Effect of hydrogen alloying on positron annihilation in Ti

Scripta METALLURGICA

Vol. 18, pp. 141-144, 1984 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

EFFECT OF HYDROGENALLOYING ON POSITRON ANiqlHILATION IN Ti

G.M. HOOdand R.J. Schultz Atomic Ener~y of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario KUJ IjU

~Received October 17, 1983) (Revised December S, 1983)

Introduction I t has recently been demonstrated (I) that positron annihilation spectroscopy (PAS) fb~ay be applied to sense effects due to hydrogen additions to Zr. In particular, i t was shown (1) that a line-shape parameter, S, derived from Doppler broadening measurements, varied smoothly with the hydrogen content of a series of quenchedZr-hyarogen alloys. In that work a result was presented which indicated that the technique might be applied to a study of hydrogen in Ti. The present work f u l f i l s the promise of that i n i t i a l measurement and shows the potential of two line-shape parameters, S and W, for the determination of hydrogen in Ti. Experimental The material used in this work was Marz grade. The supplier's analysis presented elsewhere (2) is reproduced here (Table I) for convenience. The preparation of the samples and the means of hydriding were as described previously (1). The hydrogen contents of the samp]es were determined both from the pressure drop during hydriding and, with the exception of the Ti-3.6 at% hydrogen samples, by a hydrogen-deuteriumexchange technique. The results from the two techniques agreed well.

TABLE I Composition of Marz Grade Ti used in this Work. Materials and Analyses Supplied by the Materials Research Corporation, Orangeburg, New York. ELEMENT C 0 H N B 140 420 90 15 ELEMENT Cr 3

Ni 14

Be Al 9

Zn Ga Ge Zr L L -

F

Na <.2

Nb Mo Pd L L L

Mg <.2

P .4

S Cl 12 4

Ag L

Cd In L

K Ca Ti 36 .9 M

Fe Cu Si 24 6 0

Sn Sb Te Ta W L L L L

Pt L

Au Pb L L

All values in ppm (atomic); L = < 0.05 ppm and M - major component. The order of sample treatment was (i) hydride, ( i i ) homogenize(see reference ( i ) ) , ( i i i ) quench into H20 at 273 K and (iv) quench into an HCI/H20 mixture at 200 K (3). ~asurements were made after the l a t t e r three treatments. Prior tu the quenches the samples were heated in air to 770 K and maintained at that temperature for ten minutes. The application of PAS to defect studies of metals has been discussed elsewhere (1,4); therefore only a very short account w i l l be presented here. Doppler-broadening measures the ener~ s h i f t imposed on 511 keV annihilation quanta by a velocity component of the centre of mass of the annihilating positron and electron. Usually when a positron becomes trapped in a r e l a t i v e l y negative (electrostatic attraction) or low-ion-core density defect in a metal, the fraction of annihilations with the lower velocity conduction (vis & vis ion core) electrons is

141 0036-9748/84 $3.00 + .00 Copyright (c) 1984 Pergamon Press Ltd.

142

HYDROGEN AND POSITRON A N N I H I L A T I O N

IN Ti

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2

enhanced. The r e s u l t is an a t t e n u a t i o n of the Doppler-broadening of the 511 keY photon energies which may be c h a r a c t e r i z e d in terms of c o . v e n i e n t l i n e - s h a p e paramezers. The customary line-shape parameter, S, measures the intensity of the annihilation photopeak at the centre of the broadened linewidth d i s t r i b u t i o n . An increase in S may then be identifieQ with increased positron l o c a l i z a t i o n , or trapping, at defects. A ~OOd complementary para~eter is W, which is defined to measure annihilations wizh maximums h i f t from 511 keV. A co~,~bination and comparison of measuruh~ents of S aria W can give useful insight both about defect densities and their nature (3-5). References (3) and (b) provioe details of the definition of S ane W and reference { l ) incluoes further experimental details. Results and Discussion The results of this work are presented in Figs. l an6 2 in terms of ~S and AW, which are the percentage changes in S and W, respectively, comparedto results for well annealed, nominally hydrogen-free Ti. The uncertainty levels in S ano W, based only on counting s t a t i s t i c s , are t y p i c a l l y less than 0.1 and 1.0%, respectively. The hydrogen concentrations have precision l i m i t s of the order of I0% at the lowest concentration and about 3% for the higher levels. 0

