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Devitrification behavior and mechanical property of Cu54Ni6Zr22Ti18 glass powders
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Materials Letters 62 (2008) 323 – 326 www.elsevier.com/locate/matlet
Devitrification behavior and mechanical property of Cu54Ni6Zr22Ti18 glass powders Taek-Soo Kim ⁎, J.K. Lee, J.C. Bae Advanced Materials R&D Center, Korea Institute of Industrial Technology, Incheon 406-130, South Korea Received 24 February 2007; accepted 12 May 2007 Available online 18 May 2007
Abstract Cu54Ni6Zr22Ti18 amorphous materials were prepared using gas atomization, followed by spark plasma sintering (SPS). The crystallization behavior and mechanical properties were studied using X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). With annealing the gas atomized powders to the temperatures of 837 K and 909 K, fine particles such as Cu10Zr7 and Cu51Zr14 were precipitated and embedded in the amorphous matrix, respectively. Each size of precipitates at 909 K is about 50 nm and 20 nm, respectively. Microstructural variation and mechanical properties were investigated as a function of the initial powder size and spark plasma sintering (SPS) pressure. © 2007 Elsevier B.V. All rights reserved. Keywords: Metallic glasses; Thermal behavior; Crystallization; Rapid solidification
1. Introduction Research on bulk metallic glass (BMG) has been vividly performed due to its ultra high strength more than 2 GPa at most of composition, low friction coefficient, excellent corrosion resistance, ultra high elastic modulus over 2% [1,2]. However, it is necessary for the BMG to overcome the low size limit for the industrialization, because the typical size of cast BMG is less than 15 mm in diameter so far. In addition, further enhancement of the glass forming ability – or the size – becomes slow in progress only by the alloy design. Thus, it needs to find the breakthrough by altering the cast process in to other one. Recently, powder metallurgy process (PM) is regarded as an alternative to overcome the problem, because of its rapid development. There were a couple of reports that the powders less than 100 μm in diameter were consolidated without loosing their amorphous phases [3–6]. Care to be taken during the PM processing of amorphous powders, because the amorphous powders should be consolidated within the supercooled liquid temperature range which is relatively narrow. The temperature ⁎ Corresponding author. Fax: +82 32 8500 409. E-mail address: [email protected] (T.-S. Kim). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.05.035
Fig. 1. SEM photos of Cu54Ni6Zr22Ti18 amorphous powders with 46–63 (a) and 91–150 μm (b).
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Fig. 2. DSC traces of Cu54Ni6Zr22Ti18 amorphous powders as a function of the powder size group.
is in between Tg, glass transition temperature, and Tx, crystallization temperature [7–9]. However, no systematic study on the new and low cost Cu54Ni6Zr22Ti18 composition has been reported yet. In this manuscript, spark plasma sintering (SPS) process was used to consolidate the Cu54Ni6Zr22Ti18 amorphous powders prepared by a gas atomization. The crystallization behavior and mechanical properties were investigated. 2. Experimental In order to synthesize the Cu54Ni6Zr22Ti18 amorphous powders using a high-pressure gas atomization process, the master alloy prepared by a vacuum plasma melter (VPM) was remelted in a vacuum radio-active furnace of the atomizer 200 K above the liquidus temperature down to 5.7 × 10− 5 Torr. The melt was bottom poured through a boron nitride melt delivery nozzle of 5 mm in diameter into an annular Ar gas
atomizer operating at a pressure of 5 MPa. The melt flow rate, as estimated from operating time and weight of atomized melt, was about 1.5 kg/min. Size distribution of alloy powders was measured by a conventional sieving method. In order to know the powder size effect on the glass forming ability and materials properties, the as-solidified powders were divided into four groups depending on size; group A — powders larger than 32 μm and smaller than 46 μm, group B — powders larger than 45 μm and smaller than 64 μm, group C — powders larger than 63 μm and smaller than 91 μm and group D — powders larger than 90 μm and smaller than 151 μm. The each group of powders was pre-compacted, and then consolidated by SPS method at 843 K for 60 s with an external pressure 280 MPa. The size of round shape samples SPSed is 20 mm in diameter and 5 mm in thickness in a disc. The structure of powders as atomized and bulks consolidated was characterized using a X-ray diffractometer (XRD) with monochromatic Cu-Kα radiation over 2θ range of 20°–80° at power of 5 kW in a Philips 1729. The microstructures were examined by a scanning electron microscope (SEM; JSM 5410) and transmission electron microscopy (TEM, JEM 2010). The thermal properties of the samples were studied using a differential scanning calorimetry (DSC; Perkin-Elmer DSC 7) at heating rate of 0.67 K s− 1. Mechanical properties of samples were measured at room temperature under compressive mode with a strain rate of 1 × 10− 4 s− 1. Test specimens with a dimension of 2 × 2 × 4 mm were prepared for compression tests. Fracture pattern of the tested samples was observed using scanning electron microscopy (SEM). 3. Results and discussion Fig. 1 shows the SEM photos of Cu54Ni6Zr22Ti18 amorphous powders taken from the size group of 46–63 μm (a) and 91–150 μm (b). In order to examine the surface in detail, the second figure (b) was taken with high magnification mode. Irrespective of the size, the powders as atomized shows the spherical shape and clean surface, possibly due to the clean atomization condition under vacuum of 10– 5 Torr. Corresponding to the XRD characterization, the as atomized powders
Fig. 3. XRD traces (left graph) of Cu54Ni6Zr22Ti18 amorphous samples heat treated at each temperature (right small graph).
