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Overheating effects on thermal stability and mechanical properties of Cu36Zr48Ag8Al8 bulk metallic glass
Materials and Design 31 (2010) 1029–1032
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Overheating effects on thermal stability and mechanical properties of Cu36Zr48Ag8Al8 bulk metallic glass Yanchun Zhao a,*, Shengzhong Kou a,b,*, Hongli Suo b, Renjun Wang a, Yutian Ding a a b
State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, PR China The Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing University of Technology, Beijing 100022, PR China
a r t i c l e
i n f o
Article history: Received 23 May 2009 Accepted 17 July 2009 Available online 21 July 2009
a b s t r a c t A pronounced effect of overheating is observed on the thermal stability and mechanical properties of Cu36Zr48Ag8Al8 bulk metallic glass. Higher overheated temperature enhances the thermal stability of bulk amorphous alloys, corresponding to higher specific-heat capacity and the smaller initial defect concentration. And a threshold overheating temperature is found for the fully amorphous structure. Bulk amorphous alloys exhibit good compressive plasticity at small overheat levels, whereas the compressive fracture strength and micro-hardness exhibit a significant increase first and then a slightly decrease. Mechanical properties of BMGs can be tailored in certain extent by controlling the overheated level, which is correlated with free volume and residual stresses. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) have been considered as the promising structural and functional materials because of their unique physical and mechanical properties. The new alloy compositions of BMGs have attracted great attention in the past two decades. The good glass formers, for example, Pd40Cu30Ni10P20 [1], Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy1) [2,3] and La65Al14 (Cu5/6Ag1/6)11(Ni1/2Co1/2)10 [4], were reported to have critical diameters up to several centimetres. However, in contrast to most thermoplastics, BMG liquid with higher viscosity is metastable and eventually crystallizes, which limits the processing time and cooling rate, and ultimately leads to the maximum limiting casting thickness of a BMG [5–8]. The cooling rate and processing time for fabrication which is determined by the crystallization position in the time–temperature-transformation (TTT) diagram, affect the thermomechanics and mechanical behaviors of BMGs. Besides, the melt thermal history of master alloy, i.e., the overheated level, which is closely related to thermal stability and mechanical properties of BMGs, is seldom reported. Recently, the best quaternary glass former in Zr–Cu based alloy system, Cu36Zr48Al8Ag8 BMG with a diameter up to 25 mm was prepared by copper mold injection casting. The Cu36Zr48Al8Ag8 alloy is a strong liquid with a fragility parameter m of 33, resulting in high glass forming ability (GFA) and high fracture strength with a distinct plastic strain [9]. In this paper, Cu36Zr48Al8Ag8 is fabricated by copper mould suction casting at different casting voltage. The * Corresponding authors. Address: State Key Laboratory of Gansu Advanced Nonferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, PR China. Tel.: +86 931 2973942; fax: +86 931 2806962. E-mail addresses: [email protected] (Y. Zhao), [email protected] (S. Kou). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.07.028
effect of overheated level on the thermal stability and mechanical properties of Cu36Zr48Al8Ag8 BMG is investigated under the same cooling rate and overheat time. 2. Experimental Multicomponent alloy ingots with nominal compositions of Cu36Zr48Ag8Al8 were prepared from pure elemental Cu, Zr, Al and Ag of 99.9 mass% purity by suspend melting under an argon atmosphere using a water-cooled Cu mold. The master alloy was remelted three times in order to obtain chemical homogeneity. Five bulk cylindrical rods with diameters of 3 mm were fabricated by copper mold suction casting at different casting voltage holding for 2 min, respectively. The amorphous structure of the sample was identified by X-ray diffraction with Cu Ka radiation at 40 kV to 30 mA. Thermal stability associated with glass transition, supercooled liquid region and crystallization was examined by differential scanning calorimetry (DSC) at a constant heating rate of 20 K min1 in a flowing argon atmosphere. And the liquidus temperature (Tl) was examined by differential thermal analyzer (DTA) at a constant cooling rate of 20 K min1. Mechanical properties under compressive deformation mode were conducted on a computer-controlled, servo-hydraulic MTS 810 testing machine at astrain rate of 1 104 s1 at room temperature. The Vickers hardness was measured by a MH-5 Vickers micro-hardness tester with a load of 200 g holding for 15 s. 3. Results and discussions Fig. 1 shows XRD patterns of the Cu36Zr48Ag8Al8 rods fabricated at different casting voltage. As the samples cooled by casting
