Report of International NanoSPD Steering Committee and statistics on recent NanoSPD activities

The Université de Lorraine in Metz, France, is the selected site for the 6th International Conference on Nanomaterials by Severe Plastic Deformation (NanoSPD6) following a series of five earlier conferences. This introductory paper reports on several major developments in NanoSPD activities as well as on very recent NanoSPD citation data which confirm the continued growth and expansion of this important research area. Close attention is given to the topics of workshops, conferences and seminars organized during these last three years as well as on books and reviews published prior to the NanoSPD6 conference. A special concern of the committee is in introducing and discussing the appropriate terminology to be applied in this new field of materials science and engineerin

Using finite element modelling to examine the flow process and temperature evolution in HPT under different constraining conditions

High-pressure torsion (HPT) is a metal-working technique used to impose severe plastic deformation into disc-shaped samples under high hydrostatic pressures. Different HPT facilities have been developed and they may be divided into three distinct categories depending upon the configuration of the anvils and the restriction imposed on the lateral flow of the samples. In the present paper, finite element simulations were performed to compare the flow process, temperature, strain and hydrostatic stress distributions under unconstrained, quasi-constrained and constrained conditions. It is shown there are distinct strain distributions in the samples depending on the facility configurations and a similar trend in the temperature rise of the HPT workpiece

Superplasticity of a nano-grained Mg-Gd-Y-Zr alloy processed by high-pressure torsion

While most of the reports on Mg-Gd-Y-Zr alloys report superplasticity after extrusion or friction stir processing, it is important to investigate superplasticity in these alloys after other severe plastic deformation processes having greater grain refinement capability. Accordingly, superplasticity was studied in an Mg–9Gd–4Y–0.4Zr (GW94) alloy after different high-pressure torsion (HPT) conditions. The HPT was performed at room temperature under an applied pressure of 6.0 GPa for up to 16 turns. TEM microstructural characterization revealed that the grain size was reduced from an initial value of ?8.6 ?m in the extruded condition to ?95±10 and ?85±10 nm after 8 and 16 turns, respectively. A shear punch testing method was used for evaluation of superplasticity at 573, 623, 673 and 723 K. Maximum strain rate sensitivities of ?0.51±0.05 and ?0.48±0.05 were obtained at 623 K for the material processed through 16 and 8 turns, respectively. This strain rate sensitivity and an activation energy of ?100±5 kJ mol–1 suggests the occurrence of grain boundary sliding in the superplastic regio

Effect of Long-Term Storage on Microstructure and Microhardness Stability in OFHC Copper Processed by High-Pressure Torsion

Tests are conducted to evaluate the effect of long-term storage on the microstructure and microhardness of an oxygen-free high conductivity (OFHC) copper after processing by high-pressure torsion (HPT) for various numbers of revolutions at ambient temperature. Results are presented for samples subjected to storage at room temperature through periods of either 1.25 or 7 years. The results show that an increase in storage time leads to a coarsening of the ultrafine-grained structure produced by HPT processing and a corresponding decrease in the microhardess where this is associated with the occurrence of recrystallization and grain growth. Plots of hardness against equivalent strain reveal a three-stage behavior with much lower hardness values over a range of equivalent strains of ~2-8. This behavior is similar after both storage periods but the hardness values are lower and the grain sizes are larger after storage for the longer time. The results demonstrate that long-term storage has a significantly detrimental effect on the microstructure and hardness of ultrafine-grained OFHC Cu

An investigation of the thermal stability of an Mg-Dy alloy after processing by high-pressure torsion

