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Microstructure evolution of the laser spot welded Ni-free Zr-based bulk metallic glass composites
Intermetallics 29 (2012) 92e98
Contents lists available at SciVerse ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Microstructure evolution of the laser spot welded Ni-free Zr-based bulk metallic glass composites Huei-Sen Wang a, *, Mau-Sheng Chiou a, Hou-Guang Chen a, Jason Shian-Ching Jang b, Jhen-Wang Gu a a b
Department of Materials Science and Engineering, I-Shou University, Kaohsiung 84001, Taiwan Institute of Materials Science and Engineering & Department of Mechanical Engineering, National Central University, Chung-Li 32001, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2012 Received in revised form 4 May 2012 Accepted 8 May 2012 Available online 19 June 2012
A novel Ni-free (Zr48Cu32Al8Ag8,Ta4)Si0.75 bulk metallic glass composite (BMG-C), showing an excellent combination of high strength and remarkable ductility, was laser spot welded with the pre-selected laser welding parameters. After welding, the microstructure evolution, glass forming ability (GFA) and mechanical properties of the welded samples were determined by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Vickers microhardness test. Test results showed that the parent material (PM), heat affected zone (HAZ) and weld fusion zone (WFZ) in the welds all consist of an amorphous matrix with two kinds of Ta particles: micro-sized and nano-sized. In the WFZ, during the rapid melting, the partial dissolution of micro-sized Ta particles resulted in the reduction of their volume fraction. After subsequent rapid cooling, partial micro-sized Ta particles were transformed to nano-sized particles or nano-sized particle accumulations. This transformation resulted in a slightly higher magnitude of hardness in the WFZ. Furthermore, it was found that the surrounding area of micro-sized Ta in the WFZ has better resistance to the etchant solution. In the HAZ, small amounts of Zr2Cu and nano-sized Ta tended to precipitate on the micro-sized Ta surface, and may act as heterogeneous nucleation sites. However, the small amount of precipitation in the HAZ and the micro-sized Ta transformation in the WFZ did not significantly affect the magnitude of the GFA indices, DTx, g and gm, when compared to that of un-welded BMG-C. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Glasses, metallic B. Phase transformation B. Precipitates C. Laser processing C. Welding
1. Introduction Over the past decades, much effort has been devoted to the development of bulk metallic glasses (BMGs) for both scientific research and potential applications. As a result, various BMGs [1,2], such as Pd-, Zr-, Cu-, Mg- and La-based alloys, have been found to exhibit properties superior to those of conventional crystalline materials. More recently, considering their excellent mechanical properties, high glass forming ability (GFA) and environmentally friendly raw materials used, the ZreCueAleM (M ¼ Ti, Ag, etc) BMG alloy system [3e5] has been of particular interest due to its toxic-free raw materials (Be, Ni, etc), higher strength and strong GFA as compared to other BMGs. As with all monolithic BMGs, ZreCueAleM BMG may exhibit inhomogeneous deformation below the glass-transition temperature [6], which results in the low plastic strain under compressive * Corresponding author. Tel.: þ886 7 6577711x3111; fax: þ886 7 6578444. E-mail addresses: [email protected], [email protected] (H.-S. Wang). 0966-9795/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2012.05.013
loading. To alleviate this problem, recent studies have considered the formation of bulk metallic glass composites (BMG-Cs) which combine the glassy matrix phase with a micro- or nano-scale ductile crystalline phase, such as Nb, Ta and Hf. These composites can be formed in two ways: one is to in-situ precipitate crystalline phases in the BMG matrix; the other is to ex-situ add particles separately during melting, which are then subsequently cast into a BMG matrix [1,2,6e8]. Recent studies have reported that in-situ Zr-based BMG-Cs exhibit a superior mechanical performance with up to 30% compressive plastic strain [6]. To extend the industrial applications of the novel Zr-based BMGC, the weldability of the in-situ BMG-Cs must be considered. However, in order for a composite structure in the weld fusion zone (WFZ) and heat affected zone (HAZ) to result after the welding process, BMG-C welding becomes increasingly complex when compared to that of monolithic BMGs. Over the past decade, researchers have developed several welding processes [9] for monolithic BMGs. The welding technologies studied [10e12] focused on joining Zr-based BMGs with the Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) laser
H.-S. Wang et al. / Intermetallics 29 (2012) 92e98
welding process due to its advantages of localized heating and high cooling rate. To date, very few such attempts have been made for Zr-based BMG-C welding. Therefore, in this study, an Ni-free Zr-based BMG-C toughened by in-situ precipitated Ta particles, (Zr48Cu32Al8Ag8,Ta4)Si0.75, was employed for Nd:YAG laser spot welding. The laser pulse shape was pre-selected by the empirical method, with the emphasis on the weld morphology and full penetration of a 1 mm BMG-C plate. After welding, the microstructure development, crystallization behavior, mechanical properties in the WFZ and the HAZ were investigated. 