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Formation and stability of amorphous alloys of Au-Ge-Si
FORMATION
AND
STABILITY
OF AMORPHOUS
H. S. CHENf$
and
ALLOYS
OF Au-Ge-Si*
D. TURNBULL?
Alloys of Au-Ge-Si in a narrow composition range near the eutectic were prepared in amorphous The glass transition temperature, T,, decreases slightly solid form with splat quenching techniques. with increase in gold content of the alloy but the reduced glass temperature, rp = kT,/h, was nearly independent of composition and -0.006. It was found that the addition of gold to the euteotio alloy increases the resistance of the amorphous &lloy to ~ryst&~iz&tion. This is attributed to s deereaaing rate of eryst~ll~z~tion of the me&stable y phase with excess of gold. A splat of Au ,,.r8~Gee.rz&i~.,s, alloy remained amorphous without appreciable erystallizetion up to 13’K above its T, with a heetmg rate of 20”K/min. FORMATION
ET
STABILITE
DES
ALLIAGES
AMORPHES
Au-Ge-Si
Des allirtges de Au-Ge-Si de compositions voisines de l’eutectique ont Bte prepares sous forme amorphe La temperature de transition du verre, T,, diminue leg&epar la, methode de la trempe par projection. ment quand la concentration en or de l’alliage augmente, mais la temperature rbduite du verre, r1 = kZ’,/h,, est presque independante de la composition et Bgale approximativement & 0,006. Les auteurs ont trouve que l’addition d’or B I’alliage eutectique augmente la r&.istanoe & la cristallisation de l’alli5ge amorphe. Ceoi est attribueau fait clue la vitesse de cristallislation deh phase m&a&able y decroit avec l’addition d’or. Un echantillon de l’alliage Au,,.,ze GeO.iBdS&e, demeure amorphe, sans cristallisation appreciable, jusqu’8 13°K au-dessus de sa T,, avee une vitesse de ehauffage de ZO’K/min. BILDUNG
UND
STABILITAT
AMORPHER
Au-Ge-Si-LEGIERUNGEN
Duroh Klatschkiihhmg wurden smorphe feste Au-Ge-Si-Legierungen in einem schmalen Zusammensetz~~ve~ich in der N&he des Eu~kt~~s hergestellt. Die Gl~s~mperatur T, wird mit zunehmendem Goldgehalt der Legierung etwas hiiber; jedoch ist die reduzierte Glastemper&ur, rg = kT,/h, SW 0,006,von der Zusammensetzung fast unabhiingig. Es zeigte sioh, da6 Zugabe von Gold zu der eutektischen Legierung den Widerstand der amorphen Legierung gegen Kristallisation erhiiht. Diese Eigenschaft wird der mit dem GoldtibersohuD abnehmendem K~stall~ationsges~hwindigkeit der met~t&bilen y-Phase zugeschrieben. Eine ~~~ohgek~l~ van 20°K/min ohne Au,,,,, Ge,.rz, Si&,,- Legierung blieb amorph und bei einer Heizgesch~digkeit merkliche Kristallisation bis 13°K oberhalb ihrer Glastemperatur.
INTRODUCTION
A number of metallic alloys have been quenched from the melt into an amorphous solid form.(1-3) The compositions of most of these alloys were near low lying eutectics. After the discovery of the amorphous form of the Au-Si alloy, Cohen and Turnbull(4) pointed out that a low lying eutectic composition should be especially favorable for glass formation. We have reported both thermal and rheological manifestations of the glass transition in a ternary Au-Si-Ge alloy(5) and in both binary and ternary Pd-Si based alloys.@) Partial replacement of one component in the binary systems with a third element, for example, silicon by germanium in Au-Si alloy, often enhanced the ease of formation and stability against crystallization of the glass. In the Pd-Si alloys the reduced glass temperature, T&T~= kT,/k,, where T, is the temperature at which * Received July 2, 1969. This research was supported in pert by the U.S. Office of Naval Research N00014-67-A-0298 0009 and by the Advanced Research Projects Agency, ARPASD-88 t Division of Engineering and Applied Physics, Harvard University, Cambridge, M~s~hu~tts 02138 $ Now at: Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey 07974. ACTA METALLURGICA,
VOL.
