- Email: [email protected]
Depletion of lithium due to surface oxidation: An investigation of an AlLi-sheet by Auger-spectroscopy
3ownel ef
AND ~ ~ D : ; ELSEVIER
Journal of Alloys and Compounds 255 (1997) 135-141
Depletion of lithium due to surface oxidation: An investigation of an A1-Li-sheet by Auger-spectroscopy T h o m a s Sch6berl*, Subodh K u m a r Erich Schmid lnstitut fiir Festkiirperphysik der Osterreichischen Akademie dcr Wissenschaften. JahnstraJ3e 12. A-8700 Leoben Austria
Received 28 June 1996; revised 26 August 1996
Abstract The depletion of Li beneath the surface of an AI-Li sheet as a consequence of surface oxidation is investigated by Auger electron spectroscopy. By the improved spatial resolution compared to other sampling methods it is shown that the local oxidation rate varies over a wide range on a !0 Ixm scale. The local oxidation rate gives rise to a local Li depletion combined with the formation of pores in the underlying subsurf. :e region: these pores have a strong influence on the diffusivities perpendicular to the surface. An average of the strongly varying Li concentration profiles resembles, however, the typical diffusion=like depletion curves which have been reported in previous investigations and can be described by a siPlple solution of a one dimensional diffusion equation. The results are compared with microhardness measurements: it turns out that microhardness values can give a qualitative estimate of the Li depletion as long as no high density of large pores occur in a band beneath the ,,ufface. Keyword.w Aluminium: Lithium: Auger spectroscopy: Surface oxidation: Heat treatment: Lithium depletion
I. Introduction Since the addition of only I wl% lithium (Li) to aluminium (AI) reduces the densily by 3% and increases the elastic modulus by 6% Ill, Al:~Li alloys are very auractive, especially for aerospace applications. The high strength of these alloys arises from the formation of Al~Li(5') precipitates. However, in order to I'olm these precipitates, the alloy has to be solution treated at elevated temperatures for a sufficiently long, period prior to the ageing treatment. Since at these temperatures Li has quite a high diffusivity in A! and is highly reactive with oxygen, it diffuses to the surface and reacts with the surrounding gas atmosphere during the solution treatment. An oxide layer is formed on the surface, a layer which unfortunately does not protect the alloy I~om further oxidation, leading to a Li depleted region beneath the oxide~ailoy interface. This region cannot, or only to a lesser extent, be strengthened by a subsf-$uent ageing treatment. This fact has important consequences for the fabrication, heat treatment and application of AI-Li based alloys, especially as a thin sheet material. Therefore the depletion of Li during a solution treatment has been the subject of several studies [2~171. Most of these studies rely, however, on indirect evalua*Corresponding author. 0925-8388/971517.00 © 1997 Elsevier Science S.A. All rights reserved PII S0925-8388(96~02818-6
lions of the Li depletion, mainly by microhardness menu surements, or on eddy currenl 131 and X~ray diffraction studies (e.g. 1141). We are aware of only a few studies where Li was delected directly, eog~ by a nuclear reaction analysis lechnique 111,16i. In Ihese studies the ~J,rea sampled along a cross section wa.,,, however, quite lal'ge (~-~40 p,n l up to 100 p~m); thus, if there had been inhomogenities in the Li distribution within smaller areas they could not have been detected. The Li depletion is usually observed in combination with the t)ccu~!¢¢ of pores in the vicinity of Ihe surface. The occurrence of the~e pores has been explained by a Kirkendallolike effect as a consequence of the Li depletion [ I I |. We are, however, not aware of any experimental sludy showing a direct relation between Li depletions and the formation of pores. The number of experimental methods, by which Li can be detected, especially within a small sampled volume, is limited; one of them is Auger electron spectloscopy (AES), but with severe limitations as will be shown and discussed below. AES is basically a surface sensitive technique. The common way to apply it to depth profiling is either to lake spectra while the surface is removed continuously by noble gas ion spattering, or by sampling along a cross section cut perpendicular to the surface. Due to the usu~l low sputter° ing rates (~0.1 nm/s) the first method would require
136
T. Schiiberl. S. Kmnar / Jounud t!f AIhJ.vs and Conff~ounds 255 (1997) 135-14.1
extremely long experimental times, usually leading to additional undesired effects. Thus the second method is adopted for the present investigation. The aim of this work is to measure by AES Li depletion profiles of an Al-2.8 wt% Li sheet after solution treatment. In order to test the applicability of microhardness measurements their results are compared with the AES results. As Auger spectroscopy has, to our knowledge, rarely or not yet been applied to the Li depletion effect in such AI-Li alloys, the limits of the applicability of AES are checked and discussed. Depth profiles from regions with a high density of pores are compared with those taken from regions where almost no pores could be detected by optical microscopy. We also examine the influence of a surrounding argon atmosphere during the solution treatment.
2. Experimental procedure An Al-2.8 wt% Li alloy was cast as 10 mm thick plates by melting high purity AI (99.99%) and Li (99.9%) in an argon atmosphere and let it solidify in an iron mould. The as cast plate was i~omogenized in an argon atmosphere at 550°C for 6 h. The plate was lightly ground on emery paper to remove the visible oxide layer and was cold-rolled to a I mm thick sheet. Small samples from this sheet were solution treated at 530°C for 40 rain either in air or in an argon atmosphere and quenched in water at room tempera° ture. For the "argon" treatment the ,~ample was kept inside a steel box and argon with a purity of 99.999% was passed through the box. The solution treated samples consisted of recrystallized grains with an average diameter of 50 t~m. The AES measurements were performed in a standard commercial ultra high vacuum (UHV) chamber, pumped by a turbomolecular pump and a set of triode pumps: the apparatus has ~ e n descried in detail elsewhere l lgl, Sufficiently good vacuum conditions could only be achieved by baking out the complete vacuum system, Such a bakeout means, however, an unavoidable (additional) heat treatment of the sample, Thus the temperature and the duration c,,fthe bakeout had to be chosen as a compromise between the remaining partial pressures of the residual gases, mainly of oxygen, and non°altered depth distribu° tions of Li, After a "soft" bakeout of the system at 135°C fi)r 30 h the residual pressure was below 10 '~'~'Tort for all experiments, the surface contamination by the residual gas was slow enough to run at least some complete concentration profiles without the necessity of any cleaning pr~ess, An estimate of diffusion path lengths using diffiJsion data e.g. of J l9J shows that by this bakeout the Li distributions are in no case changed over distances exceed° ing some I0 nm, The Li distributions were measured spotwis¢ by AES along several cross-sections of the samples which were mounted on a commercial x-y-z sample holder, additionally rotatable in the z~di~ction, with their polished cross
