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Calorimetric evidence for short range ordering in a metallic glass
Scripta METALLURGICA
Vol. 15, pp. I073-i076, 1981 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
CALORIMETRIC EVIDENCE FOR SHORT RANGE ORDERING IN A METALLIC GLASS.
University of Sussex,
M.G. Scott School of Engineering Brighton, England.
and Applied Sciences,
(Received May 19, 1981) Introduction A large number of metallic alloys m a y be produced in a glassy state by solidification at rates of the order of 10 6 Ks -I. These metallic glasses have attracted considerable scientific and industrial interest and now have potential applications in a number of areas (i). Like all glasses, these materials are metastable on two counts: with respect to the equilibrium crystalline state and with respect to glassy structures of lower free energy (2). This latter situation arises because the liquid to glass transformation during cooling is kinetically determined and the exact structural configuration of the glass therefore depends on the cooling rate during its formation. On subsequent heat treatment the glass can lower its free energy by relaxing structurally towards those configurations that would be produced (often hypothetically) at lower cooling rates. The changes in physical properties which accompany such relaxations in metallic glasses are often quite considerable and in many cases the optimum properties are attained only after suitable heat treatment of the as-quenched glass. The behaviour is well illustrated by some recent measurements of the elastic (Young's) modulus of various transition metal-metalloid glasses by the author and co-workers (3,4). The salient features are shown schematically in Fig i. (Identical behaviour is exhibited by many other properties including Curie temperature (5) and electrical resistivity (6).) If the as-quenched glass is annealed isothermally at a temperature T 1 the modulus (measured at room temperature) increases approximately linearly with the logarithm of time and ultimately stabilises at a value E(TI). If, however, the same glass is annealed at a lower t~nperature T2, the modulus now increases more slowly as expected but, more significantly, stabilises at a higher value E(T2). Thereafter it is possible to cycle the modulus rapidly and reversibly between E(T I) and E(T 2) simply by annealing successively at T 1 and T 2 (Fig l(b)). The above behaviour is identical to that displayed by a large number of s h o r ~ range ordering, crystalline alloys (7) and it is believed that similar ordering and disordering can occur in metallic glasses. Thus, immediately after quenching, the glass is highly disordered. During annealing at T 1 the short-range order increases until metastable equilibrium is achieved at T I. (This state is described as metastable because the equilibrium state is crystalline; it is equilibrium because the same state can be approached from both higher and lower temperatures.) If the temperature is now changed to T 2, the structure is no longer in equilibrium and the short range order increases further towards that in metastable equilibrium at T 2. Thereafter it is possible to cycle reversibly between the states of order characteristic of T 1 and T 2 or any other temperature in m u c h the same way as can be done for crystalline alloys. If this hypothesis is correct, it should be possible to detect the ordering and disordering calorimetrically. This note describes the associated enthalpy changes and shows that they are consistent with changes in short-range order.
1073 0036-9748/81/i01073-04502.00/0 Copyright (¢) 1981 Pergamon Press Ltd.
1074
SHORT RANGE ORDERING
IN M E T A L L I C GLASS
Vol.
15, No.
