Thermal stability of submicron grain structure in an Al-Sc alloy.

Severe plastic deformation (SPD) has been used over the past few decades for producing submicron grain (SMG) structures in range of metalsand alloys. Equal channel angular pressing (ECAP) is a useful process for producing these types of structures whereby the material is deformed tovery high plastic strains by passing a billet several times through the ECAP die. This process has an added advantage maintaining the initialdimensions of the billet. SMG materials produced by ECAP and related routes are useful as they usually exhibit excellent properties including highstrength and hardness, and excellent superplastic formability: these and other properties make SMG materials useful for industrial and aerospaceapplications.In this thesis, a binary aluminium alloy containing a very low concentration of scandium (0.1 wt. %) Sc alloy was investigated and compared withhigher Sc-containing alloys. The material was deformed by ECAP in the solution treated condition to an equivalent von Mises strain of 9.2 thenpre-aged at 250 0C to generate a submicron grained material containing a relatively uniform dispersion of nanosized Al3Sc dispersiods. Thethermal stability of this pre-aged microstructure was investigated by annealing at temperatures up to 450 0C resulted in continuous graincoarsening by the process of continuous recrystallization whereby the initial microstructure evolves gradually with no marked change in the grainsize distribution, texture and grain boundary character. However, extended annealing (> 1h) at 4500 C resulted in discontinuous grain coarsening(often termed recrystallization) whereby a few grains grow rapidly to eventually produce a coarse-grained final microstructure.Throughout annealing, there was a good correlation between the dispersion parameter, (f/d) where f and d is the volume fraction and the meandiameter of Al3Sc particles in the alloy, respectively, and both the mean grain size (D ) and D /D max where max D is the maximum grain diameterobserved in the microstructure. The grain structure was found to undergo moderate coarsening at the high f/d-values but converted to a coarsegrainedstructure for f/d ~