STRUCTURAL ASPECTS OF REVERSIBLE TRANSFORMATIONS IN Co-Nb-B AMORPHOUS ALLOYS

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STRUCTURAL ASPECTS OF REVERSIBLE TRANSFORMATIONS IN Co-Nb-B AMORPHOUS

ALLOYS

E. Girt, R. O’Handley, A. Mitra Ghemawat, N. Grant, P. Tomic, K. Novalija

To cite this version:

E. Girt, R. O’Handley, A. Mitra Ghemawat, N. Grant, P. Tomic, et al.. STRUCTURAL ASPECTS OF

REVERSIBLE TRANSFORMATIONS IN Co-Nb-B AMORPHOUS ALLOYS. Journal de Physique

Colloques, 1985, 46 (C8), pp.C8-551-C8-554. �10.1051/jphyscol:1985887�. �jpa-00225240�

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JOURNAL DE PHYSIQUE

Colloque C8, supplément au n°12, Tome 46, décembre 1985 page C8-551

STRUCTURAL ASPECTS OF REVERSIBLE TRANSFORMATIONS IN Co-Nb-B AMORPHOUS ALLOYS

E. Girt*, R.C. O'Handley, A. Mitra Ghemawat, N.J. Grant, P. Tomic and K. Novalija

Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.

+Institute of Physics PMF, 71000 Sarajevo, V. Putnika 43, Yougoslavia

Résumé - Des mesures de dilatométrie sur des rubans amorphes de Co80

Nb14 B6 sont présentées. Un modèle est proposé pour expliquer la transformation structurale du premier ordre observée.

Abstract - Results of dilatometric measurements on amorphous CO8 0Nb1 4B6 ribbons are presented. A model is proposed for the first-order structural transformation observed in this system.

Measurements were made on samples prepared to have different initial local strain configurations induced by tensile stress treatment.

I. INTRODUCTION

Experimental evidence points to the existence of a reversible structural trans- formation between two distinct atomic short-range orders, both of them non-crystalline, in several different Co-Nb-B glasses (1). The transformation has been observed in the range of 323 K to 423 K as a subtle, reversible change in mag- netic anisotropy. (The crystallization temperature of the glasses exceeds 673 K ) . In order to investigate the non-magnetic aspects of this transformation, we have prepared Co80Nb14B6 glasses with different initial local configurations and characterized their thermal expansion and isothermal creep behavior (with a sensi- tive dilatometer). The results of these measurements consistently suggest interest- ing structural changes in this Co80Nb14B6 glass in the temperature range over which the magnetic consequences of the transformation have been observed. First, we show that the closed dilatometric loop obtained on thermal expansion of the system close- ly follows the magnetic transformation loop observed over the same temperature range. Secondly, we found a similarity between the isothermal creep behavior of this system and that reported for the non-metalloid system Cu60Zr40 (2).

Spaepen (3) and Taub et al (4) have described the phenomenology and detailed mechanisms of homogeneous flow in metallic glasses. Girt et al (2) have studied me- tallic glasses stress-treated under homogeneous-flow conditions. They have drawn interesting conclusions about the average atomic structure in amorphous Cu60Zr40

using isothermal dilatometrie techniques (T = 473 K ) . Different applied stresses can lead to anelastic strain or can possibly alter the free volume and thus give a set of samples that show different degrees of low-temperature relaxation as described by Gibbs et al (5). By measuring the relaxation during a creep experi- ment, the viscosity can be determined. A set of viscosities are obtained which characterize the variously prepared states.

* Permanent address, Prirodno-Matematicki Fakultet, 71000 Sarajevo, V, Putnika 43, Yougoslavia.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985887

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C8-552 JOURNAL DE PHYSIQUE

11. EXPERIMENTATION AND RESULTS

Co Nb B r i b b o n s of approximately 30 pm t h i c k n e s s and 1.8 mm width were melt-sp%

A%

6a s i n g l e wheel melt-spinning device. Five p i e c e s of t h i s g l a s s (each 60 cm long) were s t r e s s - t r e a t e d simultaneously a t a c o n s t a n t temperature of 373 K f o r 24 hours. The s t r e s s e s used were 46 m a , 315 m a , 473 MPa, 585 m a and 672 MPa corresponding t o weights of 1.03 N, 7.09 N. 10.64 N. 13.16 N and 15.11 N, a t t a c h e d a t t h e end of d i f f e r e n t samples. The weights were chosen according t o t h e r e l a t i o n - ship:

W = A l n ( n ) + B

where W i s t h e weight, A, B a r e c o n s t a n t s and n i s a n i n t e g e r ( 1 , . . , 5 ) . Values of A and B were o b t a i n e d by s e t t i n g t h e s m a l l e s t weight a t 1.03 N and t h e l a r g e s t weight a t 15.11 N ( t h e maximum allowable value b e f o r e r e a c h i n g y i e l d p o i n t ) .