section perpendicular to the primary beam. The nominal diameter of the primary beam was 8 lxm, in practice the spot diameter turned out to be -10 Ixm. The primary beam current was -3 p,A at a voltage of 2 kV. The Auger electrons emitted from the sample were analysed by a cylindrical mirror analyser (CMA) in the diVidE mode (the differential signal of N registrated electrons at their respective energy E) with a 5 Volt modulation amplitude. The axis of the CMA was parallel to the primary beam. As the amplification of the secondary electron multiplier in use drifted, we used as a basic measure for the Li concentrations the relation of the signal heights of Li and AI. As we were interested primarily in the depletion of Li caused by the anneals at various conditions, an absolute determination of the Li concentrations was not necessary. The most pronounced Auger transition due to Li in the alloy was tbund at 39 eV which is reported to stem either from oxidized Li [20,21], more specifically Li:O [22], or from Li in several compounds [23]. As the peak position for Li in the alloy turned out to be almost identical to f~hat of oxidized Li and the Auger sensitivity being much larger for the oxidized Li, great care had to be taken to ascertain a clean metallic surface. The Auger peak-to-peak height at the 39 eV transition was used for the determination of relative Li contents. The most dominant Auger transition for AI has an energy of 68 eV for metallic AI and 5 ! eV for aluminium oxide. The 68 eV transition was ased to determine the particular Auger peak height ratios Li/AI as a measure lot the relative Li content, the 51 eV transition served as a control of the oxygen uptake during the experimental runs. The heavy contamination by oxygen and carbon, registered after mounting the samples into the UHV chamber, was removed by Ar°ion sputtering for I rain. Tile beam energy was 3 kV, the Ar partial pressure 6x 10 =~ Tort' resulting in a beam current density of' 25 ~A/cm ~, For the subsequent cleaning procedures in between the particular experimental runs a sputter time of 20 s was sufficient to remove all contaminants, newly adsorbed even at UHV conditions. In order to check the alteration of the Li distribution during sputtering we measured one profile repeatedly with a prolonged sputter time between the measurements. No noticeable influence on the results was fi~und as long as the sputter time did not exceed 2 rain. The Li concentration profiles were taken moving along the cross section of the specimen usually in steps of l0 gm up ~a a depth of 180 Ixm. The position of the edge was defined as the position where the signal height of the 68 eV Auger transition for AI had half its bulk value, The individual Li/AI signal height ratios along a profile were normalized by dividing them by the average of the ratios obtained from five measurements taken at various spots in the centre of that particular cross section. The microhardness measurements were performed after the AES measurements, They were done on an optical microscope with a microhardness equipment, A separate ageing treatment was not necessary since the baking out
T. Schiiberl. S. Kumar I Journal of Alloys and Compounds 255 (1997) 135-141
137
procedure in the UHV chamber before the AES investigations also caused the precipitation in the Li enriched regions. The microhardness was determined along several cross sections of the sheet cut perpendicular to the surface making indents at every 20 I~m. The values obtained were divided by average values obtained at the inner region of that particular cross section.
3. Results
(a)
3. i. Optical and scanning electron microscopy (SEM) Before mounting the samples in the UHV chamber their polished cross sections were inspected by optical microscopy. Some areas of special interest were viewed additionally by SEM. The samples contained pores already in the cold-rolled state with a size from just visible up to some I~m. In these unannealed samples the pores were distributed quite homogeneously over the whole of the cross sections. The anneals caused, however, an increased density of pores within a region of 80-100 p,m beneath the surface combined with a significant decrease of the density in the inner region of the sheet, The density of the pores and their size varied over a wide range for different cross sections. The largest pore sizes near the surface, combined with the largest scatter in size and density, were found at several cross sections of the sample solution treated in argon (Fig. in). Etching the cross sections resulted in clearly visible grain boundaries. The procedure was: etching for 2 rain in a mixture of 15 ml HNO~, 0,5 ml HF, 3 g CrVI oxide and 84 ml distilled water, then cleaning in distilled water and finally etching in a mixture of 75 ml distilled water, 5 ml HNO~, 3 ml HCI and 2 ml HF. No systematic relation between the location of grain boundaries and the distribution of the pores could be found (Fig. Ib), a relation which has been suggested qualitatively by Papazian et al. [171. Some areas of the sheet surface were (still) covered with an oxide layer with a thickness varying from a few pom up to 45 Ixm (Fig. Ic). This oxide layer was found only at the surface of the samples heat treated in air. The larger part of this layer, containing pores as in the subsurface band, was split off, most probably during the quenching process. The SEM microgmph as well as the optical microscopy suggest that splitting off the oxide layer should mainly occur at the interface clearly visible between oxide layer and alloy matrix. Then the local layer thickness is mainly determined either by the local oxidation rate on the surface (interface) or by the diffusivities along the particular paths beneath the interface or by a combination of both. We found a qualitative relation between the thickness of the oxide layer and the density of pores below the layer; increased pore, densities arc accompanied by a larger thickness of the layer, but no real quantitative relation occurred. By Auger analysis the oxide overlayer is found to consist mainly of Li, oxygen
(b)
Fig. I. Polished cross ~eczion~of ~mple~ a ~ r d i f f ~ . t hect l~at~|~t~', a) solution treated in argon, high density of pore~, etd'~d; b) ~lulte,n treated in air, etched: c) SEM microgr~Fnh with the oxide overl~yer (~ir treated sample, unetched).
and, to a smaller extent, of carbon and AI. However, the exact composition could not be determined. 3.2 Microhardness measuremems Fig. 2a shows the relative microhardness along several cross sections of the sheet in the cold-rolled state prior to the solution treatment. The values of the microhardness had fluctuations up to _+20% with a tendency of a slight softening towards the surface. For the samples heat treated in air a clearly softened subsurface zone was ob~rved reaching up to ~100 Ixm (Fig. 2b). Also the samples heat treated in argon show a softened subsurface zone which is, however, in several cases even deeper than observed with
138
7". Schiiherl. S. I(um.r t Journal oj'AIIoy.~ atld Compomld.~ 255 (1997) 135-141
120
10o 80 ".
"'....%.o/
• •
4o 20
0
,
. |
.
.
,1 sample 4, unannealed, .
.
.
,
120
80
~ 20
F
sample 5, annealed in lab air
o .,, . . . . . .
..,
120 .~'.
80 .
,~'
,
..
i
~
~ 4O
(c)
0
annealed in argon 0
20 40 60 80 I(~ 120 140 160 180 depth in micrometer
l:ig, 2, P~'olile~:~) ¢oh!:rolled ~lal¢',h) soltltio,treated in .It:¢) ~ohttloo
the air treated sample (Fig. 2c): it should t~ noted that the cross section investigated by microhardness measurement was the cross section with the largest density of pores which could be found in the argon treated sampie as well as in any other sample, The fluctuation observed with these profiles is the largest one of all. frequently exceeding ~20%.
