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Lattice location and diffusion of W implanted in Mg and Al single crystals
Nuclear Instruments Nuclear Instruments North-Holland
and Methods
in Physics Research
B63 (1992) 283-290
8 Methods
in Physics Research
Sectm B
Lattice location and diffusion of W implanted in Mg and Al single crystals R.C. da Silva a, M.F. da Silva a, I. Khubeis ‘,‘, 0. Meyer ‘, A.A. Melo b and J.C. Soares b a Departamento de Fisica, ICEN/LNETI, E.N. IO-2685 Sacauim, Portugal b Centro de Fisica Nuclear da Universidade de Lisboa, Au. Prof: Gama Pinto, 2-1699 Lisboa Codex, Portugal ’ Kernforschungszentrum Karlsruhe, Institut fiir Nukleare Festkiirperphysik, P.O. Box 3640, W-7500 Karlsruhe, Received
25 August
Germany
1991
Tungsten was implanted in Mg single crystals at 293 and 77 K. A rather high substitutional fraction, f, = 0.8, was observed after implanting W at 293 K with peak concentrations up to 0.3 at.%. With increasing peak concentration up to 2.7 at.% W, f, decreases to 0.3. Implanting 0.3 at.% W at 77 K resulted a f, value of 1, which did not change after warming up to 293 K. Annealing at 553 K causes a recovery of the host lattice, which however does not affect f,. The diffusion of W in Mg is much lower than expected from the Mg self-diffusion coefficient, which may be explained by internal oxidation of the W atoms. In the case of the WA1 system, W is completely substitutional after implantation at 293 and 7 K up to peak concentrations of about 5 at.%.
1. Introduction Al and Mg are light metals which are technologically important for a wide range of applications. Alloying with other elements is a common practice to improve specific properties. In particular, surface properties like wear and corrosion, which to a large extent determine the behavior of these materials in the working environment, have to be improved with different surface treatments. W is of potential interest to alloy with Al or Mg as it is used to improve the corrosion resistance of stainless steel [1,2]. There is also evidence that W can be used as a passivating element in Al [2,3]. It is likely to expect a similar effect for W in Mg. One important condition to improve the corrosion resistance of the host metal is that the passivating element should stay in solid solution at appreciable levels of concentration. If the concentration is too low, e.g. below 1 at.%, its influence on the corrosion properties is negligible [2]. This is a serious constraint in the case of W, since its solid solubility in Al is less than 0.03 at.% close to the melting point and it is insoluble in Mg [4]. This problem can be solved by the ion implantation technique, which is widely used to produce near surface metastable systems, in particular supersatu-
’ Permanent address: Amman, Jordan.
Dept.
of Physics,
Jordan
0168-583X/92/$05.00
0 1992 - Elsevier
Science
University,
Publishers
rated solid solutions [5]. Thus, solid solubility limits can be exceeded, although the resulting system is not in thermodynamic equilibrium and the concentration levels may be brought up to values which can significantly change the corrosion behavior. In order to understand the underlying mechanisms it is of great importance to study the lattice site location of the implanted atoms, its stability and the influence of radiation induced effects, which can control phenomena like diffusion and precipitation. In this respect WMg and WAl are potentially interesting and promising systemsyhich are still poorly understood. Earlier studies on HfMg [6,7] and HfAl [8,9] systems, formed by ion implantation, for which< complete substitutionality was formed and also on HfBe [lo] and WBe [ll] (hcp alloys), where both Hf and w occupy interstitial tetrahedral sites, lead to the expectation that W reveals a behavior similar to Hf, when implanted in Al or Mg. Previous studies on WMg used W implantation at 80 keV [12]. At this implantation energy, W was too close to the surface oxide layer, which in turn may have affected its lattice site location and stability. In the present work new experimental results are presented, for both WMg and WA1 systems, obtained by ion implantation at room temperature and at 77 K. To determine the lattice site location and the activation energy for diffusion of implanted W is the main purpose of this work. B.V. All rights resewed
284
R.C. da Siba et al. / Lattice location and diffusion of implanted W
2. Experimental
High purity Mg and Al single crystals were cut from commercially available single crystal bars and mechanically polished. The Mg (lOi0) oriented single crystals were chemically etched in concentrated (65%) nitric acid and subsequently rinsed in distilled water, The Al (110) single crystals were etched either chemically, in a solution of equal parts by volume of hydrofluoric acid and water and subsequently rinsed in distilled water, or electrochemically in a solution of equal parts by volume of concentrated 65% nitric acid and methyl alcohol and subsequently rinsed in methyl alcohol followed by ethyl alcohol [131. Three of the Mg single crystals were implanted at room temperature with W+ ions of 100, 150 and 300 keV energies to fluenees of 6 X 1014, 1.3 X 1Or5 and 1.3 X lOI6 cmb2, respectively. The W peak concentrations were determined by RBS to be 0.3, 0.3 and 2.7 at.%, respectively. One Al single crystal was implanted at room temperature with 300 keV W ions to a fluence of 1.3 X 1Or6 cm-2 the measured peak concentration was 2.5 at.%. The’Mg crystals implanted at 100 and 150 keV energy were submitted to a vacuum annealing process, starting with 72 hours at 553 K, followed by 80 min at 623 K. The 100 keV implanted crystal was further vacuum annealed at 683 K for 80 min, while the 150 keV implant was annealed at 723 K for 30 min in vacuum and at 783 for 50 min under pure He gas flow. During vacuum annealing the pressure in the annealing chamber was better than lo-’ mbar. The low temperature implantations were carried out at 77 K with 300 keV energy. The samples were mounted on a two axis goniometer, which was coupled to a 350 keV ion implanter for implantation and to a 2.5 Van de Graaff accelerator for in situ ion channeling analysis at 2.0 MeV at the KfK-INFP. The Al samples were implanted successively with W ions to fluences of 2, 4, 12, 16, 20 and 30 X 1015 at.cme2, producing W peak concentrations of 0.45, 0.80, 2.0, 2.7, 3.3 and 5.4 at.%, respectively, as measured in-situ by RBS. For the Mg sample only one implantation was performed with a fluence of 1015 at.cmw2, producing a W peak concentration of 0.3 at.%. The room temperature implanted samples were analysed with a 1.6 MeV He+ beam from the 2 MV Van de Graaff accelerator of LNETI, at Sacavem. The backscattered helium ions were detected by means of two silicon surface barrier detectors of 14 and 20 keV resolution, placed at angles of 140” and 180” to the beam direction, respectively. RBS/channeling experiments were done in the asimplanted samples and after each annealing step, in order to study the lattice site location and its stability. Aligned and random backscattering spectra were taken for several axial and planar crystallographic directions.
