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Thermal behaviour of CuTi and CuTiH amorphous powders prepared by ball milling
1398
Materials Science and Engineering, A 134 ( 1991 ) 1398-1401
Thermal behaviour of Cu-Ti and Cu-Ti-H amorphous powders prepared by ball milling M. Baricco lstituto Elettrotecnico Nazionale Gafileo Ferraris and 1NFM/GNSM, Research Unity of Torino, Corso Mass#no D'Azeglio, 42, Torino (Italy)
L. Battezzati Dipartimento di Chimica lnorganica, Chimica Fisica e Chimica dei Materali, Universit6 di Torino, Via P. Giuria, 9, Torino (Italy)
I. Soletta and L. Schiffini Dipartimento di Chimica, Universitd di Sassari, Via Vienna, Sassari (Italy)
N. Cowlam Department of Physics, University of Sheffield, SheffieM $3 7RH (U.K.)
Abstract Solid state amorphization reactions in Cu-Ti have been studied by means of DSC and structural techniques. The influence of hydrogen from the parent titanium powder on the amorphization and crystallization processes has been investigated. For Cu-Ti a diffusion-controlled process can be inferred for solid state amorphization from the parabolic trend of the heat of crystallization, as a function of the milling time. The presence of hydrogen in the alloys is found to modify the crystallization behaviour of the amorphous phase. A DSC method for the determination of the amount of hydrogen present in the alloys is given.
1. Introduction Solid state amorphization in Cu-Ti has been studied both from an experimental and a theoretical point of view [1-3]. To verify the possibility of amorphization on thermodynamic grounds, Battezzati et al. [3] have reported the calculated free energy for liquid and solid solutions. A glassforming region is predicted, ranging from 50 at.% Cu to 70 at.% Cu, but slight modifications of the free energy models for competing phases can substantially modify the prediction. In fact, Cocco et al. [1] and Politis et al. [2] found wider amorphizing ranges. A further problem, not yet extensively studied, occurs when using titanium for mechanical alloying because many batches of commercial titanium powders are partially loaded with hydrogen, which has been incorporated either during purification or embrittlement. Contaminated titanium was then used on purpose to study amorphization 0921-5093/91/$3.50
in the presence of hydrogen. It is worth recalling that hydrogen absorption caused one of the first solid state amorphization reactions ever observed in Zr3RhH [4], whereas a few systematic studies have been reported on mechanical alloying of hydrogenated materials [5, 6].
2. Experimental Elemental powders of copper and titanium have been milled in a Spex miller for various times (from 2 h to 60 h) and at two different temperatures (room temperature and - 3 0 °C). both hydrogen-free and hydrogen-contaminated titanium powders have been used. The formation of an amorphous phase and successive structural transformations on heating have been followed by X-ray [1] and neutron diffraction [6]. The thermal behaviour of the amorphous powder has been investigated by DSC analysis. Both a power © ElsevierSequoia/Printed in The Netherlands
1399
compensation (Perkin-Elmer DSC7) and a heat flux (Du Pont 1090) calorimeters have been used, at heating rates ranging from 0.083 K s 1 to 0.5 K S-l "
3. Results and discussion 3.1. C u - Ti system
Amorphization was readily achieved after 16 h of milling at room temperature in the 50 at.%~
metallic develops and is well recognized by X-ray diffraction at 16 h of milling. This phase disappears on further milling since the grinding mechanism of amorphization becomes operative (32 h). Then, new crystalline phases develop [1J. The onset temperature of crystallization given in Fig. 2 agrees with that of AH~ just discussed. For Ti6~Cu40 and CusoTis~, T~ is generally lower than that of ribbons of the same composition, i.e. 397 °C and 422 °C, respectively. This is expected for many reasons. It is well known that crystallization of an amorphous phase in the presence of crystal seeds may be strongly anticipated. Also, a catalytic effect for crystallization at the surface of highly fractured powders may be relevant. 3.2. Ol-(7~-H),system
As detailed below, the parent titanium powder contained 10 at.% H. The actual composition of the alloys differs from the previous ones by a maximum of 5 at.%, and the Ti/Cu ratio is decreased to a maximum of 0.5%. Within these limits, amorphization takes place in a similar composition range, 30 at.% ~< Cu~< 75 at.%, as for hydrogen-free materials in the same amount of time. For the sake of comparison, the hydrogenated alloys will be designated with the copper and titanium concentrations of the corresponding hydrogen-free samples. After 16 h of milling the alloys appear amorphous to X-ray and neutron diffraction except for a fraction of crystalline phase, identified as Till 2 [6]. The thermal behaviour of hydrogenated Cu60Ti40 amorphous powders is shown, as an example, in Fig. 3, as obtained from power compensation (curve (a)) and heat flux (curve (b))
1[
7°I
390
r
" 370i-
35O
42 '&
x
•
iI
[3
56
/
~x
',
330
o
m ca
~
m
/
o
',..