I0
Experimental The m e t a l l i c g l a s s u s e d in this i n v e s t i g a t i o n was F e 4 o N i 4 o B 2 0 , supplied by V a c u u m s c h m e l z e GmbH as V i t r o v a c 0040. D e t a i l e d m e a s u r e m e n t s of m o d u l u s and length changes in this g l a s s are d e s c r i b e d e l s e w h e r e (3,8). C a l o r i m e t r y was p e r f o r m e d in two instruments; a D u p o n t 1090 w i t h DSC cell and a P e r k i n - E l m e r DSC 2. To d e t e c t the changes in specific heat it was n e c e s s a r y to operate the c a l o r i m e t e r s at h i g h s e n s i t i v i t y (typically O.i mw/cm) and t h e r e f o r e c o n s i d e r able p r e c a u t i o n s w e r e r e q u i r e d to ensure good b a s e l i n e stability. Specimens of m a s s about 20 mg w e r e sealed in crimped a l u m i n i u m pans. To m i n i m i s e the transients at the b e g i n n i n g and end of thermal cycles it was n e c e s s a r y to use a reference of a p p r o x i m a t e l y the same thermal m a s s as the specimen. This was achieved by u s i n g an identical a l u m i n i u m pan filled w i t h either extra lids or w i t h approximately 20 mg of c r y s t a l l i s e d sample. .A new r e f e r e n c e was used for each n e w specimen so as to m i n i m i s e any errors due to r e l a x a t i o n of the aluminium pans themselves. Results and D i s c u s s i o n Fig 2 shows the r e l a x a t i o n spectrum obtained at a heating rate of 20 K/min. This s p e c t r u m was o b t a i n e d by heating the a s - q u e n c h e d g l a s s to 400°C, quenching to room t e m p e r a t u r e and i m m e d i a t e l y r e h e a t i n g to 4OO°C; the second run was then s u b t r a c t e d from the first to give the e n t h a l p y decrease. There was no d e t e c t able c h a n g e in specific heat between second, third and subsequent runs. To c o n f i r m that no c r y s t a l l i s a t i o n occured during the r e l a x a t i o n e x p e r i m e n t s the specimen was f i n a l l y heated at 20 K/min t h r o u g h the c r y s t a l l i z a t i o n exotherm. In no cases did either the peak position (450°C) or the a s s o c i a t e d heat r e l e a s e (5.3 kJ/mol) d i f f e r from that o b t a i n e d from a single run of an as-quenched sample. The form of the r e l a x a t i o n spectrum in Fig 2 is similar to that shown by Chen for P d - C u - S i glass. R e l a x a t i o n occurs over a wide t e m p e r a t u r e range and cannot be d e s c r i b e d by a single a c t i v a t i o n energy. The total e n t h a l p y of r e l a x a t i o n (1.25 kJ/mol) is a p p r o x i m a t e l y one quarter the heat of c r y s t a l l i s ation indicating that there is a c o n s i d e r a b l e increase in order d u r i n g relaxation. To show the effects of ordering and d i s o r d e r i n g d u r i n g thermal cycling of the glass, the following isothermal e x p e r i m e n t s w e r e performed. The a s - q u e n c h e d g l a s s was heated at the m a x i m u m rate (320 K/min in the Perkin Elmer DSC) to 38OOC and held isothermally. As expected, there was an e x o t h e r m i c heat output w h i c h fell to zero after a p p r o x i m a t e l y 15 mins. The t e m p e r a t u r e was then reduced to 330°C at the m a x i m u m rate and held; there was a further, smaller exotherm. The t e m p e r a t u r e was now increased again to 380°C; this time t h e r e was an endotherm. S u b s e q u e n t l y on cooling again to 330°C and reheating to 380°C, the e x o t h e r m and e n d o t h e r m r e s p e c t i v e l y could be r e p r o d u c e d exactly. No d e t e c t a b l e c r y s t a l l i s a t i o n occured during these experiments. To show that the effects d e s c r i b e d above d i d not arise from instrumental transients, the same e x p e r i m e n t s were c a r r i e d out using a l u m i n i u m specimens of thermal m a s s close to that of the g l a s s y specimens. Not only were the instrumental t r a n s i e n t s (dotted lines in Fig 3) insignificant, but they were in the o p p o s i t e sense to those shown by the m e t a l l i c glass. The t h e r m o g r a m s in Fig 3 are exactly those expected from short range ordering and d i s o r d e r i n g w i t h i n the glass. At 380°C the as-quenched g l a s s orders exot h e r m a l l y as the s t r u c t u r e cames into m e t a s t a b l e e q u i l i b r i u m at this temperature. W h e n the t e m p e r a t u r e is reduced to 330°C the d e g r e e of short-range order is increased, g i v i n g rise to a further exotherm. Conversely, increasing the t e m p e r a t u r e again, d e c r e a s e s the order, p r o d u c i n g an endotherm. Moreover, the d u r a t i o n of the t r a n s i e n t s is, w i t h i n e x p e r i m e n t a l limits, the same as the time taken for e q u i l i b r i u m of the Young's m o d u l u s at the same t e m p e r a t u r e s (4). W h a t these c a l o r i m e t r i c e x p e r i m e n t s do not reveal, however, is the nature of the changes in s h o r t - r a n g e order. In c r y s t a l l i n e alloys they are of two types: changes in the v a c a n c y c o n c e n t r a t i o n and changes in the chemical identity and d i s t r i b u t i o n of n e a r e s t n e i g h b o u r atom pairs. By analogy, in m e t a l l i c glasses it is likely that structural r e l a x a t i o n is a c c o m p a n i e d by changes in both the
Vol.
