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.
Highlights
Age hardening in aluminium alloys, which involves the growth of extremely fine structures ranging from a few atomic layers to precipitates as large as a few hundred nanometres, is a process over hundred years old[1, 2], and has been successfully applied on an industrial scale to strengthen light-weight metal alloys[3, 4]
We have studied the evolution of individual precipitates and their interactions using in-situ STEM
First we heated the specimen using the mechanical systems (MEMS) heater to a temperature of 520 °C to form a single-phase solid solution and switched the MEMS heater off for spontaneous cooling to the ambient temperature
Summary
Chunhui Liu[1,2], Sairam K. The evolution of the crystal structure within an individual nanoprecipiate, 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. During age-hardening, a metal alloy is heated to an elevated temperature to form a completely homogeneous solid-solution and rapidly cooled to room temperature (quenching) to form a super-saturated solid-solution From this condition, upon heating to temperatures higher than room temperature (typically around 100–200 °C for Al alloys, known as thermal ageing), the alloy decomposes resulting in the formation of a dispersion of nanoprecipitates in the matrix by the segregation of the alloy’s solute atoms. We have studied the evolution of individual precipitates and their interactions using in-situ STEM For this we used the micro-electro-mechanical systems (MEMS) based heating holder[21, 22] which allows precise control of the temperature, a very low specimen drift and high stability of the sample during heating. The heat treatment can be repeated on the same lamella and the results are reproducible
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