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Intermetallics 16 (2008) 538e543www.elsevier.com/locate/intermet

Formation of nano-scale u-phase in arc-melted micron-scaledendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine

composite during heat treatment

K.B. Kim a,*, J. Das b, M.H. Lee b, D.H. Kim c, W.H. Lee a, J. Eckert b

a Department of Advanced Materials Engineering, Sejong University, 98 Gunja-dong, Gwnagjin-gu, Seoul 143-747, Republic of Koreab IFW Dresden, Institute for Complex Materials, Dresden D-01171, Germany

c Center for Noncrystalline Materials, Department of Metallurgical Engineering, Yonsei University, 134 Schinchon-dong, Seodaemun-ku,Seoul 120-749, Republic of Korea

Received 6 September 2007; received in revised form 18 December 2007; accepted 28 December 2007

Available online 6 March 2008

Abstract

An arc-melted Zr73.5Nb9Cu7Ni1Al9.5 alloy consists of dendritic body-centered cubic solid solution and a mixture of ultrafine body-centeredtetragonal Zr2Cu-type and hexagonal close-packed MgZn2-type phases in the ultrafine matrix. This composite exhibits two exothermic reactionsupon heating to 973 K with a heating rate of 40 K/min. After the first exothermic reaction nano-scale u-phase particles have formed in the b-phasedendrites indicating a decomposition of the supersaturated solid solution upon heating. At the early stage during the decomposition of the b-phasethe u-phase is coherent with the parent phase without clear structural differences. However, semicoherency between the b- and the nano-scaleu-phase regions was observed during the growth of the u-phase. This structural semicoherency causes nano-scale twinning in the b-phasedendrites around the ordered u-phase in order to release the mismatch energy at the interfaces between the ordered u- and b-phases.� 2008 Elsevier Ltd. All rights reserved.

Keywords: A. Composites; B. Phase transformations; C. Heat treatment; D. Microstructure; E. Phase stability

1. Introduction

Along the line of the development of bulk metallic glass(BMG) composites in order to improve the limited room tem-perature ductility of monolithic BMGs, a series of Ti- andZr-based BMG in situ composites with nano-scale precipitates[1,2] or ductile dendrites [3e6] have been successfully fabri-cated. More recently, a new class of nanostructureedendritecomposites with a significant improvement of the ductilityof up to w30% without losing the strength of 1.2e2 GPahas been developed in Ti- and Zr-based alloys by addition ofb-solid solution phase stabilizers, such as Nb [7,8] and Ta[9e11]. Previous studies of Zr-based nanostructureedendritecomposites fabricated by different casting techniques have

* Corresponding author. Tel.: þ82 2 3408 3690; fax: þ82 2 3408 3664.

E-mail address: kbkim@sejong.ac.kr (K.B. Kim).

0966-9795/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.intermet.2007.12.014

pointed out that the length-scale of the microstructure ofsuch nanostructureedendrite composites control their roomtemperature mechanical properties, e.g. their strength and duc-tility [12e14]. Among these Zr-based nanostructureedendritecomposites, the correlation between the length-scale of themicrostructure and the mechanical properties has been investi-gated in detail for the nanostructureedendrite composite withoverall composition Zr73.5Nb9Cu7Ni1Al9.5 [12e14]. It hasbeen found that not only mold-cast Zr73.5Nb9Cu7Ni1Al9.5

specimens but even arc-melted samples, which experiencedan even lower cooling rate and thus an even coarser micro-structure exhibit relatively good strength of w1.2 GPa anda ductility of w12% [13]. Similarly, the recent developmentof binary TieFe nanostructureedendrite composites has alsodemonstrated good mechanical properties for arc-meltedsamples [15e18]. Consequently, arc-melting can be used asa simple and promising technique for producing the materials

Fig. 1. SEM back-scattered electron (BSE) micrographs of the arc-melted

Zr73.5Nb9Cu7Ni1Al9.5 alloy.