T.-S. Kim et al. / Materials Letters 62 (2008) 323–326
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Fig. 4. TEM bright field image and the selected area diffraction pattern (inlet) of the amorphous powders heat treated.
less than 150 μm in diameter fully consist of amorphous phases (Not shown here). Fig. 2 is the thermal properties of powders are almost same independent to the size distribution, in which the mean value of glass transition temperature (Tg), the crystallization temperature (Tx), the crystallization enthalpy (ΔH) and the extent of supercooled liquid region (ΔT = Tx − Tg) are 712 K, 767 K, 66.5 J/g and 55 K, respectively. There was no variation in the thermal properties between the powder groups, indicating a formation of fully amorphous phases again. In order identify the crystallization behavior, the amorphous powders were heat treated at the temperatures of the end of first reaction on the DSC traces (788 K), the end of second reaction (837 K), and 80 K above from the end second reaction (909 K) in Fig. 2, respectively. The last temperature of 909 K is to identify the change in the microstructure by further annealing after the second reaction. The XRD traces of the annealed specimen are as shown in Fig. 3. At the lowest temperature (788 K), reduction of amorphous halo is only appeared without any crystallizing peak. It suggests that the amorphous phases were partially devitrified into a nano-crystalline as embedded in the amorphous matrix. Further divitrifaction identifies from the sharp peaks formed after annealing at the temperature of 837 K, which corresponds to Cu10Zr7 and Cu51Zr14. It is seen a further crystallization event of Cu51Zr14 as well as an increment of peak intensity of existent Cu10Zr7 and Cu51Zr14 phases by rising the temperature up to 909 K. Fig. 4 presents the TEM bright field image and the selected area diffraction pattern (in let) of the amorphous powders. As identified at the XRD and DSC traces, the powders as atomized consist of full amorphous structure determined from no lattice image and amorphous halo, respectively. After annealing at 788 K, a lot of nanoparticles were found to form in the amorphous matrix with the sharp amorphous hallo.
Combining the XRD and EDS analysis suggested that the fine particles composed of orthorhombic Cu10Zr7. Further annealing at 837 K, coarsening of existent particles and new crystallization of hcp Cu51Zr14 occurred simultaneously. Average particle size was less than about 15 nm. At 909 K, coarsening was only proceeded without any crystallization. The size of Cu10Zr7 and Cu51Zr14 are of about 50 nm and 20 nm, respectively. The amorphous powders were SPSed as a function of the initial powder size, and then measured the density and compressive strength as seen in Fig. 5. The strength was slightly enhanced from 1.75 GPa to 1.85 GPa, as the initial powder size decreased. It is known that the crystalline materials present a higher strength with decreasing the powder size due to the microstructural refinement. In the amorphous phase, however, the glass stability will affect to the mechanical properties. Since no change in the glass forming ability was identified with the powder size distribution, it may have other effect to determine the mechanical properties of the SPSed samples. The SPS process is to sinter the materials using the pulsed thermal energy usually travels through the surface of powders. Thus, the pulsed energy can more activates the fine powders which have more surfaces rather than the coarse powders, resulting in increasing the sintering ability to the finer powders. In this work, both the strength and the densities (97.5% to 98.6%) were enhanced, as the powder sizes were reduced. The tendency corresponds to the improvement in the sintering ability. Hardness also showed the same pattern of variation.