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Y. Zhao et al. / Materials and Design 31 (2010) 1029–1032
voltage for 7, 8, 9 and 10 kV, respectively, it can be seen that the XRD patterns consist of only one broad diffuse peak between diffraction angles 30° and 45°, indicating that these samples have a glassy structure. As the casting voltage was lowered to 6 kV, though it shows a broad diffuse background, some small peaks which cannot be indexed unambiguously are detected from the pattern. Thus, the critical voltage for fully amorphous structure of Cu36Zr48Al8Ag8 is at least 6 kV, below which may have an intersection with the crystallization position. A threshold overheating temperature is found for the amorphous alloys, above which there is a drastic increase in the undercooling level and the crystallization times [10]. If the liquid is cooled from a temperature below the threshold overheating temperature the heterogeneous sites are never dissolved and induce the crystallization process static heterogeneous nucleation [11–13]. The shape and the position of the TTT curve in the temperature–time space is determined by the nucleation and growth mechanism. Crystallization changes from a nucleation controlled mechanism at high temperatures to a growth controlled mechanism at low temperatures [14]. So impurities acting as heterogeneous sites strongly affect the nucleation process in the upper part of the TTT curve. This in turn will cause the nose of the TTT curve to be at a higher temperature and shifted toward smaller crystallization time [15]. Fig. 2 shows the DSC and DTA curves of the Cu36Zr48Al8Ag8 rod cooled at different casting voltage. Tg and Tl decreased monotonously as the casting voltage was raised from 6 kV to 10 kV, whereas Tx exhibits an opposite trend. DTx and Trg increased from 102 to 108 and 0.602 to 0.605, respectively. That is, higher casting voltage enhances the thermal stability of bulk amorphous alloys. As aforementioned, the heterogeneous effects are suppressed effectively at large overheat levels. Furthermore, the viscosity decreased and homogenized structure of master alloy formed in the molten state, which is beneficial to improve the thermal stability [16]. Additionally, free volume differences may exist in nominally identical metallic glasses in the as-prepared state due to differences in their processing conditions. Cooled at higher casting voltage under the same cooling rate, more atoms have time to move to their local ordered equilibrium positions, thereby a more ordered packing atomic structure forms during cooling from the melt and, in turn, the obtained glassy sample possesses a smaller amount of free volume [17]. According to the free-volume theory, the difference of the glass transition process in the same system is relative to the difference of the excess free volume frozen [18].
Fig. 2. (a) DSC and (b) DTA curves of as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage.
Fig. 1. X-ray diffraction patterns of as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage.
The specific-heat capacity DC p is related to the reduced free volume x by DCp = bdx/dT, the parameter b is a constant. The reduced free volume x is related to the defect concentration by cD = exp (1/x) [19,20]. The change in the defect concentration is governed by the differential equation dcD =dt ¼ kr cD ðcD c0D Þ [21]. kr has the form kr = k0 exp (Ef/kBT), where the parameters Ef and k0 are constant for the same glass system [22]. The changes of the DSC curves are only related to initial defect concentration c0D [22], which corresponds to the excess free volume x0, as c0D ¼ exp ð1=x0 Þ. Thus, in Fig. 3, higher DCp correspond to the smaller c0D , which is consistent with the highly dense random packed microstructure, good glass forming ability, and higher thermal stability. From Fig. 4, a pronounced influence of overheating is observed on the mechanical behaviors. The compressive fracture strength (rcf), plastic strain (ep), elastic modulus (E) and micro-hardness (Hv) of the samples cooled at different suction voltages are in Table 1. ep increased monotonously as the casting voltage was lowered, whereas rcf, E and Hv exhibit a significant increase from 10 kV to 7 kV, then a slightly decrease at 6 kV.
Y. Zhao et al. / Materials and Design 31 (2010) 1029–1032
Fig. 3. Specific-heat capacity curves of glass transition in as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage.
