An Investigation of Novel Spraycast Al-Mg-Li-Zr-(Sc) Alloys

Abstract

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. Follow- ing hot isostatic pressing of the as-spraycast billets to close any porosity and to precipitate a ¯ne, coherent dispersoid population, forging to a true strain of 1 at 250 and 400±C led to a substantial re¯nement of the microstructure with grain sizes in the range 0.8 to 5¹m. A large intra-granular orientation gradient with distance measured using EBSD showed that at 250±C, partial dynamic recrystallisation by progressive lattice rotation led to a `necklace' structure of very ¯ne grains surrounding larger deformed grains. At 400±C, dynamic recrystallisation oc- curred 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 prop- erties of similar mechanically alloyed AA5091, and exceeding the properties of AA7010-T74. The as-forged alloys showed excellent thermal stability up to » 0.9Tm, 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±C and strain rates of 0.001-0.05s¡1.