The Effect of Dy Substitution on the Glass-Forming Abilityand Crystallization Behavior of Mg65Cu10Ni10Y10Zn5 MetallicGlass

XULIANG ZHANG, FENG XU, and GUANG CHEN

The glass-forming ability, thermal stability, and crystallization behavior of Mg65Cu10Ni10Y10-x

Zn5Dyx (x = 0, 2, and 4) alloys were investigated. The partial substitution of Dy for Y increasesthe activation energy of the first crystallization, but it decreases the glass-forming ability andthermal stability. Analyses on crystallization kinetics suggest that the substitution of Dy for Ydecreases the nucleation rate. In addition, the examination of the crystallization phase after theisothermal annealing indicated that Dy addition suppresses the formations of Mg2Cu and Mgand that it promotes the formation of MgZn crystalline phase. The decreasing glass-formingability with the Dy substitution can be attributed to the narrower supercooled liquid region,higher Gibbs free energy, and the change of crystallization products.

DOI: 10.1007/s11661-011-0854-1� The Minerals, Metals & Materials Society and ASM International 2011

I. INTRODUCTION

MG-BASED bulk metallic glasses (BMGs) haveattracted wide attention as a result of their high specificstrength, relatively low casting cost, and excellenthydrogen storage capacities.[1–3] Most previous studieson Mg-based BMGs focused on the Mg-TM-RE (TM:transition metals such as Ni, Cu, Zn, Ag, Pd, and RE:rare-earth metals such as Gd, Nd, and Y) alloysystem.[4–11] It was found that a partial substitution ofalloying elements is one of the simple and effectivemethods to improve the glass-forming ability (GFA)and/or mechanical properties of Mg-based glass-form-ing alloy systems, such as substituting Ni or Ag for Cuin the Mg-Cu-(Y,Gd),[6–8,12,13] Ag for Ni in the Mg-Cu-Ni-Gd,[14] and Nd or Gd for Y in the Mg-Cu-Y.[5,15–17]

As promising engineering materials, the thermal stabil-ity (i.e., crystallization resistance) of BMGs is alsoconsidered one of the most important aspects for theapplications, and the crystallization behavior with theincreasing temperature could be the critical point ofunderstanding the mechanisms of phase transformationfar from equilibrium.[18,19] Although some investigationsabout GFA and mechanical properties have been madewith the partial substitution of alloying elements, thecrystallization behavior needs to be further studied.

In this study, we investigate the effect of Dy substi-tution of Y in Mg65Cu10Ni10Y10Zn5 alloy on the BMGforming tendency and crystallization behavior. Theselection of Dy for partially replacing the Y element is

attributed to the differences between Dy and Y incovalent atomic radius (Dy: 0.159 nm; Y: 0.162 nm) andelectronic configuration (Dy: 4f105p66s2, Y: 4d15s2), anda similar heat of mixing against other elements and anear-zero heat of mixing for the Y-Dy binary system.The variations of thermal stability, activation energy forcrystallization, isothermal crystallization kinetics, aswell as crystallization phase with Dy substitution of Yare studied by the thermal analysis ofMg65Cu10Ni10Y10-x

Zn5Dyx (x = 0, 2, and 4) alloys.

II. EXPERIMENTAL PROCEDURES

Cu-Ni-Y and Cu-Ni-Y-Dy master alloys were firstprepared by arc melting Cu (99.99 pct, mass fraction),Ni (99.99 pct, mass fraction), Y (99.95 pct, mass frac-tion), and Dy (99.95 pct, mass fraction) under a Ti-gettered argon atmosphere in a water-cooled coppercrucible. The ingot was then remelted with pure Mg(99.95 pct) and Zn (99.99 pct) in an induction furnaceunder an argon atmosphere. Finally, cylindrical rods invarious diameters were prepared by the conventionalcopper mold casting method. The amorphous natureand phase identification of the samples were examinedby X-ray diffraction (XRD) in a diffractometer (RIG-AKU RAD-8)* with Cu Ka radiation. The thermal

stability of the samples was examined by differentialscanning calorimetry (DSC) in an argon atmosphereusing a NETZSCH** STA 449C device. Non-isothermal

XULIANG ZHANG, Graduate Student, FENG XU, AssociateProfessor, and GUANG CHEN, Professor, are with the EngineeringResearchCenter ofMaterials Behavior andDesign,Ministry ofEducation,Nanjing University of Science and Technology, Nanjing 210094,P.R. China, and with the Jiangsu Institute of Advanced Materials,Danyang 212300, Jiangsu, P.R. China. Contact e-mail: gchen@mail.njust.edu.cn

Manuscript submitted April 18, 2011.Article published online August 30, 2011

*RIGAKU is a trademark of Rigaku, Tokyo, Japan.

