In-situ STEM imaging of growth and phase change of individual CuAlX precipitates in Al alloy

Abstract

Age-hardening in Al alloys has been used for over a century to improve its mechanical properties. However, the lack of direct observation limits our understanding of the dynamic nature of the evolution of nanoprecipitates during age-hardening. Using in-situ (scanning) transmission electron microscopy (S/TEM) while heating an Al-Cu alloy, we were able to follow the growth of individual nanoprecipitates at atomic scale. The heat treatments carried out at 140, 160, 180 and 200 °C reveal a temperature dependence on the kinetics of precipitation and three kinds of interactions of nano-precipitates. These are precipitate-matrix, precipitate-dislocation, and precipitate-precipitate interactions. The diffusion of Cu and Al during these interactions, results in diffusion-controlled individual precipitate growth, an accelerated growth when interactions with dislocations occur and a size dependent precipitate-precipitate interaction: growth and shrinkage. Precipitates can grow and shrink at opposite ends at the same time resulting in an effective displacement. Furthermore, the evolution of the crystal structure within an individual nanoprecipiate, specifically the mechanism of formation of the strengthening phase, θ′, during heat-treatment is elucidated by following the same precipitate through its intermediate stages for the first time using in-situ S/TEM studies.

Precipitation during high temperature aging of Al−Cu alloys: A multiscale analysis based on first principles calculations

“…(7) and the anisotropic interfacial energy is incorporated in the third rank tensor βij(p). For variant ① of θ'', βij(p) is given by: 14) and the values in eq. (14) were chosen together with the gradient coefficient of the concentration field, κx = 0.6, to ensure that the interfacial energies of different types of interfaces are consistent with first principles calculation results and to avoid artificial fraction.…”

Precipitation in original Duralumin A-U4G versus modern 2017A alloy

“…It can be hypothesized that after artificial ageing, the historical alloy loses part of its additional dislocation density through faster recovery. The final part of the hardness curve could also be explained be the fact that alloys age faster when additional dislocations are introduced, because the dislocation network transports solute atoms through pipe diffusion [32] and thus accelerates growth and coarsening of ' precipitates, known to nucleate on dislocations [33] . This growth and coarsening will induce a decrease in hardness.…”

Transforming solid-state precipitates via excess vacancies

“…In order to examine nucleation of the θ′ phase at high spatial and temporal resolutions, we performed in situ heating experiments in the transmission electron microscope (TEM). This approach has recently been successful in characterising the evolution of precipitates embedded in a crystalline matrix at near atomic scale [23][24][25] . Using this method we achieved high nucleation rates of the θ′ phase as well as of a new precipitate phase (called η′), directly on pre-existing θ″ precipitates.…”