Positron annihilation study of the effect of hydrogen during the plastic deformation of a steel

Materials Science and Engineering, 68 (1984-1985) 191-196

191

Positron Annihilation Study of the Effect of Hydrogen D e f o r m a t i o n of a Steel

during the Plastic

RANGARAJ GOPALIENGAR, J. P. WALLACE* and R. A. ORIANI Department of Chemical Engineering and Materials Science, and Corrosion Research Center, University of Minnesota, Minneapolis, MN 55455 (U.S.A.) (Received January 16, 1984; in revised form July 24, 1984)

ABSTRACT Positron lifetime measurements were conducted on spheroidized low carbon steel (AISI 1009) deformed without hydrogen, hydrogen charged after deformation and hydrogen charged during deformation. These were measured as a function of tensile strain at a constant value o f hydrogen input fugacity (250 M N m-2). Decoration of dislocation sites with hydrogen reduced the relative intensity o f the short-lifetime component. Deformation with hydrogen in the steel increased the relative intensity o f the long-lifetime component, signifying an increase in concentration of vacancy agglomerates or microvoids.

1. INTRODUCTION Positron annihilation has proved to be a very useful and effective technique for studying the nature and binding energies of thermally produced vacancies [1, 2], the sizes and concentration of voids generated b y irradiation and b y recovery processes [3, 4], and the densities and distribution of dislocations in cold-worked, quenched and annealed states [5, 6]. The sensitivity of this technique for individual vacancies and their clusters, up to sizes of about 5 nm, is superior to that of more traditional techniques such as electrical resistometry and transmission electron microscopy. The trapping of positrons at defects where the kinetic energy and density of conduction electrons are low compared with defect-free *Present address: 299 Park Avenue, Broadway, VA 22815, U.S.A. 0025-5416/85/$3.30

material produces longer lifetimes of the positrons prior to their annihilation. The time interval is measured between the creation and the annihilation of a positron. These events are indicated b y a 1.28 MeV ? ray and a 0.5 MeV 7 ray respectively. The time interval between them is characteristic of the defect at which the positron is trapped. Excellent review articles on this technique have been recently published b y West [7], Siegel [8], Hautojarvi [9] and Byrne [10]. The potential value of positron lifetime spectrum analyses in defect-trapping studies is clear. However, three major problems exist. (a) Any spectrum deriving from random nuclear events always involves a finite statistical precision which makes complications in addition to those from the purely mathematical problem of the analysis of sums of exponentials. (b) The measured spectra amount to a convolution of an "ideal" spectrum with an instrumental resolution function which can never be exactly defined. (c) There is almost always present in the measured spectra a low intensity c o m p o n e n t arising from positron annihilation in the 22Na positron source that is unavoidable with the normally adopted sample-source-sample sandwich arrangements. The motivation for the present work was based on two facts. (1) Wallace [11] ascertained that the signal from positron trapping at dislocation cores can be much reduced b y the prior adsorption of hydrogen at the dislocations. (2) Previous work [12, 13] showed that microvoids associated with spheroidized carbide particles are present in greater numbers when straining is done in the presence of hydrogen than without it. © Elsevier Sequoia/Printed in The Netherlands

192

It therefore seemed possible that positron annihilation measurements on specimens deformed without hydrogen and subsequently charged, compared with specimens deformed during hydrogen charging, would yield information on whether hydrogen enhances the nucleation or the growth of microvoids. As will be seen subsequently, such information was not accessible by this technique. However, some insight into the effect of hydrogen on deformation-produced vacancy agglomeration has been obtained.