**NETZSCH is a trademark of Netzsch, Wittelsbacherstrasse, Selb,Germany.

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DSC curves were measured with selected heating rates of5, 10, 20, and 40 K/minute. For the isothermal analysis,the amorphous samples cut from individual as-castsamples were first heated at a rate of 50 K/minute fromthe temperature of 298 K (25 �C) to the holdingtemperature of 485 K (212 �C), and then they were heldfor a certain period of time until the completion ofcrystallization. The constituent phases of the samplesafter the isothermal annealing were identified by XRD.

III. RESULTS AND DISCUSSION

Figure 1 shows the XRD patterns taken from the cross-sectional surface of the as-castMg65Cu10Ni10Y10-xZn5Dyx(x = 0, 2, and 4) rods. The lack of sharp crystallinediffraction peaks, as well as the appearance of the broadstrong scattering near 37� (2h), indicates that the samplesare essentially amorphous.The critical diameter (Dc) underwhich the sample remains amorphous is represented by thelargest amorphous diameter. It is found that Dc ofMg65Cu10Ni10Y10-xZn5Dyx decreases gradually from6 mm for the base alloy Mg65Cu10Ni10Y10Zn5 to 3 mmfor the Mg65Cu10Ni10Y6Zn5Dy4 alloy.

Figure 2 shows the DSC traces obtained from melt-spun ribbons of Mg65Cu10Ni10Y10-xZn5Dyx (x = 0, 2,and 4) glasses during continuous heating with a heatingrate of 20 K/minute. All alloys exhibit a distinct glasstransition, followed by a broad supercooled liquidregion and then an exothermic reaction attributed tocrystallization. The sharp exothermic peak correspond-ing to the major crystallization process is located at thepeak temperature (Tp) of around 485 K. Characteristictemperatures such as Tg (glass transition temperature),Tx (crystallization temperature), Tp, Tm (solidus tem-perature), and Tl (liquidus temperature) are marked onthe DSC curves with increasing x from 0 to 4. FromFigure 2, we can find that Tg changes slightly with theincrease of x from 0 to 4. Tx decreases from 484 K(211 �C) at x = 0 to 481 K (208 �C) at x = 2, and thendecreases to 474 K (201 �C) at x = 4. Thus, DTx

(supercooled liquid region) decreases from about 51 K(–222 �C) at x = 0 to 44 K (–229 �C) at x = 4. Asmaller DTx usually implies a weaker resistance to thenucleation and growth of crystalline phases, leading to alower GFA. Thus, the small amount of substitution ofDy for Y (e.g., 4 pct) deteriorates GFA and the thermalstability of Mg65Cu10Ni10Y10Zn5 amorphous alloys. Itcan be also noticed that Tm of the samples increasesslightly with Dy addition, whereas Tl has a moresignificant increase, resulting in a wider melting range.As a result, the reduced glass transition temperature, Trg

(defined as Trg = Tg/Tl) decreases from 0.565 for x = 0to 0.542 for x = 4. This tendency is consistent with thedecrease of DTx, which suggests that the new composi-tion is farther away from the eutectic composition.However, the reduction of GFA with partial substi-

tution of Dy for Y may be attributed to a more unstableliquid structure. Compared with the alloy of Mg65Cu10Ni10Y10Zn5, Mg65Cu10Ni10Y10-xZn5Dyx (x = 2 and 4)shows the higher Tm and Tl, suggesting that thesecompositions have the lower stability of the liquidphase. This is also evidenced by comparing the Gibbsfree energy difference (DG) between the liquid andcrystal phases in the undercooled liquid region, calcu-lated by the following equation[20]:

