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Blister lid thickness measurements — A discussion in terms of the interbubble fracture model of blister formation
BLISTER LID THICKNESS MEASUREMENTS -A DWXJSSION IN TERMS OF THE INTERBUBBLE FRACTURE MODEL OF BLISTER FORMATlON J.H. EVANS Metallurgy Division, AERE, Harwell, Oxfordshiw OX11 ORA, UK
In measurements of blister lid thickness by such techniques as Rutherford back-scattering (RRS) and scanning electron microscopy (SEM) some discrepancies have been mentioned. The problem is briefly discussed and the suggestion that RRS could be measuring a stressed zone beyond the blister fracture plane is examined in terms of the gas pressure driven interbubble fracture model for bubble coalescence and blister nucleation. An extension of the model, where a microcracked region extends away from the original fracture plane, appears to provide a possible reason for local misalignment in this region and therefore supports the stressed zone proposal. The microcracked region suggested in the model both below and above the blister fracture plane also appears to fit the development of a permeable layer suggested from elastic recoil detection analysis studies of helium profiles in metals during and after blistering. A possible test of the model is proposed.
1. Introductbn A measurement of considerable interest in the study of helium-induced blister formation on metal surfaces has been that of blister lid thickness, in particular the relation between this thickness and the most probable range of the implanted helium. A number of papers have presented measurements in this area and discussed their implication on blister formation theories (e.g. [l-7]). Two main techniques have been used, Rutherford back-scattering @CBS) and scanning electron microsocopy (SEM). At low helium implant energies it has generally been found that the RBS thickness measurements, tRBS,have had values greater than R,, the most probable helium range, implying that the fracture plane could be beyond the helium peak [2-41. Using SEM measurements it is again found that the thickness, now rSEM,exceeds R, [5,6,8], but this is not unexpected since tsar measures a real physical length and must include the large helium bubble swelling which is known to exist in the implanted layer [g-11]. The effect of this swelling in the RBS measurements, where true effectively measures the number of atoms between the surface and the end of the misaligned region, should be minimal. From this outline it would seem reasonable to conclude that the relation tsEM> iRss should al-
ways hold. However, in the recent paper by Risch et al. [12], which included an investigation of both parameters for 30 and 100 keV helium ions into niobium at varying angles of incidence, this was not always true. At best they found kEM - tRBs, while at high angles of incidence for the 30 keV implantation, ram was greater than &EM.The authors considered some reasons for the discrepancy including the suggestion of Kaminsky et al. [l] that a stressed or misaligned zone exists beyond the blister fracture plane. The main purpose of the present paper is to discuss a possible way in which an extension of the interbubble fracture model for blister crack nucleation [13,14] could lead to such a zone of misalignment. 2. Theory The model to be discussed here continues an attempt to uncover possible physical processes whereby helium deposited in materials eventually causes blister formation. Previous work [13,14] concentrated on the possible modes of helium bubble growth in the absence of vacancy mobility and subsequently led to the interbubble fracture model for blistering. Briefly, it was argued that the shortage of vacancies made the bubbles highly overpressurised but this pressure could be relieved by the bubbles acquiring
Joumul of Nuclear Materials 93 & 94 (19830)745-749 @ North-Holland Publishing Company
745
746
J.H. Evans
I Blister lid thickness measurements
vacancies by the loop punching mechanism [15]. However, as more helium is deposited and bubbles grow, cooperative fracture between the bubbles suddenly becomes an easier way of relieving their overpressure, thus initiating a crack, allowing internal gas release, and causing blister formation. One area in this picture which has received only little attention is that between the initial nucleation of an internal crack and its growth into a blister cavity. As realised early on [13] the problem is that insufficient gas is released in the fracture of one or two planes of gas bubbles (when the density is as high as the observed 10’9cm-3) to cause blister lid deformation. The exact number of gas bubble planes required will obviously vary with energy and implant depth, and from blister to blister. However, for one particular case, 12 layers were estimated [13] to be required, about 10% of the total helium injected over the area of the blister. The same 10% figure emerged from a recent treatment by Kamada and Higashida [16]. Here, in contrast to their fracture model approach to the transition from a lenticular bubble to a blister, we utilise the idea of fracture between highly overpressurised bubbles to show how bubbles below and above the initial crack can release their gas into this crack. The model is outlined in figs. la-d. In fig. la the interaction of the bubbles initiates a small crack. If the bubble concentration is invariant with depth then this crack should be at the helium peak. For convenience we will assume this to be so but it must be noticed that a bubble gradient can alter this- an increase of bubble density with depth puts the fracture plane beyond the gas peak and vice versa. An interesting question at this stage is why the initial crack is parallel to the surface. There appears to be two factors: first, one might expect its general direction to be along that of the initiating gas concentration contour but secondly, and possibly of more importance, the local compressive lateral stress (LS) set up as a result of the bubble swelling [4,17] must inhibit any crack propagation perpendicular to the stress and thus to the surface. In fig. lb the crack propagates towards the final blister size. Extensive propagation of
ia)
(Cl
cd,
Fig. 1. (a) Start of fracture process to link overpressurised bubbles. Lateral stress helps to keep crack plane parallel to the surface. (b) Extension of crack to final blister size. (c) Local removal of lateral stress allows microcrack development in random directions out from original crack. (d) Microcrack propagation continues until sufficient gas has been released for blister lid deformation to be triggered.
