- Email: [email protected]
Irradiation-induced alloying in an immiscible TaTi system
Nuclear Instruments and Methods in Physics Research B 124 (I 997) 523-527 Lk!lOKil
B
BeamInteractions with Materials 8 Atoms
ELSEVIER
Irradiation-induced alloying in an immiscible Ta-Ti system Y.G. Chen, Q. Zhang, B.X. Liu * Deparrment
of Marerids
Science und Engineering.
Tsinghuu Uniuer.sity. Beijing 100084. Chinu
Received 22 November 1996; revised form received 3 January 1997 Abstract In the properly designed multilayered films according to an interfacial free energy estimation. amorphous alloys were formed upon room temperature 200 keV xenon-ion irradiation in the Ta-Ti system, although it was previously considered as a non-glass forming one because of its positive heat of formation as well as the unfavored atomic size and electronegativity. In addition, two metastable Ta-rich and Ti-rich fee phases of different lattice constants were also obtained. To explain the unusual alloying behaviors, a Gibbs-free-energy diagram of the system was constructed based on Miedema’s model. The calculated diagram showed that the interfacial free energy resulting from chemical and elastic contributions in the films could raise the initial energetic state of the multilayered films to a state higher than those of the observed amorphous and metastable crystalline phases, thus generating a driving force for alloying in this immiscible Ta-Ti system. The possible growth kinetics of the metastable crystalline phases is also discussed.
1. Introduction In the last I5 years, the alloying behavior of binary metal systems has been extensively studied by Ion Mixing (IM). Several empirical models have been proposed for predicting the alloy phase formation as well as the glassforming ability (GFA) from heat of formation (AH,), difference in structure, and difference of atomic size as well as of electronegativity, etc. [I -41. According to these models, formation of amorphous phase tends to occur in those systems with large negative A Hf and large atomic size differences. Here the large negative AH, provides a necessary driving force for transforming the original two crystalline metals into an amorphous alloy phase upon IM. A calculation based on thermodynamics of solids dealing with bulk materials form shows that a positive A Hf frequently makes the amorphous phase have higher free energy than that of an equilibrium mixture of two crystalline constituent metals, leading to a unfavoring situation for amorphization. In a scheme of multilayered films for IM to begin with, however, the situation is significantly different from the bulk form. The multilayered films include definitely some interfaces and the atoms in the interfacial layers possess some extra free energy, which should be taken into account in studying the GFA. Following this consideration. the authors’ group has recently conducted IM experiments by designing the multilayers
’ Corresponding author. Fax: + 86-10-6256-2768; email: [email protected] 0168-583X/97/$17.00
with various numbers of interfaces, or more precisely various fractions (denoted as f, = interfacial atoms/total atoms) of interfacial atoms, and succeeded in forming some amorphous alloys while the films included sufficient fractions of interface [5,6]. A large atomic size difference is considered as a favored factor. as it plays a kinetic role in preventing the growth of the possible competing crystalline phase, thus favoring amorphization. However, there were several examples that amorphous alloys were formed by IM in some systems with a small size difference [7]. According to the deduction from Hume-Rothery’s rule, a small difference in electronegativity is favored to form solid solution of simple crystalline structure which can be recrystallized during the relaxation period of IM with a fast nuclei and grow speed and this situation is therefore an unfavored factor for amorphization. This study was to test all the above mentioned factors influencing IM induced amorphization simultaneously in a specially selected system. i.e. the Ta-Ti system, which has a positive AH, ( + 2 kJ/mol), a difference in structure (Ta-bee, Ti-hcp), a small size difference (atomic size ratio close to unity) and almost identical electronegativities of two metals.
2. Experimental
procedure
According to a calculation of Gibbs-free energy based on Miedema’s model [8] and the related method [9-l I], the Ta-Ti multilayered films consisting of 16 layers with a total thickness of 50 nm were designed. Assuming an
Copyright 0 I997 Elsevier Science B.V. All rights reserved PI/ SOl68-583X(97)00097-9
Y.G. Chen et d./Nucl.
