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Viscous Flow Behavior and Thermal Properties of Bulk Amorphous Intermetallics 15 (2007) 1303e1308 www.elsevier.com/locate/intermet Viscous flow behavior and thermal properties of bulk amorphous Mg58Cu31Y11 alloy Y.C. Chang a, T.H. Hung a, H.M. Chen a, J.C. Huang a,*, T.G. Nieh b, C.J. Lee a a Institute of Materials Science and Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC b Department of Materials Science and Engineering, the University of Tennessee, Knoxville, TN 37996-2200, USA Received 26 September 2006; received in revised form 6 February 2007; accepted 19 March 2007 Available online 10 May 2007 Abstract The viscous flow behavior of the Mg58Cu31Y11 bulk amorphous rods in the supercooled viscous region is investigated using differential scan- ning calorimetry (DSC) and thermomechanical analyzer (TMA). Below the glass transition temperature, Tg, a linear thermal expansion coeffi- À6 cient of 3 Æ 1 Â 10 m/m K was obtained. In contrast, significant viscous deformation occurred as a result of a compressive load above Tg. The onset, steady state, and finish temperatures for viscous flow, determined by TMA, are slightly different from the glass transition and crystalli- zation temperatures measured by DSC. The appropriate working temperature for microforming as determined by the steady state viscous flow temperature is about 460e474 K. The effective viscosity within this temperature range is estimated to be about 107e109 Pa s, and it increases with increasing applied stress. The onset, steady state, and finish temperatures all decrease with increasing applied stress, suggesting accelerated crystallization in the present Mg58Cu31Y11 under stress. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: B. Glasses, metallic; B. Mechanical properties at high temperatures; C. Rapid solidification processing; C. Thermomechanical treatment; F. Calorimetry 1. Introduction Volkert and Spaepen in 1989 [4] reported the changes in the shear viscosity during relaxation in amorphous Pd-based bulk Metallic glassy alloys exhibit some unique physical proper- metallic glasses (BMGs). From 1995 to 1997, several studies ties such as excellent strength as compared to their corre- [5,6] examined the flow behavior of different BMGs and es- sponding crystalline counterparts. Significant plasticity can tablished their temperature and strain rate dependence. The occur in the supercooled liquid region, DTx (¼Tx À Tg, where viscous flow phenomenon is associated with the high atomic Tx is the crystallization temperature and Tg the glass transition diffusivity in the supercooled liquid region. A relationship be- temperature), resulting from a drastic drop in viscosity [1].Itis tween the non-isothermal viscous flow and the thermal expan- well known that the Mg-based Mg65Cu25Y10 system exhibits sion of a glassy alloy has been established using a free volume reasonably good glass forming ability (GFA) with a wide concept [6]. In 1999, Ye and Lu [7] applied external forces in supercooled liquid region [2]. In 2005, Ma et al. [3] doubled an attempt to retard the crystallization, thus to raise DTx.How- the critical size of the alloys from 4 to 9 mm by modifying ever, the effect of pressure became unclear and, in fact, a pres- the composition of Mg65Cu25Y10. The optimum composition sure sometimes was found to enhance crystallization and with the highest GFA was determined to be Mg58Cu31Y11. reduce DTx, for example in the Al and Zr based [8] metallic glasses. In 2001, Myung et al. [9e11] also studied the non-iso- thermal viscous flow of the Co, Pd and Zr-based glassy alloys in the glass transition range in an attempt to identify the opti- * Corresponding author. Tel.: þ886 7 5252000; fax: þ886 7 525 4099. mum temperature range for secondary processing. However, E-mail address: [email protected] (J.C. Huang). the majority of these studies were carried out by measuring 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.03.012 1304 Y.C. Chang et al. / Intermetallics 15 (2007) 1303e1308 the stressestrain curves at selected strain rates and tempera- tures using an Instron-type universal testing machine. There have been only a few attempts to study the viscous flow behavior in BMGs, including Co, Pd, Zr and Mg-based systems, using the thermomechanical analyzer (TMA) under continuous heating conditions [9e13]. For example, Busch et al. [13] reported the thermodynamic analysis and viscous flow in the Mg65Cu25Y10 amorphous alloy. Very limited data are available on the effective viscosity and stress related Intensity viscous flow behavior in the Mg-based BMGs under TMA compressive pressure. The aim of this study is to explore the viscous flow properties of the Mg58Cu31Y11 glassy alloy, and to search the optimum superplastic forming condition. 