicomponent alloy systems, such as La-, Zr-, Fe- and Cu-based alloys [1–7]. These BMGs exhibit higher strength and hardness, better corrosion resistance, comparing to their crystalline counterparts [8]. Recently, it is reported that Co43 Fe20 Ta5.5 B31.5 (numbers indicate at.%) alloy can be cast into BMG with a maximum diameter of 2 mm, and this glassy alloy exhibits the highest fracture strength of 5.3 GPa among all crystalline and glassy alloys known so far [9]. In our recent study, it has been found that Co50 Cr15 Mo14 C15 B6 alloy can be cast into fully glassy rod in diameter of 2 mm by copper mould casting and the maximum diameter for glass formation increases to at least 10 mm by adding 2% Er to this alloy [10]. This paper intends to report the thermal stability and mechanical properties of Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 glassy alloys. 2. Experimental procedure Alloy ingots were prepared by arc melting the mixtures of pure elements of Co, Cr, Mo, C, B and Er in a purified argon atmosphere. Cylindrical rods of 1–10 mm in diameter were ∗
Corresponding author. Tel.: +86 10 82316192; fax: +86 10 82314869. E-mail address: [email protected] (H. Men).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.12.095
obtained by a copper mould casting method. The structure of as-cast rods was examined by X-ray diffraction (XRD) using Cu K␣ radiation. The thermal stability and melting behaviors of the glassy alloys were investigated by a Netzsch model 404 differential scanning calorimetry (DSC) at a heating rate of 0.333 K/s. The Vickers hardness was measured with a Shangguang HX2100 microhardness tester under a load of 300 g. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-cast Co48 Cr15 Mo14 C15 B6 alloy rod of d = 2 mm and Co48 Cr15 Mo14 C15 B6 Er2 alloy rods of d = 2 mm and 10 mm, where d denotes the rod diameter. The XRD traces reveal only a broad diffuse halo, and no peaks corresponding to crystalline phases are visible, indicating the formation of a mostly amorphous phase. Fig. 2 shows DSC curves of as-cast Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 glassy rods of d = 2 mm taken at a heating rate of 0.333 K/s. Both DSC scans exhibit distinct glass transition and wide supercooled liquid region, followed by at least two exothermic events characteristic of crystallization. The glass transition temperature (Tg ) and onset temperature of the first crystallization (Tx1 ) are marked with arrows in Fig. 1. The supercooled liquid temperature region (Tx ) is defined as Tx1 − Tg . Tg and Tx1 of the Er-free alloy are 819 K and 895 K, respectively, and significantly increase to 848 K and 933 K, respectively, with the addition of 2% Er. As a result, the
H. Men et al. / Materials Science and Engineering A 449–451 (2007) 538–540
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Fig. 3. Melting and solidifying behaviors of (a) Co50 Cr15 Mo14 C15 B6 and (b) Co48 Cr15 Mo14 C15 B6 Er2 alloys.
Fig. 1. XRD patterns of as-cast Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 alloy rods with varied diameters (d).
value of Tx increases from 76 K for the Er-free alloy to 85 K for the Er-containing alloy. The present Co-based glassy alloys exhibit both high glass transition temperature and large supercooled liquid region, which are greatly favorable for extensive application of the BMGs as structural materials due to the high thermal stability. The melting and solidifying behaviors of Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 alloys are shown in Fig. 3. Two alloys exhibit a nearly identical eutectic temperature (Tm ), 1355 K for the Er-free alloy and 1350 K for the Er-containing alloy, however the apparent liquidus temperature (Tl ) of the Erfree alloy decreases from 1417 K to 1394 K with the addition of Er. As a result, the temperature span between onset and offset temperature of the melting reduces from 44 K for the Er-free alloy to 62 K for the Er-containing alloy. The reduced glass transition temperature (Trg ) is defined as Tg /Tl , and the Er-containing glassy alloy shows a slightly larger Trg than the Er-free alloy, 0.61 versus 0.58. In addition, only one major exothermic peak can be observed on the cooling traces of two alloys. Therefore, it can be confirmed that two alloy compositions are at or very close to the eutectics. With the addition of Er, the
Fig. 2. DSC curves of as-cast (a) Co50 Cr15 Mo14 C15 B6 and (b) Co48 Cr15 Mo14 C15 B6 Er2 BMGs with a diameter of 2 mm.