18, FEBRUARY
1970
the glass transition is exhibited thermally, h, is the atomic heat of vaporization and k is Boltzmann’s constant), was found to be about 0.012. T, varied rather little with composition, though it tended to increase with increase in heat liberated during liquid mixing. In this paper we will report on the composition dependence of the stability and c~stallization behavior of some Au-Ge-Si glasses. EXPERIMENTAL
A series of alloys were prepared by mixing the appropriate amount of pure gold and alloys consisting of Au,.,,Geo.174Si0.123. Amorphous films of these alloys were formed by “splat” cooling method deseribed in earlier papers. (5*7f The splats were oval shaped with lateral dimensions of approximately 2 x 3 cm and with thicknesses averaging 20 microns. The amorphous character of each splat was established by X-ray diffraction before thermal measurements were made on it. In one series of experiments, the amorphous films were annealed above its glass transition temperature in warm water. Their struetures were then examined.
261
ACTA
262
METALLURGICA,
VOL.
The glass transition temperature, T,, and the temperature T, of onset of crystallization of the amorphous alloys were measured with a differential scanning calorimeter (Perkin-Elmer DSC-1). Detailed procedures were reported in previous papers.(7*s) T, was manifested by an abrupt increase in the specific heat, CPA, of the amorphous phase during heating from below T,. A scanning rate of 20”C/min was used in all measurements.
FIQ. 1. Glass forming compositions of Au-Ge-Si alloys.
1970
70
Au(lll)
60 -
50 -
40 Yl8
(0)
RESULTS
X-ray diffraction examination showed that all splat-formed alloys in the composition range 74.0 to 79.0 at. % Au were amorphous. There was a high probability that splat-formed alloys with gold contents outside these limits would contain some crystalline precipitates in an amorphous matrix. Figure 1 shows the compositions of the alloys of Au-Si-Ge which formed entirely amorphous structures in these and in earliercK)experiments. The predominant structure of the crystallites embedded in the amorphous phases, after splatting, was a crystalline gold solution in the alloys containing >79 at. % Au and complex intermetallic structures, one designated as y, in alloys with <74 at. % Au. Upon annealing at 44”C, which is above the glass temperature, the amorphous phase crystallized to a mixture of crystalline gold solution and y. Figure 2a shows typical diffraction patterns of gold rich splats (in the example AusezGe,,106Si0_0,5)before annealing. The crystalline pattern shows a peak which may be attributed to the crystalline gold solution in a strongly preferred orientation with (111) parallel with the broad surface of the foil.
18,
30 -
F I
l-4
yz2
20 -
cl
+ AU (2001
r
0. I
0.2
0.3 sine/X
0.4
0.5
0.6
i-’
FIG. 2. Diffraction patterns of Au-Ge-Si
alloys. (a) Au-rich alloy (Au..8saCeo.lorSi,.,,~) aa spletted. (b) FHspl&ted; Au-poor alloy (Au,.,asCe,.l~8Si,.l~~)-: -.-_: annealed at 44°C for 2 min. Figure 2b compares the diffraction
patterns of an as
splatted and 44°C annealed alloy of Au,.,&e,.,,Si,,,.
The small change in peak intensity on annealing implies that the crystalline phases consisted of mixtures of very small and probably distorted crystallites. Upon heating over 200°C t’he amorphous alloy crystallized to an equilibrium Ge-Si solution, with the diamond structure, in the form of particles embedded in a crystalline gold solution. We obtainedts) Pd-Si based amorphous alloys from the melt with cooling rates as low as 102’C/sec, e.g. simply by dropping 40 micron droplets of t,he molten alloy into liquid nitrogen. However, the Au-Si-Ge alloys completely crystallized at these lower cooling rates ; they formed glasses only in the splat quenching experiments. Figure 3 shows a typical variation of the rate of heat absorption (A&) with temperature by an amorphous alloy when heated at a constant rate. First, there is the thermal manifestation of the glass transition, corresponding to an abrupt increase in heat capacity, at T,. Then at a somewhat higher temperature, T,, the metastable liquid begins to crystallize.
