Executive Summary : | The list of commercially important alloys designed using precipitation strengthening of matrix is quite large. Significant amount of effort is spent in designing these alloys often with small size, narrow size-range and uniform distribution. However considering the solid state nature of the process and kinetic limitations, absolute control on these parameters are quite rare. Recent approach of using multistage heat treatment of alloys that offer more than one type of precipitate has shown extremely encouraging results. The approach involves sequential precipitation of different structure on top of each other resulting in a core-shell structure of the precipitate. These precipitates have been found to be extremely coarsening resistant even up to 70% of the matrix melting temperature and can result in almost doubling of material strength due to finer size distribution. The idea is similar to that of size focusing using continuous injection of monomer in colloidal chemistry. However its equivalent in solid state transformations is not available until recently when matrix saturated with more than one alloying element is cooled down to precipitate one of the elements while keeping another dissolved followed by further cooling to reject second element which acts as monomer injection process as long as the new phase forms at interface of previous precipitate and matrix. Over last decade, multiple studies have shown this beyond doubt that optimum combination of chemistry, precipitate and heat treatment routine can be used to generate these core shell precipitates with extremely narrow size distribution. In this work we propose to use this approach on identifying other possible candidates for such core-shell precipitate formation in addition to one already explored which are primarily based on Al-Li-Sc alloy. The identified alloys will be optimized for chemistry and heat treatment to obtain core-shell structure in these alloys. These precipitates will be extensively characterized high-resolution imaging, diffraction and analytical techniques. The results obtained with be compared with existing data to obtain clue about key mechanisms that promote and retard the formation of these desirable core-shell structures using solid state transformation process. These information will then be used to design optimum alloy microstructure which will be tested for its mechanical response and bench marked against single stage heat treated alloy with conventional precipitate distribution. Although focus of this study will be on Al-Li based alloy systems due to availability of extensive data and its commercial relevance, however other alloys will also be explored which will be identified using available detail on possible mechanism resulting in such microstructures and the understanding gained during this study. Successful implementation of this work will be of direct interest to aerospace applications. |