Fundamental investigations of precipitation in the solid state with focus on Al-based alloys

The project 'Fundamental investigations of precipitation in the solid state with focus on Al-based alloys' started in the autumn 2013 and ended summer 2017. It employed a PhD student for almost 2 years (from September 2013 to June 2015) and a postdoc for 3 years(from April 2014 to March 2017). In addition, researchers from SINTEF Materials and Chemistry contribute to the project with expertise, experiments and atomistic calculations. The purpose of the project is to gain a fundamental understanding of precipitation in aluminum alloys and understand what lies behind the formation of the different structures that these precipitates are taking, especially how the chemical composition of the alloy acts on these precipitates. The precipitates are what make age-hardenable aluminum alloys strong, so that they can be used for example in the automotive industry. Eight Al alloys with different compositions (including Al-Cu, Al-Cu-Mg-Si, Al-Mg-Zn, Al-Cu-Mg-Si-Zn) were produced. The alloys were extruded and heat treated after advice from Hydro Aluminium, to provide a treatment that is similar to what is happening in the industry. Hardness and conductivity were measured, and we have studied the microstructure using transmission electron microscopy (TEM). High resolution studies of the atomic structure of precipitates have been done. We use the national large-scale infrastructure NORTEM in these studies. We have developed advanced techniques to study the detailed composition and structure of the precipitates. For the understanding of nucleation and growth of these precipitates, we study how the different elements added contribute to the structure and with which compositions the known precipitates are formed. We have discovered some new contexts, which are published. We have also studied strain in aluminum around the precipitates and developed a new analysis of this. As reflected in the publications from the project, we have through the whole project period had several collaborations with international researchers. We had in 2014 a five month long visit from a PhD student from Tokyo Institute of Technology in Japan, and made the foundation for a successful collaboration with this institution (ref INTPART project funded from 2016). In addition we had a visit from Warsaw in Poland, worked with researchers from Rostock in Germany and Bologna in Italy. Where necessary, we have done atomistic calculations in connection with the TEM experiments to understand the atomistic structure of the precipitates. This has been done by SINTEF researchers. We have published?. journal papers within this project and one popular publication.

To improve generic understanding of solid state precipitation, a four year study on several Al-based systems is proposed, where it is planned to combine highly advanced equipment and methods in transmission electron microscopy (TEM) with ab-initio calcula tions. The effects of cross-system substitutions, mainly in Al-Mg-Si, Al-Mg-Cu and Al-Mg-Zn, will be investigated and compared in high detail. To understand the more generic role of a solute element - the influence it has on the precipitate distribution, on morphology, and on the bulk and interface structures, the detectable changes will be quantified in high detail. The substituted fraction in single columns of the structures will be quantified. By experiments and by calculations alike, it will be invest igated how the (amount of) the introduced elements affect bonding and stability. The project relies on the newest information about atomic structures - including the interfaces - of the nanometer sized precipitates, mainly acquired by the group. Similar a nd more advanced methods / equipment as used there, will be applied in this project. The new state-of-the-art instrumentation shortly installed in in the Trondheim node of NORTEM is well suited for the challenge. The structures will be analysed by high a ngle annual dark field scanning TEM (HAADF-TEM) imaging utilizing the 1 Å resolution. The instrument allows partial occupancy in single columns to be quantified by electron energy loss spectroscopy (EELS). Stability of the structures in the hybrid alloy s ystems will be investigated through ab-initio calculations. The studies of the bonding related to stabilization will aid the understanding of why certain precipitates become possible in one system, but not in another.The work will be performed by NTNU and SINTEF in collaboration with international groups. The proposal seeks support for one PhD and one postdoctoral fellowship.