Segregation assisted grain boundary precipitation in a model Al-Zn-Mg-Cu alloy

Abstract

Understanding the composition evolution of grain boundaries and grain boundary precipitation at near-atomic scale in aluminum alloys is crucial to tailor mechanical properties and to increase resistance to corrosion and stress corrosion cracking. Here, we elucidate the sequence of precipitation on grain boundaries in comparison to the bulk in a model Al-Zn-Mg-Cu alloy. We investigate the material from the solution heat treated state (475°C), through the very early stages of aging to the peak aged state at 120°C and further into the overaged regime at 180°C. The process starts with solute enrichment on grain boundaries due to equilibrium segregation accompanied by solute depletion in their vicinity, the formation of Guinier-Preston (GP) zones in the solute-enriched grain boundary regions, and GP zones growth and transformation. The equilibrium segregation of solutes to grain boundaries during aging accelerates this sequence compared to the bulk. Analysis of the ~10 nm wide precipitate-free zones (PFZs) adjacent to the solute-enriched grain boundaries shows that the depletion zones are determined by (i) interface equilibrium segregation; (ii) formation and coarsening of the grain boundary precipitates and (iii) the diffusion range of solutes in the matrix. In addition, we quantify the difference in kinetics between grain boundary and bulk precipitation. The precipitation kinetics, as observed in terms of volume fraction, average radius, and number density, is almost identical next to the depletion zone in the bulk and far inside the bulk grain remote from any grain boundary influence. This observation shows that the region influenced by the grain boundaries does not extend beyond the PFZs.

Effect of Copper Addition on Precipitation Behavior near Grain Boundary in Al–Zn–Mg Alloy

“…There are several reports about the behavior of Stress Corrosion Cracking (SCC) resistance of AlZnMg alloys caused by Cu-segregation or Cu incorporation into GB precipitates. 21,22) According to our results, the second clusters appear distributed homogeneously inside grains in Cu-added alloys and even in the their PFZs. As the second clusters can be formed in con-PFZ without the participation of Zn atoms, SCC resistance could be affected from those fine precipitate containing Cu.…”

New approach for FIB-preparation of atom probe specimens for aluminum alloys

“…Although this technique is adequate for the study of most materials, it cannot be easily applied for the study of aluminum and its alloys: analyses of the grain boundaries composition, and investigation of potential Ga traces there, but also at other defects such as dislocations, is affected by Ga-ions implantation and diffusion due to the FIB preparation (6)(7)(8)(9)(10). Existing alternatives include electropolishing, but this technique is not site specific, or FIB preparation using a Xe+ ion beam (10)(11)(12). The last technique has shown a great efficiency, but such instruments are not widely available.…”

Interplay of Chemistry and Faceting at Grain Boundaries in a Model Al Alloy

“…GBs often see the segregation of solutes or impurities, driven by a reduction of the interfacial energy according to the Gibbs adsorption isotherm, and are favorable sites for heterogeneous nucleation of precipitates. These can form during quenching [16] or during natural or artificial aging in Al alloys [17,18]. Diffusion, segregation, and precipitation are intimately related to the local GB structure, which can result in strain localization, intergranular fracture, and corrosion [19].…”

“…The cosegregation of Mg and Zn to GBs in Al-Zn-Mg-(Cu) alloys is frequently observed experimentally. These elements possess a strong tendency to precipitate as Mg-Zn rich phase during aging [17], indicating an attractive interaction between Mg and Zn. While we are unaware of a ternary Al-Zn-Mg empirical potential, two studies [50,51] used density functional theory to study the same symmetric f120g boundary.…”