Nuclear microprobe applications in materials science

Nuclear Instruments and Methods in Physics Research B30 (1988) 474-479 North-Holland, Amsterdam

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Section IX. Nuclear microbeam applications in materials science NUCLEAR

MICROPROBE

APPLICATIONS

IN MATERIALS

SCIENCE

J.W. McMILLAN UKAEA, Hawell Labotatov, Didcot, Oxon, OX11 ORA, UK

Since its inception the nuclear microprobe has been used extensively in materials science studies. The ability of the technique to measure elements and isotopes by nuclear reaction analysis (NRA), particle induced X-ray emission (PIXE), Rutherford backscattering (RBS) and elastic recoil analysis (ERA) has proved particularly advantageous. There has been a notable increase in the exploitation of the technique to measure composition multidimensionally. Applications have involved the measurement of changes in the composition of materials during manufacture, fabrication and service, as these can strongly influence their properties. Examples have been drawn from plasma fusion research, the aero-industry, catalysis, power production, wear and the nuclear power industry.

1. Introduction Since the introduction of the first focussing nuclear microprobe by Cookson and coworkers [l] the equipment has found widespread applications in materials analysis. Studies of the movement of light elements in steels, used as fast reactor fuel cladding, led to the early introduction of nuclear microprobe methods to determine the distribution of carbon, nitrogen and boron [2-51. Over the succeeding years, as the number of nuclear microprobes has increased, these and other methods have proved invaluable in studies of materials during manufacture, fabrication and service. A succession of reviews comprehensively covers the subject [6-121. Instead of exhaustively reviewing the subject to date, the present paper examines advances and applications in a number of areas, including plasma fusion research, the aero-industry, catalysis, power production, wear and the nuclear power industry. Some of the advances in and advantages of nuclear microprobe analysis are emphasised as well as its problem solving role in materials science.

2. Applications 2.1. Plasma fusion research A considerable number of nuclear microprobe investigations have been recently reported of materials associated with plasma fusion research. Much innovative work has been described by Doyle and various collaborators [12-171. Attention has been focussed on the value of multidimensional analysis [12], the efficacy of elastic recoil analysis (ERA) [13], and external beam Rutherford backscattering analysis has been introduced for the examination of tokamak limiters

[14-161. Several studies have been reported of the 3-D analysis of titanium carbide coated graphite limiter tiles for their Ti and C distributions [12,13,15-171. RBS with 6 MeV protons has been used to follow the movement of MO, possibly as a metal droplet, from a molybdenum limiter ring to a graphite limiter, then its subsequent diffusion into the limiter and its removal from surface regions by further plasma discharges [13]. The distribution of hydrogen in silicon, exposed to a hydrogen plasma, has been determined by ERA using 20 MeV “Si4+ ions [13]. Sofield et al. [18] have employed nuclear microprobe examination of carbon disc collectors from the DITE tokamak for plasma diagnostics. Deuterium and impurity ion distributions were determined by the reaction D( 3He, p)4He and particle induced X-ray emission (PIXE), respectively. Examination of statically exposed discs allowed the radius of gyration of ions in the magnetic field to be determined, and consequently the ion energy or charge state. The examination of rotating discs allowed the temporal behaviour of the discharge to be studied. Measurements of impurity and deuterium distributions in graphite limiters from the Fontenay-aux-Roses tokamak have been reported by Engelmann and coworkers [19-211. They also describe some novel work on the determination of deuterium and tritium in thin walled microspheres with diameters of 100 to 200 pm. 120,211. The reactions employed were D(d, p)T, and T(d, n)cY. The evolution of gas during storage at various temperatures was also followed. Heck was very recently utilised 3-D analysis to study the interaction of Li, from lithium oxide plasma reactor breeder, with its stainless steel cladding [22]. The reaction ‘Li(p, cr)a: was used to determine Li, while PIXE and proton RBS were used to determine the distributions of Cr and 0, respectively. Correlations found