4. Summary Gas atomized Cu54Ni6Zr22Ti18 powders consisted of fully amorphous phases irrespective of the initial size distribution. The powders formed Cu10Zr7 and Cu51Zr14 particles by annealing at the temperatures of 788 K and 837 K, respectively, followed by only coarsening at 909 K. The final size of crystals devitrified is about 50 nm and 20 nm, respectively. The density and compressive strength of gas atomized and SPSed samples are enhanced as the powder sizes are decreased. The maximal values are about 98.6 HV and 1.85 GPa, respectively. References
Fig. 5. Relative density and stress of powders SPSed with the powder sizes.
[1] T.S. Kim, J.K. Lee, H.J. Kim, J.C. Bae, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 402 (2005) 228–233. [2] D.S. Sung, O.J. Kweon, E. Fleury, K.B. Kim, J.C. Lee, D.H. Kim, Y.C. Kim, Metal Mater. Int. 10 (2004) 575–579.
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T.-S. Kim et al. / Materials Letters 62 (2008) 323–326
[3] P.Y. Lee, S.S. Hung, J.T. Hsieh, Y.L. Lin, C.K. Lin, Intermetallics 10 (2002) 1277–1282. [4] D.J. Sordelet, E. Rozhkova, M.F. Besser, M.J. Kramer, J. Non-Cryst. Solids 334 (2003) 263–269. [5] S.-Y. Lee, T.-S. Kim, J.-K. Lee, H.-J. Kim, D.H. Kim, J.C. Bae, Intermetallics 14 (2006) 1000–1004. [6] J. Robertson, J.T. Im, I. Karaman, K.T. Hartwig, I.E. Anderson, J. NonCryst. Solids 317 (2003) 144–151.
[7] M.H. Lee, D.B. Bae, W.T. Kim, D.H. Kim, E. Rozhkova, P.B. Wheelock, D.J. Sordelet, J. Non-Cryst. Solids 315 (2003) 89–96. [8] Y.B. Kim, M.H. Park, W.Y. Jeung, J.S. Bae, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 368 (2004) 318–322. [9] J.F. Loffer, Intermetallics 11 (2003) 529–540.
Materials Letters 62 (2008) 323 – 326 www.elsevier.com/locate/matlet
Devitrification behavior and mechanical property of Cu54Ni6Zr22Ti18 glass powders Taek-Soo Kim ⁎, J.K. Lee, J.C. Bae Advanced Materials R&D Center, Korea Institute of Industrial Technology, Incheon 406-130, South Korea Received 24 February 2007; accepted 12 May 2007 Available online 18 May 2007
Abstract Cu54Ni6Zr22Ti18 amorphous materials were prepared using gas atomization, followed by spark plasma sintering (SPS). The crystallization behavior and mechanical properties were studied using X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). With annealing the gas atomized powders to the temperatures of 837 K and 909 K, fine particles such as Cu10Zr7 and Cu51Zr14 were precipitated and embedded in the amorphous matrix, respectively. Each size of precipitates at 909 K is about 50 nm and 20 nm, respectively. Microstructural variation and mechanical properties were investigated as a function of the initial powder size and spark plasma sintering (SPS) pressure. © 2007 Elsevier B.V. All rights reserved. Keywords: Metallic glasses; Thermal behavior; Crystallization; Rapid solidification
1. Introduction Research on bulk metallic glass (BMG) has been vividly performed due to its ultra high strength more than 2 GPa at most of composition, low friction coefficient, excellent corrosion resistance, ultra high elastic modulus over 2% [1,2]. However, it is necessary for the BMG to overcome the low size limit for the industrialization, because the typical size of cast BMG is less than 15 mm in diameter so far. In addition, further enhancement of the glass forming ability – or the size – becomes slow in progress only by the alloy design. Thus, it needs to find the breakthrough by altering the cast process in to other one. Recently, powder metallurgy process (PM) is regarded as an alternative to overcome the problem, because of its rapid development. There were a couple of reports that the powders less than 100 μm in diameter were consolidated without loosing their amorphous phases [3–6]. Care to be taken during the PM processing of amorphous powders, because the amorphous powders should be consolidated within the supercooled liquid temperature range which is relatively narrow. The temperature ⁎ Corresponding author. Fax: +82 32 8500 409. E-mail address: [email protected] (T.-S. Kim). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.05.035
Fig. 1. SEM photos of Cu54Ni6Zr22Ti18 amorphous powders with 46–63 (a) and 91–150 μm (b).
