Comparison of damping measurement and differential scanning calorimetry as methods of determining the glass transition temperature in metallic glasses

Materials Science and Engineering, 97 (1988) 453~,56

453

Comparison of Damping Measurement and Differential Scanning Calorimetry as Methods of Determining the Glass Transition Temperature in Metallic Glasses* H.-R. SINNING and F. HAESSNER lnstitut fiir Werkstoffe, Technische Universita't Braunschweig (F.R.G.)

Abstract Low frequency internal friction and differential scanning calorimetry (DSC) measurements were used to determine the glass transition temperature Tg of Pd4oNi4oP2o and Ni6oPd2oP2o over a large range of heating rates "]'from 0.025to lOO Kmin 1. For the higher heating rates the DSC data were analysed applying different definitions of Tx and compared with the internal friction values at lower heating rates. The best agreement between the two sets of data was obtained when Tg was defined at the end of the Cp increase for as-quenched samples and at the C? peak for relaxed samples. In this case a linear dependence of Tg vs. log i" was observed over the whole range of heating rates investigated. The relevance of these results for the experimental determination of Te in general is discussed. 1. Introduction Although much work has been done on the theory of the glass transition during the last few years, the experimental determination of the glass transition temperature (or glass temperature) Tg is still a matter of purely empirical methods for which no general theoretically well-founded rule exists. The standard method of measuring Tg, especially in metallic glasses, is differential scanning calorimetry (DSC) which makes use of the change in specific heat during the transition. However, it is not at all clear where on a given DSC trace Tg should be defined; several different recommendations can be found in the literature [1]. Another equally empirical and widely used possibility of defining Tg is the so-called "isoviscous" definition [2] which sets Tg as the temperature where the viscosity is 1012 N s m -2. Recently, we have shown for metallic glasses that on the basis of this definition a Tg determination by low frequency internal friction *Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3 7, 1987. 0025-5416/88/$3.50

measurements is possible if the oscillation frequency is lower than about 0.2 Hz [3]. This method works preferably at very low heating rates and is therefore complementary to the standard DSC method which is confined to higher heating rates. In the present paper, reports are given of a series of Tg measurements on some glasses of the P d - N i - P system using both internal friction and DSC, for heating rates ranging from 0.025 to 100 K min 1. The results of both methods are compared and the questions of whether the different definitions of Tg are compatible and whether a general recommendation in favour of one of the different possibilities of evaluating Tg from a DSC trace [1] can be given are discussed.

2. Experimental determination of Tg 2. I. Differential scanning calorimetry The DSC measurements reported in this paper were carried out at heating rates between 4 and 100 K min i with a Perkin-Elmer DSC-7 instrument. Figure 1 shows how Tg was read from a DSC curve; if a monotonic increase in specific heat Cp was observed (Fig. l(a)), the following definitions of Tg were applied: (1) onset of the Cp increase; (2) "midpoint" with half the total increase; (3) point of inflection; (4) end of the C? increase. For a relaxed material showing an additional peak in Cp (Fig. l(b)), five different definitions were tested: (1) onset of the Cp increase; (2) "midpoint" with half the increase to the peak; (3) point of inflection; (4) peak; (5) "end point" of the peak (extrapolation of the high temperature decrease to the equilibrium value). 2.2. Low frequency internal friction The internal friction measurements were performed at frequencies of about 0.I Hz using a Collette pendulum [4, 5], which is a modified type of inverted torsion pendulum characterized by a second suspension wire between the inertia member and the specimen. This instrument is especially suitable for (t'~ Elsevier Sequoia/Printed in The Netherlands

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measuring the extremely large changes in internal friction occurring during the glass transition of metallic glasses [6]. Tg was determined at the transition from predominantly anelastic to viscoelastic behaviour as shown in Fig. 2; this has been discussed in detail

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elsewhere [3]. Because of the time required for a damping measurement at such low frequencies, this method is restricted to heating rates lower than about 1 K min- 1. The measurements were performed on melt-spun Pd4oNi4oP2o (width, approximately 20 mm; thickness, approximately 0.06 mm) and on Ni6oPd2oP2o(width, 2 mm; thickness, 0.03 mm; as quenched and relaxed

