Ultra-high-resolution chemical analysis by field-ion microscopy, atom probe and position-sensitive atom-probe techniques

Ultramlcroscopy 47 (1992) 199-211 North-Holland

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Ultra-high-resolution chemical analysis by field-ion microscopy, atom probe and position-sensitive atom-probe techniques C R M G r o v e n o r , G D W Smith, A C e r e z o , J A Llddle, R A D M a c k e n z i e , P J W a r r e n , R P Setna, J M Hyde, J E Brown, I Stark and B A Shollock Department of Materials, Oxford Unwerslty, Parks Road, Oxford OX1 3PH, UK Recewed at Editorial Office 8 January 1992

This paper describes some recent results on the use of field-Ion microscopy and atom-probe techmques m the study of the fine-scale chemistry of a range of different materials It ~s shown that field-|on ~mages of the early stages of precipitation m metallurgical alloys can gwe morphological reformation before any significant contrast can be achieved by conventional transmission electron microscopy (TEM), and that the composition of these nanometer-scale particles can be accurately analysed by the use of atom-probe mlcroanalysls In addltmn, the recent development of the p o s m o n - s e n s m v e atom probe (POSAP) allows a three-dimensional composition map to be obtained of the elemental distribution m and around these parhcles In this way a more complete picture can be obtained of the morphology and chemistry of complex, fine-scale structures than Is readdy obtainable from TEM-based techmques

I. Introductlon

The a t o m - p r o b e / f i e l d - i o n microscope is a tool ideally suited for the analysis of the morphology and chemistry of metallurgical materials on the nanometer scale The basic features of this technique have been fully described elsewhere [1,2] The applications to the study of materials can be divided Into two broad areas, although they are generally used together imaging the surface structure of a polished field-ion tip, and chemical analysis of selected regions of the structure by controlled field evaporation of the tip and mass analysis of the resulting ions The potential for resolving the atomic structure of surfaces inherent in a field-ion image has been the subject of several publications, as reviewed in Miller and Smith [1] The absolute resolution limit In the image has been calculated by de CastIlho and K m g h a m [3] At normal imaging temperatures of between 20 and 80 K the resolution in a field-ion Image will always be expected to be better than 0 5 nm, and with image gas atoms of small diameter and low spe-

clmen temperatures the resolution hmlt approaches 0 2 nm When the requirement is to detect the presence and position of impurity atoms then parameters such as the ionization behavlour and the evaporation field of individual atoms on a field-ion tip of a given material become important [4] As a rule of thumb the more refractory an element IS the more brightly It will image under normal conditions It is thus much easier to detect the position of atoms of a refractory metal in a matrix of lower melting point (1 e molybdenum in steels) than, for instance, zinc in alumlnium alloys Similar problems arise when attempting to study small precipitates Only if the precipitate images In a manner significantly different from the matrix material will strong phase contrast be seen in the field-ion image Thus precipitates rich in refractory metals can be detected when extremely small, while those rich in elements like zinc and magnesium are hard to detect [14] The atom probe [1,5] is a modification of a field-ion microscope in which the chemical identity of individual field evaporated ions can be

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Fig l Schematic of a field-ion tip containing small precipitates, indicating the shape of the cone of material analysed m a conventional atom-probe experiment The diameter of the cone increases gradually as the t~p is blunted by the field-evaporation process

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C R M Grovenor et al / Ultra-high-resolution cherntcal analysts

obtained in a time-of-flight spectrometer, and the position on the s p e o m e n surface from which they were evaporated determined either by a form of selected-area aperture or by a posmon-senslttve detector [6] In the first case, a one-dimensional chemical profile is measured with lateral resolution dictated by the s~ze of the aperture when projected onto the specimen surface The defined area is typically 1-5 nm m diameter, and the

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chemical profile ~s obtained from a cyhnder of material as illustrated m fig 1 The depth resolution (and so the resolution of the chemical reformation along the cyhnder) is controlled by the plane-by-plane field-evaporation behavlour of crystalhne materials, and so Is best quoted as the mterplanar spacing m the crystal direct~on along which the analysts is taken If analysis is taken along a statable d~rectlon m, for instance, an

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b Fig 3 (a) Field-Ion image from a C u - C o s p e o m e n showing the small brightly imaging preopltates (arrowed) which are easily detected even when only 1 nm m diameter, (b) TEM image from a field-ion tip of the same material showing that these preopltates are not readily apparent

