Tanya Aycan Baser and Marcello Baricco

Transcription

1 Glss Rev.Adv.Mter.Sci. forming bility 18(2008) of (M=none, Al, Nb) bulk metllic glsses 71 GLASS FORMING ABILITY OF (M=NONE, Al, Nb) BULK METALLIC GLASSES Tny Aycn Bser nd Mrcello Bricco Diprtimento di Chimic IFM nd NIS, Università di Torino, Vi P.Giuri, Torino, Itly Received: Mrch 29, 2008 Abstrct. The effect of minor ddition of Al nd Nb on bulk glss formbility in Cu 50 lloy is investigted. Quenching rte effect ws followed compring the results obtined on ribbons nd on different sections of cone-shped ingots. The phse mixture hs been clrified for ech lloy, combining structurl, microstructurl nd composition nlyses. A fully morphous phse hs been found in ribbons, wheres only for bulk metllic glsses hs been obtined. Minor Al ddition hinders the formtion of the CuZr mrtensite phse. The effect of composition on the glss forming bilty hs been evluted with empricl prmeters, but only the clcultion of driving forces for nucletion of crystl phses, using the CALPHAD method, my explin the results. 1. INTRODUCTION Recent progress on bulk glss formtion in binry lloys hs triggered interest in esy glss forming systems [1]. Suitble minor lloying ddition is very effective for improving the glss forming bility (GFA) of binry lloys [2], such s in the cse of Al ddition to Cu-Zr binry system [3]. It hs lso been reported tht minor Nb ddition enhnces the glss forming bility of Cu-Zr system [4]. The improvement of glss forming bility is relted to the suppression of nucletion of intermetllic phses [5]. It hs been suggested tht, for bulk metllic glsses, s the Poisson rtio decreses, the mteril becomes more brittle [6,7]. Since both elementl Al nd Nb hve Poisson rtio higher thn Cu nd Zr, minor dditions of Al nd Nb in Cu-Zr system should improve the plsticity of the binry system. In this pper, the effect of Al nd Nb on GFA in (M=none, Al, Nb) lloys is reported. In prticulr, the effect of the quenching rte on phse selection is investigted. The positive effect of ddition of 4% of Al on glss formtion in Cu 50 Corresponding uthor: Mrcello Bricco, e-mil: mrcello.bricco@unito.it 2008 Advnced Study Center Co. Ltd. hs been explined on the bsis of driving forces for nucletion of crystl phses, clculted by CALPHAD method. 2. EXPERIMENTAL Mster lloys, Cu 48 Zr 48, nd Cu 48 Zr 48 ) hve been prepred from pure elements by rc melting under Ar tmosphere. Ech ingot ws remelted severl times in order to obtin good homogeneity. Melt spinning technique ws used to obtin ribbons. The liquid lloy ws cst in cone-shped copper mould in Ar tmosphere nd slices of 2 mm, 4 mm, nd 6 mm in dimeter were obtined from the s-cst ingots. In order to estimte the vrition of quenching rte on vrious slices of the cone-shped smples, experimentl dt of temperture versus time obtiend during quenching Mg-bsed lloy hve been considered [8]. From the nlysis of the rw dt, for positions where the smple thickness is 3 mm nd 6.5 mm in wedge-shped mold it turns out tht, s the dimeter doubles, cooling rte decreses to hlf. So, liner correltion between slice dimeter nd

