An investigation of novel spraycast AI-Mg-Li-Zr-(Sc) alloys

This work describes the microstructure and properties of a range of Al-(4-6)Mg(1.2-1.6)Li-(0.3-0.4)Zr-(0-0.2)Sc alloys produced at Oxford University by spraycasting. Following hot isostatic pressing of the as-spraycast billets to close any porosity and to precipitate a fine, coherent dispersoid population, forging to a true strain of I at 250 and 400 degrees C led to a substantial refinement of the microstructure with grain sizes in the range 0.8 to 5 mu m. A large intra-granular orientation gradient with distance measured using EBSD showed that at 250 degrees C, partial dynamic recrystallisation by progressive lattice rotation led to a 'necklace' structure of very fine grains surrounding larger deformed grains. At 400 degrees C, dynamic recrystallisation occurred by nucleation of new grains at prior grain boundaries and triple points. The strength of as-forged alloys was 200-350MPa and the high ductilities of up to 30% rendered the alloys amenable to post-forging cold work. A proof strength of 460MPa with 9.5% elongation was achieved in a non-heat-treatable spraycast Al-6Mg-1.3Li-0.4Zr alloy, matching the best properties of similar mechanically alloyed AA5091, and exceeding the properties of AA7010-T74. The as-forged alloys showed excellent thermal stability up to similar to 0.9T(m), with no abnormal grain growth and grain size stagnation due to Zener pinning. Finally, strain rate sensitivity testing revealed the potential for superplasticity at 400 and 500 degrees C and strain rates of 0.001-0.05s(-1).