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C R M Grot,enor et al / Ultra-high-resolution chemwal analysts

ordered crystal, a depth resolution of about 0 2 nm has been very clearly demonstrated [8] The accuracy with which individual ions are identified (and therefore the precision of the chemical analysis) depends on variations in energy of field-evaporated ions and on the length of the flight tube in a simple instrument Most modern atom probes include energy-compensating lenses, and can completely separate ionic species which do not have exactly the same mass-tocharge ratio The best mass-to-charge resolution obtained to date is in a pulsed-laser atom probe (PLAP) in which the energy spread with which ions are evaporated in conventional voltagepulsed systems can be avoided A PLAP with an 8-meter flight tube has been constructed w~th a mass resolution M / A M of over 30000 which allows separation of, for instance, 56Fe2+ and 14N+ ions because of the non-integral mass numbers of these elements [9] This ablhty is clearly important ff an experiment on the distribution of 2sSl or nitrogen in iron alloys IS to be attempted For most experiments a mass resoluhon of around 2000 (as we have in Oxford) allows precise identification of the chemical identity of each evaporated ion The POSAP has been developed m Oxford to extend the potential of the atom-probe technique to the study of materials with hlghly complex and fine-scale mlcrostructures Here the conventional one-dimensional composition profiles may not give a clear picture of the structure Measuring both the chemical identity and three-dimensional position of each ion allows the reconstruction in a computer of the detailed chemical and morphological nature of the material [7] A schematic diagram showing basic components of the detector system in the Oxford POSAP is shown in fig 2 In this equipment the spatial resoluhon for pos~t~on sensing is better than 1 nm, and this value IS usually limited by trajectory aberrations [23] In addition, the chemical specificity is reduced because of a shortening of the flight path In our system the mass resolution is currently hm~ted to 30, which means that some elements close together m the periodic table are difficult to resolve completely, copper and zinc for instance This paper gives examples of several studies of

the fine-scale chemistry and structure of materials carried out recently in Oxford Particular emphasis will be given to describing m each case the particular benefit of usmg these techniques to study these materials, rather than employing more conventional TEM-based analysis

2. Imaging of fine-scale composition variations In order to understand the very early stages of precipitation processes, it is important to be able to determine precise conditions at which precipitates are first formed as well as to investigate their morphology and composition These first precipitates are often coherent with the matrix material and so are particularly difficult to detect in TEM techniques until they have begun to coarsen If there is a significant difference in the evaporation field of the precipitate and matrbx, they can be easily detected in a field-ion image even when extremely small Fig 3a shows an example from a classic coherent precipitate system, a C u - 2 5at%Co alloy annealed at 450°C for 2 h, where a number of very small brightly imaging Co-rich precipitates can be seen in a darker Cu-rlch matrix These precipitates are not easily detected in TEM experiments [17], as can be seen in the bright-field TEM image shown In fig 3b taken of a field-ion specimen before imaging It is hard to give an accurate value for the size of these precipitates because of the magnification effects associated with relatively refractory particles in a matrix with a lower evaporation field, but an upper estimate of 1 nm can be made for the diameter of the particles in fig 3a by use of the persistence-size method described in ref [1] (p 79) Field-Ion imaging can also be used in the situation before true precipitation has occurred Fig 4 shows a field-Ion image of a rapidly solidified A I - C r alloy heat-treated at 350°C for 2 h, and a number of small brightly imaging regions can be seen The same problems with refractory particles In a relatively non-refractory matrix arise when attempting to establish the size of these features An estimate of 1-2 nm has been made in this case More detailed atom-probe mlcroanalysls

C R M Gro~enor et al / Ultra-htgh-resolutton chemtcal analysts

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shows that the bright features are rich in Cr, but they are not discrete precipitates in the sense of having a composition far from that of the matrtx material or a different crystal structure They are best described therefore as Cr-rlch clusters which may act as the nuclei for subsequent precipitation reactions [20] This use of field-ion images to identify the position of particular elements can even be extended to brightly imaging single atoms Fig 5 is a field-ion image of a ferrite lnterlath boundary in a 0 2 w t % C, 052wt% Mo steel alloy heattreated at 620°C for 100 s [21] The bright ring is formed by indwldual Mo atoms segregated to the boundary, an interpretation confirmed by atomprobe mlcroanalysis of this sample