2. P R O C E D U R E

A standard positron-lifetime-measuring apparatus [8] with a full width at half-maxim u m of 300 ps was used. Tensile test specimens were made of spheroidized AISI 1009 plain carbon steel, the dimensions of which were a thickness of 0.25 mm and a gauge length of 100 mm. The specimens were annealed in a vacuum of 10 -4 Pa at 850 °C for 15 h. Cathodic charging of the specimens was carried out in a solution of 0.1 N NaOH with 10 mg of As203 per liter as recombination poison. Prior to use the solution was subjected to pre-electrolysis for a period of 24 h, with continuous bubbling of nitrogen during this time for deaeration. The positron source was prepared by depositing droplets o f aqueous solution of 22NaC1 onto a Mylar film by means of a microliter syringe. The water was evaporated off by an IR lamp to achieve 15-20 pCi of radioactivity. After this, another Mylar film was press sealed to the Mylar film with the deposit. To determine the contributions of the source to the lifetime spectrum, measurements were made on the annealed sample by sandwiching the source between two similar specimens. The contribution of the source to the intensity of positron annihilation was only about 2%-4%. Three major steps were involved in the subsequent experimentation. (i) Firstly, two similarly annealed hydrogen-free samples were deformed together at a strain rate of 0.5 m m min -1 to the required value of strain. Positron lifetime measurements were then carried out on these specimens.

(ii) Secondly, the same two specimens after deformation were cathodically charged with hydrogen at a current density of 5 A m -2 which corresponds to an input fugacity of hydrogen of approximately 250 MN m -2 [14]. After the specimens had been charged for 2 h, t h e y were cleaned and measured for positron lifetimes and intensities. (iii) Thirdly, two other annealed samples were m o u n t e d in the Instron machine with a hydrogen-charging cell fitted to the gauge section of the specimen. Precharging was done for 2 h, after which the specimens were deformed at a rate of 0.5 m m min -1 during which charging was continued. At the desired value of strain the specimens were removed and cleaned, and measurements of positron lifetimes were carried out. The above steps were repeated for different values of strain. The rationale for carrying out these three steps in the experimentation was that step (i) would give values of positron lifetimes and intensities due to deformation without hydrogen. Step (iii) would give values of positron lifetimes and intensities as affected by both deformation and the presence of hydrogen during deformation. However, by charging the specimens used in step (i) with hydrogen after deformation and comparing their lifetime and intensity values with the lifetime and intensity values for step (iii), the effect of the presence of hydrogen during deformation on the size and concentration of the generated defects other than dislocations could be ascertained. The data were analyzed using the program Positronfit [15] modified for the CDC 6000/ 7000 series computer. In the present study, both three- and two-exponential fits were applied. Three-exponential fits were used for step (ii) and step (iii), in which hydrogen was charged after deformation and during deformation respectively. In these cases the three lifetimes extracted were ~1, r2 and the source lifetime, together with their corresponding relative intensities (see the nomenclature given in Appendix A). In cases (only for step (i)) where three-exponential fits gave large values of variance, a two-exponential fit was used, and the lifetimes extracted were ~1 and the source lifetime. The source contribution was only 3.5% with a lifetime of 1009 + 10 ps, as obtained by measurement on an annealed sample.

193

The mathematical representation of the measured spectrum is based on two important assumptions [ 15 ]. (1) The measured spectrum consists of a constant background and a number of decaying exponential functions somewhat smeared on account of finite time resolution. The smeared functions are determined by convoluting the exponentials with the resolution function of the measuring system. The spectrum obeys the Poisson distribution. (2) The resolution function can be measured as the time spectrum for two y quanta emitted simultaneously from a 6°Co source. It is, to a good approximation, a gaussian curve. The spectrum is approximated by a finite number of discrete components and is represented by

I(r) = ~ Ii exp i

(--'t \

Ti/

Ii provides a measure of the relative positron population in the various states, and the rate 1/ri reflects the effective electron density in the corresponding positron state. The extension of the analysis to situations involving simultaneous positron trapping by a variety of defects is straightforward in principle but usually difficult in practice. Hence, the inferences made from the lifetime spectra have to be made in conjunction with sample treatments based on well-established metallurgical practices and procedures.