3.3. Auger measurements
As, to our knowledge, Li concentrations in these alloys have noi yet been deter~r,ined by AES, we I~rformcd some prelimimwy experiments: 1) Consecutive Auger measuremet~ts (55) we~ performed fixing the spot of the primary beam at one point in the centre of a sputter cleaued cross section, Due to the weak 39 eV !,i signal the scatter is quite iarge, up to ± 10%, On the other hand no long time drift could be
detected, a drift caused either by oxygen adsorption or by the primary electron beam. 2) In order to check the influence of both these effects on the Li/A! AES peak height ratios, a surface, freshly sputter cleaned, was exposed to I x 10 -7 Torr 0 2 for half an hour. The resulting oxygen adsorption lead to an increase of the 39 eV signal, combined with the occurrence and increase of the 511 eV oxygen signal and the 51 eV signal due to aluminium oxide. A prolonged bombardment of this surface by the primary electron beam lead to a slow, continuous decrease of these signals• Obviously an electron stimulated deoxidation occurred both for AI and Li. Electron stimulated desorption (ESD) is often found for oxygen on metallic surfaces. Although ESD obviously occurred, it could not be detected by the mass spectrometer in use, most probably because of the very small desorption rate. Next we checked if there were any possibility to "decorate" Li by oxidation in order to obtain more pronounced 39 eV signals leading to a lowered detection limit for Li. For that purpose a depth profile was taken for a surface heavily oxidized by the procedure described above; then after sputter cleaning the surface was investigated again. No linear or systematic relation between the signal heights obtained with the clean and with the oxidized surface was found. All results confirm, however, that, even in an oxygen-free state, Li in AI shows the Auger transition at 39 eV. Prolonged sputtering combined with a consequent, deoxidizing exposure to the electron beam leads in no case to the disappearance of the respective Auger signal. During sputtering the signal is only less than half of the signal obtained promptly after switching off the sputter benin. Li seems to be sputtered pret~rentially in the present alloy, 3) In order to check to which extent the I,i/AI signal height ratios are reproducible, several prattles were measured relatedly, In most cases pronounced zigozag curves were obtained. The peak locations of these curv,'~s were otten not reproducible better than within 15 I~m. The peak to peak heights were frequently scattered up to ±20% within short distances and can therelbre not be explained only by the error of the Auger measurement itself, The existence of the pores observed in the subsurface zone (up to a ~m scale) might be one reason for the occurrence of non smooth real Li distributions. But if so, then the peak positions should be reproducible in a spatial scale of 10~m for several runs on the same profile, This was not the case, as stated above. It should be noted that during the measurements the mechanical pumps of the UHV apparatus lead to mechanical vibrations of the whole UHV apparatus and therefore to a deteriorated spatial resolution of the AES measurements, In principle the measurements could be done using the vibrationless triode pumps only, leading in fact to an improved spatial resolution, Performing the measurements without running the mechanical pumps pemmnently took, however, a lot of time; the unavoidable sputter procedures between the measurements
T. Sch6beri, S. Kumar I Journal of Alloys and Compounds 255 (1997) 135-141
required a high Ar partial pressure. Large amounts of noble gases should, however, never he pumped by triode pumps. Also, when having stopped and started aga'n, a turbomolecular pump takes at least 12 h to recover good vacuum conditions. Thus we decided to perform further standard measurements usually keeping the mechanical pumps running. In cases either of special interest or doubts in the results we made the measurements with the mechanical pumps switched off. The diagrams for the AES results should be read and interpreted as follows: the error in the relative Li concentrations is about +_10%. A value of 5% means that Li was detected but its amount was certainly below 10%. Missing or zero values in the graphs mean that the Li content was below the detection limit. Additionally to the error in the relative Li concentrations the uncertainty in the zero position (the location of the edge) must be kept in mind. Fig. 3a shows the Li concentration profile for a sample in the cold-rolled state. The fluctuations of the
120 .......... ............. .
? 60 f
.....
%f
-"
"
" - ' ~ .......
~i
V
~ 4O
(a)
'~ ~o ]sample 4, unannealed 120
1IOO6o 80
,~,,#~,.
Z
,'
i;::;: .....
.'TJ ..'.....
2o ¢ 0
.
/
.
...'
......."...
.t.
.
.
sample 5, annealed in lab air
120 '~ 100
60
0 0
20 40 60 80 100 120 140 160 180 depth in micrometer
Fig. 3. Protiles of the AES Li/AI signal height ratios: a) cold-rolled sample; b) solution treated in air; c) argon sample.
139
measured concentration values are frequently larger than can be explained by experimental uncertainty. Ignoring the fluctuations a slight Li depletion extending to a depth of ~100 I~m is obtained for the samples in the cold-rolled state. This fact should be a consequence of the homogenization treatment of the samples before rolling. The flat depletion agrees qualitatively with the results of the microhardness measurements (compare with Fig. 2a). Pronounced depletions of Li appear with all the samples solution treated, in air as well as in argon. The particular curves obtained for the samples treated in air fairly resemble each other (Fig. 3b): the Li concentration at the surface was zero or near zero for most of the profiles. The depletion depth, however, varied from 80 to 180 p,m. The largest depletion depths were found beneath those surface areas where the largest thickness of the oxide overlayer could be found, if it had not been split off. As the oxidation rate on the surface and therefore the Li depletion rate during the heat treatment might be influenced by surface roughness we investigated a sample with one side of the sheet polished to a mirror finish. The influence of the enhanced surface smoothness is rather small; the Li depletion depths are ~i00~m. On the other hand no profile with noticeably larger depths was found. Two of the profiles start from surface concentrations being remarkably higher than zero. The polishing procedure obviously results in only a few areas with a reduced oxidation rate. The results obtained for the sample solution treated in argon (Fig. 3c) show, at first sight, a situation somewhat more complicated, in general, there arises a depletion of Li during the anneals. In those cross sections where the size and density of the pores resembled the situation of the air treated sample, also the Li depth profiles are very similar. For the cross sections with large pores of high density in the subsurl~ce zone a noticeable scatter between the particular depth proliles occurred. Four principal types can be distinguished in a set of 37 profiles: Li surface concentrations of zero or near zero combined with either ahnost no Li depletion or a depletion depth up to 180 p,m, as well as Li surface concentrations up to 60%, again combined with either high or almost no depletion depth. Since the results suggest a relation between the density and size of pores in the subsurface region and the Li depletion in this zone, we measured a few profiles in a region with no visible pores and in a region with many pores of large size. The respective regions had been chosen and clearly marked under the optical microscope. These measurements were pertbtmed with the mechanical pumps switched off in order to minimize the errors in the profiles. in Fig. 4a two profiles with a mutual distance of 20 p,m are shown taken from a pore-free region of a cross section of the air treated sample. Despite only a slight Li depletion the surt~ce concentration is near zero. Taking into account the basic error of the Auger measurement itself the curves can be regarded as smooth. A completely different situation occurs with the profiles taken from an extremely
T. SehiJberl, S. Kumar i Journal qf Alloys am~ Compoumts 255 (1997) 135-141
140
120 "~ I00 8 gO ~ 4O 0
Y
Iro,,onwi ,owporo,i
| i,
.
.
.
.
120
" t0o ,.t 41)
.
(b)
] 2o 0 . . . .
.......... /'"/
I
regionwith high porosity
Fig. 4. Two neighbouring AES proliles: a) taken at a non-porous region of a ~ample heat treated in air; b) at an extremely porous region of an argon ~.mple
porous section of the sample treated in argon (Fig. 4b); the cun,¢~ obtained ate completely different. One concentration protile (solid line) is a pronounced zigozag line with no ~rong Li depletion, The suri',ce concentration is almost us high as within the bulk. Stepping only by 20 I~m along the ~urface the Li concentration is zero; the respective ~second) profile is rather smooth with a depletion depth of 100 I~m. The results confirm that pores of large si~,e are accompanied by steep concentration gradients, positive as well a~ negative when ~tepping towards the centre of the sheet and also when stepping parallel to the surface.