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tion tendency of compressive plasticity is accordant with the conjecture, whereas the fracture strength and hardness exhibits a different trend. This is due to the different residual stresses in the bulk amorphous alloys. Residual compressive stresses up to several 100 MPa can form on the surface of BMGs during the casting process [27]. Moreover, larger temperature differences induce higher thermal residual stresses under the same cooling rate. Such residual stresses will superimpose on any applied stresses, potentially affecting the fracture and fatigue behavior. That is, higher residual stresses form on the samples cooled at higher casting voltage, and thus resulting in decreases of the fracture strength, hardness and compressive plasticity. Based on the results of the present study, it is clear that the mechanical properties of bulk metallic glasses fabricated by different casting process are determined by the interaction of the free volume and residual stresses. By reducing the free volume, one can enhance fracture strength and hardness while sacrificing compressive plasticity. However, both the fracture strength, hardness and compressive plasticity can be enhanced by controlling the residual stresses. Thus, bulk amorphous alloys exhibit good compressive plasticity at small overheat levels. And as the casting voltage was lowered from 10 kV to 7 kV, the reduction of residual stresses show more effect on rcf and Hv, whereas free volume has more influence below 7 kV. Thus, careful control of free volume and residual stresses can potentially be used to improve the overall properties of bulk metallic glasses and obtain desired characteristics [26]. Finally, by controlling overheated level though a change of casting voltage, mechanical properties of BMGs can be tailored. 4. Conclusions
Fig. 4. Compressive stress–strain curves of as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage.
As aforementioned, the higher overheated alloy with lower initial defect concentration and less excess free volume, have a highly dense random packed microstructure. The deformation of metallic glasses requires the existence of free volume, i.e., extra volume relative to a fully dense glass that is frozen into the atomic structure and allows physical space for atomic movement under mechanical loading [20,23,24]. Even a tiny change in the free volume could induce a dramatic effect on flow behavior. Since free volume is needed to allow metallic glasses to deform, a reduction in free volume hinders plastic deformation [25,26]. Thus, the sample cooled at higher suction voltages may exhibit higher fracture strength and hardness, while sacrificing compressive plasticity. The varia-
Based on a study of the thermal stability and mechanical properties of Cu36Zr48Ag8Al8 bulk metallic glasses fabricated by different casting process, with specific attention paid to the effects of free volume and residual stresses variations, the effect of overheating is found. For as-cast Cu36Zr48Ag8Al8 BMG alloy, higher overheated level enhances the thermal stability and improves the crystalline threshold, while sacrificing compressive plasticity. Residual stresses makes severe affect on the samples cooled at higher casting voltage, resulting in decreases of the fracture strength, hardness and compressive plasticity. Whereas free volume of samples fabricated at lower overheated level, has greater influence upon mechanical behaviors, causing increases of fracture strength, hardness and a decrease of compressive plasticity. Finally, mechanical properties of BMGs can be tailored by controlling overheated level in certain extent. Acknowledgements The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 50371016) and the Funds for International Cooperation and Exchange of the National Natural Science Foundation of China (Grant No. 50611130629). References
Table 1 The mechanical properties of as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage. Casting voltage (kV)
rcf (MPa)
ep (%)
E (GPa)
Hv
6 7 8 9 10
1730 1865 1624 1476 1252
2.2 1.7 1.3 0.9 0.6
106.5 109.4 104.2 103.6 101.9
653.4 687.7 642.5 635.1 604.6
[1] Nishiyama N, Inoue A. Flux treated Pd–Cu–Ni–P amorphous alloy having low critical cooling rate. Mater Trans JIM 1997;38:464–72. [2] Peker A, Johnson WL. A highly processable metallic glass: Zr41.2Ti13.8Cu12.5 Ni10.0Be22.5. Appl Phys Lett 1993;63:2342–4. [3] Waniuk TA, Schroers J, Johnson WL. Critical cooling rate and thermal stability of Zr–Ti–Cu–Ni–Be alloys. Appl Phys Lett 2001;78:1213–5. [4] Jiang QK, Zhang GQ, Yang L, Wang XD, Saksl K, Franz H, et al. La-based bulk metallic glasses with critical diameter up to 30 mm. Acta Mater 2007;55:4409–18. [5] Schroers J, Nguyen T, Croopnick GA. A novel metallic glass composite synthesis method. Scr Mater 2007;56:177–80.