D i l a t o m e t r i c measurements were made under isothermal (T = 473 K) c o n d i t i o n s and a l s o under c o n d i t i o n s w i t h temperature c y c l i n g w e r t h e range f o r which t h e magnetic t r a n s f o r m a t i o n was observed (323

-

473 K). Thermal expansion AL(T311 and i s o hermal

8

flow A i ( t ) / t were measured w i t h s e n s i t i v i t i e s of approximately 1 0 and 10- , re- s p e c t i v e l y , using a p r e c i s i o n d i l a t o m e t e r having a temperature s t a b i l i t y of approximately f 0.3'~.

- 71 I I I I I I

0 300 600 900 1200 1500 I800 2100

T I M E ( s e c s )

Fig. 1 R e l a t i v e change i n l e n g t h v e r s u s time under isothermal (T = 473 I) c o n d i t i o n s f o r samples t r e a t e d w i t h f i v e d i f f e r e n t s t r e s s e s : a ) 46 MPa

,

b) 315 MPa

,

c ) 473 MPa

,

d ) 585 MPa

,

and e ) 672 m a .

Fig. 1 shows a s e t of curves o b t a i n e d from isothermal d i l a t o m e t r i c measurements o n t h e s e s t r e s s - h e a t - t r e a t e d ribbons. The experimental procedures and a n a l y s e s have been explained e a r l i e r ( 2 ) . F i r s t , we can s e e a s i m i l a r i t y between t h e s e r e s u l t s and those o b t a i n e d on i n v e s t i g a t i n g Cu Zr ( 2 ) . However, t h e r e s u l t s d i f f e r i n c h a r a c t e r from s i m i l a r measurements on the6°mef81-metalloid g l a s s , Feg,,B20. We i n t e r p r e t t h i s t o imply t h a t t h e flow mechanisms of t h e mixed Co-Nb-B g l a s s resemble more those of Cu-Zr g l a s s t h a n t h o s e of Pe-B g l a s s e s . Perhaps t h e former systems a r e more m e t a l l i c i n t h e i r bonding and have a l e s s e r degree of d i r e c t e d p-like bonds.

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The i n i t i a l c o n t r a c t i o n (At/#,

<

0) observed p r i o r t o c r e e p i n a l l but one of t h e samples i n Fig. 1 i s a m a n i f e s t a t i o n of t h e recovery of a n e l a s t i c s t r a i n o r i s due t o t h e t h e r m a l l y induced a n n i h i l a t i o n of f r e e volume. The amount of c o n t r a c t i o n d e c r e a s e s t o z e r o f o r t h e t h i r d sample (473 MPa) and t h e n i n c r e a s e s again. A f t e r t h i s i n i t i a l r e l a x a t i o n s t a g e , normal creep behavior (A0/Q

>

0) i s observed. The c o e f f i c i e n t s of v i s c o s i t y were c a l c u l a t e d f o r t h e r i b b o n s s t r e s s - t r e a t e d w i t h 473 MPa. 585 MPa and 672 MPa, f o r which we can s e p a r a t e t h e p r o c e s s of flow from t h e i n i t i a l c o n t r a c t i o n of t h e sample. Those v a l u e s of t h e v i s c o s i t y a r e given i n Table I below.

Table I

V i s c o s i t y a t 2 0 0 ~ ~ f o r amorphous CoSONb14B6 t r e a t e d w i t h v a r i o u s t e n s i l e s t r e s s e s .

