Fundamental microstructural issues associated with severe plastic deformation: Applications of transmission electron microscopy

Erika Vanessa Esquivel, University of Texas at El Paso

Abstract

This study deals with the microstructural response of several metals and alloys to severe plastic deformation (SPD) in the form of shock wave loading, impact cratering, explosive welding, and ballistic penetration. Microstructural issues that will be addressed include dynamic recrystallization, adiabatic shear bands, and microbands and microtwins. Other relevant issues are stacking fault free energy (SFE), shock wave geometry and grain boundary contributions to the deformation response. ^ The study focuses mainly on the deformation behavior correlated from the microstructural response of nickel and 304 stainless steel, but the behavior of other metals and alloys such as aluminum, copper, brass, tungsten-tantalum and steel are also discussed. These metals cover a wide range of SFE in the face centered cubic systems (FCC) as well as body centered cubic (BCC) structures. There is an emphasis on the microstructure as seen through the transmission electron microscope (TEM) but this is complemented by light microscopy to provide a more global microscopic context. ^ Observations revealed that microtwins will form in planar shock wave treatment of FCC metals and alloys above a critical shock twinning pressure, which is itself a function of SFE. In hypervelocity impact craters, microbands will form for higher SFE materials such as Al, Ni, and Cu, whereas microtwins form exclusively in lower SFE material such as brass, and a combination of both microbands and microtwins will form in materials of intermediate SFE. Both SFE and shock wave geometry influence the material behavior in response to such dynamic processes such that SFE dictates the feasibility of cross-slip and the shock wave geometry, being planar promotes slip along primary slip planes while a spherical shock wave encourages cross-slip. In ballistic penetration it has been observed that overlapping shear bands, associated with dynamic recovery and recrystallization structures allow the penetrator to ‘flow.’ In all processes of SPD grain size refinement through dynamic recrystallization has been observed. The fine grain structure would allow the material to flow in the solid state. Microstructural issues were studied in an effort to understand the mechanisms of microstructural development and evolution in extreme plastic deformation, with special emphasis on understanding the solid state flow of material facilitated by grain boundary sliding and gliding. ^ This dissertation also examined the plastic deformation extremes in the context of the framework of the conventional stress-strain diagrams as well as the severe plastic deformation regime far outside the stress-strain diagram. However, the range of studies also provide a link between these deformation regimes to create a reasonably comprehensive understanding of extreme deformation of metals and alloys in general and FCC metals and alloys in particular. ^

Subject Area

Engineering, Metallurgy|Engineering, Materials Science

Recommended Citation

Esquivel, Erika Vanessa, "Fundamental microstructural issues associated with severe plastic deformation: Applications of transmission electron microscopy" (2005). ETD Collection for University of Texas, El Paso. AAI3179341.
http://digitalcommons.utep.edu/dissertations/AAI3179341

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