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Microscopic, Spectroscopic, and Physical Techniques
Microscopic, Spectroscopic, and Physical Techniques
amorphized volume fraction can be estimated with some confidence using the resistivity data.
Irradiation of materials with energetic neutrons and charged particles leads to the production of lattice defects in the form of Frenkel pairs, anti-site defects, defect clusters, dislocation loops, and amorphous zones in the bulk and creation of sputtered atoms, adatoms, and craters near the surface. The damage phenomena may be classified into three phases: (i) initial damage event, particularly isolated collision cascades; (ii) early stages of microstructural evolution; and (iii) microstructural evolution at high displacement doses (see Radiation Damage Theory). From a historical scientific perspective, significant advances in fundamental knowledge of the underlying processes of radiation-induced property changes have been achieved (see reviews by Szenes 1992 and Schilling and Ullmaier 1994). Successful application of a variety of special experimental techniques for damage characterization has contributed immensely to this progress. Emergence of newer techniques over the years has advanced our understanding still further.
2. Internal Friction
1. Electrical Resistivity The main source of information on the temperature ranges of defect mobility and defect reactions (recombination, trapping, clustering, etc.) is the study of recovery in post-irradiation annealing measurements of electrical resistivity. Specific defect processes associated with the different annealing stages are now well established in pure metals (Takamura et al. 1982), and resistivity studies have been extended to solid solutions, intermetallics, and concentrated alloys. Residual resistivity measurements in low-temperature electron-irradiated Ni Al intermetallic compounds $ reveal a large recovery stage at 75 K, which is assigned to the mobility of Ni–Ni dumbbell interstitials. Migration of nickel vacancies leads to a significant improvement in the degree of long-range order in the intermetallic. The resistivity recovery structure observed in electron-irradiated ferritic Fe–Cr alloys consists of five peaks in the temperature range 135–230 K. The stage III peak is observed at 210 K, while the two peaks at 175 K and 195 K seem to originate from short-range vacancy migration. Doping with impurities like silicon, carbon, and nitrogen is found to suppress the recovery stages in the alloy. The influence of chromium concentration on the mobilities of interstitials and vacancies in electron-irradiated austenitic Fe–Cr–Ni alloys has been investigated through resistivity measurements. Another notable development is the ability of in situ resistivity experiments to provide information on the kinetics of irradiation-induced amorphization in metals. Evidence is found for amorphization through a direct intracascade mechanism and it turns out that the
The internal friction (IF) technique enables the detection of anelasticity relating to microscopic relaxation mechanisms in metals. For example, low frequency IF and dynamic modulus measurements have been addressed towards an understanding of the mechanism for the high mobility of dislocations in the low-temperature range of f.c.c. metals (Lauzier et al. 1993). Measurements made on ultrapure aluminum after low-temperature electron irradiation and lowtemperature plastic deformation, show a substantial decrease of the modulus and this softening is associated with the disappearance of Bordoni relaxation. This seems to result from the lubrication of dislocation motion by the deposited defects, and investigation of the annealing process shows that vacancies are responsible for this lubrication. In a study on hydrogencharged Cr–Ni and Cr–Ni–Mn steels, low-frequency IF measurements reveal five peaks in the temperature range 80–450 K. Of these, three are of relaxation nature, while the other two are of hysteretic character. Short-range migration of hydrogen is found to be responsible for the relaxation peaks. There is no indication of hydrogen-induced Snoek–Koster (SK) relaxation in the electron-irradiated alloy, which is in accordance with the known absence of SK relaxation in f.c.c. metals having low stacking fault energy. 3. Electron Microscopy Transmission electron microscopy (TEM) is the most widely used technique for microstructural characterization in the field of radiation damage. The advent of high-resolution electron microscopy (HREM) has enriched atomic-level understanding of defect processes through high-resolution lattice imaging in a wide spectrum of materials including intermetallics, quasicrystals, nanocrystals, and ceramics. A combination of cross-sectional and plane view TEM with other complementary techniques is being employed for the study of depth distribution of defects. The ability of scanning tunneling microscopy (STM) to provide atomically resolved images of defect structures in irradiated surfaces has elucidated the specific role of surfaces on damage production. Detailed and updated information on the methodology and application of different microscopes can be found in the three volumes edited by Amelinckx et al. 1997. One important advance is the use of a 400 keV heavy-ion accelerator–200 kV electron microscope link facility and in situ observation of heavy-ion damage structure in high-energy cascades. Detailed information on the cascade evolution in f.c.c. metals, like gold, copper, and nickel, has been obtained 1
Microscopic, Spectroscopic, and Physical Techniques (a)
(b)
Figure 1 (a) High-resolution electron micrograph of SiC irradiated to 10#' nmV#, taken along the [110] incident beam direction; (b) high-resolution electron micrograph of SiC irradiated to 10#( nmV#, taken along the [110] incident beam direction (reproduced from Yano et al. 1998).
