On the Size–Shape Relation of Guinier-Preston Zones and α′R-Precipitates in an Al–6.8 at% Zn Alloy

Abstract

Ellipsoidal Guinier-Preston zones and lenticular precipitates of the a;-phase grown a t 120 "C in an A1-6.8 Zn atyo alloy are examined by means of transmission electron microscopy. For each particle two characteristic lengths are determined and the corresponding two-dimensional histograms are established. It can be shown that the precipitates monotonically flatten with increasing size. Hence, the size-shape relation did not provide any indication of the phase transition.Elliptische Guinier-Preston-Zonen und lentikular Priizipitate der &-Phase, die bei 120 "C in einer A1-6,8 atyo Zn-Legierung wachsen, werden mittels Transmissionselektronenmikroskopie untersucht.Fur jedes Teilchen werden zwei charakteristische Langen bestimmt und die entsprechenden zweidimensionalen Histogramme aufgestellt. Es kann gezeigt werden, daD sich die Prazipitate mit zunehmender GroDe monoton abf lachen. Somit liefert die Beziehung, GroDe-Form, kein Anzeichen fur einen Phasenubergang.

Coherent phase stability in Al-Zn and Al-Cu fcc alloys: The role of the instability of fcc Zn

“…Precipitates in Al-Zn are observed as ellipsoidal with a polar axis parallel to the ͑111͒ direction. 23,25,26 Even the form of these precipitates could perhaps be controlled by the extremely small ͑111͒ strain in Al-Zn and therefore, would be an indirect consequence of the instability of fcc Zn found here. The instability will presumably influence all binary and multicomponent fcc-alloy systems with Zn as one constituent.…”

“…Aging of Al-Zn alloys show ͑111͒ faceting of Zn precipitates as well as a crossover at a critical particle size from spherical to oblate ellipsoid, with the short axis in ͑111͒ direction. 23,25,26 Upon going to Al-rich compositions, the SRO of Al 0.9 Zn 0.1 also peaks at ⌫, but the streaking along the ͑111͒ direction is diminished. This reduction of the ͑111͒ streaks for Al-rich compositions is a consequence of the fact that formation enthalpies of superlattices along ͑111͒ become larger ͑i.e., less stable͒ with increasing Al concentration.…”

“…The precipitation sequence involves spherical Guinier-Preston ͑GP͒ zones, 7,21 coherent ellipsoidal precipitates, and partially coherent platelets ͓with coherency along ͑111͔͒. [22][23][24][25][26] The decomposition of Al-rich Al x Cu 1Ϫx solid solutions also produces coherent GP zones. In fact, Al-Cu alloys provide the textbook example of the formation of GP zones in supersaturated solid solutions.…”

Density functional theory meets statistical physics: from the atomistic to the mesoscopic properties of alloys

“…27 and references therein), the Zn-precipitates exhibit a remarkable series of shapes and sizes. 28 Small precipitates tend to be spherical, until they reach a critical size R C of about 15-25Å(dependent on the aging temperature) at which point they become ellipsoidal/plate-like with the short axis parallel to the [111] direction. As an example, Fig.…”

On the existence field of guinier-preston zones in the AlZn system

“…This explanation, however, can be excluded, because highresolution transmission electron microscopic (HTEM) investigations, performed with these alloys over a wide range of T,, delivered in each case a uniform size distribution of the G P zones [15]. But, in full agreement with X-ray results of other authors, by means of HTEM a transition from the spherical to a disk-like shape of the GP zones was stated [15], if a certain critical radius (which is increasing with T,) is exceeded. As Guinier plot), fit well the results obtained by other authors using methods different from that applied in [14].…”