this initial crack involving only a couple of gas bubble. layers would be expected to lead to flaking. The factors controlling whether blisters or flakes form are complex but in the gas driven models of blistering are usually attributed to some mechanical property such as ductility [16,18]. The process central to the proposed model is shown schematically in fig. lc; it is suggested that the removal of the local lateral stress also removes, or partially removes, the restraint on allowed crack directions between bubbles which satisfy the crack criteria. Small microcracks can therefore be formed in a layer propagating outward from both above and below the initial crack. Clearly, extensive microcrack formation will create a myriad of interconnecting pathways and allow an appreciable fraction of gas to be released from bubbles in this region. On a local level there is a close analogy with the random crack system proposed for the release of 3He from tritides [19]. However, in the present case with no immediate pathway to the surface the gas will escape into the initial crack and, provided sufficient quantity is available, will transform it into a blister cavity, fig. Id. It is not easy to say anything definite about the dynamics of the processes in figs. la-d though experimental evidence shows that the transformation of the blister area from a flat plate to the characteristic dome is rather rapid [20,21]. This can be explained relatively easily if the local lateral stress is assumed to play an important
J.H. Ewns / Blister lid thickness measuremennb
role in locking the system into what could be a rather unstable situation. The relief of this stress after the initial crack formation could very rapidly trigger the microcrack formation described above and then immediately provide sufficient gas for blister formation. There is no doubt that any relief of lateral stress will cause deformation in the same direction as the blister formation but, as explained elsewhere [14], the strains involved seem insufficient to create the large blister lid deformations that are in fact observed. Normally in discussing the gas required for blister formation only the helium entering the blister area is considered. However, the microcracked permeable region could also extend laterally so that some gas could enter the blister cavity from this source. Certainly there is evidence (fig. 6 in ref. [22]) that a lateral permeable layer is formed at high implant temperatures. 3. Consequences
of the model
If the model outlined is correct then it could provide an explanation why RBS measurements give values greater than the blister lid thickness. One of the arguments in the development of the model is that the helium bubbles are highly overpressurised while in addition we know that the bubbles are very close together (e.g. [23]). Consequently, the small volumes between the bubbles must exist in a highly stressed state. The question to be asked is whether these volumes retain their relative orientation once the gas has been released via the microcracks. It seems hard to claim that no reorientation will take place and, although no figure can be put on the expected effects, we suggest that the misorientations could be the few degrees necessary to give the dechannelling effects seen in RBS. Instead of giving the depth of the fracture plane, RBS would then give the maximum depth of the microcracked region, i.e. T,,,, fig. Id. It is not easy on this model to estimate by how much ?RBSwill exceed R, since it will depend strongly on the gas distribution curve, and hence implant energy, and will vary from blister to blister. In addition, we cannot estimate the frac-
747
