COMPETITION BETWEEN AMORPHOUS OR INTERMETALLIC PHASE FORMATION DURING BALL-MILLING

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COMPETITION BETWEEN AMORPHOUS OR INTERMETALLIC PHASE FORMATION DURING

BALL-MILLING

M. Morris, D. Morris

To cite this version:

M. Morris, D. Morris. COMPETITION BETWEEN AMORPHOUS OR INTERMETALLIC PHASE

FORMATION DURING BALL-MILLING. Journal de Physique Colloques, 1990, 51 (C4), pp.C4-211-

C4-217. �10.1051/jphyscol:1990426�. �jpa-00230786�

COLLOQJE DE PHYSIQUE

Colloque C4, supplBment au n014, Tome 51, 15 juillet 1990

COMPETITION BETWEEN AMORPHOUS OR INTERMETALLIC PHASE FORMATION DURING BALL-MILLING

M.A. MORRIS and D.G. MORRIS

Institute of Structural Metallurgy, University of Neuchstel, Avenue de Bellevaux 51, CH-2000 Neuchdtel, Switzerland

Abstract

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I n the present work a number o f a l l o y s based on C r and Nb w i t h S i and B have been prepared by b a l l - m i l l i n g and t h e phases obtained which may be e i t h e r disordered solutions, i n t e r m e t a l l i c compounds or amorphous phases, examined. The r e l a t i o n s h i p between t h e complexity o f t h e c r y s t a l l o g r a p h i c s t r u c t u r e o f t h e compounds and t h e phases obtained has been discussed.

1

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INTRODUCTION

The usual c r i t e r i a f o r s o l i d s t a t e amorphization (1) assumes a l a r g e n e g a t i v e heat o f m i x i n g o f the elements and a much f a s t e r d i f f u s i v i t y o f one element w i t h respect t o t h e other. I n t h i s way t h e amorphization process i s considered t o be favoured by a d r i v i n g f o r c e t o mix t h e elements w i t h suppression o f the i n t e r m e t a l l i c f o r m a t i o n due t o lack o f d i f f u s i o n o f one o f t h e elements which prevents n u c l e a t i o n . Much work has been done on many a l l o y systems ( 2 ) t o produce amorphous phases from elemental powders w i t h some s t u d i e s s t a r t i n g from i n t e r m e t a l l i c compounds. Not much more has been done s t a r t i n g from elemental powders t o produce amorphous phases through i n t e r m e d i a t e compound formation.

Recent work on a Cr-Nb system ( 3 ) has shown t h a t d e s t a b i l i z a t i o n o f compounds i n t o t h e o r i g i n a l elements can occur by b a l l - m i l l i n g .

I n order t o e x p l o r e t h e e f f e c t o f compound d e s t a b i l i s a t i o n by s t r u c t u r a l d i s o r d e r caused by m i l l i n g which e v e n t u a l l y leads t o amorphous phase formation, a number o f a l l o y systems have been prepared by b a l l - m i l l i n g and t h e m i c r o s t r u c t u r e s o f t h e powders obtained have been s t u d i e d and compared.

2

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EXPERIMENTAL TECHNIQUES

The d i f f e r e n t systems chosen f o r t h i s comparative study were CrB, CrB2, CrnNb, CrSi2, NbB, NbB2 and NbSiZ. The corresponding weighed amounts o f t h e elemental powders f o r each composition were mechanically a l l o y e d i n a p l a n e t a r y b a l l - m i l l P u l v e r i s e t t e 7 w i t h a weight r a t i o b a l l s t o powder o f f o u r . The c o n t a i n e r s were loaded under an argon atmosphere and o n l y opened a t t h e end o f each r u n when t h e m i l l i n g p e r i o d was completed.

For each set o f powders the m i l l i n g process was c a r r i e d o u t f o r d i f f e r e n t periods o f time ranging between 5 t o 80 hours. Complete m i c r o s t r u c t u r a l a n a l y s i s o f a l l t h e powders was performed by X-ray d i f f r a c t i o n , SEM and TEM observations together w i t h EDS chemical analysis.

