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Materials Science and Engineering A 449–451 (2007) 118–121

Oxidation induced phase instability of Cu54Zr22Ti18Ni6bulk metallic glass

Hoon Cho a, Hanshin Choi a,∗, Hyungho Jo a,Sungchul Lim a, Byoungmoon Kim b, Changhee Lee c

a Nanomaterial Team, Advanced Material R&D Center, Korea Institute of Industrial Technology, Incheon, Republic of Koreab Research Institute of Industrial Science and Technology, Pohang, Republic of Korea

c Neomaterials and Hybrid-Process Laboratory, Hanyang University, Seoul 133-791, Republic of Korea

Received 25 August 2005; received in revised form 17 March 2006; accepted 30 March 2006

bstract

Effects of isothermal oxidation on the chemical instability which triggers crystallization of Cu54Zr22Ti18Ni6 (numbers indicate at.%) bulketallic glass were investigated. Comparing the isothermal heat flux of Cu54Zr22Ti18Ni6 bulk metallic glass within the supercooled liquid region

n differential scanning calorimetry, it could be observed that the isothermal oxidation did decrease the incubation time for crystallization. What is

urther, this amorphous phase instability was enhanced with the decrease of particle size. Through this study, chemical instability due to preferentialxidation of alloying element and resulting deviation of chemical composition in intact metallic region needs to be considered in phase evolutionf bulk metallic glass during material processing.

2006 Elsevier B.V. All rights reserved.

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eywords: Cu54Zr22Ti18Ni6 bulk metallic glass; Isothermal oxidation; Phase e

. Introduction

Among various applications using bulk metallic glass (BMG)owder, thermal spraying is very useful process in view of theaterial hybridization because useful surface properties such as

ow friction [1], high specific strength [2], and superior resis-ance for localized corrosion [3] of bulk metallic glass materialsan be effectively realized onto tough substrates. During allinds of material processing for manufacturing BMG compo-ents, the crystallization behavior of bulk metallic glass is verymportant point because as-previously remarked material prop-rties are largely dependent on phase fraction. Uncontrolledrystallinity of BMG severely increases the brittle nature. Inhe previous studies [4,5], the phase composition of vacuumlasma sprayed BMG coating was quiet different from that ofigh velocity oxy-fuel sprayed one at the similar melting degree.

rom the thermal stability for amorphous phase, this discrepancyannot be explained because cooling rates that are encountered inhermal spraying processes are generally no less than 104 K s−1

∗ Corresponding author. Tel.: +82 32 850 0135; fax: +82 32 850 0410.E-mail address: hschoi@kitech.re.kr (H. Choi).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.03.136

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6]. Thus, there must be a chemical instability for amorphoushase formation during solidification due to in-flight particlexidation in the high velocity oxy-fuel spraying in which oxy-en gas is used for oxidant. In this meaning, the crystallizationehavior needs to be considered from both thermal stabilitynd chemical stability. In this study, the effects of the oxida-ion on triggering chemical instability at the solid-state werenvestigated using Cu-base bulk metallic glass powder by com-aring the differential scanning calorimetry (DSC) thermogramsf starting BMG particles and oxidized particles.

. Experiment

Cu54Zr22Ti18Ni6 (at.%) bulk metallic glass particle was pro-uced using an inert gas atomization method and characteristicsf the particles were evaluated using X-ray diffractometry forhase identification, laser-scattering for particle size distribu-ion, differential scanning calorimetry for thermal properties (Tg

f 713 K and Tx of 770 K at the heating rate of 20 K min−1), andcanning electron microscopy for morphology. Before oxida-ion, isothermal crystallization kinetics within the supercoolediquid region of the Cu54Zr22Ti18Ni6 bulk metallic glass was

H. Cho et al. / Materials Science and Engi

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Fig. 1. Heat flux during isothermal heat treatment at each temperature.

