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Energy and orientation dependence of atom displacement in BCC metals studied by high-voltage electron microscopy
Volume 73A, number 2
PHYSICS LETTERS
3 September 1979
ENERGY AND ORIENTATION DEPENDENCE OF ATOM DISPLACEMENT IN BCC METALS STUDIED BY HIGH-VOLTAGE ELECTRON MICROSCOPY F. PHILLIPP, B. SAlLE, H. SCHMID and K. URBAN Max-Planck-Institut für Metallforschung, Institut für Physik, D 7000 Stuttgart 80, Germany Received 6 June 1979 The dependence of the atom displacement rate in Nb, Mo, and Ta on electron energy and irradiation direction has been determined at elevated temper~turesby high-voltage electron microscopy. The minimum displacement threshold energy of 24 eV (Nb), 27 eV (Mo), and 14 eV—22 eV (Ta) occurs along the (100) directions.
Introduction. Studies aiming at the determination of the minimum electron energy necessary in order to obtain a measurable atom displacement rate have so far mainly been performed by means of resistivity measurements at liquid-helium temperatures. The probabiity p(w) that an atom which has received by electron impact a kinetic energy Et and which moves in a direction w is displaced permanently can, under these conditions, be approximated by a step function: p = 1 for Et ~ Ed(~)and p = 0 for Et
that by the observation of the growth of interstitial dislocation loops which were introduced into the specimen during a pre-irradiation displacement rates down to about 10-2 b may be detected [21 Here measurements on the dependence of the displacement rate on electron energy and the direction of incidence are reported which were obtained applying this method to niobium, molybdenum, and tantalum.
ref. [1]). The resistivity method exhibits two disadvantages which make the application of other methods desirable. Firstly, relatively thick specimens (thickness z~>20 pm) have to be used. In these inelastic multiple scattering leads to such a severe broadening of the angular distribution function of an originally parallel electron beam that studies of fine details of the threshold energy surface are inhibited. Secondly, the resistivity method can only be applied at low temperatures. The question therefore arises to what extent the results obtained are relevant for atom displacement during irradiation at elevated temperatures which is particularly important from a technological of view. in thin specimens Measurements maypoint be performed (t< 0.5 pm) with high sensitivity, high angular resolution and at elevated temperatures by means of a highvoltage electron microscope (HVEM). It can be shown
electrolyte for Nb and Ta consisted of water-free Mg(Cl04)2 and methanol which excludes hydrogen absorption by the specimen during polishing [3]. Subsequently the Nb and Ta specimens were rinsed in concentrated H2 SO4 in order to remove a chemically active surface layer and to seal the specimens by a thin oxide layer against gas absorption. The specimens were preirradiated with 1 MeV (Nb), 1.1 MeV (Mo), and 1.2 MeV (Ta) electrons at 300 K (Nb, Mo) and 600 K (Ta). The subsequent loop growth measurements were carried out at 300 K (Nb), 670 K (Mo), and 600 K (Ta) under conditions where the growth rate (increase in diameter per unit time), u, was time independent. For Ta vfor ~ 1!2 and (P isvthe displacement rate), for Nb v ~ P Mo ~ P0~
•
.
Experimental. Specimens were prepared by electropolishing from high-purity foils with a residual resistivity ratio of 6900 (Nb), 40 000 (Mo), and 6 500 (Ta). A H2S04/methanol electrolyte was used for Mo. The
-
Niobium. The irradiation direction was selected so as to make an angle of about 5°with a (111), (100), and 123
Volume 73A, number 2
PHYSICS LETTERS
maximum transferred energy [eVI 20 30 40
3 September 1979
50
mar. transferred energy [eV) 30 35
25
I
~ 30 E
Nb j~91022e/m2S 300K
/
//
~20 L
10
/
//
Mo
~0
<700>
~/
-~
23elm2s
j~1.810
670K
16
‘‘
<110>
E
/A
/0
<100>
20 -
~0
0.
