Development and characterizationof high solid solution Al-Mg alloysand an immiscible Al-Bi alloyprocessed by ECAP and annealing

Abstract

The thesis is based on five papers, covering two topics: papers 1-4 mainly focus on the development and characterization of high solid solution Al-Mg alloys by room temperature ECAP and annealing, their microstructure evolution during ECAP and mechanical properties as well as the annealing response and the processing-microstructure-properties relations of the processed Al-Mg alloys; paper 5 was initiated as a first step to investigate the influence of ECAP on the size, shape and dispersion of soft particles and the role played by soft particles on the grain refinement of hardenable matrix in a hypermonotectic Al-8Bi alloy during ECAP.

The work in papers 1-4 demonstrated that room temperature ECAP combined with inter-pass annealing was an efficient methodology to process Al-7Mg alloy, which is efficient in promoting significant grain refinement and more importantly, gaining superior strength and ductility simultaneously, i.e., the presently developed Al-7Mg exhibited a superior tensile strength and ductility, i.e., UTS ~600 MPa and uniform elongation ~12%. Delicate microstructure characterization revealed that an average grain size down to ~80 nm was achieved in the Al-7Mg after multiple ECAP passes and inter-pass annealing. Though dominated by nano- and ultrafine grains, a few micron-sized grains still persisted in the final microstructure, i.e., revealing the formation of a bimodal grain structure. This nanostructured bimodal Al-7Mg alloy is particularly attractive in view of their high strength and reasonably large ductility and in a recycling perspective. Although the present result was obtained using a binary Al-7Mg as a representative model system, one can anticipate the same approach will be equally effective in other high solid solution engineering alloys containing or free of particles/dispersoids, provided that optimal annealing parameters and proper content of solid solutions were used.

In paper 5, room temperature ECAP at moderate strain (~4-5) was shown to be very effective both to disperse the soft secondary phase Bi particles and to refine the Al-matrix grain size in an as-cast Al-Bi hypermonotectic alloy. The morphologies, spatial and size distribution of soft Bi particles are modified substantially via the combination of coarsening, coalescence and fragmentation, meanwhile the presence of soft particles accelerates grain refinement of the Al matrix. The present work opens the possibility to the commercial production of monotectic aluminium alloys dispersed with soft phase X (X: Pb/Bi/Sn).