i
310
290 h
0
5
1tO
~-
15
30 Milling
time
'
60
(h)
Fig. 1. Heat of crystallization as a function of milling time. [], Cu5.Tis~ and A, Ti60Cu4o. Full triangles: sample p r e p a r e d at -
B
30
°C.
270 ~ 0
A
I 5
J
I 10
~5 1
..... MHling
i L 30 ":Jr'n~' ( h )
60
Fig. 2. Crystallization t e m p e r a t u r e as a function of milling time. rL Cus.Tis~ ~ and A Ti~.~Cua. Full Iriangles: sample prepared at - 30 °C.
1400
300 a
10C
~
.
eXO T L t
300
500
Temperature ( ~ )
Fi& 3. DSC traces for Cu~oTi4o. Curve {a): heat flux calorimeter; curve (b): power compensation calorimeter.
calorimeters. Curve (a) shows two thermal effects: a crystallization exotherm and the endothermal decomposition of T i l l 2 at higher temperature. For most of the other compositions the crystallization peak is masked by hydrogen evolution. To put in evidence the occurrence of crystallization, alloy samples must be crimped in sealable aluminium pans, so that the increase in pressure on heating the pan delays hydrogen evolution. In the heat flux calorimeter trace (Fig. 3, curve (b)) the crystallization peak is located in the same temperature range and appears similar. On the other hand, the endothermal effect is strongly depressed. The endotherm is large when using the power compensation instrument, where the sample and reference cells are separated, whereas the sample and the reference experience the same atmosphere in the heat flow calorimeter. The DSC output dp/dt may be written as [8] dp dt
dAH dt
~)ACp+AAAT
0
70
(1)
where the first term is the enthalpy change for the reaction under consideration, # is the heating rate, ACpthe specific heat difference between the final and initial states, AA is the difference in heat transfer coefficient (which depends on thermal conductivity) between sample and reference cell, and A T the temperature difference between the reference thermocouple and the heating element. The third term in eqn. (1) plays a different role in the two calorimeters because of the very high thermal conductivity of hydrogen. The same effect has been described for hydrogen evolution from Si-Ge thin films [9].
12 .
I
I
t
I
.4
'
.6
8t ,
, 1.0
XTi
Fig. 4. Apparent heat of decomposition of TiN e after crystallization of C u - T i - H amorphous alloys as a function of titanium content.
6
~4 I
o
o
o o
f .2
,
I .4
[3
I
16 .
,
Xrl
1.0
Fig. 5. Heat of crystallization as a function of titanium content. &, amorphous ribbons; zx, amorphous Cu-Ti powder; a, amorphous C u - T i - H powder.
The heat corresponding to the endothermal signal obtained for a series of C u - T i - H alloys is shown in Fig. 4 as a function of titanium content. The amount of hydrogen in the parent titanium used for alloying is determined as (10_+2)at.%H. The calibration of the calorimeter was performed by decomposing suitable amounts of pure Till 2. With the method described, a hydrogen content as low as 1 at.% could be detected in alloys prepared from other titanium batches. The A H x of hydrogenated and hydrogen free amorphous alloy powders is reported in Fig. 5, together with that of amorphous ribbons [3]. In the central composition region the values of hydrogenated alloys are definitely lower than those of hydrogen-free materials. This would imply the occurrence of different crystallization products in the two sets of samples. X-ray diffraction has been performed on samples as prepared and heated up to 435°C
1401
hydrogenated amorphous phase does not lead to hydrogenated intermetallics, but to the more stable Till 2 compound.
