No abstract

Completing a recent paper, the lattice constants of the Al‐rich f.c.c. α‐matrix (M) in equilibrium with (metastable) Guinier‐Preston (GP) zones, a[α(M/GPZ)], as well as with the β(Zn)‐equilibrium phase, a[α(M/β)], are followed during the decomposition (in most cases at RT) of ten Al‐Zn alloys ranging in zinc content between x = 8 and 48 at%. The most important results are the following. (i) In the as‐quenched state, the presence of GP zones could be distinctly proved up to x = 44 at% (but not for x = 48 at%). (ii) The unexpected values of a[α(M/GPZ)] holds for all alloys with x = 8 to 44 at%. The results obtained are discussed.

Positron lifetime τ and momentum distribution (curve shape parameter S) measurements are performed to study the decomposition in an Al5.9 at% Ag alloy during isochronal and isothermal heat treatment of water‐quenched samples. The positron studies are supplemented by small‐angle X‐ray scattering experiments. It is proved that positrons become trapped by G.P. zones being free of any vacancy‐type or dislocation‐type defect and by semicoherent γ′‐precipitates. Positron trapping rates show the coarsening of G.P. zones simultaneously with the formation and growth of γ′ during isothermal aging above 170 °C. From the temperature dependence of τ and S the transformation of η‐zones into τ‐zones is concluded. The Ag content of zones changes from about 60 at% at 120 °C (η‐zones) to 35 at% at 190 °C (η‐zones) which is in good agreement with SAXS and AP‐FIM experiments. Evidence of a reversible change of the G.P. zone composition when alternating the temperature between 190 and 120 °C is found in both positron and SAXS experiments.

The various phases and the respective ranges of their existence were determined during continuous cooling from temperatures close below the solidus line to R.T. with a rate of 2 K min-l as well as during the subsequent heating in few Al-Zn(40 wt.%)-Mg(X) alloys (X = 0 ; 0.5; 1.0; and 3.0 wt.yo) by means of X-ray phase-analysis, small angle X-ray scattering, differential thermo-analysis, optical microscopy, and X-ray micro-analysis.The most striking features are that even addition of only 0.5 wt.% Mg cause the formation of precursors of the MgZn, equilibrium phase few degrees below the solidus line.Wahrend der kontinuierlichen Abkiihlung von Temperaturen nahe unterhalb der Soliduskurve auf Raumtemperatur und wahreiid der nachfolgenden Aufheizung init 2 K min-l wurden in Al-Zn(40 Gew.yo)-Mg(X)-Legierungen (X = 0 ; 0,5; 1,O; 3,O Gew.yo) mit Hilfe der Rontgen-Phasenanalyse, der Rontgenkleinwinkelstreuung, der Differentialthermoanalyse, lichtmikroskopischer Aufnahmen und der Rontgenmikroanalyse die vcrschiedenen Phasen und deren Existenzbereiche bestimmt.Als iiberraschendes Ergebnis zeigte es sich, da13 schon ein Zusatz von nur 0,5 Gew.% Mg die Bildung von Vorlaufern der MgZn,-Gleichgewichtsphase wenige Grade unter der Soliduskurve bewirkt.

No abstract