2. Experimental process The alloy ingots used in this study were designed as an in-situ BMG composite, (Zr44Cu36Al8Ag8,Ta4)Si0.75. This alloy was prepared from pure elemental Zr, Cu, Al, Ag, Si and Ta of 99.9 wt% purity by arc melting under a Ti-gettered argon atmosphere. In order to make the composition homogeneous, this alloy has different melting processes, which can be found in the literature [1,6,7]. After the melting was completed, the liquid alloy was suctioned and cast into the water-cooled Cu mold to form a cast plate. The cast was then machined to a plate with a thickness of 1 mm, width 20 mm and length of 30 mm for Nd:YAG laser spot welding. All BMG sample plates were polished with silicon carbide paper of up to 4000-grit to remove existing oxides. The composite microstructures of the as-cast plates were characterized using X-ray diffractometry (XRD; Scintag X-400) and scanning electron microscopy (SEM; Hitachi S-4700). Prior to the laser welding tests, the samples were degreased in acetone, washed in distilled water and dried in air. The laser pulse shape used in this study (see Table 1) was pre-selected in reference to earlier studies [10e12] and based on the empirical approach, with the emphasis on the weld morphology and the possible minimum energy required to penetrate the 1 mm BMG-C plates. The test samples were then bead-on-plate laser spot welded using preselected welding parameters. During the welding process, the top and bottom surfaces of the test plates were shielded by pure argon to prevent surface oxidation. When the welding process was completed, the test plates were polished and then etched with a solution of 100 ml H2O, 5 ml H2O2 and 2 ml HF. The weld microstructure was observed by optical microscopy (OM), and a SEM equipped with an energy dispersive spectrometer (EDS). A detailed microstructure inspection was performed by transmission electron microscopy (TEM; Philip Tecnai G2). The glass transition and crystallization behaviors of the laser welds (consisting of the WFZ and HAZ) were investigated using differential scanning calorimetry (DSC; Netzsch 404C) and then compared with an un-welded plate. The heating rate for each tested sample was 20 K min1. The samples for the DSC tests were carefully taken only from the spot welds. Finally, the Vickers microhardness in the HAZ and WFZ was measured using a microhardness tester under a load of 100 g. 3. Results and discussion 3.1. Microstructure of the as-cast Zr-based BMG-Cs Prior to the welding tests, the microstructure of the sample plate from the cast was investigated. The XRD patterns (see Fig. 1(a)) of Table 1 Pre-selected laser welding parameters used in this study. Peak power (kW)
Pulse duration (ms)
Laser energy (J)
Spot size (mm)
Frequency (Hz)
2.1
4.5
8.0
0.4
2
93
the BMG-C plate showed a broad halo diffraction pattern of 30e50 which indicated the amorphous nature of the alloy matrix. In addition, the three high intensity and BCC-structured crystalline peaks suggested that Ta particles had precipitated in the amorphous matrix [6]. A further detailed examination of the Zr-based BMG-C microstructure by SEM revealed that amounts of Ta particles 5e25 mm in size could be observed in the amorphous matrix, as shown in Fig. 1(b). The TEM observation also revealed that in addition to the amorphous matrix (see Fig. 2(a)), nano-sized precipitates embedded in the amorphous matrix could be observed, as shown in Fig. 2(bed). These nano-sized precipitates were also confirmed to signify the BCC-structured Ta phase. Based on the above observations, it could be concluded that there were two types of in-situ precipitated Ta particles: micro-sized and nano-sized, distributed in the amorphous matrix. 3.2. Microstructure of the BMG-C WFZ Fig. 3(a) shows the morphology of the WFZ and the micro-sized Ta distribution in the WFZ. A fully penetrated BMG-C weld without obvious defects in the WFZ was obtained. However, when compared to that of PM or HAZ, the volume fraction of the microsized Ta were clearly reduced. Furthermore, it was found that the surrounding area of micro-size Ta in the WFZ seemed to have better resistance to the etchant solution (see Fig. 3(bed)) when compared to that in the HAZ or parent material. TEM observation further revealed that, in addition to the glassy matrix (see Fig. 4(a)), the volume fraction of the nano-sized precipitates increased, with some of them accumulating in the amorphous matrix (see Fig. 4(bed)). Due to the lower power input and high energy density characteristics of the Nd:YAG laser, the heating or cooling of the solidified BMG-C spot weld was more rapid as compared to the heating or cooling of the solidified BMG-C cast. During the rapid heating of the laser welds, there was severe localized melting of the amorphous matrix. For micro-sized Ta, because of its higher melting temperature, only partial dissolution of the Ta particles occurred; therefore, it was observed that the volume fraction of micro-sized Ta was reduced in the WFZ (see Fig. 3(a)). During the subsequent rapid cooling, the re-melted zone solidified and in-situ precipitated Ta crystalline phases in the BMG matrix. This rapid solidification rate also resulted in the massive reduction in the grain size of the re-precipitated Ta. Therefore, more nano-sized Ta formed in the amorphous matrix. However, the nano-sized Ta did not disperse uniformly throughout the glassy matrix and some nano-sized Ta particle accumulations were observed. 3.3. Microstructure of the BMG-C HAZ Several studies [10e12] which focused on joining Zr-based monolithic BMG with the Nd:YAG laser welding process all indicated that the HAZ (the area just next by the WFZ) had a tendency to crystallize. The same laser parameters used in this study as those applied in the earlier study ((Zr53Cu30Ni9Al8)Si0.5 BMG) caused the formation of massive amounts of Zr/Cu crystalline precipitates in the HAZ of the Zr-based monolithic BMG, making it brittle in nature, prone to crack formations in this area and reducing its hardness. Moreover, the GFA of the welded BMG was affected by the formation of HAZ crystallization [10]. In the present study, the pre-selected laser process parameters did not produce massive Zr/ Cu precipitates in the amorphous matrix in the HAZ. Only a few precipitates and cracks were occasionally found (see Fig. 5) at the interface between the glassy matrix and the micro-sized Ta reinforcement in the HAZ. The EDS analysis indicated that the
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Fig. 1. (a) XRD patterns of the BMG-C plate; (b) BMG-C microstructure as observed by SEM.
