- Email: [email protected]
On the phase formation during ion beam mixing of Al-Ti
Nuclear Instruments and Methods in Physics Research 1380/81 (1993) 390-39' North-Holland
Beam Interactions with Material's â.Atom.
On the phase formation during ion beam mixing of Al-Ti K- Kyllesbech Larsen
a,
S. Skovmand
a,
N . Karpe b, 3 . Bottiger
a
and R . Bormann
e
'° Institute cif Physics and Astronomy, University of Aarhus, DK-8000, Aarlurs C, D, ..ark' Department of Solid State Physics, Royal Institute of Technology, S-100 44. Steckhohn, Sweden " Institute for Materials Research, GKSS-Research Center, P.D. Box 1160, D-2054 Geesthacht, Gennaht'
The phase formation during ion beam mimig of elemental multilayered Al, _,Ti, films has been mvestigatod . The irradiations were carried o-.t with 500 keV Xe` ion_.. The whole composttionai ringe has been covered for substrate temperatures between I(10 terminal solid solutions of hip and fcc were found. Nonequilibriurn solubilit ;es of and 500 K. Between 300 and 500 K only extended fee were observed. At lower temperature an amorphous phase v as found to form for up to 60 at .% AI to hip and 25 at .^S Ti in AI-rich compositions. Gibbs free energies were calculated using the CALPHAD method, and metastublc phase diagrams were constructeà . Reasonable agreement between the predicted phase formation estimated from free energie, aad the experimental results was found . However, to thermodynamically justify the formation of the amorphous phase at low temperatures, the estimated free energies of the terminal solid solutions at low temperatures had to be increased relative to the amorphous phase, which is attributed to radiation induced defects . 1 . Introduction Eleavy-ion irradiation and ion beam mixing at low temperatures have become widely used methods to form metastable alloys . However, the detailed mechanisms involved in the phase formation is not well understood. Simulations [1,21 have shown the occurrence of atomic collision casçades whit subsequent therm~, : spikes . In these ti;crmal spikes, large diffusion-like mast, transport takes place during a short time ( « 1 ns) before couiing down to ambient temperature. At low substrate temperature, the phase formation can be assumed to be limited in time and space to the collision cascades, because only there sufficient atomic mobility may be achieved for nucleation and growth of oev pha".;cs . The formation of structurally complicated compounds, often present in the equilibrium phase diagrams of binary alloy systems Hith large negative heats of mixing, has generally been found to be suppressed during low temperature heavy-ion irradiation [3-8] . Instead, structurally simple phases like the amorphous phase, bcc, fee and hip solid solutions, and in some cases the simplest types of ordering of these as for example the C, CI (B2) structure or the Cu,,Au (Ll,) structure are formed [8-15]. 1 hcse experimental findings have been attributed 13,4,15] to difficulties in nucleation and growth of structurally complicated and ordered phases during the short time available before cooling down of a collision ca , cadc . Thermodynamically new phases can only grow if the Gibbs free energy of the system is lowered by their
formation . Thus, if kinetics allow, the phase c r phases giving the lowest Gibbs free energy should always be expected to form. From this point of view, during mixing of elemental multilayers, amorphization should only be expected if the nucleation and growth is hindered for all phases having a lower Gibbs free energy than the amorpho!