tensities of positrons in well-annealed iron (of unspecified carbon content) and in our steel (annealed at 850 °C and 10 -4 Pa for 15 h) is presented. To interpret these lifetimes, use is made of the following determinations of positron lifetimes in b.c.c, iron. For positrons in the lattice (free positrons), r values of 108 ps [16], 117 ps [17], 110 ps [4] and 114 + 2 ps [18] have been found. For positrons at monovacancies, r = 175 ps [4]. For positrons at dislocation core sites, r = 167 ps [16, 17] ; r = 142 + 5 ps for screw dislocation sites [18] and r = 165 -+ 2 ps for edge dislocation sites. For positrons at carbon-vacancy pairs, r = 160 ps [19]. Because the dislocation density in both annealed metals is certainly such that the fraction of iron atoms at dislocation cores is not more than 10 -9 and, because Kao et al. [20], have deduced that phase interfaces in steels do not trap positrons, we conclude that a r l value (Table 1) of approximately 145 ps represents an average of the contributions from positrons at carbon-vacancy pairs and in undisturbed lattice states. The small diminution of rl produced by hydrogen charging the annealed metals seems to indicate that hydrogen at a carbon-vacancy complex, for which an interaction energy of 3.3 kJ mo1-1 has been estimated [ 21], lowers the probability of trapping of a positron at the

260

3. R E S U L T S A N D D I S C U S S I O N

240

In Table 1, our evaluation of the short and long lifetimes and the associated relative in-

220 o. v 200

TABLE 1

I,-

Primary lifetime and intensities in annealed A I S I 1009 steel and iron o f 99.998% purity

.J I-rE O -r CD

Specimen

180

Short lifetime ~1 (ps)

Relative intensity 11 (%)

140

0

Without hydrogen Steel 144+-2 Iron 146 + 2

96.5 96.1

With hydrogen Steel Iron

96.2 95.8

136 +-2 139 +-2

,)

160

4

8

12

16

20

24

28

DEFORMATION(%)

Fig. 1. Plot o f short lifetime T1 vs. d e f o r m a t i o n : •", p r o c e d u r e (i), d e f o r m a t i o n (no h y d r o g e n ) ; e, procedure (ii), h y d r o g e n after d e f o r m a t i o n ; A, procedure (iii), h y d r o g e n during d e f o r m a t i o n .

194 complex or decreases somewhat the lifetime of the trapped positron. Figure 1 displays the variation with strain e o f the short-lifetime c o m p o n e n t of positrons in the steel deformed without hydrogen (procedure (i)), in the steel charged with hydrogen after the deformation (procedure (ii)) and in the steel deformed with hydrogen (procedure (iii)). Each data point represents three separate measurements. Procedure (i) produces an increase in T1 initially caused principally by positrons at dislocation sites; this is judged from the fact that hydrogen (procedure (ii)) is able to reduce the relative intensity I1 (Fig. 2) and that the monovacancies produced by deformation, which are quite mobile at room temperature [22], can be annihilated at dislocations or combine to form vacancy clusters (microvoids). Although the contribution from the latter is too small to be clearly deconvoluted from the data of procedure (i), it can be seen in Figs. 3 and 4 for specimens with hydrogen charged after deformation (procedure (ii)) partially quenching out the contribution from the dislocation sites. With increasing deformation without hydrogen r l increases sharply to about 210 ps, which is much in excess of the values associated with discrete dislocation sites or monovacancies either discrete or complexed

by carbon atoms. We note that charging in hydrogen (Figs. 1 and 2, procedure (ii)) reduces I1 anal strongly reduces T1 to levels near that of annealed material. We therefore infer that the rise in Tl(e) for procedure (i) is due to positrons at dislocation sites, and we suggest that the positron lifetime at dislocations in tangles and in cell walls formed at intermediate strains is much larger than at discrete

850

750 (e Q.

LU 3; I-._1 (.,9 Z O

650:

550

450

35C

25G 0

4

8

12

16

20

24

28

DEFORMATION(%)

Fig. 3. Plot of long lifetime 7 2 v s . deformation: o, procedure (ii), hydrogen after deformation; A, procedure (iii), hydrogen during deformation.

60 100 50

>. I-.z t~ z w _~ I-.-

90

80

40

t ILl I.Z

30

ILl

7O


5IJA ~

I-

20

,,-f, 6O

n-

10

5O 0

4

8

12

16

20

24

28

DEFORMATION(%)

0 0

4

8

12

16

20

24

28

DEFORMATIONs%)

Fig. 2. Plot of relative intensity 11 v s . deformation: m, procedure (i), deformation (no hydrogen); o, procedure (ii), hydrogen after deformation; A, procedure (iii), hydrogen during deformation.