a scanning Auger microscope, a slight improvement only, since the time for a single Auger run cannot be much shortened, especially due to the low sensitivity for Li. Moreover local changes of the Li concentrations have to be distinguished from the basic error of the Auger measurement. For this purpose a careful interpretation of the measurements is necessary. Despite these disadvantages it is shown that during a solution treatment neither the oxidation rates nor the diffusivities are constant over a whole sL. ,t. The question arises, whether the results of our investigations agree with the results already published, especially those obtained by direct Li concentration measurements. Fig. 5 shows Li distribution curves obtained by averaging a set of the measured curves for the argon treated sample (broken line) and the sample solution treated in air (solid line), respectively. For either sample the averaging results in a quite smooth line with a diffusion-like profile. Although the course of the averaged curves depends on the choice of the particular, measured profiles, smooth depletion curves result in every case when averaging at least six profiles. Within the usual error of diffusion data (see 1191 and references herein) the curves can be explained and simulated by the usual error function solution at a given, constant surface concentration [241. Thus, no principle contradiction to previous results occurs. Since the diffusion profiles, calculated at the condition of constant diffusivity in a homogeneous material, fit the averaged Li concentration profiles quite well, the Li diffusivity is obviously enhaPced along several paths combined with regions of an impeded diffusion. Moreover, concentration gradients also occur parallel to the surface, One dimensional diffusion mt×lels can therefore present only a crude approximation of the real diffnsion behaviour, The oxidation rate varies over a wide range within small distances on the surfilce, Even with the sample solution treated in air we tbund a few areas with no or almost no oxidation. These were exactly those areas where no pores were found in the subsurface band combined with almost no Li depletion. Obviously the oxidation rate depends
4, Dbcu~lon Although the applicability of AES on the measurements of Li depletions in an Al~Li alloy is limited, there are advantages of this method: the investigations can be performed in almost every standard equipment', local changes of the Li concentrations can be detected up to a spatial resolution of l0 p.m or even less. The limit of this resolution is given mainly by the diameter of the primary exciting beam. Since several types of electron guns are available wi$ beam diameters much smaller than we used, an improvement of the spatial resolution seems to be obvious. Reducing the beam diameter means, however, an enhanced current density with the consequence of a damaged surface with an altered composition. A clear disadvantage of our measurements is their consumption of time. An improvement could be the use of
120 'I 100 80 ~
H,S
40
averaged Li concentrations samples 5 and 6
0 . .
0
.
i
20 40 60 80 I00 120 140 160 181 depth in micron
Fig. 5. Averaged AES proliles of samples treated ill air {solid line} and in argon,
T. SchiJberl, S. Kumar ! Journal of Alloys and Compounds 255 (1997) 135-141
much more on the original local constitution of the surface than on the composition of the surrounding gas during a heat treatment. The variety of the diffusivities along different paths accompanied by high pore densities is obviously initiated by the great variety of the respective local oxidafon rates on the surface. Since, to our knowledge, there is a great lack in understanding the specific primary reactions on AI-Li surfaces, we plan to perform in situ oxidation experiments in this field. The application of microhardness measurements for an estimate of Li concentrations in an AI-Li alloy is severely limited. The region below the surface shows, at many cross sections, a very high density of pores of enhanced size up to depths equal to the largest depletion depths obtained by AES. The size of a particular indent strongly depends on its distances to the surrounding pores and their size; moreover it may even happen that an indent coincides with a pore, both effects leading to a large scatter in the hardness values as obtained by our measurements. Though the existence of pores is roughly related to the actual Li concentration, a hardness much lower than the real one of a compact, homogeneous alloy matrix might be ~magined. A typical misinterpretation is demonstrated for several profiles of the argon treated sample: the microhardness measurements indicate an extended soft subsurface zone. In fact the reduced (pseudo) hardness values are, to a large extent, a consequence of the pores. The depth of this porous region coincides with the depth of reduced "pseudo" hardness. Conclusions made about the Li concentration profile are in contrast to the results obtained by AES in this case. Even if the Li depletion does not lead to pores of a size visible by any kind of microscopy, there arises a deficiency of atoms in the lattice until vacancies and/or pores are, refilled by the diffusion of AI from the surface. These "missing" atoms lead in every case to a softening effect superimposed to a lowered or no precipi° late hardening owing to lowered Li concentrations. The combination of these effects makes an interpretation of hardness values in terms of Li concentrations at least doubtful.
$. Conclusions Despite its severe detection limits Auger electron spectroscopy can be applied to measure Li depletion profiles in AI-Li alloys. By this method it is possible to obtain Li depletion profiles with a spatial resolution much better than can be obtained by methods applied in previous investigations. By this improved resolution in the direction perpendicular to as well as parallel to the surface it has been shown that the local oxidation rate on the surface varies considerably. The resulting diffusion profile and the
141
density of pores beneath the respective areas depend strongly on this local rate. Averaging sets of depletion profiles results in smooth diffusion-like Li concentration profiles as obtained in previous works with much lower spatial resolution.
Acknowledgments This work was partly supported by the Austrian Fends zur F6rderung der Wissenschaftlichen Forschung under project number P 9361-TEC. Moreover the authors wish to thank Mr.~. Edeltraud Jaschouz and Mrs. Gabriele Moser for the preparation of the samples and for the performance of optical microscopy.
Re:erenees [11 K.K. Sankaran and N.J. Grant, Mater. ScL Eng., 44 (1980) 213. [2] F. Abd EI-Salam, A.I. Eatah and A. Tawfik, Phys. Status Solidi a. 75 (1983) 375. [31 J.A. Weft and A.B. Ward, Scripta metalL. I~ (1985) 367. [4] H. Ueda, A. Matsui, M. Furukawa, Y. Miura and N. Nemoto, I, jpn. h~st. Met., 49 (1985) 562. [5] D.S. McDermaid. S. Fox and H.M. Flower, European materials research society conference, Advanced mmerials research and development for transport, 26-28 Nov., 1985, Council of Europe, Strasbourg, 1985. {6] R.F. Ashton. D.S. Thompson, E.A. Starke Jr. and F.S. Lin. in C. Baker, P.J. Gregson, S.J. Harris and CJ. Peel (ed.), AluminiumLithium Ill, Institute of Metals, London, 1986, p. 66. [7l A.F. Smith, ibid, p. 164. 181 S. Fox, H.M. Flower and D.S. McDemlaid, ibid, p. 203. [9l M. Burke and J.M. Papazian, ibid, p° 287. [lOl S, Fox, H,M, Flower and D.S, Mcl~rmald, Script. Met.If,, 20 ([986) 7[, III] J.M. Papazian, R.L. Schult¢ and EN0 ABler, Metall, Trans., 17A (1986) tr)3~. [121 R.L0 Schulte, J.M° Papazian and P.N. Adler. Nut/. in.~mo,. Mtth. P/O'so ICeso,BI5 (1986) ~i~O, 113] J.M. Pap,'eian, GG. Bolt and R Shaw, Matero ScL Eng., 94 (1087) 219. 1141 M, Ahmad, Metall. Trmts,. 18A (1987)681. 1151 P. Holdway and A.W. Bowen, J. Mater. Sci., 24 (1989) 38410 1161 J.M. Papazian and R.L. Schulte, Metal/° Trans., 21A (|~OO) 39. 1171 J.M. Papazian, J.P.W.Wagner and WeD. Rooney, J. Phy~°, 48 {1987) 513. 1181 T. Sch/Jberl, Surf. Sci., 326 (1995) 267. 1191 Y. Minamino, T. Yamanc and H. Araki, Metallo Trons., 18A (1987) 1536. 120l H,14, Madden and J,E. llouston. J. ~co .~i,,i,1"echnolo, 14 (1977) 412. 1211 R.E. Clausing, D.S. Easton and G°L. Powell. Sur]~ Sci., 36 (1973) 377. 122l I.F. Ferguson, D.R. Masters. B.F. Riley and M. Turek, Le vide. les Couche~ Minces. 211 (1983) 279. [231 J,A, Bearden and A.F. Burr. Rev. M¢nl. Phys.. 39 (1967) 12'3. 124l J. Crank, The Malhematics of D~usion. Clarendon, Oxtbrd. 1975.