12-
-
8-
~
//
0 ~ 600
40
I
800 1000 electron energy [key]
~
4
Fig. 1. Loop growth rate in Nb as a function of electron energy. Irradiation direction close to (111>, (100>, and (110).
-
~
700
800
900
electron energy [keVJ
(110) crystal axis. The results are given in fig. I. The largest loop-growth rate at a given energy is obtained for irradiation close to a (100) direction. Measurable growth rates are obtained at transferred energies greater than Ed = (24 ±0.5) eV. This is in contrast to the lowest displacement threshold energy value E~’~ = 33.5 eV determined by resistivity measurements at liquid helium temperatures [41.
Fig. 3. Loop growth rate in Mo as a function of energy for irradiation along (100).
a function of the direction of the incident electrons for three different energies. In all cases a maximum occurs if the irradiation direction is parallel to a (100) crystal axis. Fig. 3 shows results of measurements for this irradiation direction as a function of energy. For the onset of measurable ioop growth Ed = (27 ±0.5) eV is obtained. Using a less sensitive method in the HVEM
Molybdenum. Fig. 2 shows the loop-growth rate as Mo . 900keV
-X
~
10
o
öOOkeV
20
£ 700keV
b
8
-
16
~
4 ~-,
6-
E •12a
~-
E
1~
T
I
T
‘T
r I
? I
o
0
I
40°
20°
<,~>
8~ 1
£
I
200
0
40<,,o>
loop growth rate vs. irradiation direction Fig. 2. Loop growth rate in Mo as a function of irradiation direction for 700 keV, 800 keV, and 900 key irradiation.
124
Volume 73A, number 2
PHYSICS LETTERS
3 September 1979
Ta • 1200 key 0 1100 keV 0 1000 keV
09 -~
c
23e/m2s
Q7
j~55-l0 600K
•
III!
50°
40°
30°
20°
10°
~111)
0° (100)
70°
I
200
30°
40° 110
Fig. 4. Loop growth rate in Ta as a function of irradiation direction for 1000 keV, 1100 keV, and 1200 keV irradiation.
Ed 30 eV was obtained for the (100> directions at 300 K [5]. Resistivity measurements at low temperature yielded E~rn= 35 eV [61. Tantalum. Fig. 4 shows the loop-growth rate as a function of the irradiation direction for three different energies. In all cases a maximum is obtained if the irradiation occurs parallel to a (100) crystal axis. Fig. 5 shows results of measurements where the irradiation direction was about 8°off a (100> direction. A relatively • maximum transferred energy [eV] 20 14 -
06
22
60
~
05
~ o~
30
T
T1
p551023 e/m2 600 K
/ I I 1
20
0 ~ 700
02 -~
18
25
800
900
/ / 1/ ~,/
1000
/
700
800
900
1000
the resistivity results on Mo and Ta [6,7]. The threshold energies for observable however, considerably loweratom than displacement the minimumare, displacement threshold energy values obtained by the resistivi,
A 7100
Discussion. In all three metals investigated a maximum loop-growth rate (displacement rate) is observed for irradiations along (100> directions. From this we can conclude that in these directions a minimum in the displacement threshold energy surface exists. This is in agreement with theoretical predictions [8] and with
ty method. A similar disagreement was found for copper [2] cadmium and zink [9]. For an explanation the following points have to be discussed: (i) The effectsuch of light impurity atoms. cesses involving impurities might leadIndirect to atompro-
ot 0
slow increase of the growth rate with energy can be recognized in the range (14 ±0.5) eV ~Ed ~ (22 ±1) eV and a fast increase at higher energies. Resistivity measurements at low temperature yielded E~m= 33 eV [7].