CHEN
AiTD
AMORPHOUS
TURNBULL:
ALLOYS
OF
263
Ati-Ge-Si
310
-2 .I -3
-4
-5
4
-6
T I
I 260
.y +” +
3oc
b
0
I
I
I
I
I
I
I
300
320
340
360
360
400
420
AU
440
T’K
FIN. 3. Thermal behavior of amorphous Au,,.~~~G~,.~,,Si,.0,b alloy. The glass temperature T,, and the onset of crystallization temperatures T,, are also shown. Ak is the rate of heat evolution. Heating rate is ZO”K/min.
Transformation to other crystalline states occurs at still higher temperatures. We found that T, increased while T, decreased, at small rates, as the gold concentration in the amorphous alloy, at a Ge-Si atom ratio fixed at 59141, increased from 74 to 80 at.%; these variations are shown in Fig. 4. The amorphous alloy could be brought, when heated of *u,.,,~0.1&.00 at the rate 20’K/min, to a temperature 13” above T, before appreciable crystallization occurred. T,, see Fig. 4, extrapolates to the value 275°K at 100% Au. DISCUSSION
The rather small variation (dT,/dx,, N - 1.O’C/ at. ‘A) of glass temperature with composition contrast with the much stronger composition dependence (~5O”C/at.%) of the liquidus temperature in the Au-Ge-Si system at near eutectic compositions. T, tends to increase somewhat with increase in the amount of heat evolved on mixing. However the reduced glass temperature T,, remains nearly constant with changing composition, at about 0.006 in the Au-Ge-Si system. Similar trends of T, with composition were found in the Pd-Si system@) but with 7rr- 0.012. Following the procedure used in our paperc6) on PdSi we have estimated the minimum undercooling required for appreciable homogeneous nucleation of crystals in undercooled Au-Ge-Si molten alloys. In the calculation we have supposed that the temperature dependence of the viscosity is as reported(5) for *%,,~0.145Si0.0% alloys and that the cooling rate is lO’W/sec. We estimate from this that the rate of homogeneous nucleation could not have been appreciable at any viscosity greater than 2 x lo3 poise
0.1 -
0.2 b(Ge.,,Si.,,
0.3
I
FIN. 4. The composition dependence of the glass transition temperature T, and temperature T, of onset of crystallization of the amorphous Au-Ge-Si alloys. Where b is the atomic concentration of Ge,.,,Si,.,, in the alloys.
The corresponding to a temperature ~350°K. liquidus temperatures of the ternary alloys were obtained from electrical resistivity measurements.(g) They are 690, 665 and 693°K respectively for and Thus the undercooling required for homogeneous crystal nucleation in these alloys appears to be at least 300 to 350°K. Alloys containing less than 74 at. % Au were predominantly y after crystallization. The increasing resistance of the alloys to crystallization with addition of gold (at more than 74 at. ‘A Au) may reflect that excess gold reduces the rate of y crystallization. It is interesting that the amorphous alloy can persist for some time above T, even with Au crystals embedded in it. It may be true that the growth of these crystals is retarded by an increased glass temperature of their matrix owing to t’he accompanying depletion of gold. REFERENCES 1. W. KLEMENT,JR., R. H. WILLEXS and POL DUWEZ, Nature 187. 869 11960). 2. R. C. &EWTD$ON, PhD. Thesis, California Institute of Technology (1966). 3. POL DUWEZ, Trans. Am. Inst. Nin. Engrs 60, 607 (1967). 4. M. H. COHENand D. TURNBULL.Nature 189. 131 (1961). 5. H. S. CHEN and D. TURNBULL,Appl. Phye.. L&t. ’ 10, 284 (1967); ibid, J. them. Phys. 48, 2560 (1968). 6. H. S. CHEN and D. TURNBULL,Acta Met. 1’7, 1021 (1969). 7. H. S. CHEN and D. TURNBULL,J. appl. Phy8. 88, 3646 (1967). H. S. CHEN and D. TURNBULL,Ada Met. 16,369 (1968). H. S. &EN, Electrical Resistivity of Alloys m their Amorphous and Liquid States, submitted to J. Phya. Chem. Solids. unpublished.