524
Instr. und Merh. in Phys. Res. B 124 (1997) 523-527
interfacial layer thickness being 5 A, the fraction of interfacial atoms was f, = 15% of the designed films. The samples were prepared in an evaporation system with a vacuum level on the order of 10m5 Pa, by alternately depositing pure Ta and Ti onto newly cleaved NaCl single crystals as substrates. The total thickness (50 nm) of the samples was to match the sum of the projected range and projected range straggling (R, + AR,) of 200 keV irradiating xenon ion. The compositions of the deposited films were controlled by adjusting the relative thickness of the constituent metals. As-deposited films were then irradiated by 200 keV xenon ions at room temperature to doses ranging from 3 X lOI to 7 X IO” Xe+/cm* in an implanter with a vacuum level on the order of IO m4 Pa. The ion beam current density was controlled to be less than 0.5 bA/cm2 to minimize the heating effect. All the samples were investigated at room temperature by Transmission Electron Microscopy (TEM) and Selected Area Diffraction (SAD) to identify the microstructures. Energy Dispersive Spectrum (EDS) analysis was employed to determine the real composition of the as-deposited films and the resultant phases in the irradiated films within an experimental error less than 5%~.
3. Results and discussion 3.1.
Formation
of amorphous
alloys
films were partially amorThe TaggTi, , multilayered phized at a dose of I X IO” Xe+/cm2, i.e., an amorphous phase coexisted with a metastable crystalline phase of fee-I structure. With increasing irradiation ion dose to 5 x
10’5
a
Xe +/cm*,
uniform
amorphous
lb show patterns of the as-deposited films amorphous phase, respectively. evolution can be summarized as formed.
Fig.
1X
la
and
phase
was
the corresponding SAD and the resulted Ta,,Ti , , The sequence of phase follows:
lO”Xe+/cm
Tas,Ti I I
RT 5 X
10”Xe+/cm2
> fee - 1 + amorphous
RT
> amorphous. In the Ta,,Ti,, multilayered films. however. after irradiating to a dose of 7 X IO” Xe+/cm*, i.e. the highest dose Table
(b)
Fi g. I. (a) and (b) show the corresponding Ta,,Ti,,
as-deposited
5 X lOI
Xe+/cm’,
SAD
films and the amorphous
patterns of the
phase at a dose of
respectively.
conducted in this study, the structure of the sample was a mixture of an amorphous phase and a polycrystalline fee-I phase, suggesting that this composition was not suitable for forming amorphous alloy. 3.2.
Formation
of metastable
crystalline
phases
Table I lists the formation of two new metastable fee phases in Ta-Ti multilayered films upon 200 keV xenon ion mixing at room temperature at different irradiation doses. The first metastable fee phase was observed together with an amorphous phase in the Ta,,Ti , , films after irradiating to a dose of I X IO” Xe+/cm*. The same and uniform metastable fee phase was formed in the Ta,,Ti,, multilayered films after ion mixing to a low dose of 3 x IO” Xe+/cm*, and the composition of this fee phase was the same of the films. i.e. Ta,,TiZ6. Table 2 is the identification of the fee phase. Taking into account the error from SAD measurement, the lattice parameter of this fee phase is 3.91 + 0.15 A. Fig. 2a and b shows the typical SAD patterns of the as-deposited and the irradiated Ta,,Ti,, films. This fee phase was named as fee-I. Another new metastable fee phase (fee-2) was observed in Ta,,Ti,,, Ta,,Ti,, and Ta,Ti,, multilayered films. The Ta2,Ti,3 films transformed into a uniform fee-2 phase at a dose of 3 X lOI Xe+/cm*, and the structure remained stable with increasing the irradiation dose to the highest dose in this study. Fig. 3 exhibits a SAD pattern of the fee-2 phase. Table 3 gives the indexing results of the, fee-2 phase and the lattice parameter was of 3.8 I rt 0. I.5 A. The
I
Phase changes of Ta-Ti “Amor.“
(4
multilayered
stands for complete
Dose
Ta,,Ti , ,
3
Ta + Ti
7
Ta + Ti
IO
fee-
films
amorphous
I + amor.
upon 200 LeV xenon
ion mixing
at room temperature.
(The
doses are in lOI
phase.)
Ta,;Ti Lb fee- I fee- 1 fee- 1
Ta,,%,
Ta2,Ti7,
fee-2
fee-2
Ta,Tig2 fee-2
fee- 2
fee-2
fee-2
fee-2
fee-2
fee-2
Xe’/cm’,
Y.G. Chen et al./Nucl.