DG ¼ DHfDTTm

� kDSf DT� T lnTm

T

� �� �½1�

Thermal analysis indicates that the melting entralpy DHf

is 7.51, 7.92, and 8.18 kJ/mol, for x = 0, 2, and 4,respectively. DSf is the melting entropy that relates toDHf by DSf = DHf / Tm. The undercooled temperatureDT is defined as DT = Tm – T, and k is the propor-tionality coefficient, which is about 0.8 for metallic glass-forming liquids. DG was calculated at T = 0.8Tm.For Mg65Cu10Ni10Y10Zn5, DG is 1.37 kJ/mol. ForMg65Cu10Ni10Y10-xZn5Dyx (x = 2 and 4), DG increasesto 1.45 and 1.49 kJ/mol, respectively, both larger thanthat of x = 0. A smaller DG implies a larger embryocritical size for nucleation. Hence, larger chemical

Fig. 1—XRD patterns of as-cast Mg65Cu10Ni10Y10-xZn5Dyx (x = 0,2, and 4) glasses.

Fig. 2—DSC curves of Mg65Cu10Ni10Y10-xZn5Dyx (x = 0, 2, and 4)glasses at a constant heating rate of 20 K/minute.

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fluctuations are required, and the stability of the liquidphase is higher.[21] From this viewpoint, the compositionx = 0 has the higher stability of the liquid phasebecause its DG is the lowest. This explains the reductionof GFA with partial substitution of Dy for Y.

The heating rate dependence of characteristic tem-peratures was investigated (listed in Table 1).

The activation energy for the first crystallization ofMg65Cu10Ni10Y10-xZn5Dyx (x = 0, 2, and 4) glasses wasdetermined by the Kissinger equation as follows[22]:

lnbT2p

¼ � Ea

RTpþ c ½2�

where b is the heating rate, Ea is the activation energy, Ris the gas constant, and c is the integration constant. Thevariation of the first Tp temperature versus b for thesealloys was measured by using the heating rates of 5, 10,20, and 40 K/minute in DSC. Figure 3 shows the plotsof ln b=T2

p as a function of 1/Tp for each alloy; the Ea

values for the first crystallization are 86, 99, and 108 kJ/molfor x = 0, 2, and 4, respectively. This indicates that theslight substitution of Dy for Y improves the activationenergy for the first crystallization and the resistance

ability to crystallization of the Mg65Cu10Ni10Y10Zn5alloy. This might be a result of the strong affinityfor Dy/Mg (DHm = –6 kJ/mol) and Dy/Cu (DHm =–22 kJ/mol), which will impose resistance to the forma-tion of the major Mg2Cu phase, possibly by blocking thepath of Mg and Cu diffusion, and indirectly increasingthe crystallization energy barrier. It should be noted thatthe substitution of Dy for Y decreases the thermalstability of the Mg65Cu10Ni10Gd10Zn5 alloy as discussedearlier. The glass with Dy addition needs higheractivation energy Ea for the crystallization, but it showsa lower first crystallization temperature Tx, thermalstability DTx, and Trg. The activation energy is notconsistent with the thermal stability based on Trg andDTx in the present study. The same phenomenon wasalso reported in the Mg-Cu-Gd system.[14]

To investigate the crystallization kinetics ofMg65Cu10Ni10Y10-xZn5Dyx (x = 0, 2, and 4) glasses,the isothermal annealing was performed at 485 K(212 �C) in the supercooled liquid region (at lowertemperatures the signal is too weak, and at highertemperatures there is a significant overlap between thesignal and the instrumental transients), which is about25 K (–248 �C) over Tg and is below Tx. Figure 4 shows

Table 1. Temperature Dependence of Characteristic Temperatures of Mg65Cu10Ni10Y10-xZn5 Dyx (x = 0, 2, and 4)

Bulk Metallic Glasses

CompositionHeating rate(K/minute) Tg (K) Tx (K) Tp (K) Tm (K) Tl (K) 4Tx (K) Trg

x = 0 5 419 463 467 719 751 44 0.55810 425 475 477 720 756 50 0.56220 433 484 489 721 766 51 0.56540 449 502 509 734 806 53 0.557

x = 2 5 416 460 464 719 753 44 0.55210 423 468 473 719 755 45 0.56020 433 481 485 722 769 48 0.56340 448 497 504 729 808 49 0.554

x = 4 5 415 451 458 721 752 36 0.55110 421 460 468 722 759 39 0.55520 430 474 478 725 793 44 0.54240 439 488 494 730 790 49 0.555