3

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RESULTS AND DISCUSSION

The X-ray spectra obtained from t h e m i l l e d powders are shown i n F i g s 1 and 2 f o r t h e Borides, S i l i c i d e s and Cr2Nb r e s p e c t i v e l y . The i n t e r e s t i n g f e a t u r e s are t h e e a r l y f o r m a t i o n of t h e Borides i n t h e case o f NbB and NbB2 w h i l s t i n t h e CrB and CrBp system t h e Borides do not form w i t h o n l y the Cr peaks being observed. Since t h e Boron ,peaks o f Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990426

C4-212 COLLOQUE DE PHYSIQUE

t h e elemental pre-mi l l e d powders were w e l l i d e n t i f i e d as t h e rhodbohedral phase, t h e i r disappearance from t h e m i l l e d powders suggest t h a t t h i s element has amorphised and o n l y c o n t r i b u t e s t o t h e much h i g h e r background observed. The decrease i n peak h e i g h t and broadening o f these peaks w i t h i n c r e a s i n g m i l l i n g t i m e i s c h a r a c t e r i s t i c o f a decrease i n c r y s t a l l i t e s i z e ( 2 ) . Also w i t h i n c r e a s i n g m i l l i n g t i m e t h e increase i n background observed c l o s e t o t h e main peaks provides evidence o f amorphisation. T h i s e f f e c t i s most pronounced a f t e r 60 hours m i l l i n g i n t h e CrB2 and NbB systems. I n t h e case o f t h e S i l i c i d e s , t h e NbSi2 system shows t h e formation o f t h e compound a f t e r o n l y 5 h o f m i l l i n g and i t s presence up t o 40 h m i l l i n g time, w h i l s t a f t e r 60 h m i l l i n g t h e quasi-disappearance o f t h e main peaks suggests t h e presence o f an amorphous phase. The CrSi2 system, however, behaves i n a r a t h e r d i f f e r e n t way: t h e format i o n o f the S i l i c i d e occurs much more s l o w l y and i s o n l y completed a f t e r 40 hours. Only a decrease i n peak h e i g h t and broadening was observed w i t h f u r t h e r m i l l i n g up t o 60 hours.

The Cr2Nb system i s a more p a r t i c u l a r case as observed from t h e appearance o f an i n t e r m e t a l l i c phase a f t e r 15 hours m i l l i n g , i t s disappearance and reappearance o f t h e Cr and Nb peaks a f t e r 25 hours m i l l i n g and the presence of an amorphous phase a f t e r 60 hours m i l l i n g . (see F i g 2).

SEM observations o f t h e powders by atomic number c o n t r a s t shows t h a t i n t h e case o f t h e Boride systems, t h e r e i s n o t a layered s t r u c t u r e present, i n s t e a d m i x i n g o f t h e elements appears t o occur l o c a l l y from t h e o u t s i d e o f t h e p a r t i c l e s w i t h t h e Boron p a r t i c l e s becoming smaller and b e t t e r d i s t r i b u t e d as a f u n c t i o n o f time (see F i g 4). This process i s continuous i n t h e case o f t h e CrB and CrBz systems where t h e compounds d i d n o t form, behaving as though s o l i d s o l u t i o n s o f t h e two elements were f i n a l l y obtained.

A t the same t i m e t h e powders appear more f r a g i l e and become smaller as they f r a c t u r e thoroughly a f t e r 60 h and 80 h m i l l i n g time. I n t h e case o f t h e NbB and NbB2 'systems, as soon as t h e Boride peaks are detected, t h e powder p a r t i c l e s appear very smalf and t o t a l l y mixed ( n e i t h e r Boron nor Nb c o n t r a s t i s seen) even though t h e same Boron d i s t r i b u t i o n was observed i n t h e e a r l y stages o f m i x i n g (see F i g 4). The s i z e o f t h e powders reaching 50-100 nm a f t e r 60 h and 80 h m i l l i n g i n d i c a t e d t h e e x t e n t o f f r a c t u r e o c c u r r i n g i n t h e f r a g i l e Borides present. The S i l i c i d e powders present much t h e same aspect as t h e Boride powders described, b u t t h e Cr2Nb system has a d i f f e r e n t l a y e r e d s t r u c t u r e w i t h i n t h e powders (see F i g 3 ) . As already described elsewhere (3) chemical a n a l y s i s o f t h e l a y e r s shows t h a t t h e g l o b a l composition necessary f o r i n t e r m e t a l l i c formation i s reached a f t e r 25 hours m i l l i n g even though the presence o f the elemental C r and Nb peaks are observed.