AND
STABILITY
OF AMORPHOUS
H. S. CHENf$
and
ALLOYS
OF Au-Ge-Si*
D. TURNBULL?
Alloys of Au-Ge-Si in a narrow composition range near the eutectic were prepared in amorphous The glass transition temperature, T,, decreases slightly solid form with splat quenching techniques. with increase in gold content of the alloy but the reduced glass temperature, rp = kT,/h, was nearly independent of composition and -0.006. It was found that the addition of gold to the euteotio alloy increases the resistance of the amorphous &lloy to ~ryst&~iz&tion. This is attributed to s deereaaing rate of eryst~ll~z~tion of the me&stable y phase with excess of gold. A splat of Au ,,.r8~Gee.rz&i~.,s, alloy remained amorphous without appreciable erystallizetion up to 13’K above its T, with a heetmg rate of 20”K/min. FORMATION
ET
STABILITE
DES
ALLIAGES
AMORPHES
Au-Ge-Si
Des allirtges de Au-Ge-Si de compositions voisines de l’eutectique ont Bte prepares sous forme amorphe La temperature de transition du verre, T,, diminue leg&epar la, methode de la trempe par projection. ment quand la concentration en or de l’alliage augmente, mais la temperature rbduite du verre, r1 = kZ’,/h,, est presque independante de la composition et Bgale approximativement & 0,006. Les auteurs ont trouve que l’addition d’or B I’alliage eutectique augmente la r&.istanoe & la cristallisation de l’alli5ge amorphe. Ceoi est attribueau fait clue la vitesse de cristallislation deh phase m&a&able y decroit avec l’addition d’or. Un echantillon de l’alliage Au,,.,ze GeO.iBdS&e, demeure amorphe, sans cristallisation appreciable, jusqu’8 13°K au-dessus de sa T,, avee une vitesse de ehauffage de ZO’K/min. BILDUNG
UND
STABILITAT
AMORPHER
Au-Ge-Si-LEGIERUNGEN
Duroh Klatschkiihhmg wurden smorphe feste Au-Ge-Si-Legierungen in einem schmalen Zusammensetz~~ve~ich in der N&he des Eu~kt~~s hergestellt. Die Gl~s~mperatur T, wird mit zunehmendem Goldgehalt der Legierung etwas hiiber; jedoch ist die reduzierte Glastemper&ur, rg = kT,/h, SW 0,006,von der Zusammensetzung fast unabhiingig. Es zeigte sioh, da6 Zugabe von Gold zu der eutektischen Legierung den Widerstand der amorphen Legierung gegen Kristallisation erhiiht. Diese Eigenschaft wird der mit dem GoldtibersohuD abnehmendem K~stall~ationsges~hwindigkeit der met~t&bilen y-Phase zugeschrieben. Eine ~~~ohgek~l~ van 20°K/min ohne Au,,,,, Ge,.rz, Si&,,- Legierung blieb amorph und bei einer Heizgesch~digkeit merkliche Kristallisation bis 13°K oberhalb ihrer Glastemperatur.
INTRODUCTION
A number of metallic alloys have been quenched from the melt into an amorphous solid form.(1-3) The compositions of most of these alloys were near low lying eutectics. After the discovery of the amorphous form of the Au-Si alloy, Cohen and Turnbull(4) pointed out that a low lying eutectic composition should be especially favorable for glass formation. We have reported both thermal and rheological manifestations of the glass transition in a ternary Au-Si-Ge alloy(5) and in both binary and ternary Pd-Si based alloys.@) Partial replacement of one component in the binary systems with a third element, for example, silicon by germanium in Au-Si alloy, often enhanced the ease of formation and stability against crystallization of the glass. In the Pd-Si alloys the reduced glass temperature, T&T~= kT,/k,, where T, is the temperature at which * Received July 2, 1969. This research was supported in pert by the U.S. Office of Naval Research N00014-67-A-0298 0009 and by the Advanced Research Projects Agency, ARPASD-88 t Division of Engineering and Applied Physics, Harvard University, Cambridge, M~s~hu~tts 02138 $ Now at: Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey 07974. ACTA METALLURGICA,
VOL.