3. One-dimensional atom-probe analyses The first example of a conventional atom-probe analysis is taken from the same material as shown m fig 5 but after a longer heat treatment, 28 h at 620°C This steel contains small Mo-rich carbide precipitates, and it is important when developing an understanding of the precipitation processes in this commercial alloy to determine their pre-

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Fig 4 Field-ion tmage of an A I - C r alloy heat-treated to allow the formahon of brightly tmagmg clusters of Cr atoms (arrowed)

Fig 5 Field-ion image of a lath boundary m a terntlc steel showing the brightly imaging Mo atoms segregated to the boundary plane A very small M2C precipitate can also be seen, which is very effectwe at pinning the boundary

clse composition even though they are only about 10 nm in diameter Fig 6a shows composition profiles for Fe, C and Mo taken during an evaporation sequence through one of these precipitates The field-ion image in fig 6b is of another such precipitate to illustrate that the Mo-rlch carbides image very brightly From the atomprobe data the precipitate can be determined to have the composition 65 2 + 1 4 at% Mo, 32 7 + 1 4 at% C and 2 1 _+ 0 4 at% Fe, while the matrix has composition Fe, 0 13 + 0 03 at% Mo, 0 01 _+ 0 01 at% C The precipitate is thus of the M2C type, but containing a small amount of Fe as well as Mo This low level of iron could not have been rehably detected in a TEM-based experiment because of the proximity of large quantities of iron in the matrLx The low level of Mo and C in the matrLx has also been accurately measured These composition profiles also illustrate the excellent depth resolution available m the atomprobe technique, with the interface between the matrLx and MozC (which we would expect to be essentially atomically abrupt) having an apparent

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width of less than 0 5 nm on both s~des of the p r e o p l t a t e This is a classic example of the use of a chemlcally abrupt interface to estabhsh the

intrinsic resolution of an experimental technique In fact, the apparent abruptness of these parttcular interfaces m an atom-probe experiment ~s

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Fig 6 (a) Composition profile through an M2C precipitate about 8 nm in diameter m a ferntlC steel The c o m p o s m o n of the precipitate has been accurately determined, and the highly abrupt matrLx/partlcle interfaces can be clearly seen (h) Field-ion mlcrograph of a similar precipitate m the same materml showing the brightly ~maglng Mo-rlch phase at the bottom left of the imaged region

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Fig 8 (a) Comparison of the zinc concentration profile across a grain boundary in 7150 alloy samples obtained In a STEM (a I) [11] and an atom-probe (a z) experiment T h e improved resolution achievable In the atom probe gives a much clearer indication of the strong segregation of zinc (b) T E M mlcrographs of the field-ion tip containing a grain boundary before and after analysis in the atom probe

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probably limited by the curvature of the Mo2C/ matrix interface The second example of one-dimensional atom-probe m~croanalysls is taken from a spmodally decomposed high-purity F e - 2 6 % C r - 3 % N I alloy which we are using as a model alloy to study the degradation of the mechamcal properties of the ferrlte phase of commercml duplex stainless steels The observed embrlttlement has been hnked to the spmodal decomposition process, and can have a very severe effect on the fracture toughness of these ~mportant lndustrml materials [18] After a heat treatment at 450°C for 1000 h, a strong composition variation is revealed in the one-&menslonal composition profiles shown in fig 7 The chrommm and iron concentrations vary out-of-phase with one another, and a wavelength of the composition fluctuauon of ~ 10 nm has been estimated from cahbratlon of the number of ions collected from evaporahon of a single atomic plane from a field-ion specimen of the same material imaged at the same voltage However, while this kind of data can give information on the composition excursions developed during the decomposition reaction, no clear picture of the morphology of the m~crostructure is gamed Thls problem will be addressed further below A final example of the power of a simple atom-probe analysis m the study of the fine-scale chemistry of materials ~s g~ven m fig 8 Here a comparison is made of composmon profiles across grain boundaries m 7150 alummmm alloys obtained in a V G HB501 STEM [10] and an atomprobe The importance of this kind of analysis lies m the need to develop an understanding of the stress-corrosion cracking (SCC) behawour of th~s alloy, a phenomenon which seems to be very strongly dependent on the local chemistry around grain boundaries [11] With an electron-probe dmmeter of between 1 and 2 nm, the resolution of the segregated Zn at the boundary in the STEM is relatively poor, certainly no better than the beam diameter In the atom probe the essentially plane-by-plane resolution available when the analysis &rect~on is chosen to be approximately normal to the boundary plane means that a much sharper segregation profile is obtained, more consistent with the expected equlhbrmm

