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Laser solid forming Zr-based bulk metallic glass
Intermetallics 22 (2012) 110e115
Contents lists available at SciVerse ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Laser solid forming Zr-based bulk metallic glass Gaolin Yang a, Xin Lin a, *, Fencheng Liu a, Qiao Hu a, Liang Ma a, Jinfu Li b, Weidong Huang a, * a b
State Key Laboratory of Solidification Processing, Northwest Polytechnical University, Xi’an 710072, China School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2011 Received in revised form 2 October 2011 Accepted 5 October 2011 Available online 27 November 2011
The crystallization characteristics of Zr55Al10Ni5Cu30 bulk metallic glasses BMGs during pulsed laser surface melting (PLSM) were examined, and the crystallization behavior during Laser solid forming (LSF) of Zr55Al10Ni5Cu30 BMGs with the pre-laid powder method on the amorphous substrates was further investigated. It was found that the BMG could keep the amorphous state after PLSM with six pulses and crystallization began to occur in heat-affected zone (HAZ) after PLSM with twelve pulses. There was no crystallization occurred in the deposit with one and two layers during LSF, and the volume fraction of amorphous phase in the deposit with seven layers deposit was about 92.44%. The crystallization degree did not increase remarkably with the increasing of deposited layers. The crystallization mainly occurred in HAZ during PLSM and LSF. A physical model was proposed to describe laser solid forming of BMGs, which explained the formation mechanism of BMGs during laser solid forming. It is shown that the crystallization during the PLSM and LSF process was mainly caused by the accumulation of structural relaxation in the HAZ. The size of HAZ should be smaller than the thickness of single pulsed laser deposited layer during LSF of BMGs without crystallization. Based on the present model and experiment results, we can reckon that bulk metallic glasses could be achieved by LSF without size limitation. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: B. glasses, metallic B. phase transformation C. laser processing C. melting
1. Introduction Bulk metallic glasses (BMGs) have attracted much attention for their exceptional physical, chemical, and mechanical properties [1]. The BMGs are usually prepared by copper mold casting technique. However, the dimension of as-cast BMGs is limited by the critical casting diameter using this method, which restricts the widespread application of the BMGs [2]. Laser solid forming (LSF) is a novel additive manufacturing technology which can be used to form high performance and fully dense metal parts in any geometry [3]. The formation of the small volume of molten pool in a relative large substrate generally presents high heating and cooling rates in LSF. The cooling rate of the molten pool during LSF using the continuous wave laser is more than 3000 C s1 [4], and it will be higher for LSF using pulsed laser. That is to say, the cooling rate of the molten pool during LSF is much higher than the critical cooling rates for the preparation of BMGs, which are generally from 1 C s1 to 100 C s1 according to the BMGs’ composition [5]. Therefore LSF should be a potential candidate to prepare BMGs without limit of critical dimensions [6]. A series of laser processing technologies have applied to prepare metallic glasses, such as laser welding [7], laser surface * Corresponding authors. Tel.: þ86 29 88494510; fax: þ86 29 88494001. E-mail addresses: [email protected] (X. Lin), [email protected] (W. Huang). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.10.008
melting [8] and laser cladding [9]. All of them indicated that BMGs can avoid crystallization during laser processing when using the suitable processing parameters. To date, some researchers have attempted to prepare BMGs with laser processing technology. Unfortunately, none of them could prove that laser processing technology can effectively prepare BMGs without size limitation. Zheng et al. have prepared Fe58Cr15Mn2B16C4Mo2Si1W1Zr1 BMG alloy in a size of 10 mm 10 mm 10 mm by LSF and found that there was a serious crystallization in the asdeposited alloy [10]. Sun et al. have used a laser deposition process to deposit one layer Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 metallic glass on a amorphous substrates of the same nominal composition, and found that reducing the heat input during laser processing can result in the formation of amorphous deposited layer and the near elimination of the crystalline HAZ in the amorphous substrates, but the thickness of their deposited layer was only about 0.2 mm [6]. In this paper, the crystallization characteristics of Zr55Al10Ni5Cu30 BMGs during pulsed laser surface melting (PLSM) were first investigate, and then the LSF using pulsed laser with the prelaid powder method was used to prepare Zr55Al10Ni5Cu30 BMGs, the crystallization behaviors of Zr55Al10Ni5Cu30 BMGs during LSF was further investigated. Finally, a physical model was proposed to describe the LSF of BMGs.
G. Yang et al. / Intermetallics 22 (2012) 110e115
2. Experimental procedures Zr55Al10Ni5Cu30 (atomic percent) alloys ingots were prepared by arc melting with a mixture of pure Zr(99.95 wt%), Al(99.99 wt%), Ni(99.99 wt%), Cu(99.99 wt%) under an argon atmosphere. The amorphous plates with the thickness of 2.1 mm were produced by water cooled copper mold casting using the homogenized ingots. These plates were further cut into small plates with the dimensions of 5 12 2.1 mm3, which were used as the substrates. The substrate surfaces were ground with 600 grit SiC paper and cleaned with acetone prior to pulsed laser surface melting and laser solid forming process. Argon atomized Zr55Al10Ni5Cu30 powder in a size range of 5e150 mm was used for LSF of Zr55Al10Ni5Cu30 BMGs. The laser surface melting and laser solid forming experiments were performed in a glove box with argon shielding gas using a 300 W Nd:YAG pulsed laser. The diameter of laser beam was 1 mm. The nominal single pulsed energy was 80 J with pulse duration of 3 ms. The pulsed frequency was 0.1 Hz. As for PLSM of the BMGs, the surface of the amorphous plates was remelted at single point by pulsed laser for one, six, twelve and twenty pulses, respectively. As for LSF of the BMGs, the pre-laid powder method was used. During LSF, a layer of the powder with the thickness of 0.2 mm was first laid in the amorphous substrate or pre-deposited layer, then this powder layer was melted point by point along the pre-set trajectory by pulsed laser beam and resolidified to form a solid deposited layer bonded metallurgically with the substrate or pre-deposited layer. Thus, a 3-D BMGs can be produced by this repeated laser deposition layer by layer. Fig. 1 shows the LSF process of BMGs. The laser scanning speed was 0.095 mm s1. The interval between laser deposited tracks was 0.95 mm. In present work, the deposits with one, two, four and seven laser deposited layers were prepared respectively. The crystallization state of the substrate, powder and the deposit with seven layers was characterized by X-ray diffraction (X’Pert MPD PRO, XRD). Phase transformations in the heating process of the deposit with seven layers and the substrate were studied by differential scanning calorimetry (NETZSCH STA 449C, DSC) at a heating rate of 20 C/min. The microstructural characteristics of the samples were first examined using the metallographic microscope after etched with a mixture of 10 ml H2O, 10 ml HNO3 and 1 ml HF. Detail microstructural examination was further performed by scanning electron microscopy (Tescan VEGAII LMH, SEM) and transmission electron microscopy (Philip Tecnai F 30G2, TEM). The oxygen contents of the powder and substrate were measured by LECO TC600 oxygenenitrogen analyzer. 3. Results The microstructures of the Zr55Al10Ni5Cu30 BMGs by PLSM are shown in Fig. 2(a). There is no obvious difference in the microstructure between the remelted zone and substrate for PLSM with one and six pulses. There only show some streamlines in the
Fig. 1. Schematic diagram of the LSF of BMGs.