AND ~ ~ D : ; ELSEVIER
Journal of Alloys and Compounds 255 (1997) 135-141
Depletion of lithium due to surface oxidation: An investigation of an A1-Li-sheet by Auger-spectroscopy T h o m a s Sch6berl*, Subodh K u m a r Erich Schmid lnstitut fiir Festkiirperphysik der Osterreichischen Akademie dcr Wissenschaften. JahnstraJ3e 12. A-8700 Leoben Austria
Received 28 June 1996; revised 26 August 1996
Abstract The depletion of Li beneath the surface of an AI-Li sheet as a consequence of surface oxidation is investigated by Auger electron spectroscopy. By the improved spatial resolution compared to other sampling methods it is shown that the local oxidation rate varies over a wide range on a !0 Ixm scale. The local oxidation rate gives rise to a local Li depletion combined with the formation of pores in the underlying subsurf. :e region: these pores have a strong influence on the diffusivities perpendicular to the surface. An average of the strongly varying Li concentration profiles resembles, however, the typical diffusion=like depletion curves which have been reported in previous investigations and can be described by a siPlple solution of a one dimensional diffusion equation. The results are compared with microhardness measurements: it turns out that microhardness values can give a qualitative estimate of the Li depletion as long as no high density of large pores occur in a band beneath the ,,ufface. Keyword.w Aluminium: Lithium: Auger spectroscopy: Surface oxidation: Heat treatment: Lithium depletion
I. Introduction Since the addition of only I wl% lithium (Li) to aluminium (AI) reduces the densily by 3% and increases the elastic modulus by 6% Ill, Al:~Li alloys are very auractive, especially for aerospace applications. The high strength of these alloys arises from the formation of Al~Li(5') precipitates. However, in order to I'olm these precipitates, the alloy has to be solution treated at elevated temperatures for a sufficiently long, period prior to the ageing treatment. Since at these temperatures Li has quite a high diffusivity in A! and is highly reactive with oxygen, it diffuses to the surface and reacts with the surrounding gas atmosphere during the solution treatment. An oxide layer is formed on the surface, a layer which unfortunately does not protect the alloy I~om further oxidation, leading to a Li depleted region beneath the oxide~ailoy interface. This region cannot, or only to a lesser extent, be strengthened by a subsf-$uent ageing treatment. This fact has important consequences for the fabrication, heat treatment and application of AI-Li based alloys, especially as a thin sheet material. Therefore the depletion of Li during a solution treatment has been the subject of several studies [2~171. Most of these studies rely, however, on indirect evalua*Corresponding author. 0925-8388/971517.00 © 1997 Elsevier Science S.A. All rights reserved PII S0925-8388(96~02818-6
lions of the Li depletion, mainly by microhardness menu surements, or on eddy currenl 131 and X~ray diffraction studies (e.g. 1141). We are aware of only a few studies where Li was delected directly, eog~ by a nuclear reaction analysis lechnique 111,16i. In Ihese studies the ~J,rea sampled along a cross section wa.,,, however, quite lal'ge (~-~40 p,n l up to 100 p~m); thus, if there had been inhomogenities in the Li distribution within smaller areas they could not have been detected. The Li depletion is usually observed in combination with the t)ccu~!¢¢ of pores in the vicinity of Ihe surface. The occurrence of the~e pores has been explained by a Kirkendallolike effect as a consequence of the Li depletion [ I I |. We are, however, not aware of any experimental sludy showing a direct relation between Li depletions and the formation of pores. The number of experimental methods, by which Li can be detected, especially within a small sampled volume, is limited; one of them is Auger electron spectloscopy (AES), but with severe limitations as will be shown and discussed below. AES is basically a surface sensitive technique. The common way to apply it to depth profiling is either to lake spectra while the surface is removed continuously by noble gas ion spattering, or by sampling along a cross section cut perpendicular to the surface. Due to the usu~l low sputter° ing rates (~0.1 nm/s) the first method would require
136
T. Schiiberl. S. Kmnar / Jounud t!f AIhJ.vs and Conff~ounds 255 (1997) 135-14.1
extremely long experimental times, usually leading to additional undesired effects. Thus the second method is adopted for the present investigation. The aim of this work is to measure by AES Li depletion profiles of an Al-2.8 wt% Li sheet after solution treatment. In order to test the applicability of microhardness measurements their results are compared with the AES results. As Auger spectroscopy has, to our knowledge, rarely or not yet been applied to the Li depletion effect in such AI-Li alloys, the limits of the applicability of AES are checked and discussed. Depth profiles from regions with a high density of pores are compared with those taken from regions where almost no pores could be detected by optical microscopy. We also examine the influence of a surrounding argon atmosphere during the solution treatment.
2. Experimental procedure An Al-2.8 wt% Li alloy was cast as 10 mm thick plates by melting high purity AI (99.99%) and Li (99.9%) in an argon atmosphere and let it solidify in an iron mould. The as cast plate was i~omogenized in an argon atmosphere at 550°C for 6 h. The plate was lightly ground on emery paper to remove the visible oxide layer and was cold-rolled to a I mm thick sheet. Small samples from this sheet were solution treated at 530°C for 40 rain either in air or in an argon atmosphere and quenched in water at room tempera° ture. For the "argon" treatment the ,~ample was kept inside a steel box and argon with a purity of 99.999% was passed through the box. The solution treated samples consisted of recrystallized grains with an average diameter of 50 t~m. The AES measurements were performed in a standard commercial ultra high vacuum (UHV) chamber, pumped by a turbomolecular pump and a set of triode pumps: the apparatus has ~ e n descried in detail elsewhere l lgl, Sufficiently good vacuum conditions could only be achieved by baking out the complete vacuum system, Such a bakeout means, however, an unavoidable (additional) heat treatment of the sample, Thus the temperature and the duration c,,fthe bakeout had to be chosen as a compromise between the remaining partial pressures of the residual gases, mainly of oxygen, and non°altered depth distribu° tions of Li, After a "soft" bakeout of the system at 135°C fi)r 30 h the residual pressure was below 10 '~'~'Tort for all experiments, the surface contamination by the residual gas was slow enough to run at least some complete concentration profiles without the necessity of any cleaning pr~ess, An estimate of diffusion path lengths using diffiJsion data e.g. of J l9J shows that by this bakeout the Li distributions are in no case changed over distances exceed° ing some I0 nm, The Li distributions were measured spotwis¢ by AES along several cross-sections of the samples which were mounted on a commercial x-y-z sample holder, additionally rotatable in the z~di~ction, with their polished cross
section perpendicular to the primary beam. The nominal diameter of the primary beam was 8 lxm, in practice the spot diameter turned out to be -10 Ixm. The primary beam current was -3 p,A at a voltage of 2 kV. The Auger electrons emitted from the sample were analysed by a cylindrical mirror analyser (CMA) in the diVidE mode (the differential signal of N registrated electrons at their respective energy E) with a 5 Volt modulation amplitude. The axis of the CMA was parallel to the primary beam. As the amplification of the secondary electron multiplier in use drifted, we used as a basic measure for the Li concentrations the relation of the signal heights of Li and AI. As we were interested primarily in the depletion of Li caused by the anneals at various conditions, an absolute determination of the Li concentrations was not necessary. The most pronounced Auger transition due to Li in the alloy was tbund at 39 eV which is reported to stem either from oxidized Li [20,21], more specifically Li:O [22], or from Li in several compounds [23]. As the peak position for Li in the alloy turned out to be almost identical to f~hat of oxidized Li and the Auger sensitivity being much larger for the oxidized Li, great care had to be taken to ascertain a clean metallic surface. The Auger peak-to-peak height at the 39 eV transition was used for the determination of relative Li contents. The most dominant Auger transition for AI has an energy of 68 eV for metallic AI and 5 ! eV for aluminium oxide. The 68 eV transition was ased to determine the particular Auger peak height ratios Li/AI as a measure lot the relative Li content, the 51 eV transition served as a control of the oxygen uptake during the experimental runs. The heavy contamination by oxygen and carbon, registered after mounting the samples into the UHV chamber, was removed by Ar°ion sputtering for I rain. Tile beam energy was 3 kV, the Ar partial pressure 6x 10 =~ Tort' resulting in a beam current density of' 25 ~A/cm ~, For the subsequent cleaning procedures in between the particular experimental runs a sputter time of 20 s was sufficient to remove all contaminants, newly adsorbed even at UHV conditions. In order to check the alteration of the Li distribution during sputtering we measured one profile repeatedly with a prolonged sputter time between the measurements. No noticeable influence on the results was fi~und as long as the sputter time did not exceed 2 rain. The Li concentration profiles were taken moving along the cross section of the specimen usually in steps of l0 gm up ~a a depth of 180 Ixm. The position of the edge was defined as the position where the signal height of the 68 eV Auger transition for AI had half its bulk value, The individual Li/AI signal height ratios along a profile were normalized by dividing them by the average of the ratios obtained from five measurements taken at various spots in the centre of that particular cross section. The microhardness measurements were performed after the AES measurements, They were done on an optical microscope with a microhardness equipment, A separate ageing treatment was not necessary since the baking out
T. Schiiberl. S. Kumar I Journal of Alloys and Compounds 255 (1997) 135-141
137
procedure in the UHV chamber before the AES investigations also caused the precipitation in the Li enriched regions. The microhardness was determined along several cross sections of the sheet cut perpendicular to the surface making indents at every 20 I~m. The values obtained were divided by average values obtained at the inner region of that particular cross section.