525
Instr. und Meth. in Phys. Res. B 124 (1997) 523-527
Table 2 Identification of the fee-1 phase formed in the Ta,,Ti,, multilayered films by room temperature 200 LieVion irradiation to a dose of 1 X 10” Xe+/cm*. (I~~~_, = 3.91 ;i
2.21 I .95 I.38 1.19 1.13 0.98 0.90 0.87 0.80
(hkl)
4,lC (A)
Intensity
Ill 200 220 311 222 400 331 420 422
2.26 1.96 1.38 1.18 1.13 0.98 0.90 0.87 0.79
strong strong strong medium medium weak medium medium medium
Fig. 3. A typical SAD pattern of the fee-2 phase formed in Ta>,Ti,, films upon IM at a dose of 3X lOI Xe+/cm’.
fee-2 phases were also obtained in the Ta,,Ti,, and TasTi,, films upon ion irradiation and their lattice parameters were both of 3.80 f 0.15 A. 3.3. Discussion Following the suggestion of Miedema et al. [8,9], the free-energy change of the formation of a solid solution and amorphous phases can be written in details. For a solid solution phase, AC;,=AH,C+AH,e+AH;-TAS,
(1)
in which AH’ is the chemical contribution and it can be given by AH’=Mx AAV2/‘f AB, where M is an amplitude reflecting the magnitude of the electron redistribution interaction, xA and VA are the atomic concentration and volume of atom A, and fAB is a function accounting for the degree to which atoms A are surrounded by atoms B; AH’ is the elastic contribution and can be derived from AH’ =xAxa(xaLAinB +xALainA>, where Li in j is the elastic contribution to the heat of solution i in j, A Hs is the structural contributions and the expression is A Hs =
E(F) - x,E,(Z,) -x,E,(Z,), where 2 is the average number of electrons per atom and E, EA and E, are the lattice stabilities of the alloy, the pure component A and the pure component B, respectively; AS is the entropy change and can be taken as AS = -I?[ x,ln( xA) + x,In(x,)], in which R is the gas constant. For an amorphous phase, AC,=
AHc +
~(x,T,,,
+x~T,.,)
-
TA,s,
(2)
where cr is an empirical constant equal to 3.5 J/(mol K) and T,,; is the melting point of the component metal i (i = A, B). According to Gerkema et al. [l I], the interfacial free energy was calculated as A’%,, = S, x a,, ,
(3)
in which S, is the surface area occupied by one mole interfacial atoms; cAB is the interfacial energy between A and B metals and can be estimated as a,, = 0.15( aA,0 + aB.O) + ~h~~;:v~ic~;c~t=;;~ energy of metal i, and uchchem Fig. 4 is a calculated Gibbs-free energy diagram of the Ta-Ti system. The values of the parameters used in the calculation were listed in Table 4 [8,12]. Fig. 4 shows that
Table 3 Indexing results of the fee-2 phase formed in the TaZ,Ti,, multilayered films at a dose of I X IO” Xe+/cm’ and the lattice = 3.81 i parameter was Indexed lo be ~~~~~~ de,,
(a)
@I
Fig. 2. (a) and (b) are the typical SAD patterns of the Ta,,Ti,, as-deposited films and the fee-I phase at a dose of 3X lOi Xe+ /cm’. respectively.
2.20 1.89 I.35 1.15 1.10 0.95 0.88 0.85 0.78
(A)
(hkl)
4,,,
III
2.20 1.91 I.35 1.15 1.10 0.95 0.87 0.85 0.78
200 220 311 222 400 331 420 422
(9
Intensity strong strong strong medium medium weak weak medium medium
Y.G. Chen et al./Nucl.
526
Instr. ad
Meth. in Phys. Res. B 124 (1997) 523-527
estingly, in some other Ta-based alloy systems, amorphous alloys were also obtained at the Ta-rich side, e.g. Ta-Pu amorphous alloys formed by sputtering technique [ 131, Ta,,Au,, and Ta,,Cu,, amorphous films synthesized both by solid-state reaction [ 14,151. The next issue is the growth kinetics of the two fee phases upon ion mixing. The fee-I phase that emerged at the Ta end, was formed through a transformation of bccto-fee, which seemed a little difficult. According to a proposed two-step reverse martensitic phase transfotmation mechanism of bee -+ hcp + fee [l6], the calculated lattice parameter of the fee-I phase was ufcc_, = 3.93 A, which was in agreement with the experimental result. For the fee-2 phase, according to a very recent report [17] that sputtered Ti films possessed fee structure instead of its normal hcp structure and the lattice parameter of the new fee structure was indexed to be afcc = 4.20 .& According to Vegard’s law, a lattice parameter of a fee Ti-based solid solution can be calculated as a = (3.90 I0.2) A, which was compatible with the present results by IM. The Ti-rich fee-2 phase was therefore considered as a fee Ti-based super-saturated solid solution. Incidentally, it is noted that there were two crystalline phases reported in the literature [ 181 and their structures were of hexagonal and orthorhombit, respectively. We therefore concluded that the above two fee structured metastable phases were new ones in this Ta-Ti system.
