Helium bubbles in an amorphous FeNiMoB alloy

Scripta METALLURGICA

Vol. 16, pp. 11-14, 1982 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

HELIUM BUBBLES IN AN AMORPHOUSFe-Ni-Mo-B ALLOY

H. Van Swijgenhoven (a), G. Knuyt (a), L.M. Stals (a), S.E. Donnelly (b) (a) Materials Physics Group, Limburgs U n i v e r s i t a i r Centrum, B-3610 Diepenbeek (Belgium) (b) I . R . I . S . Facult~s U n i v e r s i t a i r e s , B-5000 Namur (Belgium)

(Received June 29, 1981) (Revised October 30, 1981) INTRODUCTION As a continuation of r a d i a t i o n damage studies on amorphous a l l o y s we report in t h i s paper on the observation of helium gas bubbles in Fe40 Ni38 Mo4 BI8 (Metglas 2826 MB) f o l l o w i n g 5 keV He+-ion i r r a d i a t i o n at room temperature. I t has previously been observe# [I~21] that argon gas bubbles form in Metglas 2826 MB during 5 keV Ar+-ion i r r a d i a t i o n . The aim of the present research was to i n v e s t i g a t e whether gas bubbles also form during He+-ion i r r a d i a t i o n . OBSERVATIONS AND DISCUSSION The amorphous a l l o y was obtained from A l l i e d Chemical Corporation (Morristown, N.Y., U.S.A.) in the form of sheets 40 ~m t h i c k . Specimens were i r r a d i a t e d with He+-ions at room temperature with a Physical Electronics gun in the dose ranqe~ 1016 to 1018 ions/cm 2 _ and at fluxes of 1014 He+-ions/cm2s and 5.1014 He+-ions/cm2s. The observations were made in a P h i l i p s EM-4OO-HTG transmission electron microscope. The specimens were thinned e l e c t r o l y t i c a l l y p r i o r to i r r a d i a ~ o n i n a s o l u t i o n of acetic and p e r c h l o r i c acid in the volume r a t i o 10:! at -15°C. Fig. i shows the CTEM images of amorphous specimens a f t e r i r r a d i a t i o n with 5 keV He+-ions at room temperature to doses between 2.1016 and 1018 He+-ions/cm 2. An image of as received material is also included. I t can be ~een that at 2.1016 ions/cm 2 no v i s i b l e gas bubbles are present, whereas at 4.1016 ions/cm ~ a large number of very small gas bubbles with a mean diameter of 1.4 nm can be observed. The mean diameter of the gas bubbles increases with increasing dose up to 33 nm at21018 ions/cm 2. At the intermediate doses the mean diameters are : 2.1 nm at 5.1016 ions/cm , 2.4 nm at 1017 ions/cm 2, 4.3 nm at 2.1017 ions/cm 2. I f one assumes that the bubbles are in e q u i l i b r i u m with the surface tension of the material then from the bubble diameter and bubble density one finds u~ing the Van der Waals law that the helium retention is nearly 50 percent at 4.1016 ions/cm 2 and 5 percent at 1018 ions/cm 2. Fig. 2 shows the r e s u l t of p o s t - i r r a d i a t i o n annealing at a dose of 5.1016 ions/cm 2. The specimen was heated at 200°C for lh in the heating stage of the electron microscope. I f one compares the bubble d i s t r i b u t i o n a f t e r p o s t - i r r a d i a t i o n annealing at 200°C ( f i g . 2) with the bubble d i s t r i b u t i o n at the same dose without annealing ( f i g . I ) , then one may conclude that heating at 200°C has no e f f e c t on the bubble d i s t r i b u t i o n . This indicates that i f s t r u c t u r a l r e l a x a t i o n does occur, which would most probably lead to an increased bubble diameter: no clear e f f e c t on the bubble d i s t r i b u t i o n can be observed. I t can also be an i n d i c a t i o n that there is no s t r u c t u r a l r e l a x a t i o n in helium doped specimens. Annealing in the heating holder of the microscope at the nominal value of 250°C results in sharpening and s p l i t t i n g of the d i f f u s e d i f f r a c t i o n h a l o ' s . This e f f e c t is associated with the formation of small domains with enhanced contrast in the transmission image. Although 250°C is a rather low temperature f o r c r y s t a l l i z a t i o n of t h i s p a r t i c u l a r a l l o y , the domains are believed to be metastable c r y s t a l l i n e phases. I t i s , therefore, impossible with t h i s material to study the behaviour of gas bubbles in the amorphous phase over a broader temperature range. On the basis of the present r e s u l t s we would l i k e to present the f o l l o w i n g t e n t a t i v e model f o r i n e r t gas bubble formation in amorphous a l l o y s . Upon penetration of a 5 keV He+-ion