segregation behawour The orientation of the gram boundary with respect to the analysis direction, the axis of the field-ion t~p, is shown in fig 8b The two dark-field T E M images show the tip before and after the atom-probe analys~s, and it is clear that the gram boundary has intersected the analysed region of the tip The reason for the dramatic difference in the actual weight percentage of zinc in the two profiles shown in fig 8a Is not understood, but it is difficult to see how the low level measured in the STEM experiment can be explained given that there is more than 6wt% of zinc in this alloy Even If a considerable fraction of the zinc is tied up in large precipitates it would be surprising if the matrix level fell as low as is suggested by thls STEM data These three examples show that conventional atom-probe mlcroanalysls can be used to g~ve very-h~gh-resolut~on chemical lnformauon on a range of materials In all these cases, it would currently be e~ther extremely difficult or impossible to obtain a similar quality of chemical reformation on these fine-scale composition variations using electron-microscopy techmques A large number of other examples of the excellent resolution that can be obtained m conventional atomprobe experiments on a very wide range of materials can be found in refs [1,2]

4. Position-sensitive atom-probe microanalysis As has already been mentioned, it is sometimes important to be able to analyse the chemistry of a material with very high spatial resolution in more than one dimension Th~s ~s particularly true if one is to be able to build up an accurate picture of fine-scale percolated structures for example, or those in which interfaces may be stepped or have lnhomogeneous dlStrlbutlons of segregated species These kinds of structures cannot be analysed effectively by techniques which integrate chemical data through the thickness of a T E M foil or those in which the composlt~onal data is essentmlly one-&menslonal Here we will give several examples of the use of our umque P O S A P in the study of the three-dlmen-

C R M Grovenor et al / Ultra-htgh-resolutton chemtcal analysts

slonal chemistry of a variety of different materials The first example continues the work on the structure of the 7150 alumlnlum alloys, and IS a study of how the matrix precipitates change in size, shape and composition during ageing Fig 9 shows isoconcentration renderings of POSAP data for Mg and Zn taken from two samples, the first peak aged and the second after a retrogression and reageing ( R R A ) treatment [19] This renderlng IS made up by defining the volume of material within which the concentration of each element exceeds a chosen value, 10 at% In all four of the pictures In fig 9 The volumes analysed are in the form of cylinders 19 nm across and 8 nm deep for

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the peak aged material, and 27 nm across and 8 nm deep for the more heavily aged The precipitates can clearly be seen to be much larger and less numerous in the more heavily aged material as expected, but we can also investigate the composition change as ageing proceeds This is especially important because the R R A treatment ~s thought to alter the precipitate compositions, and hence influence the SCC susceptibility of the alloys In these particular measurements, the precipitates in the peak-aged material have compositions m the range 65-80 at% A1 and zinc to magnesium ratios in the range 0 5-1 2 After the R R A treatment a distribution of precipitate sizes can be seen m figs 9c and 9d The smaller

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Fig 9 Isosurface renderings of POSAP data showing the morphology of precipitates m 7150 a l u m m l u m samples after two different heat treatments (a, b) Zn and Mg 10% lSOsurfaces in peak-aged material, (c, d) similar lSOsurfaces m retrogression and reaged material These images have been scaled so that they are presented at exactly the same magmficatlon The change m the size distribution of the precipitates can be clearly seen, and from this data the composition of each preopltate can be analysed

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Fig 10 Rendering of POSAP data plotting the lsosurface at 50% Co from a Cu-2 5% Co alloy heat treated for 2 h at 723 K to grow Co-rich preopltates The smaller features are mdwldual volume elements, and Identify the presence of pairs of Co ions

(dissolving) precipitates have compositions relatively d e p l e t e d m the solute elements, 7 5 - 8 5 a t % A1 while the larger (growmg) precipitates have compositions in the r a n g e 4 5 - 5 5 a t % A1 a n d zinc to m a g n e s i u m ratios very close to 0 66 T h e s e data indicate that a d r a m a t t c change has occ u r r e d in the prectpltate compositions d u r i n g the R R A t r e a t m e n t , a n d analysts of this kind could not have b e e n o b t a i n e d o n precipitates of this size u s m g T E M - b a s e d t e c h n i q u e s A similar e x p e r i m e n t can be carried out o n the fine Co-rich precipitates shown m the field-ion image in fig 3 A P O S A P e x p e r i m e n t o n this same m a t e r i a l reveals, as expected, that the small spherical precipitates are rich m Co, as shown in the lSOsurface r e n d e r i n g usmg a Co level of 50%, fig 10 T h e composition of even these small volu m e s of m a t e r i a l can be accurately d e t e r m i n e d by analysis of the P O S A P data It ts f o u n d that the average prectpltate composition, once they have b e e n aged to d i a m e t e r s in excess of 2 nm, ts C o - 5 a t % C u , in excellent a g r e e m e n t with prevt-