Multilayer
40
Ta
TI concentratm
Fio TL:Ti
WJ
60
II
(at 90)
4. A typical calculated Gibbs-free-energy
diagram
of the
system. The dashed line shows the initial energetic levels
of the as-deposited multilayers.
the free energy of the amorphous phase and the two terminal solid solutions is well above that of the reference state (an equilibrium mixture of the two constituent metals), implying an unfavorable thermodynamic condition for alloying in the system. However, IM begins with multilayered films consisting definitely of a certain number of interfaces and the atoms in the interfacial layers possess some extra free energy. The interfacial free energy contribution therefore was taken into account to the above calculation of the free energy and it did elevate the multilayered films to a state of higher free energy than those of the amorphous and other crystalline phases involved, thus providing a necessary driving force for forming Ta-Ti alloys. From the figure, one can see that the Ta content range favoring amorphization is at Ta-rich side, which is in good agreement with the IM experimental results. On the other hand, it is worthwhile mentioning that the absence of the amorphous phase in Ti-rich composition side upon IM was due to the competition between the amorphous and the existing me&table fee phase of simple structure, which was able to nuclei and grow, thus resulting in hindering amorphization at this side. It is therefore believed that the free energy calculation can serve as a semi-quantitative explanation for amorphization in the Ta-Ti system. Inter-
4. Concluding
remarks
(I) Amorphous alloys were formed by ion mixing in the Ta-Ti system, though the positive A Hf, a small size difference and a small difference in electronegativity have been considered as unfavored factors in conventional glass forming techniques. (2) A Gibbs-free-energy diagram of the Ta-Ti system was established by calculating the free-energy curves of the multilayered films and other related phases. The calculated diagram, including the consideration of interfacial energy for the multilayers, can give an appropriate interpretation to the formation of the metastable phases.
Table 4 Values of the parameters used in Gibbs-free-energy Parameter
T,, [K]
Ta
3287
Ti
1943
Amorphous
-
As-deposited
-
M [kJ/(mol -
cm’f]
V’/’
calculation in the Ta-Ti [cm*]
y
system. I’-.’
means the data were not used L,, inri [U/mol]
S, [ IO5 m’]
u. [mJ/m*J
Lri inra [k.J/mol]
4.89
_
3150
_
2.20
4.82
_
2100
_
5
-
2.16 _
1.14 _
i-5
+6
_
+5
+6
_
films Solid solution
_
1.14
0
-
Y.G. Chen et al./Nucl.
Instr. and Meth. in Phys. Res. B 124 (1997) 523-527
Acknowledgements The authors are grateful to the National Natural Science Foundation of China and the Administration of Tsinghua University for the financial aid to this study.
References
[II G.
Hagg, Z. Phys. Chem. B 12 (1931) 3.
121J.A. Alonso and S. Simozar, Solid State Commun. 48 (1983) 765.
Dl W. Hume-Rothery,
R.E. Smallman and C.M. Haworth, Structure of Metals and Alloys, 5th Ed. (Institute of Metals, London, 1969). [41 B.X. Liu, W.L. Johnson, M.-A. Nicolet and S.S. Lau, Appl. Phys. Lett. 42 (1983) 45. 151 0. Jin. Z.J. Zhang and B.X. Liu, Appl. Phys. Lett. 67 (1995) 1524. bl 0. Jin, Z.J. Zhang and B.X. Liu, J. Appl. Phys. 78 (1995) 149.