-c phase . This requirement has formed ihc basis for several att milts to predict amorphization from known or est :~ira~~ : thermodynamic data (10-17]. A common approach in predicting the metastable phase formation has beer, t, use semiempirical models, like that of Micdema [18] or CALPHAD calculations (19,20], to estimate Gibbs free energies for all relevant phases . Among the. simple phases which may nucleate and grow, the ones which minimize the Gibbs free energy of the system are predicted to form . The minimalization can be performed using the restriction that the composition should be homogeneous throughout the entire volume (a polymorphous construction) or allowing for full compositional segregation (the common-tangent construction). For several binary metallic alloys, like Ni-Zr [12,10], Co-Zr [12,16], Fc-Zr [12,16], AI-1Vb (13] and AI-Ni [15], this method has successfully been used to predict the compositional ranges for which amorphization is achieved by low temperature ion beam mixing using heavy ions . It should, how, .ver, be remarked that this method heavily rely on available thermodynamic data, which in many cases a , e not accurately known . During heavy-ion irradiation, a large number of crystal defects are created. Their p-esence will increase the Gibbs
0168-583X/93/$06.(1) © 1993 - Elsevier Science Publishers B.V, All rights reserved
K Krllesbcclr Larsen cr ,rl. / Plinse for"narron du , rrrg wa beam -ngof ,11-T,
391
free energy, particularly at low temperatures where defect annihilation is slow . Even in cases where relatively good estimates of the thermodynamic data fc. the unirradiated phases are available, the important and very complicated problem of how to adjust them
properly to take rronequilibrium concentrations of defects into account has so far not been solved . In this work, we have investigated the low temperature phase formation in the AI-Ti system during ion beam mixing with 500 keV Xe' ions . Only three simfee ple phases, hop, and the amorphous phase, are ob~crvcd between 100 and 500 K. CALPHAD calculations show that the awuipltads pltdse at no compositian has the lowest free energy of the three. In spite of
this, amorphization is observed during mixing at 100 and 200 K. Thermodynamically, this can only be achieved if the free energies of the crystalline solid
solutions of fcc and hcp are increased relative to the amorphous phase at the lowest temperatures, due to radiation induced defects. 2. Experimental details Using two independent electron-gun evaporation sources, the AI-Ti thin film multilayers were produced. The purim:r of the AI and Ti source materials
were 3N and 5N, respectively . A typical fi!nr consisted of about 20 layers having a total thick,tess of about 800 A, the individuae layers no'. _;seeding 60 f1. Thr; pressure, measured before a,id after the depositions, did not exceed 2 x 10 fi Pa. Sapphire single crystals partly covered with a thin NaCI layer were used as suhstratcs. Using Ruthprford tiaer" scattering spectrometry (RBS), the film compositions were measured wi+h an --
accuracy of 2% and the homogeneity was checked . Irradiation with 500 keV Xc t ions were performc-d in
Fig. 1 . Diffraction patterns (a) Ice solid solution (Tir3 Ai s7, IM w th 2x IOr° Xe'/cm` at 290 K) ; (b) the amorphous phase (Ti-A'781 IM with 2x10"' Xe'/cm` at 114 FJ ; (c) hcp solid solution (Ti 44 A1 5n, IM with 2X 1016 Xe+/tM2 at 200K); (d) hcp+fcc solid solutions (Ti, 4 Al ,, IM with ":x 10" Xe'/cm' at 4tH) K). identified. Fig. 2 sum-iarize- the experinrcntai results,, sh .rwinr the phases formed during ion beam mixing for the r stole composition range and substrate temperatures between 100 and 500 K. In this temperature
range, only terminal solid solutions, fee for the AI-rich alloys and hcp for the Ti-rich alloys, and the amorphous phase appear ; no intermetallic compounds are
5
a vacuum of about 4 x 10- Pa, with a dose of 2 x :0"' ions /cm` to ensure complete mixing of the elemental layers . The current was kept below 0.8 p.A/cm and the samples were mounted with heat conducting paste in order to avoid sample heating during trie irradiation.
- .0c
2
After irradiations, the parts of the films having a thin NaCl layer between the film and the sapphire were floated off in de-ionized water and mounted on copper grids. Using a Philips CNî-20 (200 kV) tra,tsmission electron microscope (TEM), the phases formed during the irradiation were identified . The amorphous phase exhibited featureless micrographs and broad diffraction haloes.
x
Typical diffraction patterns arc shown in fig . 1, from which the phases formed during ion beam mixing were
.T1
0 E-
1
-
0
0
0
0
0
ox
0.
400
0
0
0
O
0
ox
oX
0
0
0
O
0
0 ox
,c
x
0
O
0
0
0
x
x
0
0
0
0.
.x
x
c. y
a
~
500
200 100 .
0
3. Experimental results and thermodynamic calculations
--
0
0
. .