J. W. McMillan /Applications in materials science suggest that lithium chromate forms during the attack of the steel by lithium oxide. 2.2. Aero-industry A number of aerospace related materials problems that have been investigated by nuclear microprobe analysis have been outlined by D’Agastino and co-workers [23]. The failure of welds in titanium has been shown to be associated with attack by oxygen rather than hydrogen or carbon. The segregation of lithium in brazes when employed in the zero gravity environment of the Skylab orbiter has been studied. Also, nuclear microprobe analysis has been used to study the stress corrosion of aluminium alloys by measuring hydrogen distributions. Lithium/aluminium alloys are coming into prominence for aero space applications. Schulte et al. [24] have reported on studies of the loss of lithium from the surface of two alloys. The sectioned specimens were examined using the reaction 7Li(3He, po)gBe using 2.5 MeV 3He ions. A good correlation was demonstrated between lithium loss and microhardness. Work has very recently been reported by Degreve et al. [25] on the quantitative analysis of intermetallic phases in Al/Li alloys by several probe techniques in combination. Nuclear microprobe examination was based on irradiation with 2.8 MeV protons. Proton induced gamma emission (PIGE) was used to determine Li and Al through measurement of the 472 keV gamma rays from the former and the 843 and 1013 keV gamma rays from the latter. PIXE was used to determine Zn in a Li/Al/ Zn alloy. A new phase, designated 7, with a composition close to that of Al,ZnLi3 was identified. A ternary phase in Al/Li/Si alloys, T phase, was confirmed to have a composition of Al 2 Li 3Si *. Composite materials for aircraft offer weight and cost savings particularly when bonded by adhesives [26]. To study the long term performance of adhesively bonded graphite/epoxy composites, they were subjected to exposure to an atmosphere containing deuterated water vapour. The degree and rate of moisture absorption was determined by nuclear microprobe examination of sectioned samples, by measuring deuterium using the reaction DoHe, P)~H~. Good agreement was obtained between experiment and prediction using Fickian diffusion. The ability of nuclear microprobe analysis to measure isotopic tracers was a clear advantage in this application. Nuclear microprobe analysis has also been used for the investigation of aircraft engine components. Olabanji and Calvert have studied the efficacy of yttrium in improving the adhesion of oxide scales on engine turbine blades made from Co/Cr/Al/Y alloys when operated at 1080°C [27]. PIXE was used to measure the enhancement of the Y concentration at the surface.

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McMillan [28] has examined wear particles from jet engine gear boxes, that had been collected on magnetic traps. The source of the particles was identified through carbon analysis, as the only difference between the two gear trains involved was their degree of carburisation. The results for two wear particles were 0.29 & 0.02% and 0.31 & OS%, whereas the gears had carbon contents of 0.31 + 0.1% and 0.11% + 0.04%. The investigations allowed the prompt replacement of the failing component. 2.3. Catalysis In reviewing the role of nuclear physics for materials technology, Conlon has discussed the role of accelerator methods in the examination of catalysts [29]. Essentially accelerator methods can be used to character&e catalysts during their production and use. Early work by Cairns and Cookson [30], although nonmicrobeam, illustrates the power of PIXE in assessing the quality of car exhaust catalysts based on a special aluminium containing steel, and the resistance of different formulations to Pb poisoning. Another type of car exhaust catalyst, a ceramic honeycomb coated with alumina containing Pt, Rh and Pd, has been examined by microbeam PIXE to study its performance [31]. While the deposition of Pb at the catalyst surface could be observed the distribution of the active constituents was essentially unchanged during service. The coking and regeneration of catalysts used for hydrothermal reforming of hydrocarbons has been studied using nuclear microprobe techniques. Wright et al. [32] have examined the distribution of carbon and palladium in a variety of sectioned catalyst pellets observing variations in carbon concentration in accord with predicted behaviour. Sofield et al. [33] have performed microbeam ERA on the same materials to determine hydrogen profiles; the incident beam was 2.5 MeV 3He+ ions. Combination of the results allowed the estimation of C/H ratios. Maggiore [34] et al. have investigated the chemical stability of dispersed catalysts in fuel cell electrodes using 2.5 MeV protons for microbeam BBS. They were able to establish that the V component of an intermetallic catalyst of Pt and V migrated when subjected to severe operating conditions in phosphoric acid. 2.4. Power production Several of aspects of conventional power production including heat exchanger performance, weld production and fly-ash constitution have been the subjects of nuclear microprobe investigations. The mechanism of the oxidation of ferritic steel heat exchanger tubes has been studied by successive exposure to steam of natural isotopic abundance oxygen IX. APPLICATIONS IN MATERIALS SCIENCE