precipitates were rich in Cu and Zr. Both the EDS results and those of earlier studies [10,12] indicated that the precipitates more likely signified the Zr2Cu phase. The prior formation of Zr2Cu could be attributed to: ZreCu are major constituents; ZreCu has a high negative mixing heat and a massive difference in electro-negativity [13]; and small-sized Cu atoms which are easily diffused within a matrix [13]. Furthermore, it was found that the Zr2Cu phase precipitated more favorably at the interface between the glassy matrix and reinforced Ta particle. More detailed TEM analysis indicated that, in addition to the glassy matrix (see Fig. 6(a)), Zr2Cu, micro-sized and nano-sized Ta (see Fig. 6(bed)) embedded in the matrix, some nano-sized Ta formed at the micro-sized Ta surface (see Fig. 6(eeg)). The Zr2Cu and nano-sized Ta tended to precipitate on the micro-sized Ta surface and may have acted as heterogeneous nucleation sites [1].
3.4. Glass forming ability of welded BMG-C To investigate the glass transition and crystallization behaviors of the BMG-C laser welds, DSC was used. Fig. 7 shows the DSC traces measured from the Zr-based BMG-C PM and the laser welds at a heating rate of 20 K min1. The characteristic temperatures were defined as glass transition temperature, Tg; crystallization temperature, Tx; and liquidus temperature, Tl. Using Tg, Tx and Tl, the values of the GFA indices of BMG [1,6,10], DTx (DTx ¼ Tx e Tg), g (g ¼ Tx/(Tg þ Tl)) and gm (gm ¼ (2Tx Tg)/Tl), were obtained (see Table 2). As the test results in Fig. 7 and Table 2 show, Tg, Tx and Tl and the GFA indices of the spot welds and parent material exhibited a similar magnitude. The earlier studies [10,12] used the same laser welding parameters to weld a (Zr53Cu30Ni9Al8)Si0.5 BMG, producing the massive crystallization in the HAZ of the welds. The test results indicated
H.-S. Wang et al. / Intermetallics 29 (2012) 92e98
95
Fig. 2. TEM observation of BMG-C parent material (PM): (a) Bright field image (BFI) and selected area diffraction pattern (SAD) of amorphous matrix; (b) BFI and (c) dark field image (DFI) of nano-sized precipitates embedded in the amorphous matrix; (d) corresponding SAD pattern of nano-sized precipitates identified as the Ta phase.
Fig. 3. (a) The cross-section of the weld. After etching, observations of the surrounding areas of micro-size Ta in the (b) Parent material (PM); (c) heat affected zone (HAZ); and (d) weld fusion zone (WFZ).
96
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Fig. 4. TEM observation of BMG-C WFZ: (a) BFI and SAD of amorphous matrix; (b) BFI; (c) DFI of nano-sized precipitate accumulation in the amorphous matrix; and (d) corresponding SAD pattern of nano-sized precipitates identified as the Ta phase.
that the GFA of the welded BMGs was affected by the crystallization formation in the amorphous matrix of the welds. In this study, Zr/ Cu precipitates did occasionally form at the interface between the glassy matrix and the micro-sized Ta reinforcement in the HAZ, but the amount was quite small. The lower Zr/Cu precipitate formation in the HAZ could be attributed to the Zr-based BMG-C having a higher GFA index (DTx was about 79 K), when compared to that of the (Zr53Cu30Ni9Al8)Si0.5 BMG (DTx was about 71 K) [10]. Many researchers [e.g., 1, 13] have indicated that the alloy with the higher GFA index tended to have a higher glass forming ability, thus making it easier to obtain the
glass structure at a lower cooling rate. Furthermore, part of the micro-sized Ta in the WFZ transformed into nano-sized Ta; however, this transformation did not seem to significantly affect the GFA of the welds. 3.5. Microhardness test results of welded BMG-C Fig. 8 shows the results of the microhardness tests for the PM, HAZ and WFZ. It was observed that the HAZ of the weld had a similar hardness to the PM due to the very few crystalline precipitates or cracks observed in this area.
Fig. 5. A few precipitates and cracks were occasionally found at the interface between the glassy matrix and the micro-sized Ta reinforcement in the HAZ.
H.-S. Wang et al. / Intermetallics 29 (2012) 92e98
97
Fig. 6. TEM observation of BMG-C HAZ: (a) BFI and SAD of amorphous matrix; (b) BFI; (c) DFI of nano-sized precipitates embedded in the amorphous matrix; (d) corresponding SAD pattern of nano-sized precipitates identified as the Ta phase. (e) BFI; (f) DFI of nano-sized Ta formed at the micro-sized Ta surface; and (g) corresponding SAD pattern of nano-sized precipitates signifying the Ta phase.