1200
electron energy (key]
Fig. 5. Loop growth rate in Ta as a function of energy for irradiation direction close to (100).
displacement at energies which are below threshold in purematerials. However, so far no impurity mechanism has been proposed which would allow to explain the HVEM results quantitatively or even qualitatively. For recent discussions on this subject see refs. [2,7,9—12]. (ii) The difference in angular resloution. The HVEM125
Volume 73A, number 2
PHYSICS LETTERS
method may allow to observe effects of very fine details in the threshold energy surface where the threshold energy drops to very low values. In resistivity measurements the respective displacements might, due to the wide spread of the angular distribution function of the electrons, contribute too little to obtain a measurable effect. A decision on this point requires absolute values of the displacement cross section which are very diffIcult to obtain at elevated temperatures. (iii) A temperature dependence of the displacement process. The influence of lattice vibrations may lead to the stabiity.of Frenkel pairs which are unstable during low-temperature irradiation. The result would be a displacement probability increasing with temperature. More work is required in order to allow a decision as to what extent each of the mentioned points contributes to the results obtained by the HVEM method.
126
3 September 1979
References [1] P. Vajda, Rev. Mod. Phys. 49 (1977) 481.
[21 N. Yoshida and K. Urban, Phys. Lett. 63A (1977) 381. [31 Schober andY. Sorajic, Metallography 6(1973)183. [41 T. K. Faber, Thesis, Univ. Stuttgart (1974).
[5] K. Izul, S. Furuno, H. Otsu, 1. Nishida and H. Maeta, Proc. 9th Intern. Congr. EM (Toronto, 1978) Vol. 1, p. 354. A. Lucasson and P. Lucas-
[6] F. Maury, P. Vajda, M. Biget,
son, Radiat. Eff. 25 (1975) 175.
[71 M. Maury, Vajda,820. A. Lucasson son,Biget, Phys.F.Rev. B19 P. (1979)
and P. Lucas[8] C. Erginsoy, G.H. Vineyard and A. Englert, Phys. Rev. 133 (1964) A595. Karim, M.E. Whitehead, M.H. Loretto and R.E. Smaliman, Acta Metall. 26 (1978) 975. [10] F. P. Vajda, Eff.Maury, 10(1971) 239. A. Lucasson and P. Lucasson, Radiat.
[91 A.S.A.
[11] H. Schmid and K. Urban, Proc. 5th Intern. Conf. HVEM (Kyoto, 1977) p. 527. [12] P. Vajda, F. Maury, A. Lucasson and P. Lucasson, Phys. Lett. 68A (1978) 95.
PHYSICS LETTERS
3 September 1979
ENERGY AND ORIENTATION DEPENDENCE OF ATOM DISPLACEMENT IN BCC METALS STUDIED BY HIGH-VOLTAGE ELECTRON MICROSCOPY F. PHILLIPP, B. SAlLE, H. SCHMID and K. URBAN Max-Planck-Institut für Metallforschung, Institut für Physik, D 7000 Stuttgart 80, Germany Received 6 June 1979 The dependence of the atom displacement rate in Nb, Mo, and Ta on electron energy and irradiation direction has been determined at elevated temper~turesby high-voltage electron microscopy. The minimum displacement threshold energy of 24 eV (Nb), 27 eV (Mo), and 14 eV—22 eV (Ta) occurs along the (100) directions.
Introduction. Studies aiming at the determination of the minimum electron energy necessary in order to obtain a measurable atom displacement rate have so far mainly been performed by means of resistivity measurements at liquid-helium temperatures. The probabiity p(w) that an atom which has received by electron impact a kinetic energy Et and which moves in a direction w is displaced permanently can, under these conditions, be approximated by a step function: p = 1 for Et ~ Ed(~)and p = 0 for Et
that by the observation of the growth of interstitial dislocation loops which were introduced into the specimen during a pre-irradiation displacement rates down to about 10-2 b may be detected [21 Here measurements on the dependence of the displacement rate on electron energy and the direction of incidence are reported which were obtained applying this method to niobium, molybdenum, and tantalum.