Whenever necessary, in order to detect any deviation from substitutional occupation, angular scans were carried out. Energy windows in the implanted W region and in the host metal were selected at energies corresponding to the implantation depths, in order to obtain the relative backscattering yields as a function of the tilt angle through the channeling direction. These angular scan curves are characterized by two parameters: the normalized minimum yield Xmin, which is the yield obtained for perfect alignment of the beam and crystalline directions, normalized to the yield obtained for random orientation; and the experimental critical angle q,,,, this being the half angular width at half height of the angular scan curve. The apparent substitutional fraction for a particular crystalline direction is defined by f$ = (l
-xL)/(l
-x$:in)7
(1)
where the superscripts i and h refer to the impurity and host atoms, respectively. The fraction estimated in this way represents the true substitutional fraction only if the displaced fraction (1 -fJ is randomly distributed [14]. Otherwise, the lattice positions of the non substitutional fractions can be derived only by comparison with the results of Monte Carlo simulations, done for the same geometry as in the experiment.
3. Results 3. I. The WMg system Angular yield curves of W and Mg (100 keV W in Mg at RT) through the (11~0) and (lOi0) axes and (0001) plane are shown in fig. 1. Within the experimental errors the dips of the W atoms are always shallower than the ones corresponding to the Mg host atoms, while the critical angles are rather similar. For such a case eq. (1) may be used to evaluate the substitutional fraction. Table 1 summarizes the experimental channeling parameters X2: and X,$,, together with the values derived for the substitutional fraction, f,, along each axis and plane. The data show that a major fraction (about 80%) of the implanted W atoms must reside in substitutional sites of the Mg host. The stability of the W site occupation was studied in annealing experiments. Table 1 shows that the Mg host lattice recovers by about 40% after annealing at 553 K. Further annealing at 623 IS lead to a slight increase of the dechanneling yield, while annealing at 683 K caused a large increase of the disorder. The lattice site location, however is not strongly affected, f, and qt,, change only slightly. This implies that the positions of the W atoms in the Mg lattice are stable in this temperature range. The complete angular scans of W and Mg taken after the annealing at 623 K for the
R. C. da S&a et al. / Lattice location and diffusion of implanted W
(4
285
0.3at.s1 TM-Tp=RT
?#,pO.22~ x,,,,, ~0.66 1
-1
Tilt
-2
Tii
hg?e
Angle
(deg)
2
(de:)
Fig. 1. Angular scans for Mg and W, measured at RT through the (a) (ll%>, (b) (lO‘iO>axial directions and (c) (0001) plane, in the as-implanted state.
same three major axial and planar directions (fig. 2) show that, in spite of the high annealing temperature, the results are similar to those at TA= 293 K as shown in fig. 1. After high fluence 300 keV W implantation in Mg
at room temperature, aligned and random RBS spectra (fig. 3) show that the substitutional fraction decreased to f, = 0.3 4 0.1, as the W concentration was increased to 2.5 at.%. In order to minimize the influence of the surface on
Table 1 Minimum yields from the host Mg and the implanted W ions, and substitutional fractions f, along the indicated crystalline directions as measured at room temperature for the 100 keV W implantation, and after annealing at temperature TA. =A
293 K
Direction
%%I
xKiii,
fs
%“/ps
q&
(llZO>
0.244 f 0.005 0.378 + 0.005 0.647 + 0.005
0.404 * 0.019 0.445 f 0.020 0.664 + 0.019
0.79 f 0.03 0.8lk 0.04 0.95 IO.04
0.51 f 0.03 0.39 + 0.03 0.24 f 0.03
0.49 f 0.04 0.35 + 0.04 0.22 + 0.03
0.134 f 0.005 0.239 rt 0.005 0.482+ 0.005
0.297 + 0.020 0.455 + 0.020 0.551+ 0.020
0.81&-0.04 0.72 + 0.04 0.87 f 0.04
@OOl)
0.184 f 0.005 0.290 + 0.005 0.466 f 0.005
0.336~0.020 0.449 + 0.020 0.507 * 0.020
0.81& 0.04 0.78 + 0.04 0.92 f 0.04
0.50 + 0.03 0.36 k 0.03 0.21+ 0.03
0.47 + 0.04 0.33 + 0.04 0.16+ 0.04
(llZO>
0.342 i 0.005
0.465 i 0.020
0.81 i: 0.04
(ioio) @OOl>
553 K (72 h)
(llZO>
(ioio) (0001)
623 K (80 min) 683 K (80 min)
(llZO>
(ioio)
R.C. da Silva et al. / Lattice location and diffusion of implanted W
286
0.3atr
K
T.-BR3K
(b)
1
-1
Tilt
Ang(ie
(deg)
k-sY-77 Tilt
Angle
(deg)
Fig. 2. Angular scans for Mg and W, measured at RT through the (a) (llzO>, (b) (lOjO) axial directions and (c) (0001) plane, after annealing at 623 K.
shallow implants as discussed above, a further in situ experiment has been performed at high implantation energy (300 keV) and low temperature (77 K). From the results (shown in figs. 4.la, b) a substitutional fraction f, = 1 was obtained. Additionally, after an:i1.6MeV
x 2 z.sot.% l,,=T,=RT
‘He+
w
4L-
w
c-\ “0 ‘;;”
i
V
;r
-0 ‘2 >-
.
q
1
1
I+
_,,u q
random
.