15, No. I0
SHORT RANGE ORDERING IN METALLIC GLASS
1075
nature and distribution of free volume (there are no atom-size vacancies) and the local atomic co-ordinations. Egami has proposed the terms topological short-range order (TSRO) and chemical short-range order (CSRO) to describe these two extreme situations (iO). The author has shown elsewhere that there are significant differences between the kinetics of initial relaxation of the asquenched glass and those of the subsequent ordering and disordering ( 4 ) . These differences arise because whereas the as-quenched glass contains a significant excess free volume, the subsequent ordering and disordering involves no detectable volume change and is believed to arise almost entirely from changes in chemical short range order. In this respect metallic glasses behave very similarly to crystalline short-range ordering alloys (7). Conclusions By differential scanning calorimetry it has been ordering phenomenon occurs in metallic glasses. On glass orders exothermally. Thereafter increases in ordering (endothermic) and decreases in temperature
shown that a short-range annealing, the as-quenched temperature lead to disto ordering (exothermic).
Ac knowl ed~ ement s This work was supported by the U.K. Science Research Council and the U.S. Office of Naval Research (contract no. N-OOOI4-78-G-OO39). I am grateful to Prof. R.W. Cahn for his advice on short-range ordering in crystalline alloys. References I. 2.
F.E. Luborsky and L.A. Johnson, Journal de Physique, 41 C8-820 (1980). R.W. Hopper and D.R. Uhlmann, Metallic Glasses, eds. J.J. Gilman and H.J. Leamy, p. 128, ASM (1978). 3. A. Kursumovi~', E. Girt, M.G. Scott and R.W. Cahn, Scripta Metall., 14 1303 (198o). 4. M.G. Scott and A. Kursumovzc, submitted for publication to Acta Metall. 5. C.D. Graham Jr. and T. Egami, Rapidly Quenched Metals III, ed. B. Cantor, Vol II, p.96. The Metals Society, London (1978). 6. M. Balanzat, Scripta Metall., 14. 173, (1980). 7. L. Trieb and and G.G. Veith, Aeta Metall., 26, 185 (1978). 8. A. Kursumovlc, M.G. Scott and R.W. Cahn, Scr-Tpta Metall., 14, 1245 (1980). 9. H.S. Chen and E. Coleman, Appl. Phys. Lett. 28, 245 1976). iO. T. Egami, Mat. Res. Bull., 13, 557 (1978).
FIG i. Schematic representation of the change in Young's modulus during, (a) isothermal annealing of the as-quenched metallic glass, (b) subsequent temperature cycling.
EC I
Io) log time
EIT ) E(T~)
72
I--
J time
1076
SHORT RANGE ORDERING
IN METALLIC
GLASS
Vol.
15, No.
1.0
g t--
0.5
0 100
300
200
&O0
T (°C) FIG 2. Thermal relaxation (Dupont 1090 DSC).
spectrum of F e 4 o N i 4 o B 2 0
+10
~+5
at 20 K/min.
--a.q380°C -..~30°C .... _380°C .........no
\
E
sample
%%%%
o
=
0 / / !
4--"
O ¢-
i
-5 !
i i
0
I
I
I
5
10
15
t (min) FIG 3. Thermal transients at 380°C and 330°C for the glass F e 4 o N i 4 o B 2 0 . The dotted line shows the instrumental transients at the same temperatures.
i0
Vol. 15, pp. I073-i076, 1981 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
CALORIMETRIC EVIDENCE FOR SHORT RANGE ORDERING IN A METALLIC GLASS.