Fig. 2. DSC trace of the arc-melted micron-scale dendrite reinforced

Zr73.5Nb9Cu7Ni1Al9.5 ultrafine composite recorded at a heating rate of

40 K/min.

539K.B. Kim et al. / Intermetallics 16 (2008) 538e543

without any need for a subsequent casting process. Therefore,it is necessary to further investigate possible modifications ofthe microstructure of the nanostructureedendrite compositesobtained by arc-melting. One of the easy approaches to modifythe microstructure is a heat treatment using the metastabilityof the nanostructureedendrite composites.

However, no investigations regarding the phase transfor-mation of the nanostructureedendrite composites upon heattreatment have been described in detail so far. Since it isa well known skill to improve the mechanical properties inmany conventional alloys by heat treatment, i.e. by the forma-tion of GP zones in Al alloys [19,20], the investigation of thestructural evolution in the nanostructureedendrite compositesmay give a hint for a further enhancement of the mechanicalproperties compared to the as-solidified state.

For this study, an arc-melted Zr73.5Nb9Cu7Ni1Al9.5 nano-structureedendrite composite was selected to investigate itsstructural evolution. In the following, we will focus on the de-composition of the supersaturated b-solid solution dendritesduring heat treatment.

2. Experimental procedure

A Zr73.5Nb9Cu7Ni1Al9.5 ingot (w20 mm in diameter andw10 mm in height) was prepared by arc-melting a mixtureof high purity elements (Zr: 99.8 wt.%; Nb: 99.8 wt.%; Ni:99.95 wt.%; Cu and Al: 99.999 wt.%) in an Ar atmosphere.The ingot was turned and melted four times to ensure chem-ical homogeneity. The phase identification was performed ina Siemens D 500 X-ray diffractometer (XRD) using Cu Karadiation scanning from 30� � 2q� 80�. The thermal stabilitywas investigated with a PerkineElmer Diamond differentialscanning calorimeter (DSC) using Al2O3 pans under high pu-rity Ar and constant-rate heating at 40 K/min up to 973 K. Themicrostructure was investigated with a Zeiss DSM 962 scan-ning electron microscope (SEM) operated at 30 keV after sur-face etching with a mixture of 98 ml HNO3 and 2 ml HF for4 s, and a Philips CM20 transmission electron microscope(TEM) at 200 keV coupled with energy-dispersive X-ray anal-ysis (EDX, Noran). Thin foils for TEM were prepared by theconventional method of slicing and grinding, followed by ion-milling (BAL-TEC RES 010). In order to obtain high qualityof the chemistry using EDX, nano-size beam (w10 nm) hasbeen used at least five times with high efficiency of morethan 80%. Furthermore, the TEM sample has been tiltedw13� to the perpendicular direction to the EDX detector.

3. Results and discussion

Fig. 1 shows SEM back-scattered electron (BSE) micro-graph of the arc-melted Zr73.5Nb9Cu7Ni1Al9.5 alloy. Theoverall microstructure displayed in Fig. 1 reveals that the mi-cro-scale dendrites are homogeneously distributed in thematrix. The volume fraction of the dendrites is around 80e85%. Moreover, the bright contrast of the dendrites in theback-scattered image indicates that they contain a largeramount of heavy elements compared to the matrix. However,

it is very difficult to observe the microstructure of the matrixdue to the limited resolution of conventional SEM. The de-tailed microstructure of the matrix will be investigated usingTEM.

Fig. 2 shows a DSC trace of the arc-melted micron-scaledendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine composite.There are two exothermic reactions upon constant-rate heatingwith 40 K/min up to 970 K. The onset temperatures (Txo1 andTxo2) and the heat releases (DH1 and DH2) of the first and thesecond exothermic reactions are 702 K and 32.4 J/g, and921 K and 4.3 J/g, respectively. The heat treatment tempera-tures selected to understand the structural evolution of themicron-scale dendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultra-fine composite during the first exothermic reaction are alsoindicated by the arrows in Fig. 2.

Fig. 3 shows typical XRD patterns of the arc-melted andheat treated micron-scale dendrite reinforced Zr73.5Nb9Cu7

Ni1Al9.5 ultrafine composites. The predominant crystalline

Fig. 3. XRD patterns of the arc-melted and heat treated micron-scale dendrite

reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine composite.