tion, F, of bubbles that are bisected by the some bubbles, and gas, must microcracks; remain in this region. However, to get an order of magnitude estimate we can take the example quoted previously where for 30 keV helium into molybdenum (R, = 1060 A), about 10% of the injected gas was required to form the blister. Translating this into depth about the gas peak gives (fRBS- RP) - 15OA if F were 100% and about 600 %, if F were 25%. These figures do not appear too far away from the values found in practice. Although no direct evidence exists to support the proposed model, there are some results which could be consistent with it. For example, Thomas and Wilson have given evidence of local reorientation in regions of high gas content [24], while the same effect has been noted by Johnson and Mazey [25]. More recently the Canadian group [26-281 have obtained interesting data on helium profiles in several metals using Elastic Recoil Detection Analysis, and have demonstrated that at the onset of blister formation the peak helium concentration can drop and on further helium dose the depth profiles widen, sometimes forming two peaks. The results have been explained in terms of the release of helium via a helium saturated permeable layer [29]; we propose that this layer is identical with the microcracked region described above and is formed by the mechanism of interbubble fracture between the bubbles. The effect of further dose after blistering is interesting; according to the model, as more gas is implanted the critical combination of bubble size concentration and gas pressure for interbubble fracture will be reached at the more outlying parts of the original gas profile. In other words the microcrack front will continue to move, with one front moving towards the metal surface and the other moving into the material below the original crack. If the blister lid has remained intact during this period, and the complicating features of blister lid heating [30] are avoided by low beam currents, then further growth might be expected as more gas escapes into the blister cavity. However, once the top crack front coincides with the metal surface an effective permeable layer is formed
748
J.H. Evans
/ Blister lid thickness measurements
over the whole implanted range and gas can escape directly by this route. The end effect of this microcracking model is entirely consistent with the suggestion of Roth et al. [31] that in a broad distribution of implanted ions the gas can diffuse out of pores and small cracks. Obviously the movement of the metal surface by sputtering will be an important parameter in determining the dose at which the permeable zone front coincides with the surface. There is one further experimental observation for which the proposed model could provide an explanation, and which leads to a possible test of the model. Occasionally if flaking occurs after blistering then blister craters are observed on the fracture plane of the flake, e.g. fig. 2. (The same phenomenon is also found sometimes if a large blister is formed over earlier small blisters.) If the microcracked region is very brittle-not an unreasonable assumption -then the shocks of handling, or flake production, could disturb and remove this region; the result would be the observed crater relative to the original fracture plane. The alternative and perhaps more straightforward explanation is obviously in terms of two fracture planes, one for the flake at the gas peak and the second for the blisters beyond the peak. Fortunately there appears to be a way of testing these two possibilities: if the two-
Fig. 2. Example of blister Markers are 1 pm.
craters on floor of flaked
area.