Acknowledgments G. Cocco (University of Sassari) and S. Enzo (University of Venezia) are gratefully acknowledged for useful discussions. Progetto Finalizzato CNR "Materiali speciali per tecnologie avanzate" is acknowledged for financial support under contracts 89.00623.68 and 89.00605.68. b
References 2@
2@
Fig. 6. X-ray diffractograms of Cu60Ti40 amorphous alloys: (a) hydrogenated; (b) hydrogen free. Curves (c) and (d) refer to samples (a) and (b), respectively, after heating up to the completion of crystallization. Insert: neutron diffraction patterns of samples (a) and (b). Symbols: o, TizCu; o, Cu4Ti3; +, Cu4Ti; I1, CuTi; v , Cu3Ti; 0 , Till 2.
(Fig. 6). Neutron diffraction of amorphous powder is also shown in the insert of Fig. 6, where the hydrogen contamination is well evidenced by the slope of the background [6]. As expected, different crystallization products are found in hydrogenated and hydrogen-free amorphous Cu60Tia0:TiH 2 and Cu3Ti in the former and CunTi, 6 - C u T i , C u a T i 3 and cubic CuTi 2 in the latter. Cu3Ti , here stabilized by the presence of TiH2, was already found in splat-cooled foils as a metastable phase [10]. The crystallization of the
1 G. Cocco, I. Soletta, S. Enzo, M. Magini and N. Cowlam, J. de Phys., C4-51 (1990) 181. 2 C. Politis and W. L. Johnson, J. Appl. Phys., 60 (1986) 1147. 3 L. Battezzati, M. Baricco, G. Riontino and I. Soletta, J. de Phys., C4-51 (1990)79. 4 W. L. Johnson, Prog. Mater. Sci., 30 (1986) 81. 5 C. Suryanarayana, R. Sunderasan and F. H. Froes, Int. Symp. on Solid State Powder Processing, Indianapolis, October 1989. 6 P. K. Ivison, I. Soletta, G. Cocco, S. Enzo, L. Battezzati and N. Cowlam, Mater. Sci. Eng., A128 (1991) this volume. 7 E. Hellstern and L. Schultz, J. Appl. Phys., 63 (1988) 1408. 8 J. Sestak, in Comprehensive Analytical Chemistry, Vol. XII-D, Elsevier, Amsterdam, 1984. 9 L. Battezzati, F. Demichelis, C. F. Pirri and E. Tresso, J. Appl. Phys., in press. 10 B. C. Giessen and D. 8zymanski, J. AppL Cryst., 4 (1971) 257.
Materials Science and Engineering, A 134 ( 1991 ) 1398-1401
Thermal behaviour of Cu-Ti and Cu-Ti-H amorphous powders prepared by ball milling M. Baricco lstituto Elettrotecnico Nazionale Gafileo Ferraris and 1NFM/GNSM, Research Unity of Torino, Corso Mass#no D'Azeglio, 42, Torino (Italy)
L. Battezzati Dipartimento di Chimica lnorganica, Chimica Fisica e Chimica dei Materali, Universit6 di Torino, Via P. Giuria, 9, Torino (Italy)
I. Soletta and L. Schiffini Dipartimento di Chimica, Universitd di Sassari, Via Vienna, Sassari (Italy)
N. Cowlam Department of Physics, University of Sheffield, SheffieM $3 7RH (U.K.)
Abstract Solid state amorphization reactions in Cu-Ti have been studied by means of DSC and structural techniques. The influence of hydrogen from the parent titanium powder on the amorphization and crystallization processes has been investigated. For Cu-Ti a diffusion-controlled process can be inferred for solid state amorphization from the parabolic trend of the heat of crystallization, as a function of the milling time. The presence of hydrogen in the alloys is found to modify the crystallization behaviour of the amorphous phase. A DSC method for the determination of the amount of hydrogen present in the alloys is given.
1. Introduction Solid state amorphization in Cu-Ti has been studied both from an experimental and a theoretical point of view [1-3]. To verify the possibility of amorphization on thermodynamic grounds, Battezzati et al. [3] have reported the calculated free energy for liquid and solid solutions. A glassforming region is predicted, ranging from 50 at.% Cu to 70 at.% Cu, but slight modifications of the free energy models for competing phases can substantially modify the prediction. In fact, Cocco et al. [1] and Politis et al. [2] found wider amorphizing ranges. A further problem, not yet extensively studied, occurs when using titanium for mechanical alloying because many batches of commercial titanium powders are partially loaded with hydrogen, which has been incorporated either during purification or embrittlement. Contaminated titanium was then used on purpose to study amorphization 0921-5093/91/$3.50
in the presence of hydrogen. It is worth recalling that hydrogen absorption caused one of the first solid state amorphization reactions ever observed in Zr3RhH [4], whereas a few systematic studies have been reported on mechanical alloying of hydrogenated materials [5, 6].
