A SOFT X-RAY INVESTIGATION OF THE ELECTRONIC STATES OF METASTABLE PHASES IN Al-Ag ALLOYS

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A SOFT X-RAY INVESTIGATION OF THE

ELECTRONIC STATES OF METASTABLE PHASES IN Al-Ag ALLOYS

N. Negm, L. Watson, A. Szasz

To cite this version:

N. Negm, L. Watson, A. Szasz. A SOFT X-RAY INVESTIGATION OF THE ELECTRONIC STATES

OF METASTABLE PHASES IN Al-Ag ALLOYS. Journal de Physique Colloques, 1987, 48 (C9),

pp.C9-1033-C9-1036. �10.1051/jphyscol:19879186�. �jpa-00227302�

J O U R N A L D E P H Y S I Q U E

Colloque C9, suppl6ment au n012, Tome 48, dhcembre 1987

A SOFT X-RAY INVESTIGATION OF THE ELECTRONIC STATES OF METASTABLE PHASES IN A1-Ag ALLOYS

N.Z. NEGM"), L.M. W A T S O N and A. S Z A S Z ' ~ )

Department of Metallurgy, University of Strathclyde, 48 North Portland Street, GB-Glasgow G1 IXN, Great-Britain

ABSTRACT

The e l e c t r o n i c s t r u c t u r e o f A1-Ag a l l o y s i n d i f f e r e n t metastable s t a t e s was i n v e s t i g a t e d by s o f t x-ray emission spectroscopy (SXES). A c o r r e l a t i o n i s suggested between t h e l o c a l s t a b i l i t y o f t h e d i f f e r e n t aged s t a t e s and t h e i r e l e c t r o n i c s t r u c t u r e .

INTRODUCTION.

Many i n v e s t i g a t i o n s have been c a r r i e d o u t i n t o t h e reasons f o r t h e m e t a s t a b i l - i t y o f t h e d i f f e r e n t s t a t e s o f age-hardenable aluminium a l l o y s [I]. The f o r m a t i o n mechanism o f metastable phases i s dependent on many f a c t o r s

121

one o f which i s t h e e l e c t r o n i c s t r u c t u r e o f t h e a l l o y . I n most experimental s t u d i e s o f t h e e l e c t r o n i c s t r u c t u r e o f a l l o y s o n l y a l l o y c o n c e n t r a t i o n i s considered, however t h e e f f e c t o f metastable phases on t h e e l e c t r o n i c s t r u c t u r e has been shown t o have s i g n i f i c a n t e f f e c t s [3,4].

EXPERIMENTAL.

Aluminium L emission s p e c t r a from a s e r i e s o f A1-Ag a l l o y s s u b j e c t e d t o v a r i o u s heat tr2~Zment.s were measured u s i n g a concave g r a t i n g s o f t x-ray emission spectrometer d e s c r i b e d i n d e t a i l elsewhere [ 5 ] . The specimens were a t t a c h e d t o a water-cooled copper anode u s i n g a c o l d s e t t i n g conducting cement and t h e s p e c t r a were e x c i t e d by e l e c t r o n bombardment w i t h low i n p u t power (2.5kV and 3mA). The e l e c t r o n gun used a n i c k e l f i l a m e n t coated w i t h Bag and SrO t o minimise h e a t i n g by r a d i a t i o n . The vacuum i n t h e spectrometer was 10- t o r r d u r i n g o p e r a t i o n .

M u l t i p l e scanning was used w i t h t h e sample b e i n g scraped c l e a n under vacuumn a f t e r each scan. The d a t a were summed and processed b y computer.

The a l l o y s were prepared from h i g h p u r i t y (99.999%) m a t e r i a l s which were m e l t e d i n alumina c r u c i b l e s and s t i r r e d w i t h an alumina rod. The c a s t samples were c o l d r o l l e d and g i v e n an homogenising anneal a t 550°C f o r 24 hours. The specimens were s o l u t i o n t r e a t e d a t 550°C, water quenched t o produce a s u p e r s a t u r a t e d s o l i d s o l u t i o n (SSSS) and g i v e n t h e r e q i : i r e d ageing t r e a t m e n t j u s t p r i o r t o mounting i n t h e spectrometer. The p a r t i c u l a r stage i n t h e f o r m a t i o n o f p r e c i p i t a t e s due t o t h e d i f f e r e n t h e a t t r e a t m e n t s was determined u s i n g d i f f e r e n t i a l t h e r m a l a n a l y s i s ( D T A ) and microhardness measurements.

Some h e a t i n g o f t h e samples under t h e e l e c t r o n beam d u r i n g measurement o f t h e s p e c t r e was unavoidable and c o n s t i t u t e s an ' i n s i t u ' ageing t r e a t m e n t . Use was

(''present address:Department of Physics. Souhag University. Souhag. Egypt

(2'0n leave of absence from Laboratory of Surface and Interface Physics. Eotvos University. Budapest.

Muzeum krt. 6-8. H-1088 Budapest, Hungary

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879186

C9-1034 JOURNAL DE PHYSIQUE

made of this to monitor the effect of the ageing process on the electronic struct- ure by plotting individual scans which took half an hour to complete. The surface temperature of the sample under the electron beam was calculated using the formula of Lang and Baer [6J to be 70DC. Hardness measurements of samples susceptible to ageing at this temperature after study confirmed that this is a reasonable estimate.

In all cases the SXES presented here are the A1 L emission bands which relate to the partial density of states of 'st and 'd7'2ymmetry at the aluminium sites. The intensities of the spectra were all normalised at the point of maximum intensity. Emission spectra from silver in the alloys was of too low an intensity for accurate examination.