3. Results
(a)
3. i. Optical and scanning electron microscopy (SEM) Before mounting the samples in the UHV chamber their polished cross sections were inspected by optical microscopy. Some areas of special interest were viewed additionally by SEM. The samples contained pores already in the cold-rolled state with a size from just visible up to some I~m. In these unannealed samples the pores were distributed quite homogeneously over the whole of the cross sections. The anneals caused, however, an increased density of pores within a region of 80-100 p,m beneath the surface combined with a significant decrease of the density in the inner region of the sheet, The density of the pores and their size varied over a wide range for different cross sections. The largest pore sizes near the surface, combined with the largest scatter in size and density, were found at several cross sections of the sample solution treated in argon (Fig. in). Etching the cross sections resulted in clearly visible grain boundaries. The procedure was: etching for 2 rain in a mixture of 15 ml HNO~, 0,5 ml HF, 3 g CrVI oxide and 84 ml distilled water, then cleaning in distilled water and finally etching in a mixture of 75 ml distilled water, 5 ml HNO~, 3 ml HCI and 2 ml HF. No systematic relation between the location of grain boundaries and the distribution of the pores could be found (Fig. Ib), a relation which has been suggested qualitatively by Papazian et al. [171. Some areas of the sheet surface were (still) covered with an oxide layer with a thickness varying from a few pom up to 45 Ixm (Fig. Ic). This oxide layer was found only at the surface of the samples heat treated in air. The larger part of this layer, containing pores as in the subsurface band, was split off, most probably during the quenching process. The SEM microgmph as well as the optical microscopy suggest that splitting off the oxide layer should mainly occur at the interface clearly visible between oxide layer and alloy matrix. Then the local layer thickness is mainly determined either by the local oxidation rate on the surface (interface) or by the diffusivities along the particular paths beneath the interface or by a combination of both. We found a qualitative relation between the thickness of the oxide layer and the density of pores below the layer; increased pore, densities arc accompanied by a larger thickness of the layer, but no real quantitative relation occurred. By Auger analysis the oxide overlayer is found to consist mainly of Li, oxygen
(b)
Fig. I. Polished cross ~eczion~of ~mple~ a ~ r d i f f ~ . t hect l~at~|~t~', a) solution treated in argon, high density of pore~, etd'~d; b) ~lulte,n treated in air, etched: c) SEM microgr~Fnh with the oxide overl~yer (~ir treated sample, unetched).
and, to a smaller extent, of carbon and AI. However, the exact composition could not be determined. 3.2 Microhardness measuremems Fig. 2a shows the relative microhardness along several cross sections of the sheet in the cold-rolled state prior to the solution treatment. The values of the microhardness had fluctuations up to _+20% with a tendency of a slight softening towards the surface. For the samples heat treated in air a clearly softened subsurface zone was ob~rved reaching up to ~100 Ixm (Fig. 2b). Also the samples heat treated in argon show a softened subsurface zone which is, however, in several cases even deeper than observed with
138
7". Schiiherl. S. I(um.r t Journal oj'AIIoy.~ atld Compomld.~ 255 (1997) 135-141
120
10o 80 ".
"'....%.o/
• •
4o 20
0
,
. |
.
.
,1 sample 4, unannealed, .
.
.
,
120
80
~ 20
F
sample 5, annealed in lab air
o .,, . . . . . .
..,
120 .~'.
80 .
,~'
,
..
i
~
~ 4O
(c)
0
annealed in argon 0
20 40 60 80 I(~ 120 140 160 180 depth in micrometer
l:ig, 2, P~'olile~:~) ¢oh!:rolled ~lal¢',h) soltltio,treated in .It:¢) ~ohttloo
the air treated sample (Fig. 2c): it should t~ noted that the cross section investigated by microhardness measurement was the cross section with the largest density of pores which could be found in the argon treated sampie as well as in any other sample, The fluctuation observed with these profiles is the largest one of all. frequently exceeding ~20%.