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[6] Zhang B, Zhao DQ, Pan MX, Wang WH, Greer AL. Amorphous metallic plastic. Phy Rev Lett 2005;94:205502–6. [7] Waniuk TA, Schroers J, Johnson WL. Timescales of crystallization and viscous flow of the bulk glass-forming Zr–Ti–Ni–Cu–Be alloys. Phys Rev B 2003;67:184203–12. [8] Schroers J, Johnson WL. Highly processable bulk metallic glass-forming alloys in the Pt–Co–Ni–Cu–P system. Appl Phys Lett 2004;84:3666–8. [9] Zhang QS, Zhang W, Inoue A. Preparation of Cu36Zr48Ag8Al8 bulk metallic glass with a diameter of 25 mm by copper mold casting. Mater Trans 2007;48:629–31. [10] Mukherjee S, Zhou Z, Schroers J, Johnson WL, Rhim WK. Overheating threshold and its effect on time–temperature-transformation diagrams of zirconium based bulk metallic glasses. Appl Phys Lett 2004;84:5010–2. [11] Drehman AJ, Greer AL. Kinetics of crystal nucleation and growth in Pd40Ni40P20 glass. Acta Metall 1984;32:323–32. [12] Nishiyama N, Inoue A. Supercooling investigation and critical cooling rate for glass formation in Pd–Cu–Ni–P alloy. Acta Mater 1999;47:1487–95. [13] Shen TD, Schwarz RB. Bulk ferromagnetic glasses prepared by flux melting and water quenching. Appl Phys Lett 1999;75:49–51. [14] Schroers J, Busch R, Johnson WL. Repeated crystallization in undercooled Zr41Ti14Cu12.5Ni10Be22.5 liquids. Appl Phys Lett 2000;76:2343–5. [15] Kim YJ, Busch R, Johnson WL, Rulison AJ, Rhim WK. Experimental determination of a time–temperature-transformation diagram of the undercooled Zr41Ti14Cu12.5Ni10Be22.5 alloy using the containerless electrostatic processing technique. Appl Phys Lett 1996;68:1057–9. [16] Schroers J, Wu Y, Johnson WL. Heterogeneous Influences on the crystallization of Pd43Ni10Cu27P20. Philos Mag A 2002;82:1207–17.
[17] Jiang WH, Liu FX, Wang YD, Zhang HF, Choo H, Liaw PK. Comparison of mechanical behavior between bulk and ribbon Cu-based metallic glasses. Mater Sci Eng A 2006;430:350–4. [18] Van den Beukel A, Sietsma J. The glass transition as a free volume related kinetic phenomenon. Acta Metall Mater 1990;38:383–9. [19] Cohen MH, Turnbull D. Molecular transport in liquids and glasses. J Chem Phys 1959;31:1164–9. [20] Spaepen F. A microscopic mechanism for steady state inhomogeneous flow in metallic glasses. Acta Metall 1977;25:407–15. [21] Taso SS, Spaepen F. Structural relaxation of a metallic glass near equilibrium. Acta Metall 1985;33:881–9. [22] Wen P, Tang MB, Pan MX, Zhao DQ, Zhang Z, Wang WH. Calorimetric glass transition in bulk metallic glass forming Zr–Ti–Cu–Ni–Be alloys as a freevolume-related kinetic phenomenon. Phys Rev B 2003;67:212201–5. [23] Argon AS. Plastic deformation in metallic glasses. Acta Metall 1979;27:47–58. [24] Steif PS, Spaepen F. Hutchinson JW. Strain localization in amorphous metals Acta Metall 1982;30:447–55. [25] Wu TW, Spaepen F. The relation between enbrittlement and structural relaxation of an amorphous metal. Philos Mag B 1990;61:739–50. [26] Launey ME, Busch R, Kruzic JJ. Effect of free volume changes and residual stresses on the fatigue and fracture behavior of a Zr–Ti–Ni–Cu–Be bulk metallic glass. Acta Mater 2008;56:500–10. [27] Aydiner CC, Ustundag E, Prime MB, Peker A. Modeling and measurement of residual stresses in a bulk metallic glass plate. J Non-Cryst Solids 2003;316:82–95.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Overheating effects on thermal stability and mechanical properties of Cu36Zr48Ag8Al8 bulk metallic glass Yanchun Zhao a,*, Shengzhong Kou a,b,*, Hongli Suo b, Renjun Wang a, Yutian Ding a a b
State Key Laboratory of Gansu Advanced Non-ferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, PR China The Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing University of Technology, Beijing 100022, PR China
a r t i c l e
i n f o
Article history: Received 23 May 2009 Accepted 17 July 2009 Available online 21 July 2009