The same homogeneity o f g l o b a l composition i s observed a f t e r 60 h m i l l i n g even though t h e d i f f e r e n t l a y e r s present s l i g h t l y higher concentrations o f Chromium and Niobium (see F i g 5 ) . At t h i s stage t h e presence o f an amorphous phase i s detected by X-ray d i f f r a c t i o n , suggesting once more t h a t t h e amorphous phase forms from a m i x t u r e o f s o l i d s o l u t i o n s o f t h e two elements.

The presence o f amorphous phases i n these powders was f u r t h e r i n v e s t i g a t e d by TEM observations made d i r e c t l y on t h e powders ( t h e i r small sizes, 50-200 nm, made t h i s p o s s i b l e w i t h o u t any t h i n n i n g being necessary). As seen i n F i g s 3 and 5, a l l t h e powders i n v e s t i g a t e d show t h e presence o f some c r y s t a l l i n e p a r t i c l e s embedded i n an amorphous m a t r i x a f t e r 60 hours o f m i l l i n g . This i s i n complete agreement w i t h t h e X-ray s p e c t r a observed. Amorphization i s not complete unless a completely rounded background i s observed, t h e presence o f any c r y s t a l 1 i t e s w i l l a l l o w some sharpness t o be observed i n t h e peaks o f the spectrum. The powders t h a t had a smaller number o f c r y s t a l l i t e s were those from t h e CrB2, NbSi2 and Cr2Nb systems i n agreement w i t h t h e most rounded X-ray s p e c t r a observed a f t e r 60 hours m i l l i n g . Also measurements o f c r y s t a l l i t e sizes made from t h e w i d t h a t h a l f maximum o f t h e peaks u s i n g t h e Scherrer f o r m u l a show t h a t both t h e CrB2

CrB

M A

Cr

n

^{40 HOURS}

60 HOURS

20 HOURS

5 HOURS

-.-W ~ ~

60 HOURS

S

60 HOURS

100

F i g 1. X-ray spectra from MA powders a f t e r d i f f e r e n t m i l l i n g periods. Note t h e formation o f NbB and NbSi2 compounds and t h e lack o f formation o f CrB and CrB2.

C4-214 COLLOQUE DE PHYSIQUE

Cr2Nb

M A

CDS CPS

400

-

^{5 HOURS 400}

^-

^Cr 25 HOURS

300

-

200

-

Cr,Nb

400

- ¹

^{15 HOURS} _CPS

400 300

-

Cr 60 HOURS

200

-

^Nb

l00

-

J

100

9r

³⁴

F i g 2. X-ray s p e c t r a from m i l l e d elemental powders o f composition Cr2Nb.

F i g 3. a) and b) NbSi2 Ma powders. c ) and d) Cr2Nb powders a f t e r m i l l i n g f o r 60 hours i n a l l cases. Note t h e layered s t r u c t u r e s i n t h e Cr2Nb system. Amorphous m a t r i x w i t h some c r y s t a l l i t e s are observed i n both types o f powders by TEM.

F i g 4. Observed m i l l e d powders by atomic number c o n t r a s t i n t h e

SEM.

Note t h e Boron d i s t r i b u t i o n i n a)and b ) f o r t h e CrB system and t h e much smaller powder s i z e a f t e r NbB2 formation occurs.

F i g 5. TEM observations o f a) CrB2 and b) NbB powders a f t e r 60 hours m i l l i n g . Note t h e d i f f u s e r i n g s c h a r a c t e r i s t i c of t h e amorphous m a t r i x and t h e c r y s t a l l i t e s observed i n Dark F i e l d .

C4-216 COLLOQUE DE PHYSIQUE

and NbSi2 systems have t h e smallest values at 3 nm, t h i s being c o n s i s t e n t w i t h a more complete amorphization process observed.

D e t a i l e d comparison o f t h e amorphization paths f o l l o w e d by these systems shows t h a t i n some cases t h e formation o f a more s t a b l e compound occurs f i r s t (as i n t h e case o f NbB, NbB2 and NbSiz w i t h t h e h i g h e s t n e g a t i v e enthalpy o f compound f o r m a t i o n between -250 and -140 KJ/mol) f u r t h e r m i l l i n g r e s u l t i n g i n t h e formation o f an amorphous phase c o n t a i n i n g c r y s t a l l i t e s o f the remaining compound. However i n t h e case o f t h e CrB, CrB2 and Cr2Nb systems t h e amorphization process occurs d i r e c t l y from t h e m i x i n g o f t h e elements w i t h no intermediate compound being present a t t h e t i m e o f amorphization. Whichever t h e path f o l lowed, t h e amorphizat i o n process seems t o be a continuous one w i t h i n c r e a s i n g volume f r a c t i o n o f amorphous phase as a f u n c t i o n o f m i l l i n g time.