ref. [1]). The resistivity method exhibits two disadvantages which make the application of other methods desirable. Firstly, relatively thick specimens (thickness z~>20 pm) have to be used. In these inelastic multiple scattering leads to such a severe broadening of the angular distribution function of an originally parallel electron beam that studies of fine details of the threshold energy surface are inhibited. Secondly, the resistivity method can only be applied at low temperatures. The question therefore arises to what extent the results obtained are relevant for atom displacement during irradiation at elevated temperatures which is particularly important from a technological of view. in thin specimens Measurements maypoint be performed (t< 0.5 pm) with high sensitivity, high angular resolution and at elevated temperatures by means of a highvoltage electron microscope (HVEM). It can be shown
electrolyte for Nb and Ta consisted of water-free Mg(Cl04)2 and methanol which excludes hydrogen absorption by the specimen during polishing [3]. Subsequently the Nb and Ta specimens were rinsed in concentrated H2 SO4 in order to remove a chemically active surface layer and to seal the specimens by a thin oxide layer against gas absorption. The specimens were preirradiated with 1 MeV (Nb), 1.1 MeV (Mo), and 1.2 MeV (Ta) electrons at 300 K (Nb, Mo) and 600 K (Ta). The subsequent loop growth measurements were carried out at 300 K (Nb), 670 K (Mo), and 600 K (Ta) under conditions where the growth rate (increase in diameter per unit time), u, was time independent. For Ta vfor ~ 1!2 and (P isvthe displacement rate), for Nb v ~ P Mo ~ P0~
•
.
Experimental. Specimens were prepared by electropolishing from high-purity foils with a residual resistivity ratio of 6900 (Nb), 40 000 (Mo), and 6 500 (Ta). A H2S04/methanol electrolyte was used for Mo. The
-
Niobium. The irradiation direction was selected so as to make an angle of about 5°with a (111), (100), and 123
Volume 73A, number 2
PHYSICS LETTERS
maximum transferred energy [eVI 20 30 40
3 September 1979
50
mar. transferred energy [eV) 30 35
25
I
~ 30 E
Nb j~91022e/m2S 300K
/
//
~20 L
10
/
//
Mo
~0
<700>
~/
-~
23elm2s
j~1.810
670K
16
‘‘
<110>
E
/A
/0
<100>
20 -
~0
0.
12-
-
8-
~
//
0 ~ 600
40
I
800 1000 electron energy [key]
~
4
Fig. 1. Loop growth rate in Nb as a function of electron energy. Irradiation direction close to (111>, (100>, and (110).
-
~
700
800
900
electron energy [keVJ
(110) crystal axis. The results are given in fig. I. The largest loop-growth rate at a given energy is obtained for irradiation close to a (100) direction. Measurable growth rates are obtained at transferred energies greater than Ed = (24 ±0.5) eV. This is in contrast to the lowest displacement threshold energy value E~’~ = 33.5 eV determined by resistivity measurements at liquid helium temperatures [41.
Fig. 3. Loop growth rate in Mo as a function of energy for irradiation along (100).
a function of the direction of the incident electrons for three different energies. In all cases a maximum occurs if the irradiation direction is parallel to a (100) crystal axis. Fig. 3 shows results of measurements for this irradiation direction as a function of energy. For the onset of measurable ioop growth Ed = (27 ±0.5) eV is obtained. Using a less sensitive method in the HVEM
Molybdenum. Fig. 2 shows the loop-growth rate as Mo . 900keV
-X
~
10
o
öOOkeV
20
£ 700keV
b
8
-
16
~
4 ~-,
6-
E •12a
~-
E
1~
T
I
T
‘T
r I
? I
o
0
I
40°
20°
<,~>
8~ 1
£
I
200
0
40<,,o>
loop growth rate vs. irradiation direction Fig. 2. Loop growth rate in Mo as a function of irradiation direction for 700 keV, 800 keV, and 900 key irradiation.
124
Volume 73A, number 2
PHYSICS LETTERS
3 September 1979
Ta • 1200 key 0 1100 keV 0 1000 keV
09 -~
c
23e/m2s
Q7
j~55-l0 600K
•
III!
50°
40°
30°
20°
10°
~111)
0° (100)
70°
I
200
30°
40° 110
Fig. 4. Loop growth rate in Ta as a function of irradiation direction for 1000 keV, 1100 keV, and 1200 keV irradiation.