(loio)
101 3
300
i”i
a
aligned
\
400 Channel
500
750
850
Number
Fig. 3. Random and (lOiO> aligned spectra of a Mg single crystal implanted with a high fluence of 300 keV W ions at RT, measured at RT.
nealing to room temperature the substitutional fraction stays constant as shown in fig. 4.2. The analysis at RT was performed using 1.6 and 2 MeV He ions (see figs. 4.2 a, b). The analysis at 1.6 MeV reveals reduced minimum yield values, which indicates that the main defect structure in the implanted region are dislocations. The comparison of the W concentration depth profiles of the 100 keV implanted samples before and after annealing for 72 hours at 553 K (fig. 5) shows that, although diffusion occurs, the process is very slow. The derived coefficient is approximately 3 X 101’ cm2 s-t, which is several orders of magnitude lower than the Mg self diffusion coefficient at the same temperature (2 x lo-l3 cm* s-l) [15]. After subsequent annealing at higher temperatures (80 min at 623 and 683 K) no further diffusion was observed. Similar results were obtained with a Mg single crystal implanted with 150 keV W ions at room temperature. This system was submitted to a sequence of annealing treatments under a vacuum pressure better than lo-’ mbar, starting with a long pre-annealing of 72 hours at 553 K, and proceeding with short stages at 623 K (80 min) and 723 K (30 min). A final stage at 783 K (50 min), under pure He gas flow, was also carried
R.C. da Silva et al. / Lattice location and diffusion of implanted W
< 15 >.3ats w r”=T,=77K
o.y_%, Y-
fb) 1
.
(ioio)
0 100
Fig. 4.1. (a) Random
287
3feV I-
w
alQnad
-1
200 Channel
and (1070)
aligned
Tilt An&
Number spectra
(deg)’
of a Mg single crystal implanted with 300 keV ions and (b) corresponding scan curves, measured at 77 K.
angular
out. The RBS data show that the W concentration depth profiles after annealing do not change, hence no noticeable diffusion was observed. 3.2. The WA1 - system
1
.
0 200
300
(b)
400 500 750 Channel Number
2YeV
650
The in situ measured (110) aligned and random spectra at 77 K of the various 300 keV implantation doses are shown in fig. 6. With increasing the concentration of W from 0.45 to 5.4 at.%, the RBS yield did not show any evidence for amorphization of the Al host, and the substitutional fraction f, remains constant and is equal to 1. In order to check the system stability, RBS channeling measurements were repeated
ke’ A as-implanted
6
,i
P
:
;:;;;sUyd
rz
.
T,=553K
, ‘i,,i 7
100
200 Channel
300 42: Number
2c
Fig. 4.2. Random and aligned spectra in the (a) (1120) and (b) (lOi0) channeling directions for the same sample as in fig. 4.1, measured at RT.
0 depth
50 ( 101scm-2)
Fig. 5. Comparison of tungsten depth profiles before annealing at TA = 553 K for 72 hours.
100 and after
R.C. da Silva et al. / Lattice location and diffusion of implanted W
288
x
10
0.45.ts w
T.=T,=77K
.
(110)
aligned
260
300
Channel
IC)
425
200
300
425
Channel
Number
Channel
Number
x2 2.7ut.s w T.=T,=77K
2YaV'He'
Channel
525
Number
Number
Fig. 6. Random and (110) aligned RBS spectra for WA1 recorded in silu at 77 K, after 300 keV W ion implantations, to (a) 0.45, (b) 0.80, (c) 2.7 and (d) 5.4 at.%. The derived substitutional fraction is always equal to one.
for the highest concentration, after annealing to RT. No noticeable change of f, was observed. Further, one Al single crystal implanted with 300 keV W ions to 2.5 at.% at RT was also studied. Random and aligned spectra obtained for each of the major axial directions (IlO), (100) and (111) (fig. 71, always gave f, = 1, thus indicating that W is also fully substitutional in the Al lattice.
4. Discussion and conclusions The RBS/channeling results show a remarkably high substitutional fraction of 0.8 for W implanted in Mg at room temperature. The angular scans for the major crystallographic directions exclude that a dominant fraction of W atoms resides in any of the regular interstitial sites of the Mg lattice. Occupation of octahedral sites would imply the observation of flux peaks for all studied directions, while occupation of tetrahedral sites would require the observation of structures like double flux peaking in the (1120) direction and pronounced narrowing in the (lOi0) direction, which is not the case. Additionally, as the critical angles
derived from the angular scan curves for W and Mg are not significantly different, the conclusion is that all W atoms are located in substitutional sites. The substitutional fraction decreases with concentration. While f, = 0.8 at 0.3 at.% W, it decreases to 0.3 at 2.7 at.% W. This is in agreement with earlier measurements where a f, value of 0.35 was observed for 0.8 at.% W implanted at RT and 400 keV. Implantation at 80 keV resulted in reduced substitutional fraction! probably as a large fraction of W is located in the 330 A thick oxide layer [12]. The lattice site occupied by the W atoms after implantation is remarkably stable. Annealing up to 623 K, does not induce significant changes in the main channeling parameters, implying that the diffusion process does not affect the site location of the implanted W. Implantation at 77 K yields an f,value of 1, for 0.3 at.% W in Mg, which does not change after annealing to 293 K. W diffuses extremely slowly in the Mg lattice, the diffusion being 3 x 10-l’ cm* s-l at 553 K, which is about 4 orders of magnitude lower than the Mg selfdiffusion coefficients (- 2 x lo-l3 cm* sP1 at the same temperature). As the diffusion occurs during the host
R.C. da Silva et al. / Lattice location and diffusion of implanted W
289
. (100) al@nmd
200
300
400
Channel
500
750 Number
0200
850
-tr 300
400
Channel
500
“I+ ‘5( Numt ,e
b,,_ m random
Loo
300
400
Channel
500
Nut-n:
Fig. 7. Random and aligned RBS spectra along (a) ]( llO>, Cb) (100) and (c) (111) channeling directions, for an Al implanted with 300 keV W ions at RT to a maximum concentration of 2.5 at.%.