2 72 T. A. Bser nd M. Bricco Fig. 1. XRD ptterns of Cu 50 lloy: ribbon (), slices of s-cst smple with dimeter of 2 mm slice (b), 4 mm (c), 6 mm (d) nd mster lloy (e). Symbols correspond to Cu 10 (Ο), CuZr mrtensite, bsic structure ( ), CuZr mrtensite, super structure (p), CuZr cubic (o) nd CuZr 2 ( ) phses. quenching rte cn be considered for coneshped mold. X-ry diffrction (XRD) (Cu K α ) ws performed to exmine the structure. The microstructure ws studied using opticl microscope nd composition nlysis ws crried out with EDS. Therml nlysis ws performed by differentil scnning clorimetry (DSC) t heting rte of 10 C/min. 3. RESULTS AND DISCUSSION The effect of the quenching rte on the glss formtion in Cu 50 lloy ws studied by XRD nd the results re reported in Fig. 1. The XRD pttern of the mster lloy cn be fully indexed with Cu 10, CuZr 2 nd CuZr mrtensic phse [9,10]. In fct, high temperture CuZr cubic phse might undergo mrtensitic trnsformtion on cooling below 140 C [11]. According to the phse identifiction, there is no equilibrium of phses even in the mster lloy. The sme phse mixture ws observed in the slices of the s-cst smple with dimeter of 6 mm nd 4 mm, wheres only CuZr cubic nd CuZr mrtensitic phses were observed in the 2 mm slice. The opticl microgrph of the Cu 50 mster lloy is shown in Fig. 2. EDS nlysis showed tht phse I is enriched in Zr (55 t.%), wheres n enrichment in Cu is observed in phse II (64 t.%). So, these phses hve been ssigned to CuZr 2 nd Cu 10, respectively. The needle shped phse III might be ssocited to the CuZr mrtensite. The microstructures observed in the 6 mm, 4 mm, nd 2 mm slices of the s-cst smple re shown Figs. 2b, 2c, nd 2d, respectively. As the ingot dimeter decresed, becuse of the incresed quenching rte, the grin size decresed nd the microstructure is refined. XRD results obtined on system re shown in Fig. 3. The pttern of the mster lloy cn be fully indexed considering CuZr cubic, Cu 10, CuZr 2 nd AlCuZr crystlline phses. CuZr cubic, Cu 10 nd AlCuZr phses were lso observed in the 6 mm nd 4 mm slices of the s-cst smple, wheres only the Cu 10 phse embedded in n morphous mtrix ws observed in the 2 mm slice of the ingot. Phse identifiction shows tht minor Al ddition to Cu 50 hinders the formtion of CuZr mrtensite, suggesting tht Cu 10 is the first nucleting phse on cooling. The microstructure of the mster lloy is shown in Fig. 4. EDS nlysis showed tht only phse I contins Al, so phse I, II nd III hve been ssigned to AlCuZr, CuZr 2, nd Cu Zr crystl structures, respectively. Finer microstructures hve been 10 7 observed in 6 mm slice (Fig. 4b) nd 4 mm slice (Fig. 4c), wheres no crystls hve been observed in the 2 mm slice of the s-cst smple. XRD ptterns of smples re reported in Fig. 5. CuZr mrtensite nd unknown phses hve been observed in the mster lloy, s well s in the 6 mm nd 4 mm slices of the scst smple. In the XRD pttern of slice with 2 mm dimeter, min contributions from CuZr cubic phse were found, together with dditionl diffrction peks due to the mrtensite phse, without evidence of n morphous phse. A fully morphous phse hs been obtined by rpid solidifiction for ll compositions (Figs. 1, 3, nd 5). The DSC trces of crystlliztion of the morphous ribbons nd of the (Cu Zr ) Al bulk smples re shown in Fig. 6. Glss trnsition (T g ) nd crystlliztion (T x ) tempertures nd het of crystllistion ( H x ) re summrised in Tble 1. The melting (T m ) nd liquidus (T l ) tempertures nd the het of melting ( H m ) for vrious compositions hve been determined by high temperture DSC nd the results re collected in Tble 1. Al ddition to Cu 50 morphous lloy increses the cystllistion temperture, wheres Nb ddition decreses it. As the dimeter of the bulk smples incresed, the intensity of the crystlliztion event decresed, becuse of the reduced morphous

