Crater formation on the surface of metals and alloys during high power ion beam processing

5 Pages·0.635 MB·English

by  Shulov V.A., Nochovnaya N.A.

NuclearInstrumentsandMethodsinPhysicsResearchB148(1999)154–158
Crater formation on the surface of metals and alloys during high
power ion beam processing
V.A. Shulov a,*, N.A. Nochovnaya b
aMoscowAviationInstitute,4Volokolamskoyeshosse,125871Moscow,RussianFederation
bAll-RussianInstituteofAviationMaterials,17RadioStreet,107005Moscow,RussianFederation
Abstract
Thee(cid:128)ectofhigh-power,pulsedion-beamirradiationandvariousmethodsofthepreliminarysurfacetreatmenton
thecraterformationprocesswasexaminedusingAugerelectronspectroscopy,X-raydi(cid:128)ractionanalysis,andscanning
electron microscopy. The crater distribution density, sizes and shape, along with their microhardness and chemical
compositioninsideandoutsidethemweredetermined.Asaresultoftheseexperiments,themostprobablemechanisms
of crater formation on the surface of refractory alloys were established. (cid:211) 1999 Elsevier Science B.V. All rights re-
served.
PACS: 34.50.D
Keywords: Ionbeam; Craterformation; Serviceproperties; Surface
1. Introduction characteristics of irradiated components are the
most important factors. The objective of the
It is well known that crater formation on the present research is the investigation of such e(cid:128)ects
solid surfaces, when irradiating them by high- as the preliminary surface treatment and irradia-
power pulsed, ion-beams (HPPIB) [1] leads to the tion conditions (particularly pulsed ion current
catastrophic deterioration of the property level density) on the crater formation process.
(fatigue strength, oxidation resistance, salt corro-
sion resistance etc.) [2–4] of irradiated compo-
nents. As a result, the determination of crater 2. Experimental
formation mechanisms and the development of
methods allowing to decrease the negative influ- The studied samples were made of bars, pro-
enceof already formed craters upon the operating duced of the following refractory alloys: VT8M
(Ti–6Al–3.7Mo–0.2Fe–0.3Si), VT9 (Ti–7Al–
3.8Mo–0.2Fe–2.5Zr–0.3Si), VT18U (Ti–6A1–
*Correspondingauthor.Tel.:[phone];fax:+7095 3.4Mo–0.2Fe–4.5Zr–1.5Nb–0.25Si–3Sn), VT25U
1582977;e-mail:roman@fipc.ru (Ti–7A1–2.5Mo–1.5W–0.2Fe–2.5Zr–0.25Si),
0168-583X/98/$–seefrontmatter (cid:211) 1999ElsevierScienceB.V.Allrightsreserved.
PII:S0168-583X(98)00845-3
V.A.Shulov,N.A.Nochovnaya/Nucl.Instr.andMeth.inPhys.Res.B148(1999)154–158 155
EP866sh (Fe–2.5Ni–16.5Cr–1.6Mo–1W–0.3Nb–
5.5Co–0.6Si), and EP718ID (Ni–33Fe–2.4Ti–
16Cr–1.4A1–5.2Mo–3.5W–1.5Nb–0.6Mn–0.3Si)
withtheuseofmachining.Theirradiationofthese
targets (14·7·3 mm3) were carried out using the
TEMPaccelerator[4]bycarbon(70%)andproton
ion beams under the following conditions: ion
energy – 300 keV (E), pulse duration – 50 ns (s),
andtheioncurrent densityinapulse(j)aswellas
thenumberofpulses(n)werevariedfromj(cid:136)20A
cm(cid:255)2, n(cid:136)1 up to j(cid:136)220 A cm(cid:255)2, n(cid:136)100. After
theirradiationsometargetswerestudiedbyAuger
electron spectroscopy (AES), X-ray di(cid:128)raction
analysis, scanning electron microscopy (SEM),
opticalmicroscopy(OM),andmicrohardness(H )
l Fig. 1. SEM-micrographs of VT9 alloy surface treated by
measurements. Other irradiated samples were
HPPIB(j(cid:136)120–140Acm(cid:255)2;n(cid:136)3pulses):a,b–round-shaped
subjected to vacuum annealing for 10–60 min at craterwiththeconvexityandconcavityinthecentre;c–round-
the given operating temperature for each alloy, in shapedmulti-ringcrater;d–adjoiningcrater.
particular, VT8M, VT9-500(cid:176)C; VT18U, VT25U-
550(cid:176)C; EP866sh-600(cid:176)C, and EP718ID-650(cid:176)C).