onset ) for the Er-free alloy onset temperature of solidification (Tsol reduces by about 38 K, from 1388 K to 1350 K. It is implied that the undercooling ability of the Er-free liquid is enhanced due to the addition of Er. In other words, the undercooled liquid of the Er-containing alloy is more stable than that of the Er-free alloy. In general, the increase of Trg and the enhanced stability of undercooled liquid indicate better glass-forming ability. The microhardness (Hv ) is determined to be about 1.56 GPa for the Er-free BMG with a diameter of 2 mm and 1.58 GPa for the Er-containing BMG with a diameter of 10 mm. A tensile yield strength (σ y ) can be estimated according to σ y = 3Hv . Based on the microhardness, the tensile yield strength of the two glassy alloys can be determined to be about 4.7 GPa, which approximates that of Co–Fe–Ta–B BMG. The microhardness of the Co-based BMGs is much higher than 0.7 GPa of Ti–Zr–Cu–Ni–Be BMG [11] and 0.66 GPa of Cu–Zr–Ti BMG [6]. Therefore, the present Co-based BMGs with ultra-high strength and strong GFA may promise extraordinary application potential as structural materials. The values of Tg and Tx of the Er-free glassy alloy increase apparently from 819 K and 76 K to 848 K and 85 K, respectively, with the addition of 2% Er, suggesting that the glassy phase and supercooled liquid of the Er-containing alloy are more stable. The atomic diameter of Er is the largest among Co, Cr, Mo, C, B and Er: 0.125 nm, 0.128 nm, 0.140 nm, 0.077 nm, 0.097 nm and 0.175 nm, respectively, and it is believed that the addition of a small amount of Er can lead to a more densely packed atomic configuration of amorphous phase and liquid. Therefore, the undercooled liquid of the Er-free alloy is stabionset due to the addition lized, as reflected by the reduction of Tsol of Er, and the Er-containing alloy exhibits a relatively higher Trg . The large value of Trg reflects a low nucleating rate in the undercooled liquid, and a low critical cooling rate for glass formation. According to ref. [12], the critical cooling rate can be estimated to be less than 40 K/s for the Er-containing alloy, much lower than about 1000 K/s of the Er-free alloy. As such, the Ercontaining alloy exhibits much stronger GFA than the Er-free alloy.
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H. Men et al. / Materials Science and Engineering A 449–451 (2007) 538–540
4. Summary In summary, Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 glassy alloys exhibit high thermal stability. Tg , Tx1 and Trg of the Er-free glassy alloy are 819 K, 76 K and 0.58, respectively, and increase to 848 K, 85 K and 0.61, respectively, with the addition of 2 at.% Er. The microhardness is as high as about 1.56 GPa for the Er-free BMG and 1.58 GPa for the Er-containing BMG, respectively.
References [1] [2] [3] [4] [5] [6] [7] [8]
Acknowledgement This work was supported by the National Nature Science Foundation of China (Grant Nos. 50225103 and 50471001).
[9] [10] [11] [12]
A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 31 (1991) 177. T. Zhang, A. Inoue, T. Masumoto, Mater. Trans. JIM 32 (1991) 1005. A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. Z.P. Lu, C.T. Liu, J.R. Thompson, W.D. Porter, Phys. Rev. Lett. 92 (2004) 245503–2455031. V. Ponnambalam, S.J. Poon, G.J. Shiflet, J. Mater. Res. 19 (2004) 1320. A. Inoue, W. Zhang, T. Zhang, K. Kurosaka, Mater. Trans. 42 (2001) 1149. D.H. Xu, G. Duan, W.L. Johnson, Phys. Rev. Lett. 92 (2004) 245504–2455041. A. Inoue, B.L. Shen, A.R. Yavari, A.L. Greer, J. Mater. Res. 18 (2003) 1487. A. Inoue, B.L. Shen, H. Koshiba, H. Kato, A.R. Yavari, Acta Mater. 52 (2004) 1631. H. Men, S.J. Pang, T. Zhang, J. Mater. Res. 21 (2006) 958. J.M. Park, Y.C. Kim, W.T. Kim, D.H. Kim, Mater. Trans. 45 (2004) 595. X.H. Lin, W.L. Johnson, J. Appl. Phys. 78 (1995) 6514.