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remelted zone which indicated the occurrence of the remelting phenomena during PLSM. When the pulse number is more than twelve, there shows a featureless white remelted zone surrounded by distinct crystalline heat affected zones (HAZ), which consisted mainly of the sparse clusters of spherulites. The morphology of the spherulites presented a central nucleation and rapid radial growth pattern (Fig. 2(c)), which is similar with the observation in laser processed Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMGs [6]. Especially, the number and size of clusters of spherulites increased with the laser pulse. As for the featureless white remelted zone, it could be reckoned that it should still keep the amorphous state according to the previous researches [6,11]. The microstructures of the deposits with one, two, four and seven layers are shown in Fig. 2(b). Similar to PLSM with one and six pulses, there is also no obvious difference in the microstructure between the deposited layers and the substrate for LSF with one and two deposited layers. When the four layers were deposited, there shows a series of featureless white deposited zones separated by distinct crystalline bands, which consisted mainly of the clusters of dendrites (Fig. 2(d)). It can be seen that the crystalline bands lay between the adjacent deposited layers and traces. That means that these crystalline bands should result from the crystallization in the HAZ in the pre-deposited layer during laser depositing adjacent layer, the featureless white deposited zones could be amorphous. It can be found from Fig. 2(b) that the deposit with seven deposited layers contains about 92.44% (volume fraction) amorphous phase, and the crystallization behavior has no obvious change with increasing the deposited layers. . For verifying the crystallization characteristics in the deposits, Fig. 3 gives the TEM bright-field images and the diffraction patterns at the interface between the adjacent layers for the deposit with seven deposited layers. It can be seen from Fig. 3(a) that there is a sudden change between the amorphous zone and crystalline zone. Fig. 3(b) shows the high magnification image and diffraction pattern of an amorphous zone. Fig. 3(c) shows the high magnification image and diffraction patterns of a crystalline zone. There are the facecentered cubic Al5Ni3Zr2 phase particles with the average size of about 270 nm in amorphous matrix in the crystalline zone. It should be indicated that these nano-size particles generally appeared near the micro-size close-packed hexagonal dendrites (Fig. 3(d)). For further making clear these crystallization characteristics in the HAZ in the adjacent layer, Fig. 4 give a high magnification SEM image at the interface between the adjacent layers for the deposit with seven deposited layers. It can be found from Fig. 4 that there exist the crystal particles with the average size of about 300 nm near the clusters of dendrites, which is consistent with the above TEM results. Thus, based on the mentioned above, it can be reckoned that the featureless white deposited zones in Fig. 2(b) should be amorphous. Fig. 5 shows the XRD patterns of the substrate, powders and the deposit with the seven layers. It can be seen that the XRD pattern of the as-cast substrate exhibits a broad amorphous peak, and the XRD patterns for both powder and the deposit also present a broad amorphous peaks, but there are a few weak sharp Al5Ni3Zr2 crystal peaks in their XRD patterns. That means that the powder and deposit mainly consist of amorphous and a little Al5Ni3Zr2 crystallization phase. The volume fractions of amorphous phase in the powder and deposit are about 96.29 1.96at% and 84.50 11.02at % according to Fig. 5. It can be seen that the volume fractions of amorphous in the deposit estimated by XRD pattern is similar with that measured by optical observation. The DSC curves of the deposit with seven layers and the substrate are shown in Fig. 6. The glass transition temperature (Tg) of the Zr55Al10Ni5Cu30 substrate is 400 C, its onset crystallization temperature (Tx) is 485 C. The Tx of deposit with seven layers is low than that of substrate which should result from that there exist
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Fig. 2. (a) Microstructures of the Zr55Al10Ni5Cu30 BMGs by PLSM with one, six, twelve and twenty pulsed laser irradiations; (b) Microstructures of the deposits with one, two, four and seven layers. (c) SEM of twenty times irradiations. (d) SEM of seven deposited layers.
Fig. 3. TEM bright-field images of seven deposited layers. (a) an amorphous zone. (b) a crystallization zone. (c) a junction area of amorphous zone and crystallization zone. SA1, SA2, SA3 and SA4 are selected area diffraction patterns for marked positions. They show that amorphous zone is amorphous phase without crystallization and crystallization zone is face-centered cubic Al5Ni3Zr2 phase crystal particle and micro-size close-packed hexagonal dendrite in amorphous matrix.
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Fig. 6. DSC curves of the deposit with seven layers and the substrate. Fig. 4. SEM images of seven deposited layers. Upright part is crystal particles similar as the crystal particles shown in Fig. 3. Left part is amorphous.
a little crystallization in deposit with seven layers. The volume fraction of amorphous phase of the deposit is calculated to be 93.96 wt% by comparing the crystallization latent heat of the deposit and the substrate. It is interesting to note that, even the nano-size crystal particles cannot be clearly seen in the metallographic microscope image (Fig. 2(b)), the amorphous phase content estimated from Fig. 2(b) is acceptable as the volume fraction of the nano-size crystal particles is very small. 4. Discussion 4.1. Simulation of temperature distribution during PLSM The DSC results of Zr55Al10Ni5Cu30 in our previous work [11] revealed that the melting temperature(Tm) of Zr55Al10Ni5Cu30 is 834 C. The temperature distribution of the substrate during PLSM with a single pulse was simulated by a three-dimensional finite element model using Comsol software. The simulation results are shown in Fig. 7. It can be seen that there is a zone between the isothermal lines of Tm and Tg as shown in Fig. 7(a) in the substrate
during PLSM. This zone corresponded to the HAZ of the molten pool. That means the HAZ should be most likely to occur crystallization if there is enough structural relaxation or enough low cooling rate in the HAZ. It is consistent with the experimental results by PLSM as shown in Fig. 2(a). Fig. 7(b) shows that the average heating and cooling rates from Tg to Tm at point a shown in Fig. 7(a), which is at the center of the top center of the molten pool, were 4.3 106 C s1 and 3.7 104 C s1 respectively. While the critical cooling rate for the amorphous formation of Zr65Al7.5Ni10Cu17.5 is 1.5 C s1 [12]. Since the crystallization of BMGs is a kinetic process, the enough rapid heating or cooling rate can avoid the crystallization of BMGs [13]. Thus, in rapid heating process, the amorphous alloy will transform to supercooled liquid at Tg, and form the superheat liquid at Tm. During rapid cooling process, the melted liquid could transform to amorphous under Tg again [1]. That is to say, there is no phase transformation happened at Tm. For this reason, the remelted zone in present work was defined as the zone whose temperature is higher than Tm during PLSM and LSF. The HAZ was redefined as the zone where the temperature is higher than Tg but lower than Tm during PLSM and LSF. The zone with the temperature lower than Tg during PLSM and LSF was called thermo-stable zone. 4.2. Crystallization behaviors in remelted zone, HAZ and thermostable zone during PLSM
Fig. 5. XRD patterns of the substrate and powder and micro-focused XRD patterns of the substrate and the deposit with seven layers.
