The glass transition temperature of A (Ni5Pd5) 82P18 amorphous alloy

Scripta METALLURGICA

Vol. ii, pp. 603-606, 1977 Printed in the United States

Pergamon Press, Inc

THE GLASS TRANSITION TEMPERATURE OF A (Ni5Pd 5) 82P18 AMORPHOUS ALLOY

P.G. Boswell Department of Mining and Metallurgical Engineering University of Queensland St. Lucia, Brisbane, Australia.

(Received April 25, 1977) Introduction Thermal evidence for a glass transition in a metallic amorphous alloy was first reported by Chen (i) using an Au770Ge136Si94 alloy and subsequently the glass transition temperatures, Tg, of numerous other metallic glasses have been measured. Chen (i) also found a small dependence of T~ on the imposed linear heating rate, ~, such that Tg for his splatquenched Au770Ge136Si94~lass increased by =1.5 K on increasing 8 from 0.042 to 0.67 K/s (2.5 to 40 K/rain). However, recently Chen (2) and Lewis and Davies (3) reported a considerably stronger dependence of Tg on 8B for a (Ni5Pd5)80P20 glass whereby Tg increased by =I6K for an eightfold increase in ~ (from 0.17 to 1.33K/s; i0 to 80 K/min). In this communication an attempt will be made to interpret these and other results describing the heating rate dependence of measured Tg temperatures for NiPdP metallic glasses produced by liquid quenching. Exl0erimental Results Figure 1 gives reported Tg temperatures, plotted as a function of 8 and log (~), for a (Ni5P~)80P20 glass (2,3) together with T_ temperatures measured in this laboratory for aTwO (Ni5P~)82~I8 glass. Table I specifies the~experimental procedures adopted in each case. points need to be raised in regard to the plotted data, namely a) the methods of estimating Tg and b) corrections for systematic errors arising in the course of thermal analysis experiments. In the case of the (Ni5P~)82P18 glass, the Tg temperature for a given heating rate was taken to be the temperature at which the specific heat of the transforming glass attained a value equal to one half the total specific heat change accompanying the g l a s s - l i q ~ d transformation. This temperature is roughly equi~ralent (4) to the temperature at the onset of the rapid decrease in the enthalpy relaxation at the glass-liquid transition. Decreases of this type are displayed by Chef's (2) heat of relaxation spectra for a filamentary cast (Ni5P~) 82P18 glass ribbon, so Tg temperatures could be estimated from these spectra. Unfortunately Lewis and Davies (3) omitted mentioning their procedure for estimating the Tg temperatures for their splat-quenched (NisPd5)80P20 alloy. Corrections to measured T

temperatures

With regard to corrections to the measured Tg temperatures, we observe that of the data plotted in Figure 1 only those obtained in the course of the present investigation have been corrected for systematic errors using the method outlined by Strella and Erhardt (5). qhese authors demonstrated theoretically that the error in Tg, or indeed for any transformation temperature obtained by thermal analysis, is composed of a sample error and a machine-path error. The sample error is negligible for specimens, such as metallic alloys, having a high thermal diffueivity so it can be ignored. The machine-path error, however, is not insignificant and it has been reported to be as large as =6 K per i K/s (1,5), being of the same order of magnitude as the observed increases in Tg with ~ (= 10K per 1 K/s; see Figure i). The machine path error can be obtained for a given thermal analysis unit by measuring the difference between the indicated and true transformation temperatures of a phase transition having a very small overall activation energy for transformation (implying that kinetic delay times are negligible). Suitable transformations include the magnetic change at the Curie te~i~erature of some metals (e.g. Ni) and alloys and the melting of some pure metals (e.g. Pb (6))at the

603

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(Ni, Pd,),_xPx @ •

X-- 0-20 X-O-f

c (z) kdm~e4 /

585

j

580

t.swJsa,_,, ~

f

D.~o.~)

.,

/

"350

~/~,j.~.,

r~,

///

I--

I 1 (~ ". KS-'

0

FIG. I.

~

I -1

l 0 log (~"). KS-'

The heating rate dependence of the observed glass transition temperatures for (Ni5Pd 5) 80P20 and (Ni5P ~ ) 82P18 glasses.

.TABLE 1 ~he Glass Transition Temperatures for (Ni5Pd5)l_ x P Details x --

Ref.

•18

this study

•20

Chen(2)

•20

Lewis and Davies (3)

Specimen

Exptl. method

uT (K_~

Glasses : Procedural

measurement error(K)

machine error

AH a (kJ/mole)

filamentary cast ribbon (=10~m thick)

DTA

temp.at half total change in specific heat

+ 2.0 to + 3.0

corrected for using m.pt. of Pb

"

DSC

onset of rapid decrease of relaxation enthalpy

+_ 1.5

uncorrected

=460

+ 1.0

uncorrected

=330

splat-quenched DSC foil

?