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J. W. McMillan

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/ Applications

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and then to steam enriched in ‘*O [8]. Nuclear microprobe examination of the sectioned tubes, using the reaction ‘*O(p, a)“5 and PIXE, allows the simultaneous determination of the distributions of “0 and transition metals, Fe and Cr, respectively. Oxidation can be controlled by oxygen or iron diffusion through the oxide film. Similar work h,as been reported on the oxidation of advanced gas cooled reactor boiler steels using both ‘*O enriched CO, and D,O [35]. Both illustrate the importance of the isotopic sensitivity of nuclear microprobe analysis. Welds play an important role in many industrial applications. Nuclear microprobe examination of welds in nuclear power plant has been described [36,37]. Commonly the light element distributions can be determined by NRA while alloying element distributions are determined by PIXE. Recently, failure of a weld in a steam pipe from conventional electricity generating plant was investigated by nuclear microprobe analysis. The distribution of carbon and chromium were measured, by the reaction ‘*C(d, p)13C and PIXE, across the boundary between the parent alloy, 0.5% Cr/Mo/V, and the weld, 2.25% Cr/l% MO. The results are presented in fig. 1. The decarburisation of the parent metal and the carburisation of the weld metal are both clearly visible. The carbon distribution correlated well with microhardness measurements. To eliminate this type of

failure, without changing the alloys, experiments have been conducted to find conditions that minimise carbon movement during welding and post-weld heat treatment. The distribution of trace elements from coal in combustion products is of interest environmentally and for metal recovery. Nuclear microprobe analysis, in combination with other techniques, has been employed to determine the constitution of fly-ash particles [38,39]. Fly ash particles, from a Yugoslavian power station, were embedded in epoxy resin and polished prior to determination of the distribution of various elements across their diameter by PIXE. Magnetic and nonmagnetic particles could be identified from their K, Ca and Fe distributions.

2.5. Wear While not extensively investigated by nuclear microprobe methods, wear is an area of interest particularly as ion implantation has been used to improve the wear properties of many materials [29,40]. Hartley [4O], has described the influence of the implantation of both light and heavy ions on the wear of metals. He has employed microbeam RBS with 2 MeV 4He+ ions to examine the distribution of Pb, Sn and Ag in steels before and after

f. W. MeMillan / Applications in materials science

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wear tests and finds that while Sn and Ag are lost, Pb is retained. The use of nuclear microprobe analysis to examine wear particles has already been described in section 2.2. 2.6. Nuclear power industry Since its inception nuclear microprobe analysis has continued to play an important role in the investigation of materials problems in the nuclear power industry. Four recent applications have been reported by McMillan and co-workers [41]. Three demonstrate the particular importance of the isotopic sensitivity of the technique in an industry where the isotopic abundance of elements may be disturbed. The diffusion of tritium in austenitic steels was measured by the nuclear microprobe examination of sectioned specimens using the reaction T(d, n)a when irradiating with 1.5 MeV deuterons and measuring emitted neutrons at 0 O. An excellent sensitivity of 0.1 ppm was achieved and loss of tritium could be avoided by holding beam current densities below 0.5 pA pm-*. Caution is always essential when examining materials for hydrogen and its isotopes. Subsequent work on ferritic steels was unsuccessful when using the same