The heating and cooling process caused by a pulsed laser irradiation will lead to a heat shock to the amorphous substrate. During a heat shock, the zone in the BMGs whose temperature was higher than Tg can pass through supercooled liquid phase zone, but will occur relaxation which would be inclined to crystallization when the enough relaxation is experienced [14]. For the remelted zone, the relaxation produced by the first heat shock would be remelted during the second pulse laser irradiation as the temperature in remelted zone was higher than Tm. So the relaxation in remelted zone would not be accumulated during laser irradiation with multiple pulses. However, for the HAZ, its temperature was lower than Tm and higher than Tg during the heat shock, and the relaxation was accumulated during multiple pulse laser irradiation. The immoderate relaxation will result in crystallization, which could be realized in the HAZ when it experienced laser irradiation with the enough pulse. For thermo-stable zone where the temperature was lower than Tg, there would be no crystallization during multiple pulse laser irradiation, since the previous isothermal anneal experiments of Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMGs showed that a distinct
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Fig. 7. The simulation result of the temperature distribution of the substrate with a single pulsed laser irradiation. The substrate was 2 mm thick, just the top half was shown in the picture. (a) The temperature distribution for 3.4 ms after irradiation. (b) Thermal history at three points shown in (a). a, b and g are all at the axial symmetry axis of temperature field. a is at the surface of the specimen, b is at the bottom of the remelting zone and g is at the bottom of the heat affected zone.
crystallization produced in Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMGs at Tg needed several hours [15] while the operation time when the local temperature of the BMGs is higher that Tg is only 17.2 ms during single pulse laser irradiation. In addition, Tg of BMGs will vary with the heating or cooling rate, and the higher heating or cooling rate will lead to the increase of Tg [15]. If the Tg measured by DSC method was named Tg0. The real Tg in PLSM should be higher than Tg0. Thus, the thermo-stable zone which was defined by Tg0 should not be suffered from crystallization. Thus, as shown in Fig. 2(a), the HAZ would first keep no visible crystallization after PLSM with a little pulses, but get more and more crystallization when the pulse number was more than six, while the remelted zone and thermostable zone could keep no crystallization. If BMGs can avoid crystallization after the PLSM with one pulse, the accumulation of the relaxation in the HAZ will be the main factor affecting the crystallization during laser processing BMGs with multiple pulses. If the heat shock in HAZ was named as the effective heat shock (EHS), the main factor affecting the LSF of the BMGs should be the maximum number of EHS that BMGs can undertake to keep no crystallization. 4.3. Physic model for LSF of BMGs The schematic diagram of the cross section of the deposit along the laser scanning direction during laser solid forming was shown in Fig. 8. It can be seen that the subsequent pulsed laser depositing will affect the crystallization behaviors of the pre-deposited layers. For example, the laser depositing of the hatched zone with three traces which is adjacent to A zone in Fig. 8 can bring EHS to A zone. But the size of HAZ is small, so only a little subsequent pulsed laser depositing can bring EHS to A zone. When the distance of the depositing zone to A zone is large enough, no more EHS will be brought to A zone during subsequent LSF. If A zone could still keep amorphous at this time, it will not occur crystallization any more in the subsequent LSF process. Thus, if the number of EHS that each deposited layer experienced during whole LSF process is lower than the maximum number of EHS for occurring crystallization, the BMGs would be prepared by LSF.
Thus, LSF of the BMGs without crystallization depends on the number of EHS that the pre-deposited amorphous layers experienced (NEP) and their structural relaxations induced by each EHS. The NEP depends on the size of HAZ of the subsequent deposited layers and the layer thickness during single pulse laser deposition. If the size of the HAZ is much smaller than the layer thickness during single pulse laser deposition, the NEP was only one for most of parts in the HAZ of the deposited layers and two or three for the parts of the HAZ in the overlapping zone between the adjacent deposited layers. If the size of the HAZ is larger than the thickness during single pulse laser deposition, the NEP will increase with increasing deposited layers remarkably. In addition, the structural relaxation in the pre-deposited amorphous layers depends on the heating and cooling rates during the current and subsequent laser deposition process. The enough high heating and cooling rate could induce little structural relaxation. However, The higher heating and cooling rate generally means the less energy input and the shorter existing time of the remelted zone, which will not only induce the lower deposition efficiency but also lead to the smaller size of the HAZ. So, to avoid the occurrence of the crystallization during LSF of the BMGs, a good combination of NEP and their structural relaxations induced by each EHS should be found.
Fig. 8. The schematic diagram of the cross section of LSF BMGs.