52-l
[7] F. Pan, Y.G. Chen, Z.J. Zhang and B.X. Liu, J. Non-Cryst. Solids 194 (1996) 305. [8] A.R. Miedema, A.K. Niessen, F.R. de Boer, R. Boom and W.C.M. Mattens, Cohesion in Metals: Transition Metal Alloys (North-Holland, Amsterdam, 1989). [9] J.A. Alonso, L.J. Gallego and S. Simozar, Nuovo Cimento 12 (1990) 587. [IO] Z.J. Zhang, 0. Jin and B.X. Liu, Phys. Rev. B 51 (1995) 8076. [ll] J. Gerkema and A.R. Miedema, Surf. Sci. 124 (1981) 351. [ 121 Periodic Table of the Elements (Sargent-Welch Sci. Co.). [13] H.F. Rizzo, T. Zocco, T.B. Massalski, M. Nastasi and A. Echevenia, Metal. Mat. Trans. A 25 (1994) 1579. [14] F. Pan, Y.G. Chen and B.X. Liu, Appl. Phys. Lett. 67 (1995) 780. (151 E. Ma, J.-H. He, P.J. Schilling, F. Pan and B.X. Liu, Mater. Res. Sot. Symp. Proc. 398 (1996) 221. [I61 Z.J. Zhang and B.X. Liu, J. Phys. Condens. Matter 6 (1994) 2647. [I71 C.H. Liu, W.Z. Li and H.D. Li, Mater. Lett. 20 (1994) 325. [18] T.B. Massalski, Binary Alloy Phase Diagrams, Vol. 3 (ASM Intern., 1990) p. 3431.
B
BeamInteractions with Materials 8 Atoms
ELSEVIER
Irradiation-induced alloying in an immiscible Ta-Ti system Y.G. Chen, Q. Zhang, B.X. Liu * Deparrment
of Marerids
Science und Engineering.
Tsinghuu Uniuer.sity. Beijing 100084. Chinu
Received 22 November 1996; revised form received 3 January 1997 Abstract In the properly designed multilayered films according to an interfacial free energy estimation. amorphous alloys were formed upon room temperature 200 keV xenon-ion irradiation in the Ta-Ti system, although it was previously considered as a non-glass forming one because of its positive heat of formation as well as the unfavored atomic size and electronegativity. In addition, two metastable Ta-rich and Ti-rich fee phases of different lattice constants were also obtained. To explain the unusual alloying behaviors, a Gibbs-free-energy diagram of the system was constructed based on Miedema’s model. The calculated diagram showed that the interfacial free energy resulting from chemical and elastic contributions in the films could raise the initial energetic state of the multilayered films to a state higher than those of the observed amorphous and metastable crystalline phases, thus generating a driving force for alloying in this immiscible Ta-Ti system. The possible growth kinetics of the metastable crystalline phases is also discussed.
1. Introduction In the last I5 years, the alloying behavior of binary metal systems has been extensively studied by Ion Mixing (IM). Several empirical models have been proposed for predicting the alloy phase formation as well as the glassforming ability (GFA) from heat of formation (AH,), difference in structure, and difference of atomic size as well as of electronegativity, etc. [I -41. According to these models, formation of amorphous phase tends to occur in those systems with large negative A Hf and large atomic size differences. Here the large negative AH, provides a necessary driving force for transforming the original two crystalline metals into an amorphous alloy phase upon IM. A calculation based on thermodynamics of solids dealing with bulk materials form shows that a positive A Hf frequently makes the amorphous phase have higher free energy than that of an equilibrium mixture of two crystalline constituent metals, leading to a unfavoring situation for amorphization. In a scheme of multilayered films for IM to begin with, however, the situation is significantly different from the bulk form. The multilayered films include definitely some interfaces and the atoms in the interfacial layers possess some extra free energy, which should be taken into account in studying the GFA. Following this consideration. the authors’ group has recently conducted IM experiments by designing the multilayers
’ Corresponding author. Fax: + 86-10-6256-2768; email: [email protected] 0168-583X/97/$17.00
with various numbers of interfaces, or more precisely various fractions (denoted as f, = interfacial atoms/total atoms) of interfacial atoms, and succeeded in forming some amorphous alloys while the films included sufficient fractions of interface [5,6]. A large atomic size difference is considered as a favored factor. as it plays a kinetic role in preventing the growth of the possible competing crystalline phase, thus favoring amorphization. However, there were several examples that amorphous alloys were formed by IM in some systems with a small size difference [7]. According to the deduction from Hume-Rothery’s rule, a small difference in electronegativity is favored to form solid solution of simple crystalline structure which can be recrystallized during the relaxation period of IM with a fast nuclei and grow speed and this situation is therefore an unfavored factor for amorphization. This study was to test all the above mentioned factors influencing IM induced amorphization simultaneously in a specially selected system. i.e. the Ta-Ti system, which has a positive AH, ( + 2 kJ/mol), a difference in structure (Ta-bee, Ti-hcp), a small size difference (atomic size ratio close to unity) and almost identical electronegativities of two metals.