T e n s i l e S t r e s s V ' c o s i t y

( m a ) 1

lo-''

n o i s e )

The absence of an i n i t i a l s t a g e c o n t r a c t i o n i n t h e 473 MPa sample could imply t h a t a f t e r i t s p r e p a r a t i o n e i t h e r i ) t h e r e was no a n e l a s t i c s t r a i n o r t h e r e was no f r e e volume t o be r e l a x e d o r i i ) t h a t t h e sample had r e l a x e d on b r i n g i n g i t t o 2 0 0 ' ~ f o r isothermal study (a p r o c e s s t h a t t a k e s l e s s t h a n 1 0 sec.) (Ref. 2 ) . I n o r d e r t o determine which of t h e s e g o s s i b i l i t i e s dominates, we examined t h e thermal expan- s i o n of t h e samples up t o 200 C. It must be noted t h a t t h e s e thermal expansion measurements examine t h e s t r e s s - p r e p a r e d samples over a time-temperature regime t o t h a t shown i n Fig. 1. Three c h a r a c t e r i s t i c behaviors a r e shown i n Fig. 2 (a. b. c ) . (We do not include r e s u l t s f o r samples s t r e s s e d w i t h 46 MPa and 672 MPa because t h e i r behavior i s v e r y s i m i l a r t o those of samples s t r e s s e d w i t h 315 MPa and 585 MPa). I n t h e s e measurements, t h e thermal expansion of samples s t r e s s e d w i t h 473 MPa i s d r a s t i c a l l y d i f f e r e n t from t h a t of t h e o t h e r samples.

F i g u r e s 2 a and c show what we would c a l l a more normal thermal expansion behavior. I n t h e s e cases, w h i l e t h e expansion i s n o t l i n e a r ( p o s s i b l y due t o a gradu- a l s t r u c t u r a l r e l a x a t i o n over t h e temperature range s t u d i e d ) , t h e behavior i s monotonic on h e a t i n g . However, i n Fig. 2b (473 m a ) , t h e s t r u c t u r a l r e l a x a t i o n

seems t o occur i n two f a i r l y well-defined temperature ranges, around 8 0 ' ~ and 1 5 0 ~ ~ . I n f a c t , t h i s i n c r e a s e of flow a c t u a l l y e l o n g a t e s t h i s p a r t i c u l a r sample much more t h a n t h e o t h e r t r e a t e d samples. The thermal expansion behavior of t h e 473 MPa sam- p l e seems t o support t h e a l t e r n a t i v e t h a t , p r i o r t o t h e isothermal study, apprecia- b l e r e l a x a t i o n a s w e l l a s enhanced flow occured. Thus we cannot d i s t i n g u i s h between t h e two a l t e r n a t i v e s f o r t h e absence of c o n t r a c t i o n i n t h e 473 MPa sample.

I n t h i s work, we i n v e s t i g a t e d t h e amorphous Co Nb B system using t h e methods of p r e c i s i o n isothermal and normal d i l a t o m e t r y . Wh%e

ibtg

normal v i s c o u s flow(AL/e

) 0) and s t r u c t u r a l r e l a x a t i o n (A! /J!

<

0 ) have been observed i n i s o t h e r m a l d i l a t o m e t r y , samples p r e - t r e a t e d w i t h a s p e c i f i c s t r e s s show no i n i t i a l c o n t r a c t i o n . Samples p r e - t r e a t e d t o have l a r g e f r e e volumes o r a n e l a s t i c s t r a i n ( a s d i r e c t e d by i n i t i a l c o n t r a c t i o n i n i s o t h e r m a l c r e e p s t u d i e s ) show h y s t e r e s i s i n t h e i r thermal expansion t h a t i s r e m i n i s c e n t of thermo-magnetic i n d i c a t i o n s of a s t r u c t u r a l transformation i n t h e s e g l a s s y a l l o y s .

We wish t o thank M. McHenry f o r h e l p f u l d i s c u s s i o n s and Z. ^Majs t o v i c f o r t e c h n i c a l a s s i s t a n c e . Prof. G i r t g r a t e f u l l y acknowledges t h e support of a F u l l b r i g h t Fellowship. The work a t MIT was supported by t h e U. S. O f f i c e of Naval Re search.

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C8-554 JOURNAL DE PHYSIQUE

.

I I I I I

0 0 4 0 8 0 120 160 2 0 0 2 4 0

T E M P E R A T U R E ( ' C ) 5

4

H

*

-?

,

OT

a

I

0

Fig, 2 R e l a t i v e change i n l e n g t h v e r s u s temperature f o r samples t r e a t e d w i t h : a ) 315 MPa b ) 473 MPa and c ) 585 MPa.

I I I I -7

-1

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0 3 -

. *

.

2 - *

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4 0 8 0 120 160 200 24

1 ) B. W. Corb, R. C. O'Bandley, J. Megnsar, and N. J. Grant, Phys. Rev.

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