Table 1

EDX analysis results for the dendrites and the ultrafine matrix in the arc-

melted micron-scale dendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine

composite

Phase Elements (at.%)

Zr Nb Cu Ni Al

Dendrite 74.8� 2.0 8.2� 1.7 5.9� 1.2 0.2� 0.1 10.9� 1.3

Zr2Cu-type 59.4� 1.8 3.7� 1.1 29.3� 1.5 0.4� 0.1 7.2� 1.2

MgZn2-type 57.4� 1.3 1.8� 1.2 31.6� 1.1 3.4� 0.3 5.8� 0.9

540 K.B. Kim et al. / Intermetallics 16 (2008) 538e543

phase is a body-centered cubic (bcc) b-phase in the arc-meltedsample. The peak position of the (110) plane of the b-phaseappears at a higher 2q value (¼36.44�) than for the standardZr b-phase (¼35.79�) [21]. This suggests that the b-solidsolution phase of the micron-scale dendrites in the Zr73.5Nb9

Cu7Ni1Al9.5 ultrafine composite contains a significant amountof other elements. Besides, there are weak diffraction peaksfrom the small volume fraction of the matrix. Due to the lim-ited resolution of the conventional XRD experiments per-formed in this study it is difficult to identify the crystallinephases in the matrix. The XRD pattern after heat treatment

Fig 4. TEM bright-field image (a) and selected area diffraction patterns [(b)e(d)

ultrafine composite; (b) [111] zone axis of the dendritic b-solid solution phase a

zone axis of the MgZn2-type phase in the matrix.

at 723 K exhibits a slight change in the width of the diffractionpeaks of the (110) plane of the b-solid solution phase com-pared to that of the arc-melted sample. It is well known thatthe width of the diffraction peaks is linked with the grainsize and the strain of the respective phase [22]. Hence, moredetailed information regarding possible structural changes inthe dendrites is expected from the TEM investigation. TheXRD pattern obtained after heat treatment at 823 K shows ad-ditional and overlapping diffraction peaks, which correspondto the u-phase. In addition, one can observe a shift of thepeak position of the (110) plane of the b-solid solution phasetowards lower 2q values, i.e. 36.3� and 36.1�, after heat treat-ment at 723 and 823 K, with increasing heat treatment temper-ature. Altogether, these results suggest that the formation ofthe u-phase occurs by the decomposition of the supersaturatedb-solid solution phase in the dendrites through the first exo-thermic reaction visible in the DSC trace. Due to the largediffraction intensities of the b-solid solution and the u-phase,it is difficult to resolve the diffraction intensities from thephase corresponding to the matrix by XRD.

] from the arc-melted micron-scale dendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5

nd (c) [111] zone axis of the Zr2Cu-type phase in the matrix and (d) [101]

Fig. 5. SEM BSE micrographs [(a) and (b)] from the dendrites of the arc-

melted micron-scale dendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine com-

posite after heat treatment at 723 and 823 K, respectively.

Fig. 6. TEM bright-field image (a), corresponding selected area diffraction pat-

tern (b) and a HRTEM image (c) from the dendrites of the arc-melted micron-

scale dendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine composite after heat

treatment at 723 K.

541K.B. Kim et al. / Intermetallics 16 (2008) 538e543

Fig. 4 shows a bright-field TEM image (a) and selected areadiffraction patterns [(b)e(d)] for the arc-melted micron-scaledendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine composite.The bright-field image reveals that the ultrafine matrix consistsof two crystalline phases with a typical lamellar eutectic struc-ture [23]. The selected area diffraction pattern obtained fromthe dendrites in Fig. 4(b) shows the [111] zone axis of theb-solid solution phase. The selected area diffraction patternsfrom the ultrafine matrix are identified as the [111] and [101]zone axes of body-centered tetragonal (bct) Zr2Cu-type andhexagonal close-packed (hcp) MgZn2-type crystalline phases.These findings reveal that the crystalline phases in the presentcomposite are b-Zr solid solution phase in the dendrites anda mixture of Zr2Cu-type and MgZn2-type phases in the ultra-fine matrix.