20 40 60 80 A1 - Concentration [at .%j -
2-11
100
Fig . 2. The phase formation observed at room temperature of Ti-AI after 500keV Xe' irradiation as a function of composition and substrate temperatur, The. symbols denote (0) hcp-Ti(à.0, (a) the amorphous phase, and (x) fcc-AI(Ti). Tic. METAL MODIFICATION (c)
392
K Kyllesbech Larsen et al. / Phase formation during ionbeam miring of AI-7ï Gibbs free energies are very often small and in practice at low temperatures the metastable phase diagrams are quite independent of temuerature . Only when some phases have very different entropies or are very close in energy do temperature considerations have to be taken into vccount. how~c , +t -hould be noted that the calculation of the free energies is base nn the experimentally deter-
mined thermodynamic data obtained on equilibrium phases or on metastable phases, whose properties are not influenced by nonequilibrium concentrations of structural defects . Therefore, applications of the calculates face energy curves to highly nonequilibrium processes sdch as irradiation have to be undertaken with caution. Nevertheless, for irradiation temperatures between Pig. 3. The Dibbs free energy of various Ti-Al phases at 6,73 BC calculated by the CALPÜAD method. The top of the upper part shows a section of the polymorphous phase diagra.n with only the hcp and fcc solid solutions. Also, in the bottom of the upper part, a section of the common-tangent constructed phase diagram is shown.
observed . Both single-Nh?se and two-phase regions are observed . In crag to understand the phase formation and its correlation to thermodynamic parameters, the Gibbs free energies were calculated using the CALPHAD method for the various stable and metastable phases [,19,20]. The calculations and a discussion are described in more detail in ref. [21]. The amorphous phase is treated as an extension of the undercooled liquid be-
300 and 500 K a reasonable agreement exists between the experimentally observed phase formation (fig . 2) and Gibbs free energy curves in fig . 3, in particular if the common-tangent rule is applied in order to defee scribe the coexistence of the hcp and the solid solutiors. The experimentally observed concentration limits of the phase regions (hcp, fcc and hcp + fee) agree within a few % to that uepacted in fig. 3, the
slightly higher Al concentrations of the experimentally observed two-phase field, in particular at higher irradiation temperatures, is most probably related to uncertainty in calculating the stability of the fee phase, wA»ch
exhibits only a very narrow homogeneity range in the equilibrium state. The good agreement between the experimental results and the thermodynamic calculations indicates that with respect to equilibrium state the relative stabilities of hcp and fcc solid solutions are not substantially
llow the glass temperature. The results are shown in fig . 3, where the compo.Aion dependence of the Gibbs free energy is displayed for the various phases of the
altered by irradiation at temperatures between 300 and 500 K. Further, one can conclude that the free energy of the amorphous phase has to be higher than those of
Al-Ti system at 673 K. Above the curves, sections of two metastable phase diagrams considering only the fee, hcp and amorphous ptiases corresponding to a polymorphous construction ltop) and a common-tan-
the hcp and fcc solid solution, in -gfeement with thermodynamic calculations . Otheewise, the amorph,us phase should be iormed in this temperature ran, , As the . amorphous phase has a higher entropy
gent construction (bottom) are shown.
than the crystalline solid solutions it should be less stable at 'tower temperatures . Therefore, the formation of the amorphous phase cannot be explained by tem-
4. Discussion
perature dependence of the thermodynamic functions . Only if the free energies of the crystalline solid solutions are increased relative to the amorphous phase at !ow tempe,s.urcs, can the formation of the amorphous
When comparing the experimental phase Formation with the predictions from a metastablc phase diagram, the temperature scales cannot be directly compared. During low temperature ion irradiation the phase formation is Eelieved to take place during the cooling down of the cascaaes to ambient temperature . Thus the phase formation occurs at an elevated temperature compared to the ambient temperature at which the substrate was held during the experiment. Fortunately, the differences in the temperature dependence of the
phase be justuieu th ;" rmod,n,tmically. This effect should be more important . .t low temperatures where the annihilation of radiation induced defects is slow.