4,O-

ji•

J

50-

--)/

20-

/o

f +

\ \ <~

<3

,

/

IC

A

\

+

/

03

5.C

"x\\i \,

IO.C

8\\•

o'~ 15.0 I

,

I

I

J

O I.O 2.0 5.0 HYDROGEN CONCENTRATION (At %)

FIG. l The influence of heat treatment on the response of the S parameter to the hydrogen content of m-Ti. The symbols are associated as follows: ~ and +, measurements for the homogenizedand Z73 K quenched states, r e s p e c t i v e l y ; O a n d O , measurements in the '200 K quenched state' from data sets l and 2, respectively. The arrow indicates the position of the datum a f t e r a requench (to 200 K) of the T i - l . 4 at% hydrogen sample.

i

,

I

,

I

,

I

,

o I.O 2.0 5.o HYDROGEN CONCENTRATION (At %) FIG. 2 The influence of heat treatment on the response of the W parameter to the ~@drogen content of ~-Ti. The symbols are associated as per the Fig. l caption. The sy~ol qD, denotes the datum for the T i - l . 4 at% ~drogen sample after a requench to 200 K.

The results are, basically, from two sets of measurements. The f i r s t set was ma~e on samples doped with up to 2.4 at% hydrogen. The second set was made four months l a t e r , on the same samples, in their oriyinal 200 K quench state. Also included in the second set were data for a Ti-3.6 at% hydrogen alloy and new results for the Ti 1.4 at% ~drogen samples, following a requench to ZOO K. Within the uncertainty of the measurements, the AS and AW values were found to be reproducible; no s i g n i f i c a n t time dependence of those parameters - following prolonged exposure at 295 K - has

k:ol.

18,

.~o.

2

ItYI)ROGILNANI) t~OSITRON :\5~:<[f{II.AFI()N IN TL

113

been detected. Background interference problems obviated tl~e acquisition of meaninsfu] W data for the 273 K quench state. The W parameter is p a r t i c u l a r l y susceptible to sucn conditions (3,6) which, for the most part, have r e l a t i v e l y l i t t l e effect on S.

Two evident features of this work are that the response of S and W to the hydrogen content of Ti increases with increased specimen cooling rate at~d that at hi9her hydrogen levels, S and W tend towards saturation values. These aspects may be unaerstoom - at least in p r i n c i p l e in terms of the p r o b a b i l i t y of positron trapping at defects associated with hydrides (1). As the dispersion of hydrides, for a given hydrogen content, becomes increasingly f ine, the positron trapping p r o b a b i l i t y w i l l increase and tend towards unity (saturation trapping) at s u f f i c i e n t l y high hydrogen concentrations. I t is to be expected, on general ~rounds, that tn~ degree of hydride dispersion w i l l increase with increased hydrogen supersaturation (codlin9 r a t e ) . The l a t t e r result has been v e r i f i e d experih~entally for the Zr-nyorogen system (7). The r e l a t i v e l y slow response of S to hydrogen additions at low hydrogen levels for the 273 K quench, compared to s i m i l a r results for x-Zr (see Fig. 1 of reference ( I ) ) , is probdoly r e l a t e d to the generally higher solid s o l u b i l i t y levels of hydrusen in ~-Ti at a ~iven temperature (8,9). For a given quench bath temperature anu hydrogen content, the higher solvus temperature (T s) f o r x-Zr w i l l lead to faster cooling throubh Ts and hence a greater degree of supersaturation. The response of the positron to the presence of hydrogen in quenched Ti-hydrugen alloys is understandable in terms of both the low density of the hydrides, compared to the Ti {,atrix, and the probable formation of m i s f i t dislocations to relieve the excessive stresses gen~rateu by t h e i r formation (lO). The density of the fcc e-hydride, which appears to be the form h~st l i k e l y to be present, is about 17% lower than that of ~-Ti ( l l ) .