3 Glss forming bility of (M=none, Al, Nb) bulk metllic glsses 73 Fig. 2. Opticl microgrphs for: Cu 50 mster lloy (), slices of s-cst smple with dimeter of 6 mm (b) 4 mm (c) nd 2 mm (d). Fig. 3. XRD ptterns of lloy: ribbon (), slices of s-cst smple with dimeter of 2 mm slice (b), 4 mm (c), 6 mm (d) nd mster lloy (e). Symbols correspond to Cu 10 (Ο), CuZr 2 ( ), CuZr cubic (o), nd AlCuZr (*) phses. Fig. 5. XRD ptterns of lloy: ribbon (), slices of s-cst smple with dimeter of 2 mm slice (b), 4 mm (c), 6 mm (d) nd mster lloy (e). Symbols correspond to CuZr mrtensite, bsic structure (r), CuZr mrtensite, super structure (p), CuZr cubic (o), nd unknown ( ) phses. Fig. 4. Opticl microgrph for: mster lloy (), slices of s-cst smple with dimeter of 6 mm (b) nd 4 mm (c). frction due to the decresed quenching rte (Tble 1). The GFA of lloys my be defined on the bsis of empiricl prmeters [12], such s T=T x -T g, T rg =T g /T l nd γ= T x /(T g +T l ). The ddition of 4% Al nd Nb to Cu 50 does not ffect significntly the supercooled liquid region ( T), but slight enhncement of T rg nd γ ws obtined for lloy with respect to Cu 50 nd (Tble 1). In this cse, empiricl prmeter cnnot fully explin the significnt increse of GFA due to the Al ddition to Cu 50,

4 74 T. A. Bser nd M. Bricco Tble 1. Therml dt obtined from DSC trces: T m =melting temperture, T l =liquidus temperture, H m =enthphy of fusion, T g =glss trnsition temperture, T x = crystllistion temperture, T=T x -T g, H x = enthlpy of crystllistion. Composition T m T l H m T g T x T H x Amorphicity T rg γ ( C) ( C) (J g -1 ) ( C) ( C) ( C) (J g -1 ) (%) (T g /T l ) b (T x / (T g +T l )) b Cu (ribbon) (ribbon) (ribbon) (2 mm) (4 mm) (6 mm) Dt obtined from mster lloys. b T rg nd γ were defined in Ref [12]. Fig. 6. DSC trces for the crystllistion of Cu 50 ribbon (), ribbon (b), ribbon (c), s-cst smple tken from the slices with dimeter of 2 mm (d), 4 mm (e) nd 6 mm (f). Fig. 7. Driving force for nucletion of crystl phses s function of temperture, clculted ccording to the CALPHAD method. so thermodynmic nlysis of GFA hs been crried out by the CALPHAD method [13]. The driving force for nucletion of crystl phse form the undercooled liquid cn be esily estimted if description of the free energy of both phses is vilble s function of temperture nd composition. For ternry systems, thermodynmic description cn be obtined by interpoltion of the ssessed binries. In order to understnd the role of thermodynmics on glss formtion in the investigted systems, recently s-