18, FEBRUARY
1970
the glass transition is exhibited thermally, h, is the atomic heat of vaporization and k is Boltzmann’s constant), was found to be about 0.012. T, varied rather little with composition, though it tended to increase with increase in heat liberated during liquid mixing. In this paper we will report on the composition dependence of the stability and c~stallization behavior of some Au-Ge-Si glasses. EXPERIMENTAL
A series of alloys were prepared by mixing the appropriate amount of pure gold and alloys consisting of Au,.,,Geo.174Si0.123. Amorphous films of these alloys were formed by “splat” cooling method deseribed in earlier papers. (5*7f The splats were oval shaped with lateral dimensions of approximately 2 x 3 cm and with thicknesses averaging 20 microns. The amorphous character of each splat was established by X-ray diffraction before thermal measurements were made on it. In one series of experiments, the amorphous films were annealed above its glass transition temperature in warm water. Their struetures were then examined.
261
ACTA
262
METALLURGICA,
VOL.
The glass transition temperature, T,, and the temperature T, of onset of crystallization of the amorphous alloys were measured with a differential scanning calorimeter (Perkin-Elmer DSC-1). Detailed procedures were reported in previous papers.(7*s) T, was manifested by an abrupt increase in the specific heat, CPA, of the amorphous phase during heating from below T,. A scanning rate of 20”C/min was used in all measurements.
FIQ. 1. Glass forming compositions of Au-Ge-Si alloys.
1970
70
Au(lll)
60 -
50 -
40 Yl8
(0)
RESULTS
X-ray diffraction examination showed that all splat-formed alloys in the composition range 74.0 to 79.0 at. % Au were amorphous. There was a high probability that splat-formed alloys with gold contents outside these limits would contain some crystalline precipitates in an amorphous matrix. Figure 1 shows the compositions of the alloys of Au-Si-Ge which formed entirely amorphous structures in these and in earliercK)experiments. The predominant structure of the crystallites embedded in the amorphous phases, after splatting, was a crystalline gold solution in the alloys containing >79 at. % Au and complex intermetallic structures, one designated as y, in alloys with <74 at. % Au. Upon annealing at 44”C, which is above the glass temperature, the amorphous phase crystallized to a mixture of crystalline gold solution and y. Figure 2a shows typical diffraction patterns of gold rich splats (in the example AusezGe,,106Si0_0,5)before annealing. The crystalline pattern shows a peak which may be attributed to the crystalline gold solution in a strongly preferred orientation with (111) parallel with the broad surface of the foil.
18,
30 -
F I
l-4
yz2
20 -
cl
+ AU (2001
r
0. I
0.2
0.3 sine/X
0.4
0.5
0.6
i-’
FIG. 2. Diffraction patterns of Au-Ge-Si
alloys. (a) Au-rich alloy (Au..8saCeo.lorSi,.,,~) aa spletted. (b) FHspl&ted; Au-poor alloy (Au,.,asCe,.l~8Si,.l~~)-: -.-_: annealed at 44°C for 2 min. Figure 2b compares the diffraction
patterns of an as
splatted and 44°C annealed alloy of Au,.,&e,.,,Si,,,.
The small change in peak intensity on annealing implies that the crystalline phases consisted of mixtures of very small and probably distorted crystallites. Upon heating over 200°C t’he amorphous alloy crystallized to an equilibrium Ge-Si solution, with the diamond structure, in the form of particles embedded in a crystalline gold solution. We obtainedts) Pd-Si based amorphous alloys from the melt with cooling rates as low as 102’C/sec, e.g. simply by dropping 40 micron droplets of t,he molten alloy into liquid nitrogen. However, the Au-Si-Ge alloys completely crystallized at these lower cooling rates ; they formed glasses only in the splat quenching experiments. Figure 3 shows a typical variation of the rate of heat absorption (A&) with temperature by an amorphous alloy when heated at a constant rate. First, there is the thermal manifestation of the glass transition, corresponding to an abrupt increase in heat capacity, at T,. Then at a somewhat higher temperature, T,, the metastable liquid begins to crystallize.
