Microstructure and texture evolution during continuous recrystallisation of a commercial Al-Fe-Si alloy

• Main Author: Davies, R. K.
• Published: Swansea University 1997
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636391

The work contained in this thesis is based on a detailed investigation of continuous recrystallisation in commercial aluminium alloys. There are two main objectives to this work. 1. To examine the characteristic differences between discontinuous and continuous recrystallisation in terms of softening kinetics, development of microstructure and evolution of texture. 2. To elucidate the role of second-phase particles during subgrain growth and propose a theory to explain the mechanism of continuous recrystallisation. Three commercial Al-Fe-Si alloys were cold rolled to 80%-97% reductions and then annealed isochronally for 30 minutes at temperatures between 150°C and 400°C. The softening kinetics were determined by monitoring the evolution of microhardness, and microstructures were characterised by using colour optical microscopy. X-ray diffraction and electron backscatter diffraction were used to determine bulk texture and microtexture respectively. Transmission electron microscopy was used to assess subgrain and particle sizes and to examine the interaction of particles with subboundaries during subgrain growth. Continuous recrystallisation was only present in AA8079 after 97% cold reduction. Three characteristic features were observed with its occurrence; a very gradual softening of the material over a large temperature range, a continuous transformation of the substructure into a grain structure in the absence of nucleation and growth and a strengthening of the S and Cu rolling texture components. The alloy contained a distribution of both large and small FeAl<SUB>3</SUB> second-phase particles. The small particles which restricted growth by pinning subboundaries at lower temperatures subsequently coarsen at higher temperatures allowing homogeneous subgrain growth, the rate of which is controlled by the rate of particle coarsening. Conversely, the large particles did not act as sites for nucleation. In the present material, continuous recrystallisation is explained by two mechanisms: subgrain growth controlled by particle coarsening and normal grain growth commencing from a very fine grain structure.