Crystallization and thermophysical properties of Cu46Zr47Al6Co1 bulk metallic glass Crystallization and thermophysical properties of Cu46Zr47Al6Co1 bulk metallic glass Kang Wu, Ran Li, and Tao Zhang C[.]
Crystallization and thermophysical properties of Cu46Zr47Al6Co1 bulk metallic glass Kang Wu, Ran Li, and Tao Zhang Citation: AIP Advances 3, 112115 (2013); doi: 10.1063/1.4832235 View online: http://dx.doi.org/10.1063/1.4832235 View Table of Contents: http://aip.scitation.org/toc/adv/3/11 Published by the American Institute of Physics AIP ADVANCES 3, 112115 (2013) Crystallization and thermophysical properties of Cu46 Zr47 Al6 Co1 bulk metallic glass Kang Wu, Ran Li,a and Tao Zhang Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China (Received October 2013; accepted November 2013; published online 13 November 2013) Phase evolution of two-step crystallization and the subsequent B2-phase transformation was presented in Cu46 Zr47 Al6 Co1 bulk metallic glass (BMG) during heating process Thermophysical properties, i.e the thermal diffusivity and the specific heat capacity, of the BMG in amorphous solid state and supercooled liquid state as well as its crystalline counterparts were measured from room temperature to 1070 K The thermal conductivity was also calculated through combination of the data of the thermal diffusivity and the specific heat capacity The possible influence of the C 2013 Author(s) All crystallization on the thermophysical properties was discussed article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4832235] I INTRODUCTION Recently, CuZr-based bulk metallic glasses (BMGs) and their BMG composite materials have received intense interests due to superior mechanical properties (e.g significant plasticity and work hardening behaviors), relative low cost and controllable tailoring for microstructure.1–6 Pauly et al reported that tensile deformation can induce nanocrytallization of B2 CuZr phase in a series of Cu-Zr-Al BMGs, and the formation of the nanocrystals and their twinning results in slight tensile ductility at low strain rates.2 Wu et al introduced micrometer-size B2 CuZr phase (10∼100 μm) into Zr-Cu-Al-Co glassy matrix and the resulting composites showed large tensile stain up to 8%.3 This kind of BMG composites (BMGCs), called shape-memory BMGCs,4 with distinct work hardening behavior, large tensile ductility and tunable composite microstructure provides greatly potential applications as advanced structural materials The correlations between morphology of B2 CuZr phase (including size, distribution, and volume fraction) and mechanical properties of CuZr-based BMGCs were also investigated.5–7 The tensile ductility of CuZr-based BMGCs can be improved up to ∼14% by morphological optimization of the B2 phase.8 Many methods have been provided to control microstructure and improve ductility of CuZrbased BMGCs, e.g optimal selection of alloy composition,9, 10 casting parameter adjustment by controlling melting current/time and cooling rate,10, 11 inoculation by introduction of refractory metals,5, 12 melt adjustment by remelting,8 and induced crystallization by selective laser melting.13 Among these methods, the selective laser melting (SLM) with significant merits, e.g quick production of samples with unlimited size and intricate shape, is a promising technique for synthesis of BMGs and BMG composite materials.14 The optimal choice of alloy compositions and processing parameters to accurately control crystallization from metallic glass-forming liquid is still a great challenge due to the metastable nature of this kind of materials.14–16 Computer simulation of thermal history during SLM processing can be greatly helpful for understanding the mechanism of glass formation and crystallization in resulting samples and determining the optimal conditions The specific heat capacity, the thermal diffusivity and the thermal conductivity are ones of the most important a Authors to whom correspondence should be addressed E-mail: liran@buaa.edu.cn (R L.); Tel: +86-10-82316192; Fax: +86-10-82339705 2158-3226/2013/3(11)/112115/8 3, 112115-1 C Author(s) 2013 112115-2 Wu, Li, and Zhang AIP Advances 3, 112115 (2013) thermophysical parameters for the simulation.17 Zacharia et al investigated the effect of the constant and temperature-dependent thermophysical parameters on the computer simulation.18 The results indicated that the choice of the thermophysical properties can significantly influence the development of the weld pool K.Mundra et al further revealed the important role of the thermophysical properties in the modeling of laser melting.19 The simulation of peak temperature and melting pool geometry can be strongly influenced by the thermal conductivity and the specific capacity However, the thermophysical properties were only reported in several selective BMG systems with superior