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THERMODYNAMIC EVALUATION of the Cu-Ti SYSTEM in VIEW of SOLID STATE AMORPHIZATION REACTIONS L

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THERMODYNAMIC EVALUATION of the Cu-Ti SYSTEM in VIEW of SOLID STATE AMORPHIZATION REACTIONS L THERMODYNAMIC EVALUATION OF THE Cu-Ti SYSTEM IN VIEW OF SOLID STATE AMORPHIZATION REACTIONS L. Battezzati, M. Baricco, G. Riontino, I. Soletta To cite this version: L. Battezzati, M. Baricco, G. Riontino, I. Soletta. THERMODYNAMIC EVALUATION OF THE Cu- Ti SYSTEM IN VIEW OF SOLID STATE AMORPHIZATION REACTIONS. Journal de Physique Colloques, 1990, 51 (C4), pp.C4-79-C4-85. 10.1051/jphyscol:1990409. jpa-00230769 HAL Id: jpa-00230769 https://hal.archives-ouvertes.fr/jpa-00230769 Submitted on 1 Jan 1990 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. ~OLLOQUEDE PHYSIQUE Colloque C4, suppl6ment au 11'14, Tome 51, 15 juillet 1990 THERMODYNAMIC EVALUATION OF THE Cu-Ti SYSTEM IN VIEW OF SOLID STATE AMORPHIZATION REACTIONS L. BATTEZZATI* , M. BARICCO*~ *""" , G. RIONTINO" *"* and I. SOLETTA** * '~ipartimento di Chimica Inorganics, Chimica Fisica e Chimica dei Materiali, Universitd di Torino, Italy ""~stitutoElettrotecnico Nazionale Galileo Perraris, Torino, Italy *.* Dipartimento di Chimica, Universitd di Sassari, Italy *et* INPM, Unita' di Ricerca di Torino, Italy Resumk -La description thermodynamique du sist6me Cu-Ti est reconsideree, car les calculs he la littgrature, bien que donnant une bonne representation du diagramme de phase, ne prevoient pas la possibilit6 de l'amorphisation. Une nouvelle courbe d'knergie libre pour la phase liquide est prohos6e, prenent en compte un excks de chaleur spkcifique de melange. Cette dernikre quantit6 a 6t6 obtenue pour quelques compositions de la difference entre la chaleur de fusion et la chaleur de cristallisation de rubans amorphes. On montre que la stabilit6 de la phase liquide cro'lt lorsque la temperature decroit. Les courbes T et To pour Cu-Ti sont calcul6es, et 1' intervalle d'amorphisation discut86. Abstract - The thermodynamic description of the Cu-Ti system is revised as current evaluations of it, while giving a reasonable fit to the phase diagram, do not predict the possibility of amorphization. A new free energy curve for the liquid phase is derived accounting for an excess specific heat of mixing. This latter quantity has been obtained for a few compositions from the difference between the heat of fusion and the heat of crystallization of melt spun ribbons. An increase in stability of the liquid phase on decreasing temperature is shown. Tg and To curves for Cu- Ti are calculated and the glass forming range is discussed. 1 - INTRODUCTION Cu-Ti has been one of the first metal-metal systems to show good glass forming tendency by liquid quenching /l/. Recently, it has been found that amorphization is possible in Cu-Ti either by reacting the pure solid elements /2/ or by grinding two intermetallic compounds /3/. The amorphizing range is wide and covers also compositions of intermetallic compounds. The Cu-Ti phase diagram displays steep liquidus curves in the terminal regions and a number of relatively low melting compounds in its central part; here the liquid field extends to low temperature with a shallow liquidus curve. As the intermetallics melt within a narrow range between 1281 K and 1153 K, it may be inferred that they are of similar and limited stability . This condition should certainly favour solid state amorphization. In spite of the vast evidence reported for vitrification in Cu-Ti, current evaluations of its thermodynamic properties, while giving a reasonable fit to the phase diagram, do not predict the possibility of amorphization by any technique, as the free energy of the liquid, extrapolated to the undercooled regime, never falls below that of competing solid solutions /4/. This implies that the locus of equal free energy between the liquid and solid solutions (To curves) stands well above the glass transition temperature. Under these circumstances, glassy alloys would never be produced, because a homogeneous close-packed phase would form at all compositions. On the other hand, the ability of amorphizing in the solid state indicates an enhanced stability of the liquid phase with respect to extended solid solutions. We reconsider here the thermodynamics of the Cu-Ti system with attention to the description of the free energy of the liquid phase. Experimental data are reported on the heat of fusion and crystallization of some alloys and T curves are calculated in order to make a comparison with the experiment&? Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990409 COLLOQUE DE PHYSIQUE glass forming range. 