through in situ observation of vacancy clusters. These studies show that clustered defects can be used as a probe to detect local fluctuations of defect concentration. Results on the size and lifetime distribution of clustered defects and their dependence on the mass of the incident ion, beam flux, and irradiation temperature have been reported (Szenes 1992). Irradiation effects in low-activation structural materials like silicon carbide have attracted recent attention. Neutron damage in SiC up to a fluence of 1.9i10#( nmV# (E 0.1 MeV) has been investigated by high-resolution electron microscopy (Yano et al. 1998). Figure 1(a) shows the high-resolution micrograph of SiC irradiated to 1.0i10#' nmV#, and the lattice image of β-SiC is clearly visible for the whole area without any detectable disturbance of lattice fringes. Alternatively, in the specimen irradiated to 1.0i10#( nmV#, a change in stacking sequence is observed along f111g as seen in Fig. 1(b). Detailed analysis based on a multislice image simulation indicates that the defect formed above the critical fluence is an interstitial-type dislocation loop on o111q, with a thickness of one SiC layer and an average loop diameter of about 15 nm. Preservation of crystallinity in SiC is indicated up to the highest fluence studied, with no evidence of voids or amorphization. Studies on the formation of nanoporous layers on ion-bombarded metal surfaces and radiation blistering are of technological importance. In a significant study (Johnson et al. 1999) a combination of cross-sectional TEM and scanning electron microscopy (SEM) has been employed to investigate the mechanisms of radiation blistering in copper implanted with helium ions above and below the critical dose. Bubble structures directly associated with blistering have been identified and their depth dependence determined. 2
Figure 2 shows the blister morphology in a heliumirradiated copper surface, as seen by SEM. In Fig. 2(a) it can be seen that there is no preferential decoration of grain boundaries and blister morphology is independent of grain orientation. In some cases, blister lids have been blown off, to reveal highly pitted cavity floors (see Fig. 2(b)). The cavity structures at the edges of blister lids, seen under SEM, match those in the heavily cavitated buried layer observed in transverse sections using TEM. Systematic microstructural characterization of neutron-irradiated pressure vessel steels containing copper has been reported using atom probe–field ion microscopy (APFIM). A high density of small, roughly spherical or disk-shaped copper precipitates has been observed. APFIM examination has also revealed complex and diverse reactions in neutronirradiated pressure vessel steel welds. The results show that the ultrafine size of the particles and the spatial distribution of various elements have an influence on the increase in ductile–brittle transition temperature upon irradiation of these steels (see Nuclear Reactors: Pressure Vessel Steels). Such a material characterization would be difficult but for the use of APFIM. Convergent-beam electron diffraction (CBED) studies of ion-irradiated phases of α-ferrite and M C #$ ' in welds of 9Cr–1Mo steel show two characteristic features in CBED patterns which are sensitive to the concentration of point defects, and this offers an attractive method for point-defect identification. Salient features of atomically resolved surface imaging have been reported in STM results for ion impacts on platinum under 4.5 keV neon and xenon irradiation. These results show that the surface damage consists of clusters of adatoms juxtaposed to a vacancy crater near the point of ion impact. STM
Microscopic, Spectroscopic, and Physical Techniques (a)
(b)
30 lm
3 lm
Figure 2 SEM micrographs of radiation blisters in copper implanted with helium to the critical dose. (a) General view of the implanted surface at low magnification. Three grain boundaries meeting at a point near the top of the picture are evident; (b) five intact blisters near a blister crater (reproduced from Johnson et al. 1999). empty clusters He–V clusters (He/V = 1)
400
180
300
160 s(ps)
s(ps)
500
200
single vacancy multi-helium complex
140 120
100
100 0
20
40 60 Cluster size, nvac
80
(a)
0
2
4 He/V ratio
6
8
(b)
Figure 3 (a) Variation of computed positron lifetime as a function of cluster size for pure vacancy clusters and He–V clusters in Ni; (b) variation of computed positron lifetime as a function of He\V ratio for multiple helium–single vacancy complexes in Ni (reproduced from Viswanathan and Amarendra 1991).
evidence of microexplosions, in the form of large craters, has been found in a number of heavy-ion impacts. It is important to note that STM results of ion impacts on surfaces have helped to validate some of the results of molecular dynamics simulations, which have now reached a reliable level of accuracy (Averback and Ghaly 1997).