Besides structural detects such as vacancies, dislocations and grain boundat ies also chemical disorder may be introduced by irradiation. In solid solution phases the latter is related to a decrease of short-range order which is especially expected in alloy systems like Ti-AI
K. Kyllesb-°h Lcr- : et al /Phase formation during ion beam mixing ofAI-Ti
exhibiting a high enthalpy of mixing for solid solutions. By increasing the free energy of the crystalline solid solutio-s at low temperatures by a few kJ/ g-at . in order to take this effect into account, the formation of the amorphous phase can be predicted in the experimentally observed concentration range. It is interesting to nose that similar composition -n;es for metastable phase formation have been observed by rechanical alloying of Ti-Al powder blends and ball milling of Ti-a[un.inides In particular, hcp solid solutions have been observed for AI concentrations up to 60 at.%, whereas for AI contents higher than 75 at.% the fee structure is formed upon milling . By lowering the milling intensity partial amorphization is achieved, in particular for the composition of 50 at.% Al, which is within. ihc range of amorphous phase formation observed by lo-temperature ion irradiation . A detailed comparison of the results and the conclusions with respect to mechanism of phase selection in both processes will be published in a separate paper. Acknowledgement One of us (N .K .) gratefully acknowledges support from the Swedish Natural Research Council (NFR). References [i] M.W . Gurnian and J.H. Kinney, J. Nucl. Mater. 103/104 (1981) 1319. [2] W .E . King and R . Benedek, J . Nucl . Mater . 117 (1983) 26 . [3] M . Nastasi and J .W. Mayer, Mater . Sci . Rep. 6 (1991) 1 .
393
[4] J .L . Brimhall, H.E. Kissinger and L .A. Charlot, Radiat . Eff. 77 (1983) 237. [5] L S . Hung, M. Nastasi, J . Gyulai and J.W . Mayer, Appl . Phys . Leu . 42 (1983) 672 . [6] M . Nastasi, D . Lillienfeld, H .H. Johnson and J .W. Mayer, Appl . Phys. Lett. 42 (1986) 672 . [7] O .Z . Hong, D .A. Lillienfeld and J.W . Mayer, J . Appl. Phys. 64 (1988) 4478. [8] D .M. Follstaedt and J .A. Knapp, J . Appl. Phys . 59 (1986) 1756. [9] K . Hohmuth, V. Hegira and B . Rauschenberg, Nucl . Imtr . ane Meth . B39 (1989) 100 . [10] C . Jaouen, J .P. Riviere and J . Delafond, Nucl . Instr. and Meth. B19/20 (1980 549. [I1] C . Jaouen, J.P . Riviere, J. Delafond, L. Thome, F. Pons, R. Danielou, J . Fon:enille and E . Ligeon, J . Appl . Phys . 65 (1989) 1499. [12] J . Bottiger, K . Dyrbye, K. Pampers and R . Poulsen, Philos . Mag A59 (1989) 569 . [13] K. Pampus, K . Pyrbye, B . Torp and R. Bormann, J. Mater. Res . 4 (1989) 1385. [14] L.U . Aaen Andersen, J. Brettiger and K. Dyrbye, Nucl. Instr . and Meth . B51 (1990) 125 . [15] K . Kyllesbech Larsen, N . Karge, J .B. Bottiger and R . Bormann, J. Mater . Res . 7 (1992) 861 . [16] J .M . Lopez, J .A. Alonso and L .J. Gallego, Phys. Rev . B36 (1987) 3716 . [17] H .M . Fe ;nandez, M . Barricco, L. Batteiazzi and L.J . Gallego, J . Alloys Comp. 184 (1992) 139. [18] F .R. de Boer, R. Mattgins, W.C.M. Miedema and A.R. Niessen, Cohesion in Metais (North-Holland, Amsterdam, 1989). [19] R . Bormann, F. Gärtner and K. Zöltzer. J 1,ess-Common Met. 140 (1988) 335. [20] R . Bormann and K . Zöltzer, Phys. Status Solidi A131 (1992) 691 . [21] M . Oehring, T . Klassen and R . Bormann, submitted to J. Mater. Res.