CHEN
AiTD
AMORPHOUS
TURNBULL:
ALLOYS
OF
263
Ati-Ge-Si
310
-2 .I -3
-4
-5
4
-6
T I
I 260
.y +” +
3oc
b
0
I
I
I
I
I
I
I
300
320
340
360
360
400
420
AU
440
T’K
FIN. 3. Thermal behavior of amorphous Au,,.~~~G~,.~,,Si,.0,b alloy. The glass temperature T,, and the onset of crystallization temperatures T,, are also shown. Ak is the rate of heat evolution. Heating rate is ZO”K/min.
Transformation to other crystalline states occurs at still higher temperatures. We found that T, increased while T, decreased, at small rates, as the gold concentration in the amorphous alloy, at a Ge-Si atom ratio fixed at 59141, increased from 74 to 80 at.%; these variations are shown in Fig. 4. The amorphous alloy could be brought, when heated of *u,.,,~0.1&.00 at the rate 20’K/min, to a temperature 13” above T, before appreciable crystallization occurred. T,, see Fig. 4, extrapolates to the value 275°K at 100% Au. DISCUSSION
The rather small variation (dT,/dx,, N - 1.O’C/ at. ‘A) of glass temperature with composition contrast with the much stronger composition dependence (~5O”C/at.%) of the liquidus temperature in the Au-Ge-Si system at near eutectic compositions. T, tends to increase somewhat with increase in the amount of heat evolved on mixing. However the reduced glass temperature T,, remains nearly constant with changing composition, at about 0.006 in the Au-Ge-Si system. Similar trends of T, with composition were found in the Pd-Si system@) but with 7rr- 0.012. Following the procedure used in our paperc6) on PdSi we have estimated the minimum undercooling required for appreciable homogeneous nucleation of crystals in undercooled Au-Ge-Si molten alloys. In the calculation we have supposed that the temperature dependence of the viscosity is as reported(5) for *%,,~0.145Si0.0% alloys and that the cooling rate is lO’W/sec. We estimate from this that the rate of homogeneous nucleation could not have been appreciable at any viscosity greater than 2 x lo3 poise
0.1 -
0.2 b(Ge.,,Si.,,
0.3
I
FIN. 4. The composition dependence of the glass transition temperature T, and temperature T, of onset of crystallization of the amorphous Au-Ge-Si alloys. Where b is the atomic concentration of Ge,.,,Si,.,, in the alloys.
The corresponding to a temperature ~350°K. liquidus temperatures of the ternary alloys were obtained from electrical resistivity measurements.(g) They are 690, 665 and 693°K respectively for and Thus the undercooling required for homogeneous crystal nucleation in these alloys appears to be at least 300 to 350°K. Alloys containing less than 74 at. % Au were predominantly y after crystallization. The increasing resistance of the alloys to crystallization with addition of gold (at more than 74 at. ‘A Au) may reflect that excess gold reduces the rate of y crystallization. It is interesting that the amorphous alloy can persist for some time above T, even with Au crystals embedded in it. It may be true that the growth of these crystals is retarded by an increased glass temperature of their matrix owing to t’he accompanying depletion of gold. REFERENCES 1. W. KLEMENT,JR., R. H. WILLEXS and POL DUWEZ, Nature 187. 869 11960). 2. R. C. &EWTD$ON, PhD. Thesis, California Institute of Technology (1966). 3. POL DUWEZ, Trans. Am. Inst. Nin. Engrs 60, 607 (1967). 4. M. H. COHENand D. TURNBULL.Nature 189. 131 (1961). 5. H. S. CHEN and D. TURNBULL,Appl. Phye.. L&t. ’ 10, 284 (1967); ibid, J. them. Phys. 48, 2560 (1968). 6. H. S. CHEN and D. TURNBULL,Acta Met. 1’7, 1021 (1969). 7. H. S. CHEN and D. TURNBULL,J. appl. Phy8. 88, 3646 (1967). H. S. CHEN and D. TURNBULL,Ada Met. 16,369 (1968). H. S. &EN, Electrical Resistivity of Alloys m their Amorphous and Liquid States, submitted to J. Phya. Chem. Solids. unpublished.