324
T.-S. Kim et al. / Materials Letters 62 (2008) 323–326
Fig. 2. DSC traces of Cu54Ni6Zr22Ti18 amorphous powders as a function of the powder size group.
is in between Tg, glass transition temperature, and Tx, crystallization temperature [7–9]. However, no systematic study on the new and low cost Cu54Ni6Zr22Ti18 composition has been reported yet. In this manuscript, spark plasma sintering (SPS) process was used to consolidate the Cu54Ni6Zr22Ti18 amorphous powders prepared by a gas atomization. The crystallization behavior and mechanical properties were investigated. 2. Experimental In order to synthesize the Cu54Ni6Zr22Ti18 amorphous powders using a high-pressure gas atomization process, the master alloy prepared by a vacuum plasma melter (VPM) was remelted in a vacuum radio-active furnace of the atomizer 200 K above the liquidus temperature down to 5.7 × 10− 5 Torr. The melt was bottom poured through a boron nitride melt delivery nozzle of 5 mm in diameter into an annular Ar gas
atomizer operating at a pressure of 5 MPa. The melt flow rate, as estimated from operating time and weight of atomized melt, was about 1.5 kg/min. Size distribution of alloy powders was measured by a conventional sieving method. In order to know the powder size effect on the glass forming ability and materials properties, the as-solidified powders were divided into four groups depending on size; group A — powders larger than 32 μm and smaller than 46 μm, group B — powders larger than 45 μm and smaller than 64 μm, group C — powders larger than 63 μm and smaller than 91 μm and group D — powders larger than 90 μm and smaller than 151 μm. The each group of powders was pre-compacted, and then consolidated by SPS method at 843 K for 60 s with an external pressure 280 MPa. The size of round shape samples SPSed is 20 mm in diameter and 5 mm in thickness in a disc. The structure of powders as atomized and bulks consolidated was characterized using a X-ray diffractometer (XRD) with monochromatic Cu-Kα radiation over 2θ range of 20°–80° at power of 5 kW in a Philips 1729. The microstructures were examined by a scanning electron microscope (SEM; JSM 5410) and transmission electron microscopy (TEM, JEM 2010). The thermal properties of the samples were studied using a differential scanning calorimetry (DSC; Perkin-Elmer DSC 7) at heating rate of 0.67 K s− 1. Mechanical properties of samples were measured at room temperature under compressive mode with a strain rate of 1 × 10− 4 s− 1. Test specimens with a dimension of 2 × 2 × 4 mm were prepared for compression tests. Fracture pattern of the tested samples was observed using scanning electron microscopy (SEM). 3. Results and discussion Fig. 1 shows the SEM photos of Cu54Ni6Zr22Ti18 amorphous powders taken from the size group of 46–63 μm (a) and 91–150 μm (b). In order to examine the surface in detail, the second figure (b) was taken with high magnification mode. Irrespective of the size, the powders as atomized shows the spherical shape and clean surface, possibly due to the clean atomization condition under vacuum of 10– 5 Torr. Corresponding to the XRD characterization, the as atomized powders
Fig. 3. XRD traces (left graph) of Cu54Ni6Zr22Ti18 amorphous samples heat treated at each temperature (right small graph).
T.-S. Kim et al. / Materials Letters 62 (2008) 323–326
325
Fig. 4. TEM bright field image and the selected area diffraction pattern (inlet) of the amorphous powders heat treated.