a b s t r a c t A pronounced effect of overheating is observed on the thermal stability and mechanical properties of Cu36Zr48Ag8Al8 bulk metallic glass. Higher overheated temperature enhances the thermal stability of bulk amorphous alloys, corresponding to higher specific-heat capacity and the smaller initial defect concentration. And a threshold overheating temperature is found for the fully amorphous structure. Bulk amorphous alloys exhibit good compressive plasticity at small overheat levels, whereas the compressive fracture strength and micro-hardness exhibit a significant increase first and then a slightly decrease. Mechanical properties of BMGs can be tailored in certain extent by controlling the overheated level, which is correlated with free volume and residual stresses. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Bulk metallic glasses (BMGs) have been considered as the promising structural and functional materials because of their unique physical and mechanical properties. The new alloy compositions of BMGs have attracted great attention in the past two decades. The good glass formers, for example, Pd40Cu30Ni10P20 [1], Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy1) [2,3] and La65Al14 (Cu5/6Ag1/6)11(Ni1/2Co1/2)10 [4], were reported to have critical diameters up to several centimetres. However, in contrast to most thermoplastics, BMG liquid with higher viscosity is metastable and eventually crystallizes, which limits the processing time and cooling rate, and ultimately leads to the maximum limiting casting thickness of a BMG [5–8]. The cooling rate and processing time for fabrication which is determined by the crystallization position in the time–temperature-transformation (TTT) diagram, affect the thermomechanics and mechanical behaviors of BMGs. Besides, the melt thermal history of master alloy, i.e., the overheated level, which is closely related to thermal stability and mechanical properties of BMGs, is seldom reported. Recently, the best quaternary glass former in Zr–Cu based alloy system, Cu36Zr48Al8Ag8 BMG with a diameter up to 25 mm was prepared by copper mold injection casting. The Cu36Zr48Al8Ag8 alloy is a strong liquid with a fragility parameter m of 33, resulting in high glass forming ability (GFA) and high fracture strength with a distinct plastic strain [9]. In this paper, Cu36Zr48Al8Ag8 is fabricated by copper mould suction casting at different casting voltage. The * Corresponding authors. Address: State Key Laboratory of Gansu Advanced Nonferrous Metal Materials, Lanzhou University of Technology, Lanzhou 730050, PR China. Tel.: +86 931 2973942; fax: +86 931 2806962. E-mail addresses: [email protected] (Y. Zhao), [email protected] (S. Kou). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.07.028
effect of overheated level on the thermal stability and mechanical properties of Cu36Zr48Al8Ag8 BMG is investigated under the same cooling rate and overheat time. 2. Experimental Multicomponent alloy ingots with nominal compositions of Cu36Zr48Ag8Al8 were prepared from pure elemental Cu, Zr, Al and Ag of 99.9 mass% purity by suspend melting under an argon atmosphere using a water-cooled Cu mold. The master alloy was remelted three times in order to obtain chemical homogeneity. Five bulk cylindrical rods with diameters of 3 mm were fabricated by copper mold suction casting at different casting voltage holding for 2 min, respectively. The amorphous structure of the sample was identified by X-ray diffraction with Cu Ka radiation at 40 kV to 30 mA. Thermal stability associated with glass transition, supercooled liquid region and crystallization was examined by differential scanning calorimetry (DSC) at a constant heating rate of 20 K min1 in a flowing argon atmosphere. And the liquidus temperature (Tl) was examined by differential thermal analyzer (DTA) at a constant cooling rate of 20 K min1. Mechanical properties under compressive deformation mode were conducted on a computer-controlled, servo-hydraulic MTS 810 testing machine at astrain rate of 1 104 s1 at room temperature. The Vickers hardness was measured by a MH-5 Vickers micro-hardness tester with a load of 200 g holding for 15 s. 3. Results and discussions Fig. 1 shows XRD patterns of the Cu36Zr48Ag8Al8 rods fabricated at different casting voltage. As the samples cooled by casting
