Enhancement of structural relaxation in amorphous PdSi following stress reduction

Scripta

METALLURGICA

V o l . 13, pp. 8 8 3 - 8 8 6 , 1979 Printed in t h e U . S . A .

Pergamon Press Ltd. All rights reserved.

ENHANCEMENT OF STRUCTURAL RELAXATION IN AMORPHOUS PdSi FOLLOWING STRESS REDUCTION

A.I. Taub* and F. Spaepen Division of Applied Sciences Harvard University, Cambridge, MA 02138

(Received

June

21,

1979)

Introduction Several investigators have examined the effect of thermal annealing on the creep properties of metallic glasses (i). Most of these tests were conducted at constant stress (2). The few tests involving stress changes were used only to investigate stress-strain rate relations or anelastic behavior (3,4,5). In this study, the effect of stress changes on annealing behavior was examined. Results Using the creep apparatus described previously (6), the isothermal flow rate of several Pd82Si18 wires was measured as a function of time. Figure 1 shows the data for three as-quenched specimens undergoing a 227 ° C anneal. The data is expressed in terms of the "apparent" viscosity n a = 1/3 ~/c, where £ is the total strain rate including the anelastic transients. Curves A and B represent constant stress tests. Curve C is for a specimen that underwent a stress doubling halfway into the anneal. The lower apparent viscosity observed at the beginning of the higher stress anneals, reflects only the difference in the anelastic contribution to the flow rate. That is, the higher the stress, the greater the anelastic flow and therefore the lower the "apparent" viscosity. When the anelastic transients decay (approximately 1 - 2 days), the curves merge. It can be seen then, that neither the magnitude of the stress, nor an increase in the load during the anneal, affect the relaxation behavior. On the other hand, structural annealing has been found to depend strongly on load removal, as shown in Figure 2a. The data obtained after the decay of the anelastic transients, show that while load increases have no visible effect on the relaxation kinetics, load removals result in an anomalous relaxation. The contributiQn of anelastic recovery to the structural relaxation saturated at a viscosity of 4.2 x 1016 Ns/m L. Subsequent load cycles over an even larger stress range (6 - 251 MPa compared with 67 - 190 MPa), did not increase the viscosity noticeably (Figure 2b). As a verification that the anomalous viscosity increases are not associated with the instantaneous elastic perturbations created by abruptly removing loads, the load was removed and immediately reapplied with no observable effect on annealing behavior.

*I.B.M. Predoctoral Fellow, 1978-79.

883 0036-9748/79/090883-04502.00/0 Copyright (c) 1 9 7 9 P e r g a m o n Press

Ltd.

884

STRUCTURAL RELAXATION

IN AMORPHOUS PdSi

Vol. 13, No. 9

3xi0 I!

---A

(%1

E

,//~/

3xio"

z

INCREASE

0 &-

I 3xi014 0

I 50

I 100

I 150

I 200

TI M E (hours) FIG. 1 227 ° C annealing behavior for three as-quenched specimens under different stresses. Curve C is for a specimen that underwent a load change midway into the anneal. Discussion The enhancement of relaxation following stress reduction seems to be associated with the recovery of the anelastic strain. The degree of anomalous relaxation should then depend on the amount of anelastic recovery. In our tests, all the anelastic strain is recovered and the effect on the viscosity is dramatic. In a similar set of experiments, Murata et al. (5) permitted only a small fraction of the anelastic strain to return, by reapplying the load after only a few minutes of recovery. They observed no anomalies in flow behavior, as expected. The enhancement of relaxation can be explained qualitatively by employing the free volume model for flow ( 7 ) . Spaepen (8) has suggested that there should exist certain, as yet undefined, "defects" or local atomic configurations which, when activated, will permit structural relaxations that reduce the free volume and therefore increase the viscosity of the glass. By analogy with crystalline materials, in which a supersaturation of vacancies can be expelled by coalescence of the vacancies into discs which then collapse, these relaxation sites may be a local collection of free volume in a critical configuration, that can also collapse elastically, transmitting the volume to the surface. It is conceivable that some sites which produce anelastic flow, can act as this type of relaxation site. When activated, a restoring force is established as an elastic stress field around the site. T h i s may result in a localized redistribution of the free volume. Normally, when the load is removed, and the anelastic strain recovered, the free volume distribution will return to its unstressed state. However, at some anelastic sites, the region may elastically collapse about the free volume, increasing local ordering, before redistribution can occur. This would result in a transfer of free volume to the surface and an increase in the overall viscosity. The later loading cycles (Figure 2b) spanned a larger stress range but had no effect on the annealing behavior. This saturation may be due either to the inability of sites on the low and high sides of the anelastic activation energy spectrum (9) to act as relaxation sites, or to the decrease in overall mobility at the high viscosity which inhibits localized free volume redistributions.

Vol.

13, No. 9

STRUCTURAL RELAXATION

IN AMORPHOUS PdSi

885

3x1011

rRESS IPe) 67

129

67

67

190

| I |

\

I I %

1

E

\

\

%.. . . . . . . .

.),

FIG.

\\

~n 3xlO Z

/

// / /

/

I

3xll

100

0

200

2a

I

[

I

I

300

400

500

575

TIME (hours)

lmpo)

r

kl

I I I I

3x101TITRESS

I

129 ', 67

% %

I 190

12..9 )

%

'~ 6

251

251

%

I

f

Ii11 /I

3=1o;' ,"

s ....

iii~

/ I

FIG. 2b

I

3x1015[ 575

I

I

]

I

I

I

I

675

77~

875

975

1075

1175

1275

TI ME (hours) FIG.

2

227 ° C a n n e a l i n g behavior under stress changes. Dotted lines are actual data. Heavy line indicates the course of the "true" viscosity, i.e., with the estimated anelastic contribution to the flow excluded. (a) Initial loading cycles showing enhanced relaxation on load removal. (b) S u b s e q u e n t loading cycles with no observed change in viscosity.

886

STRUCTURAL

RELAXATION

IN A M O R P H O U S

PdSi

Vol.

13,

No.

Acknowledgement We want to thank Professor D. Turnbull for many helpful suggestions, and M.F. Ashby and R.B. Stephens for useful discussions. This work has been supported by the Office of Naval Research under Contract No. N00014-77-C-0002. References (i) (2) (3) (4) (5) (6) (7) (8) (9)

F. Spaepen and D. Turnbull, "Metallic Glasses", American Society of Metals, Cleveland, Ohio (1978), p. 114. J.C. Gibeling and W.D. Nix, Scripta Met. 12, 919 (1978). J. Logan and M.F. Ashby, Acta Mat. 22, 1047 (1974). R. Maddin and T. Masumoto, Mat. Sci. Eng. 9, 153 (1972). T. Murata, H. Kimura and T. Masumoto, 8cripta Mat. iO, 705 (1976). A.I. Taub and F. Spaepen, Scripta Met. 13, 195 (1979). F. Spaepen, Acta Met. 25, 407 (1977). F. Spaepen, J. Non-Cryst. Solids 31, 207 (1978). A.S. Argon and H.Y. Kuo, submitted to J. Non-Cryst. Solids.

9