2. Experimental Elemental powders of copper and titanium have been milled in a Spex miller for various times (from 2 h to 60 h) and at two different temperatures (room temperature and - 3 0 °C). both hydrogen-free and hydrogen-contaminated titanium powders have been used. The formation of an amorphous phase and successive structural transformations on heating have been followed by X-ray [1] and neutron diffraction [6]. The thermal behaviour of the amorphous powder has been investigated by DSC analysis. Both a power © ElsevierSequoia/Printed in The Netherlands
1399
compensation (Perkin-Elmer DSC7) and a heat flux (Du Pont 1090) calorimeters have been used, at heating rates ranging from 0.083 K s 1 to 0.5 K S-l "
3. Results and discussion 3.1. C u - Ti system
Amorphization was readily achieved after 16 h of milling at room temperature in the 50 at.%~
metallic develops and is well recognized by X-ray diffraction at 16 h of milling. This phase disappears on further milling since the grinding mechanism of amorphization becomes operative (32 h). Then, new crystalline phases develop [1J. The onset temperature of crystallization given in Fig. 2 agrees with that of AH~ just discussed. For Ti6~Cu40 and CusoTis~, T~ is generally lower than that of ribbons of the same composition, i.e. 397 °C and 422 °C, respectively. This is expected for many reasons. It is well known that crystallization of an amorphous phase in the presence of crystal seeds may be strongly anticipated. Also, a catalytic effect for crystallization at the surface of highly fractured powders may be relevant. 3.2. Ol-(7~-H),system
As detailed below, the parent titanium powder contained 10 at.% H. The actual composition of the alloys differs from the previous ones by a maximum of 5 at.%, and the Ti/Cu ratio is decreased to a maximum of 0.5%. Within these limits, amorphization takes place in a similar composition range, 30 at.% ~< Cu~< 75 at.%, as for hydrogen-free materials in the same amount of time. For the sake of comparison, the hydrogenated alloys will be designated with the copper and titanium concentrations of the corresponding hydrogen-free samples. After 16 h of milling the alloys appear amorphous to X-ray and neutron diffraction except for a fraction of crystalline phase, identified as Till 2 [6]. The thermal behaviour of hydrogenated Cu60Ti40 amorphous powders is shown, as an example, in Fig. 3, as obtained from power compensation (curve (a)) and heat flux (curve (b))
1[
7°I
390
r
" 370i-
35O
42 '&
x
•
iI
[3
56
/
~x
',
330
o
m ca
~
m
/
o
',..
i
310
290 h
0
5
1tO
~-
15
30 Milling
time
'
60
(h)
Fig. 1. Heat of crystallization as a function of milling time. [], Cu5.Tis~ and A, Ti60Cu4o. Full triangles: sample p r e p a r e d at -
B
30
°C.
270 ~ 0
A
I 5
J
I 10
~5 1
..... MHling
i L 30 ":Jr'n~' ( h )
60
Fig. 2. Crystallization t e m p e r a t u r e as a function of milling time. rL Cus.Tis~ ~ and A Ti~.~Cua. Full Iriangles: sample prepared at - 30 °C.
1400
300 a
10C
~
.
eXO T L t
300
500
Temperature ( ~ )
Fi& 3. DSC traces for Cu~oTi4o. Curve {a): heat flux calorimeter; curve (b): power compensation calorimeter.
calorimeters. Curve (a) shows two thermal effects: a crystallization exotherm and the endothermal decomposition of T i l l 2 at higher temperature. For most of the other compositions the crystallization peak is masked by hydrogen evolution. To put in evidence the occurrence of crystallization, alloy samples must be crimped in sealable aluminium pans, so that the increase in pressure on heating the pan delays hydrogen evolution. In the heat flux calorimeter trace (Fig. 3, curve (b)) the crystallization peak is located in the same temperature range and appears similar. On the other hand, the endothermal effect is strongly depressed. The endotherm is large when using the power compensation instrument, where the sample and reference cells are separated, whereas the sample and the reference experience the same atmosphere in the heat flow calorimeter. The DSC output dp/dt may be written as [8] dp dt
dAH dt
~)ACp+AAAT
0
70
(1)
where the first term is the enthalpy change for the reaction under consideration, # is the heating rate, ACpthe specific heat difference between the final and initial states, AA is the difference in heat transfer coefficient (which depends on thermal conductivity) between sample and reference cell, and A T the temperature difference between the reference thermocouple and the heating element. The third term in eqn. (1) plays a different role in the two calorimeters because of the very high thermal conductivity of hydrogen. The same effect has been described for hydrogen evolution from Si-Ge thin films [9].