Table 1 lists the EDX analysis results of the dendrites andthe ultrafine matrix. The dendrites exhibit a higher content ofNb (w8.2 at.%) and Al (w10.9 at.%) compared to both theZr2Cu-type and MgZn2-type phases in the ultrafine matrix.In contrast, the Zr2Cu-type phase has a very low Ni content(w0.4 at.%) and a higher content of Nb (w3.74 at.%) com-pared to the MgZn2-type phase in the ultrafine matrix,

suggesting that the content of Ni and Nb is important to con-trol the formation of the crystalline phases in the ultrafine ma-trix during solidification. These EDX results agree well withthe contrast observed in the SEM BSE images [Fig. 1(b)].As described in Fig. 1, the bright contrast of the dendrites iscaused by the high scattering intensity from the heavy ele-ments, e.g. Zr and Nb.

Fig. 5 shows SEM BSE micrographs [(a) and (b)] from thedendrites of the nanostructureedendrite composite after heattreatment at 723 K and 823 K, respectively. In the BSE micro-graph in Fig. 5(a) obtained after heating to 723 K, only very

Fig. 7. TEM bright-field image (a) and corresponding selected area diffraction patterns [(b) and (c)] from the dendrites of the arc-melted micron-scale dendrite

reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine composite after heat treatment at 823 K; the circles in (c) indicate the structural mismatch between the ordered u- and

b-Zr solid solution phases.

542 K.B. Kim et al. / Intermetallics 16 (2008) 538e543

weak contrast is visible in the dendrites. After heating up to823 K, homogeneously distributed nano-scale precipitates canbe found in the dendrites as shown in Fig. 5(b). This indicatesa decomposition of the supersaturated b-solid solution phasethrough the first exothermic reaction.

Fig. 6 shows a TEM bright-field image (a), a correspondingselected area diffraction pattern (b) and a high resolution trans-mission electron microscopy (HRTEM) image (c) from thedendrites of the arc-melted micron-scale dendrite reinforcedZr73.5Nb9Cu7Ni1Al9.5 ultrafine composite after heat treatmentat 723 K. The overall microstructure in Fig. 6(a) exhibits thedendrites and the ultrafine matrix. The TEM bright-field imagefrom the dendrites exhibits no clear contrast. However, the cor-responding selected area diffraction pattern of the dendrites inFig. 6(b) exhibits diffuse diffraction streaks along the [111]zone axis of the b-solid solution. Such streaks can often befound for very small lattice strains [24] during the formationof the u-phase, suggesting that the decomposition of the b-solid solution phase is accompanied by local straining. Similardiffuse diffraction streaks were also reported for rapidly solid-ified Ti60Cr10Al20 upon the formation of the u-phase froma supersaturated b-Ti solid solution phase [25]. Therefore, itis possible to assume that the formation of the u-phase inour specimen occurs with a very small lattice mismatch tothe b-solid solution phase. Furthermore, the HRTEM imagein Fig. 6(c) obtained along the [111] zone axis of the b-solidsolution phase show no clearly mismatched lattice fringesbetween the b-solid solution and u-phases phase. This is pre-sumably because the structural deviation between the b- andthe u-phases is very small along the [111] zone axis of theb-solid solution phase [25].

Fig. 7 shows a TEM bright-field image (a) and the corre-sponding selected area diffraction patterns [(b) and (c)] ofthe micron-scale dendrite reinforced Zr73.5Nb9Cu7Ni1Al9.5