2. Experimental
procedure
According to a calculation of Gibbs-free energy based on Miedema’s model [8] and the related method [9-l I], the Ta-Ti multilayered films consisting of 16 layers with a total thickness of 50 nm were designed. Assuming an
Copyright 0 I997 Elsevier Science B.V. All rights reserved PI/ SOl68-583X(97)00097-9
Y.G. Chen et d./Nucl.
524
Instr. und Merh. in Phys. Res. B 124 (1997) 523-527
interfacial layer thickness being 5 A, the fraction of interfacial atoms was f, = 15% of the designed films. The samples were prepared in an evaporation system with a vacuum level on the order of 10m5 Pa, by alternately depositing pure Ta and Ti onto newly cleaved NaCl single crystals as substrates. The total thickness (50 nm) of the samples was to match the sum of the projected range and projected range straggling (R, + AR,) of 200 keV irradiating xenon ion. The compositions of the deposited films were controlled by adjusting the relative thickness of the constituent metals. As-deposited films were then irradiated by 200 keV xenon ions at room temperature to doses ranging from 3 X lOI to 7 X IO” Xe+/cm* in an implanter with a vacuum level on the order of IO m4 Pa. The ion beam current density was controlled to be less than 0.5 bA/cm2 to minimize the heating effect. All the samples were investigated at room temperature by Transmission Electron Microscopy (TEM) and Selected Area Diffraction (SAD) to identify the microstructures. Energy Dispersive Spectrum (EDS) analysis was employed to determine the real composition of the as-deposited films and the resultant phases in the irradiated films within an experimental error less than 5%~.
3. Results and discussion 3.1.
Formation
of amorphous
alloys
films were partially amorThe TaggTi, , multilayered phized at a dose of I X IO” Xe+/cm2, i.e., an amorphous phase coexisted with a metastable crystalline phase of fee-I structure. With increasing irradiation ion dose to 5 x
10’5
a
Xe +/cm*,
uniform
amorphous
lb show patterns of the as-deposited films amorphous phase, respectively. evolution can be summarized as formed.
Fig.
1X
la
and
phase
was
the corresponding SAD and the resulted Ta,,Ti , , The sequence of phase follows:
lO”Xe+/cm
Tas,Ti I I
RT 5 X
10”Xe+/cm2
> fee - 1 + amorphous
RT
> amorphous. In the Ta,,Ti,, multilayered films. however. after irradiating to a dose of 7 X IO” Xe+/cm*, i.e. the highest dose Table
(b)
Fi g. I. (a) and (b) show the corresponding Ta,,Ti,,
as-deposited
5 X lOI
Xe+/cm’,
SAD
films and the amorphous
patterns of the
phase at a dose of
respectively.
conducted in this study, the structure of the sample was a mixture of an amorphous phase and a polycrystalline fee-I phase, suggesting that this composition was not suitable for forming amorphous alloy. 3.2.
Formation
of metastable
crystalline
phases
Table I lists the formation of two new metastable fee phases in Ta-Ti multilayered films upon 200 keV xenon ion mixing at room temperature at different irradiation doses. The first metastable fee phase was observed together with an amorphous phase in the Ta,,Ti , , films after irradiating to a dose of I X IO” Xe+/cm*. The same and uniform metastable fee phase was formed in the Ta,,Ti,, multilayered films after ion mixing to a low dose of 3 x IO” Xe+/cm*, and the composition of this fee phase was the same of the films. i.e. Ta,,TiZ6. Table 2 is the identification of the fee phase. Taking into account the error from SAD measurement, the lattice parameter of this fee phase is 3.91 + 0.15 A. Fig. 2a and b shows the typical SAD patterns of the as-deposited and the irradiated Ta,,Ti,, films. This fee phase was named as fee-I. Another new metastable fee phase (fee-2) was observed in Ta,,Ti,,, Ta,,Ti,, and Ta,Ti,, multilayered films. The Ta2,Ti,3 films transformed into a uniform fee-2 phase at a dose of 3 X lOI Xe+/cm*, and the structure remained stable with increasing the irradiation dose to the highest dose in this study. Fig. 3 exhibits a SAD pattern of the fee-2 phase. Table 3 gives the indexing results of the, fee-2 phase and the lattice parameter was of 3.8 I rt 0. I.5 A. The
I
Phase changes of Ta-Ti “Amor.“
(4
multilayered
stands for complete
Dose
Ta,,Ti , ,
3
Ta + Ti
7
Ta + Ti
IO
fee-
films
amorphous
I + amor.
upon 200 LeV xenon
ion mixing
at room temperature.