5 Glss forming bility of (M=none, Al, Nb) bulk metllic glsses 75 sessed Cu-Zr phse digrm [14] hs been considered s bse system. In this cse, specific thermodynmic description for the undercooled liquid nd morphous phses hs been introduced, in order to tke into ccount experimentl vlues of glss trnsition. In order to obtin informtion on the effect of the ddition of 4% of Al nd Nb to Cu 50 on the driving force for nucletion of crystlline Cu-Zr phses, the ssessed Al-Cu [15], Al- Zr [16], Cu-Nb [17] nd Nb-Zr [18] systems hve been selected. The results of the clculted driving forces for nucletion of CuZr cubic, CuZr 2, nd Cu 10 crystl phses from the undercooled liquid re reported in Fig. 7 s function of the temperture. Becuse of the lck of dt on the thermodynmic description of CuZr mrtensite nd of ternry phses, only Cu-Zr equilibrium crystl phses hve been considered. As consequence of the thermodynmic description of the undercooled liquid, which includes n excess het cpcity, the driving force increses on undercooling, reches mximum nd then decreses for lower tempertures. For ll phses, the highest driving force is obtined for liquid lloy with composition Cu 50. During the quenching of the liquid lloy, CuZr cubic is the first phse showing tendency to nuclete, but CuZr 2 shows the highest driving force for nucletion for n high degree of undercooling. The ddition of 4% of Nb nd Al to Cu 50 reduces the driving force for nucletion, but this effect ppers more importnt for Al ddition. Even if the phse selection might be modified for kinetic resons, the thermodynmic nlysis of driving forces for nucletion of crystl phses my explin the observed increse of GFA due to the Al ddition to Cu CONCLUSIONS The effect of minor dditions of Al nd Nb on glssforming bility of Cu 50 hs been investigted. The effect of the quenching rte on bulk glss formtion hs been followed considering ribbons nd sliced smples with different dimeters, tken from cone-shped s-cst ingot. All ribbons were morphous. Fully crystlline bulk smples hve been obtined for Cu 50 nd. For, smples with incresing dimeter showed decresing morphous frction, becuse of the reduced quenching rte. Therml dt hve been collected from DSC mesurements. Empiricl GFA prmeters cnnot explin the increse in GFA due to the ddition of Al to Cu 50. So, the driving forces for nucletion of crystl phses hve been clculted by the CALPHAD method, showing tht Al ddition leds to decrese of them with respect to the binry system. ACKNOWLEDGEMENTS This work ws performed for Regione Piemonte, Nnomt, COFIN/MIUR _002 nd MCRTN-CT REFERENCES [1] J. Ds, M. B. Tng, K. B. Kim, R. Theissmnn, F. Bier, W. H. Wng nd J. Eckert // Phys. Rev. Lett. 94 (2005) [2] W. H. Wng, Q. Wei nd H. Y. Bi // Appl. Phys. Lett. 71 (1997) 58. [3] W. H. Wng, J. J. Lewondowski nd A. L. Greer // J. Mter Res. 20 (2005) [4] J. Xi, J. Qing, Y. Wng, Q. Wng nd C. Dong // Appl. Phys. Lett. 88 (2006) [5] P. Yu, H. Y. Bi, M. B. Tng nd W. L. Wng // J. Non-Crys. Solids 351 (2005) [6] Y. Zhng nd A. L. Greer // J. All. nd Comp (2007) 2. [7] J. Eckert, J. Ds, K. B. Kim, F. Bier, M.B. Tng, W.H. Wng nd Z.F. Zhng // Intermetllics 14 (2006) 876. [8] N. H. Pryds, M. Eldrup, M. Ohnum, A. S. Pedersen, J. Httel nd S. Linderoth // Mter. Trns. JI1 (2000) [9] D. Schryvers, G. S. Firstov, J. W. Seo, J. Vn Humbeeck nd Y. N. Kovl // Script Mter. 27 (1997) [10] Z. Y. Liu, M. Aindow, J. A. Hriljc nd I. P. Jones // J. Metstble nd Nnocryst. Mter. 223 (2001) 360. [11] Y. N. Kovl, G. S. Firstov nd A. V. Kotko // Script Metll. Mter. 27 (1992) [12] D. Xu, G. Dun nd W. L. Johnson // Phys. Rev. Lett. 92 (2004) [13] M. Plumbo, G. Cccimni, E. Bosco nd M. Bricco // Clphd 25 (2001) 625. [14] T. Abe, M. Shimono, M. Ode nd H. Onoder // Act Mter. 54 (2006) 909. [15] N. Sunders, Al-Cu system, In: I. Ansr, A. T. Dinsdle, M. H. Rnd, COST 507: Thermochemicl Dtbse for Light Metl Alloys (Europen Communities, Luxemburg, 1998), p. 28.

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