G. Yang et al. / Intermetallics 22 (2012) 110e115
The relative small volume remelted zone was mainly cooled by relative large volume substrate which was further cooled by the external conditions such as the surrounding gas convection and the large copper fixture. This leads to a high cooling rate in the remelted zone and the HAZ. However, a continuous or high dense energy inputs in the remelted zone will lead to a rise in the temperature of the substrate, which will make a decrease of cooling rate in the remelted zone and the HAZ and further cause the crystallization in the HAZ. This effect is named as the cumulative effect of heat input (CEH). So, during LSF with the pulses laser, the pulsed frequency should be reduced to make sure that the substrate and the pre-deposited layers could be cooled to a suitable lower temperature just before every pulse laser irradiation. 4.4. Crystallization behaviors in the deposit As the analysis above, the crystallization in the deposit could mainly occur in HAZ of every deposited layer. The shape of single pulse laser deposited layer can be estimated by the shape of crystalline band in the deposit. It can be seen from Fig. 2(b) that the layer thickness during single pulse laser depositing was much larger than the size of HAZ. Thus, most of parts in the pre-deposited layer did not experience the EHS. Part of the HAZ experienced the EHS one time. The HAZ in the overlapping zone between the adjacent layers got the more crystallization as it experienced two or three EHSes. The relative small volume HAZ also results in that there was no obvious change in the morphology of the crystalline band with the increase of the deposited layers. This also means that when a large size BMGs was prepared by LSF, the crystallization would not be aggravated during the multi-layer deposition process. It is interesting to note that Fig. 2(b) shows the crystallization only happened in the deposited region during LSF. There is no crystallization in the substrate. Comparing Fig. 2(c) with Fig. 2(d), it also can be seen that the crystallization in the crystalline bands in the deposit during LSF is worse than that in the HAZ of the substrate during PLSM. On the one hand, this may be related to the low CEH in the substrate since it generally has a better cooling environment compared with the deposited region. On the other hand, it is well known that the oxygen content has an important effect on the glass forming ability of Zr-based BMGs, The oxygen content should be less than 0.025 wt.% for the fabrication of the ZreTieCueNieAi alloy BMGs [16]. Since there exists a possible increase in the oxygen content during the powder preparation, the oxygen contents of the powder and the substrate were further measured. The measured oxygen contents of the powder and the substrate were 0.2 wt.% and 0.02 wt.% respectively. So the worse crystallization in the deposited region was more probably caused by the higher oxygen content in the deposit. It should be indicated that the processing parameters in present work were not optimized. Based on the above results, it can be reckoned that LSF technique could be used to prepare BMGs without the limitation of critical dimension. Since LSF is a free form fabrication technique [3], the BMGs could be prepared with any size and any shape under a certain precision through LSF. 5. Conclusions The possibility of preparing Zr-based BMGs by laser solid forming (LSF) without size limitation was investigated by experiments and further analyzed by the physic model. The main results are summarized as follows: (1) Zr55Al10Ni5Cu30 BMGs could kept the amorphous state after pulsed laser surface melting (PLSM) with six pulses and the crystallization occurred after PLSM with twelve and twenty
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pulses. There was no crystallization occurred in the deposit with one and two layers deposits during LSF, and the volume fraction of amorphous phase in the deposit with seven layers deposit was about 92.44%. The crystallization degree did not increase remarkably with the increasing of deposited layers during laser solid forming. The crystallization only occurred in the heat-affected zone (HAZ) during PLSM and LSF of BMGs. (2) The remelted zone was defined as the zone whose temperature is higher than Tm, the HAZ was redefined as the zone where the temperature is higher than Tg but lower than Tm, and the zone with the temperature lower than Tg was called thermo-stable zone during laser processing BMGs. There is no crystallization in remelted zone and thermo-stable zone during laser processing. The crystallization during the laser processing was mainly caused by the accumulation of structural relaxation in the HAZ. (3) The adaptation of of laser pulse number and the structural relaxations induced by each laser pulse is the key factor for LSF of BMGs without crystallization. The siz of HAZ should be smaller than the thickness of single pulsed laser deposited layer during LSF of BMGs without crystallization. (4) Combining with the physical model, LSF of BMGs without crystallization and the limitation of critical dimensions has been proved to be feasible. Acknowledgments The work is funded by the Research Program of National Natural Science Foundation of China (No. 50971102), China Postdoctoral Scientific Fund (No. 20090461312) and the fund of the State Key Laboratory of Solidification Processing in NPU (Nos. 16-TZ-2007, 39QZ-2009 and 05-BZ-2010). References [1] Byrne CJ, Eldrup M. Bulk metallic glasses. Science 2008;321:502e3. [2] Lu ZP, Liu CT, Thomspon JR, Porter WD. Structural amorphous steels. Phys Rev Lett 2004;92(24):245503-1e245503-4. [3] Lin X, Yue TM, Yang HO, Huang WD. microstructure and phase evolution in laser rapid forming of a functionally graded Ti-Rene88DT alloy. Acta Mater 2006;54:1901e15. [4] Tan H, Chen J, Lin X, Zhao XM, Huang WD. Research on molten pool temperature in the process of laser rapid forming. Mater Sci Forum 2007; 2301:546e9. [5] Telford M. The case for bulk metallic glass. Mater Today 2004;7:36e43. [6] Sun H, Flores KM. Microstructural analysis of a laser-processed Zr-based bulk metallic glass. Metall Mater Trans A 2010;41A:1752e7. [7] Kawahito Y, Terajima T, Kimura H, Kuroda T, Nakata K, Katayama S. Highpower fiber laser welding and its application to metallic glass Zr55Al10Ni5Cu30. Mater Sci Eng.B 2008;148:105e9. [8] Ma F, Yang JJ, Zhu XN, Liang CY, Wang HS. Femtosecond laser-induced concentric ring microstructures on Zr-based metallic glass. Appl Surf Sci 2010;256:3653e60. [9] Yue TM, Su YP, Yang HO. Laser cladding of Zr65Al7.5Ni10Cu17.5 amorphous alloy on magnesium. Mater Lett 2007;61:209e12. [10] Zheng B, Zhou Y, Smugeresky JE, Lavernia EJ. Processing and behavior of Febased metallic glass components via laser-engineered net shaping. Metall Mater Trans A 2009;40A:1235e45. [11] Liu WW, Lin X, Yang GL, Yang HO, Li WDJF. Huang. Influence of glass forming ability of alloy on crystallization in heat-affected zone by laser remelting Zr based bulk metallic glasses. Chin J Lasers 2010;37:2931e6. [12] Inoue A, Zhang T, Nishiyama N, Ohba K, Masumoto T. Preparation of 16 mm diameter rod of amorphous Zr65Al7.5Ni10Cu17.5 alloy Mater. Trans 1993;34: 1234e7. [13] Mukherjee S, Schroers J, Zhou Z, Johnson WL, Rhim WK. Viscosity and specific of volume of bulk metallic galss-forming alloys and their correlation with glass forming ability. Acta Mater 2004;52:3689e95. [14] Wang HR, Gao YL, Min GH, Hui XD, Ye YF. Primary crystallization in rapidly solidified Zr70Cu20Ni10 alloy from a supercooled liquid region. Phys Lett A 2003;314:81e7. [15] Suryanarayana C, Inoue A. Bulk mettallic glasses. New York: CRC Press; 2011. [16] Lin XH, Johnson WL, Rhim WK. Effect of oxygen impurity on crystallization of an undercooled bulk glass forming ZreTieCueNieAi alloy. Mater Trans 1997; 38:473e7.