Thermal stability and microhardness of new Co-based bulk metallic glasses H. Men ∗ , S.J. Pang, T. Zhang Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, PR China Received 22 August 2005; received in revised form 2 October 2005; accepted 5 December 2005
Abstract In this paper, thermal stability and microhardness of Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 (numbers indicate at.%) glassy alloys were investigated. The two glassy alloys have high thermal stability. Glass transition temperature, supercooled liquid temperature range and reduced glass transition temperature are 819 K, 76 K and 0.58, respectively, for the Er-free alloy and 848 K, 85 K and 0.61, respectively, for the Er-containing alloy. The two glassy alloys exhibit high Vickers hardness of about 1.56 GPa and 1.58 GPa, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glass; Co-based alloys; Thermal stability; Microhardness
1. Introduction In the past decades, a great number of bulk metallic glasses (BMGs) had been synthesized in multicomponent alloy systems, such as La-, Zr-, Fe- and Cu-based alloys [1–7]. These BMGs exhibit higher strength and hardness, better corrosion resistance, comparing to their crystalline counterparts [8]. Recently, it is reported that Co43 Fe20 Ta5.5 B31.5 (numbers indicate at.%) alloy can be cast into BMG with a maximum diameter of 2 mm, and this glassy alloy exhibits the highest fracture strength of 5.3 GPa among all crystalline and glassy alloys known so far [9]. In our recent study, it has been found that Co50 Cr15 Mo14 C15 B6 alloy can be cast into fully glassy rod in diameter of 2 mm by copper mould casting and the maximum diameter for glass formation increases to at least 10 mm by adding 2% Er to this alloy [10]. This paper intends to report the thermal stability and mechanical properties of Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 glassy alloys. 2. Experimental procedure Alloy ingots were prepared by arc melting the mixtures of pure elements of Co, Cr, Mo, C, B and Er in a purified argon atmosphere. Cylindrical rods of 1–10 mm in diameter were ∗
Corresponding author. Tel.: +86 10 82316192; fax: +86 10 82314869. E-mail address: [email protected] (H. Men).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.12.095
obtained by a copper mould casting method. The structure of as-cast rods was examined by X-ray diffraction (XRD) using Cu K␣ radiation. The thermal stability and melting behaviors of the glassy alloys were investigated by a Netzsch model 404 differential scanning calorimetry (DSC) at a heating rate of 0.333 K/s. The Vickers hardness was measured with a Shangguang HX2100 microhardness tester under a load of 300 g. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-cast Co48 Cr15 Mo14 C15 B6 alloy rod of d = 2 mm and Co48 Cr15 Mo14 C15 B6 Er2 alloy rods of d = 2 mm and 10 mm, where d denotes the rod diameter. The XRD traces reveal only a broad diffuse halo, and no peaks corresponding to crystalline phases are visible, indicating the formation of a mostly amorphous phase. Fig. 2 shows DSC curves of as-cast Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 glassy rods of d = 2 mm taken at a heating rate of 0.333 K/s. Both DSC scans exhibit distinct glass transition and wide supercooled liquid region, followed by at least two exothermic events characteristic of crystallization. The glass transition temperature (Tg ) and onset temperature of the first crystallization (Tx1 ) are marked with arrows in Fig. 1. The supercooled liquid temperature region (Tx ) is defined as Tx1 − Tg . Tg and Tx1 of the Er-free alloy are 819 K and 895 K, respectively, and significantly increase to 848 K and 933 K, respectively, with the addition of 2% Er. As a result, the
H. Men et al. / Materials Science and Engineering A 449–451 (2007) 538–540
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Fig. 3. Melting and solidifying behaviors of (a) Co50 Cr15 Mo14 C15 B6 and (b) Co48 Cr15 Mo14 C15 B6 Er2 alloys.
Fig. 1. XRD patterns of as-cast Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 alloy rods with varied diameters (d).
value of Tx increases from 76 K for the Er-free alloy to 85 K for the Er-containing alloy. The present Co-based glassy alloys exhibit both high glass transition temperature and large supercooled liquid region, which are greatly favorable for extensive application of the BMGs as structural materials due to the high thermal stability. The melting and solidifying behaviors of Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 alloys are shown in Fig. 3. Two alloys exhibit a nearly identical eutectic temperature (Tm ), 1355 K for the Er-free alloy and 1350 K for the Er-containing alloy, however the apparent liquidus temperature (Tl ) of the Erfree alloy decreases from 1417 K to 1394 K with the addition of Er. As a result, the temperature span between onset and offset temperature of the melting reduces from 44 K for the Er-free alloy to 62 K for the Er-containing alloy. The reduced glass transition temperature (Trg ) is defined as Tg /Tl , and the Er-containing glassy alloy shows a slightly larger Trg than the Er-free alloy, 0.61 versus 0.58. In addition, only one major exothermic peak can be observed on the cooling traces of two alloys. Therefore, it can be confirmed that two alloy compositions are at or very close to the eutectics. With the addition of Er, the
Fig. 2. DSC curves of as-cast (a) Co50 Cr15 Mo14 C15 B6 and (b) Co48 Cr15 Mo14 C15 B6 Er2 BMGs with a diameter of 2 mm.