ii 0036-9748/82/010011-04503.00/0 Copyright (c) 1982 Pergamon Press Ltd.

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He IN AN A M O R P H O U S ALLOY

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Fig. 1 : a. CTEM image with SAD of as-received amorphous Metglas 2826 MB b. CTEM imaoe with SAD of amorphous Metglas 2826 MB after implantation to a dose of 2.1016 He+-ions/cm2 (flux : 1014 He+-ions/cm2s). CTEM image with SAD of amorphous Metglas 2826 r~ and bubble distribution after implantation to the following doses : c-d : 4.1016 He+-ions/cm2 (flux : 1014 He+-ions/cm2s) e - f : 5.1016 He+-ions/cm2 (flux 1014 He+-ions/cmLs)

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He IN AN AlvlORPHOUS ALLOY

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200100 ~ m m

i!i~'ii~

dimmrm2.4nm

0818143240

rue

h

300-

Immr:43 nm

200 100

j

2s3ss2es7.e.

8 1624324048566472808896nm I

Fig. 1 (continued) CTEM image with implantation to g-h : 1.101~ i - j : 2.1011 k-I : 1018

SAD of amorphous Metglas 2826 MB and bubble d i s t r i b u t i o n following dRses : He+-ions/cm L ( f l u x : 1014 He+-ions/cm2s) He+-ions/cm 2 ( f l u x 1014 He+-ions/cmZs} He+-ions/cm 2 ( f l u x 5.1014 He+-ions/cm2s)

after

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He IN AN AMORPHOUS

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into the material a collision cascade containing on average about 50 displaced atoms in a cascade volume of about 106 ~3 [3] is made. In the collision cascade localized positive and negative density variation similar to i n t e r s t i t i a l s and vancancies in crystalline material are produced. At the places of negative density variation helium gas atoms may be captured thus leading to the formation of a bubble nucleus. Stabilization of a negative density variation, which in fact can also be described as positive excess volume, by helium atoms in order to form a bubble nucleus is a necessary condition for the model, because i t has been shown that vacancy type defects are unstable in amorphous alloys [4]. The nucleus may then grow by further attraction of excess volume and helium atoms. CONCLUSION The observation of inert gas bubbles in amorphous Metglas 2826 MB following 5 keV Ar+-ion irradiation which has been reported previously [1,2], is confirmed by similar experiments with 5keV He+-ions. The diameter of the He-bubbles increases with increasing dose. A possible mechanism for inert gas bubble formation in amorphous alloys consists of the s t a b i l i zation by inert gas atoms of positive excess volume ("vacancy") created in the collision cascade and of further growth of these nuclei by the capture of additional positive excess volume and gas atoms.

7 ~ I m=2.1m I

Ut61432 m

Fig. 2 : CTEM image with SAD of amorphous Metglas 2826 MB and bubble diameter distribution after implantation to 5.1016 He+-ions/cm2 (flux : 1014 He+-ions/cmZs) and postirradiation annealing at 200°C for lh. REFERENCES [1] [2] [3] [4]

H. Van Swijgenhoven, J. Moens, J. Vanoppen and L.M. Stals, Scripta M e t . , ~ 629 (1981). H Van Swijgenhoven, J. Vanoppen and L.M. Stals, phys. stat. sol.(a) 67 ~v (1981). Y. Yamamuraand Y. Kitazoe, Rad. Eff. 39, 251 (1978). C.H. Bennett, P. Chaudhari, V. Moruzzi and P. Steinhardt, Phil. Mag A 40, 484 (1979).

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