2 - EXPERIMENTAL Cu BTi34, Cu60Ti40, CU~~T~~~,TigOCu10 alloys hkve been prepared by arc mefting the pure elemen S omogeneity was ensured by remelting the alloy buttons several times. Pieces of crushed alloys were melted and spun onto a copper wheel under helium to produce amorphous ribbons. Amorphization was almost complete in all cases as checked by x-ray diffraction. Few milligrams of ribbon were used to determine the heat of crystallization of the amorphous alloy by DSc at .5 K/s heating rate. Fast heating (5 K/s) revealed the occurrence of a glass transition in Cu QTi and Cug6Tig4. The heat of fusion of the alloys was measured with a %iga% DTA apparatus by comparison of melting and solidification peaks with those of pure A1 and Cu. The accuracy of the results is lower than that obtained with standard techniques, however the deviations were mantained within a band of 10%. 3 - RESULTS The experiments devised in this work aim at determining the specific heat difference between liquid and solid phases, A~pl-sfrom where AHf is the heat of fusion at the melting point, Tf, and AH, is the heat of crystallization of the amorphous alloy. Tx is the .temperature of the maximum of the DSc peak. If the amorphous p ase is considered as an highly undercooled liquid, an average value for ACp -S is calculated between Tf and T . The heat of fusion was determined for the CU~~T~~~eutectic (13.1 kJ/mol), tEe congruent CuTi (15.8 kJ/mol) and the Cu3Ti (13.3 kJ/mol) and Ti2Cu (13.2 kJ/mol) compounds which decompose peritecticafiy at a few degrees below the liquidus. The heats of crystallization are 6.5, 4.6, 6.5 and 7.2 kJ/mol respectively. They pertain to alloys completely amorphous under x-ray diffraction apart from CU~~T~~~where a small crystalline fraction was detected, In this case the heat of crystallization is slightly underestimated. The onset crystallization temperatures correspond to those reported in /l/. From eq. (l), the average specific heat has been obtained as given in Fig. 1. Fig. 1 - Average specific heat difference between liquid and solid phases for some Cu-Ti alloys. Dashed line: parabolic fit to the data. All values are. substantial a d comparable with those collected for other glass-forming alloys /5/. ACp for pure Cu and Ti are also reported, as calculated in the next section. If the reference state in Fig, 1 is changed from the specific heat of solid elements to that of liquid elements, the excess specific heat of mixing, ACp, is obtained. For the purpose of extending the calculation to all compositions, the points. have been fitted by a parabola, with the awareness that it overestimates the quantity for very dilute solutions. The mixing effect on ACp is remarkable at intermediate concentration as expected for a system undergoing ordering. The chosen trend is symmetrical with respect to composition within the experimental error. However a skewness towards higher Cu content, in accordance to that of the Cowley short range order parameter /6/, may be possible. In fact, it can be caused simply by a slight change in the properties of the reference pure elements which carry a strong uncertainty, as discussed below. 4 - DETAILS OF THE CALCULATION The free energy of the solution phases is written as where GA and GB are the lattice sf bilities of the pure elements taken from Kaufmann /7/ and Saunders /8/, AGid is the ideal free energy of mixing and AG~~an excess term. As we are going to describe the free energy of the liquid in undercooling regime, the reference state of pure Cu is calculated at every temperature accounting for the difference between a constant specific heat for the liquid and that of crystal phases /g/. The Kauzmann temperature of vanishing entropy,TK, often considered as an ideal glass transition temperature, is 257 K for Cu and the average AcP1-' is 4.7 (Fig.l ) . The assessment of the reference state for pure Ti in undercooling regime is more difficult. A recent collection of data /10/ reports a value of 8 + 3 J/mol K for AC~~-~at the melting point and a constant specific heat for the liquid. Therefore, the extrapolation of the entropy to low temperatures leads to TK= 0.62 T whereas extrapolation of the actual glas: transition temperature in %;nary systems to zero Ti concentration gives Tp - 0.3 T Ill/. So, the specific heat of liquid undercooled Ti appears overestlmated. If TK is taken at 0.25Tf for Ti as for other pure metals /12/, an effective speclfic heat difference between liquid and DTi of 5.4 J/mol K is derived. The specific heat of aTi was taken from /10/ and extrapolated to its melting point; the specific heat of fcc Ti was considered equal to that of aTi.
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