Fig. 7. DSC traces measured from the Zr-based BMG-C PM and the laser welds.
Furthermore, it was found that the WFZ had a slightly higher hardness. It was thought that the increased hardness may have been associated with the amount of micro-sized Ta particles transformed into nano-sized particles. Presently, available explanations for the mechanism of the increased hardness in the BMG-C WFZ caused by the size distribution or volume fraction of the reinforcement are rather limited. Considering the mechanical properties, such as strength or hardness, of the composite materials are closely related to the size distribution or volume fraction of the embedded reinforcement. Especially, when more nano-sized Ta particles form in the BMG-C matrix, they might make more effective barriers which inhibit the shear band propagation [1] and
increase the related mechanical properties mentioned above. Hence, in this study, when the laser spot re-melting process of the BMG-C was conducted, the size distribution of the Ta particles and the volume fraction of the nano-sized Ta particles were changed, which could account for the slight increase in hardness. Table 2 Tg, Tx and Tl, and developed GFA indices of the tested welds.
Parent metal Welds
Tg
Tx
Tl
DTx
g
gm
709.9 708.0
789.1 786.1
1244 1243
79.20 78.10
0.4039 0.4029
0.6980 0.6953
98
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3. In the HAZ, small amounts of Zr2Cu and nano-sized Ta tended to precipitate on the micro-sized Ta surface and may have acted as heterogeneous nucleation sites. 4. The pre-selected laser parameters did not significantly affect the magnitude of the GFA indices, DTx, g and gm, when compared to that of un-welded BMG-C, although small amounts of precipitation in the HAZ, as well as micro-sized Ta transformation in the WFZ occurred.
References
Fig. 8. Microhardness tests in the PM, HAZ and WFZ.
4. Conclusions A novel Ni-free (Zr48Cu32Al8Ag8,Ta4)Si0.75 bulk metallic glass composite (BMG-C) weld was fabricated in this study by means of the Nd:YAG pulse laser spot welding process with pre-selected laser parameters. After the welding, the microstructure evolution, glass forming ability (GFA) and mechanical properties of the welded samples were investigated. The results are summarized as follows: 1. Parent material (PM), heat affected zone (HAZ) and weld fusion zone (WFZ) in the welds all consisted of an amorphous matrix and two kinds of Ta particles: micro-sized and nano-sized. 2. In the WFZ, some micro-sized Ta transformed to nano-sized particles or nano-sized particle accumulations. This transformation a slightly higher magnitude of hardness in the WFZ.
[1] Suryanarayana C, Inoue A. Bulk metallic glasses. Boca Raton, FL, U.S.A: CRC Press, Taylor & Francis Group; 2011. [2] Miller M, Liaw P. Bulk metallic glasses an overview. New York: Springer Science þ Business Media, LLC; 2008. [3] Freels M, Wang GY, Zhang W, Liaw PK, Inoue A. Cyclic compression behavior of a CueZreAleAg bulk metallic glass. Intermetallics 2011;19(8):1174e83. [4] Zhang W, Zhang Q, Qin C, Inoue A. Synthesis and properties of CueZreAgeAl glassy alloys with high glass-forming ability. Materials Science and Engineering: B 2008;148(1e3):92e6. [5] Jiang QK, Wang XD, Xp Nie, Zhang GQ, Ma H, Fecht HJ, et al. Zre(Cu, Ag)eAl bulk metallic glasses. Acta Materialia 2008;56(8):1785e96. [6] Jang JSC, Jian SR, Pan DJ, Wu YH, Huang JC, Nieh TG. Thermal and mechanical characterizations of a Zr-based bulk metallic glass composite toughened by insitu precipitated Ta-rich particles. Intermetallics 2010;18(4):560e4. [7] Qin CL, Zhang W, Asami K, Kimura H, Wang XM, Inoue A. A novel Cu-based BMG composite with high corrosion resistance and excellent mechanical properties. Acta Materialia 2006;54(14):3713e9. [8] Gebert A, Kuehn U, Baunack S, Mattern N, Schultz L. Pitting corrosion of zirconium-based bulk glass-matrix composites. Materials Science and Engineering: A 2006;415(1e2):242e9. [9] Kawamura Y, Shoji T, Ohno Y. Welding technologies of bulk metllic glasses. Journal of Non-Crystalline Solids 2003;317:152e7. [10] Wang HS, Chiou MS, Chen HG, Jang JSC. The effects of initial welding temperature and welding parameters on the crystallization behaviors of laser spot welded Zr-based bulk metallic glass. Materials Chemistry and Physics 2011;129(1e2):547e52. [11] Wang HS, Chen HG, Jang JSC. Microstructure evolution in Nd:YAG laserwelded (Zr53Cu30Ni9Al8)Si0.5 bulk metallic glass alloy. Journal of Alloys and Compounds 2010;495(1):224e8. [12] Wang HS, Chen HG, Jang JSC, Chiou MS. Combination of a Nd:YAG laser and a liquid cooling device to (Zr53Cu30Ni9Al8)Si0.5 bulk metallic glass welding. Materials Science and Engineering: A 2010;528(1):338e41. [13] Jang JSC, Tsao SF, Chang LJ, Huang JC, Liu CT. Nanocrystallization of Zr61Al7.5Cu17.5Ni10Si4 metallic glass. Intermetallics 2009;17:56e64.