3.3. Auger measurements
As, to our knowledge, Li concentrations in these alloys have noi yet been deter~r,ined by AES, we I~rformcd some prelimimwy experiments: 1) Consecutive Auger measuremet~ts (55) we~ performed fixing the spot of the primary beam at one point in the centre of a sputter cleaued cross section, Due to the weak 39 eV !,i signal the scatter is quite iarge, up to ± 10%, On the other hand no long time drift could be
detected, a drift caused either by oxygen adsorption or by the primary electron beam. 2) In order to check the influence of both these effects on the Li/A! AES peak height ratios, a surface, freshly sputter cleaned, was exposed to I x 10 -7 Torr 0 2 for half an hour. The resulting oxygen adsorption lead to an increase of the 39 eV signal, combined with the occurrence and increase of the 511 eV oxygen signal and the 51 eV signal due to aluminium oxide. A prolonged bombardment of this surface by the primary electron beam lead to a slow, continuous decrease of these signals• Obviously an electron stimulated deoxidation occurred both for AI and Li. Electron stimulated desorption (ESD) is often found for oxygen on metallic surfaces. Although ESD obviously occurred, it could not be detected by the mass spectrometer in use, most probably because of the very small desorption rate. Next we checked if there were any possibility to "decorate" Li by oxidation in order to obtain more pronounced 39 eV signals leading to a lowered detection limit for Li. For that purpose a depth profile was taken for a surface heavily oxidized by the procedure described above; then after sputter cleaning the surface was investigated again. No linear or systematic relation between the signal heights obtained with the clean and with the oxidized surface was found. All results confirm, however, that, even in an oxygen-free state, Li in AI shows the Auger transition at 39 eV. Prolonged sputtering combined with a consequent, deoxidizing exposure to the electron beam leads in no case to the disappearance of the respective Auger signal. During sputtering the signal is only less than half of the signal obtained promptly after switching off the sputter benin. Li seems to be sputtered pret~rentially in the present alloy, 3) In order to check to which extent the I,i/AI signal height ratios are reproducible, several prattles were measured relatedly, In most cases pronounced zigozag curves were obtained. The peak locations of these curv,'~s were otten not reproducible better than within 15 I~m. The peak to peak heights were frequently scattered up to ±20% within short distances and can therelbre not be explained only by the error of the Auger measurement itself, The existence of the pores observed in the subsurface zone (up to a ~m scale) might be one reason for the occurrence of non smooth real Li distributions. But if so, then the peak positions should be reproducible in a spatial scale of 10~m for several runs on the same profile, This was not the case, as stated above. It should be noted that during the measurements the mechanical pumps of the UHV apparatus lead to mechanical vibrations of the whole UHV apparatus and therefore to a deteriorated spatial resolution of the AES measurements, In principle the measurements could be done using the vibrationless triode pumps only, leading in fact to an improved spatial resolution, Performing the measurements without running the mechanical pumps pemmnently took, however, a lot of time; the unavoidable sputter procedures between the measurements
T. Sch6beri, S. Kumar I Journal of Alloys and Compounds 255 (1997) 135-141
required a high Ar partial pressure. Large amounts of noble gases should, however, never he pumped by triode pumps. Also, when having stopped and started aga'n, a turbomolecular pump takes at least 12 h to recover good vacuum conditions. Thus we decided to perform further standard measurements usually keeping the mechanical pumps running. In cases either of special interest or doubts in the results we made the measurements with the mechanical pumps switched off. The diagrams for the AES results should be read and interpreted as follows: the error in the relative Li concentrations is about +_10%. A value of 5% means that Li was detected but its amount was certainly below 10%. Missing or zero values in the graphs mean that the Li content was below the detection limit. Additionally to the error in the relative Li concentrations the uncertainty in the zero position (the location of the edge) must be kept in mind. Fig. 3a shows the Li concentration profile for a sample in the cold-rolled state. The fluctuations of the
120 .......... ............. .
? 60 f
.....
%f
-"
"
" - ' ~ .......
~i
V
~ 4O
(a)
'~ ~o ]sample 4, unannealed 120
1IOO6o 80
,~,,#~,.
Z
,'
i;::;: .....
.'TJ ..'.....
2o ¢ 0
.
/
.
...'
......."...
.t.
.
.
sample 5, annealed in lab air
120 '~ 100
60
0 0
20 40 60 80 100 120 140 160 180 depth in micrometer
Fig. 3. Protiles of the AES Li/AI signal height ratios: a) cold-rolled sample; b) solution treated in air; c) argon sample.
139
measured concentration values are frequently larger than can be explained by experimental uncertainty. Ignoring the fluctuations a slight Li depletion extending to a depth of ~100 I~m is obtained for the samples in the cold-rolled state. This fact should be a consequence of the homogenization treatment of the samples before rolling. The flat depletion agrees qualitatively with the results of the microhardness measurements (compare with Fig. 2a). Pronounced depletions of Li appear with all the samples solution treated, in air as well as in argon. The particular curves obtained for the samples treated in air fairly resemble each other (Fig. 3b): the Li concentration at the surface was zero or near zero for most of the profiles. The depletion depth, however, varied from 80 to 180 p,m. The largest depletion depths were found beneath those surface areas where the largest thickness of the oxide overlayer could be found, if it had not been split off. As the oxidation rate on the surface and therefore the Li depletion rate during the heat treatment might be influenced by surface roughness we investigated a sample with one side of the sheet polished to a mirror finish. The influence of the enhanced surface smoothness is rather small; the Li depletion depths are ~i00~m. On the other hand no profile with noticeably larger depths was found. Two of the profiles start from surface concentrations being remarkably higher than zero. The polishing procedure obviously results in only a few areas with a reduced oxidation rate. The results obtained for the sample solution treated in argon (Fig. 3c) show, at first sight, a situation somewhat more complicated, in general, there arises a depletion of Li during the anneals. In those cross sections where the size and density of the pores resembled the situation of the air treated sample, also the Li depth profiles are very similar. For the cross sections with large pores of high density in the subsurl~ce zone a noticeable scatter between the particular depth proliles occurred. Four principal types can be distinguished in a set of 37 profiles: Li surface concentrations of zero or near zero combined with either ahnost no Li depletion or a depletion depth up to 180 p,m, as well as Li surface concentrations up to 60%, again combined with either high or almost no depletion depth. Since the results suggest a relation between the density and size of pores in the subsurface region and the Li depletion in this zone, we measured a few profiles in a region with no visible pores and in a region with many pores of large size. The respective regions had been chosen and clearly marked under the optical microscope. These measurements were pertbtmed with the mechanical pumps switched off in order to minimize the errors in the profiles. in Fig. 4a two profiles with a mutual distance of 20 p,m are shown taken from a pore-free region of a cross section of the air treated sample. Despite only a slight Li depletion the surt~ce concentration is near zero. Taking into account the basic error of the Auger measurement itself the curves can be regarded as smooth. A completely different situation occurs with the profiles taken from an extremely
T. SehiJberl, S. Kumar i Journal qf Alloys am~ Compoumts 255 (1997) 135-141
140
120 "~ I00 8 gO ~ 4O 0
Y
Iro,,onwi ,owporo,i
| i,
.
.
.
.
120
" t0o ,.t 41)
.
(b)
] 2o 0 . . . .
.......... /'"/
I
regionwith high porosity
Fig. 4. Two neighbouring AES proliles: a) taken at a non-porous region of a ~ample heat treated in air; b) at an extremely porous region of an argon ~.mple
porous section of the sample treated in argon (Fig. 4b); the cun,¢~ obtained ate completely different. One concentration protile (solid line) is a pronounced zigozag line with no ~rong Li depletion, The suri',ce concentration is almost us high as within the bulk. Stepping only by 20 I~m along the ~urface the Li concentration is zero; the respective ~second) profile is rather smooth with a depletion depth of 100 I~m. The results confirm that pores of large si~,e are accompanied by steep concentration gradients, positive as well a~ negative when ~tepping towards the centre of the sheet and also when stepping parallel to the surface.