(The
doses are in lOI
phase.)
Ta,;Ti Lb fee- I fee- 1 fee- 1
Ta,,%,
Ta2,Ti7,
fee-2
fee-2
Ta,Tig2 fee-2
fee- 2
fee-2
fee-2
fee-2
fee-2
fee-2
Xe’/cm’,
Y.G. Chen et al./Nucl.
525
Instr. und Meth. in Phys. Res. B 124 (1997) 523-527
Table 2 Identification of the fee-1 phase formed in the Ta,,Ti,, multilayered films by room temperature 200 LieVion irradiation to a dose of 1 X 10” Xe+/cm*. (I~~~_, = 3.91 ;i
2.21 I .95 I.38 1.19 1.13 0.98 0.90 0.87 0.80
(hkl)
4,lC (A)
Intensity
Ill 200 220 311 222 400 331 420 422
2.26 1.96 1.38 1.18 1.13 0.98 0.90 0.87 0.79
strong strong strong medium medium weak medium medium medium
Fig. 3. A typical SAD pattern of the fee-2 phase formed in Ta>,Ti,, films upon IM at a dose of 3X lOI Xe+/cm’.
fee-2 phases were also obtained in the Ta,,Ti,, and TasTi,, films upon ion irradiation and their lattice parameters were both of 3.80 f 0.15 A. 3.3. Discussion Following the suggestion of Miedema et al. [8,9], the free-energy change of the formation of a solid solution and amorphous phases can be written in details. For a solid solution phase, AC;,=AH,C+AH,e+AH;-TAS,
(1)
in which AH’ is the chemical contribution and it can be given by AH’=Mx AAV2/‘f AB, where M is an amplitude reflecting the magnitude of the electron redistribution interaction, xA and VA are the atomic concentration and volume of atom A, and fAB is a function accounting for the degree to which atoms A are surrounded by atoms B; AH’ is the elastic contribution and can be derived from AH’ =xAxa(xaLAinB +xALainA>, where Li in j is the elastic contribution to the heat of solution i in j, A Hs is the structural contributions and the expression is A Hs =
E(F) - x,E,(Z,) -x,E,(Z,), where 2 is the average number of electrons per atom and E, EA and E, are the lattice stabilities of the alloy, the pure component A and the pure component B, respectively; AS is the entropy change and can be taken as AS = -I?[ x,ln( xA) + x,In(x,)], in which R is the gas constant. For an amorphous phase, AC,=
AHc +
~(x,T,,,
+x~T,.,)
-
TA,s,
(2)
where cr is an empirical constant equal to 3.5 J/(mol K) and T,,; is the melting point of the component metal i (i = A, B). According to Gerkema et al. [l I], the interfacial free energy was calculated as A’%,, = S, x a,, ,
(3)
in which S, is the surface area occupied by one mole interfacial atoms; cAB is the interfacial energy between A and B metals and can be estimated as a,, = 0.15( aA,0 + aB.O) + ~h~~;:v~ic~;c~t=;;~ energy of metal i, and uchchem Fig. 4 is a calculated Gibbs-free energy diagram of the Ta-Ti system. The values of the parameters used in the calculation were listed in Table 4 [8,12]. Fig. 4 shows that
Table 3 Indexing results of the fee-2 phase formed in the TaZ,Ti,, multilayered films at a dose of I X IO” Xe+/cm’ and the lattice = 3.81 i parameter was Indexed lo be ~~~~~~ de,,
(a)
@I
Fig. 2. (a) and (b) are the typical SAD patterns of the Ta,,Ti,, as-deposited films and the fee-I phase at a dose of 3X lOi Xe+ /cm’. respectively.