Contents lists available at SciVerse ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Microstructure evolution of the laser spot welded Ni-free Zr-based bulk metallic glass composites Huei-Sen Wang a, *, Mau-Sheng Chiou a, Hou-Guang Chen a, Jason Shian-Ching Jang b, Jhen-Wang Gu a a b
Department of Materials Science and Engineering, I-Shou University, Kaohsiung 84001, Taiwan Institute of Materials Science and Engineering & Department of Mechanical Engineering, National Central University, Chung-Li 32001, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 March 2012 Received in revised form 4 May 2012 Accepted 8 May 2012 Available online 19 June 2012
A novel Ni-free (Zr48Cu32Al8Ag8,Ta4)Si0.75 bulk metallic glass composite (BMG-C), showing an excellent combination of high strength and remarkable ductility, was laser spot welded with the pre-selected laser welding parameters. After welding, the microstructure evolution, glass forming ability (GFA) and mechanical properties of the welded samples were determined by a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Vickers microhardness test. Test results showed that the parent material (PM), heat affected zone (HAZ) and weld fusion zone (WFZ) in the welds all consist of an amorphous matrix with two kinds of Ta particles: micro-sized and nano-sized. In the WFZ, during the rapid melting, the partial dissolution of micro-sized Ta particles resulted in the reduction of their volume fraction. After subsequent rapid cooling, partial micro-sized Ta particles were transformed to nano-sized particles or nano-sized particle accumulations. This transformation resulted in a slightly higher magnitude of hardness in the WFZ. Furthermore, it was found that the surrounding area of micro-sized Ta in the WFZ has better resistance to the etchant solution. In the HAZ, small amounts of Zr2Cu and nano-sized Ta tended to precipitate on the micro-sized Ta surface, and may act as heterogeneous nucleation sites. However, the small amount of precipitation in the HAZ and the micro-sized Ta transformation in the WFZ did not significantly affect the magnitude of the GFA indices, DTx, g and gm, when compared to that of un-welded BMG-C. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Glasses, metallic B. Phase transformation B. Precipitates C. Laser processing C. Welding
1. Introduction Over the past decades, much effort has been devoted to the development of bulk metallic glasses (BMGs) for both scientific research and potential applications. As a result, various BMGs [1,2], such as Pd-, Zr-, Cu-, Mg- and La-based alloys, have been found to exhibit properties superior to those of conventional crystalline materials. More recently, considering their excellent mechanical properties, high glass forming ability (GFA) and environmentally friendly raw materials used, the ZreCueAleM (M ¼ Ti, Ag, etc) BMG alloy system [3e5] has been of particular interest due to its toxic-free raw materials (Be, Ni, etc), higher strength and strong GFA as compared to other BMGs. As with all monolithic BMGs, ZreCueAleM BMG may exhibit inhomogeneous deformation below the glass-transition temperature [6], which results in the low plastic strain under compressive * Corresponding author. Tel.: þ886 7 6577711x3111; fax: þ886 7 6578444. E-mail addresses: [email protected], [email protected] (H.-S. Wang). 0966-9795/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2012.05.013
loading. To alleviate this problem, recent studies have considered the formation of bulk metallic glass composites (BMG-Cs) which combine the glassy matrix phase with a micro- or nano-scale ductile crystalline phase, such as Nb, Ta and Hf. These composites can be formed in two ways: one is to in-situ precipitate crystalline phases in the BMG matrix; the other is to ex-situ add particles separately during melting, which are then subsequently cast into a BMG matrix [1,2,6e8]. Recent studies have reported that in-situ Zr-based BMG-Cs exhibit a superior mechanical performance with up to 30% compressive plastic strain [6]. To extend the industrial applications of the novel Zr-based BMGC, the weldability of the in-situ BMG-Cs must be considered. However, in order for a composite structure in the weld fusion zone (WFZ) and heat affected zone (HAZ) to result after the welding process, BMG-C welding becomes increasingly complex when compared to that of monolithic BMGs. Over the past decade, researchers have developed several welding processes [9] for monolithic BMGs. The welding technologies studied [10e12] focused on joining Zr-based BMGs with the Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) laser
H.-S. Wang et al. / Intermetallics 29 (2012) 92e98
welding process due to its advantages of localized heating and high cooling rate. To date, very few such attempts have been made for Zr-based BMG-C welding. Therefore, in this study, an Ni-free Zr-based BMG-C toughened by in-situ precipitated Ta particles, (Zr48Cu32Al8Ag8,Ta4)Si0.75, was employed for Nd:YAG laser spot welding. The laser pulse shape was pre-selected by the empirical method, with the emphasis on the weld morphology and full penetration of a 1 mm BMG-C plate. After welding, the microstructure development, crystallization behavior, mechanical properties in the WFZ and the HAZ were investigated. 