4. Positron Annihilation Spectroscopy Positron annihilation spectroscopy (PAS) is an established technique for the detection of vacancy-type defects in materials in the concentration range 10V( to 10V% (Brandt and Dupasquier 1983). The Dopplerbroadened annihilation line shape measures the elec-
tron momentum distribution around the positron, while positron lifetime experiments provide a measure of the electron density at the positron site. Both types of measurements, i.e., lifetime and line-shape, are defect-specific. The lifetime of the trapped positron is sensitively dependent on the size and even the configurational structure of the vacancy clusters. Positron lifetime increases as a function of cluster size. For large clusters (nv 50), positron lifetime saturates and this sets the limit for size dependence. This aspect has been exploited over the years to provide detailed information on the early stages of vacancy clustering, over a size range below the resolution limit of TEM. In this respect, PAS provides a nice complement to TEM in the study of void nucleation in irradiated materials. Through high-resolution lifetime spectroscopy, it is 3
Resolution\ sensitivity
Technique
Point defect clusters
Solute– defect complexes
Solute clusters
Strength
Limitation
TEM
2 nm
Yes ( 2 nm diam)
No
Yes ( 2 nm diam)
Most widely used technique for microstructure
Specimen thickness
HREM
Atomic resolution "0.1 nm
Yes
Yes, if with lattice strain
Yes
High resolution lattice imaging
Careful interpretation
SEM
5 nm
Surface clusters of large size
Surface morphology (3-D view)
Low spatial resolution
Positron annihilation
Monovacancy onwards; Concentration 10V(
Small angle scattering
nm size range
Vacancy clusters (nV 50)
Solute– vacancy complex
Yes, if the cluster contains a vacancy
High defect specificity
Only vacancytype defects
Yes
No
Yes
Quantitative microstructure
Information averaged over large volume
Ion scattering\ channeling
Concentrations 10V$
Small clusters
Yes
Small clusters
Defect profile; lattice location
Not sensitive to low concentrations
Perturbed angular correlation
Atomic environment of the probe atom
Small clusters
Yes
Small clusters
Structure and geometry of small clusters
Not suitable for extended defects. Care in interpretation
Mossbauer spectroscopy
Local atomic environment
Yes
Yes
Atomistic information
Only specific systems. Care in interpretation
Internal friction
Concentrations 10−&
Interstitial clusters
Yes
Small clusters
Information on defect relaxation and diffusion constants
Only selected defects
FIM
Atomic resolution
Yes
Yes
Yes
Single atom sensitivity
Limited volume examined
STM
Atomic resolution
Yes
Yes
Yes
Atomically resolved surface imaging
Only conducting samples
AFM
20 nm
No
Large solute clusters
Physical and chemical characterization of the surface
Microscopic, Spectroscopic, and Physical Techniques
4 Table 1 A comparative assessment of various microscopic, spectroscopic, and physical techniques.
Microscopic, Spectroscopic, and Physical Techniques
0.15
is small in the open-structured B2 aluminides. For detailed information on the application of PAS to materials science problems, see Jean et al. (1997).
0.10
5. Small-angle Scattering
14 12
8
p/leff
p(GPa)
10
pHe 6
0.05
4
Dp
2 0
2c/rB 0
1
2
3
0.00
rB (nm)
Figure 4 Helium pressure pHe plotted as a function of mean bubble radius rB in helium-irradiated Ni. The filled circles correspond to experimental values. The dashed curve is the equilibrium bubble pressure for comparison. Also shown is the overpressure ∆p l pHek2γ\rB (reproduced from Ullmaier 1991).