I t was suggested e a r l i e r ( I ) , for the Zr/ilydroyen system, that positrons may be trapped at e i t h e r the hydrides, per s~, or tile dislocation arrays associated with them, or both. In t h i s work a study of both S and W data allows some discrimination of the probable nature of the defect with which the positron is i n t e r a c t i n g . This arises frohL the c h a r a c t e r i s t i c line-shape features which d i f f e r e n t types of defect nave, a result of the u i f f e r e n t electron d i s t r i b u t i o n s experienced by the positron (see for example, references 4 and 12). Thus i t has been detem,lined (13), for e s s e n t i a l l y equivalent spectrometry conditions, that vacancy defects and dislocations in ~-Ti give rise to ±W/±S ratios for vacancies and dislocations which are ,luch smaller than those observed in the quenched Ti-hydrogen a llo y - see Table I I . TAULE II Ratio of Changes in S and W Parameters, ~iS/~W, Induced by Various Treatments of I n i t i a l l y Well-Annealed Ti Sample Treatment

Probable Defects

-~W/~S

4U% thickness reduction

dislocations anQ point defects

1.b

As above plus I0 min at 6UU K to eliminate point defects

dislocations

1.b

Electron i r r a d i a t e d

vacancies

1.7

Hydrided to 3.6 at% hydrogen and quenched from 770 to 20U K

hydrides and dislocations

3.~

All of the measurements reported here were made with a 22Na positron source

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HYDROGEN AND POSITRON AN~qlHILATION IN Ti

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The observation indicates that positrons are not preaominantly annihilating fro~L aislocations; they are probably annihilating from a staze within U~e hydriqe i t s e l f . I n i z i a l considerations suggest that the result is in sympathy wizh so~ae prelimindry work on o r i e n t ~ i u n effects on positron annihilation from specifically oriented sinsle crystals of pure ano hydrogen-doped ~-Zr (13). Acknowledgeraents The authors acknowleoge the assistance of C.E. Coleman ~n~ J.G. Bryson with t~drieiny samples and J. Mislan, H. Herrington ana L. Junop for providing the hydrogen analyses for Zhe isotope exchange method. Acknowledgements are also made to C.E. E l l s anu C.t. Coleman for useful discussions. REFERENCES 1. G.M. Hood, R.J. Schultz , Scripa Met. 16, 1359 (19B2). 2. G,M. Hood, M. Eldrup and N.J. Pedersen, Proc. 5tn I n t l . Conf. on Positron Annihilation, 1979, Eds. R.R. Hasiguti ana K. Fuoiwara (The Japan Inst. of metals, Senaai, Japan), p. 751. 3. G.M. Hood and R.J. Schultz, Can. J. Phys. 6u, I l l 7 (19~2) 4. R.N. West, in Positrons in Solids, Ed. P. llautojarvi, Topics in Current Physics, 12, 1979. 5. G.M. Hood and R.J. Schultz, Phil. Ma9. A, to be published. 6. G.M. Hood and R.J. Schultz, J. Nucl. t4ater. 96, 15 (19Bl). 7. See reference to A.R. Daniel in C.E. Ells, J. Nucl. Mater. 2B, 129 (19bCi). 8. N.E. Paton, B.S. Hickman and D.H. Leslie, Met. Trans. 2, 2791 (1971). 9. C.E. Coleman and J.F.R. Ambler, Can. i,~et. Quart. 17, 81 (197~). lO. J.C. Williams, Proc. I n t l . Conf. on Effect of Hydrogen on t#~e Behaviour of Materials, Jackson Lake, USA, publ. TMS AIME, p. 367, (1976). 11. W.B. Pearson, Handbook of Lattice Spacings and Structures of 14etals, (Peryamon Press, Oxford, 1964). 12. M. Eldrup, O.E. Mogensen and J.H. Evans, J. Phys. F. 6, 499 (197~). 13. G.M. Hood and R.J. Schultz, unpublished work.