Fig. 4. Plot of relative intensity

12 v s . d e f o r m a t i o n : o, pro-

procedure (ii), hydrogen after deformation; A , c e d u r e (iii), hydrogen during d e f o r m a t i o n .

195 dislocations. H o w e v e r , at e ~ 24% t h e m u c h t i g h t e r cell walls again p r o d u c e smaller T1 values f o r p o s i t r o n s in d i s l o c a t i o n sites. T h e r e is at p r e s e n t no i n f o r m a t i o n in the l i t e r a t u r e to s u p p o r t this a d m i t t e d l y ad h o c e x p l a n a t i o n o f o u r results. D e f o r m a t i o n w i t h h y d r o g e n (Fig. 1, p r o c e d u r e (iii)) is v e r y little d i f f e r e n t f r o m t h a t o b t a i n e d w i t h p r o c e d u r e (ii), i.e. t h e p r e s e n c e o f h y d r o g e n during d e f o r m a t i o n does n o t sensibly c h a n g e t h e e f f e c t of deform a t i o n o n the d i s l o c a t i o n - u n r e l a t e d f e a t u r e s o f T1. T h e long-lifetime c o m p o n e n t T2 o f the posit r o n s increases s t r o n g l y in i n t e n s i t y (Fig. 4, p r o c e d u r e (ii)) relative to d i s l o c a t i o n - u n r e l a t e d features. Values o f 300 ps a n d a b o v e are c h a r a c t e r i s t i c o f v a c a n c y clusters [ 1 9 ] . I t is v e r y i n t e r e s t i n g t h a t , a l t h o u g h 12 increases w i t h increasing d e f o r m a t i o n , the m e a n value o f r2 decreases. If, as is e x p e c t e d , the l i f e t i m e o f p o s i t r o n s at v a c a n c y clusters increases w i t h increasing size, o u r d a t a i n d i c a t e t h a t increasing d e f o r m a t i o n leads to m o r e n u m e r o u s v a c a n c y clusters of smaller m e a n size. This seems physically reasonable because vacancy a n n i h i l a t i o n is a p r o c e s s c o m p e t i t i v e w i t h v a c a n c y clustering, a n d the p r o b a b i l i t y o f r e p e a t e d e n c o u n t e r s b e t w e e n vacancies n e e d e d t o f o r m larger clusters m u s t d e c r e a s e in regions o f large local d i s l o c a t i o n d e n s i t y such as cell walls. C o n s i d e r i n g p r o c e d u r e (iii), we o b s e r v e (Fig. 3) t h a t h y d r o g e n during d e f o r m a t i o n p r o d u c e s smaller m e a n values o f ~2 at a n y o n e value o f d e f o r m a t i o n , m e a n i n g t h a t h y d r o g e n favors smaller sizes o f v a c a n c y clusters. A t t h e s a m e t i m e , h y d r o g e n during d e f o r m a t i o n causes a larger n u m b e r o f vac a n c y clusters (Fig. 4) at a given strain. This is c o n s i s t e n t w i t h t h e s t a b i l i z a t i o n of clusters b y h y d r o g e n o w i n g t o t h e lowering of t h e surface free e n e r g y o f a m i c r o v o i d surface b y a d s o r p tion of hydrogen.

4. CONCLUSIONS (1) T h e s h o r t l i f e t i m e T1 in well-annealed iron a n d steel c o n t a i n s c o n t r i b u t i o n s f r o m c a r b o n - v a c a n c y pairs a n d f r o m b u l k states. (2) T h e initial increase in t h e s h o r t lifet i m e r l w i t h strain is m a i n l y d u e to dislocat i o n sites; t h e p o s i t r o n l i f e t i m e in d i s l o c a t i o n sites is s t r o n g l y a f f e c t e d b y t h e a r r a n g e m e n t o f t h e dislocations.