Aftercompletingofthepointedheattreatmentthe (Fig. 1(a)) and the concave centre (Fig. 1(b));
studying cycle was repeated again. In this case it round-shaped multi-rings craters (Fig. 1(c); ad-
should be noted that some of targets were also joining craters (Fig. 1(d)); craters in the form of
subjected to various treatments as grinding, pol- ellipse, crack and disk including also faceted and
ishing, diamond smoothing, surface plastic defor- dent-shapedones.Analysisoftheresultspresented
mation, chemical etching, ion implantation, and in Tables 1 and published in Refs. [5–7] gives a
oxidation prior to irradiating by HPPIB. The possibility to trace the e(cid:128)ect of irradiation condi-
craterdistributiondensity(q),theirdiameter(D ) tions on the type, size, and distribution density of
av
and form along with the microhardness and craters through the surface. It is obvious that the
chemical composition inside and outside them ioncurrentdensity(j)increasefrom40upto110A
were determined during the performed studies. In cm(cid:255)2leadstotheincreaseofthecraterdistribution
addition to these studies, there were some pre- density (q) from 0 up to 1100–1400 mm(cid:255)2. The
scratched samples, which were also irradiated and tendency to the decrease of j down to 200–500
investigated. mm(cid:255)2 and the growth of craters takes place with
the further increasing j (120fi180 A cm(cid:255)2). How-
ever,avalueofqisfirstlyincreasedwithariseofa
3. Results and discussion pulses number (n(cid:136)1, 2 ,3) and then it decreased
(n(cid:136)5–7). It is impossible to establish any depen-
The results of the performed studied allow to dence between q and n values if the ion current
divideallcratersintotwogroupsdependingonthe density achieves high values (j>160–180 A cm(cid:255)2).
stage of their formation. They are the primary On the base of the fixed experimental results one
craters formed after the single-pulse irradiation more important conclusion can be made, that the
and the secondary craters fixed either after the formation of round-shaped and faceted craters
multiple-or single-pulse irradiation and at the occurs mainly when irradiation with the ion cur-
stage of the final heat treatment (annealing after rentdensityof100–150Acm(cid:255)2 andtheformation
HPPIB irradiation). Depending on the crater of dent-shaped and faceted craters and cracks
shape they can be classified into the following takes place at very high values of the ion current
types:round-shapedcraterswiththeconvexcentre density (Fig. 2(a)). The depth of the primary
156 V.A.Shulov,N.A.Nochovnaya/Nucl.Instr.andMeth.inPhys.Res.B148(1999)154–158
Table 1
DistributiondensityandaveragediameterofcratersformedonVT9alloysurface(E(cid:136)300keV;s(cid:136)50ns)
Ioncurrentdensity Pulsenumber Craterdistributionden- Averagediameterofcra- Averagedepthof
j,Acm(cid:255)2 n,pulses sityq,mm(cid:255)2 tersD ,lm cratersh ,lm
av av
60 1 0 0 0
80 1 420(cid:139)20 5–10 1.5(cid:139)0.2
100 1 580(cid:139)40 5–40 1.5(cid:139)0.2
120 1 340(cid:139)30 15–50 2.0(cid:139)0.4
160 1 260(cid:139)20 28–38 2.2(cid:139)0.8
60 2 0 0 0
80 2 200(cid:139)20 5–7 0.5(cid:139)0.2
100 2 1200(cid:139)200 5–7 0.5(cid:139)0.2
120 2 1060(cid:139)200 10–40 1.0(cid:139)0.4
60 3 0 0 0
80 3 1480(cid:139)200 5–15 1.0(cid:139)0.4
100 3 1100(cid:139)200 5–15 1.0(cid:139)0.4
120 3 1100(cid:139)200 10–40 1.0(cid:139)0.4
160 3 160(cid:139)20 20–60 2.0(cid:139)0.2
60 5 0 0 0
80 5 920(cid:139)100 5–10 0.5(cid:139)0.2
100 5 1100(cid:139)200 10–20 1.0(cid:139)0.2
120 5 500(cid:139)100 25–60 1.0(cid:139)0.2
160 5 260(cid:139)30 30–70 2.0)0.5
60 7 0 0 0
80 7 660(cid:139)70 10–40 0.5(cid:139)0.2
100 7 900(cid:139)100 20–50 1.0(cid:139)0.2
120 7 680(cid:139)50 20–60 1.5(cid:139)0.3
160 7 460(cid:139)30 15–25 2.0)0.5
craters achieves 1–2 lm while the secondary cra- bombarding the already melted surface of targets
ters have the depth of not more, than 0.1–0.5 lm. byerodedparticles.Table1illustratesthee(cid:128)ectof
The microhardness values near craters are signifi- theioncurrentdensityontheaveragesizeofformed
cantly lower than the appropriate H values, fixed craters.Itisobviousthattheriseofjfrom80upto
l
on the ‘‘free’’ surface. H (craters) is equal to H 180 Acm(cid:255)2 leads to theaverage diameter increase
l l
(surface) only after the final annealing of irradi- (D (cid:136)5fi100 lm) of formed craters, however, if
a
ated samples. j(cid:136)160–180 A cm(cid:255)2 the surface craters sizes are
The chemical composition of craters being changeddependingonanumberofpulses.Thein-
formed as a result of high-current irradiation tensivegrowthofnewphases(carbides,oxides,and
(j>160–180Acm(cid:255)2)andalsothesecondarycraters oxicarbidesofSn,Zr,Mo,W,Si,Al)withincraters
are similar to the chemical composition of the with the changed composition is observed during
studied alloys. Meanwhile, if HPPIB processing the final annealing. It is explained by the rise of
wascarriedoutattheaveragevaluesofj(100–150A microhardness within craters as a result of the
cm(cid:255)2),craterscontainedaconsiderableamountof vacuum annealing. In this case the formation of
Mo,Zr,Si(titaniumalloys)andAl,Mo,W(steels separate1–20lmcrystalscanoccurwithincraters.