Ilc . METAL MODIFICATION (0
Beam Interactions with Material's â.Atom.
On the phase formation during ion beam mixing of Al-Ti K- Kyllesbech Larsen
a,
S. Skovmand
a,
N . Karpe b, 3 . Bottiger
a
and R . Bormann
e
'° Institute cif Physics and Astronomy, University of Aarhus, DK-8000, Aarlurs C, D, ..ark' Department of Solid State Physics, Royal Institute of Technology, S-100 44. Steckhohn, Sweden " Institute for Materials Research, GKSS-Research Center, P.D. Box 1160, D-2054 Geesthacht, Gennaht'
The phase formation during ion beam mimig of elemental multilayered Al, _,Ti, films has been mvestigatod . The irradiations were carried o-.t with 500 keV Xe` ion_.. The whole composttionai ringe has been covered for substrate temperatures between I(10 terminal solid solutions of hip and fcc were found. Nonequilibriurn solubilit ;es of and 500 K. Between 300 and 500 K only extended fee were observed. At lower temperature an amorphous phase v as found to form for up to 60 at .% AI to hip and 25 at .^S Ti in AI-rich compositions. Gibbs free energies were calculated using the CALPHAD method, and metastublc phase diagrams were constructeà . Reasonable agreement between the predicted phase formation estimated from free energie, aad the experimental results was found . However, to thermodynamically justify the formation of the amorphous phase at low temperatures, the estimated free energies of the terminal solid solutions at low temperatures had to be increased relative to the amorphous phase, which is attributed to radiation induced defects . 1 . Introduction Eleavy-ion irradiation and ion beam mixing at low temperatures have become widely used methods to form metastable alloys . However, the detailed mechanisms involved in the phase formation is not well understood. Simulations [1,21 have shown the occurrence of atomic collision casçades whit subsequent therm~, : spikes . In these ti;crmal spikes, large diffusion-like mast, transport takes place during a short time ( « 1 ns) before couiing down to ambient temperature. At low substrate temperature, the phase formation can be assumed to be limited in time and space to the collision cascades, because only there sufficient atomic mobility may be achieved for nucleation and growth of oev pha".;cs . The formation of structurally complicated compounds, often present in the equilibrium phase diagrams of binary alloy systems Hith large negative heats of mixing, has generally been found to be suppressed during low temperature heavy-ion irradiation [3-8] . Instead, structurally simple phases like the amorphous phase, bcc, fee and hip solid solutions, and in some cases the simplest types of ordering of these as for example the C, CI (B2) structure or the Cu,,Au (Ll,) structure are formed [8-15]. 1 hcse experimental findings have been attributed 13,4,15] to difficulties in nucleation and growth of structurally complicated and ordered phases during the short time available before cooling down of a collision ca , cadc . Thermodynamically new phases can only grow if the Gibbs free energy of the system is lowered by their
formation . Thus, if kinetics allow, the phase c r phases giving the lowest Gibbs free energy should always be expected to form. From this point of view, during mixing of elemental multilayers, amorphization should only be expected if the nucleation and growth is hindered for all phases having a lower Gibbs free energy than the amorpho!-c phase . This requirement has formed ihc basis for several att milts to predict amorphization from known or est :~ira~~ : thermodynamic data (10-17]. A common approach in predicting the metastable phase formation has beer, t, use semiempirical models, like that of Micdema [18] or CALPHAD calculations (19,20], to estimate Gibbs free energies for all relevant phases . Among the. simple phases which may nucleate and grow, the ones which minimize the Gibbs free energy of the system are predicted to form . The minimalization can be performed using the restriction that the composition should be homogeneous throughout the entire volume (a polymorphous construction) or allowing for full compositional segregation (the common-tangent construction). For several binary metallic alloys, like Ni-Zr [12,10], Co-Zr [12,16], Fc-Zr [12,16], AI-1Vb (13] and AI-Ni [15], this method has successfully been used to predict the compositional ranges for which amorphization is achieved by low temperature ion beam mixing using heavy ions . It should, how, .ver, be remarked that this method heavily rely on available thermodynamic data, which in many cases a , e not accurately known . During heavy-ion irradiation, a large number of crystal defects are created. Their p-esence will increase the Gibbs
