An investigation was conducted to examine the mechanical behavior and microstructure evolution during deformation of ultrafine-grained pure magnesium at low temperatures within the temperature range of 296 -373 K. Discs were processed by high-pressure torsion until saturation in grain refinement. Dynamic hardness testing revealed a gradual increase in strain rate sensitivity up to m ≈ 0.2. High ductility was observed in the ultrafine-grained magnesium including an exceptional elongation of ~360% in tension at room temperature and stable deformation in micropillar compression. Grain coarsening and an increase in frequency of grain boundaries with misorientations in the range 15°  45° occurred during deformation in tension. The experimental evidence, when combined with an analysis of the deformation behavior, suggests that grain boundary sliding plays a key role in low strain rate deformation of pure magnesium when the grain sizes are at and below ~5 µm.

High purity copper was processed by high-pressure torsion (HPT) at room temperature and then stored at room temperature for periods of up to 6 weeks to investigate the effect of self-annealing. Hardness measurements were recorded both at 48 h after HPT processing and after various storage times. The results show the occurrence of recovery near the edges of the discs after processing through 1/2 and 1 turn and this leads to a significant drop in the measured hardness values which is accompanied by microstructural evidence for abnormal grain growth. Conversely, there was no recovery, and therefore no hardness drops, after processing through 5 and 10 turns. X-ray line profile analysis was used to determine the crystallite sizes and dislocation densities at 1 h after HPT and after storage for different times.The results show a good thermal stability in high purity Cu after processing through more than 1 turn of HPT but care must be exercised in recording hardness measurements when processing through only fractional or very small numbers of turns.

No abstract

An extruded ZK60 magnesium alloy was processed by high-pressure torsion (HPT) at room temperature for up to 5 turns under a constant compressive pressure of 2.0 GPa with a rotation speed of 1 rpm. This processing produced an average grain size of ~700 nm. The grain size distributions and textures were examined by electron backscatter diffraction (EBSD) and this revealed some multi-modality in the microstructure at different stages of straining with fractions of both coarse grains and ultrafine grains. EBSD analysis at the mid-radius positions of unprocessed and HPT-processed materials revealed a gradual evolution from a prismatic {101 ത 0} fiber to an ultimate basal {0001} fiber texture with the c-axis parallel to the normal direction.The majority of grain boundaries had misorientations larger than 15 o throughout the processing.The strain hardening tended towards a reasonable hardness homogeneity with a hardenability exponent, η, of 0.07 up to strains of ~20 and with a subsequent hardness saturation at Hv ≈ 125.

We examine the effect of grain size on the dynamic failure tantalum during laser-shock compression and release and identify a significant effect of grain size on spall strength,which is opposite the prediction of the Hall-Petch relationship: monocrystals have a higher spall strength than polycrystals, which, in turn, are stronger in tension than ultrafine grain sized specimens. Post-shock characterization reveals ductile failure which evolves by void nucleation, growth, and coalescence. Whereas in the monocrystal the voids grow in the interior, nucleation is both intra and intergranular in the poly and UFG crystals. The fact that spall is primarily intergranular in both poly and nanocrystalline samples is strong evidence for higher growth rates of intergranular voids, which have a distinctly oblate spheroid shape in contrast with intragranular voids, which are more spherical. Consistent with prior literature and theory we also identify an increase with spall strength with strain rate from 6x10 6 to 5x10 7 s-1. Molecular dynamics calculations agree with the experimental results and also predict grain-boundary separation in the spalling of polycrystals as well as an increase in spall strength with strain rate. An analytical model based on the kinetics of nucleation and growth of intra and intergranular voids and extending the