possible to differentiate between various types of defects. The association of an experimentally resolved lifetime with a specific defect has been made possible by the progress in ab initio calculations of positron density distribution and annihilation rates. Reliable lifetime calculations are now available for a variety of defects such as monovacancies, pure vacancy clusters, impurity decorated vacancy clusters, and solute clusters. As an illustrative example (Viswanathan and Amarendra 1991), Fig. 3(a) shows the variation of computed positron lifetime τ as a function of cluster size for undecorated and helium-decorated vacancy clusters, where the effect of helium decoration on τ is clearly seen. Fig. 3(b) shows positron lifetime sensitivity to multiple helium decoration of a vacancy through the dependence of τ on He\V ratio. In the case of large bubbles, model calculations indicate that positron lifetime is solely dependent on the gas density inside the bubble. Systematic experiments and analysis in conjunction with theoretical results have led to qualitative and quantitative understanding of the nucleation and growth of helium bubbles in a number of irradiated metals, dilute alloys, and austenitic steels. Properties of atomic vacancies have been investigated in electronirradiated intermetallic compounds like transitionmetal aluminides, NiZr, and NiTi. These studies show a strong temperature dependence of positron lifetime in vacancies for close-packed intermetallics, whereas it
Small-angle neutron scattering (SANS) and smallangle x-ray scattering (SAXS) are excellent tools for the characterization of those inhomogeneities whose dimensions are in the nanometer range, such as precipitates, voids, and gas bubbles (see Metallurgy: Small-angle Scattering). The scattering is an elastic interaction related to the difference in the scattering length density between the scattering center and the matrix. Important microstructural parameters of the scattering objects, such as their size distribution, density, volume fraction, and specific surface area, can be determined from an analysis of the intensity patterns. The main advantage of neutrons compared with x rays is the low absorption and larger beam size, and also the fact that neutron cross-sections depend on the nuclear rather than the electronic structure. The latter makes it possible for SANS to discriminate between different isotopes of the same element, allowing contrast variation measurements. This aspect has been utilized in the SANS study on nickel implanted with 1200 appm of $He and %He (Ullmaier 1991). SANS measurements on nickel in postimplantation annealing have revealed a bimodal distribution of the bubble population: small bubbles in the bulk matrix which grow under conditions of vacancy deficit and large bubbles in the vicinity of grain boundaries. The salient feature of this study is the experimental determination of helium atom density (gas pressure) in the bubble from SANS data, based on the contrast variation technique. Figure 4 shows the variation of helium pressure pHe in the bubble and its comparison with the equilibrium pressure value of 2γ\rB(γ being the surface free energy). The striking result in Fig. 4 is that the gas pressure in small bubbles is in excess of that expected for equilibrium bubbles over the size range investigated. Even at the highest annealing temperature, corresponding to 70% of the melting temperature, bubbles in the bulk matrix contain an overpressure of 3 GPa. Existence of overpressure, which has been corroborated by independent positron annihilation experiments on helium-implanted nickel, has strong implications for bubblecoarsening mechanisms. SAXS has also been extensively used for a detailed insight into radiation effects. For example, a SAXS study of silica (amorphous SiO ) irradiated with light # ions showed that structural modification in the material is due to preserved primary atomic size defects, thereby demonstrating the value of the technique for studying irradiated amorphous structures (see Glazer 1997). 5
Microscopic, Spectroscopic, and Physical Techniques 6. Summary and Conclusion Salient information obtained in recent years through the application of microscopic, spectroscopic, and physical techniques for radiation damage studies has been highlighted, based on selected examples. Table 1 gives a comparative critical assessment of the merits and de-merits of various techniques. It is clear that with the availability of different classes of microscopes today, electron microscopy will play a leading role in the characterization of damage and microstructure at different levels of understanding. It should be emphasized at the same time that a judicious combination of techniques for the study of the same material or specimen is the need of the hour. Conclusions based on correlated results derived from the complementary strength of each technique will be of great value towards a comprehensive and definitive understanding of the complex issues associated with radiation damage phenomena.