(3) D e c o r a t i o n o f d i s l o c a t i o n sites w i t h hydrogen strongly reduces the contribution ( b u t still p e r m i t s s o m e c o n t r i b u t i o n ) to I1 f r o m d i s l o c a t i o n sites. (4) D e f o r m a t i o n o f t h e steel o c c u r r i n g in t h e p r e s e n c e o f h y d r o g e n favors larger n u m bers o f smaller sizes o f v a c a n c y clusters c o m pared with deformation occurring without hydrogen.

ACKNOWLEDGMENTS We are g r a t e f u l t o P r o f e s s o r J. T. Waber, N o r t h w e s t e r n University, f o r useful discussions a n d f o r t h e privilege o f reading his p a p e r [19] p r i o r to p u b l i c a t i o n . This w o r k was s u p p o r t e d b y t h e U.S. Dep a r t m e n t o f E n e r g y t h r o u g h the C o r r o s i o n R e s e a r c h Center, U n i v e r s i t y of M i n n e s o t a , under Contract DE-AC02-79ER10450.

REFERENCES 1 B.T.A. McKee, W. Trifthauser and A. T. Stewart, Phys. Rev. Lett., 28 (1972) 358. 2 I. K. McKenzie, T. L. K. Hoo, A. B. McDonald and B. T. A. McKee, Phys. Rev. Lett., 19 (1967) 946. 3 O. Mogensen, K. Petersen, R. M. J. Cotterill and B. Hudson, Nature (London), 239 (1972) 98. 4 P. Hautojarvi, T. Judin, A. Vehanen, J. Yli Kauppila, J. Johansson, J. Verdone and P. Moser, Solid State Commun., 29 (1979) 855. 5 R. M. J. Cotterill, M. J. Petersen, G. Trumpy and J Traff, J. Phys. F, 2 (1972) 459. 6 M. L. Johnson, S. Saterlie, D. Boice and J. G. Byrne, Phys. Status Solidi A, 48 (1978) 551. 7 R. N. West, Adv. Phys., 22 (1973) 263. 8 R. W. Siegel, Scr. MetaIL, 14 (1980) 15. 9 P. Hautojarvi, Top. Curr. Phys., 12 (1979) 1-22. 10 J. G. Byrne, Metall. Trans. A, 10 (1979) 791. 11 J. P. Wallace, unpublished research, 1980. 12 R. A. Oriani and P. H. Josephic, Scr. Metall., 13 (1979) 469. 13 H. Cialone and J. Asaro, Metall. Trans. A, 10 (1979) 367. 14 R. A. Oriani and P. H. Josephic, Acta Metall., 27 (1979) 957. 15 P. Kirkegaard and M. Eldrup, Cornput. Phys. Commun., 3 (1972) 240. 16 F. Van Brabander, D. Segers, M. Donkens and L. Donkens-Vanpraet, in P. G. Coleman, S. C. Sharma and L. M. Diana (eds.), Positron Annihilation, North-Holland, Amsterdam, 1982, p. 472. 17 P. Hautojarvi, A. Vehanen and V. S. Mikhalenkov, Appl. Phys., 11 (1976) 191.

196 18 Y. K. Park, J. T. Waber, C. L. Snead, Jr., K. G. Lynn, M. Meshii and C. G. Park, to be published. 19 P. Hautojarvi, in P. G. Coleman, S. C. Sharma and L. M. Diana (eds.), Positron Annihilation, NorthHolland, Amsterdam, 1982, p. 389. 20 P.-W. Kao, S. Panchonadeeswaran and J. G. Byrne, Metall. Trans. A, 13 (1982) 1177. 21 J. P. Hirth, Metall. Trans. A, 11 (1980) 6. 22 H. E. Shaefer, K. Maier, M. Weller, D. Herlack, A. Seeger and J. Diehl, Scr. Metall., 11 (1977) 803.

APPENDIX A: NOMENCLATURE

I1 12

T1 72

measured relative intensity of annihilation of positrons contributing to ~1 measured relative intensity of annihilation of positrons contributing to T2 measured short lifetime of positrons measured long lifetime of positrons