nvestigated using differential scanning calorimetry in order tovaluate the effects of oxidation on the crystallization kinetics.or the oxidation test, as-received particles were divided intooarse particle (mean particle size of 90 �m) and fine particlemean particle size of 20 �m). After that, the BMG particles weresothermally oxidized for 1 h at 523, 623, 713, 720, 740, and70 K using a thermogravimetry analyzer. The oxidation kinet-cs of the BMG particle showed the parabolic rate law. After oxi-ation, phase composition and thermal properties were observedsing the X-ray diffractometry and the differential scanningalorimetry. Also, the plane-view and the cross-sectional mor-hologies of the oxidized BMG particles were examined using acanning electron microscope equipped with an energy disper-ion spectroscope.

. Results and discussion

Fig. 1 shows isothermal heat flux of Cu54Zr22Ti18Ni6 bulketallic glass within the supercooled liquid region which is

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Fig. 2. Variations of thermal properties of coarse particle (a) and fine

neering A 449–451 (2007) 118–121 119

elow crystallization temperature of 770 K and above glassransition temperature of 713 K. Within the empirical scopef time, any exothermic peak is not observed at both 730 and40 K. However, the incubation time required for starting crys-allization and the duration time for completing crystallizationre markedly reduced with the increase of temperature from 750o 770 K. In the case of 770 K, no incubation time is observed asxpected from continuous heating in DSC. It is noted that thereas no crystallization in the particles which was isothermallyeat treated below 740 K in this study. DSC thermographsor both isothermally oxidized coarse and fine BMG particlesan be seen in Fig. 2. Notable changes in thermal propertieshich resulted from isothermal oxidation can be observed. In

he case of coarse particle, double-step crystallization behaviorhich is typical of Cu54Zr22Ti18Ni6 bulk metallic glass cane seen in the oxidized particles below 713 K. However, therimary crystallization is absent at both 720 and 740 K. For thexidized particle at 770 K, no exothermic peak is observed asxpected from the isothermal heat flux at the same temperatureFig. 1). From the results of both 720 and 740 K, intact metallicolid part of the oxidized particle was partially crystallizeduring isothermal oxidation. And this might be due to changef chemical composition and resulting decrease of activationnergy for ordering to form crystalline phase. This oxidationnduced chemical instability for the irreversible crystallizations further enhanced in fine particles. It can be deduced that

arked crystallization occurred during isothermal oxidationrom isothermal oxidation temperature of 713 K. This particleize effect on the acceleration in the crystallization may resultsrom its small mass: as the particle size is decreased, healing

ower in chemical composition of a stable oxide phase forminglement near the oxide/metal interface is low and changingn chemical composition of the remaining metallic solidart is rapid at the same consumption of a specific alloying

particle (b) according to the isothermal oxidation temperature.

120 H. Cho et al. / Materials Science and Engineering A 449–451 (2007) 118–121

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Fig. 3. Phase compositions of oxidized particles at different oxidation temp

lement due to oxide film growth. Therefore, rapid change ofhemical composition in intact metallic part may affect amor-hous phase stability and/or crystallization kinetics by changingtomic mobility in modified chemistry whether transition inxide phase species occurs or not. Phase evolutions for fine par-icle and coarse particle are compatible to the results of DSC as

hown in Fig. 3. Diffraction from oxide film is prominent fromhe isothermal oxidation temperature of 713 K for fine particlehile marked difference in diffraction pattern of coarse particle

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ig. 4. Characteristic plane-view and cross-sectional morphologies of fine particle (according to isothermal oxidation temperature.

es (�: tetragonal-ZrO2; ©: CuO). (a) Fine particle and (b) coarse particle.

s observed from 740 K. Considering the broadening degree iniffraction pattern of tetragonal zirconia and metastability ofetragonal zirconia phase, it can be deduced that nanostructuredetragonal zirconia having a grain size less than the critical ones considered to be a major oxide film of Cu54Zr22Ti18Ni6 bulk

etallic glass within the scope of this study. As a matter of fact,

alculated grain size of oxidized tetragonal zirconia followingcherrer equation was 12–15 nm. Characteristic plane-viewnd cross-sectional features of oxidized particles according to