0168-583X/93/$06.(1) © 1993 - Elsevier Science Publishers B.V, All rights reserved
K Krllesbcclr Larsen cr ,rl. / Plinse for"narron du , rrrg wa beam -ngof ,11-T,
391
free energy, particularly at low temperatures where defect annihilation is slow . Even in cases where relatively good estimates of the thermodynamic data fc. the unirradiated phases are available, the important and very complicated problem of how to adjust them
properly to take rronequilibrium concentrations of defects into account has so far not been solved . In this work, we have investigated the low temperature phase formation in the AI-Ti system during ion beam mixing with 500 keV Xe' ions . Only three simfee ple phases, hop, and the amorphous phase, are ob~crvcd between 100 and 500 K. CALPHAD calculations show that the awuipltads pltdse at no compositian has the lowest free energy of the three. In spite of
this, amorphization is observed during mixing at 100 and 200 K. Thermodynamically, this can only be achieved if the free energies of the crystalline solid
solutions of fcc and hcp are increased relative to the amorphous phase at the lowest temperatures, due to radiation induced defects. 2. Experimental details Using two independent electron-gun evaporation sources, the AI-Ti thin film multilayers were produced. The purim:r of the AI and Ti source materials
were 3N and 5N, respectively . A typical fi!nr consisted of about 20 layers having a total thick,tess of about 800 A, the individuae layers no'. _;seeding 60 f1. Thr; pressure, measured before a,id after the depositions, did not exceed 2 x 10 fi Pa. Sapphire single crystals partly covered with a thin NaCI layer were used as suhstratcs. Using Ruthprford tiaer" scattering spectrometry (RBS), the film compositions were measured wi+h an --
accuracy of 2% and the homogeneity was checked . Irradiation with 500 keV Xc t ions were performc-d in
Fig. 1 . Diffraction patterns (a) Ice solid solution (Tir3 Ai s7, IM w th 2x IOr° Xe'/cm` at 290 K) ; (b) the amorphous phase (Ti-A'781 IM with 2x10"' Xe'/cm` at 114 FJ ; (c) hcp solid solution (Ti 44 A1 5n, IM with 2X 1016 Xe+/tM2 at 200K); (d) hcp+fcc solid solutions (Ti, 4 Al ,, IM with ":x 10" Xe'/cm' at 4tH) K). identified. Fig. 2 sum-iarize- the experinrcntai results,, sh .rwinr the phases formed during ion beam mixing for the r stole composition range and substrate temperatures between 100 and 500 K. In this temperature
range, only terminal solid solutions, fee for the AI-rich alloys and hcp for the Ti-rich alloys, and the amorphous phase appear ; no intermetallic compounds are
5
a vacuum of about 4 x 10- Pa, with a dose of 2 x :0"' ions /cm` to ensure complete mixing of the elemental layers . The current was kept below 0.8 p.A/cm and the samples were mounted with heat conducting paste in order to avoid sample heating during trie irradiation.
- .0c
2
After irradiations, the parts of the films having a thin NaCl layer between the film and the sapphire were floated off in de-ionized water and mounted on copper grids. Using a Philips CNî-20 (200 kV) tra,tsmission electron microscope (TEM), the phases formed during the irradiation were identified . The amorphous phase exhibited featureless micrographs and broad diffraction haloes.
x
Typical diffraction patterns arc shown in fig . 1, from which the phases formed during ion beam mixing were
.T1
0 E-
1
-
0
0
0
0
0
ox
0.
400
0
0
0
O
0
ox
oX
0
0
0
O
0
0 ox
,c
x
0
O
0
0
0
x
x
0
0
0
0.
.x
x
c. y
a
~
500
200 100 .
0
3. Experimental results and thermodynamic calculations
--
0
0
. .