analysis conditions because of concentration changes during irradiation. Examination of high bum up fast reactor fuel pins using nuclear microprobe analysis revealed that radiogenie r3C, produced in oxide fuel by the reaction 160(n, cr)13C, is released from the fuel and diffuses into the steel cladding. The reactions 13C(d, p)t4C and ‘*C(d, p)13C were used for the simultaneous detennination of 13C and ‘*C, respectively. The radial variation of bum up of “B in B4C fast reactor control pins can be measured using the nuclear microprobe and the reaction 7Li(p, a)ol, if the bumup product 7Li does not diffuse [10,41]. This behaviour was observed until recently, when metallographically revealed black rings in radial sections, subsequently proved, on nuclear microprobe examination, to be coincident with the high concentrations of ‘Li shown in fig. 2. Higher “B bum-up and higher operating temperatures led to 7Li movement. The Harwell nuclear microprobe has been used to investigate the contamination of steel surfaces by actinides [41-431. A combination of RBS, PIXE, and NRA allowed the sensitive detection of actinides, 0.01 pg cm-‘, the determination of medium mass elements and light elements, respectively. Heavy plutonium conIX. APPLICATIONS IN MATERIALS SCIENCE

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tamination was shown to be associated with light element inclusions and penetrated into such regions to depths of greater than 2 pm. Contamination was also found to agglomerate at physical imperfections such as pits. Work by Olabanji and Calvert [44] on the nuclear microprobe examination of oxide films on Fe 9% Cr alloys with additions of Si at 0.3% and 0.6% is a reminder of the numerous applications of the technique to examine the oxidation of reactor steels and alloys [SJO]. In the above study, proton RRS was employed to examine the stoichiometry of the films, while PIXE revealed that Si accumulated near the oxide/metal interface. The present of Si improves oxide adherence and corrosion resistance.

3. Conclusions While some aspects of the application of nuclear microprobe analysis in materials science have been neglected, for instance thin foil and film measurements [45-471, the extensive use of the technique is evident. Important features of the technique that have been exploited to a greater degree in recent years include its multidimensional analytical capability and the value of combined studies involving RRS, PIXE, NRA, and ERA. Exploitation of these features will continue to expand, and as improvements in resolving power are also introduced new applications areas will emerge, for instance grain boundary studies. However, simple low resolution measurements will often suffice. Although advances in technique should be encouraged, unless they are economically attractive or technically vital they will fail to attract materials problems. This review was undertaken as part of the Underlying Research Programme of the UKAEA.

