On the influence of various heat treatments and impurity additions on structure changes in Al‐Zn‐Mg alloys

Abstract

I n the last two decades a lot of investigations were performed to obtain more insight in which manner the decomposition structure of the age-bardenable alloys can be directed in a wanted way, that means to attain finely distributed precipitation of the 7'-phase and a narrow precipitation free zone (PFZ) at the grain boundaries (GB) in order to improve their mechanical properties and to diminish their stress corrosion sensitivity. The most applied niethods to put this into practice are two-or even morestep ageing treatments. Since quite recently the author together with others ( L~F F -LER, KOVACS, LENDVAI) published a review about these problems only the most striking features will be presented in a concise manner. 1 ) Not only G.P. zones, but also a dense system of finely distributed precipitates of the partially coherent metastable q'-phase results in good mechanical properties.2) The q'-phase cannot form homogeneously, that means its origination necessarily needs assistence by suitable sites for heterogeneous nucleation. The number of these nucleation sites has to be large enough in order to yield, eventually, a dense system of precipitation of the 7'-phase.3) The simplest method to realize this is a preageing treatment performed, e.g., a t RT, in the course of which G.P. zones are nucleated and grow. Thus the treatment "Homogenization/Quench to RTIPreaging a t RTIFinal ageing at elevated temperatures" is widely technically applied. 4) G.P. zones can serve as nucleation sites for the 7'-phase in two ways. a ) G.P. zones overcritical in size can directly be transformed into precipitates of the q'-phase (sequence G.P. zones/q'-phase). This mechanism most probably operates in cases the hardness and volume fraction of the precipitates do not drop during the ageing a t elevated teniperatures to the asquenched state. This sequence is typical for Al-Zn-Mg alloys with zinc contents larger than about 2.5 at.76 and/ or in cases the pre-ageing treatment was performed a t higher temperatures (e.g. "C).b) Concentration fluctuations left behind dissolving zones can also assist the heterogeneous nucleation of 7'-phase. lndications that this mechanism is in fact operating are that during the ageing a t elevated temperatures the hardness values etc. do drop to the as-quenched state and subsequently increase and that samples directly quenched to elevated temperatures do not show a significant increase in hardness. 5 ) Below the temperature of homogeneous nucleation of precipitation (Thom.n.)G.P. zones are the first decomposition product. I n the Al-Zn-Mg system is placed about 200 K below the boundary of the range of homogeneity. The heterogeneous formation of precipitates of the ql-phase can take place in the temperature l)

Positron lifetime spectroscopy and decomposition processes in commercial Al-Zn-Mg-based alloys

“…The difference between the values obtained by the two microscopic mechanisms that explain the effect of the composition on the migration of the solutes were discussed in [31]. Alloys 7005 and 7012 have almost the same magnesiumto-zinc ratios; the main points of difference lie in: (a) the absolute value of the Mg content (1.4 wt% in alloy 7005, 2 wt% in alloy 7012), a factor that seems to influence the diffusion processes rather strongly in special cases [30,32]; (b) the quaternary element (Mn in alloy 7005, Cu in alloy 7012). It is known that additions of Cu to Al-Zn-Mg alloys favour the formation of GP II zones and delay the rate of η -precipitation, while Mn acts in the opposite way [33].…”