fracture plane theory is correct then the underside of the removed flake should follow the profile of the parts remaining and have raised portions corresponding to the craters. On the other hand, if there is a common fracture plane and the microcrack idea is correct the removed flake should itself contain craters at blister sites. As far as is known this point has not yet been tested. References [l] M. Kaminsky, S.K. Das and G. Fenske, J. Nucl. Mater. 59 (1976) 86. [2] J. Roth, R. Behrisch and B.M.U. Scherzer, J. Nucl. Mater. 53 (1974) 147. [3] J. Roth, Conf. on Appl. of Ion Beams to Materials, Warwick 1975, Inst. Phys. Conf. Ser. 28 (1976) p. 280. [4] M. Risch, J. Roth and B.M.U. Scherzer, in: Proc. Intern. Symp. on Plasma Wall Interactions, Jiilich, 1976 (Pergamon, 1977) p. 391. [S] R.G. St-Jacques, J.G. Martel, B. Terreault, G. Veilleux, S.K. Das, M. Kaminsky and G. Fenske, J. Nucl. Mater. 63 (1976) 273. [6] S.K. Das, M. Kaminsky and G. Fen&e, J. Nucl. Mater. 76/77 (1978) 215. [7] G. Fenske, S.K. Das, M. Kaminsky and G.H. Miley, J. Nucl. Mater. 76/77 (1978) 247. [8] M. Braun, J.L. Whitton and B. Emmoth, J. Nucl. Mater. 85/86 (1979) 1091. [9] R.S. Blewer and W. Beezhold, Radiation Effects 19 (1973) 49. [lo] R.G. St-Jacques, F. Veilleux, J.G. Martel and B. Terresult, Radiation Effects 47 (1980) 233. [ll] R.G. St-Jacques, G. Veilleux and B. Terreault, Nucl. Instrum. Methods 170 (1980) 461. [12] M.R. Risch, J. Roth and B.M.U. Scherzer, J. Nucl. Mater. 82 (1979) 220. [13] J.H. Evans, J. Nucl. Mater. 68 (1977) 129. [14] J.H. Evans, J. Nucl. Mater. 76P7 (1978) 228. [15] G.W. Greenwood, A.J.E. Foreman and D.E. Rimmer, J. Nucl. Mater, 4 (1959) 305. [16] K. Kamada and Y. Higashida, J. Appl. Phys. 50 (1979) 4131. [17] E.P. EerNisse and S.T. Picraux, J. Appl. Phys. 48 (1977) 9. [18] K. Kamada and H. Naramoto, Radiation Effects 42 (1979) 209. [19] J.H. Evans, J. Nucl. Mater. 79 (1979) 249. [20] G.J. Thomas and W. Bauer, J. Nucl. Mater. 63 (1976) 280. [21] J.I. Bennetch, M.L. Sattler, L.L. Schiestle Horton, J.A. Horton and W.A. Jesser, J. Nucl. Mater. 85/86 (1979)665. [22] K. Sone, M. Saidoh, R. Yamada and H. Ohtsuka, J. Nucl. Mater. 76177 (1978) 240.
J.H. Evans / Blister lid thickness measurements [23] D.J. Mazy, B.L. Eyre, J.H. Evans, SK. Erents and G.M. McCracken, J. Nucl. Mater. 64 (1977) 145. [24] G.J. Thomas and K.J. Wilson, Trans. Am. Nucl. Sot. 27 (1977) 273. [25l P.B. Johnson and D.J. Maxey, J. Nucl. Mater. 93/94 (1980) 721. [26] B. Terreault, J.G. Martel, R.G. St-Jacques, G. Veilleux, J. L’Ecuyer, C. Brassard, C. Cardinal, L. Deschenes and J.P. Labrie, J. Nucl. Mater. 68 (1977) 334. [27] B. Terreault, R.G. St-Jacques, G. Veilleux, J.G. Mattel,
[28]
[29] [30] [31]
749
J. L’Ecuyer, C. Brassard and C. Cardinal, Can. J. Phys. 56 (1978) 235. B. Terreault, G. Abel, J.G. Mattel, R.G. St-Jacques, J.P. Labrie and J. L’Ecuyer, J. Nucl. Mater. 76/77 (1978) 249. B. Terreault, J. Nucl. Mater. 93/94 (1980) 797. S.K. Deb and D.K. Sood, Bull. Am. Phys. Sot. 23 (1978) 1062. J. Roth, R. Behrisch and B.M.U. Scherzer, J. Nucl. Mater. 57 (1975) 365.
In measurements of blister lid thickness by such techniques as Rutherford back-scattering (RRS) and scanning electron microscopy (SEM) some discrepancies have been mentioned. The problem is briefly discussed and the suggestion that RRS could be measuring a stressed zone beyond the blister fracture plane is examined in terms of the gas pressure driven interbubble fracture model for bubble coalescence and blister nucleation. An extension of the model, where a microcracked region extends away from the original fracture plane, appears to provide a possible reason for local misalignment in this region and therefore supports the stressed zone proposal. The microcracked region suggested in the model both below and above the blister fracture plane also appears to fit the development of a permeable layer suggested from elastic recoil detection analysis studies of helium profiles in metals during and after blistering. A possible test of the model is proposed.