a scanning Auger microscope, a slight improvement only, since the time for a single Auger run cannot be much shortened, especially due to the low sensitivity for Li. Moreover local changes of the Li concentrations have to be distinguished from the basic error of the Auger measurement. For this purpose a careful interpretation of the measurements is necessary. Despite these disadvantages it is shown that during a solution treatment neither the oxidation rates nor the diffusivities are constant over a whole sL. ,t. The question arises, whether the results of our investigations agree with the results already published, especially those obtained by direct Li concentration measurements. Fig. 5 shows Li distribution curves obtained by averaging a set of the measured curves for the argon treated sample (broken line) and the sample solution treated in air (solid line), respectively. For either sample the averaging results in a quite smooth line with a diffusion-like profile. Although the course of the averaged curves depends on the choice of the particular, measured profiles, smooth depletion curves result in every case when averaging at least six profiles. Within the usual error of diffusion data (see 1191 and references herein) the curves can be explained and simulated by the usual error function solution at a given, constant surface concentration [241. Thus, no principle contradiction to previous results occurs. Since the diffusion profiles, calculated at the condition of constant diffusivity in a homogeneous material, fit the averaged Li concentration profiles quite well, the Li diffusivity is obviously enhaPced along several paths combined with regions of an impeded diffusion. Moreover, concentration gradients also occur parallel to the surface, One dimensional diffusion mt×lels can therefore present only a crude approximation of the real diffnsion behaviour, The oxidation rate varies over a wide range within small distances on the surfilce, Even with the sample solution treated in air we tbund a few areas with no or almost no oxidation. These were exactly those areas where no pores were found in the subsurface band combined with almost no Li depletion. Obviously the oxidation rate depends
4, Dbcu~lon Although the applicability of AES on the measurements of Li depletions in an Al~Li alloy is limited, there are advantages of this method: the investigations can be performed in almost every standard equipment', local changes of the Li concentrations can be detected up to a spatial resolution of l0 p.m or even less. The limit of this resolution is given mainly by the diameter of the primary exciting beam. Since several types of electron guns are available wi$ beam diameters much smaller than we used, an improvement of the spatial resolution seems to be obvious. Reducing the beam diameter means, however, an enhanced current density with the consequence of a damaged surface with an altered composition. A clear disadvantage of our measurements is their consumption of time. An improvement could be the use of
120 'I 100 80 ~
H,S
40
averaged Li concentrations samples 5 and 6
0 . .
0
.
i
20 40 60 80 I00 120 140 160 181 depth in micron
Fig. 5. Averaged AES proliles of samples treated ill air {solid line} and in argon,
T. SchiJberl, S. Kumar ! Journal of Alloys and Compounds 255 (1997) 135-141
much more on the original local constitution of the surface than on the composition of the surrounding gas during a heat treatment. The variety of the diffusivities along different paths accompanied by high pore densities is obviously initiated by the great variety of the respective local oxidafon rates on the surface. Since, to our knowledge, there is a great lack in understanding the specific primary reactions on AI-Li surfaces, we plan to perform in situ oxidation experiments in this field. The application of microhardness measurements for an estimate of Li concentrations in an AI-Li alloy is severely limited. The region below the surface shows, at many cross sections, a very high density of pores of enhanced size up to depths equal to the largest depletion depths obtained by AES. The size of a particular indent strongly depends on its distances to the surrounding pores and their size; moreover it may even happen that an indent coincides with a pore, both effects leading to a large scatter in the hardness values as obtained by our measurements. Though the existence of pores is roughly related to the actual Li concentration, a hardness much lower than the real one of a compact, homogeneous alloy matrix might be ~magined. A typical misinterpretation is demonstrated for several profiles of the argon treated sample: the microhardness measurements indicate an extended soft subsurface zone. In fact the reduced (pseudo) hardness values are, to a large extent, a consequence of the pores. The depth of this porous region coincides with the depth of reduced "pseudo" hardness. Conclusions made about the Li concentration profile are in contrast to the results obtained by AES in this case. Even if the Li depletion does not lead to pores of a size visible by any kind of microscopy, there arises a deficiency of atoms in the lattice until vacancies and/or pores are, refilled by the diffusion of AI from the surface. These "missing" atoms lead in every case to a softening effect superimposed to a lowered or no precipi° late hardening owing to lowered Li concentrations. The combination of these effects makes an interpretation of hardness values in terms of Li concentrations at least doubtful.
$. Conclusions Despite its severe detection limits Auger electron spectroscopy can be applied to measure Li depletion profiles in AI-Li alloys. By this method it is possible to obtain Li depletion profiles with a spatial resolution much better than can be obtained by methods applied in previous investigations. By this improved resolution in the direction perpendicular to as well as parallel to the surface it has been shown that the local oxidation rate on the surface varies considerably. The resulting diffusion profile and the
141
density of pores beneath the respective areas depend strongly on this local rate. Averaging sets of depletion profiles results in smooth diffusion-like Li concentration profiles as obtained in previous works with much lower spatial resolution.
Acknowledgments This work was partly supported by the Austrian Fends zur F6rderung der Wissenschaftlichen Forschung under project number P 9361-TEC. Moreover the authors wish to thank Mr.~. Edeltraud Jaschouz and Mrs. Gabriele Moser for the preparation of the samples and for the performance of optical microscopy.
Re:erenees [11 K.K. Sankaran and N.J. Grant, Mater. ScL Eng., 44 (1980) 213. [2] F. Abd EI-Salam, A.I. Eatah and A. Tawfik, Phys. Status Solidi a. 75 (1983) 375. [31 J.A. Weft and A.B. Ward, Scripta metalL. I~ (1985) 367. [4] H. Ueda, A. Matsui, M. Furukawa, Y. Miura and N. Nemoto, I, jpn. h~st. Met., 49 (1985) 562. [5] D.S. McDermaid. S. Fox and H.M. Flower, European materials research society conference, Advanced mmerials research and development for transport, 26-28 Nov., 1985, Council of Europe, Strasbourg, 1985. {6] R.F. Ashton. D.S. Thompson, E.A. Starke Jr. and F.S. Lin. in C. Baker, P.J. Gregson, S.J. Harris and CJ. Peel (ed.), AluminiumLithium Ill, Institute of Metals, London, 1986, p. 66. [7l A.F. Smith, ibid, p. 164. 181 S. Fox, H.M. Flower and D.S. McDemlaid, ibid, p. 203. [9l M. Burke and J.M. Papazian, ibid, p° 287. [lOl S, Fox, H,M, Flower and D.S, Mcl~rmald, Script. Met.If,, 20 ([986) 7[, III] J.M. Papazian, R.L. Schult¢ and EN0 ABler, Metall, Trans., 17A (1986) tr)3~. [121 R.L0 Schulte, J.M° Papazian and P.N. Adler. Nut/. in.~mo,. Mtth. P/O'so ICeso,BI5 (1986) ~i~O, 113] J.M. Pap,'eian, GG. Bolt and R Shaw, Matero ScL Eng., 94 (1087) 219. 1141 M, Ahmad, Metall. Trmts,. 18A (1987)681. 1151 P. Holdway and A.W. Bowen, J. Mater. Sci., 24 (1989) 38410 1161 J.M. Papazian and R.L. Schulte, Metal/° Trans., 21A (|~OO) 39. 1171 J.M. Papazian, J.P.W.Wagner and WeD. Rooney, J. Phy~°, 48 {1987) 513. 1181 T. Sch/Jberl, Surf. Sci., 326 (1995) 267. 1191 Y. Minamino, T. Yamanc and H. Araki, Metallo Trons., 18A (1987) 1536. 120l H,14, Madden and J,E. llouston. J. ~co .~i,,i,1"echnolo, 14 (1977) 412. 1211 R.E. Clausing, D.S. Easton and G°L. Powell. Sur]~ Sci., 36 (1973) 377. 122l I.F. Ferguson, D.R. Masters. B.F. Riley and M. Turek, Le vide. les Couche~ Minces. 211 (1983) 279. [231 J,A, Bearden and A.F. Burr. Rev. M¢nl. Phys.. 39 (1967) 12'3. 124l J. Crank, The Malhematics of D~usion. Clarendon, Oxtbrd. 1975.