2. Experimental process The alloy ingots used in this study were designed as an in-situ BMG composite, (Zr44Cu36Al8Ag8,Ta4)Si0.75. This alloy was prepared from pure elemental Zr, Cu, Al, Ag, Si and Ta of 99.9 wt% purity by arc melting under a Ti-gettered argon atmosphere. In order to make the composition homogeneous, this alloy has different melting processes, which can be found in the literature [1,6,7]. After the melting was completed, the liquid alloy was suctioned and cast into the water-cooled Cu mold to form a cast plate. The cast was then machined to a plate with a thickness of 1 mm, width 20 mm and length of 30 mm for Nd:YAG laser spot welding. All BMG sample plates were polished with silicon carbide paper of up to 4000-grit to remove existing oxides. The composite microstructures of the as-cast plates were characterized using X-ray diffractometry (XRD; Scintag X-400) and scanning electron microscopy (SEM; Hitachi S-4700). Prior to the laser welding tests, the samples were degreased in acetone, washed in distilled water and dried in air. The laser pulse shape used in this study (see Table 1) was pre-selected in reference to earlier studies [10e12] and based on the empirical approach, with the emphasis on the weld morphology and the possible minimum energy required to penetrate the 1 mm BMG-C plates. The test samples were then bead-on-plate laser spot welded using preselected welding parameters. During the welding process, the top and bottom surfaces of the test plates were shielded by pure argon to prevent surface oxidation. When the welding process was completed, the test plates were polished and then etched with a solution of 100 ml H2O, 5 ml H2O2 and 2 ml HF. The weld microstructure was observed by optical microscopy (OM), and a SEM equipped with an energy dispersive spectrometer (EDS). A detailed microstructure inspection was performed by transmission electron microscopy (TEM; Philip Tecnai G2). The glass transition and crystallization behaviors of the laser welds (consisting of the WFZ and HAZ) were investigated using differential scanning calorimetry (DSC; Netzsch 404C) and then compared with an un-welded plate. The heating rate for each tested sample was 20 K min1. The samples for the DSC tests were carefully taken only from the spot welds. Finally, the Vickers microhardness in the HAZ and WFZ was measured using a microhardness tester under a load of 100 g. 3. Results and discussion 3.1. Microstructure of the as-cast Zr-based BMG-Cs Prior to the welding tests, the microstructure of the sample plate from the cast was investigated. The XRD patterns (see Fig. 1(a)) of Table 1 Pre-selected laser welding parameters used in this study. Peak power (kW)
Pulse duration (ms)
Laser energy (J)
Spot size (mm)
Frequency (Hz)
2.1
4.5
8.0
0.4
2
93
the BMG-C plate showed a broad halo diffraction pattern of 30e50 which indicated the amorphous nature of the alloy matrix. In addition, the three high intensity and BCC-structured crystalline peaks suggested that Ta particles had precipitated in the amorphous matrix [6]. A further detailed examination of the Zr-based BMG-C microstructure by SEM revealed that amounts of Ta particles 5e25 mm in size could be observed in the amorphous matrix, as shown in Fig. 1(b). The TEM observation also revealed that in addition to the amorphous matrix (see Fig. 2(a)), nano-sized precipitates embedded in the amorphous matrix could be observed, as shown in Fig. 2(bed). These nano-sized precipitates were also confirmed to signify the BCC-structured Ta phase. Based on the above observations, it could be concluded that there were two types of in-situ precipitated Ta particles: micro-sized and nano-sized, distributed in the amorphous matrix. 3.2. Microstructure of the BMG-C WFZ Fig. 3(a) shows the morphology of the WFZ and the micro-sized Ta distribution in the WFZ. A fully penetrated BMG-C weld without obvious defects in the WFZ was obtained. However, when compared to that of PM or HAZ, the volume fraction of the microsized Ta were clearly reduced. Furthermore, it was found that the surrounding area of micro-size Ta in the WFZ seemed to have better resistance to the etchant solution (see Fig. 3(bed)) when compared to that in the HAZ or parent material. TEM observation further revealed that, in addition to the glassy matrix (see Fig. 4(a)), the volume fraction of the nano-sized precipitates increased, with some of them accumulating in the amorphous matrix (see Fig. 4(bed)). Due to the lower power input and high energy density characteristics of the Nd:YAG laser, the heating or cooling of the solidified BMG-C spot weld was more rapid as compared to the heating or cooling of the solidified BMG-C cast. During the rapid heating of the laser welds, there was severe localized melting of the amorphous matrix. For micro-sized Ta, because of its higher melting temperature, only partial dissolution of the Ta particles occurred; therefore, it was observed that the volume fraction of micro-sized Ta was reduced in the WFZ (see Fig. 3(a)). During the subsequent rapid cooling, the re-melted zone solidified and in-situ precipitated Ta crystalline phases in the BMG matrix. This rapid solidification rate also resulted in the massive reduction in the grain size of the re-precipitated Ta. Therefore, more nano-sized Ta formed in the amorphous matrix. However, the nano-sized Ta did not disperse uniformly throughout the glassy matrix and some nano-sized Ta particle accumulations were observed. 3.3. Microstructure of the BMG-C HAZ Several studies [10e12] which focused on joining Zr-based monolithic BMG with the Nd:YAG laser welding process all indicated that the HAZ (the area just next by the WFZ) had a tendency to crystallize. The same laser parameters used in this study as those applied in the earlier study ((Zr53Cu30Ni9Al8)Si0.5 BMG) caused the formation of massive amounts of Zr/Cu crystalline precipitates in the HAZ of the Zr-based monolithic BMG, making it brittle in nature, prone to crack formations in this area and reducing its hardness. Moreover, the GFA of the welded BMG was affected by the formation of HAZ crystallization [10]. In the present study, the pre-selected laser process parameters did not produce massive Zr/ Cu precipitates in the amorphous matrix in the HAZ. Only a few precipitates and cracks were occasionally found (see Fig. 5) at the interface between the glassy matrix and the micro-sized Ta reinforcement in the HAZ. The EDS analysis indicated that the
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Fig. 1. (a) XRD patterns of the BMG-C plate; (b) BMG-C microstructure as observed by SEM.