Another p o i n t t o discuss i s t h e general aspect o f t h e powders. Only t h e Cr2Nb powders present t h e layered s t r u c t u r e c h a r a c t e r i s t i c o f c o l d welding caused by g l o b a l p l a s t i c i t y . I n a l l other cases m i x i n g o f the elements occurred a f t e r f r a c t u r i n g , w i t h l o c a l d i T f u s i o n from t h e o u t s i d e towards t h e i n s i d e o f t h e powders b u t no l o c a l p l a s t i c i t y producing welding. For t h i s reason i n these most f r a g i l e powders, t h e energy produced by t h e impact w i t h the b a l l s w i l l not be s t o r e d i n t h e powders b u t instead w i l l be spent i n the c r e a t i o n o f the new f r a c t u r e surfaces. This i s most e v i d e n t i n t h e case where t h e e a r l y f o r m a t i n g o f t h e very f r a g i l e Borides or S i l i c i d e s produce m i l l e d powders which are t e n times smaller than those i n t h e Cr2Nb system. Also i n t h e case o f t h e systems where compound formation occurs, t h e chemical composition o f t h e powders has reached t h e g l o b a l value, t h e r e f o r e any f u r t h e r m i l l i n g should o n l y r e s u l t i n l o c a l chemical d i s o r d e r which w i l l d e s t a b i l i z e t h e compound. I n t h i s way t h e amorphization process by b a l l - m i l l i n g can be seen as a continuous increase i n t h e volume f r a c t i o n o f zones w i t h l o c a l d i s o r d e r o f t h e c r y s t a l l o g r a p h i c s t r u c t u r e s . With i n c r e a s i n g m i l l i n g time t h e complex c r y s t a l l o g r a p h i c s t r u c t u r e s w i l l f i n a l l y disappear g i v i n g way t o t h e amorphous phase. We can say t h a t t h i s d e s t r u c t i o n o f t h e chemical order corresponds t o t h e increase i n enthalpy and entropy o f t h e powders by the impact w i t h t h e b a l l s . This increase i n enthalpy and entropy w i l l be h i g h e s t i n t h e systems where more p l a s t i c i t y and welding o f t h e powders i s observed (i.e.

i n t h e case o f t h e l a y e r e d s t r u c t u r e s ) t h e r e f o r e t h e chemical i n s t a b i l i t y w i l l be more pronounced. The f a c t t h a t no compound formation occurs i n t h e case o f t h e CrB and CrB2 systems w i t h heats o f formation o f -75 and -94 kJ/mol r e s p e c t i v e l y w h i l s t t h e d i r e c t format i o n o f t h e amorphous phase from t h e elements takes p1 ace, suggests t h a t chemical d i s o r d e r o c c u r r i n g from t h e e a r l y stages o f m i l l i n g prevents compound formation i n t h e case of c e r t a i n complex s t r u c t u r e s . This means t h a t n u c l e a t i o n o f t h e compound i s not p o s s i b l e because e i t h e r t h e c r i t i c a l a c t i v a t i o n energy necessary f o r n u c l e a t i o n i s t o o h i g h or t h e c r i t i c a l nucleus s i z e i s never reached. This i s i l l u s t r a t e d by t h e f a c t t h a t t h e compound CrSi2 forms a f t e r m i l l i n g times between 10 t o 40 hours even though i t s enthalpy o f formation, AHf = -8OKJlmo1, i s intermediate t o those o f CrB and &B2.

I t could be p o s s i b l e t o a t t r i b u t e t h e d i f f e r e n t n u c l e a t i o n a b i l i t y t o t h e d i f f e r e n t d i f f u s i v i t y o f Boron compared t o t h a t o f S i l i c o n ( s i n c e a l l t h r e e are^ l i n e compounds). On t h e other hand such an argument cannot be h e l d r e s p o n s i b l e f o r t h e slower n u c l e a t i o n o f NbB2 which i s not a l i n e compound (15 hours) compared t o NbB ( 5 hours) which i s a l i n e compound and has a less negative enthalpy o f formation. Also t h e case o f Cr2Nb where t h e complex cubic Laves phase cannot form by m i l l i n g even though t h e range o f compositions a t which t h i s compound should form i s reached ( 3 ) suggests t h a t it i s n o t t h e d i f f e r e n t d i f f u s i v i t y o f Chromium and Niobium which prevents n u c l e a t i o n o f the compound b u t r a t h e r t h e complexity o f t h e s t r u c t u r e . C r y s t a l s t r u c t u r e plays an important r o l e i n t h e process o f n u c l e a t i o n since it i s r e l a t e d t o t h e i n t e r f a c e energy o f t h e nucleus and t h e r e f o r e t h e c r i t i c a l energy f o r compound n u c l e a t i o n . I n t h e case o f more complex s t r u c t u r e s t h i s i n t e r f a c e energy w i l l increase more q u i c k l y w i t h increased s t r u c t u r a l d i s o r d e r o f t h e