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voltage for 7, 8, 9 and 10 kV, respectively, it can be seen that the XRD patterns consist of only one broad diffuse peak between diffraction angles 30° and 45°, indicating that these samples have a glassy structure. As the casting voltage was lowered to 6 kV, though it shows a broad diffuse background, some small peaks which cannot be indexed unambiguously are detected from the pattern. Thus, the critical voltage for fully amorphous structure of Cu36Zr48Al8Ag8 is at least 6 kV, below which may have an intersection with the crystallization position. A threshold overheating temperature is found for the amorphous alloys, above which there is a drastic increase in the undercooling level and the crystallization times [10]. If the liquid is cooled from a temperature below the threshold overheating temperature the heterogeneous sites are never dissolved and induce the crystallization process static heterogeneous nucleation [11–13]. The shape and the position of the TTT curve in the temperature–time space is determined by the nucleation and growth mechanism. Crystallization changes from a nucleation controlled mechanism at high temperatures to a growth controlled mechanism at low temperatures [14]. So impurities acting as heterogeneous sites strongly affect the nucleation process in the upper part of the TTT curve. This in turn will cause the nose of the TTT curve to be at a higher temperature and shifted toward smaller crystallization time [15]. Fig. 2 shows the DSC and DTA curves of the Cu36Zr48Al8Ag8 rod cooled at different casting voltage. Tg and Tl decreased monotonously as the casting voltage was raised from 6 kV to 10 kV, whereas Tx exhibits an opposite trend. DTx and Trg increased from 102 to 108 and 0.602 to 0.605, respectively. That is, higher casting voltage enhances the thermal stability of bulk amorphous alloys. As aforementioned, the heterogeneous effects are suppressed effectively at large overheat levels. Furthermore, the viscosity decreased and homogenized structure of master alloy formed in the molten state, which is beneficial to improve the thermal stability [16]. Additionally, free volume differences may exist in nominally identical metallic glasses in the as-prepared state due to differences in their processing conditions. Cooled at higher casting voltage under the same cooling rate, more atoms have time to move to their local ordered equilibrium positions, thereby a more ordered packing atomic structure forms during cooling from the melt and, in turn, the obtained glassy sample possesses a smaller amount of free volume [17]. According to the free-volume theory, the difference of the glass transition process in the same system is relative to the difference of the excess free volume frozen [18].
Fig. 2. (a) DSC and (b) DTA curves of as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage.
Fig. 1. X-ray diffraction patterns of as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage.
The specific-heat capacity DC p is related to the reduced free volume x by DCp = bdx/dT, the parameter b is a constant. The reduced free volume x is related to the defect concentration by cD = exp (1/x) [19,20]. The change in the defect concentration is governed by the differential equation dcD =dt ¼ kr cD ðcD c0D Þ [21]. kr has the form kr = k0 exp (Ef/kBT), where the parameters Ef and k0 are constant for the same glass system [22]. The changes of the DSC curves are only related to initial defect concentration c0D [22], which corresponds to the excess free volume x0, as c0D ¼ exp ð1=x0 Þ. Thus, in Fig. 3, higher DCp correspond to the smaller c0D , which is consistent with the highly dense random packed microstructure, good glass forming ability, and higher thermal stability. From Fig. 4, a pronounced influence of overheating is observed on the mechanical behaviors. The compressive fracture strength (rcf), plastic strain (ep), elastic modulus (E) and micro-hardness (Hv) of the samples cooled at different suction voltages are in Table 1. ep increased monotonously as the casting voltage was lowered, whereas rcf, E and Hv exhibit a significant increase from 10 kV to 7 kV, then a slightly decrease at 6 kV.
Y. Zhao et al. / Materials and Design 31 (2010) 1029–1032
Fig. 3. Specific-heat capacity curves of glass transition in as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage.