lattice recovery stage at 553 K, it is assumed that it is coupled to a defect release process. One possible explanation for such a low mobility could be the formation of a complex W-O molecule by internal oxidation, which is less mobile than a single ion in the lattice. Oxygen is a good candidate for such a complex due to the high affinity of W for oxygen and also to the presence of an oxygen source, the surface oxide layer. Similar behavior has been microscopically identified using hafnium implanted samples [16-181. In the case of W&l system, W is completely substitutional regardless of the implantation temperature or dose, even when the concentration is as high as several atomic percent. This points to a complete miscibility of W and Al in this concentration range, far above the solid solubility limit in the thermodynamic equilibrium. The present data, together with earlier work in polycrystalline Al samples with very high concentrations (up to 1.5 at.%) of 40 keV W ions [3], indicate that an even higher solid solubility of W in Al with remarkable stability may be expected. In fact, the system formed at very high doses was stable up to 683 K and the forma-
single crystal
tion of a different alloy phase was observed only above this temperature. Therefore the behavior of both WMg and WA1 systems are different. While W in Mg has a high substitutional fraction only at low concentrations it is completely substitutional in Al up to concentrations of 5 at.%. Comparing these systems to the closely related HfMg system [6,7,16-181 and HfAl system [5,8,9] some simdarities can be found. Hf is 100% substitutional, in both Mg and Al, at room temperature implantation. The Hf substitutionality in Al is also independent of fluence and implantation temperature, within the concentration ranges studied. Almost no diffusion of Hf in Mg can be detected, up to temperatures very near the melting temperature of the lattice. Here, the combined use of RBS/channeling and PAC techniques showed that Hf has a strong interaction with the oxygen coming from the native oxide layer. Internal oxidation takes place when the implantation energy is such that the Hf profile overlaps with the surface oxide layer. Additional investigations on the Mg based alloys produced by ion implantation should be pursued.
290
R.C. da Silva et al. / Lattice
location
Acknowledgements This work has been partially supported by JNICT (Portugal) and BMFT (Germany) under the bilateral agreement of 1990. One of us (R.C.S.) thanks LNETI for a leave of absence for staying at the KfK, Karlsruhe.
References [l] N. Bui, A. Irhzu, F. Dabosi and Y. Limouzin-Maire, Corros. 39 (1983) 491. [2] B.A. Shaw, T.L. Fritz, G.D. Davis and W.C. Moshier, J. Electrochem. Sot. 137 (1990) 1317. [3] RX. da Silva, M.F. da Silva, A.A. Melo, J.C. Soares, E. LeitHo and M. Barbosa, Nucl. Instr. and Meth. B50 (1990) 423. [4] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958). [5] 0. Meyer and A. Turos, Mater. Sci. Rep. 2 (1987) 371. [6] M.F. da Siiva, M.R. da Silva, E.J. Alves, A.A. Melo, J.C. Soares, P.M. Winand and R. Vianden, in: Surface Engineering, NATO AS1 Series E85 (Nijhoff, Dordrecht, 19841 p. 74. [7] M.R. da Silva, A.A. Melo, J.C. Soares, M.F. da Silva and R. Vianden, Nucl. Instr. and Meth. B15 (1986) 944.
and diffusion
of implanted
W
[8] M.B. Kurup, K.G. Prasad, R.P. Sharma and D.O. Boerma, Nucl. Instr. and Meth. 813 (1986) 68. 191D.O. Boerma, P.J.M. Smulders, KG. Prasad, M.M. Cruz, R.C. da Silva, and F. Pleiter, J. Less-Common Metals 145 (1988) 487. [lo] E.N. Kaufmann, K. Krien, J.C. Soares and K. Freitag, Hyperfine Interactions 1 (1976) 485. [ll] R. Vianden and E.N. Kaufmann, Nucl. Instr. and Meth. 149 (1978) 393. 1121 M.R. da S&a, PhD Thesis, Technical University of Lisbon (1987) pp. 61-66, unpublished. [13] I. Kbubeis, R. Gerber and 0. Meyer, Radiat. Effects and Def. Solids 115 (1990) 193. [141 L.C. Feldmann, J.W. Mayer and S.T. Picraux, Materials Analysis by Ion Channeling (Academic Press, New York, 1982). 1151P. Shewmon, Trans. AIME 206 (19561 918 or A.M. Brown and M.F. Ashby, Correlations for Diffusion Constants, Acta Metal. 28 (1980) 1085. [I61 M.M. Cruz, A.A. Melo, M.F. da Silva and J.C. Soares, Hyperfine Interactions 34 (1987) 223. 1171 M.M. Cruz, A.A. Mel?, J.C. Soares, M.F. da Silva and R. Vianden, Nucl. Instr. and Meth. B19/20 (1987) 200. llS1 M.M. Cruz, A.A. Melo and J.C. Soares, in: Nuclear Physics Applications on Materials Science, NATO AS1 Series El44 (Kluwer, Dordrecht, 1987) p. 263.