2.20 1.89 I.35 1.15 1.10 0.95 0.88 0.85 0.78
(A)
(hkl)
4,,,
III
2.20 1.91 I.35 1.15 1.10 0.95 0.87 0.85 0.78
200 220 311 222 400 331 420 422
(9
Intensity strong strong strong medium medium weak weak medium medium
Y.G. Chen et al./Nucl.
526
Instr. ad
Meth. in Phys. Res. B 124 (1997) 523-527
estingly, in some other Ta-based alloy systems, amorphous alloys were also obtained at the Ta-rich side, e.g. Ta-Pu amorphous alloys formed by sputtering technique [ 131, Ta,,Au,, and Ta,,Cu,, amorphous films synthesized both by solid-state reaction [ 14,151. The next issue is the growth kinetics of the two fee phases upon ion mixing. The fee-I phase that emerged at the Ta end, was formed through a transformation of bccto-fee, which seemed a little difficult. According to a proposed two-step reverse martensitic phase transfotmation mechanism of bee -+ hcp + fee [l6], the calculated lattice parameter of the fee-I phase was ufcc_, = 3.93 A, which was in agreement with the experimental result. For the fee-2 phase, according to a very recent report [17] that sputtered Ti films possessed fee structure instead of its normal hcp structure and the lattice parameter of the new fee structure was indexed to be afcc = 4.20 .& According to Vegard’s law, a lattice parameter of a fee Ti-based solid solution can be calculated as a = (3.90 I0.2) A, which was compatible with the present results by IM. The Ti-rich fee-2 phase was therefore considered as a fee Ti-based super-saturated solid solution. Incidentally, it is noted that there were two crystalline phases reported in the literature [ 181 and their structures were of hexagonal and orthorhombit, respectively. We therefore concluded that the above two fee structured metastable phases were new ones in this Ta-Ti system.
Multilayer
40
Ta
TI concentratm
Fio TL:Ti
WJ
60
II
(at 90)
4. A typical calculated Gibbs-free-energy
diagram
of the
system. The dashed line shows the initial energetic levels
of the as-deposited multilayers.
the free energy of the amorphous phase and the two terminal solid solutions is well above that of the reference state (an equilibrium mixture of the two constituent metals), implying an unfavorable thermodynamic condition for alloying in the system. However, IM begins with multilayered films consisting definitely of a certain number of interfaces and the atoms in the interfacial layers possess some extra free energy. The interfacial free energy contribution therefore was taken into account to the above calculation of the free energy and it did elevate the multilayered films to a state of higher free energy than those of the amorphous and other crystalline phases involved, thus providing a necessary driving force for forming Ta-Ti alloys. From the figure, one can see that the Ta content range favoring amorphization is at Ta-rich side, which is in good agreement with the IM experimental results. On the other hand, it is worthwhile mentioning that the absence of the amorphous phase in Ti-rich composition side upon IM was due to the competition between the amorphous and the existing me&table fee phase of simple structure, which was able to nuclei and grow, thus resulting in hindering amorphization at this side. It is therefore believed that the free energy calculation can serve as a semi-quantitative explanation for amorphization in the Ta-Ti system. Inter-
4. Concluding
remarks
(I) Amorphous alloys were formed by ion mixing in the Ta-Ti system, though the positive A Hf, a small size difference and a small difference in electronegativity have been considered as unfavored factors in conventional glass forming techniques. (2) A Gibbs-free-energy diagram of the Ta-Ti system was established by calculating the free-energy curves of the multilayered films and other related phases. The calculated diagram, including the consideration of interfacial energy for the multilayers, can give an appropriate interpretation to the formation of the metastable phases.
Table 4 Values of the parameters used in Gibbs-free-energy Parameter
T,, [K]
Ta
3287
Ti
1943
Amorphous
-
As-deposited
-
M [kJ/(mol -
cm’f]
V’/’
calculation in the Ta-Ti [cm*]
y
system. I’-.’
means the data were not used L,, inri [U/mol]
S, [ IO5 m’]
u. [mJ/m*J
Lri inra [k.J/mol]
4.89
_
3150
_
2.20
4.82
_
2100
_
5
-
2.16 _
1.14 _
i-5
+6
_
+5
+6
_
films Solid solution
_
1.14
0
-
Y.G. Chen et al./Nucl.
Instr. and Meth. in Phys. Res. B 124 (1997) 523-527
Acknowledgements The authors are grateful to the National Natural Science Foundation of China and the Administration of Tsinghua University for the financial aid to this study.
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