ANELASTICITY AND STRUCTURAL RELAXATION OF THE METALLIC GLASS Fe40 Ni40 B20

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ANELASTICITY AND STRUCTURAL RELAXATION

OF THE METALLIC GLASS Fe40 Ni40 B20

R. Kamel, N. Gobran, S. Saleh, B. Hanna

To cite this version:

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JOURNAL DE PHYSIQUE

CoZZoque CS, suppl&ment au nO1O, Tome 4 2 , octobre 1981 page CS-505

A N E L A S T I C I T Y AND STRUCTURAL RELAXATION OF THE METALLIC GLASS F e 4 0 Ni40 B 2 0

R. Kamel, N.K. Gobran, S. Saleh and B.F. Hanna

Physics Department, Faculty o f Science, University of Cairo, G z a , Egypt

Abstract.- Internal friction, Q", and magnetic energy loss, t a n 6

,

were used to trace atomic rearrangements in metallic a s s Fe Nigo Bgo during the course of its transformation to the crystglline p ase. An internal friction peak showed itself in the relaxation spectrum before the onset of crystallization, associated with a drop in the magnetic energy loss. From an is thermal study of the time decay at different temperatures of Q-? and t a n 8

,

and using the cross-cut method, the atomic rearrangements effected by thermal lzygting were found to be activated by an energy of 0.97 X 10 J. The concept of a quantum void called vacy capable of migration and producing local order in the amorphous structure is here presented and identified.

1. Introduction.- Early changes in physical. properties were found (l) to take place during annealing at temperatures far below the crystal- lization temperature. These changes are attributed to certain atomic rearrangements that are not necessarily associated with incipient

It is the aim of the present paper to throw some light o n the kinetics of the atomic mechanism involved in the struct- ural relaxation taking place prior to the amorphous-crystalline tran- sition in the metallic glass Feg0 NikO B20.

2. Experimental and results.- The amorphous material in the form of short ribbons were electrostatically excited to lateral vibrations which were traced by a micro-vibration detector circuit previously

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described. The internal friction was measured at different tempera- tures in vacuo from the width of the resonance curve.

Internal friction showed a rather small monotonic increase with temperature on which a small relaxation peak around 400

K

was found. Approaching the recrystallization temperature, the Q-' increased sharply reaching a maximum value at about 640

K,

followed by a pron- ounced decrease, see fig,(l-a). The Q-' peaks did not show up in the cooling run indicating the irreversibility of the process in action. I n order to test the relaxation origin of the first Q-' peak, the amorphous samples were given isothermal heat pulses at the prescribed

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C5-506 JO'JRNAL DE PHYSIQUE

Fig. l : Temperature dependence of internal friction, Q", and magnetic energy loss, t a n

,

of Fe40Ni40B20 while heating u p and cooling down. M 2 0 1 , 0015. b C

.-

.F

cro10

-

Temperature T (K) o Heating Cooling 3

=

66 Hz

-

'

f

t a )

temperatures 353,

373

and 403 K for different periods of time after which the internal friction was determined.

A

decrease in Q-' was observed with annealing time, fig.(2-a), and was accentuated by incr- easing the annealing temperature, The cross-cut method was used to determine the equivalent times, t, and temperatures, T, characteriz- ing the relaxation process concerned, Based on a first order chemi- cal rate equation describing this relaxation process, equivalent times and temperatures giving a certain degree of reaction might be represented by;

t exp (

-E /

k T )

=

constant

.

A n activation energy o f 0.95 x 10'19 J was determined from the average slopes o f the straight lines presently obtained relating (In t ) versus (l/T), figure ( 2 - 8 ) .

T h e monotonic exponential increase with temperature o f the back- ground Q-'

,

fig.( l-a), due to thermally activated internal friction was found t o be activated by an energy of

0.58

x 1 0 - l ~ J.