: 623 K; b: 730 K; c: 770 K) and coarse particle (d: 623 K; e: 730 K; f: 770 K)

H. Cho et al. / Materials Science and Engineering A 449–451 (2007) 118–121 121

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ig. 5. Arrhenius plot for parabolic oxidation rate constant at both transient oxid

article size and isothermal oxidation temperature can be seenn Fig. 4. Microstructures of oxidized fine particle having meaniameter of about 20 �m are different from those of coarsearticle of about 90 �m. In the case of plane-view morphologyf fine particle, smooth oxide film is covered on BMG particlet low temperature of 623 K and small particulate protrusionsre present on the surface at the increased temperature of 730 K.

hen the oxidation temperature is further increased to 770 K,evere crack is observed with the coarsened islands of oxide pro-rusions. Through the cross-sectional morphology of the crackedxidized particle which shows thick oxide layer is producedlong the crack, cracking was not due to the thermal expansionoefficient mismatch induced thermal stress during cooling butue to the stress which is induced by the growth of the thermallyrown oxides during isothermal oxidation. On the other hand,his propensity seems to be retarded for the coarse particle.rack is also present on the surface of coarse particle at the

sothermal temperature of 770 K. Crack seems to be nucleatednd propagated along the thickening protrusions. According toBR (Pilling–Bedworth ratio) which can be an indicator of stressegree resulting from oxide film growth, compressive stressesults from the growth of oxide film which has a lower densityhan bulk metallic glass. At sufficiently high temperature, stressan be relaxed by plastic flow of softened BMG but furtherncrease of oxide film thickness and/or recover of stiffness byrystallization may make oxide film be buckled. Otherwise,tress field along in-depth direction may be changed to favorrack on surface of oxide scale considering inward growth ofxide scale and spherical geometry of oxidizing particle. Crack

idth becomes wider and deeper as internal oxide film growsn the naked metallic surfaces along the crack. Conclusively,his characteristic features of the oxidized Cu–Zr–Ti–Ni BMGarticle is accelerated by both increase of isothermal oxidation

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stage and steady-state oxidation stage. (a) Fine particle and (b) coarse particle.

emperature and decrease of particle size. For isothermal oxida-ion kinetics, parabolic oxidation behavior was observed for theMG particle and the activation energy can be calculated usingslope between log Kp and reciprocal temperature in Arrheniuslot as shown in Fig. 5. Considering change of the parabolicate constant with isothermal oxidation time and temperature,lots for transient stage and steady-state stage are separated.ransient stage and steady-state activation energies of finearticle are same to those of coarse particle. Enhanced oxidationehavior due to small mass of fine particle can be identified.

. Conclusion

In this study, amorphous phase stability of Cu54Zr22Ti18Ni6ulk metallic glass was evaluated in view of thermo-chemicalnstability. Within the glass transition temperature, crystalliza-ion of intact solid part in the particle is accelerated due tosothermal oxidation because preferential oxidation of Zr alloy-ng element results in change of chemical composition andesulting decrease in the activation energy for crystallization.his reaction is accelerated in fine particle due to low reser-oir capacity of an alloying element against consumption of thelement during oxide film growth.

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1] D.H. Lee, J.E. Evetts, Acta Mater. 32 (1984) 1035–1043.2] A. Inoue, Bulk Amorphous Alloys: Practical Characteristics and Applica-

tions, Trans Tech Publications Ltd, Switzerland, 1999.

C.A. Carmichael, J.L. Wright, Intermetallics 10 (2002) 1157–1162.4] H. Choi, J. Kim, C. Lee, K. Lee, J. Mater. Sci. Lett. 40 (2005) 3873–3875.5] H. Choi, S. Yoon, G. Kim, H. Jo, C. Lee, Scripta Mater. 53 (2005) 125–130.6] H. Jones, Mater. Lett. 26 (1996) 133–136.