The Effect of Small Additions of Si on the Structure of an Al−2 at% Zn−1 at% Mg Mother Alloy

Abstract

The structures of specimens from an Al−2 at% Zn−1 at% Mg mother alloy with additions of Si, (Mn + Si), and (Fe + Si), are investigated by TEM and microprobe for two different heat treatments (HT) namely direct quench (DQ) to 160 °C and a two‐step ageing treatment (TSA) (pre‐ageing at RT for 7d) to explain the striking differences in the course of the hardness curve obtained during the ageing at 160 °C. It turned out that these differences are mainly due to the fact whether or not fairly large needles of the β″(Mg2Si)‐phase can form.

The effect of additions of Mn and Cr and the applied solidification technique on the structure of an Al-2.2 at% Zn-1.4 at% Mg alloy

“…Si by forming Mg,Si, diminishing in this manner the strength of the alloy) ; (ii) they can stabilize the Guinier-Preston (GP) zones as, e.g., reported for additions of Ag [ 11 and Cu [2] ; (iii) they may assist the heterogeneous nucleation of metastable phases more stable than GP zones, e.g. q'(MgZn,)-phase, if the alloy is directly aged a t temperatures above the upper limit temperature of the formation of G P zones, as known for additions of Si [3]; and (iv) they can capture vacancies, retarding in this manner the formation of G P zones, as known, e.g., for additions of Si, V, and Ti liecently, the effect of additions of afew elements, namely Si, Cr, Mn, and Fe, single and pairwise, on the mechanical properties of an A1-1.9 atyo Zn-1.3 a t % Mg mother alloy was investigated by Korngiebel and coworkers [3, 5 to 81. Especially the effect of additions of Mn and/or Cr was of interest, because these elements considerablyraised the hardness and in addition, according to the literature, they may improve the response of the Al-Zn-Mg alloys against stress corrosion cracking.…”