1. Introductbn A measurement of considerable interest in the study of helium-induced blister formation on metal surfaces has been that of blister lid thickness, in particular the relation between this thickness and the most probable range of the implanted helium. A number of papers have presented measurements in this area and discussed their implication on blister formation theories (e.g. [l-7]). Two main techniques have been used, Rutherford back-scattering @CBS) and scanning electron microsocopy (SEM). At low helium implant energies it has generally been found that the RBS thickness measurements, tRBS,have had values greater than R,, the most probable helium range, implying that the fracture plane could be beyond the helium peak [2-41. Using SEM measurements it is again found that the thickness, now rSEM,exceeds R, [5,6,8], but this is not unexpected since tsar measures a real physical length and must include the large helium bubble swelling which is known to exist in the implanted layer [g-11]. The effect of this swelling in the RBS measurements, where true effectively measures the number of atoms between the surface and the end of the misaligned region, should be minimal. From this outline it would seem reasonable to conclude that the relation tsEM> iRss should al-
ways hold. However, in the recent paper by Risch et al. [12], which included an investigation of both parameters for 30 and 100 keV helium ions into niobium at varying angles of incidence, this was not always true. At best they found kEM - tRBs, while at high angles of incidence for the 30 keV implantation, ram was greater than &EM.The authors considered some reasons for the discrepancy including the suggestion of Kaminsky et al. [l] that a stressed or misaligned zone exists beyond the blister fracture plane. The main purpose of the present paper is to discuss a possible way in which an extension of the interbubble fracture model for blister crack nucleation [13,14] could lead to such a zone of misalignment. 2. Theory The model to be discussed here continues an attempt to uncover possible physical processes whereby helium deposited in materials eventually causes blister formation. Previous work [13,14] concentrated on the possible modes of helium bubble growth in the absence of vacancy mobility and subsequently led to the interbubble fracture model for blistering. Briefly, it was argued that the shortage of vacancies made the bubbles highly overpressurised but this pressure could be relieved by the bubbles acquiring
Joumul of Nuclear Materials 93 & 94 (19830)745-749 @ North-Holland Publishing Company
745
746
J.H. Evans
I Blister lid thickness measurements
vacancies by the loop punching mechanism [15]. However, as more helium is deposited and bubbles grow, cooperative fracture between the bubbles suddenly becomes an easier way of relieving their overpressure, thus initiating a crack, allowing internal gas release, and causing blister formation. One area in this picture which has received only little attention is that between the initial nucleation of an internal crack and its growth into a blister cavity. As realised early on [13] the problem is that insufficient gas is released in the fracture of one or two planes of gas bubbles (when the density is as high as the observed 10’9cm-3) to cause blister lid deformation. The exact number of gas bubble planes required will obviously vary with energy and implant depth, and from blister to blister. However, for one particular case, 12 layers were estimated [13] to be required, about 10% of the total helium injected over the area of the blister. The same 10% figure emerged from a recent treatment by Kamada and Higashida [16]. Here, in contrast to their fracture model approach to the transition from a lenticular bubble to a blister, we utilise the idea of fracture between highly overpressurised bubbles to show how bubbles below and above the initial crack can release their gas into this crack. The model is outlined in figs. la-d. In fig. la the interaction of the bubbles initiates a small crack. If the bubble concentration is invariant with depth then this crack should be at the helium peak. For convenience we will assume this to be so but it must be noticed that a bubble gradient can alter this- an increase of bubble density with depth puts the fracture plane beyond the gas peak and vice versa. An interesting question at this stage is why the initial crack is parallel to the surface. There appears to be two factors: first, one might expect its general direction to be along that of the initiating gas concentration contour but secondly, and possibly of more importance, the local compressive lateral stress (LS) set up as a result of the bubble swelling [4,17] must inhibit any crack propagation perpendicular to the stress and thus to the surface. In fig. lb the crack propagates towards the final blister size. Extensive propagation of
ia)
(Cl
cd,
Fig. 1. (a) Start of fracture process to link overpressurised bubbles. Lateral stress helps to keep crack plane parallel to the surface. (b) Extension of crack to final blister size. (c) Local removal of lateral stress allows microcrack development in random directions out from original crack. (d) Microcrack propagation continues until sufficient gas has been released for blister lid deformation to be triggered.