The alloy compositions investigated were Al-8, 12 and 16at% Ag. The ageing treatments the alloys were subjected to to produce the various metastable states were the following.

I. SSSS

2. SSSS 140°C for 22 hrs and quenched 3. SSSS 200°C for 32 mins and quenched 4. SSSS 300°C for 3 hrs and quenched 5. SSSS 300°C for 3 hrs slowly cooled RESULTS AND DISCUSSION

The sequence of evolution of metastable states on ageing a SSSS of silver in aluminium is the following

[ 7 ] :

SSSS

⁺

spherical GP zones (Q)

+

disordered (partially redissolved) GP zones

(E )+

hexagonal intermediate A12Ag-type precipitates (Y')

+

equilibrium A1 Ag

precipitates (Y). The inltlal state of the alloys investigated was $he eta phase and by suitable heat treatments the different intermediate metastable phases can be produced and monitored using DTA. Heat treating at 300°C and slowly cooling produces the equilibrium precipitate (y) with a small amount of y' phase remaining.

Only the alloys subjected to heat treatments 1 and 3 showed any change due to the in-situ heat treatment of the electron beam and in these cases individual

TABLE I

Condition* Composition State* Position of

EF*

Half-band-width* Position of MC*

at% Ag eVtO . ^{lev eVt0.2eV eV*O.leV}

5

8

Y (Y') 73.1 9.7 68.55

5 12 Y (Y') 73.1

^9.9

68.70

5 16 Y (Y') 73.5 10.1 68.50

* Values at start of the measuring process.

scans were recorded. For the other heat treatments, the temperature was too low or the time too short during recording compared with the external heat treatments to have much effect and the average of several scans was used. Changes in the electronic structure due to the different metastable states is apparent from the results given in Table I.

The relative stability of a metastable phase depends on the energy difference between that phase and the equilibrium one. One factor contributing to the stabil- ity is the electronic structure

[ 8 , 9 ] .

Assuming that the partial density of 'p' states in the valence band does not counterballance any energy change, the elec- tronic contribution to the free energy can be monitored by measuring the energy of the mass centre (MC) of the area under the SXES curve. The position of the MC will be independent of the effects of the normalisation of the spectra and of instru- mental effects. The energy of the MC's for the alloys studied are plotted against the estimated phases existing in the alloys in Figure 1. The similarity between the alloys of different silver concentrations is apparent. In the early stages of GP zone formation the MC position is low indicating a strong electronic

contribution to the metastability, lower than in the phase. The important stabilising factor for the coherent GP zones is the electronic structure this is supported by other investigations [3,4,10]. The endothermic processes accompanying the production of the partially disordered phase

( E )

is accompanied by an increase in the MC energy and the stability of this phase must be controlled by the increase in entropy. In the next stage of precipitation

( y ' )

the MC position again

decreases while the hardness increases indicating the formation of another meta- stable state stabilised by the electronic structure. At higher temperatures the equilibrium precipitate is formed, the dominating stabilising factor is again configurational and the electronic energy (the MC) reverts to a value close to that of pure aluminium since the electronic structure is little disturbed by the small amount of the precipitated grains of the gamma phase.

. ^{. . . .} .

^,

^. _Al.

phase

ssss 7' 7 ^t Y' r

Figure 1

JOURNAL DE PHYSIQUE

CONCLUSION.

The s y s t e m a t i c i n v e s t i g a t i o n o f t h e A1-Ag age hardenable a l l o y s p r o v i d e s evidence f o r t h e b a s i c r o l e t h a t e l e c t r o n i c s t r u c t u r e p l a y s i n s t a b i l i s i n g t h e p r e c u r s o r s t a t e s i n t h e f o r m a t i o n o f e q u i l i b r i u m p r e c i p i t a t e s and perhaps goes some way t o e x p l a i n i n g t h e enigma mentioned i n a r e c e n t r e v i e w by Cohen [ll] concerning t h e m e t a s t a b i l i t y o f these age hardening s t a t e s .

ACKNOWLEDGEMENTS

The a u t h o r s a r e i n d e b t e d t o M r J K o l l a r o f Eotvos U n i v e r s i t y , Budapest, f o r c a r r y i n g o u t t h e DTA measurements.

REFERENCES

[l] See f o r example t h e r e g u l a r Conferences on Age-hardenable aluminium a l l o y s . The l a s t was

-

4 t h Conf. CAAA, B a l a t o n f u r e d , Hungary, 1986.

[ 2 ] C o t r e l l A H (Ed) An I n t r o d u c t i o n t o M e t a l l u r g y , Edward Arnold, London, 1967 [ 3 ] Kertesz L, Kojnok J and Szasz A; Cryst. L a t t . D e f e c t s

9,

219, 1982.

[ 4 ] Szasz A, Kertesz L, Hajdu J and K o l l a r J; Aluminium

61,

515, 1985.

[ 5 ] Watson L M, Dimond R K and Fabian D J; J.Sci.Instrum.

44,

506, 1967.

[ 6 ] Lang J K and Baer Y; Rev.Sci.1nst.r. 50, 221, 1979.

[ 7 ] Kwarciak J; J.Therm.Ana1.

30,

177, 1985.

[ 8 ] Hume-Rothery W and Raynor G V: The s t r u c t u r e o f m e t a l s and a l l o y s , Institute o f Metals, 1954.

[ 9 ] P e t t i f o r D G; J.Phys.C.(Sol.State Phys)

3,

367, 1970.

[ l o ]

Kertesz L and Ahmed M A; J.Phys.F (Met.Phys)

13,

2235, 1983.

[ll] Cohen J 8; S o l i d S t a t e Physics

2,

131, 1986.