Contents lists available at SciVerse ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Laser solid forming Zr-based bulk metallic glass Gaolin Yang a, Xin Lin a, *, Fencheng Liu a, Qiao Hu a, Liang Ma a, Jinfu Li b, Weidong Huang a, * a b
State Key Laboratory of Solidification Processing, Northwest Polytechnical University, Xi’an 710072, China School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 June 2011 Received in revised form 2 October 2011 Accepted 5 October 2011 Available online 27 November 2011
The crystallization characteristics of Zr55Al10Ni5Cu30 bulk metallic glasses BMGs during pulsed laser surface melting (PLSM) were examined, and the crystallization behavior during Laser solid forming (LSF) of Zr55Al10Ni5Cu30 BMGs with the pre-laid powder method on the amorphous substrates was further investigated. It was found that the BMG could keep the amorphous state after PLSM with six pulses and crystallization began to occur in heat-affected zone (HAZ) after PLSM with twelve pulses. There was no crystallization occurred in the deposit with one and two layers during LSF, and the volume fraction of amorphous phase in the deposit with seven layers deposit was about 92.44%. The crystallization degree did not increase remarkably with the increasing of deposited layers. The crystallization mainly occurred in HAZ during PLSM and LSF. A physical model was proposed to describe laser solid forming of BMGs, which explained the formation mechanism of BMGs during laser solid forming. It is shown that the crystallization during the PLSM and LSF process was mainly caused by the accumulation of structural relaxation in the HAZ. The size of HAZ should be smaller than the thickness of single pulsed laser deposited layer during LSF of BMGs without crystallization. Based on the present model and experiment results, we can reckon that bulk metallic glasses could be achieved by LSF without size limitation. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: B. glasses, metallic B. phase transformation C. laser processing C. melting
1. Introduction Bulk metallic glasses (BMGs) have attracted much attention for their exceptional physical, chemical, and mechanical properties [1]. The BMGs are usually prepared by copper mold casting technique. However, the dimension of as-cast BMGs is limited by the critical casting diameter using this method, which restricts the widespread application of the BMGs [2]. Laser solid forming (LSF) is a novel additive manufacturing technology which can be used to form high performance and fully dense metal parts in any geometry [3]. The formation of the small volume of molten pool in a relative large substrate generally presents high heating and cooling rates in LSF. The cooling rate of the molten pool during LSF using the continuous wave laser is more than 3000 C s1 [4], and it will be higher for LSF using pulsed laser. That is to say, the cooling rate of the molten pool during LSF is much higher than the critical cooling rates for the preparation of BMGs, which are generally from 1 C s1 to 100 C s1 according to the BMGs’ composition [5]. Therefore LSF should be a potential candidate to prepare BMGs without limit of critical dimensions [6]. A series of laser processing technologies have applied to prepare metallic glasses, such as laser welding [7], laser surface * Corresponding authors. Tel.: þ86 29 88494510; fax: þ86 29 88494001. E-mail addresses: [email protected] (X. Lin), [email protected] (W. Huang). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.10.008
melting [8] and laser cladding [9]. All of them indicated that BMGs can avoid crystallization during laser processing when using the suitable processing parameters. To date, some researchers have attempted to prepare BMGs with laser processing technology. Unfortunately, none of them could prove that laser processing technology can effectively prepare BMGs without size limitation. Zheng et al. have prepared Fe58Cr15Mn2B16C4Mo2Si1W1Zr1 BMG alloy in a size of 10 mm 10 mm 10 mm by LSF and found that there was a serious crystallization in the asdeposited alloy [10]. Sun et al. have used a laser deposition process to deposit one layer Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 metallic glass on a amorphous substrates of the same nominal composition, and found that reducing the heat input during laser processing can result in the formation of amorphous deposited layer and the near elimination of the crystalline HAZ in the amorphous substrates, but the thickness of their deposited layer was only about 0.2 mm [6]. In this paper, the crystallization characteristics of Zr55Al10Ni5Cu30 BMGs during pulsed laser surface melting (PLSM) were first investigate, and then the LSF using pulsed laser with the prelaid powder method was used to prepare Zr55Al10Ni5Cu30 BMGs, the crystallization behaviors of Zr55Al10Ni5Cu30 BMGs during LSF was further investigated. Finally, a physical model was proposed to describe the LSF of BMGs.
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2. Experimental procedures Zr55Al10Ni5Cu30 (atomic percent) alloys ingots were prepared by arc melting with a mixture of pure Zr(99.95 wt%), Al(99.99 wt%), Ni(99.99 wt%), Cu(99.99 wt%) under an argon atmosphere. The amorphous plates with the thickness of 2.1 mm were produced by water cooled copper mold casting using the homogenized ingots. These plates were further cut into small plates with the dimensions of 5 12 2.1 mm3, which were used as the substrates. The substrate surfaces were ground with 600 grit SiC paper and cleaned with acetone prior to pulsed laser surface melting and laser solid forming process. Argon atomized Zr55Al10Ni5Cu30 powder in a size range of 5e150 mm was used for LSF of Zr55Al10Ni5Cu30 BMGs. The laser surface melting and laser solid forming experiments were performed in a glove box with argon shielding gas using a 300 W Nd:YAG pulsed laser. The diameter of laser beam was 1 mm. The nominal single pulsed energy was 80 J with pulse duration of 3 ms. The pulsed frequency was 0.1 Hz. As for PLSM of the BMGs, the surface of the amorphous plates was remelted at single point by pulsed laser for one, six, twelve and twenty pulses, respectively. As for LSF of the BMGs, the pre-laid powder method was used. During LSF, a layer of the powder with the thickness of 0.2 mm was first laid in the amorphous substrate or pre-deposited layer, then this powder layer was melted point by point along the pre-set trajectory by pulsed laser beam and resolidified to form a solid deposited layer bonded metallurgically with the substrate or pre-deposited layer. Thus, a 3-D BMGs can be produced by this repeated laser deposition layer by layer. Fig. 1 shows the LSF process of BMGs. The laser scanning speed was 0.095 mm s1. The interval between laser deposited tracks was 0.95 mm. In present work, the deposits with one, two, four and seven laser deposited layers were prepared respectively. The crystallization state of the substrate, powder and the deposit with seven layers was characterized by X-ray diffraction (X’Pert MPD PRO, XRD). Phase transformations in the heating process of the deposit with seven layers and the substrate were studied by differential scanning calorimetry (NETZSCH STA 449C, DSC) at a heating rate of 20 C/min. The microstructural characteristics of the samples were first examined using the metallographic microscope after etched with a mixture of 10 ml H2O, 10 ml HNO3 and 1 ml HF. Detail microstructural examination was further performed by scanning electron microscopy (Tescan VEGAII LMH, SEM) and transmission electron microscopy (Philip Tecnai F 30G2, TEM). The oxygen contents of the powder and substrate were measured by LECO TC600 oxygenenitrogen analyzer. 3. Results The microstructures of the Zr55Al10Ni5Cu30 BMGs by PLSM are shown in Fig. 2(a). There is no obvious difference in the microstructure between the remelted zone and substrate for PLSM with one and six pulses. There only show some streamlines in the
Fig. 1. Schematic diagram of the LSF of BMGs.