and Methods
in Physics Research
B63 (1992) 283-290
8 Methods
in Physics Research
Sectm B
Lattice location and diffusion of W implanted in Mg and Al single crystals R.C. da Silva a, M.F. da Silva a, I. Khubeis ‘,‘, 0. Meyer ‘, A.A. Melo b and J.C. Soares b a Departamento de Fisica, ICEN/LNETI, E.N. IO-2685 Sacauim, Portugal b Centro de Fisica Nuclear da Universidade de Lisboa, Au. Prof: Gama Pinto, 2-1699 Lisboa Codex, Portugal ’ Kernforschungszentrum Karlsruhe, Institut fiir Nukleare Festkiirperphysik, P.O. Box 3640, W-7500 Karlsruhe, Received
25 August
Germany
1991
Tungsten was implanted in Mg single crystals at 293 and 77 K. A rather high substitutional fraction, f, = 0.8, was observed after implanting W at 293 K with peak concentrations up to 0.3 at.%. With increasing peak concentration up to 2.7 at.% W, f, decreases to 0.3. Implanting 0.3 at.% W at 77 K resulted a f, value of 1, which did not change after warming up to 293 K. Annealing at 553 K causes a recovery of the host lattice, which however does not affect f,. The diffusion of W in Mg is much lower than expected from the Mg self-diffusion coefficient, which may be explained by internal oxidation of the W atoms. In the case of the WA1 system, W is completely substitutional after implantation at 293 and 7 K up to peak concentrations of about 5 at.%.
1. Introduction Al and Mg are light metals which are technologically important for a wide range of applications. Alloying with other elements is a common practice to improve specific properties. In particular, surface properties like wear and corrosion, which to a large extent determine the behavior of these materials in the working environment, have to be improved with different surface treatments. W is of potential interest to alloy with Al or Mg as it is used to improve the corrosion resistance of stainless steel [1,2]. There is also evidence that W can be used as a passivating element in Al [2,3]. It is likely to expect a similar effect for W in Mg. One important condition to improve the corrosion resistance of the host metal is that the passivating element should stay in solid solution at appreciable levels of concentration. If the concentration is too low, e.g. below 1 at.%, its influence on the corrosion properties is negligible [2]. This is a serious constraint in the case of W, since its solid solubility in Al is less than 0.03 at.% close to the melting point and it is insoluble in Mg [4]. This problem can be solved by the ion implantation technique, which is widely used to produce near surface metastable systems, in particular supersatu-
’ Permanent address: Amman, Jordan.
Dept.
of Physics,
Jordan
0168-583X/92/$05.00
0 1992 - Elsevier
Science
University,
Publishers
rated solid solutions [5]. Thus, solid solubility limits can be exceeded, although the resulting system is not in thermodynamic equilibrium and the concentration levels may be brought up to values which can significantly change the corrosion behavior. In order to understand the underlying mechanisms it is of great importance to study the lattice site location of the implanted atoms, its stability and the influence of radiation induced effects, which can control phenomena like diffusion and precipitation. In this respect WMg and WAl are potentially interesting and promising systemsyhich are still poorly understood. Earlier studies on HfMg [6,7] and HfAl [8,9] systems, formed by ion implantation, for which< complete substitutionality was formed and also on HfBe [lo] and WBe [ll] (hcp alloys), where both Hf and w occupy interstitial tetrahedral sites, lead to the expectation that W reveals a behavior similar to Hf, when implanted in Al or Mg. Previous studies on WMg used W implantation at 80 keV [12]. At this implantation energy, W was too close to the surface oxide layer, which in turn may have affected its lattice site location and stability. In the present work new experimental results are presented, for both WMg and WA1 systems, obtained by ion implantation at room temperature and at 77 K. To determine the lattice site location and the activation energy for diffusion of implanted W is the main purpose of this work. B.V. All rights resewed
284
R.C. da Siba et al. / Lattice location and diffusion of implanted W
2. Experimental
High purity Mg and Al single crystals were cut from commercially available single crystal bars and mechanically polished. The Mg (lOi0) oriented single crystals were chemically etched in concentrated (65%) nitric acid and subsequently rinsed in distilled water, The Al (110) single crystals were etched either chemically, in a solution of equal parts by volume of hydrofluoric acid and water and subsequently rinsed in distilled water, or electrochemically in a solution of equal parts by volume of concentrated 65% nitric acid and methyl alcohol and subsequently rinsed in methyl alcohol followed by ethyl alcohol [131. Three of the Mg single crystals were implanted at room temperature with W+ ions of 100, 150 and 300 keV energies to fluenees of 6 X 1014, 1.3 X 1Or5 and 1.3 X lOI6 cmb2, respectively. The W peak concentrations were determined by RBS to be 0.3, 0.3 and 2.7 at.%, respectively. One Al single crystal was implanted at room temperature with 300 keV W ions to a fluence of 1.3 X 1Or6 cm-2 the measured peak concentration was 2.5 at.%. The’Mg crystals implanted at 100 and 150 keV energy were submitted to a vacuum annealing process, starting with 72 hours at 553 K, followed by 80 min at 623 K. The 100 keV implanted crystal was further vacuum annealed at 683 K for 80 min, while the 150 keV implant was annealed at 723 K for 30 min in vacuum and at 783 for 50 min under pure He gas flow. During vacuum annealing the pressure in the annealing chamber was better than lo-’ mbar. The low temperature implantations were carried out at 77 K with 300 keV energy. The samples were mounted on a two axis goniometer, which was coupled to a 350 keV ion implanter for implantation and to a 2.5 Van de Graaff accelerator for in situ ion channeling analysis at 2.0 MeV at the KfK-INFP. The Al samples were implanted successively with W ions to fluences of 2, 4, 12, 16, 20 and 30 X 1015 at.cme2, producing W peak concentrations of 0.45, 0.80, 2.0, 2.7, 3.3 and 5.4 at.%, respectively, as measured in-situ by RBS. For the Mg sample only one implantation was performed with a fluence of 1015 at.cmw2, producing a W peak concentration of 0.3 at.%. The room temperature implanted samples were analysed with a 1.6 MeV He+ beam from the 2 MV Van de Graaff accelerator of LNETI, at Sacavem. The backscattered helium ions were detected by means of two silicon surface barrier detectors of 14 and 20 keV resolution, placed at angles of 140” and 180” to the beam direction, respectively. RBS/channeling experiments were done in the asimplanted samples and after each annealing step, in order to study the lattice site location and its stability. Aligned and random backscattering spectra were taken for several axial and planar crystallographic directions.
