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Severe plastic deformation in micrometer scale sample volumes is investigated by performing large strain machining inside a scanning electron microscope. Digital image correlation of in situ secondary electron images of the deformation zone is used to measure the mechanics of deformation. The effect of length scales on microstructure evolution is characterized using orientation imaging microscopy/electron backscattered diffraction analysis of the deformation zone. A geometrically necessary switchover in microstructure evolution mechanisms in small length scales is identified and attributed to a coupled effect of geometric confinement and high strain gradients. A phenomenological model capturing the switchover is proposed. It is shown that contrary to conventional perception, a high crystallographic curvature caused by strain gradients impedes grain refinement in small length scales, resulting in an anomalous suppression of grain refinement and fragmentation. [ABSTRACT FROM AUTHOR]

Abstract: Severe plastic deformation of titanium at near-ambient temperature has been realized using large-strain machining. The material that received the highest levels of strain, the chip, is found to have a nanocrystalline structure with ∼100nm sized grains. This microstructure is finer than that observed in titanium following equal channel angular pressing at elevated temperatures. The machined surface of the workpiece is found to have a graded microstructure composed of ultrafine grains at and near the surface and a preponderance of twins further into the subsurface. The parameters of the large-strain deformation field, namely strain rate and strain, are obtained using an adaptation of a particle image velocimetry technique. The microstructure characteristics are shown to be a consequence of the deformation field parameters, which activate both twinning and dislocation-mediated mechanisms of plastic flow. [Copyright &y& Elsevier]

Abstract: We present the properties of aluminum 6061 alloys of various tempers, severely deformed in plane strain at room temperature, by machining. Various values of strain are introduced into the chip, in a single pass, by varying the rake angle of the tool. Chips cut from peak-aged 6061 (T6 condition) are composed of finer microstructures and possess higher hardness than peak-aged 6061 subjected to equal channel angular pressing at elevated temperatures. Thermal stability of the chips with different levels of strain is analyzed by studying evolution of Vickers micro-hardness and microstructure after different heat treatments. Chips produced from the peak-aged temper and over-aged temper soften following heat treatment while those from the solution-treated state first gain strength before softening. The results are rationalized based on prior studies of the characteristics and kinetics of precipitation and coarsening in Al–Mg–Si systems. The observations also suggest processing routes for consolidation of the chips into bulk forms while retaining the UFG microstructure with enhanced mechanical properties. [Copyright &y& Elsevier]

The emergence of shear inhomogeneity across grain boundaries during severe shear deformation in Cu at the micrometer length-scales is demonstrated through in situ imaging inside a scanning electron microscope. The deformation mechanics during machining is found to be orientation dependent. Microstructure refinement from severe plastic deformation in confined volumes is entirely suppressed in certain orientations but results in a characteristic ultrafine structure in others. Rampant ductile fracture is found to nucleate at some grain boundaries due to elevated dislocation densities, and this failure mechanism is imprinted on the freshly generated surface, which limits the precision of machining-based processes (e.g., diamond turning) in nanometrically-smooth roughness profiles. Unlabelled Image • The homogeneity of severe plastic deformation at micrometer length-scales is orientation-dependent. • The deformed structure from large shear strains spans from ultrafine grains to that entirely unrefined. • The orientation of the loading determines the kinetics of dislocation annihilation and the resulting microstructure. • Ductile fracture due to shear inhomogeneity across grain boundaries limits the integrity of inherited surfaces. • Fundamental limits emerge on the suitability of technologies such as diamond turning in fabricating optical components. [ABSTRACT FROM AUTHOR]

The aliphatic spacer length connected to the rigid mesogenic cores is shown to control the structural organization and mechanical actuation in response to light-stimulus in azobenzene-functionalized liquid crystalline polymers networks. The spacer lengths in the mesogenic host (non-photochromic) and in the photo-active azobenzene-functionalized cross linker are parametrically varied to create monodomain liquid crystalline samples. A suite of thermomechanical, photomechanical and structural characterization is used to characterize the binary co-polymers. The photomechanical responses are compared by calculating a figure-of-merit – Photocompliance (Cp). This parameter, Cp, which characterizes the incremental strain generated in response to unit intensity of irradiation (W/cm 2 ), is found to correlate strongly with the S xray order parameter and the D-spacings. We show that compact (smaller D) and higher ordering of the mesogens that result in copolymers with longer flexible spacers manifest greater photomechanical activity. [ABSTRACT FROM AUTHOR]