20 40 60 80 A1 - Concentration [at .%j -
2-11
100
Fig . 2. The phase formation observed at room temperature of Ti-AI after 500keV Xe' irradiation as a function of composition and substrate temperatur, The. symbols denote (0) hcp-Ti(à.0, (a) the amorphous phase, and (x) fcc-AI(Ti). Tic. METAL MODIFICATION (c)
392
K Kyllesbech Larsen et al. / Phase formation during ionbeam miring of AI-7ï Gibbs free energies are very often small and in practice at low temperatures the metastable phase diagrams are quite independent of temuerature . Only when some phases have very different entropies or are very close in energy do temperature considerations have to be taken into vccount. how~c , +t -hould be noted that the calculation of the free energies is base nn the experimentally deter-
mined thermodynamic data obtained on equilibrium phases or on metastable phases, whose properties are not influenced by nonequilibrium concentrations of structural defects . Therefore, applications of the calculates face energy curves to highly nonequilibrium processes sdch as irradiation have to be undertaken with caution. Nevertheless, for irradiation temperatures between Pig. 3. The Dibbs free energy of various Ti-Al phases at 6,73 BC calculated by the CALPÜAD method. The top of the upper part shows a section of the polymorphous phase diagra.n with only the hcp and fcc solid solutions. Also, in the bottom of the upper part, a section of the common-tangent constructed phase diagram is shown.
observed . Both single-Nh?se and two-phase regions are observed . In crag to understand the phase formation and its correlation to thermodynamic parameters, the Gibbs free energies were calculated using the CALPHAD method for the various stable and metastable phases [,19,20]. The calculations and a discussion are described in more detail in ref. [21]. The amorphous phase is treated as an extension of the undercooled liquid be-
300 and 500 K a reasonable agreement exists between the experimentally observed phase formation (fig . 2) and Gibbs free energy curves in fig . 3, in particular if the common-tangent rule is applied in order to defee scribe the coexistence of the hcp and the solid solutiors. The experimentally observed concentration limits of the phase regions (hcp, fcc and hcp + fee) agree within a few % to that uepacted in fig. 3, the
slightly higher Al concentrations of the experimentally observed two-phase field, in particular at higher irradiation temperatures, is most probably related to uncertainty in calculating the stability of the fee phase, wA»ch
exhibits only a very narrow homogeneity range in the equilibrium state. The good agreement between the experimental results and the thermodynamic calculations indicates that with respect to equilibrium state the relative stabilities of hcp and fcc solid solutions are not substantially
llow the glass temperature. The results are shown in fig . 3, where the compo.Aion dependence of the Gibbs free energy is displayed for the various phases of the
altered by irradiation at temperatures between 300 and 500 K. Further, one can conclude that the free energy of the amorphous phase has to be higher than those of
Al-Ti system at 673 K. Above the curves, sections of two metastable phase diagrams considering only the fee, hcp and amorphous ptiases corresponding to a polymorphous construction ltop) and a common-tan-
the hcp and fcc solid solution, in -gfeement with thermodynamic calculations . Otheewise, the amorph,us phase should be iormed in this temperature ran, , As the . amorphous phase has a higher entropy
gent construction (bottom) are shown.
than the crystalline solid solutions it should be less stable at 'tower temperatures . Therefore, the formation of the amorphous phase cannot be explained by tem-
4. Discussion
perature dependence of the thermodynamic functions . Only if the free energies of the crystalline solid solutions are increased relative to the amorphous phase at !ow tempe,s.urcs, can the formation of the amorphous
When comparing the experimental phase Formation with the predictions from a metastablc phase diagram, the temperature scales cannot be directly compared. During low temperature ion irradiation the phase formation is Eelieved to take place during the cooling down of the cascaaes to ambient temperature . Thus the phase formation occurs at an elevated temperature compared to the ambient temperature at which the substrate was held during the experiment. Fortunately, the differences in the temperature dependence of the
phase be justuieu th ;" rmod,n,tmically. This effect should be more important . .t low temperatures where the annihilation of radiation induced defects is slow.
