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

Abstract

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.