glass-forming ability (GFA), like Zr-Al-Ni-Cu, Zr-Ti-Cu-Ni-Be, and Pd-Cu-Ni-P,20–23 while few reports were presented for CuZr-based metallic glasses so far.24 In the present work, a model material, i.e Cu46 Zr47 Al6 Co1 glass-forming alloy, was chosen in CuZr-based BMG system according to the former papers.3, 13 The crystallization process of the Cu46 Zr47 Al6 Co1 BMG was identified The thermal diffusivity and the specific heat capacity of this BMG in amorphous solid state and supercooled liquid state as well as its crystalline counterparts were measured in the temperature range from 300 K to 1070 K by laser flash method and differential scanning calorimetry (DSC), respectively The thermal conductivity of the BMG and its crystalline counterparts was also calculated Possible relationships between the constituent phases and the thermophysical properties were discussed II EXPERIMENTAL Ingots of Cu46 Zr47 Al6 Co1 alloy were prepared by arc-melting mixtures of pure metals of Zr (99.9 wt.%), Cu (99.99 wt.%), Al (99.999 wt.%), and Co (99.9 wt.%) in a Ti-gettered high purified argon atmosphere The homogenization can be ensured by melting at least four times Pieces of the ingots were re-melted and injected into copper molds to form two kinds of samples: rods of mm in diameter and plates of mm in thickness and 12 mm in width Amorphous structure of the samples was confirmed using a Bruker AXS D8 X-ray diffractometer (XRD) with Cu Ka radiation (not shown here) Thermal stability, phase transformation, and melting/solidification behaviors of the alloy were evaluated using a Netzsch DSC 404 C at a heating/cooling rate of 0.167 K/s The samples can be protected from oxidation using a high vacuum system To study constituent phases precipitated from the glassy matrix during crystallization, the amorphous samples were heated up to a target temperature at a rate of 0.167 K/s and then cooled to room temperature at the same rate using the same DSC equipment The density (ρ) was measured by Archimedes’ method in 1,1,2,2-TBE (tetrabromoethane) using a Sartorius BSA224S balance The thermal diffusivity was measured from 300 K to 1070 K using a ULVAC-RICO TC-7000H standard laser flash thermal constants analyzer The upper limit of the temperature is very close to the melting temperature of the BMG (1141 K) The dimension of the BMG sample for the measurement is 10 mm in diameter and mm in thickness The surface was firstly polished with SiC abrasive grinding papers of up to grit 1200, and then blackened by spraying graphite to improve the absorption of laser beam The sample chamber was evacuated to low pressure less than 10−2 Pa during the whole measurement The sample was heated up to every test temperature at a rate of 0.167 K/s At least triple data were obtained at each test temperature to ensure the accuracy The “t1/2 method” was adopted to calculated the thermal diffusivity (α).25 For an ideal homogeneous sample under one-dimensional conduction, impulse input, adiabatic boundary and constant properties, the α can be expressed by: α = 1.37 l2 π 2t , (1) 1/2 where l is the thickness of specimen, and t1/2 is the interval required for the back surface to reach half of the maximum temperature raise after the impulse input of laser The specific heat capacity (Cp ) of the BMG in amorphous solid state and supercooled liquid state as well as its crystalline counterparts was measured using a Netzsch STA 449F3 Platinum crucibles and a sapphire standard were adopted The sample was heated up to 1070 K at a constant heating rate of 0.167 K/s during the measurement Firstly, a baseline of an empty Platinum crucible was measured Then, the sapphire standard and the sample were measured in the same conditions, 112115-3 Wu, Li, and Zhang AIP Advances 3, 112115 (2013) respectively The Cp of the sample can be calculated by the following equation:26 C p,sample (T ) = Hsample − H pan m sapphir e · · C p,sapphir e (T ), Hsapphir e − H pan m sample (2) where mi is the mass, Hi is the heat-flow signal and C p,i (T ) is the specific heat capacity of the sample or sapphire The thermal conductivity (κ) was calculated by combination of the above data of the specific heat capacity and the thermal diffusivity for the sample in different states as the following equation: κ = ρC p α (3) III RESULTS AND DISCUSSION A Crystallization of Cu46 Zr47 Al6 Co1 bulk metallic glass Figure 1(a) shows a typical DSC curve of the Cu46 Zr47 Al6 Co1 BMG at a heating rate of 0.167 K/s The sample exhibits distinct glass transition followed by a supercooled liquid region prior to two-step crystallization An intensive exothermic peak corresponding to partial devitrification from the supercooled liquid can be observed at ∼750 K The second exothermic peak at ∼875 K can be distinguished by enlarging the region B (circled by dot line shown in Fig 1(a)) The glass transition