Bibliography Amelinckx S, Van Dyck D, Van Landuyt J, Van Tandeloo G 1997 Handbook of Microscopy: Applications in Materials Science, Physics and Chemistry. VCH, Weinheim, Vols. I–III Averback R S, Ghaly M 1997 Fundamental aspects of defect production in solids. Nucl. Instrum. Methods Phys. Res. B. 127–128, 1–11
Brandt W, Dupasquier A 1983 Positron Solid State Physics. North Holland, Amsterdam Glazer A M 1997 Proceedings of the 10th International Conference on Small Angle Scattering, Brazil. J. Appl. Crystallogr. 30, 569–888 Jean Y C, Eldrup M, Schrader D M, West R N 1997 Positron Annihilation (ICPA–11). Mater. Sci. Forum. 255–7, 1–807 Johnson P B, Thomson R W, Reader K 1999 TEM and SEM studies of radiation blistering in helium implanted copper. J. Nucl. Mater. 273, 117–29 Lauzier J, Gremand G, Benoit W 1993 Proceedings of 6th European Conf. of the Academy of Mining and Metallurgy on Internal Friction and Ultrasonic Attenuation in Solids. Mater. Sci. Forum 119–21, 183–8 Schilling W, Ullmaier H 1994 In: Cahn R W, Haasen P, Kramer E J (eds.) Materials Science and Technology. VCH, Weinheim, Germany, 10B, Part II, pp. 179–249 Szenes G 1992 Physics of irradiation effects in metals (PM ’91). Mater. Sci. Forum 97–99, 1–789 Takamura J-I, Doyama M, Kiritani M 1982 Point Defects and Defect Interactions in Metals. North Holland, Amsterdam Ullmaier H 1991 In: Donnelly S E, Evans J H (eds.) Fundamental Aspects of Inert Gases in Solids, NATO ASI Series B 279. Plenum, New York, pp. 277–85 Viswanathan B, Amarendra G 1991 In: Donnelly S E, Evans J H (eds.) Fundamental Aspects of Inert Gases in Solids, NATO ASI Series B 279. Plenum, New York, pp. 209–19 Yano T, Miyazaki H, Akiyoshi M, Iseki T 1998 X-ray diffractometry and high resolution electron microscopy of neutron irradiated SiC to a fluence of 1.9i10#( n\m#. J. Nucl. Mater. 253, 78–86
B. Viswanathan
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 5661–5666 6
amorphized volume fraction can be estimated with some confidence using the resistivity data.
Irradiation of materials with energetic neutrons and charged particles leads to the production of lattice defects in the form of Frenkel pairs, anti-site defects, defect clusters, dislocation loops, and amorphous zones in the bulk and creation of sputtered atoms, adatoms, and craters near the surface. The damage phenomena may be classified into three phases: (i) initial damage event, particularly isolated collision cascades; (ii) early stages of microstructural evolution; and (iii) microstructural evolution at high displacement doses (see Radiation Damage Theory). From a historical scientific perspective, significant advances in fundamental knowledge of the underlying processes of radiation-induced property changes have been achieved (see reviews by Szenes 1992 and Schilling and Ullmaier 1994). Successful application of a variety of special experimental techniques for damage characterization has contributed immensely to this progress. Emergence of newer techniques over the years has advanced our understanding still further.
2. Internal Friction
1. Electrical Resistivity The main source of information on the temperature ranges of defect mobility and defect reactions (recombination, trapping, clustering, etc.) is the study of recovery in post-irradiation annealing measurements of electrical resistivity. Specific defect processes associated with the different annealing stages are now well established in pure metals (Takamura et al. 1982), and resistivity studies have been extended to solid solutions, intermetallics, and concentrated alloys. Residual resistivity measurements in low-temperature electron-irradiated Ni Al intermetallic compounds $ reveal a large recovery stage at 75 K, which is assigned to the mobility of Ni–Ni dumbbell interstitials. Migration of nickel vacancies leads to a significant improvement in the degree of long-range order in the intermetallic. The resistivity recovery structure observed in electron-irradiated ferritic Fe–Cr alloys consists of five peaks in the temperature range 135–230 K. The stage III peak is observed at 210 K, while the two peaks at 175 K and 195 K seem to originate from short-range vacancy migration. Doping with impurities like silicon, carbon, and nitrogen is found to suppress the recovery stages in the alloy. The influence of chromium concentration on the mobilities of interstitials and vacancies in electron-irradiated austenitic Fe–Cr–Ni alloys has been investigated through resistivity measurements. Another notable development is the ability of in situ resistivity experiments to provide information on the kinetics of irradiation-induced amorphization in metals. Evidence is found for amorphization through a direct intracascade mechanism and it turns out that the
The internal friction (IF) technique enables the detection of anelasticity relating to microscopic relaxation mechanisms in metals. For example, low frequency IF and dynamic modulus measurements have been addressed towards an understanding of the mechanism for the high mobility of dislocations in the low-temperature range of f.c.c. metals (Lauzier et al. 1993). Measurements made on ultrapure aluminum after low-temperature electron irradiation and lowtemperature plastic deformation, show a substantial decrease of the modulus and this softening is associated with the disappearance of Bordoni relaxation. This seems to result from the lubrication of dislocation motion by the deposited defects, and investigation of the annealing process shows that vacancies are responsible for this lubrication. In a study on hydrogencharged Cr–Ni and Cr–Ni–Mn steels, low-frequency IF measurements reveal five peaks in the temperature range 80–450 K. Of these, three are of relaxation nature, while the other two are of hysteretic character. Short-range migration of hydrogen is found to be responsible for the relaxation peaks. There is no indication of hydrogen-induced Snoek–Koster (SK) relaxation in the electron-irradiated alloy, which is in accordance with the known absence of SK relaxation in f.c.c. metals having low stacking fault energy. 