this initial crack involving only a couple of gas bubble. layers would be expected to lead to flaking. The factors controlling whether blisters or flakes form are complex but in the gas driven models of blistering are usually attributed to some mechanical property such as ductility [16,18]. The process central to the proposed model is shown schematically in fig. lc; it is suggested that the removal of the local lateral stress also removes, or partially removes, the restraint on allowed crack directions between bubbles which satisfy the crack criteria. Small microcracks can therefore be formed in a layer propagating outward from both above and below the initial crack. Clearly, extensive microcrack formation will create a myriad of interconnecting pathways and allow an appreciable fraction of gas to be released from bubbles in this region. On a local level there is a close analogy with the random crack system proposed for the release of 3He from tritides [19]. However, in the present case with no immediate pathway to the surface the gas will escape into the initial crack and, provided sufficient quantity is available, will transform it into a blister cavity, fig. Id. It is not easy to say anything definite about the dynamics of the processes in figs. la-d though experimental evidence shows that the transformation of the blister area from a flat plate to the characteristic dome is rather rapid [20,21]. This can be explained relatively easily if the local lateral stress is assumed to play an important
J.H. Ewns / Blister lid thickness measuremennb
role in locking the system into what could be a rather unstable situation. The relief of this stress after the initial crack formation could very rapidly trigger the microcrack formation described above and then immediately provide sufficient gas for blister formation. There is no doubt that any relief of lateral stress will cause deformation in the same direction as the blister formation but, as explained elsewhere [14], the strains involved seem insufficient to create the large blister lid deformations that are in fact observed. Normally in discussing the gas required for blister formation only the helium entering the blister area is considered. However, the microcracked permeable region could also extend laterally so that some gas could enter the blister cavity from this source. Certainly there is evidence (fig. 6 in ref. [22]) that a lateral permeable layer is formed at high implant temperatures. 3. Consequences
of the model
If the model outlined is correct then it could provide an explanation why RBS measurements give values greater than the blister lid thickness. One of the arguments in the development of the model is that the helium bubbles are highly overpressurised while in addition we know that the bubbles are very close together (e.g. [23]). Consequently, the small volumes between the bubbles must exist in a highly stressed state. The question to be asked is whether these volumes retain their relative orientation once the gas has been released via the microcracks. It seems hard to claim that no reorientation will take place and, although no figure can be put on the expected effects, we suggest that the misorientations could be the few degrees necessary to give the dechannelling effects seen in RBS. Instead of giving the depth of the fracture plane, RBS would then give the maximum depth of the microcracked region, i.e. T,,,, fig. Id. It is not easy on this model to estimate by how much ?RBSwill exceed R, since it will depend strongly on the gas distribution curve, and hence implant energy, and will vary from blister to blister. In addition, we cannot estimate the frac-
747
tion, F, of bubbles that are bisected by the some bubbles, and gas, must microcracks; remain in this region. However, to get an order of magnitude estimate we can take the example quoted previously where for 30 keV helium into molybdenum (R, = 1060 A), about 10% of the injected gas was required to form the blister. Translating this into depth about the gas peak gives (fRBS- RP) - 15OA if F were 100% and about 600 %, if F were 25%. These figures do not appear too far away from the values found in practice. Although no direct evidence exists to support the proposed model, there are some results which could be consistent with it. For example, Thomas and Wilson have given evidence of local reorientation in regions of high gas content [24], while the same effect has been noted by Johnson and Mazey [25]. More recently the Canadian group [26-281 have obtained interesting data on helium profiles in several metals using Elastic Recoil Detection Analysis, and have demonstrated that at the onset of blister formation the peak helium concentration can drop and on further helium dose the depth profiles widen, sometimes forming two peaks. The results have been explained in terms of the release of helium via a helium saturated permeable layer [29]; we propose that this layer is identical with the microcracked region described above and is formed by the mechanism of interbubble fracture between the bubbles. The effect of further dose after blistering is interesting; according to the model, as more gas is implanted the critical combination of bubble size concentration and gas pressure for interbubble fracture will be reached at the more outlying parts of the original gas profile. In other words the microcrack front will continue to move, with one front moving towards the metal surface and the other moving into the material below the original crack. If the blister lid has remained intact during this period, and the complicating features of blister lid heating [30] are avoided by low beam currents, then further growth might be expected as more gas escapes into the blister cavity. However, once the top crack front coincides with the metal surface an effective permeable layer is formed
748
J.H. Evans
/ Blister lid thickness measurements
over the whole implanted range and gas can escape directly by this route. The end effect of this microcracking model is entirely consistent with the suggestion of Roth et al. [31] that in a broad distribution of implanted ions the gas can diffuse out of pores and small cracks. Obviously the movement of the metal surface by sputtering will be an important parameter in determining the dose at which the permeable zone front coincides with the surface. There is one further experimental observation for which the proposed model could provide an explanation, and which leads to a possible test of the model. Occasionally if flaking occurs after blistering then blister craters are observed on the fracture plane of the flake, e.g. fig. 2. (The same phenomenon is also found sometimes if a large blister is formed over earlier small blisters.) If the microcracked region is very brittle-not an unreasonable assumption -then the shocks of handling, or flake production, could disturb and remove this region; the result would be the observed crater relative to the original fracture plane. The alternative and perhaps more straightforward explanation is obviously in terms of two fracture planes, one for the flake at the gas peak and the second for the blisters beyond the peak. Fortunately there appears to be a way of testing these two possibilities: if the two-
Fig. 2. Example of blister Markers are 1 pm.
