Structure of the grain boundary region in an Al–15 at.% Zn and Al–2.0 at.% Zn–1.3 at.% Mg alloy aged at elevated temperatures

Syneček, V.; Simmerska, M.; Bartuška, P.; Löffler, H.; Gerlach, R.

doi:10.1002/crat.2170181010

Cryst. Res. Technol.

1983

DOI: 10.1002/crat.2170181010

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Structure of the grain boundary region in an Al–15 at.% Zn and Al–2.0 at.% Zn–1.3 at.% Mg alloy aged at elevated temperatures

V. Syneček

M. Simmerska

P. Bartuška

et al.

Abstract: The resultsof TEM investigations are presented of the effects of various solution and ageing treatments, cooling and quenching rates as well as of tensile stress a t elevated temperatures on the microstructure of the grain boundary (GB) region of the examined Al-Zn and AlZn-Mg alloys.A pronounced influence was found of partial coherency of GB stable ,8(Zn) precipitates with the a matrix in the Al-Zn alloy on the asymmetry of Zn depleted GB region and on the shape of GB precipitates. The role of GB's as sources… Show more

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Cited by 5 publications

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

Löffler

1984

Cryst. Res. Technol.

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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)

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

Löffler

1984

Cryst. Res. Technol.

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The structure of the “influenced grain boundary region” of an Al‐12at.% Zn alloy in dependence on quenching conditions

Siebert

Kroggel

1985

Cryst. Res. Technol.

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It is known t h a t in t h e Al-Zn alloys t h e nucleation and growth of t h e G.P. zones diiringisothermal ageing in t h e RT-range considerably depend on t h e applied quenching treatm e n t and often certain "irregularities", e.g. an inversion of t h e decomposition r a t e with increasing homogenization temperature ( T s ) , a r e observed. Various conceptions were tlaveloped in order t o explain these inversions, b u t none of them can consistently interpret t h e observed niain features. For more details see SIEBERT and KROGCEL e t al. It is of interest for t h e present work to mention t h a t various models were developed to explain t h e structure of t h e precipitation free zone (PFZ). They a r e mainly based on suggesting t h a t t h e profile of t h e vacancies or t h a t of t h e solute atoms or both in t h e GB-region tlct,ermine tho width and t h e striicture of t h e P F Z (see e.g. S Y N E C E K e t 81.). It was t h e idea of tho authors to gain more information o n t h e effect of t h e GR o n t h e structure changes proceeding in its vicinity on investigating not only t h e width and stmrctiire of t h o P F Z , 1)iit tho details of t h e striictiire changes occurring from t h e regions vory close to t h e GB np to t h e midgrain in dependence o n varioiis quenching conditions (T, and qticnching rate, wq).F o r this reason we selected a n A l -1 2 a t .~o Z n alloy produced from materials of purity 4N and aged t h e samples of thickness 0.12 nnn, after various quenching treatments, at 100 "C for 1 h in order t o a t t a i n precipitates suitable to be determined in size sufficiently accurately. T h e samples were annealed at T, = 300 "C or 490 "C for 0.5 h , quenched to RT into water, fluid W250, or oil, stored at RT for 10 s, a n d finally aged at 100 "C for 1 h. T h e foils investigated in t h e T E M of t y p e T'ESLA B S 540 operating at 120 kV were electrolyticallg thinned hy t h e well-known window method.The main results are presented in Figure 1. Note that v : ] > wTz50 > wyter (for details of the quenching rates see STEBWRT). The following yields seem worth to be particularly emphasized.

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

Salawa

Löffler

Korngiebel

1986

phys. stat. sol. (a)

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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.

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Structure of the grain boundary region in an Al–15 at.% Zn and Al–2.0 at.% Zn–1.3 at.% Mg alloy aged at elevated temperatures

Cited by 5 publications

References 28 publications

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

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

The structure of the “influenced grain boundary region” of an Al‐12at.% Zn alloy in dependence on quenching conditions

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

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