precipitates were rich in Cu and Zr. Both the EDS results and those of earlier studies [10,12] indicated that the precipitates more likely signified the Zr2Cu phase. The prior formation of Zr2Cu could be attributed to: ZreCu are major constituents; ZreCu has a high negative mixing heat and a massive difference in electro-negativity [13]; and small-sized Cu atoms which are easily diffused within a matrix [13]. Furthermore, it was found that the Zr2Cu phase precipitated more favorably at the interface between the glassy matrix and reinforced Ta particle. More detailed TEM analysis indicated that, in addition to the glassy matrix (see Fig. 6(a)), Zr2Cu, micro-sized and nano-sized Ta (see Fig. 6(bed)) embedded in the matrix, some nano-sized Ta formed at the micro-sized Ta surface (see Fig. 6(eeg)). The Zr2Cu and nano-sized Ta tended to precipitate on the micro-sized Ta surface and may have acted as heterogeneous nucleation sites [1].
3.4. Glass forming ability of welded BMG-C To investigate the glass transition and crystallization behaviors of the BMG-C laser welds, DSC was used. Fig. 7 shows the DSC traces measured from the Zr-based BMG-C PM and the laser welds at a heating rate of 20 K min1. The characteristic temperatures were defined as glass transition temperature, Tg; crystallization temperature, Tx; and liquidus temperature, Tl. Using Tg, Tx and Tl, the values of the GFA indices of BMG [1,6,10], DTx (DTx ¼ Tx e Tg), g (g ¼ Tx/(Tg þ Tl)) and gm (gm ¼ (2Tx Tg)/Tl), were obtained (see Table 2). As the test results in Fig. 7 and Table 2 show, Tg, Tx and Tl and the GFA indices of the spot welds and parent material exhibited a similar magnitude. The earlier studies [10,12] used the same laser welding parameters to weld a (Zr53Cu30Ni9Al8)Si0.5 BMG, producing the massive crystallization in the HAZ of the welds. The test results indicated
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Fig. 2. TEM observation of BMG-C parent material (PM): (a) Bright field image (BFI) and selected area diffraction pattern (SAD) of amorphous matrix; (b) BFI and (c) dark field image (DFI) of nano-sized precipitates embedded in the amorphous matrix; (d) corresponding SAD pattern of nano-sized precipitates identified as the Ta phase.
Fig. 3. (a) The cross-section of the weld. After etching, observations of the surrounding areas of micro-size Ta in the (b) Parent material (PM); (c) heat affected zone (HAZ); and (d) weld fusion zone (WFZ).
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Fig. 4. TEM observation of BMG-C WFZ: (a) BFI and SAD of amorphous matrix; (b) BFI; (c) DFI of nano-sized precipitate accumulation in the amorphous matrix; and (d) corresponding SAD pattern of nano-sized precipitates identified as the Ta phase.
that the GFA of the welded BMGs was affected by the crystallization formation in the amorphous matrix of the welds. In this study, Zr/ Cu precipitates did occasionally form at the interface between the glassy matrix and the micro-sized Ta reinforcement in the HAZ, but the amount was quite small. The lower Zr/Cu precipitate formation in the HAZ could be attributed to the Zr-based BMG-C having a higher GFA index (DTx was about 79 K), when compared to that of the (Zr53Cu30Ni9Al8)Si0.5 BMG (DTx was about 71 K) [10]. Many researchers [e.g., 1, 13] have indicated that the alloy with the higher GFA index tended to have a higher glass forming ability, thus making it easier to obtain the
glass structure at a lower cooling rate. Furthermore, part of the micro-sized Ta in the WFZ transformed into nano-sized Ta; however, this transformation did not seem to significantly affect the GFA of the welds. 3.5. Microhardness test results of welded BMG-C Fig. 8 shows the results of the microhardness tests for the PM, HAZ and WFZ. It was observed that the HAZ of the weld had a similar hardness to the PM due to the very few crystalline precipitates or cracks observed in this area.
Fig. 5. A few precipitates and cracks were occasionally found at the interface between the glassy matrix and the micro-sized Ta reinforcement in the HAZ.
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Fig. 6. TEM observation of BMG-C HAZ: (a) BFI and SAD of amorphous matrix; (b) BFI; (c) DFI of nano-sized precipitates embedded in the amorphous matrix; (d) corresponding SAD pattern of nano-sized precipitates identified as the Ta phase. (e) BFI; (f) DFI of nano-sized Ta formed at the micro-sized Ta surface; and (g) corresponding SAD pattern of nano-sized precipitates signifying the Ta phase.
Fig. 7. DSC traces measured from the Zr-based BMG-C PM and the laser welds.