12 .
I
I
t
I
.4
'
.6
8t ,
, 1.0
XTi
Fig. 4. Apparent heat of decomposition of TiN e after crystallization of C u - T i - H amorphous alloys as a function of titanium content.
6
~4 I
o
o
o o
f .2
,
I .4
[3
I
16 .
,
Xrl
1.0
Fig. 5. Heat of crystallization as a function of titanium content. &, amorphous ribbons; zx, amorphous Cu-Ti powder; a, amorphous C u - T i - H powder.
The heat corresponding to the endothermal signal obtained for a series of C u - T i - H alloys is shown in Fig. 4 as a function of titanium content. The amount of hydrogen in the parent titanium used for alloying is determined as (10_+2)at.%H. The calibration of the calorimeter was performed by decomposing suitable amounts of pure Till 2. With the method described, a hydrogen content as low as 1 at.% could be detected in alloys prepared from other titanium batches. The A H x of hydrogenated and hydrogen free amorphous alloy powders is reported in Fig. 5, together with that of amorphous ribbons [3]. In the central composition region the values of hydrogenated alloys are definitely lower than those of hydrogen-free materials. This would imply the occurrence of different crystallization products in the two sets of samples. X-ray diffraction has been performed on samples as prepared and heated up to 435°C
1401
hydrogenated amorphous phase does not lead to hydrogenated intermetallics, but to the more stable Till 2 compound.
Acknowledgments G. Cocco (University of Sassari) and S. Enzo (University of Venezia) are gratefully acknowledged for useful discussions. Progetto Finalizzato CNR "Materiali speciali per tecnologie avanzate" is acknowledged for financial support under contracts 89.00623.68 and 89.00605.68. b
References 2@
2@
Fig. 6. X-ray diffractograms of Cu60Ti40 amorphous alloys: (a) hydrogenated; (b) hydrogen free. Curves (c) and (d) refer to samples (a) and (b), respectively, after heating up to the completion of crystallization. Insert: neutron diffraction patterns of samples (a) and (b). Symbols: o, TizCu; o, Cu4Ti3; +, Cu4Ti; I1, CuTi; v , Cu3Ti; 0 , Till 2.
(Fig. 6). Neutron diffraction of amorphous powder is also shown in the insert of Fig. 6, where the hydrogen contamination is well evidenced by the slope of the background [6]. As expected, different crystallization products are found in hydrogenated and hydrogen-free amorphous Cu60Tia0:TiH 2 and Cu3Ti in the former and CunTi, 6 - C u T i , C u a T i 3 and cubic CuTi 2 in the latter. Cu3Ti , here stabilized by the presence of TiH2, was already found in splat-cooled foils as a metastable phase [10]. The crystallization of the
1 G. Cocco, I. Soletta, S. Enzo, M. Magini and N. Cowlam, J. de Phys., C4-51 (1990) 181. 2 C. Politis and W. L. Johnson, J. Appl. Phys., 60 (1986) 1147. 3 L. Battezzati, M. Baricco, G. Riontino and I. Soletta, J. de Phys., C4-51 (1990)79. 4 W. L. Johnson, Prog. Mater. Sci., 30 (1986) 81. 5 C. Suryanarayana, R. Sunderasan and F. H. Froes, Int. Symp. on Solid State Powder Processing, Indianapolis, October 1989. 6 P. K. Ivison, I. Soletta, G. Cocco, S. Enzo, L. Battezzati and N. Cowlam, Mater. Sci. Eng., A128 (1991) this volume. 7 E. Hellstern and L. Schultz, J. Appl. Phys., 63 (1988) 1408. 8 J. Sestak, in Comprehensive Analytical Chemistry, Vol. XII-D, Elsevier, Amsterdam, 1984. 9 L. Battezzati, F. Demichelis, C. F. Pirri and E. Tresso, J. Appl. Phys., in press. 10 B. C. Giessen and D. 8zymanski, J. AppL Cryst., 4 (1971) 257.