T h e oscilloscope method was used t o investigate the magnetic hysteresis loop o f the sample. Samples were placed as the core of a magnetizing coil with a n induction coil wound directly around it.The area of the hysteresis loop was taken as a measure of the magnetic energy dissipation, t a n

b

,

The sample was heated while i n its expe- rimental test position from room temperature t o about 750

K,

and the magnetic energy loss was continuously observed during the heating u p

/'*

Y

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nme of anreailng t (h) 1 0 0 O l T

t k ,

Fig, 2 : Decay o f nor- malized Q-' and tan with annealing time at different constant temperatures.

T h e straight lines re- present equivalent times and temperatures obtained from the cross cuts shown by dashed lines.

and subsequent cooling down runs.

T h e area of the hysteresis loop showed a starting decrease at relatively l o w temperatures followed by a temperature independent stage, Approaching the recrystallization temperature, t a n

6 ,

exper- ienced a sharp drop i n the loop area at about 6 0 0 K

,

figure ( 1 -b). Increasing further the temperature, the magnetic energy loss increa- sed excessively showing a peak value near

640

K.

After passing the recrystallization temperature, the loop area diminished progressively t o nearly a zero value, and the magnetic hysteresis loop remained vanishingly small while the sample was subsequently cooled down. Figure (l-b) represents a typxcal result with the arrows representxng the direction of the heating run.

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JOURNAL DE PHYSIQUE

3.

Discussion.- T h e observed early drop in magnetic energy loss and the associated internal friction peak of F e

40

Nipo B20 showing up . t t

nearly the same low range o f annealing temperatures are thoughi i o

be due to atomic rearrangements involving the motion of some kind of structural defect existing i n the amorphous phase($). The average activation energy (

0.97

x 1 0 " ~ J ) obtained rrom isothermal tests uslng mechanical and magnetic investigation tools i s thought t o char- acterize the migration of a small unit o f free volume causing atomic tilting, thus enhancing topological short range order (TSRO), which does not discriminate between metal and metalloid atoms

( 5 ) .

~ ~ c ~ e n ~ s ~ , o ~ , a t o ~ i ~ 4 $ & & ~ n g S a ~ ~ - ' f S , R 0 , :

In crystalline solids, the free volume associated with the rise o f temperature shows u p as vaca- nt lattice sites in thermal equilibrium. The concentration o f these sites increases until the solid struture melts and the concept of a lattice vacancy becomes irrelevant. Analogous to the case o f liquids, the free volume existing i n amorphous solids h a s n o definite shape i m p r e g n a t ~ n g the random close-packed atomic structure(6). I n order t o interpret the annealing behaviour of a n amorphous metallic compou- nd, one should take into consideration the fate o f this free volume until it ultimately transforms t o a number of discrete vacancies i n the metallic structure after crystallization

.

Atomic tilting lea- ding t o TSRO might thus be considered as effected by some sort of mig- ration and

/

or elimination of a fraction of this free volume result- ing in the minimization of internal free energy.

Accordingly, we suggest the possible existence o f a unit o r quantum of void(lf) denoted by a vacy '1, capable o f migrating and

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causing local atomic tilting,

A

vacy might be thought of as a small- sized void dissociated in two dimensions, I n the usual undisturbed state, its motion is anisotropic and restricted to the plane

in

which it lies. However, it might move, spiraling in a snaky way, normal to its plane when an atom is made t o penetrate through its center. The migration of a vacy by one atomic diameter produces a tilting of the

atom by a distance equivalent to the vacy's thickness,

A

schematic

representatxon of the void migration and the associated atomic tilt

is shown in figure (3). The presently observed activation energy,

0.97 x 10-l9 J, activating topological short range order could thus be assigned to the energy activating vacy migration in the amorphous structure.

(l) H . S . Ghen, H.J. Leamy and M. Barmatz ; J. Non-cryst, Solids,

2

444

(1970).

( 2 ) T. Egami ; Mat. Res. Bull.

,

S ,

557

11978).

( 3 )

Y.L. Yousef,

E.A.

El-Salam,

M.M.

Saadeldin and S.M. Khalil ;

J. Phys, (E),

2 ,

829 (1970).

( 4 )

J . Frenkel ; Kinetic Theory of Liquidsm p0174 (Dover Publica

tions) New York

(1955).

( 5 )

T.

Egami ; Mat, Sci, Eng,,

32,

293 (1978).