craters on floor of flaked
area.
fracture plane theory is correct then the underside of the removed flake should follow the profile of the parts remaining and have raised portions corresponding to the craters. On the other hand, if there is a common fracture plane and the microcrack idea is correct the removed flake should itself contain craters at blister sites. As far as is known this point has not yet been tested. References [l] M. Kaminsky, S.K. Das and G. Fenske, J. Nucl. Mater. 59 (1976) 86. [2] J. Roth, R. Behrisch and B.M.U. Scherzer, J. Nucl. Mater. 53 (1974) 147. [3] J. Roth, Conf. on Appl. of Ion Beams to Materials, Warwick 1975, Inst. Phys. Conf. Ser. 28 (1976) p. 280. [4] M. Risch, J. Roth and B.M.U. Scherzer, in: Proc. Intern. Symp. on Plasma Wall Interactions, Jiilich, 1976 (Pergamon, 1977) p. 391. [S] R.G. St-Jacques, J.G. Martel, B. Terreault, G. Veilleux, S.K. Das, M. Kaminsky and G. Fenske, J. Nucl. Mater. 63 (1976) 273. [6] S.K. Das, M. Kaminsky and G. Fen&e, J. Nucl. Mater. 76/77 (1978) 215. [7] G. Fenske, S.K. Das, M. Kaminsky and G.H. Miley, J. Nucl. Mater. 76/77 (1978) 247. [8] M. Braun, J.L. Whitton and B. Emmoth, J. Nucl. Mater. 85/86 (1979) 1091. [9] R.S. Blewer and W. Beezhold, Radiation Effects 19 (1973) 49. [lo] R.G. St-Jacques, F. Veilleux, J.G. Martel and B. Terresult, Radiation Effects 47 (1980) 233. [ll] R.G. St-Jacques, G. Veilleux and B. Terreault, Nucl. Instrum. Methods 170 (1980) 461. [12] M.R. Risch, J. Roth and B.M.U. Scherzer, J. Nucl. Mater. 82 (1979) 220. [13] J.H. Evans, J. Nucl. Mater. 68 (1977) 129. [14] J.H. Evans, J. Nucl. Mater. 76P7 (1978) 228. [15] G.W. Greenwood, A.J.E. Foreman and D.E. Rimmer, J. Nucl. Mater, 4 (1959) 305. [16] K. Kamada and Y. Higashida, J. Appl. Phys. 50 (1979) 4131. [17] E.P. EerNisse and S.T. Picraux, J. Appl. Phys. 48 (1977) 9. [18] K. Kamada and H. Naramoto, Radiation Effects 42 (1979) 209. [19] J.H. Evans, J. Nucl. Mater. 79 (1979) 249. [20] G.J. Thomas and W. Bauer, J. Nucl. Mater. 63 (1976) 280. [21] J.I. Bennetch, M.L. Sattler, L.L. Schiestle Horton, J.A. Horton and W.A. Jesser, J. Nucl. Mater. 85/86 (1979)665. [22] K. Sone, M. Saidoh, R. Yamada and H. Ohtsuka, J. Nucl. Mater. 76177 (1978) 240.
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