Whenever necessary, in order to detect any deviation from substitutional occupation, angular scans were carried out. Energy windows in the implanted W region and in the host metal were selected at energies corresponding to the implantation depths, in order to obtain the relative backscattering yields as a function of the tilt angle through the channeling direction. These angular scan curves are characterized by two parameters: the normalized minimum yield Xmin, which is the yield obtained for perfect alignment of the beam and crystalline directions, normalized to the yield obtained for random orientation; and the experimental critical angle q,,,, this being the half angular width at half height of the angular scan curve. The apparent substitutional fraction for a particular crystalline direction is defined by f$ = (l
-xL)/(l
-x$:in)7
(1)
where the superscripts i and h refer to the impurity and host atoms, respectively. The fraction estimated in this way represents the true substitutional fraction only if the displaced fraction (1 -fJ is randomly distributed [14]. Otherwise, the lattice positions of the non substitutional fractions can be derived only by comparison with the results of Monte Carlo simulations, done for the same geometry as in the experiment.
3. Results 3. I. The WMg system Angular yield curves of W and Mg (100 keV W in Mg at RT) through the (11~0) and (lOi0) axes and (0001) plane are shown in fig. 1. Within the experimental errors the dips of the W atoms are always shallower than the ones corresponding to the Mg host atoms, while the critical angles are rather similar. For such a case eq. (1) may be used to evaluate the substitutional fraction. Table 1 summarizes the experimental channeling parameters X2: and X,$,, together with the values derived for the substitutional fraction, f,, along each axis and plane. The data show that a major fraction (about 80%) of the implanted W atoms must reside in substitutional sites of the Mg host. The stability of the W site occupation was studied in annealing experiments. Table 1 shows that the Mg host lattice recovers by about 40% after annealing at 553 K. Further annealing at 623 IS lead to a slight increase of the dechanneling yield, while annealing at 683 K caused a large increase of the disorder. The lattice site location, however is not strongly affected, f, and qt,, change only slightly. This implies that the positions of the W atoms in the Mg lattice are stable in this temperature range. The complete angular scans of W and Mg taken after the annealing at 623 K for the
R. C. da S&a et al. / Lattice location and diffusion of implanted W
(4
285
0.3at.s1 TM-Tp=RT
?#,pO.22~ x,,,,, ~0.66 1
-1
Tilt
-2
Tii
hg?e
Angle
(deg)
2
(de:)
Fig. 1. Angular scans for Mg and W, measured at RT through the (a) (ll%>, (b) (lO‘iO>axial directions and (c) (0001) plane, in the as-implanted state.
same three major axial and planar directions (fig. 2) show that, in spite of the high annealing temperature, the results are similar to those at TA= 293 K as shown in fig. 1. After high fluence 300 keV W implantation in Mg
at room temperature, aligned and random RBS spectra (fig. 3) show that the substitutional fraction decreased to f, = 0.3 4 0.1, as the W concentration was increased to 2.5 at.%. In order to minimize the influence of the surface on
Table 1 Minimum yields from the host Mg and the implanted W ions, and substitutional fractions f, along the indicated crystalline directions as measured at room temperature for the 100 keV W implantation, and after annealing at temperature TA. =A
293 K
Direction
%%I
xKiii,
fs
%“/ps
q&
(llZO>
0.244 f 0.005 0.378 + 0.005 0.647 + 0.005
0.404 * 0.019 0.445 f 0.020 0.664 + 0.019
0.79 f 0.03 0.8lk 0.04 0.95 IO.04
0.51 f 0.03 0.39 + 0.03 0.24 f 0.03
0.49 f 0.04 0.35 + 0.04 0.22 + 0.03
0.134 f 0.005 0.239 rt 0.005 0.482+ 0.005
0.297 + 0.020 0.455 + 0.020 0.551+ 0.020
0.81&-0.04 0.72 + 0.04 0.87 f 0.04
@OOl)
0.184 f 0.005 0.290 + 0.005 0.466 f 0.005
0.336~0.020 0.449 + 0.020 0.507 * 0.020
0.81& 0.04 0.78 + 0.04 0.92 f 0.04
0.50 + 0.03 0.36 k 0.03 0.21+ 0.03
0.47 + 0.04 0.33 + 0.04 0.16+ 0.04
(llZO>
0.342 i 0.005
0.465 i 0.020
0.81 i: 0.04
(ioio) @OOl>
553 K (72 h)
(llZO>
(ioio) (0001)
623 K (80 min) 683 K (80 min)
(llZO>
(ioio)
R.C. da Silva et al. / Lattice location and diffusion of implanted W
286
0.3atr
K
T.-BR3K
(b)
1
-1
Tilt
Ang(ie
(deg)
k-sY-77 Tilt
Angle
(deg)
Fig. 2. Angular scans for Mg and W, measured at RT through the (a) (llzO>, (b) (lOjO) axial directions and (c) (0001) plane, after annealing at 623 K.
shallow implants as discussed above, a further in situ experiment has been performed at high implantation energy (300 keV) and low temperature (77 K). From the results (shown in figs. 4.la, b) a substitutional fraction f, = 1 was obtained. Additionally, after an:i1.6MeV
x 2 z.sot.% l,,=T,=RT
‘He+
w
4L-
w
c-\ “0 ‘;;”
i
V
;r
-0 ‘2 >-
.
q
1
1
I+
_,,u q
random
.