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remelted zone which indicated the occurrence of the remelting phenomena during PLSM. When the pulse number is more than twelve, there shows a featureless white remelted zone surrounded by distinct crystalline heat affected zones (HAZ), which consisted mainly of the sparse clusters of spherulites. The morphology of the spherulites presented a central nucleation and rapid radial growth pattern (Fig. 2(c)), which is similar with the observation in laser processed Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMGs [6]. Especially, the number and size of clusters of spherulites increased with the laser pulse. As for the featureless white remelted zone, it could be reckoned that it should still keep the amorphous state according to the previous researches [6,11]. The microstructures of the deposits with one, two, four and seven layers are shown in Fig. 2(b). Similar to PLSM with one and six pulses, there is also no obvious difference in the microstructure between the deposited layers and the substrate for LSF with one and two deposited layers. When the four layers were deposited, there shows a series of featureless white deposited zones separated by distinct crystalline bands, which consisted mainly of the clusters of dendrites (Fig. 2(d)). It can be seen that the crystalline bands lay between the adjacent deposited layers and traces. That means that these crystalline bands should result from the crystallization in the HAZ in the pre-deposited layer during laser depositing adjacent layer, the featureless white deposited zones could be amorphous. It can be found from Fig. 2(b) that the deposit with seven deposited layers contains about 92.44% (volume fraction) amorphous phase, and the crystallization behavior has no obvious change with increasing the deposited layers. . For verifying the crystallization characteristics in the deposits, Fig. 3 gives the TEM bright-field images and the diffraction patterns at the interface between the adjacent layers for the deposit with seven deposited layers. It can be seen from Fig. 3(a) that there is a sudden change between the amorphous zone and crystalline zone. Fig. 3(b) shows the high magnification image and diffraction pattern of an amorphous zone. Fig. 3(c) shows the high magnification image and diffraction patterns of a crystalline zone. There are the facecentered cubic Al5Ni3Zr2 phase particles with the average size of about 270 nm in amorphous matrix in the crystalline zone. It should be indicated that these nano-size particles generally appeared near the micro-size close-packed hexagonal dendrites (Fig. 3(d)). For further making clear these crystallization characteristics in the HAZ in the adjacent layer, Fig. 4 give a high magnification SEM image at the interface between the adjacent layers for the deposit with seven deposited layers. It can be found from Fig. 4 that there exist the crystal particles with the average size of about 300 nm near the clusters of dendrites, which is consistent with the above TEM results. Thus, based on the mentioned above, it can be reckoned that the featureless white deposited zones in Fig. 2(b) should be amorphous. Fig. 5 shows the XRD patterns of the substrate, powders and the deposit with the seven layers. It can be seen that the XRD pattern of the as-cast substrate exhibits a broad amorphous peak, and the XRD patterns for both powder and the deposit also present a broad amorphous peaks, but there are a few weak sharp Al5Ni3Zr2 crystal peaks in their XRD patterns. That means that the powder and deposit mainly consist of amorphous and a little Al5Ni3Zr2 crystallization phase. The volume fractions of amorphous phase in the powder and deposit are about 96.29 1.96at% and 84.50 11.02at % according to Fig. 5. It can be seen that the volume fractions of amorphous in the deposit estimated by XRD pattern is similar with that measured by optical observation. The DSC curves of the deposit with seven layers and the substrate are shown in Fig. 6. The glass transition temperature (Tg) of the Zr55Al10Ni5Cu30 substrate is 400 C, its onset crystallization temperature (Tx) is 485 C. The Tx of deposit with seven layers is low than that of substrate which should result from that there exist
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Fig. 2. (a) Microstructures of the Zr55Al10Ni5Cu30 BMGs by PLSM with one, six, twelve and twenty pulsed laser irradiations; (b) Microstructures of the deposits with one, two, four and seven layers. (c) SEM of twenty times irradiations. (d) SEM of seven deposited layers.
Fig. 3. TEM bright-field images of seven deposited layers. (a) an amorphous zone. (b) a crystallization zone. (c) a junction area of amorphous zone and crystallization zone. SA1, SA2, SA3 and SA4 are selected area diffraction patterns for marked positions. They show that amorphous zone is amorphous phase without crystallization and crystallization zone is face-centered cubic Al5Ni3Zr2 phase crystal particle and micro-size close-packed hexagonal dendrite in amorphous matrix.
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Fig. 6. DSC curves of the deposit with seven layers and the substrate. Fig. 4. SEM images of seven deposited layers. Upright part is crystal particles similar as the crystal particles shown in Fig. 3. Left part is amorphous.
a little crystallization in deposit with seven layers. The volume fraction of amorphous phase of the deposit is calculated to be 93.96 wt% by comparing the crystallization latent heat of the deposit and the substrate. It is interesting to note that, even the nano-size crystal particles cannot be clearly seen in the metallographic microscope image (Fig. 2(b)), the amorphous phase content estimated from Fig. 2(b) is acceptable as the volume fraction of the nano-size crystal particles is very small. 4. Discussion 4.1. Simulation of temperature distribution during PLSM The DSC results of Zr55Al10Ni5Cu30 in our previous work [11] revealed that the melting temperature(Tm) of Zr55Al10Ni5Cu30 is 834 C. The temperature distribution of the substrate during PLSM with a single pulse was simulated by a three-dimensional finite element model using Comsol software. The simulation results are shown in Fig. 7. It can be seen that there is a zone between the isothermal lines of Tm and Tg as shown in Fig. 7(a) in the substrate
during PLSM. This zone corresponded to the HAZ of the molten pool. That means the HAZ should be most likely to occur crystallization if there is enough structural relaxation or enough low cooling rate in the HAZ. It is consistent with the experimental results by PLSM as shown in Fig. 2(a). Fig. 7(b) shows that the average heating and cooling rates from Tg to Tm at point a shown in Fig. 7(a), which is at the center of the top center of the molten pool, were 4.3 106 C s1 and 3.7 104 C s1 respectively. While the critical cooling rate for the amorphous formation of Zr65Al7.5Ni10Cu17.5 is 1.5 C s1 [12]. Since the crystallization of BMGs is a kinetic process, the enough rapid heating or cooling rate can avoid the crystallization of BMGs [13]. Thus, in rapid heating process, the amorphous alloy will transform to supercooled liquid at Tg, and form the superheat liquid at Tm. During rapid cooling process, the melted liquid could transform to amorphous under Tg again [1]. That is to say, there is no phase transformation happened at Tm. For this reason, the remelted zone in present work was defined as the zone whose temperature is higher than Tm during PLSM and LSF. The HAZ was redefined as the zone where the temperature is higher than Tg but lower than Tm during PLSM and LSF. The zone with the temperature lower than Tg during PLSM and LSF was called thermo-stable zone. 4.2. Crystallization behaviors in remelted zone, HAZ and thermostable zone during PLSM
Fig. 5. XRD patterns of the substrate and powder and micro-focused XRD patterns of the substrate and the deposit with seven layers.