3. Electron Microscopy Transmission electron microscopy (TEM) is the most widely used technique for microstructural characterization in the field of radiation damage. The advent of high-resolution electron microscopy (HREM) has enriched atomic-level understanding of defect processes through high-resolution lattice imaging in a wide spectrum of materials including intermetallics, quasicrystals, nanocrystals, and ceramics. A combination of cross-sectional and plane view TEM with other complementary techniques is being employed for the study of depth distribution of defects. The ability of scanning tunneling microscopy (STM) to provide atomically resolved images of defect structures in irradiated surfaces has elucidated the specific role of surfaces on damage production. Detailed and updated information on the methodology and application of different microscopes can be found in the three volumes edited by Amelinckx et al. 1997. One important advance is the use of a 400 keV heavy-ion accelerator–200 kV electron microscope link facility and in situ observation of heavy-ion damage structure in high-energy cascades. Detailed information on the cascade evolution in f.c.c. metals, like gold, copper, and nickel, has been obtained 1
Microscopic, Spectroscopic, and Physical Techniques (a)
(b)
Figure 1 (a) High-resolution electron micrograph of SiC irradiated to 10#' nmV#, taken along the [110] incident beam direction; (b) high-resolution electron micrograph of SiC irradiated to 10#( nmV#, taken along the [110] incident beam direction (reproduced from Yano et al. 1998).
through in situ observation of vacancy clusters. These studies show that clustered defects can be used as a probe to detect local fluctuations of defect concentration. Results on the size and lifetime distribution of clustered defects and their dependence on the mass of the incident ion, beam flux, and irradiation temperature have been reported (Szenes 1992). Irradiation effects in low-activation structural materials like silicon carbide have attracted recent attention. Neutron damage in SiC up to a fluence of 1.9i10#( nmV# (E 0.1 MeV) has been investigated by high-resolution electron microscopy (Yano et al. 1998). Figure 1(a) shows the high-resolution micrograph of SiC irradiated to 1.0i10#' nmV#, and the lattice image of β-SiC is clearly visible for the whole area without any detectable disturbance of lattice fringes. Alternatively, in the specimen irradiated to 1.0i10#( nmV#, a change in stacking sequence is observed along f111g as seen in Fig. 1(b). Detailed analysis based on a multislice image simulation indicates that the defect formed above the critical fluence is an interstitial-type dislocation loop on o111q, with a thickness of one SiC layer and an average loop diameter of about 15 nm. Preservation of crystallinity in SiC is indicated up to the highest fluence studied, with no evidence of voids or amorphization. Studies on the formation of nanoporous layers on ion-bombarded metal surfaces and radiation blistering are of technological importance. In a significant study (Johnson et al. 1999) a combination of cross-sectional TEM and scanning electron microscopy (SEM) has been employed to investigate the mechanisms of radiation blistering in copper implanted with helium ions above and below the critical dose. Bubble structures directly associated with blistering have been identified and their depth dependence determined. 2
Figure 2 shows the blister morphology in a heliumirradiated copper surface, as seen by SEM. In Fig. 2(a) it can be seen that there is no preferential decoration of grain boundaries and blister morphology is independent of grain orientation. In some cases, blister lids have been blown off, to reveal highly pitted cavity floors (see Fig. 2(b)). The cavity structures at the edges of blister lids, seen under SEM, match those in the heavily cavitated buried layer observed in transverse sections using TEM. Systematic microstructural characterization of neutron-irradiated pressure vessel steels containing copper has been reported using atom probe–field ion microscopy (APFIM). A high density of small, roughly spherical or disk-shaped copper precipitates has been observed. APFIM examination has also revealed complex and diverse reactions in neutronirradiated pressure vessel steel welds. The results show that the ultrafine size of the particles and the spatial distribution of various elements have an influence on the increase in ductile–brittle transition temperature upon irradiation of these steels (see Nuclear Reactors: Pressure Vessel Steels). Such a material characterization would be difficult but for the use of APFIM. Convergent-beam electron diffraction (CBED) studies of ion-irradiated phases of α-ferrite and M C #$ ' in welds of 9Cr–1Mo steel show two characteristic features in CBED patterns which are sensitive to the concentration of point defects, and this offers an attractive method for point-defect identification. Salient features of atomically resolved surface imaging have been reported in STM results for ion impacts on platinum under 4.5 keV neon and xenon irradiation. These results show that the surface damage consists of clusters of adatoms juxtaposed to a vacancy crater near the point of ion impact. STM
Microscopic, Spectroscopic, and Physical Techniques (a)
(b)
30 lm
3 lm
Figure 2 SEM micrographs of radiation blisters in copper implanted with helium to the critical dose. (a) General view of the implanted surface at low magnification. Three grain boundaries meeting at a point near the top of the picture are evident; (b) five intact blisters near a blister crater (reproduced from Johnson et al. 1999). empty clusters He–V clusters (He/V = 1)
400
180
300
160 s(ps)
s(ps)
500
200
single vacancy multi-helium complex
140 120
100
100 0
20
40 60 Cluster size, nvac
80
(a)
0
2
4 He/V ratio
6
8
(b)
Figure 3 (a) Variation of computed positron lifetime as a function of cluster size for pure vacancy clusters and He–V clusters in Ni; (b) variation of computed positron lifetime as a function of He\V ratio for multiple helium–single vacancy complexes in Ni (reproduced from Viswanathan and Amarendra 1991).