(loio)
101 3
300
i”i
a
aligned
\
400 Channel
500
750
850
Number
Fig. 3. Random and (lOiO> aligned spectra of a Mg single crystal implanted with a high fluence of 300 keV W ions at RT, measured at RT.
nealing to room temperature the substitutional fraction stays constant as shown in fig. 4.2. The analysis at RT was performed using 1.6 and 2 MeV He ions (see figs. 4.2 a, b). The analysis at 1.6 MeV reveals reduced minimum yield values, which indicates that the main defect structure in the implanted region are dislocations. The comparison of the W concentration depth profiles of the 100 keV implanted samples before and after annealing for 72 hours at 553 K (fig. 5) shows that, although diffusion occurs, the process is very slow. The derived coefficient is approximately 3 X 101’ cm2 s-t, which is several orders of magnitude lower than the Mg self diffusion coefficient at the same temperature (2 x lo-l3 cm* s-l) [15]. After subsequent annealing at higher temperatures (80 min at 623 and 683 K) no further diffusion was observed. Similar results were obtained with a Mg single crystal implanted with 150 keV W ions at room temperature. This system was submitted to a sequence of annealing treatments under a vacuum pressure better than lo-’ mbar, starting with a long pre-annealing of 72 hours at 553 K, and proceeding with short stages at 623 K (80 min) and 723 K (30 min). A final stage at 783 K (50 min), under pure He gas flow, was also carried
R.C. da Silva et al. / Lattice location and diffusion of implanted W
< 15 >.3ats w r”=T,=77K
o.y_%, Y-
fb) 1
.
(ioio)
0 100
Fig. 4.1. (a) Random
287
3feV I-
w
alQnad
-1
200 Channel
and (1070)
aligned
Tilt An&
Number spectra
(deg)’
of a Mg single crystal implanted with 300 keV ions and (b) corresponding scan curves, measured at 77 K.
angular
out. The RBS data show that the W concentration depth profiles after annealing do not change, hence no noticeable diffusion was observed. 3.2. The WA1 - system
1
.
0 200
300
(b)
400 500 750 Channel Number
2YeV
650
The in situ measured (110) aligned and random spectra at 77 K of the various 300 keV implantation doses are shown in fig. 6. With increasing the concentration of W from 0.45 to 5.4 at.%, the RBS yield did not show any evidence for amorphization of the Al host, and the substitutional fraction f, remains constant and is equal to 1. In order to check the system stability, RBS channeling measurements were repeated
ke’ A as-implanted
6
,i
P
:
;:;;;sUyd
rz
.
T,=553K
, ‘i,,i 7
100
200 Channel
300 42: Number
2c
Fig. 4.2. Random and aligned spectra in the (a) (1120) and (b) (lOi0) channeling directions for the same sample as in fig. 4.1, measured at RT.
0 depth
50 ( 101scm-2)
Fig. 5. Comparison of tungsten depth profiles before annealing at TA = 553 K for 72 hours.
100 and after
R.C. da Silva et al. / Lattice location and diffusion of implanted W
288
x
10
0.45.ts w
T.=T,=77K
.
(110)
aligned
260
300
Channel
IC)
425
200
300
425
Channel
Number
Channel
Number
x2 2.7ut.s w T.=T,=77K
2YaV'He'
Channel
525
Number
Number
Fig. 6. Random and (110) aligned RBS spectra for WA1 recorded in silu at 77 K, after 300 keV W ion implantations, to (a) 0.45, (b) 0.80, (c) 2.7 and (d) 5.4 at.%. The derived substitutional fraction is always equal to one.
for the highest concentration, after annealing to RT. No noticeable change of f, was observed. Further, one Al single crystal implanted with 300 keV W ions to 2.5 at.% at RT was also studied. Random and aligned spectra obtained for each of the major axial directions (IlO), (100) and (111) (fig. 71, always gave f, = 1, thus indicating that W is also fully substitutional in the Al lattice.
4. Discussion and conclusions The RBS/channeling results show a remarkably high substitutional fraction of 0.8 for W implanted in Mg at room temperature. The angular scans for the major crystallographic directions exclude that a dominant fraction of W atoms resides in any of the regular interstitial sites of the Mg lattice. Occupation of octahedral sites would imply the observation of flux peaks for all studied directions, while occupation of tetrahedral sites would require the observation of structures like double flux peaking in the (1120) direction and pronounced narrowing in the (lOi0) direction, which is not the case. Additionally, as the critical angles
derived from the angular scan curves for W and Mg are not significantly different, the conclusion is that all W atoms are located in substitutional sites. The substitutional fraction decreases with concentration. While f, = 0.8 at 0.3 at.% W, it decreases to 0.3 at 2.7 at.% W. This is in agreement with earlier measurements where a f, value of 0.35 was observed for 0.8 at.% W implanted at RT and 400 keV. Implantation at 80 keV resulted in reduced substitutional fraction! probably as a large fraction of W is located in the 330 A thick oxide layer [12]. The lattice site occupied by the W atoms after implantation is remarkably stable. Annealing up to 623 K, does not induce significant changes in the main channeling parameters, implying that the diffusion process does not affect the site location of the implanted W. Implantation at 77 K yields an f,value of 1, for 0.3 at.% W in Mg, which does not change after annealing to 293 K. W diffuses extremely slowly in the Mg lattice, the diffusion being 3 x 10-l’ cm* s-l at 553 K, which is about 4 orders of magnitude lower than the Mg selfdiffusion coefficients (- 2 x lo-l3 cm* sP1 at the same temperature). As the diffusion occurs during the host
R.C. da Silva et al. / Lattice location and diffusion of implanted W
289
. (100) al@nmd
200
300
400
Channel
500
750 Number
0200
850
-tr 300
400
Channel
500
“I+ ‘5( Numt ,e
b,,_ m random
Loo
300
400
Channel
500
Nut-n:
Fig. 7. Random and aligned RBS spectra along (a) ]( llO>, Cb) (100) and (c) (111) channeling directions, for an Al implanted with 300 keV W ions at RT to a maximum concentration of 2.5 at.%.