ous a t o m - p r o b e a n d n e u t r o n - s c a t t e r i n g experim e n t s [12,13], but here t h e r e is no p r o b l e m with matrtx c o n t r i b u t i o n s to the c o p p e r level in the precipitate [12] or necessity to make arbitrary a s s u m p t i o n s a b o u t the d i s t r i b u t i o n of precipitate sizes before estimates can be m a d e of the composition [13] Images of the kind shown in fig 10 are butlt u p by a c c u m u l a t i n g the ions of Co a n d Cu in volume e l e m e n t s ~ 0 3 x 0 3 x 0 3 n m 3 so that most of these e l e m e n t s c o n t a i n either 2 or 3 ions T h u s with a n lSOsurface c o n c e n t r a t i o n level of 50% Co a n d noticing that the smallest features in fig 10 are the size of mdlvldual volume elements, it is clear that we are able to identify the presence of pairs of Co ions in the analysed volume (the smallest possible cluster)) as well as to study the larger preclpttates of the Co-rich phase T h e analysis of composition fluctuations m splnodally d e c o m p o s e d structures has b e e n shown m fig 7, b u t to gain a complete tdea of the structure of this kind of m a t e r i a l requires the t h r e e - d t m e n s l o n a l analysis only achtevable by us-

Fig 11 Rendering of POSAP data plotting the isosurface at 40% Cr from an Fe-26% Cr-% NI alloy which is strongly spmodally decomposed The interconnected nature of the Cr-nch regions is clearly shown m this kmd of representation of a three-dimensional structure

C R M Grouenor et al / Ultra-htgh-resolunon chemtcal analysts

lng field-ion-microscope evaporation sequences [22] or more conveniently by the P O S A P Fig 11 is a 40% Cr isosurface representation of P O S A P data taken from an F e - 2 6 % C r - 3 % N I alloy aged at 500°C for 500 h The Cr-rlch regions clearly identified in the conventional atom-probe profiles are seen to form a sponge-like interconnected structure, and the Fe-rlch regions can also be

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shown to have the same kind of structure This 1s thus an example of the use of the three-dimensional resolution potential of the P O S A P technique to elucidate the structure of what ts effectively a nanoscale composite matertal A final example of P O S A P analysis is taken from work on G a I n A s / I n P multi-quantum well samples H e r e the problem of interest is to inves-

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Fig 12 POSAP composmon maps showing the distribution of Ga, In, As and P m a section about 1 nm m depth through the interface between an InP barrier and a GaInAs well The chemical abruptness of the interface can be examined by presenting this same data m the form shown m fig 13, where the intermixing between the well and the b a m e r is clear

C R M Grovenor et al / Ultra-htgh-resolutton

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S T E M and P O S A P analysis in the study of these quantum well interfaces has shown that considerable consistency can be achieved in the data obtained on the same samples by the careful application of these techniques, but that the best intrinsic resolution is achieved by the POSAP [16]

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tlgate the morphology and chemistry of the interfaces between the active G a I n A s layers and the InP barriers The abruptness of this interface is believed to have a strong Influence on the electronic properties of the materials This particular sample was grown by metallorganic chemical vapour deposition ( M O C V D ) at 650°C, and is nominally lattice matched layers of Ga46In54As of thickness 10 nm separated by 33 nm-thick layers of InP The sample preparation for atomprobe analysis of this kind of material is particularly difficult, and has been described elsewhere [15] Fig 12 shows POSAP composition maps of the distribution of the four elements across a single interface between a G a I n A s well on the right and an InP barrier on the left The data can also be presented as a composition profile for each of the four elements, fig 13, in which the InP and G a I n A s layers are easily identified but the chemical abruptness of the interface can also be studied It can be seen that the interface is chemically diffuse over a distance of more than 5 rim, I e significant intermLxing of the elements has occurred during growth This klnd of intermtxlng is certain to have important consequences for the electronic performance of this structure, but as yet insufficient data of this quality has been collected to make direct correlations of mlcrostructural features with properties A direct comparison of the use of high-resolution TEM,

