Positron Studies of the Chemical Composition of Guinier‐Preston Zones in AlZn Alloys

Abstract

Various decomposed Al-Zn alloys are investigated by positron lifetime and angular correlation technique in order to study the potential of positron method in estimating the composition of Guinier-Preston zones. From experiments the curve shape parameters, S, the core annihilation rate, A,, and the number of free conduction electrons, z, derived from the zeros of the one-dimensional Fourier-transform of the angular correlation curve are estimated. The response of the various annihilation parameters to zone composition, positron affinity, and alloying effects (lattice change, charge transfer) is discussed. The number of free electrons per atom reflects most clearly the chemical composition of the GP zones.Verschiedene entmischte A1-Zn-Legierungen werden mit der Positronenlebensdauer-und Winkelkorrelationstechnik untersucht, um das Potential der Positronenmethode zur Bestimmung der Komposition von Guinier-Preston-Zonen zu untersuchen. Aus den Experimenten wird der Kurvenformparameter S, die Annihilationsrate mit gebundenen Elektronen und die Zahl von freien Leitungselektronen z, die aus den Nullstellen der Fouriertransformierten der Winkelkorrelationskurve abgeleitet wird, bestimmt. Es wird die Empfindlichkeit der verschiedenen Annihilationsparameter beziiglich Zonenkomposition, Positronenaffinitat und Legierungseffekten (Gitteranderung, Ladungsiibergang) diskutiert. Die Zahl freier Elektronen pro Atom widerspiegelt am deutlichsten die chemische Zusammensetzung der GP-Zonen.

A positron study of guinier-preston zones and γ′-precipitates in an AlAg alloy

“…A similar situation is found for G.P. zones in Al-Zn alloys where AS" = 0.78 + 0.03, A 1 = 0.81 + 0.15, but cZn = (70 + 2) at% [33]. As we have discussed above, c.& > cAg is a consequence of the positron affinity of Ag.…”

“…These effects and the potential and limits of the positron method in estimating the composition of G.P. zones were discussed in detail by Dlubek and Gerber [33].…”

“…However, the difference in the lattice parameter uGp of G.P. zones compared with the pure metals A1 and Ag causes a similar effect and may be included in the value E in (12) (see [33]). Using [2], ---AP-FIM experiments of Osamura et al [3] (uGp -aAl)/uAI = 0.2% and (aGP -aAg)/aAg = -0.8% [36,371 we have estimated that due to this lattice mismatch c i g may exceed cAg by about 5 at%.…”

“…Using [2], ---AP-FIM experiments of Osamura et al [3] (uGp -aAl)/uAI = 0.2% and (aGP -aAg)/aAg = -0.8% [36,371 we have estimated that due to this lattice mismatch c i g may exceed cAg by about 5 at%. As discussed in [33] the number of free electrons per atom z estimated from positron experiments is a very useful parameter for estimating cLg since from all of the annihilation parameters z is most weakly affected by the positron affinity. z may be calculated from the Fermi momentum and the true lattice parameter of the material under investigation assuming the validity of the free-electron theory.…”

Positron/Positronium Annihilation as a Probe for the Chemical Environment of Free Volume Holes in Polymers

“…Here, a semiquantitative evaluation of the composition of these nanometer size coherent clusters has been achieved through a simple scaling of the DB line shape parameter, S, of the alloy constituents. 12 (S is a routinely used integral † University of Bristol. ‡ ITA Institut Ko ¨then.…”