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Fig. 4. As for Fig. 3, but for relaxed Ni6oPd2oP20, showing a peak in the specific heat. for 2 h at 573 K), which were kindly supplied by Dr. A. R. Yavari (Institut National Polytechnique de Grenoble) and Professor S. Steeb (Max-Planck-lnstitut ftir Metallforschung, Stuttgart) respectively. For the internal friction measurements, samples about 1 cm in length and 2 mm in width were cut from the ribbons. 3. Results Figure 3 shows the glass transition temperatures obtained with both methods on as-quenched Pd4oNi,oP2o and Ni6oPd2oP20 as a function of heating rate T. Except for the strongly scattered data of the "onset" and "midpoint" of the Cp increase for Ni6oPd2oP2o in Fig. 3(b), a linear dependence of Tg vs. log 7~ is observed for each evaluation. A fit between the results of both methods, however, is only obtained if the definition (4) of Tg at the end of the Cp increase is applied to the DSC data. Using this definition, a single straight line may be drawn in both diagrams which fits the data from both damping measurements and DSC within the experimental error. A linear relationship between Tg and log T over almost four decades in heating rate is then obtained for both Pd4oNi4oP2o and NiroPd2oP2o. For relaxed NiroPd2oP2o showing a peak in specific heat (Fig. l(b)), the definition of 7"8 at the peak itself gives the best fit to the internal friction data (Fig. 4), although some more measurements on this material are necessary to settle this beyond doubt. Again a linear relation between Tg and log ~ over

The experimental results presented above have shown that information about the glass transition may be obtained over a very large range of heating rates by combining DSC with low frequency internal friction measurements. For the materials studied here, the results suggest that the most useful definition of Tg in DSC is at the end of the Cp increase for a curve such as that in Fig. l(a) and at the peak for a curve as in Fig. l(b). The combined results may also be taken as further experimental evidence in favour of a general compatibility of the (empirical) isoviscous and calorimetric definitions of the glass transition. Since an answer to the practically important question of this compatibility on theoretical grounds is still missing, we can only treat this question phenomenologically. This has been done in some detail elsewhere [3], including possible objections from the literature. We have come to the conclusion that there is no principal objection against the claim that the definition of Tg at a certain value of viscosity (underlying the internal friction method) is compatible with the more common determination of Tg by a calorimetric measurement. The present results fit this picture. Another important point is whether the recommendations of defining Tg at the end of the C, increase (Fig. l(a)) or at the Cp peak (Fig. l(b)), as suggested by our data on Pd4oNi4oP20 and Ni6oPd2oP2o, are of more general significance, For the first case (Fig. l(a)), we have already pointed out [3] that the computer simulations of Cunat [7] based on a statistical approach to the thermodynamic properties of the liquid and glassy states indeed support this interpretation from a completely independent point of view. However, the definition of Tg at the C, peak (Fig. l(b)) has not been tested by Cunat. We therefore hope that further experiments involving a comparison of different methods of Tg determination as well as a continuation of computer simulation studies will help to clarify the question of how Tg should be determined from DSC measurements. Acknowledgments We wish to thank Professor S. Steeb and Dr. A. R. Yavari for the supply of specimens as well as P. Pfeiffer and Dr. E. Woldt for performing the DSC measurements. This work was supported by the Deutsche Forschungsgemeinschaft.

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References 1 M. Lasocka, J. Mater. Sci., 15(1980) 1283. 2 R. Parthasarathy, K. J. Rao and C. N. R. Rao, Chem. Soc. Rev., 12 (1983) 361. 3 H.-R. Sinning and F. HaeBner, J. Non-Cryst. Solids, 93 (1987) 53.

4 G. Collette, C. Roederer and C. Crussard, M+m. S¢i. Rev. Mbtall., 58 (1961) 61. 5 H.-R. Sinning, J. Phys. E, 19 (1986) 866. 6 H.-R. Sinning and F. HaeBner, Set. Metall., 20 (1986) 1541. 7 C. Cunat, Thdse d'Etat, Universit~ de Nancy I, 1985.