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tion tendency of compressive plasticity is accordant with the conjecture, whereas the fracture strength and hardness exhibits a different trend. This is due to the different residual stresses in the bulk amorphous alloys. Residual compressive stresses up to several 100 MPa can form on the surface of BMGs during the casting process [27]. Moreover, larger temperature differences induce higher thermal residual stresses under the same cooling rate. Such residual stresses will superimpose on any applied stresses, potentially affecting the fracture and fatigue behavior. That is, higher residual stresses form on the samples cooled at higher casting voltage, and thus resulting in decreases of the fracture strength, hardness and compressive plasticity. Based on the results of the present study, it is clear that the mechanical properties of bulk metallic glasses fabricated by different casting process are determined by the interaction of the free volume and residual stresses. By reducing the free volume, one can enhance fracture strength and hardness while sacrificing compressive plasticity. However, both the fracture strength, hardness and compressive plasticity can be enhanced by controlling the residual stresses. Thus, bulk amorphous alloys exhibit good compressive plasticity at small overheat levels. And as the casting voltage was lowered from 10 kV to 7 kV, the reduction of residual stresses show more effect on rcf and Hv, whereas free volume has more influence below 7 kV. Thus, careful control of free volume and residual stresses can potentially be used to improve the overall properties of bulk metallic glasses and obtain desired characteristics [26]. Finally, by controlling overheated level though a change of casting voltage, mechanical properties of BMGs can be tailored. 4. Conclusions
Fig. 4. Compressive stress–strain curves of as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage.
As aforementioned, the higher overheated alloy with lower initial defect concentration and less excess free volume, have a highly dense random packed microstructure. The deformation of metallic glasses requires the existence of free volume, i.e., extra volume relative to a fully dense glass that is frozen into the atomic structure and allows physical space for atomic movement under mechanical loading [20,23,24]. Even a tiny change in the free volume could induce a dramatic effect on flow behavior. Since free volume is needed to allow metallic glasses to deform, a reduction in free volume hinders plastic deformation [25,26]. Thus, the sample cooled at higher suction voltages may exhibit higher fracture strength and hardness, while sacrificing compressive plasticity. The varia-
Based on a study of the thermal stability and mechanical properties of Cu36Zr48Ag8Al8 bulk metallic glasses fabricated by different casting process, with specific attention paid to the effects of free volume and residual stresses variations, the effect of overheating is found. For as-cast Cu36Zr48Ag8Al8 BMG alloy, higher overheated level enhances the thermal stability and improves the crystalline threshold, while sacrificing compressive plasticity. Residual stresses makes severe affect on the samples cooled at higher casting voltage, resulting in decreases of the fracture strength, hardness and compressive plasticity. Whereas free volume of samples fabricated at lower overheated level, has greater influence upon mechanical behaviors, causing increases of fracture strength, hardness and a decrease of compressive plasticity. Finally, mechanical properties of BMGs can be tailored by controlling overheated level in certain extent. Acknowledgements The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 50371016) and the Funds for International Cooperation and Exchange of the National Natural Science Foundation of China (Grant No. 50611130629). References
Table 1 The mechanical properties of as-cast Cu36Zr48Ag8Al8 samples fabricated at different suction casting voltage. Casting voltage (kV)
rcf (MPa)
ep (%)
E (GPa)
Hv
6 7 8 9 10
1730 1865 1624 1476 1252
2.2 1.7 1.3 0.9 0.6
106.5 109.4 104.2 103.6 101.9
653.4 687.7 642.5 635.1 604.6
[1] Nishiyama N, Inoue A. Flux treated Pd–Cu–Ni–P amorphous alloy having low critical cooling rate. Mater Trans JIM 1997;38:464–72. [2] Peker A, Johnson WL. A highly processable metallic glass: Zr41.2Ti13.8Cu12.5 Ni10.0Be22.5. Appl Phys Lett 1993;63:2342–4. [3] Waniuk TA, Schroers J, Johnson WL. Critical cooling rate and thermal stability of Zr–Ti–Cu–Ni–Be alloys. Appl Phys Lett 2001;78:1213–5. [4] Jiang QK, Zhang GQ, Yang L, Wang XD, Saksl K, Franz H, et al. La-based bulk metallic glasses with critical diameter up to 30 mm. Acta Mater 2007;55:4409–18. [5] Schroers J, Nguyen T, Croopnick GA. A novel metallic glass composite synthesis method. Scr Mater 2007;56:177–80.
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