onset ) for the Er-free alloy onset temperature of solidification (Tsol reduces by about 38 K, from 1388 K to 1350 K. It is implied that the undercooling ability of the Er-free liquid is enhanced due to the addition of Er. In other words, the undercooled liquid of the Er-containing alloy is more stable than that of the Er-free alloy. In general, the increase of Trg and the enhanced stability of undercooled liquid indicate better glass-forming ability. The microhardness (Hv ) is determined to be about 1.56 GPa for the Er-free BMG with a diameter of 2 mm and 1.58 GPa for the Er-containing BMG with a diameter of 10 mm. A tensile yield strength (σ y ) can be estimated according to σ y = 3Hv . Based on the microhardness, the tensile yield strength of the two glassy alloys can be determined to be about 4.7 GPa, which approximates that of Co–Fe–Ta–B BMG. The microhardness of the Co-based BMGs is much higher than 0.7 GPa of Ti–Zr–Cu–Ni–Be BMG [11] and 0.66 GPa of Cu–Zr–Ti BMG [6]. Therefore, the present Co-based BMGs with ultra-high strength and strong GFA may promise extraordinary application potential as structural materials. The values of Tg and Tx of the Er-free glassy alloy increase apparently from 819 K and 76 K to 848 K and 85 K, respectively, with the addition of 2% Er, suggesting that the glassy phase and supercooled liquid of the Er-containing alloy are more stable. The atomic diameter of Er is the largest among Co, Cr, Mo, C, B and Er: 0.125 nm, 0.128 nm, 0.140 nm, 0.077 nm, 0.097 nm and 0.175 nm, respectively, and it is believed that the addition of a small amount of Er can lead to a more densely packed atomic configuration of amorphous phase and liquid. Therefore, the undercooled liquid of the Er-free alloy is stabionset due to the addition lized, as reflected by the reduction of Tsol of Er, and the Er-containing alloy exhibits a relatively higher Trg . The large value of Trg reflects a low nucleating rate in the undercooled liquid, and a low critical cooling rate for glass formation. According to ref. [12], the critical cooling rate can be estimated to be less than 40 K/s for the Er-containing alloy, much lower than about 1000 K/s of the Er-free alloy. As such, the Ercontaining alloy exhibits much stronger GFA than the Er-free alloy.
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H. Men et al. / Materials Science and Engineering A 449–451 (2007) 538–540
4. Summary In summary, Co50 Cr15 Mo14 C15 B6 and Co48 Cr15 Mo14 C15 B6 Er2 glassy alloys exhibit high thermal stability. Tg , Tx1 and Trg of the Er-free glassy alloy are 819 K, 76 K and 0.58, respectively, and increase to 848 K, 85 K and 0.61, respectively, with the addition of 2 at.% Er. The microhardness is as high as about 1.56 GPa for the Er-free BMG and 1.58 GPa for the Er-containing BMG, respectively.
References [1] [2] [3] [4] [5] [6] [7] [8]
Acknowledgement This work was supported by the National Nature Science Foundation of China (Grant Nos. 50225103 and 50471001).
[9] [10] [11] [12]
A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 31 (1991) 177. T. Zhang, A. Inoue, T. Masumoto, Mater. Trans. JIM 32 (1991) 1005. A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. Z.P. Lu, C.T. Liu, J.R. Thompson, W.D. Porter, Phys. Rev. Lett. 92 (2004) 245503–2455031. V. Ponnambalam, S.J. Poon, G.J. Shiflet, J. Mater. Res. 19 (2004) 1320. A. Inoue, W. Zhang, T. Zhang, K. Kurosaka, Mater. Trans. 42 (2001) 1149. D.H. Xu, G. Duan, W.L. Johnson, Phys. Rev. Lett. 92 (2004) 245504–2455041. A. Inoue, B.L. Shen, A.R. Yavari, A.L. Greer, J. Mater. Res. 18 (2003) 1487. A. Inoue, B.L. Shen, H. Koshiba, H. Kato, A.R. Yavari, Acta Mater. 52 (2004) 1631. H. Men, S.J. Pang, T. Zhang, J. Mater. Res. 21 (2006) 958. J.M. Park, Y.C. Kim, W.T. Kim, D.H. Kim, Mater. Trans. 45 (2004) 595. X.H. Lin, W.L. Johnson, J. Appl. Phys. 78 (1995) 6514.