2. Experimental procedure 20 30 40 50 60 70 80 2 Theta The Mg58Cu31Y11 (in at%) alloy rod with a diameter of 4 mm was prepared by a copper mold injection casting tech- Fig. 1. X-ray diffraction patterns of the as-quenched samples. nique in an argon atmosphere with pure Mg and pre-alloyed CueY ingots as the starting materials. The composition of range is about 66 K. Comparison of the thermal properties is the samples was verified by using energy dispersive spectros- made in Table 1 between the current Mg Cu Y BMG copy (EDS). The amorphous nature of the as-quenched BMG 58 31 11 and the classic Mg Cu Y alloy. It can be seen in Table 1 rod sample was confirmed using X-ray diffractometry (XRD, 65 25 10 that Mg Cu Y seems to be closer to the ternary eutectic Siemens D5000) with a monochromatic Cu Ka radiation. 65 25 10 composition with a smaller DT (¼T À T , where T and T The basic thermal properties were measured in a continuous l l m l m are the liquidus and solidus temperatures, respectively). How- heating mode with a heating rate of 10 K/min by differential ever, Ma et al. [3] pointed out that a large change of T , as a re- scanning calorimetry (DSC, TA Instruments DSC 2920). The l sult of a minor change in composition, is a result of a steep temperature and heat flow of the DSC were calibrated by using liquidus slope for a typical deep eutectic. The composition pure In and Zn standard samples. Cu pans were used for both of the best glass former is about 5 at% away from the eutectic the samples and the reference. A thermomechanical analyzer point, on the Cu-rich side. In this study, we found that the T (TMA, Perkin Elmer Diamond) was employed to measure rg value (¼T /T ) of the Mg Cu Y alloy is not much different the temperature dependence of the effective viscosity, relative g l 58 31 11 from, and actually is slightly smaller than, that of the displacement (where the displacement is the absolute value of Mg Cu Y alloy, consistent with another report [3].How- contraction plus the penetration depth), and steady state vis- 65 25 10 ever, the g value [¼T /(T þ T )] of Mg Cu Y is higher. cous flow temperature as a function of applied compression x g l 58 31 11 In fact, the maximum diameter of the Mg Cu Y BMG stress and heating rate. Due to the brittle nature of the current 58 31 11 can reach 12 mm, which is greater than the 7 mm for the Mg Cu Y BMG, compressive mode was used throughout. 58 31 11 Mg Cu Y rod. Stresses of 0.8, 2.4, 7.1, 117.8, and 318.5 kPa were applied by 65 25 10 the TMA ceramic flat-end probe which was 3.0 mm in diam- eter. The test specimens used for TMA measurements were cy- lindrical rods 4 mm in diameter and 4 mm in length, which corresponds to a probe to sample area ratio of 0.56. The heat- T ing rate was fixed at 10 K/min, the same as that used for DSC. m The effective linear expansion coefficient (aL), or the speci- men displacement response under the flat-end probe compres- sion was measured in a temperature range of 300e600 K, from which the effective viscosity of the alloy was extracted. Tg Tx 3. Results and discussion The X-ray diffraction pattern from the as-cast rod Exothermic (arb. unit) Mg58Cu31Y11 sample is displayed in Fig. 1, which indicates that the alloy is amorphous. The DSC thermogram of the alloy measured at a heating rate of 10 K/min is shown in Fig. 2. From the DSC curve, it is evident that there is a distinct glass 400 500 600 700 800 transition with one exothermic crystallization peak and an- Temperature (K) other one for melting peak. The supercooled temperature Fig. 2. DSC thermograms of the BMGs obtained at a heating rate of 10 K/min. Y.C. Chang et al. / Intermetallics 15 (2007) 1303e1308 1305 Table 1 Table 2 Thermal properties of the Mg58Cu31Y11 and Mg65Cu25Y10 melt-spun ribbons Viscous flow behavior of the Mg58Cu31Y11 BMGs obtained from TMA at obtained from DSC at a heating rate of 10 K/min a heating rate of 10 K/min Tg (K) Tx (K) DTx Tm (K) Tl (K) DTm Trg g Applied DLmax (mm) DLmax/DL0 Tonset (K) Tvs (K) Tfinish (K) (K) (K) stress (kPa) Mg58 413 479 66 711 754 43 0.547 0.410 0.8 60.9 1.52 450 474 475 Cu31Y11 2.4 162.6 4.07 447 471 473 Mg65 407 462 55 708 738 30 0.551 0.403 7.1 265.3 6.63 441 469 472 Cu25Y10 117.8 748.6 18.72 435 466 470 318.5 923.3 23.08 430 460 467 Temperature dependence of the relative displacement of the w10À4 m/m K [9]) is expected to be overshadowed by the as-cast bulk amorphous Mg58Cu31Y11 alloys measured by TMA operated in a compression mode at various stress levels large viscous compressive strain ranging from 1% to 25%).
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