ultrafine composite after heat treatment at 823 K. The TEMbright-field image of the dendrites in Fig. 7(a) shows clearcontrast, indicating that the precipitates of 5e20 nm in sizeare homogeneously distributed. The diffraction patterns ofthe [001] and [012] zone axes of the b-solid solution phasein Fig. 7(b) and (c) show well overlapped diffraction peakswith the [1e12] and [2e10] zone axes of the u-phase indicat-ing a structural coherency between the u- and b-solid solutionphases. Furthermore, there are several diffraction peaks fromthe superlattice scattering of the u-phase, as indicated bythe squares in Fig. 7(b). This indicates ordering of the u-phaseafter the first exothermic reaction. However, the detailed ob-servation of the selected area diffraction patterns in Fig. 7(c)reveals that the diffraction spots of the ordered u-phaseshow a slight mismatch with the diffraction spots of the b-solid solution phase, as indicated by circles, suggesting theformation of a semicoherent interface between these phases.Thus, the differences in the selected area diffraction patternsin Figs. 6(b) and 7(c) suggest that the structural deviation ofthe ordered u-phase from the b-phase in the dendrites in-creases during the progress of the decomposition of the b-solidsolution and the growth of the u-phase. Moreover, the value ofthe lattice parameter of the u-phase measured a¼ 5.13 A andc¼ 3.12 A, is quite similar to the known structural relationshipbetween the b-solid solution and u-phase confirming thestructural semicoherency [26]. The EDX analysis results ofthe b-solid solution and the ordered u-phase in the micron-scale dendrites reveal that the b-solid solution contains a higher

Fig. 8. HRTEM image from the dendrites of the arc-melted micron-scale den-

drite reinforced Zr73.5Nb9Cu7Ni1Al9.5 ultrafine composite after heat treatment

at 823 K; the inset shows an enlarged HRTEM image for the nano-scale

twinned region.

543K.B. Kim et al. / Intermetallics 16 (2008) 538e543

content of Al (w14.4 at.%) compared to the ordered u-phase.This suggests that the formation of the nano-scale u-phasepossibly occurs through the redistribution of Al in the mi-cron-scale dendrites during heat treatment.

Fig. 8 shows a HRTEM image from the dendrites after heattreatment at 823 K. One can clearly observe the differences inthe lattice fringes of the ordered u-phase, as indicated by thesquares, compared to those of the b-phase. Furthermore, thereare nano-scale twinned lattice fringes around the orderedu-phase that precipitated from the b-solid solution, as indi-cated by circles. A HRTEM image of the nano-scale twins isgiven in the inset of Fig. 8. These observations suggest thatthe mismatch strains of the ordered u-phase in the b-solidsolution, as indicated by the selected area diffraction patternsin Fig. 7(b) and (c), can be released by nano-scale twinning ofthe b-solid solution.

4. Summary

The formation of a micron-scale dendrite reinforced Zr73.5

Nb9Cu7Ni1Al9.5 ultrafine composite by arc-melting and itsstructural evolution upon heat treatment were investigated.The as-prepared microstructure reveals a homogeneous distri-bution of micron-scale bcc b-Zr solid solution dendrites witha volume fraction of 80e85% embedded in an ultrafine matrixconsisting of a mixture of bct Zr2Cu-type and hcp MgZn2-typephases. Upon heating to elevated temperatures two exothermicreactions appear in the DSC scan. At the early stage of the firstexothermic reaction, i.e. after heating to 723 K, the formationof a u-phase precipitated from the highly supersaturated b-phase dendrites was confirmed by selected area diffraction.However, the visible contrast in TEM and the lattice fringesin HRTEM investigations of the u-phase in the dendrites aredifficult to be resolved due to the small structural deviations

between the u- and b-Zr solid solution phases indicatingstructural coherency between these phases. After further heattreatment at 823 K the formation of spherical nano-scale or-dered u-phase particles was observed in the dendrites. Thenano-scale ordered u-phase is semicoherent to the b-Zr solidsolution phase. These results indicate that the interfaces be-tween the u- and b-phases change from coherent to semicoher-ent interfaces during the growth of the u-phase. Furthermore,the formation of a nano-scale twinned microstructure wasobserved around the nano-scale ordered u-phase. This suggeststhat the structural mismatch due to the semicoherency betweenthe ordered u- and b-Zr solid solution phases promotes nano-scale twinning in order to release the mismatch energy.

Acknowledgements

This work was supported by the Korea Science and Engi-neering Foundation (KOSEF) grant funded by the Koreagovernment (MOST) (R01-2007-000-10549-0), the GlobalResearch Laboratory Program of Korea Ministry of Scienceand Technology, and the EU within the framework of theResearch Training Network on ductile bulk metallic glasscomposites (MRTN-CT-2003-504692).

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