m a t r i x due t o m i l l i n g . I n some cases t h i s increased i n t e r f a c e energy can be compensated b y t h e i n c r e a s e i n volume energy produced by t h e excess o f d e f e c t s . As an example we observe t h e n u c l e a t i o n o f NbB2 w i t h a hexagonal s t r u c t u r e (C32) and AHf = -250 KJ/mol a f t e r 15 hours m i l l i n g , b u t i n t h e CrB2 case w i t h t h e same c r y s t a l s t r u c t u r e b u t AHf = -94 KJ/mol, t h e compound never forms. I n a s i m i l a r way- we can e x p l a i n t h e f a s t e r f o r m a t i o n of NbB than NbB2 ( o f s i m i l a r hHf) on t h e assumption t h a t t h e i n t e r f a c e energy o f t h e NbB s t r u c t u r e i s lower.

Since s u f f i c i e n t l y l o n g m i l l i n g times f i n a l l y lead t o t h e f o r m a t i o n o f amorphous phases t h i s i n d i c a t e s t h a t t h e f r e e energy o f t h e system can be lowered i n t h i s way d u r i n g m i l l i n g whichever p a t h i s f o l l o w e d . I f we c o n s i d e r t h e s m a l l e r c r y s t a l l i t e s i z e s measured f r o m t h e X-ray spectra, t h e systems NbSi2 and CrB2 are c o n s i s t e n t w i t h a more complete amorphization process. Only t h e more o r l e s s d i f f u s e r i n g s and t h e n a n o c r y s t a l s observed w i t h i n t h e powders by TEM g i v e a f u l l u n d e r s t a n d i n g o f t h e e x t e n t t o which an amorphous phase can c o e x i s t w i t h e i t h e r t h e compound o r t h e s o l i d s o l u t i o n o f t h e elements r e s p e c t i v e l y . T h i s i n d i c a t e s t h a t t h e a m o r p h i z a t i o n process does n o t occur u n i f o r m l y t h r o u g h o u t t h e sample but r a t h e r must depend on t h e l o c a l s t r u c t u r a l d i s o r d e r c r e a t e d as t h e impact w i t h t h e b a l l i n c r e a s e s t h e energy l o c a l l y .

I f t h e amorphous phases form i n o r d e r t o reduce t h e f r e e energy o f t h e systems i t must be a d m i t t e d t h a t t h e h i g h energy s t a t e s corresponding t o a d i s o r d e r e d s o l i d s o l u t i o n o f t h e elements ( i n some cases) o r t o a d i s o r d e r e d d e s t a b i l i z e d compound ( i n o t h e r s ) must be e q u i v a l e n t . Indeed t h e c r y s t a l l i t e s i z e s observed b y TEM i n t h e powders NbSi2 o r CrBz are s i m i l a r , i n d i c a t i n g t h a t t h e amorphous phase i s formed a f t e r b r e a k i n g down t h e c r y s t a l l i t e s i z e t o t h e same e x t e n t i n t h e compound or s o l i d s o l u t i o n cases, r e s p e c t i v e l y .

4

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CONCLUSIONS

Amorphous phases are formed i n a l l t h e systems s t u d i e d which compete w i t h n a n o c r y s t a l l i n e s t r u c t u r e s o f e i t h e r t h e i n t e r m e d i a t e compound o r t h e s o l i d s o l u t i o n o f t h e element. I t seems t h a t i n these cases, t h e major aspect h i n d e r i n g compound f o r m a t i o n i s r e l a t e d t o t h e c o m p l e x i t y o f t h e c r y s t a l s t r u c t u r e s and n o t t o t h e d i f f e r e n t d i f f u s i v i t y o f t h e two elements.

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