Fig. 3—Heating rate dependence of Tp for Mg65Cu10Ni10Y10-x

Zn5Dyx (x = 0, 2, and 4) bulk metallic glasses.Fig. 4—Isothermal DSC curves of Mg65Cu10Ni10Y10-xZn5Dyx(x = 0, 2, and 4) glasses (annealing temperature 485 K).

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the corresponding DSC curves obtained during theisothermal annealing. Each DSC curve exhibits anincubation period followed by an exothermic peak, whichcorresponds to the crystallization of the amorphousphase. The bell-shaped exothermic traces reveal that thefirst exothermic reaction of all the studied alloys isattributed to nucleation and growth of crystallinephases.[17]Moreover, with the addition of theDy element,the exothermic peak width increases slightly, indicatingthat the growth rate of crystals must be sluggish.

The kinetics of such isothermal transformations canbe analyzed by using the Johnson–Mehl–Avrami (JMA)equation[23]:

ln½� lnð1� XÞ� ¼ n ln kþ n lnðt� sÞ ½3�

where X is the transformed volume fraction at time t, s isthe incubation time, and n is the Avrami exponent. Thecombination of different growth mechanisms will pro-duce different Avarami exponents, and the results areshown in Figure 5. The values of n for Mg65Cu10Ni10Y10-xZn5Dyx (x = 0, 2, and 4) glasses are deter-mined to be 2.9, 2.5, and 2.0, respectively. The highest nvalue of 2.9 for Mg65Cu10Ni10Y10Zn5 glass reflects thenucleation-controlled crystallization process with simul-taneous existence of the nucleation process and three-dimensional growth of nuclei.[24] With the addition ofDy, the n value for the glasses decreases to 2.0–2.5,indicating the crystallization process with a sluggishgrowth rate. In addition, this value is typical forcrystallization governed by the diffusion-controlledgrowth of spherical grains, the nucleation rate of whichis low at low temperatures, but increases to a constantrate at higher temperatures.

To study the crystalline phases formed during thecrystallization process, the as-cast Mg65Cu10Ni10Y10-x

Zn5Dyx (x = 0, 2, and 4) glassy samples were annealedisothermally for 20 minutes at 500 K (227 �C), which isthe temperature just after the completion of the primarycrystallization process. Figure 6 shows the XRD pat-terns of the annealed samples. From these patterns, it isknown that Mg2Cu and Mg are the crystalline phasesafter the isothermal annealing treatment for the x = 0

and x = 2 glassy alloys, whereas Mg2Cu and MgZn arethe crystalline phases for the x = 4 glassy alloy. It canbe seen that the peak of Mg2Cu crystalline phases areapparently reduced with the addition of Dy, implyingthat Dy addition effectively suppressed the formation ofthe Mg2Cu crystalline phase.

IV. CONCLUSIONS

The influences of the partial substitution of Dy for Yin Mg65Cu10Ni10Y10Zn5 glass on the glass-formingability, thermal stability, and crystallization behaviorare studied. The results reveal that Dy increases theactivation energy for the first crystallization, but itdecreases the glass-forming ability and thermal stability.Analyses of crystallization kinetics give evidence that thenucleation rate decreases with the substitution of Dy forY. The examination of the crystallization phase after theisothermal annealing indicates that Dy addition caneffectively suppress the formation of Mg2Cu, Mg andpromote the formation of the MgZn crystalline phase.The decreased GFA is a result of increasing the Gibbsfree energy difference between the liquid and crystal.

ACKNOWLEDGMENTS

This work was supported by the National NaturalScience Foundation of China (Grant No. 50871054),the Specialized Research Fund for the Doctoral Pro-gram of Higher Education of China (Grant No.20093219110035), Natural Science Foundation of Ji-angsu Province (BK2007213), and NUST ResearchFunding (2010ZDJH010).

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