The heating and cooling process caused by a pulsed laser irradiation will lead to a heat shock to the amorphous substrate. During a heat shock, the zone in the BMGs whose temperature was higher than Tg can pass through supercooled liquid phase zone, but will occur relaxation which would be inclined to crystallization when the enough relaxation is experienced [14]. For the remelted zone, the relaxation produced by the first heat shock would be remelted during the second pulse laser irradiation as the temperature in remelted zone was higher than Tm. So the relaxation in remelted zone would not be accumulated during laser irradiation with multiple pulses. However, for the HAZ, its temperature was lower than Tm and higher than Tg during the heat shock, and the relaxation was accumulated during multiple pulse laser irradiation. The immoderate relaxation will result in crystallization, which could be realized in the HAZ when it experienced laser irradiation with the enough pulse. For thermo-stable zone where the temperature was lower than Tg, there would be no crystallization during multiple pulse laser irradiation, since the previous isothermal anneal experiments of Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMGs showed that a distinct
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Fig. 7. The simulation result of the temperature distribution of the substrate with a single pulsed laser irradiation. The substrate was 2 mm thick, just the top half was shown in the picture. (a) The temperature distribution for 3.4 ms after irradiation. (b) Thermal history at three points shown in (a). a, b and g are all at the axial symmetry axis of temperature field. a is at the surface of the specimen, b is at the bottom of the remelting zone and g is at the bottom of the heat affected zone.
crystallization produced in Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMGs at Tg needed several hours [15] while the operation time when the local temperature of the BMGs is higher that Tg is only 17.2 ms during single pulse laser irradiation. In addition, Tg of BMGs will vary with the heating or cooling rate, and the higher heating or cooling rate will lead to the increase of Tg [15]. If the Tg measured by DSC method was named Tg0. The real Tg in PLSM should be higher than Tg0. Thus, the thermo-stable zone which was defined by Tg0 should not be suffered from crystallization. Thus, as shown in Fig. 2(a), the HAZ would first keep no visible crystallization after PLSM with a little pulses, but get more and more crystallization when the pulse number was more than six, while the remelted zone and thermostable zone could keep no crystallization. If BMGs can avoid crystallization after the PLSM with one pulse, the accumulation of the relaxation in the HAZ will be the main factor affecting the crystallization during laser processing BMGs with multiple pulses. If the heat shock in HAZ was named as the effective heat shock (EHS), the main factor affecting the LSF of the BMGs should be the maximum number of EHS that BMGs can undertake to keep no crystallization. 4.3. Physic model for LSF of BMGs The schematic diagram of the cross section of the deposit along the laser scanning direction during laser solid forming was shown in Fig. 8. It can be seen that the subsequent pulsed laser depositing will affect the crystallization behaviors of the pre-deposited layers. For example, the laser depositing of the hatched zone with three traces which is adjacent to A zone in Fig. 8 can bring EHS to A zone. But the size of HAZ is small, so only a little subsequent pulsed laser depositing can bring EHS to A zone. When the distance of the depositing zone to A zone is large enough, no more EHS will be brought to A zone during subsequent LSF. If A zone could still keep amorphous at this time, it will not occur crystallization any more in the subsequent LSF process. Thus, if the number of EHS that each deposited layer experienced during whole LSF process is lower than the maximum number of EHS for occurring crystallization, the BMGs would be prepared by LSF.
Thus, LSF of the BMGs without crystallization depends on the number of EHS that the pre-deposited amorphous layers experienced (NEP) and their structural relaxations induced by each EHS. The NEP depends on the size of HAZ of the subsequent deposited layers and the layer thickness during single pulse laser deposition. If the size of the HAZ is much smaller than the layer thickness during single pulse laser deposition, the NEP was only one for most of parts in the HAZ of the deposited layers and two or three for the parts of the HAZ in the overlapping zone between the adjacent deposited layers. If the size of the HAZ is larger than the thickness during single pulse laser deposition, the NEP will increase with increasing deposited layers remarkably. In addition, the structural relaxation in the pre-deposited amorphous layers depends on the heating and cooling rates during the current and subsequent laser deposition process. The enough high heating and cooling rate could induce little structural relaxation. However, The higher heating and cooling rate generally means the less energy input and the shorter existing time of the remelted zone, which will not only induce the lower deposition efficiency but also lead to the smaller size of the HAZ. So, to avoid the occurrence of the crystallization during LSF of the BMGs, a good combination of NEP and their structural relaxations induced by each EHS should be found.
Fig. 8. The schematic diagram of the cross section of LSF BMGs.