temperature (Tg ), the onset points of the first crystallization (Tx1 ) and the second one (Tx2 ) can be determined to be 693 K, 752 K, and 849 K, respectively An endothermic peak is found at a higher temperature of ∼960K A possible inverse eutectoid transformation might be responsible for the endothermic reaction, like other reports.5, 9, 13 The peak temperature of the phase transformation (TTP ) is 961 K Considering possible influence of devitrification behavior on the thermophysical properties, the phase evolution during crystallization was investigated for the Cu46 Zr47 Al6 Co1 BMG According to the thermal characteristics of the DSC curve in Fig 1(a), three temperature points, i.e the endpoints of the first crystallization event (810 K), the second one (900 K), and the phase transformation stage (910 K) were chosen as heat treatment (HT) temperature Figure 1(b) shows the XRD patterns of the annealed Cu46 Zr47 Al6 Co1 BMG samples After the first crystallization stage, only Cu10 Zr7 phase can be identified in the annealed sample (HT at 810 K) For the amorphous sample annealed at a higher temperature of 900 K, additional crystalline phases of CuZr2 and AlCu2 Zr are precipitated from the matrix except for Cu10 Zr7 phase However, when the annealing temperature is raised up to 990 K, most of Cu10 Zr7 and CuZr2 phases are disappeared and replaced by CuZr (B2) phase AlCu2 Zr phase is still detected in the annealed sample The results indicate that the endothermic peak near 960 K is corresponding to the phase transformation of the inverse eutectoid reaction from Cu10 Zr7 and CuZr2 phases to CuZr phase The constituent phases in the ingot and the amorphous samples annealed at a higher temperature up to the liquidus temperature were also checked Similar to the sample annealed at 990 K, all the samples exhibit the same constituent phases of B2 CuZr and AlCu2 Zr (not shown here) Although B2 CuZr phase is a metastable phase at room temperature in Cu-Zr binary system,27 the B2 CuZr phase can be stabilized to room temperature in our annealed samples because of the existence of Co.28 It is also greatly beneficial for the in situ fabrication of CuZr-based BMGCs The amorphous samples annealed at 1070 K are regarded as the stable crystalline solid of this alloy for the following measurements of the thermophysical properties B Thermal diffusivity of Cu46 Zr47 Al6 Co1 bulk metallic glass Figure shows the temperature dependence of the thermal diffusivity (α) for the Cu46 Zr47 Al6 Co1 BMG and its crystalline counterparts The α of the amorphous solid increases monotonically with increasing temperature from 300 K to 680 K The temperature coefficient is ∼0.005 m2 K−1 s−1 The α of the BMG at room temperature is 1.94 × 10−6 m2 /s, comparable to the values of Zr-, Pd- and Fe-based metallic glasses (1.6–2.6 × 10−6 m2 /s) 20–24 The α of the supercooled liquid 112115-4 Wu, Li, and Zhang AIP Advances 3, 112115 (2013) FIG (a) A typical DSC curve of the Cu46 Zr47 Al6 Co1 bulk metallic glass at a heating rate of 0.167 K/s (b) XRD patterns of as-cast Cu46 Zr47 Al6 Co1 bulk metallic glass and the samples annealed at 810 K, 900 K and 990 K 112115-5 Wu, Li, and Zhang AIP Advances 3, 112115 (2013) FIG Temperature dependence of the thermal diffusivity (α) for Cu46 Zr47 Al6 Co1 bulk metallic glass and its crystalline counterparts increases steadily with increasing temperature, like the change of the amorphous solid The similar tendency was also found in supercooled water.29, 30 As temperature raises beyond Tx1 , the crystalline phase begins to be significantly precipitated into the matrix The α of the devitrified alloy with changeable constituent phases and microstructure is characterized by fast increase with a large positive temperature coefficient until the inverse eutectoid reaction, denoted as Cu10 Zr7 + CuZr2 → CuZr, at ∼960 K The α of the crystalline solid also shows a roughly linear dependence on temperature from 300 K to 1070 K, like the change of the crystalline counterparts in other metallic glass systems.20, 21, 23 The α of the crystalline solid at room temperature is 4.46 × 10−6 m2 /s, much larger than the value of the amorphous solid C Special heat capacity of Cu46 Zr47 Al6 Co1 bulk metallic glass Figure shows the specific heat capacity (Cp ) of the Cu46 Zr47 Al6 Co1 BMG in amorphous solid state and supercooled liquid state as well as its crystalline counterparts The Cp of the amorphous solid changes insignificantly in the temperature range from room temperature to Tg 31, 32 The temperature dependence of the Cp in the supercooled liquid region exhibits a half λ shape, like the change in other metallic glass systems following a l/T2 law provided by Kubaschewski et al.: 32–34 C p,mol (T ) = 3R + aT + bT −2 , (4) where Cp,mol is the heat capacity per mole; R is the gas constant; a or b is a constant With increasing temperature