University of Sussex,
M.G. Scott School of Engineering Brighton, England.
and Applied Sciences,
(Received May 19, 1981) Introduction A large number of metallic alloys m a y be produced in a glassy state by solidification at rates of the order of 10 6 Ks -I. These metallic glasses have attracted considerable scientific and industrial interest and now have potential applications in a number of areas (i). Like all glasses, these materials are metastable on two counts: with respect to the equilibrium crystalline state and with respect to glassy structures of lower free energy (2). This latter situation arises because the liquid to glass transformation during cooling is kinetically determined and the exact structural configuration of the glass therefore depends on the cooling rate during its formation. On subsequent heat treatment the glass can lower its free energy by relaxing structurally towards those configurations that would be produced (often hypothetically) at lower cooling rates. The changes in physical properties which accompany such relaxations in metallic glasses are often quite considerable and in many cases the optimum properties are attained only after suitable heat treatment of the as-quenched glass. The behaviour is well illustrated by some recent measurements of the elastic (Young's) modulus of various transition metal-metalloid glasses by the author and co-workers (3,4). The salient features are shown schematically in Fig i. (Identical behaviour is exhibited by many other properties including Curie temperature (5) and electrical resistivity (6).) If the as-quenched glass is annealed isothermally at a temperature T 1 the modulus (measured at room temperature) increases approximately linearly with the logarithm of time and ultimately stabilises at a value E(TI). If, however, the same glass is annealed at a lower t~nperature T2, the modulus now increases more slowly as expected but, more significantly, stabilises at a higher value E(T2). Thereafter it is possible to cycle the modulus rapidly and reversibly between E(T I) and E(T 2) simply by annealing successively at T 1 and T 2 (Fig l(b)). The above behaviour is identical to that displayed by a large number of s h o r ~ range ordering, crystalline alloys (7) and it is believed that similar ordering and disordering can occur in metallic glasses. Thus, immediately after quenching, the glass is highly disordered. During annealing at T 1 the short-range order increases until metastable equilibrium is achieved at T I. (This state is described as metastable because the equilibrium state is crystalline; it is equilibrium because the same state can be approached from both higher and lower temperatures.) If the temperature is now changed to T 2, the structure is no longer in equilibrium and the short range order increases further towards that in metastable equilibrium at T 2. Thereafter it is possible to cycle reversibly between the states of order characteristic of T 1 and T 2 or any other temperature in m u c h the same way as can be done for crystalline alloys. If this hypothesis is correct, it should be possible to detect the ordering and disordering calorimetrically. This note describes the associated enthalpy changes and shows that they are consistent with changes in short-range order.
1073 0036-9748/81/i01073-04502.00/0 Copyright (¢) 1981 Pergamon Press Ltd.
1074
SHORT RANGE ORDERING
IN M E T A L L I C GLASS
Vol.
15, No.