less than 150 μm in diameter fully consist of amorphous phases (Not shown here). Fig. 2 is the thermal properties of powders are almost same independent to the size distribution, in which the mean value of glass transition temperature (Tg), the crystallization temperature (Tx), the crystallization enthalpy (ΔH) and the extent of supercooled liquid region (ΔT = Tx − Tg) are 712 K, 767 K, 66.5 J/g and 55 K, respectively. There was no variation in the thermal properties between the powder groups, indicating a formation of fully amorphous phases again. In order identify the crystallization behavior, the amorphous powders were heat treated at the temperatures of the end of first reaction on the DSC traces (788 K), the end of second reaction (837 K), and 80 K above from the end second reaction (909 K) in Fig. 2, respectively. The last temperature of 909 K is to identify the change in the microstructure by further annealing after the second reaction. The XRD traces of the annealed specimen are as shown in Fig. 3. At the lowest temperature (788 K), reduction of amorphous halo is only appeared without any crystallizing peak. It suggests that the amorphous phases were partially devitrified into a nano-crystalline as embedded in the amorphous matrix. Further divitrifaction identifies from the sharp peaks formed after annealing at the temperature of 837 K, which corresponds to Cu10Zr7 and Cu51Zr14. It is seen a further crystallization event of Cu51Zr14 as well as an increment of peak intensity of existent Cu10Zr7 and Cu51Zr14 phases by rising the temperature up to 909 K. Fig. 4 presents the TEM bright field image and the selected area diffraction pattern (in let) of the amorphous powders. As identified at the XRD and DSC traces, the powders as atomized consist of full amorphous structure determined from no lattice image and amorphous halo, respectively. After annealing at 788 K, a lot of nanoparticles were found to form in the amorphous matrix with the sharp amorphous hallo.
Combining the XRD and EDS analysis suggested that the fine particles composed of orthorhombic Cu10Zr7. Further annealing at 837 K, coarsening of existent particles and new crystallization of hcp Cu51Zr14 occurred simultaneously. Average particle size was less than about 15 nm. At 909 K, coarsening was only proceeded without any crystallization. The size of Cu10Zr7 and Cu51Zr14 are of about 50 nm and 20 nm, respectively. The amorphous powders were SPSed as a function of the initial powder size, and then measured the density and compressive strength as seen in Fig. 5. The strength was slightly enhanced from 1.75 GPa to 1.85 GPa, as the initial powder size decreased. It is known that the crystalline materials present a higher strength with decreasing the powder size due to the microstructural refinement. In the amorphous phase, however, the glass stability will affect to the mechanical properties. Since no change in the glass forming ability was identified with the powder size distribution, it may have other effect to determine the mechanical properties of the SPSed samples. The SPS process is to sinter the materials using the pulsed thermal energy usually travels through the surface of powders. Thus, the pulsed energy can more activates the fine powders which have more surfaces rather than the coarse powders, resulting in increasing the sintering ability to the finer powders. In this work, both the strength and the densities (97.5% to 98.6%) were enhanced, as the powder sizes were reduced. The tendency corresponds to the improvement in the sintering ability. Hardness also showed the same pattern of variation.
4. Summary Gas atomized Cu54Ni6Zr22Ti18 powders consisted of fully amorphous phases irrespective of the initial size distribution. The powders formed Cu10Zr7 and Cu51Zr14 particles by annealing at the temperatures of 788 K and 837 K, respectively, followed by only coarsening at 909 K. The final size of crystals devitrified is about 50 nm and 20 nm, respectively. The density and compressive strength of gas atomized and SPSed samples are enhanced as the powder sizes are decreased. The maximal values are about 98.6 HV and 1.85 GPa, respectively. References
Fig. 5. Relative density and stress of powders SPSed with the powder sizes.
[1] T.S. Kim, J.K. Lee, H.J. Kim, J.C. Bae, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 402 (2005) 228–233. [2] D.S. Sung, O.J. Kweon, E. Fleury, K.B. Kim, J.C. Lee, D.H. Kim, Y.C. Kim, Metal Mater. Int. 10 (2004) 575–579.
326
T.-S. Kim et al. / Materials Letters 62 (2008) 323–326
[3] P.Y. Lee, S.S. Hung, J.T. Hsieh, Y.L. Lin, C.K. Lin, Intermetallics 10 (2002) 1277–1282. [4] D.J. Sordelet, E. Rozhkova, M.F. Besser, M.J. Kramer, J. Non-Cryst. Solids 334 (2003) 263–269. [5] S.-Y. Lee, T.-S. Kim, J.-K. Lee, H.-J. Kim, D.H. Kim, J.C. Bae, Intermetallics 14 (2006) 1000–1004. [6] J. Robertson, J.T. Im, I. Karaman, K.T. Hartwig, I.E. Anderson, J. NonCryst. Solids 317 (2003) 144–151.
[7] M.H. Lee, D.B. Bae, W.T. Kim, D.H. Kim, E. Rozhkova, P.B. Wheelock, D.J. Sordelet, J. Non-Cryst. Solids 315 (2003) 89–96. [8] Y.B. Kim, M.H. Park, W.Y. Jeung, J.S. Bae, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 368 (2004) 318–322. [9] J.F. Loffer, Intermetallics 11 (2003) 529–540.