Several examples have been given to illustrate the use of a t o m - p r o b e / f i e l d - i o n microscopy analysis in the study of materials which contain veryfine-scale composition fluctuations It has been shown that small precipitates and clusters of solute atoms can be identified in field-ion images, and their chemistry and morphology studied with the P O S A P Clusters as small as a pair of solute atoms can be reliably studied, and an accurate measurement made of the composition of precipitates as small as 1 nm in diameter The structure of a wide variety of interfaces can also be investigated with very high spatial resolution using either the conventional atom-probe or the P O S A P Grain boundaries and lnterphase interfaces have been analysed, and p h e n o m e n a like gramboundary segregation and chemical diffuseness can be studied in some detail We believe that these examples have demonstrated that the combination of high spatial resolution and good chemical specificity makes atomprobe mlcroanalysls very powerful in the study of materials on the nanometer scale, providing information on the morphology as well as the chemistry of samples whxch cannot be investigated simply by TEM-based techniques

Acknowledgements The authors are pleased to acknowledge support and encouragement from a large number of sources Professor Sir Peter Hlrsch FRS is acknowledged for the provision of laboratory faclhties, and the S E R C for support during the course of this work Materials and financial support for particular projects described in this p a p e r were

C R M Grovenor et al / Ultra-htgh-resolunon chemical analysts

p r o v i d e d by T h e R o y a l S o c i e t y ( A C ) , RAEF a r n b o r o u g h (B A S ), D r M C h i s h o l m a n d O a k Ridge National Laboratories and the Leathersellers Scholarship of Jesus College, Oxford (RP S), Alcan International (P J W ) , The P l e s s e y C o m p a n y (J A L ) a n d T h e C e n t r a l E l e c t n o t y R e s e a r c h L a b o r a t o r y (J E B )

[8] [9] [10] [11] [12] [13] [14] [15]

References [16] [1] M K Mdler and G D W Smith, Atom Probe Mlcroanalysis Principles and Apphcatlons to Materials Research (Materials Research Sooety, Pittsburgh, 1989) [2] T T Tsong, Atom Probe Field Ion Microscopy (Cambridge University Press, Cambridge, 1990) [3] C M C de Castllho and D R gangham, J Phys D 20 (1987) 116 [4] T T Tsong, Surf So 10 (1968) 303 [5] E W Muller, J A Pamtz and SB McLean, Rev So Instrum 39 (1968) 83 [6] A Cerezo, T J Godfrey and G D W Smith, Rev Scl Instrum 59 (1988) 862 [7] A Cerezo, T J Godfrey, C R M Grovenor, M G Hetherlngton, R M Hoyle, J P Jakubovlcs, J A Llddle,

[17] [18]

[19] [20] [21] [22] [23]

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G D W Smxth and G M Worrall, J Microscopy 154 (1989) 215 M K Mdler and J A Horton, Scr Metall 20 (1986) 1125 J Llu, C Wu and T T Tsong, Surf So 246 (1991) 157 J R Plckens and T J Largan, Metall Trans A 18 (1987) 1735 J Crompton, Alcan International, private communication H Wendt and P Haasen, Scr Metall 19 (1985) 1053 W Wagner, Acta Metall Mater 12 (1990) 2711 S S Brenner, J Kowahk and Hua Mlng-Jlan, Surf Scl 246 (1991) 210 J A Llddle, A Norman, A Cerezo and C R M Grovenor, J Phys (Pans) 49 (1988) C6-509 J A Llddle, N J Long and A K Petford-Long, Mater Character 25 (1990) 157 M F Chxsholm, PhD Thesis, Carnegie-Mellon University, Pittsburgh, 1986 Proc Int Workshop on Intermediate Temperature Embrlttlement Processes in Duplex Stainless Steels, Mater Scl Technol 6 (1990) J K Park and A J Ardell, Metall Trans A 15 (1984) 1531 B A Shollock, A Cerezo, E D Boyes, B Cantor and G D W Smith, Mater So Eng 98 (1988)197 I Stark and G D W Smith, m Phase Transformations, Ed G W Lorlmer (Institute of Metals, London, 1988) S S Brenner, M K Mdler and W A Sofia, Scr Metall 16 (1982) 831 A R Waugh and E D Boyes, Surf Scl 61 (1976) 109