evidence of microexplosions, in the form of large craters, has been found in a number of heavy-ion impacts. It is important to note that STM results of ion impacts on surfaces have helped to validate some of the results of molecular dynamics simulations, which have now reached a reliable level of accuracy (Averback and Ghaly 1997).
4. Positron Annihilation Spectroscopy Positron annihilation spectroscopy (PAS) is an established technique for the detection of vacancy-type defects in materials in the concentration range 10V( to 10V% (Brandt and Dupasquier 1983). The Dopplerbroadened annihilation line shape measures the elec-
tron momentum distribution around the positron, while positron lifetime experiments provide a measure of the electron density at the positron site. Both types of measurements, i.e., lifetime and line-shape, are defect-specific. The lifetime of the trapped positron is sensitively dependent on the size and even the configurational structure of the vacancy clusters. Positron lifetime increases as a function of cluster size. For large clusters (nv 50), positron lifetime saturates and this sets the limit for size dependence. This aspect has been exploited over the years to provide detailed information on the early stages of vacancy clustering, over a size range below the resolution limit of TEM. In this respect, PAS provides a nice complement to TEM in the study of void nucleation in irradiated materials. Through high-resolution lifetime spectroscopy, it is 3
Resolution\ sensitivity
Technique
Point defect clusters
Solute– defect complexes
Solute clusters
Strength
Limitation
TEM
2 nm
Yes ( 2 nm diam)
No
Yes ( 2 nm diam)
Most widely used technique for microstructure
Specimen thickness
HREM
Atomic resolution "0.1 nm
Yes
Yes, if with lattice strain
Yes
High resolution lattice imaging
Careful interpretation
SEM
5 nm
Surface clusters of large size
Surface morphology (3-D view)
Low spatial resolution
Positron annihilation
Monovacancy onwards; Concentration 10V(
Small angle scattering
nm size range
Vacancy clusters (nV 50)
Solute– vacancy complex
Yes, if the cluster contains a vacancy
High defect specificity
Only vacancytype defects
Yes
No
Yes
Quantitative microstructure
Information averaged over large volume
Ion scattering\ channeling
Concentrations 10V$
Small clusters
Yes
Small clusters
Defect profile; lattice location
Not sensitive to low concentrations
Perturbed angular correlation
Atomic environment of the probe atom
Small clusters
Yes
Small clusters
Structure and geometry of small clusters
Not suitable for extended defects. Care in interpretation
Mossbauer spectroscopy
Local atomic environment
Yes
Yes
Atomistic information
Only specific systems. Care in interpretation
Internal friction
Concentrations 10−&
Interstitial clusters
Yes
Small clusters
Information on defect relaxation and diffusion constants
Only selected defects
FIM
Atomic resolution
Yes
Yes
Yes
Single atom sensitivity
Limited volume examined
STM
Atomic resolution
Yes
Yes
Yes
Atomically resolved surface imaging
Only conducting samples
AFM
20 nm
No
Large solute clusters
Physical and chemical characterization of the surface
Microscopic, Spectroscopic, and Physical Techniques
4 Table 1 A comparative assessment of various microscopic, spectroscopic, and physical techniques.
Microscopic, Spectroscopic, and Physical Techniques
0.15
is small in the open-structured B2 aluminides. For detailed information on the application of PAS to materials science problems, see Jean et al. (1997).
