Inhomogeneous microstructural evolution of pure iron during high-pressure torsion

A Bridgman anvil apparatus (unconstrained high-pressure torsion configuration) was used to shear samples of ultra high purity iron (99.99999%) in torsion under pressure (0.5 GPa). The evolution of the resulting microstructure was characterized by focused ion beam imaging (FIB), electron backscattered diffraction mapping (EBSD) and automated crystal orientation mapping in a transmission electron microscope (ACOM-TEM). The initial large grains (100 μm) were fragmented first; with the resulting deformation causing low angle dislocation boundaries followed by a gradual increase in their misorientation, finally leading to the formation of fine structural or microstructural elements (substructure) with high misorientations. These final structural elements (about 220 nm) could be considered as grains, if viewed from the conventional terms to describe grain (as that being entirely delimited by continuous HAB). One possible mechanism of formation could be continuous dynamic recrystallization. Furthermore deformation caused by the tests led to continuous misorientation in the grains whereas a significant continuous misorientation gradient in the grains was highlighted. The formation of predominant sub-micronic crystalline structure resulted in a significant increase in strength while good ductility was maintained. At a microscopic scale there was evidence that the refinement first occurred in the near surface layer of the sample and then propagated through the whole volume with a necessary condition of adhesion at the interfaces sample/anvils.

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► Three techniques (FIB, EBSD, and ACOM-TEM) are used to characterize highly deformed high purity iron grade.
► Fine microstructure is obtained at low pressure (0.5 GPa), after 1 rev. with the unconstrained HPT.
► Fine microstructure generation is correlated with contact conditions.
► Microstructural evolution mechanism is assimilated to continuous dynamic recrystallisation.
► Significant continuous misorientation gradient in the grains is highlighted.