Besides structural detects such as vacancies, dislocations and grain boundat ies also chemical disorder may be introduced by irradiation. In solid solution phases the latter is related to a decrease of short-range order which is especially expected in alloy systems like Ti-AI
K. Kyllesb-°h Lcr- : et al /Phase formation during ion beam mixing ofAI-Ti
exhibiting a high enthalpy of mixing for solid solutions. By increasing the free energy of the crystalline solid solutio-s at low temperatures by a few kJ/ g-at . in order to take this effect into account, the formation of the amorphous phase can be predicted in the experimentally observed concentration range. It is interesting to nose that similar composition -n;es for metastable phase formation have been observed by rechanical alloying of Ti-Al powder blends and ball milling of Ti-a[un.inides In particular, hcp solid solutions have been observed for AI concentrations up to 60 at.%, whereas for AI contents higher than 75 at.% the fee structure is formed upon milling . By lowering the milling intensity partial amorphization is achieved, in particular for the composition of 50 at.% Al, which is within. ihc range of amorphous phase formation observed by lo-temperature ion irradiation . A detailed comparison of the results and the conclusions with respect to mechanism of phase selection in both processes will be published in a separate paper. Acknowledgement One of us (N .K .) gratefully acknowledges support from the Swedish Natural Research Council (NFR). References [i] M.W . Gurnian and J.H. Kinney, J. Nucl. Mater. 103/104 (1981) 1319. [2] W .E . King and R . Benedek, J . Nucl . Mater . 117 (1983) 26 . [3] M . Nastasi and J .W. Mayer, Mater . Sci . Rep. 6 (1991) 1 .
393
[4] J .L . Brimhall, H.E. Kissinger and L .A. Charlot, Radiat . Eff. 77 (1983) 237. [5] L S . Hung, M. Nastasi, J . Gyulai and J.W . Mayer, Appl . Phys . Leu . 42 (1983) 672 . [6] M . Nastasi, D . Lillienfeld, H .H. Johnson and J .W. Mayer, Appl . Phys. Lett. 42 (1986) 672 . [7] O .Z . Hong, D .A. Lillienfeld and J.W . Mayer, J . Appl. Phys. 64 (1988) 4478. [8] D .M. Follstaedt and J .A. Knapp, J . Appl. Phys . 59 (1986) 1756. [9] K . Hohmuth, V. Hegira and B . Rauschenberg, Nucl . Imtr . ane Meth . B39 (1989) 100 . [10] C . Jaouen, J .P. Riviere and J . Delafond, Nucl . Instr. and Meth. B19/20 (1980 549. [I1] C . Jaouen, J.P . Riviere, J. Delafond, L. Thome, F. Pons, R. Danielou, J . Fon:enille and E . Ligeon, J . Appl . Phys . 65 (1989) 1499. [12] J . Bottiger, K . Dyrbye, K. Pampers and R . Poulsen, Philos . Mag A59 (1989) 569 . [13] K. Pampus, K . Pyrbye, B . Torp and R. Bormann, J. Mater. Res . 4 (1989) 1385. [14] L.U . Aaen Andersen, J. Brettiger and K. Dyrbye, Nucl. Instr . and Meth . B51 (1990) 125 . [15] K . Kyllesbech Larsen, N . Karge, J .B. Bottiger and R . Bormann, J. Mater . Res . 7 (1992) 861 . [16] J .M . Lopez, J .A. Alonso and L .J. Gallego, Phys. Rev . B36 (1987) 3716 . [17] H .M . Fe ;nandez, M . Barricco, L. Batteiazzi and L.J . Gallego, J . Alloys Comp. 184 (1992) 139. [18] F .R. de Boer, R. Mattgins, W.C.M. Miedema and A.R. Niessen, Cohesion in Metais (North-Holland, Amsterdam, 1989). [19] R . Bormann, F. Gärtner and K. Zöltzer. J 1,ess-Common Met. 140 (1988) 335. [20] R . Bormann and K . Zöltzer, Phys. Status Solidi A131 (1992) 691 . [21] M . Oehring, T . Klassen and R . Bormann, submitted to J. Mater. Res.
Ilc . METAL MODIFICATION (0