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The relative small volume remelted zone was mainly cooled by relative large volume substrate which was further cooled by the external conditions such as the surrounding gas convection and the large copper fixture. This leads to a high cooling rate in the remelted zone and the HAZ. However, a continuous or high dense energy inputs in the remelted zone will lead to a rise in the temperature of the substrate, which will make a decrease of cooling rate in the remelted zone and the HAZ and further cause the crystallization in the HAZ. This effect is named as the cumulative effect of heat input (CEH). So, during LSF with the pulses laser, the pulsed frequency should be reduced to make sure that the substrate and the pre-deposited layers could be cooled to a suitable lower temperature just before every pulse laser irradiation. 4.4. Crystallization behaviors in the deposit As the analysis above, the crystallization in the deposit could mainly occur in HAZ of every deposited layer. The shape of single pulse laser deposited layer can be estimated by the shape of crystalline band in the deposit. It can be seen from Fig. 2(b) that the layer thickness during single pulse laser depositing was much larger than the size of HAZ. Thus, most of parts in the pre-deposited layer did not experience the EHS. Part of the HAZ experienced the EHS one time. The HAZ in the overlapping zone between the adjacent layers got the more crystallization as it experienced two or three EHSes. The relative small volume HAZ also results in that there was no obvious change in the morphology of the crystalline band with the increase of the deposited layers. This also means that when a large size BMGs was prepared by LSF, the crystallization would not be aggravated during the multi-layer deposition process. It is interesting to note that Fig. 2(b) shows the crystallization only happened in the deposited region during LSF. There is no crystallization in the substrate. Comparing Fig. 2(c) with Fig. 2(d), it also can be seen that the crystallization in the crystalline bands in the deposit during LSF is worse than that in the HAZ of the substrate during PLSM. On the one hand, this may be related to the low CEH in the substrate since it generally has a better cooling environment compared with the deposited region. On the other hand, it is well known that the oxygen content has an important effect on the glass forming ability of Zr-based BMGs, The oxygen content should be less than 0.025 wt.% for the fabrication of the ZreTieCueNieAi alloy BMGs [16]. Since there exists a possible increase in the oxygen content during the powder preparation, the oxygen contents of the powder and the substrate were further measured. The measured oxygen contents of the powder and the substrate were 0.2 wt.% and 0.02 wt.% respectively. So the worse crystallization in the deposited region was more probably caused by the higher oxygen content in the deposit. It should be indicated that the processing parameters in present work were not optimized. Based on the above results, it can be reckoned that LSF technique could be used to prepare BMGs without the limitation of critical dimension. Since LSF is a free form fabrication technique [3], the BMGs could be prepared with any size and any shape under a certain precision through LSF. 5. Conclusions The possibility of preparing Zr-based BMGs by laser solid forming (LSF) without size limitation was investigated by experiments and further analyzed by the physic model. The main results are summarized as follows: (1) Zr55Al10Ni5Cu30 BMGs could kept the amorphous state after pulsed laser surface melting (PLSM) with six pulses and the crystallization occurred after PLSM with twelve and twenty
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pulses. There was no crystallization occurred in the deposit with one and two layers deposits during LSF, and the volume fraction of amorphous phase in the deposit with seven layers deposit was about 92.44%. The crystallization degree did not increase remarkably with the increasing of deposited layers during laser solid forming. The crystallization only occurred in the heat-affected zone (HAZ) during PLSM and LSF of BMGs. (2) The remelted zone was defined as the zone whose temperature is higher than Tm, the HAZ was redefined as the zone where the temperature is higher than Tg but lower than Tm, and the zone with the temperature lower than Tg was called thermo-stable zone during laser processing BMGs. There is no crystallization in remelted zone and thermo-stable zone during laser processing. The crystallization during the laser processing was mainly caused by the accumulation of structural relaxation in the HAZ. (3) The adaptation of of laser pulse number and the structural relaxations induced by each laser pulse is the key factor for LSF of BMGs without crystallization. The siz of HAZ should be smaller than the thickness of single pulsed laser deposited layer during LSF of BMGs without crystallization. (4) Combining with the physical model, LSF of BMGs without crystallization and the limitation of critical dimensions has been proved to be feasible. Acknowledgments The work is funded by the Research Program of National Natural Science Foundation of China (No. 50971102), China Postdoctoral Scientific Fund (No. 20090461312) and the fund of the State Key Laboratory of Solidification Processing in NPU (Nos. 16-TZ-2007, 39QZ-2009 and 05-BZ-2010). References [1] Byrne CJ, Eldrup M. Bulk metallic glasses. Science 2008;321:502e3. [2] Lu ZP, Liu CT, Thomspon JR, Porter WD. Structural amorphous steels. Phys Rev Lett 2004;92(24):245503-1e245503-4. [3] Lin X, Yue TM, Yang HO, Huang WD. microstructure and phase evolution in laser rapid forming of a functionally graded Ti-Rene88DT alloy. Acta Mater 2006;54:1901e15. [4] Tan H, Chen J, Lin X, Zhao XM, Huang WD. Research on molten pool temperature in the process of laser rapid forming. Mater Sci Forum 2007; 2301:546e9. [5] Telford M. The case for bulk metallic glass. Mater Today 2004;7:36e43. [6] Sun H, Flores KM. Microstructural analysis of a laser-processed Zr-based bulk metallic glass. Metall Mater Trans A 2010;41A:1752e7. [7] Kawahito Y, Terajima T, Kimura H, Kuroda T, Nakata K, Katayama S. Highpower fiber laser welding and its application to metallic glass Zr55Al10Ni5Cu30. Mater Sci Eng.B 2008;148:105e9. [8] Ma F, Yang JJ, Zhu XN, Liang CY, Wang HS. Femtosecond laser-induced concentric ring microstructures on Zr-based metallic glass. Appl Surf Sci 2010;256:3653e60. [9] Yue TM, Su YP, Yang HO. Laser cladding of Zr65Al7.5Ni10Cu17.5 amorphous alloy on magnesium. Mater Lett 2007;61:209e12. [10] Zheng B, Zhou Y, Smugeresky JE, Lavernia EJ. Processing and behavior of Febased metallic glass components via laser-engineered net shaping. Metall Mater Trans A 2009;40A:1235e45. [11] Liu WW, Lin X, Yang GL, Yang HO, Li WDJF. Huang. Influence of glass forming ability of alloy on crystallization in heat-affected zone by laser remelting Zr based bulk metallic glasses. Chin J Lasers 2010;37:2931e6. [12] Inoue A, Zhang T, Nishiyama N, Ohba K, Masumoto T. Preparation of 16 mm diameter rod of amorphous Zr65Al7.5Ni10Cu17.5 alloy Mater. Trans 1993;34: 1234e7. [13] Mukherjee S, Schroers J, Zhou Z, Johnson WL, Rhim WK. Viscosity and specific of volume of bulk metallic galss-forming alloys and their correlation with glass forming ability. Acta Mater 2004;52:3689e95. [14] Wang HR, Gao YL, Min GH, Hui XD, Ye YF. Primary crystallization in rapidly solidified Zr70Cu20Ni10 alloy from a supercooled liquid region. Phys Lett A 2003;314:81e7. [15] Suryanarayana C, Inoue A. Bulk mettallic glasses. New York: CRC Press; 2011. [16] Lin XH, Johnson WL, Rhim WK. Effect of oxygen impurity on crystallization of an undercooled bulk glass forming ZreTieCueNieAi alloy. Mater Trans 1997; 38:473e7.