above the first crystallization stage, the Cp value decreases rapidly due to the precipitation of Cu10 Zr7 phase from the supercooled liquid The Cp of the crystalline solid increases slowly with increasing temperature up to around 800 K, then it decreases The value is comparable to that of the corresponding amorphous solid like the change in other noncrystalline solids.35 112115-6 Wu, Li, and Zhang AIP Advances 3, 112115 (2013) FIG Temperature dependence of the specific heat capacity (Cp ) for Cu46 Zr47 Al6 Co1 bulk metallic glass and its crystalline counterparts FIG Temperature dependence of the thermal conductivity (κ) for Cu46 Zr47 Al6 Co1 bulk metallic glass and its crystalline counterparts 112115-7 Wu, Li, and Zhang AIP Advances 3, 112115 (2013) D Thermal conductivity of Cu46 Zr47 Al6 Co1 bulk metallic glass Figure shows the temperature dependence of the thermal conductivity (κ) for the Cu46 Zr47 Al6 Co1 BMG and its crystalline counterparts The κ of the BMG in amorphous solid state is roughly correlated with temperature from 300 K to Tg with a positive linear coefficient of 0.016 Wm−1 K−2 The κ of the CuZr-based BMG at room temperature is 5.7 W m−1 K−1 , comparable to the values of other reported Zr- and Cu-based metallic glasses but less than the ones of Fe-, Co-, and Pt-based metallic glasses.20–24 The κ increases dramatically at Tg from 11.8 W m−1 K−1 to 18.6 W m−1 K−1 , and then decreases after the first crystallization due to the discontinuous change of the Cp during glass transition and crystallization The κ of the crystalline solid at room temperature is 13.0 Wm−1 K−1 , almost three times of that of the CuZr-based BMG at the same temperature The κ of the crystalline solid shows a peak value of 25.7 Wm−1 K−1 at ∼800 K, similar to the change of the Cp of the crystalline solid In both of the temperature ranges (below and above 800 K), the κ is roughly linear on temperature with coefficients of 0.026 Wm−1 K−2 and −0.023 Wm−1 K−2 , respectively These data provide us fundamental information for not only scientific research of thermophysical properties of CuZr-based BMGs but also emerging industrial application of selective laser melting technique for the construction of this kind of materials IV CONCLUSION In this study, the two-step crystallization of the Cu46 Zr47 Al6 Co1 BMG was clarified as: 1st 2nd Am → Am. + Cu 10 Zr7 → Cu 10 Zr7 + Cu Zr2 + AlCu Zr The subsequent inverse eutectoid reaction was identified for the formation of stable crystalline counterpart at room temperature, consisting of CuZr (B2) and AlCu2 Zr Furthermore, the thermal diffusivity and the specific heat capacity of the BMG in different states (i.e amorphous solid, supercooled liquid and its crystallization counterparts) were evaluated from room temperature to 1070 K The values of the thermal conductivity were also calculated according to the data of the thermal diffusivity and the specific heat capacity The thermal diffusivity and conductivity of the amorphous solid show roughly linear dependence on temperature, while the specific heat capacity keeps a rough constant The thermal conductivity exhibits two discontinuous changes due to glass transition and crystallization while the thermal diffusivity changes continuously through the supercooled liquid region After the first crystallization stage, the thermal diffusivity and conductivity increase rapidly with increasing temperature and reach the same value of the stable crystalline counterpart at around 960 K due to the formation of B2 CuZr phase The specific heat capacity of the crystalline solid is similar to the value of the amorphous state, exhibiting weak temperature dependence ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grant Nos 51071008 and 51131002), the Program for New Century Excellent Talents in University (NCET), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry J Das, M B Tang, K B Kim, R Theissmann, F Baier, W H Wang, and J Eckert, Phys Rev Lett 94, 205501 (2005) Pauly, S Gorantla, G Wang, U Kăuhn, and J Eckert, Nat Mater 9, 473–477 (2010) Y Wu, Y Xiao, G L Chen, C T Liu, and Z P Lu, Adv Mater 22, 2770–2773 (2010) D C Hofmann, Science 329, 1294 (2010) Z Q Liu, R Li, G Liu, W H Su, H Wang, Y Li, 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(2013) Crystallization and thermophysical properties of Cu46 Zr47 Al6 Co1 bulk metallic glass Kang Wu, Ran Li,a and Tao Zhang Key Laboratory of Aerospace Materials and Performance (Ministry of Education),... Co1 bulk metallic glass at a heating rate of 0.167 K/s (b) XRD patterns of as-cast Cu46 Zr47 Al6 Co1 bulk metallic glass and the samples annealed at 810 K, 900 K and 990 K 112115-5 Wu, Li, and. .. Cu46 Zr47 Al6 Co1 bulk metallic glass and its crystalline counterparts FIG Temperature dependence of the thermal conductivity (κ) for Cu46 Zr47 Al6 Co1 bulk metallic glass and its crystalline