The current investigation presents a breakdown analysis of the elastoplastic behavior of commercially pure aluminum pre-strained via severe plastic deformation (SPD) and tested in tension. The tensile samples selected, owing to their prior SPD, had the gradient microstructure from the fragmentation stage as well as a homogeneous equiaxed structure from the steady-state regime. Except for the regions of low deformation and steady-state of SPD, the microstructures of pre-strained material were largely dominated by the presence of geometrically necessary boundaries. During tensile straining, dislocation strengthening contributed more to the overall material strength in fragmentation stage samples, while grain boundary strengthening played a major role in the steady-state stage samples. The dislocation density evolution rate-based approach predicted a more active role for the dynamic annihilation/recovery events in the SPD material when compared to the fully recrystallized condition. This could explain the drastic drop in tensile elongation of the pre-strained material as well as the enhancement in uniform and post-necking elongation with the gradual increase in the amount of pre-strain. A plastic instability condition based on the dislocation density evolution approach successfully accounted for the observed stable deformation limits in the coarse- and fine-grained material.

Severe plastic deformation imposed under high hydrostatic pressure introduces a considerable dislocation substructure in metals from the early stages of deformation, ultimately resulting in grain fragmentation. Characterization and quantification of the substructure require methods with a sufficiently high angular and spatial resolution to reveal the local heterogeneities in orientation differences and the length scales of the substructure. However, the statistical relevance of the observations should be assured which requires relatively large fields of view. In present work, the evolution of dislocation substructures during static and dynamic high pressure torsion processing of commercially pure aluminum is examined. Orientation data obtained by electron backscatter diffraction using two different mapping step sizes are utilized to assess the detection of the dislocation substructures and boundaries during the grain fragmentation stage. Accumulation of distortion in the crystal produces an increase in measurement noise at each pixel which is estimated using Kamaya's plots. The storage of dislocations and related angular misfits reduces the peak height of the probability density distribution of misorientation gradients, moves the peak to higher misorientation gradients and widens the distribution. Superposition of double Rayleigh distributions over the combined dislocation boundary data predicts a slightly higher median for the frequency of geometrically necessary boundaries and larger misorientation gradients across these boundaries in dynamically deformed material. In incidental dislocation boundaries, higher misorientation gradients are only observed at lower equivalent strains. Buildup of shear strain leads to the deterioration in the quality of the fitting to a double Rayleigh distribution and is linked to the complex evolution pattern of the dislocation boundaries. Finally, in statically deformed material, anisotropy in the substructure evolution is observed in the shear and radial planes.

Manufacturing of ultrafine-grained (UFG) or nanocrystalline (NC) metals via a single step top-down approach imposing severe plastic deformation (SPD) is one of the most promising ways to achieve superior properties such as high strength and superplastic forming capability. Nonetheless, the lack of relevant data on their post-SPD performance in different test environments makes it difficult to fully understand their mechanical behavior. While characterizing the tensile behavior, almost all of the previous reports are limited to the discussion on the plastic performance of the material in terms of elongation to failure and corresponding strength, with only a few studies discussing the effect of grain fragmentation on work hardening response of the material. In the present work, a comprehensive analysis is presented in terms of the uniform and post-necking mechanical behavior of the ultrafine-grained material. Commercially pure aluminum is subjected to high pressure torsion (HPT) deformation with strains ranging from very low levels (γ ≈ 2.1) to values well in the saturation regime (γ ≈ 25.1). When tested in uniaxial tension, the strength increases monotonously. The uniform elongation improves with the imposed HPT strain, though remains lower than the value of the initial material. Based on the slopes of the stress-strain curve, three distinct zones are identified, i.e. uniform deformation, post-necking-1, and post-necking-2. With accumulating SPD deformation, the material shows enhanced pre-necking strength and ductility; while post-necking material fails early and at lower strength levels. The post-necking response is observed to be highly microstructure dependent: a lower grain size augments the resistance for micro-crack propagation and thus the ductility, however, once initiated, a crack propagates much faster in fine-grained than in coarse-grained HPT processed material.

Severe plastic deformation (SPD) of metals to obtain ultra-fine or even nano-sized grains has proven to be an interesting concept explored over the last few decades. However, the mechanical behavior of SPD metals and the underlying microstructural phenomena are not fully understood yet. In present work, commercially pure aluminum was subjected to high pressure torsion (HPT) deformation with strains ranging from very low levels to values well in the steady-state microstructure regime. The mechanical properties of the HPT processed samples were determined using tensile tests on miniature samples using full-field strain mapping. Orientation imaging microscopy (OIM) was utilized to follow the progression of grain refinement and texture as a function of imposed SPD. Local orientation based misorientation gradients helped to perform statistical boundary analysis and determine the fractions of incidental and geometrically necessary dislocation (GND) boundaries and local GND densities. From probability density distributions of the misorientation gradients two different stages of microstructural evolution, namely, fragmentation and saturation, could be discerned. The strength increased monotonously and the uniform elongation, though lower than the value of the annealed material, enhanced with the imposed strain in HPT. The post-necking response was observed to be highly microstructure dependent, where a lower grain size augmented the resistance for micro-crack propagation and enhanced the elongation-to-failure. In addition, the work hardening response corresponding to the yield point displayed maxima coinciding with the onset of the saturation stage. Anisotropy in fracture strain, observed between the axial and radial directions in a disk-like HPT sample, reduced with the randomization of shear texture, while higher intensities of the C {100}<110> orientation was considered responsible for the lower elongation-to-failure along the radial direction.