Ed 30 eV was obtained for the (100> directions at 300 K [5]. Resistivity measurements at low temperature yielded E~rn= 35 eV [61. Tantalum. Fig. 4 shows the loop-growth rate as a function of the irradiation direction for three different energies. In all cases a maximum is obtained if the irradiation occurs parallel to a (100) crystal axis. Fig. 5 shows results of measurements where the irradiation direction was about 8°off a (100> direction. A relatively • maximum transferred energy [eV] 20 14 -
06
22
60
~
05
~ o~
30
T
T1
p551023 e/m2 600 K
/ I I 1
20
0 ~ 700
02 -~
18
25
800
900
/ / 1/ ~,/
1000
/
700
800
900
1000
the resistivity results on Mo and Ta [6,7]. The threshold energies for observable however, considerably loweratom than displacement the minimumare, displacement threshold energy values obtained by the resistivi,
A 7100
Discussion. In all three metals investigated a maximum loop-growth rate (displacement rate) is observed for irradiations along (100> directions. From this we can conclude that in these directions a minimum in the displacement threshold energy surface exists. This is in agreement with theoretical predictions [8] and with
ty method. A similar disagreement was found for copper [2] cadmium and zink [9]. For an explanation the following points have to be discussed: (i) The effectsuch of light impurity atoms. cesses involving impurities might leadIndirect to atompro-
ot 0
slow increase of the growth rate with energy can be recognized in the range (14 ±0.5) eV ~Ed ~ (22 ±1) eV and a fast increase at higher energies. Resistivity measurements at low temperature yielded E~m= 33 eV [7].
1200
electron energy (key]
Fig. 5. Loop growth rate in Ta as a function of energy for irradiation direction close to (100).
displacement at energies which are below threshold in purematerials. However, so far no impurity mechanism has been proposed which would allow to explain the HVEM results quantitatively or even qualitatively. For recent discussions on this subject see refs. [2,7,9—12]. (ii) The difference in angular resloution. The HVEM125
Volume 73A, number 2
PHYSICS LETTERS
method may allow to observe effects of very fine details in the threshold energy surface where the threshold energy drops to very low values. In resistivity measurements the respective displacements might, due to the wide spread of the angular distribution function of the electrons, contribute too little to obtain a measurable effect. A decision on this point requires absolute values of the displacement cross section which are very diffIcult to obtain at elevated temperatures. (iii) A temperature dependence of the displacement process. The influence of lattice vibrations may lead to the stabiity.of Frenkel pairs which are unstable during low-temperature irradiation. The result would be a displacement probability increasing with temperature. More work is required in order to allow a decision as to what extent each of the mentioned points contributes to the results obtained by the HVEM method.
126
3 September 1979
References [1] P. Vajda, Rev. Mod. Phys. 49 (1977) 481.
[21 N. Yoshida and K. Urban, Phys. Lett. 63A (1977) 381. [31 Schober andY. Sorajic, Metallography 6(1973)183. [41 T. K. Faber, Thesis, Univ. Stuttgart (1974).
[5] K. Izul, S. Furuno, H. Otsu, 1. Nishida and H. Maeta, Proc. 9th Intern. Congr. EM (Toronto, 1978) Vol. 1, p. 354. A. Lucasson and P. Lucas-
[6] F. Maury, P. Vajda, M. Biget,
son, Radiat. Eff. 25 (1975) 175.
[71 M. Maury, Vajda,820. A. Lucasson son,Biget, Phys.F.Rev. B19 P. (1979)
and P. Lucas[8] C. Erginsoy, G.H. Vineyard and A. Englert, Phys. Rev. 133 (1964) A595. Karim, M.E. Whitehead, M.H. Loretto and R.E. Smaliman, Acta Metall. 26 (1978) 975. [10] F. P. Vajda, Eff.Maury, 10(1971) 239. A. Lucasson and P. Lucasson, Radiat.
[91 A.S.A.
[11] H. Schmid and K. Urban, Proc. 5th Intern. Conf. HVEM (Kyoto, 1977) p. 527. [12] P. Vajda, F. Maury, A. Lucasson and P. Lucasson, Phys. Lett. 68A (1978) 95.