Furthermore, it was found that the WFZ had a slightly higher hardness. It was thought that the increased hardness may have been associated with the amount of micro-sized Ta particles transformed into nano-sized particles. Presently, available explanations for the mechanism of the increased hardness in the BMG-C WFZ caused by the size distribution or volume fraction of the reinforcement are rather limited. Considering the mechanical properties, such as strength or hardness, of the composite materials are closely related to the size distribution or volume fraction of the embedded reinforcement. Especially, when more nano-sized Ta particles form in the BMG-C matrix, they might make more effective barriers which inhibit the shear band propagation [1] and
increase the related mechanical properties mentioned above. Hence, in this study, when the laser spot re-melting process of the BMG-C was conducted, the size distribution of the Ta particles and the volume fraction of the nano-sized Ta particles were changed, which could account for the slight increase in hardness. Table 2 Tg, Tx and Tl, and developed GFA indices of the tested welds.
Parent metal Welds
Tg
Tx
Tl
DTx
g
gm
709.9 708.0
789.1 786.1
1244 1243
79.20 78.10
0.4039 0.4029
0.6980 0.6953
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3. In the HAZ, small amounts of Zr2Cu and nano-sized Ta tended to precipitate on the micro-sized Ta surface and may have acted as heterogeneous nucleation sites. 4. The pre-selected laser parameters did not significantly affect the magnitude of the GFA indices, DTx, g and gm, when compared to that of un-welded BMG-C, although small amounts of precipitation in the HAZ, as well as micro-sized Ta transformation in the WFZ occurred.
References
Fig. 8. Microhardness tests in the PM, HAZ and WFZ.
4. Conclusions A novel Ni-free (Zr48Cu32Al8Ag8,Ta4)Si0.75 bulk metallic glass composite (BMG-C) weld was fabricated in this study by means of the Nd:YAG pulse laser spot welding process with pre-selected laser parameters. After the welding, the microstructure evolution, glass forming ability (GFA) and mechanical properties of the welded samples were investigated. The results are summarized as follows: 1. Parent material (PM), heat affected zone (HAZ) and weld fusion zone (WFZ) in the welds all consisted of an amorphous matrix and two kinds of Ta particles: micro-sized and nano-sized. 2. In the WFZ, some micro-sized Ta transformed to nano-sized particles or nano-sized particle accumulations. This transformation a slightly higher magnitude of hardness in the WFZ.
[1] Suryanarayana C, Inoue A. Bulk metallic glasses. Boca Raton, FL, U.S.A: CRC Press, Taylor & Francis Group; 2011. [2] Miller M, Liaw P. Bulk metallic glasses an overview. New York: Springer Science þ Business Media, LLC; 2008. [3] Freels M, Wang GY, Zhang W, Liaw PK, Inoue A. Cyclic compression behavior of a CueZreAleAg bulk metallic glass. Intermetallics 2011;19(8):1174e83. [4] Zhang W, Zhang Q, Qin C, Inoue A. Synthesis and properties of CueZreAgeAl glassy alloys with high glass-forming ability. Materials Science and Engineering: B 2008;148(1e3):92e6. [5] Jiang QK, Wang XD, Xp Nie, Zhang GQ, Ma H, Fecht HJ, et al. Zre(Cu, Ag)eAl bulk metallic glasses. Acta Materialia 2008;56(8):1785e96. [6] Jang JSC, Jian SR, Pan DJ, Wu YH, Huang JC, Nieh TG. Thermal and mechanical characterizations of a Zr-based bulk metallic glass composite toughened by insitu precipitated Ta-rich particles. Intermetallics 2010;18(4):560e4. [7] Qin CL, Zhang W, Asami K, Kimura H, Wang XM, Inoue A. A novel Cu-based BMG composite with high corrosion resistance and excellent mechanical properties. Acta Materialia 2006;54(14):3713e9. [8] Gebert A, Kuehn U, Baunack S, Mattern N, Schultz L. Pitting corrosion of zirconium-based bulk glass-matrix composites. Materials Science and Engineering: A 2006;415(1e2):242e9. [9] Kawamura Y, Shoji T, Ohno Y. Welding technologies of bulk metllic glasses. Journal of Non-Crystalline Solids 2003;317:152e7. [10] Wang HS, Chiou MS, Chen HG, Jang JSC. The effects of initial welding temperature and welding parameters on the crystallization behaviors of laser spot welded Zr-based bulk metallic glass. Materials Chemistry and Physics 2011;129(1e2):547e52. [11] Wang HS, Chen HG, Jang JSC. Microstructure evolution in Nd:YAG laserwelded (Zr53Cu30Ni9Al8)Si0.5 bulk metallic glass alloy. Journal of Alloys and Compounds 2010;495(1):224e8. [12] Wang HS, Chen HG, Jang JSC, Chiou MS. Combination of a Nd:YAG laser and a liquid cooling device to (Zr53Cu30Ni9Al8)Si0.5 bulk metallic glass welding. Materials Science and Engineering: A 2010;528(1):338e41. [13] Jang JSC, Tsao SF, Chang LJ, Huang JC, Liu CT. Nanocrystallization of Zr61Al7.5Cu17.5Ni10Si4 metallic glass. Intermetallics 2009;17:56e64.