and nickel-base alloys). Moreover, it was revealed These crystals can have various forms (dendrites,
thatsomecraterscontainedtheelements,typicalof cubicandpyramidal crystals,Fig.2(b).Theinten-
a structural material of the cathode system (for sive growth of crystals depends on the di(cid:128)erent
example, Sn). It points to the fact of the cathode conditionsofheattransferatthecratersboundaries
erosion under HPPIB irradiation e(cid:128)ect, and when andthehighresidualstressesnearcraters.
V.A.Shulov,N.A.Nochovnaya/Nucl.Instr.andMeth.inPhys.Res.B148(1999)154–158 157
Some results of the second stage of experi-
ments are presented in Table 2 and Fig. 3. It is
evident, that the preliminary diamond smoothing
of sample surfaces allows to obtain the ‘‘crater-
free’’ surface when irradiating by using any ion
current density (Fig. 3(a)). The surface of the sim-
ilar prescatched samples after the irradiation con-
tainscratershowever,theyareconcentratedalong
the scratch (Fig. 3(b)). One more important result
of these experiments is the formation of craters,
separate areas of which were not melted if the
targetsweresubjected tothe preliminarydiamond
smoothing and final chemical etching(Fig. 3(c)).
According to our experimental and published
data [5–7] the variety of the following crater for-
mationmechanismscanbepossible:theformation
of a gas-plasma phase during the ablation of sur-
face volatiles; the bombardment of the melted
surface by the accelerator structure elements; and
the availability of nonstable conditions during
meltingduetothedi(cid:128)erenceinphaseandchemical
compositions, impurity distributions and disloca-
tion densities of targets.
The formulated crater formation mechanisms
give a possibilitytoproposeanumberof methods
to reduce this surface phenomenon including:
changes in ion accelerator electrode material (in
particular, the target material instead of steel);
Fig. 2. SEM-micrographs of VT18U alloy surface treated by target surface preparation such as fine polishing;
HPPIB(a–j(cid:136)180–200Acm(cid:255)2,n(cid:136)3pulses);b–j(cid:136)120–140A
follow-on HPPIB treatment at low ion current
cm(cid:255)2, n(cid:136)3 pulses) with the following annealing (b –
density (j<60 A cm(cid:255)2) for titanium alloys and
T(cid:136)550(cid:176)C).
Table 2
E(cid:128)ectoftheinitialsurfacetreatmentonthecraterformationprocessduringtheirradiationbyHPPIB(E(cid:136)300keV;j(cid:136)120–140A
cm(cid:255)2;s(cid:136)50ns;n(cid:136)1pulse)
Treatment Craterdensity,q,mm(cid:255)2,(cid:139)40mm(cid:255)2
VT8M VT9 VT18U EP866sh EP718ID
Grinding 290 320 270 120 150
Polishing 60 90 70 70 90
Diamondsmoothing ) ) ) ) )
Plasticdeformation 480 460 420 210 24
Chemicaletching 450 500 430 290 310
Oxidation 420 380 410 280 330
IonimplantationbySm 330 340 360 140 160
158 V.A.Shulov,N.A.Nochovnaya/Nucl.Instr.andMeth.inPhys.Res.B148(1999)154–158
4. Conclusion
The major conclusions from this study can be
summarized as follows:
1. Craterformationprocesstakesplaceonthesur-
faceofsolidsduringirradiationbyHPPIB.This
phenomenonisduetothecathodematerialero-
sion (technological factor) and the nonstability
in the physical and chemical state of the irradi-
ated surface.
2. In order to reduce the negative e(cid:128)ect of crater
formation upon the properties of targets can
be proposed: changing the accelerator diode
system material; to use the fine polishing when
preparingthesurfacefortheirradiation;tocar-
ryoutthefinalHPPIBtreatmentatlowioncur-
rent density (crater-free irradiation); and to
perform the final heavy ion implantation.
Acknowledgements
This work was supported by the Russian State
Committee of Higher Education.
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