I0
Experimental The m e t a l l i c g l a s s u s e d in this i n v e s t i g a t i o n was F e 4 o N i 4 o B 2 0 , supplied by V a c u u m s c h m e l z e GmbH as V i t r o v a c 0040. D e t a i l e d m e a s u r e m e n t s of m o d u l u s and length changes in this g l a s s are d e s c r i b e d e l s e w h e r e (3,8). C a l o r i m e t r y was p e r f o r m e d in two instruments; a D u p o n t 1090 w i t h DSC cell and a P e r k i n - E l m e r DSC 2. To d e t e c t the changes in specific heat it was n e c e s s a r y to operate the c a l o r i m e t e r s at h i g h s e n s i t i v i t y (typically O.i mw/cm) and t h e r e f o r e c o n s i d e r able p r e c a u t i o n s w e r e r e q u i r e d to ensure good b a s e l i n e stability. Specimens of m a s s about 20 mg w e r e sealed in crimped a l u m i n i u m pans. To m i n i m i s e the transients at the b e g i n n i n g and end of thermal cycles it was n e c e s s a r y to use a reference of a p p r o x i m a t e l y the same thermal m a s s as the specimen. This was achieved by u s i n g an identical a l u m i n i u m pan filled w i t h either extra lids or w i t h approximately 20 mg of c r y s t a l l i s e d sample. .A new r e f e r e n c e was used for each n e w specimen so as to m i n i m i s e any errors due to r e l a x a t i o n of the aluminium pans themselves. Results and D i s c u s s i o n Fig 2 shows the r e l a x a t i o n spectrum obtained at a heating rate of 20 K/min. This s p e c t r u m was o b t a i n e d by heating the a s - q u e n c h e d g l a s s to 400°C, quenching to room t e m p e r a t u r e and i m m e d i a t e l y r e h e a t i n g to 4OO°C; the second run was then s u b t r a c t e d from the first to give the e n t h a l p y decrease. There was no d e t e c t able c h a n g e in specific heat between second, third and subsequent runs. To c o n f i r m that no c r y s t a l l i s a t i o n occured during the r e l a x a t i o n e x p e r i m e n t s the specimen was f i n a l l y heated at 20 K/min t h r o u g h the c r y s t a l l i z a t i o n exotherm. In no cases did either the peak position (450°C) or the a s s o c i a t e d heat r e l e a s e (5.3 kJ/mol) d i f f e r from that o b t a i n e d from a single run of an as-quenched sample. The form of the r e l a x a t i o n spectrum in Fig 2 is similar to that shown by Chen for P d - C u - S i glass. R e l a x a t i o n occurs over a wide t e m p e r a t u r e range and cannot be d e s c r i b e d by a single a c t i v a t i o n energy. The total e n t h a l p y of r e l a x a t i o n (1.25 kJ/mol) is a p p r o x i m a t e l y one quarter the heat of c r y s t a l l i s ation indicating that there is a c o n s i d e r a b l e increase in order d u r i n g relaxation. To show the effects of ordering and d i s o r d e r i n g d u r i n g thermal cycling of the glass, the following isothermal e x p e r i m e n t s w e r e performed. The a s - q u e n c h e d g l a s s was heated at the m a x i m u m rate (320 K/min in the Perkin Elmer DSC) to 38OOC and held isothermally. As expected, there was an e x o t h e r m i c heat output w h i c h fell to zero after a p p r o x i m a t e l y 15 mins. The t e m p e r a t u r e was then reduced to 330°C at the m a x i m u m rate and held; there was a further, smaller exotherm. The t e m p e r a t u r e was now increased again to 380°C; this time t h e r e was an endotherm. S u b s e q u e n t l y on cooling again to 330°C and reheating to 380°C, the e x o t h e r m and e n d o t h e r m r e s p e c t i v e l y could be r e p r o d u c e d exactly. No d e t e c t a b l e c r y s t a l l i s a t i o n occured during these experiments. To show that the effects d e s c r i b e d above d i d not arise from instrumental transients, the same e x p e r i m e n t s were c a r r i e d out using a l u m i n i u m specimens of thermal m a s s close to that of the g l a s s y specimens. Not only were the instrumental t r a n s i e n t s (dotted lines in Fig 3) insignificant, but they were in the o p p o s i t e sense to those shown by the m e t a l l i c glass. The t h e r m o g r a m s in Fig 3 are exactly those expected from short range ordering and d i s o r d e r i n g w i t h i n the glass. At 380°C the as-quenched g l a s s orders exot h e r m a l l y as the s t r u c t u r e cames into m e t a s t a b l e e q u i l i b r i u m at this temperature. W h e n the t e m p e r a t u r e is reduced to 330°C the d e g r e e of short-range order is increased, g i v i n g rise to a further exotherm. Conversely, increasing the t e m p e r a t u r e again, d e c r e a s e s the order, p r o d u c i n g an endotherm. Moreover, the d u r a t i o n of the t r a n s i e n t s is, w i t h i n e x p e r i m e n t a l limits, the same as the time taken for e q u i l i b r i u m of the Young's m o d u l u s at the same t e m p e r a t u r e s (4). W h a t these c a l o r i m e t r i c e x p e r i m e n t s do not reveal, however, is the nature of the changes in s h o r t - r a n g e order. In c r y s t a l l i n e alloys they are of two types: changes in the v a c a n c y c o n c e n t r a t i o n and changes in the chemical identity and d i s t r i b u t i o n of n e a r e s t n e i g h b o u r atom pairs. By analogy, in m e t a l l i c glasses it is likely that structural r e l a x a t i o n is a c c o m p a n i e d by changes in both the
Vol.