lattice recovery stage at 553 K, it is assumed that it is coupled to a defect release process. One possible explanation for such a low mobility could be the formation of a complex W-O molecule by internal oxidation, which is less mobile than a single ion in the lattice. Oxygen is a good candidate for such a complex due to the high affinity of W for oxygen and also to the presence of an oxygen source, the surface oxide layer. Similar behavior has been microscopically identified using hafnium implanted samples [16-181. In the case of W&l system, W is completely substitutional regardless of the implantation temperature or dose, even when the concentration is as high as several atomic percent. This points to a complete miscibility of W and Al in this concentration range, far above the solid solubility limit in the thermodynamic equilibrium. The present data, together with earlier work in polycrystalline Al samples with very high concentrations (up to 1.5 at.%) of 40 keV W ions [3], indicate that an even higher solid solubility of W in Al with remarkable stability may be expected. In fact, the system formed at very high doses was stable up to 683 K and the forma-
single crystal
tion of a different alloy phase was observed only above this temperature. Therefore the behavior of both WMg and WA1 systems are different. While W in Mg has a high substitutional fraction only at low concentrations it is completely substitutional in Al up to concentrations of 5 at.%. Comparing these systems to the closely related HfMg system [6,7,16-181 and HfAl system [5,8,9] some simdarities can be found. Hf is 100% substitutional, in both Mg and Al, at room temperature implantation. The Hf substitutionality in Al is also independent of fluence and implantation temperature, within the concentration ranges studied. Almost no diffusion of Hf in Mg can be detected, up to temperatures very near the melting temperature of the lattice. Here, the combined use of RBS/channeling and PAC techniques showed that Hf has a strong interaction with the oxygen coming from the native oxide layer. Internal oxidation takes place when the implantation energy is such that the Hf profile overlaps with the surface oxide layer. Additional investigations on the Mg based alloys produced by ion implantation should be pursued.
290
R.C. da Silva et al. / Lattice
location
Acknowledgements This work has been partially supported by JNICT (Portugal) and BMFT (Germany) under the bilateral agreement of 1990. One of us (R.C.S.) thanks LNETI for a leave of absence for staying at the KfK, Karlsruhe.
References [l] N. Bui, A. Irhzu, F. Dabosi and Y. Limouzin-Maire, Corros. 39 (1983) 491. [2] B.A. Shaw, T.L. Fritz, G.D. Davis and W.C. Moshier, J. Electrochem. Sot. 137 (1990) 1317. [3] RX. da Silva, M.F. da Silva, A.A. Melo, J.C. Soares, E. LeitHo and M. Barbosa, Nucl. Instr. and Meth. B50 (1990) 423. [4] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958). [5] 0. Meyer and A. Turos, Mater. Sci. Rep. 2 (1987) 371. [6] M.F. da Siiva, M.R. da Silva, E.J. Alves, A.A. Melo, J.C. Soares, P.M. Winand and R. Vianden, in: Surface Engineering, NATO AS1 Series E85 (Nijhoff, Dordrecht, 19841 p. 74. [7] M.R. da Silva, A.A. Melo, J.C. Soares, M.F. da Silva and R. Vianden, Nucl. Instr. and Meth. B15 (1986) 944.
and diffusion
of implanted
W
[8] M.B. Kurup, K.G. Prasad, R.P. Sharma and D.O. Boerma, Nucl. Instr. and Meth. 813 (1986) 68. 191D.O. Boerma, P.J.M. Smulders, KG. Prasad, M.M. Cruz, R.C. da Silva, and F. Pleiter, J. Less-Common Metals 145 (1988) 487. [lo] E.N. Kaufmann, K. Krien, J.C. Soares and K. Freitag, Hyperfine Interactions 1 (1976) 485. [ll] R. Vianden and E.N. Kaufmann, Nucl. Instr. and Meth. 149 (1978) 393. 1121 M.R. da S&a, PhD Thesis, Technical University of Lisbon (1987) pp. 61-66, unpublished. [13] I. Kbubeis, R. Gerber and 0. Meyer, Radiat. Effects and Def. Solids 115 (1990) 193. [141 L.C. Feldmann, J.W. Mayer and S.T. Picraux, Materials Analysis by Ion Channeling (Academic Press, New York, 1982). 1151P. Shewmon, Trans. AIME 206 (19561 918 or A.M. Brown and M.F. Ashby, Correlations for Diffusion Constants, Acta Metal. 28 (1980) 1085. [I61 M.M. Cruz, A.A. Melo, M.F. da Silva and J.C. Soares, Hyperfine Interactions 34 (1987) 223. 1171 M.M. Cruz, A.A. Mel?, J.C. Soares, M.F. da Silva and R. Vianden, Nucl. Instr. and Meth. B19/20 (1987) 200. llS1 M.M. Cruz, A.A. Melo and J.C. Soares, in: Nuclear Physics Applications on Materials Science, NATO AS1 Series El44 (Kluwer, Dordrecht, 1987) p. 263.