EBSD Characterisation of the microstructure of metals following hot working
Prof. Mark Rainforth
IMMPETUS, Department of Engineering Materials, The University of Sheffield
17:15 - 18:15 Tuesday 26 January 2010 TUG P2 W.M. Rainforth, P. Cizek, F. Bai, E.J. Palmiere, P. Davies, B.P. Wynne, D. Randman
The development of physically-based models of microstructural evolution during the hot working of metals requires knowledge of the internal state variables, such as microstructure, texture and dislocation substructure characteristics, over a range of deformation conditions. Characterisation of this microstructural evolution has traditionally been undertaken using TEM. However, TEM is not only very time consuming, but also samples a very small amount of material. The problem remains-the structure at the nanometer level controls the properties of the macroscopic material; but is the structure we measure in the TEM representative of the structure in the whole material, often with tonnages of material? This problem is often ignored, but the advent of high resolution electron back scatter diffraction (EBSD) means that statistically meaningful amounts of material can be assessed, but with nanometer scale resolution. The talk will look at three case studies, steels, titanium and magnesium alloys:
1) In steels, deformation is undertaken in the austenite phase field, but transformation of the austenite to a variety of transformation products during cooling eradicates the hot deformed microstructure. A model Fe-30wt%Ni based alloy, which retains a stable austenitic structure at room temperature, will be reported. Evolution of the microstructure and crystallographic texture was characterised in detail using EBSD and some comparison with TEM will be provided.
2) The process route for near-α titanium alloy forgings determines the microstructures and textures observed in the final product, which has implications for properties such as creep and fatigue. The microstructural evolution of Timetal 834, hot worked in the α+β phase field, will be presented. Again, understanding is difficult because the high temperature β phase, which is present in large volumes at hot working temperatures, is lost through transforms on subsequent cooling. A key aspect of this work is the development of a computer program which allows the reconstruction the microtexture of the parent β phase prior to the transformation by analysing EBSD maps of the transformed structure.
3) Elektron 675 is a new alloy based on the magnesium-yttrium-gadolinium ternary system, developed by Magnesium Elektron Ltd. Relatively low strains initiates a significant amount of dynamically recrystallised grains which form a necklace structures around the grain boundaries. The texture obtained from the non-recrystallised regions showed a typical rolling texture common in magnesium alloys, with the c-axis aligned to the normal direction; however the texture taken from the recrystallised grains showed a strong randomisation of orientations. The dominant mechanism of dynamic recrystallisation appears to be 'Continuous Dynamic Recrystallisation by Progressive Lattice Rotation'
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