0.10
5. Small-angle Scattering
14 12
8
p/leff
p(GPa)
10
pHe 6
0.05
4
Dp
2 0
2c/rB 0
1
2
3
0.00
rB (nm)
Figure 4 Helium pressure pHe plotted as a function of mean bubble radius rB in helium-irradiated Ni. The filled circles correspond to experimental values. The dashed curve is the equilibrium bubble pressure for comparison. Also shown is the overpressure ∆p l pHek2γ\rB (reproduced from Ullmaier 1991).
possible to differentiate between various types of defects. The association of an experimentally resolved lifetime with a specific defect has been made possible by the progress in ab initio calculations of positron density distribution and annihilation rates. Reliable lifetime calculations are now available for a variety of defects such as monovacancies, pure vacancy clusters, impurity decorated vacancy clusters, and solute clusters. As an illustrative example (Viswanathan and Amarendra 1991), Fig. 3(a) shows the variation of computed positron lifetime τ as a function of cluster size for undecorated and helium-decorated vacancy clusters, where the effect of helium decoration on τ is clearly seen. Fig. 3(b) shows positron lifetime sensitivity to multiple helium decoration of a vacancy through the dependence of τ on He\V ratio. In the case of large bubbles, model calculations indicate that positron lifetime is solely dependent on the gas density inside the bubble. Systematic experiments and analysis in conjunction with theoretical results have led to qualitative and quantitative understanding of the nucleation and growth of helium bubbles in a number of irradiated metals, dilute alloys, and austenitic steels. Properties of atomic vacancies have been investigated in electronirradiated intermetallic compounds like transitionmetal aluminides, NiZr, and NiTi. These studies show a strong temperature dependence of positron lifetime in vacancies for close-packed intermetallics, whereas it
Small-angle neutron scattering (SANS) and smallangle x-ray scattering (SAXS) are excellent tools for the characterization of those inhomogeneities whose dimensions are in the nanometer range, such as precipitates, voids, and gas bubbles (see Metallurgy: Small-angle Scattering). The scattering is an elastic interaction related to the difference in the scattering length density between the scattering center and the matrix. Important microstructural parameters of the scattering objects, such as their size distribution, density, volume fraction, and specific surface area, can be determined from an analysis of the intensity patterns. The main advantage of neutrons compared with x rays is the low absorption and larger beam size, and also the fact that neutron cross-sections depend on the nuclear rather than the electronic structure. The latter makes it possible for SANS to discriminate between different isotopes of the same element, allowing contrast variation measurements. This aspect has been utilized in the SANS study on nickel implanted with 1200 appm of $He and %He (Ullmaier 1991). SANS measurements on nickel in postimplantation annealing have revealed a bimodal distribution of the bubble population: small bubbles in the bulk matrix which grow under conditions of vacancy deficit and large bubbles in the vicinity of grain boundaries. The salient feature of this study is the experimental determination of helium atom density (gas pressure) in the bubble from SANS data, based on the contrast variation technique. Figure 4 shows the variation of helium pressure pHe in the bubble and its comparison with the equilibrium pressure value of 2γ\rB(γ being the surface free energy). The striking result in Fig. 4 is that the gas pressure in small bubbles is in excess of that expected for equilibrium bubbles over the size range investigated. Even at the highest annealing temperature, corresponding to 70% of the melting temperature, bubbles in the bulk matrix contain an overpressure of 3 GPa. Existence of overpressure, which has been corroborated by independent positron annihilation experiments on helium-implanted nickel, has strong implications for bubblecoarsening mechanisms. SAXS has also been extensively used for a detailed insight into radiation effects. For example, a SAXS study of silica (amorphous SiO ) irradiated with light # ions showed that structural modification in the material is due to preserved primary atomic size defects, thereby demonstrating the value of the technique for studying irradiated amorphous structures (see Glazer 1997). 5
Microscopic, Spectroscopic, and Physical Techniques 6. Summary and Conclusion Salient information obtained in recent years through the application of microscopic, spectroscopic, and physical techniques for radiation damage studies has been highlighted, based on selected examples. Table 1 gives a comparative critical assessment of the merits and de-merits of various techniques. It is clear that with the availability of different classes of microscopes today, electron microscopy will play a leading role in the characterization of damage and microstructure at different levels of understanding. It should be emphasized at the same time that a judicious combination of techniques for the study of the same material or specimen is the need of the hour. Conclusions based on correlated results derived from the complementary strength of each technique will be of great value towards a comprehensive and definitive understanding of the complex issues associated with radiation damage phenomena.
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B. Viswanathan
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 5661–5666 6