15, No. I0
SHORT RANGE ORDERING IN METALLIC GLASS
1075
nature and distribution of free volume (there are no atom-size vacancies) and the local atomic co-ordinations. Egami has proposed the terms topological short-range order (TSRO) and chemical short-range order (CSRO) to describe these two extreme situations (iO). The author has shown elsewhere that there are significant differences between the kinetics of initial relaxation of the asquenched glass and those of the subsequent ordering and disordering ( 4 ) . These differences arise because whereas the as-quenched glass contains a significant excess free volume, the subsequent ordering and disordering involves no detectable volume change and is believed to arise almost entirely from changes in chemical short range order. In this respect metallic glasses behave very similarly to crystalline short-range ordering alloys (7). Conclusions By differential scanning calorimetry it has been ordering phenomenon occurs in metallic glasses. On glass orders exothermally. Thereafter increases in ordering (endothermic) and decreases in temperature
shown that a short-range annealing, the as-quenched temperature lead to disto ordering (exothermic).
Ac knowl ed~ ement s This work was supported by the U.K. Science Research Council and the U.S. Office of Naval Research (contract no. N-OOOI4-78-G-OO39). I am grateful to Prof. R.W. Cahn for his advice on short-range ordering in crystalline alloys. References I. 2.
F.E. Luborsky and L.A. Johnson, Journal de Physique, 41 C8-820 (1980). R.W. Hopper and D.R. Uhlmann, Metallic Glasses, eds. J.J. Gilman and H.J. Leamy, p. 128, ASM (1978). 3. A. Kursumovi~', E. Girt, M.G. Scott and R.W. Cahn, Scripta Metall., 14 1303 (198o). 4. M.G. Scott and A. Kursumovzc, submitted for publication to Acta Metall. 5. C.D. Graham Jr. and T. Egami, Rapidly Quenched Metals III, ed. B. Cantor, Vol II, p.96. The Metals Society, London (1978). 6. M. Balanzat, Scripta Metall., 14. 173, (1980). 7. L. Trieb and and G.G. Veith, Aeta Metall., 26, 185 (1978). 8. A. Kursumovlc, M.G. Scott and R.W. Cahn, Scr-Tpta Metall., 14, 1245 (1980). 9. H.S. Chen and E. Coleman, Appl. Phys. Lett. 28, 245 1976). iO. T. Egami, Mat. Res. Bull., 13, 557 (1978).
FIG i. Schematic representation of the change in Young's modulus during, (a) isothermal annealing of the as-quenched metallic glass, (b) subsequent temperature cycling.
EC I
Io) log time
EIT ) E(T~)
72
I--
J time
1076
SHORT RANGE ORDERING
IN METALLIC
GLASS
Vol.
15, No.
1.0
g t--
0.5
0 100
300
200
&O0
T (°C) FIG 2. Thermal relaxation (Dupont 1090 DSC).
spectrum of F e 4 o N i 4 o B 2 0
+10
~+5
at 20 K/min.
--a.q380°C -..~30°C .... _380°C .........no
\
E
sample
%%%%
o
=
0 / / !
4--"
O ¢-
i
-5 !
i i
0
I
I
I
5
10
15
t (min) FIG 3. Thermal transients at 380°C and 330°C for the glass F e 4 o N i 4 o B 2 0 . The dotted line shows the instrumental transients at the same temperatures.
i0