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in materials science

[8] J.W. McMiIlan, Analysis of Non-metals in Metals, ed., G. Kraft (Walder de Gruyter and Co., Berlin and New York, 1981) p. 173. [9] J.A. Cookson, Nucl. Instr. and Meth. 181 (1981) 115. [lo] J.W. McMiIIan, F.C.W. Pummery and P.M. Pollard, Nucl. Instr. and Meth. 197 (1982) 171. [ll] G.J.F. Legge, Nucl. Instr. and Meth. B3 (1984) 561. [12] B.L. Doyle, Nucl. Instr. and Meth. B15 (1986) 654. [13] B.L. Doyle and N.D. Wing, IEEE Trans. Nucl. Sci. NS-30 (1983) 1214. [14] B.L. Doyle, Nucl. Instr. and Meth. 218 (1983) 29. [15] B.L. Doyle, J. Vat. Sci. Technol. A3 (1985) 1374. [16] H.F. Dylla, M.A. Uhickson, P.H. LaMarche, D.K. Owens and B.L. Doyle, J. Vat. Sci. Technol. A3 (1985) 105. [17] B.L. Doyle, R.T. McGrath and A.E. Portau, Nucl. Instr. and Meth. B22 (1987) 34. [18] C.J. Sofield, G.M. McCracken, L.B. BridwelI, J. Shea, E.S. Hotson and S.K. Erents, Nucl. Instr. and Meth. 191 (1981) 383. [19] C. Engelmann and J. Bardy, Nucl. Instr. and Meth. 218 (1983) 209. [20] C. Engelmann and J. Bardy, Rapport CEA-R5340 (1986). [21] C. Engehnamr and R. Dei-Cas, CLEFS-CEA, Revue Scientifique et Technique du Commissariat a L’Energic Atomic No. 4. (Janvier 1987) p 22. [22] D. Heck, these Proceedings (Nuclear Microprobe Technology and Applications) Nucl. Instr. and Meth. B30 (1988) 486. [23] M.D. D’Agastino, E.A. Kamykowski, F.J. Kuehne, G.M. Padawer, E.J. Schneid, R.L. Shulte, M.C. Stauber and F.R. Swanson, J. Radioanal. Chem. 43 (1978) 421. [24] R.L. Shulte, J.M. Papazian and P.N. Alder, Nucl. Instr. and Meth B15 (1986) 550. [25] F. Degreve, B. Dubost, A. Dubus, N.A. Thome, F. Bodart and G. Demortier, Conf&ence AI-Li IV (Bagnolet, Paris, lo-12 Juin 1987). [26] R.L. Schuhe and R.J. DeIasi, IEEE Trans. Nucl. Sci. NS-28 (1981) 1841. [27] S.O. Olabanji and J.M. Calvert, Nucl. Instr. and Meth. BlO/ll (1985) 700. [28] J.W. McMillan, unpublished work. [29] T.W. ConIon, Contemp. Phys. 26 (1985) 521. [30] J.A. Cairns and J.A. Cookson, Nucl. Instr. and Meth 168 (1980) 511. [31] J.A. Cookson, Applications of High Energy Ion Microbeams, ed. G.W. Grime and F. Watt (Adam Hilger Ltd., Bristol, UK, 1987) p. 294. [32] C.J. Wright, J.W. McMiIIan and J.A. Cookson, J.C.S. Chem. Commun. (1979) 968. [33] C.J. Sofield, L.B. BridweII and C.J. Wright, Nucl. Instr. and Meth. 191 (1981) 379. [34] C.J. Maggiore, T.M. Benjamin, P.J. Hyde, P.S.Z. Rogers, S. Srinivasan, J. Tesmer, D.S. Woolum and D.S. Burnett, IEEE Trans. Nucl. Sci. NS-30 (1983) 1224. [35] A.M. Pritchard, N.E.W. Hartley, J.F. Singleton and A.E. TruswelI, Corrosion Sci. 20 (1980) 1. [36] J.W. McMilIan, P.M. Hirst, F.C.W. Pummery, J. Huddleston and T.B. Pierce, Nucl. Instr. and Meth. 149 (1978) 83. [37] J.A. Cookson, Proc. 7th Divisional Conf. European Phys. Sot., Nuclear Physics Methods in Materials Research eds. K. Bethge, H. Baumann, H. Jex and F. Rauch (F.R. Vieweg, Braunschweig, FRG, 1980)‘~. 145.

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[43] J.W. McMillan, P.M. Pollard and F.C.W. Pummery, Nucl. Instr. and Meth. B15 (1986) 394. [44] SO. Olabanji and J.M. Calvert, IEEE Trans. Nucl. Sci. NS-30 (1983) 1246. [45] C.J. Sofield, J.A. Cookson, J.M. Freeman and K. Parthasaradhi, Nucl. Instr. and Meth. 138 (1976) 411. [46] C.J. Sofield, L.B. Bridwell, N.E.B. Cowem, N.R.S. Tait and D.W.L. Tolfree, Nucl. Instr. and Meth. 186 (1981) 505. [47] J.A. Cookson, Harwell Report, AERE-R12699 (1987).

IX. APPLICATIONS

IN MATERIALS

SCIENCE