480 +30

equilibrium melting point. It is necessary to measure the error over the temperature range of the transformation under investigation and this can be achieved by interpolating errors measured at higher and lower temperatures. Occasionally however, it is possible to select a reference transformation that takes place at a temperature which is close to the mlknown transition temperature. In the case of the (Ni5Pd5)82P18 glass, Tg = 580K so the melting point of Pb (600.4K) is a suitable reference point. The corrected Tg temperature for each heating rate is then obtained by subtracting the corresponding machine-path error.

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Analysis and Discussion The heating rate dependence of T Inspection of Figure 1 (left-hand side) demonstrates that the corrected Tg temperatures obtained in this laboratory for a (Ni5Pds)82PI8 glass do not lie upon a straight line on the T u versus 8 plot. On the other hand, the tmcorrected data for (Ni5Pd5)8oP20 glasses reported b@ Chen (2) and by Lewis and Davies (3) fall on straight lines indica£ing linear T~- 8 relationships. Replotting the corrected and ~ c o r r e c t e d Tg temperatures as functio~n of log (8), as in Figure 1 (right-hand side) it is readily appreclated that the corrected data for a (Ni5Pd5)82PI8 glass now lie on a straight line, within experimental error, while the two sets of ~ncorrected data for the (Pd5Ni5)80P20 glass) do not. The non-linear Tg-8, linear Tg - log (8) behaviour described here for the (Ni~d5)82P18 alloy has also been observed for a IV-VI (Ge15Te85(4)) and a V-VI glass (As2Se3(7) ). However, the linear Tg - 8, behaviour displayed by the reported (2,3) data for a (Ni5P~)80P20 glass appears neither to have been referred to nor discussed previously. Strella and Erhardt (5) have shown theoretically that the machine error for most types of thermal analysis apparati will be proportional to 8 and experimental measurements of the error confirm this prediction (1,5). It is therefore likely that the observed (2,3) linear Tg-8 behaviour for (Ni5Pd5)80P20is an apparent effect brought about by systematic machine errors which masked the true non-linear Tg-8 behaviour. The apparent activation enthalpy

for relaxation at T

Moynihan et al (7,8) have demonstrated that AH*, the activation enthalpy for the relaxation time controlling the structural enthalpy relaxation at T g is given by: 2 AH* = 2. 303 R T (d log (8) / dT ) {i} g g provided the Tg temperatures are measured from heat capacity heating curves obtained by heating the glass at a rate 8, equal to the cooling rate u, through the transition region. Here Tg is some temperature located in the middle of the transition region and R is the gas constant. Equation {i} predicts that for_8 = ~, then Tg is proportional to log (8) and the gradient of the Tg - log (8) plot is =2. 303 R Tg 2 / AH , . In the case of the NiPdP glasses under consideration, 8 << e(typically 8 < 1.5K/s and e > 104 K/s ) so any observed linear Tg-log (8) behaviour will give, in conjumction with Equation {i}, an apparent activation enthalpy, AH a, for the relaxation at Tg. Experiments conducted using oxide glasses (7,8) have shown that for these materials, AH* =AH D the activation enthalpy for the shear viscosity at Tg. In the case of metallic glasses having B << u it is anticipated that AH a will be less than or equal to AH* and that AH a approaches AH* as S approaches e(4). This establishes a theoretical upper limit to AH a as follows :

Hence:

AH a ~

AH* = AH ~

AH a ~

AH ~

(for 8 < ~) {2}

The activation enthalpy for the viscous deformation of a (Ni5Pd5)80P20 glass has been estimated using the temperature dependence of the creep rate (3). It was found that AH~ = 5900 R T2/(T-392) 2 =460 kJ/mole over the range of the measured T~ temperatures for the (Ni5Pd5)80P20 glass (580-600K, see Figure i). The AH a value for the closely related (Ni5Pd5)82P18 glass is, from Figure 1 and Equation {i}, equal to 480 + 30 kJ/mole so the Equality {2} holds to within experimental error. It has therefore been shown, for the first time, that the relaxation time controlling the structural enthalpy relaxation of a metallic glass at Tg displays an activation enthalpy equal to the activation enthalpy for the shear viscosity. Acknowledgements The author is indebted to Professor G.A. Chadwick Research Grants Committee for financial support.

for guidance,

and to the Australian

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References i. 2. 3. 4. 5. 6. 7. 8.

H.S. Chen and D.A. Turnbull, J.Phys.Chem. 48, 2560 (1968). H.S. Chert, Appl.Phys.Lett. 29, 328 (1976). B.G. Lewis and H.A. Davies,Proc.Intl.Syn~.Struct.Non-Cryst.Materials (University of Cambridge, 1976). Taylor and Francis; in press. M. Lasocka, Mater.Sci.Engng. 23, 173 (1976). S. Strella and P.F. Erhardt, J.AppI.Poly.Sci. 1-3, 1373 (1969). S.J. Peppiatt and J.R. Sambles, Proc.Roy.Soc. A345, 387 (1975). C.T. Moynihan, A.J. Easteal and J. Wilder, J.Phys.Chem. 78, 2673 (1974). C.J. Moynihan, A.J. Easteal, M.A. DeBolt and J. Tucker, J.Am.Ceram.Soc. 59, 12 (1976).

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