Microstructure Evolution and Aging Kinetics of Al-Mg-Si and Al-Mg-Si-Sc Alloys Processed by ECAP

A study was carried out on a ECAP processed Sc-containing Al-Mg-Si alloy and on a reference 6082 alloy to investigated grain structure evolution during severe plastic deformation and post-ECAP aging behaviour. The results showed that the mechanism of ultrafine structure development was substantially unchanged with respect to a reference Sc-free alloy. Also the aging sequence and precipitation kinetics of the two alloys revealed to be comparable. The ECAP processed samples of the 6082 reference alloy showed a clear recrystallization peak at temperatures in the range 315-360°C, depending on the amount of strain experienced, whereas the Sc-containing alloy retained its ultrafine structure up to temperatures well exceeding 450°C, under the conditions reproduced in a DSC temperature scan.

High-resolution observations of metastable γ’ precipitation in Al-15at.%Ag

It is currently well established that, for an Al-Ag alloy quenched from the α phase and aged within the metastable solvus, the aging sequence is: supersaturated α → GP zones → γ’ → γ (Ag2Al). While GP zones and plate-shaped γ’ are metastable phases, continuously distributed in the matrix, formation of the equilibrium phase γ takes place at grain boundaries by discontinuous precipitation (DP). The crystal structure of both γ’ and γ is hep with the following orientation relationship with respect to the fee α matrix: {0001}γ′,γ // {111}α, <1120>γ′,γ, // <110>α.The mechanisms and kinetics of continuous matrix precipitation (CMP) in dilute Al-Ag alloys have been studied in considerable detail. The quantitative description of DP kinetics, however, has received less attention. The present contribution reports the microstructural evolution resulting from aging an Al-Ag alloy with Ag content higher than those previously reported in the literature, focusing the observations of γ' plate-shaped metastable precipitates.

Transmission Electron Microscopy Study of Precipitation in a Nickel-Beryllium Alloy

The aging sequence in precipitation hardening Ni-2 w/o Be has been studied by transmission electron microscopy as a function of time at temperatures of 400°C, 565°C, 650°C and 750°C. The microstructural changes at different stages in the aging sequence were analyzed by a combination of electron diffraction and dark field microscopy. Prior to aging, the alloy was solution annealed at 1100°C for 6 hrs. Subsequent examination of the solution annealed material showed no evidence of either continuous or discontinuous precipitation.The precipitation sequence following aging at 400°C and 565 °C was found to be:This is analogous to that reported for the widely studied copper-beryllium1 and copper-zinc-beryllium2 systems. Precipitation commences with the nucleation of coherent platlet G. P. zones parallel to {001} matrix planes. As aging proceeds, the intermediate semicoherent γ precipitate is formed continuously from the G. P. zones.