An Mg-0.41Dy (wt%) alloy was successfully processed by high-pressure tension (HPT) through 5 turns at room temperature. The evolution of the recrystallization microstructure and the texture and mechanical properties of the deformed alloy were investigated after annealing at 200 and 400 °C for 1 h using Electron Backscatter Diffraction (EBSD) and Vickers measurements. The recrystallization temperature and activation energy were evaluated using Differential Scanning Calorimetry (DSC). Processing by HPT led to significant grain refinement with an average grain size of ~0.5 ± 0.1 μm which increased to ~1.2 ± 0.8 μm after annealing at 400 °C. This slow increase in grain size at a high temperature demonstrates a good thermal stability of the microstructure. The alloy exhibited two main fiber textures after HPT processing: firstly a typical basal fiber (φ1 = 0–360°, Φ = 0° and φ2 = 0–60°) and secondly a fiber localized at φ1 = 180°, Φ = 60° and φ2 = 0–90°. These textures were retained after annealing at 400 °C. There was no change in the microhardness value after annealing at 200 °C (41 ± 1 Hv) and only a minor decrease after annealing at 400 °C (38.4 ± 0.5 Hv). The DSC results showed that the temperature associated with the recrystallization process increased with increasing heating rate and the activation energy for recrystallization was measured as ~25 kJ mol−1

An investigation of strain softening phenomenon in Al-0.1% Mg alloy during high-pressure torsion processing

An Al-0.1% Mg alloy was processed by high-pressure torsion (HPT) at room temperature. The Al-0.1% Mg alloy displays strain softening phenomenon through hardness evolution: the hardness values in the disc centre area are higher than at the disc edge area after 1/2, 1 and 3 turns, and the size of the hard region in the disc centre gradually reduces as the numbers of turns increases from 1/2 to 3 turns. The hardness values evolve towards homogeneity along the disc diameters after 5 and 10 turns. Electron backscatter diffraction (EBSD) and X-ray line profile analysis suggest that the lower hardness values at the disc edge area in the Al-0.1% Mg alloy are related to a recovery / recrystallization mechanism where the material is subjected to heavy straining

Recrystallization in an Mg-Nd alloy processed by high-pressure torsion: a calorimetric analysis

Differential scanning calorimetry (DSC) was used to evaluate the recrystallization temperature and activation energy for an Mg-1.43Nd (wt.%) alloy after severe plastic deformation by high-pressure torsion (HPT) at room temperature up to 10 turns. The recrystallization kinetics were determined from DSC analysis. The results show that the recrystallization temperature increases with increasing heating rate and decreases with increasing numbers of HPT turns. Severe plastic deformation by HPT significantly reduces the recrystallization temperature. The estimated activation energy for recrystallization was in the range of ~ 84-89 kJ mol-1

On the microstructure and mechanical properties of an Fe-10Ni-7Mn martensitic steel processed by high-pressure torsion.

High-pressure torsion (HPT) processing was applied to an Fe-10Ni-7Mn (wt.%) martensitic steel at room temperature and the grain size was reduced from an initial value of ~5.5 μm to an ultrafine value of ~185 nm for the ferritic phase and around 30 nm for the austenitic phase after 20 HPT turns. The microstructure and mechanical properties of the as-processed material were evaluated using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), field emission scanning electron microscopy (FESEM), microhardness measurements and tensile testing. In addition, annealing of an as-processed specimen was analyzed by differential scanning calorimetry (DSC). The results show that HPT processing increases the hardness and ultimate tensile strength to ~690 Hv and ~2230 MPa, respectively, but the ductility is decreased from ~16.5% initially to ~6.4% and ~3.1% after 10 and 20 turns, respectively. The hardness distributions and EBSD images show that a reasonably homogeneous microstructure is formed when applying a sufficient level of pressure and torsional strain. The DSC results demonstrate that processing by HPT reduces the start and finish temperatures of the reverse transformation of martensite to austenite and there is continuous re-crystallization after the recovery process

Effect of grain size and crystallographic structure on the corrosion and tribocorrosion behaviour of a CoCrMo biomedical grade alloy in simulated body fluid

CoCrMo alloys are used in hip and knee replacements due to their excellent long-term survival rates. However, high failure rates have recently been observed associated with adverse tissue reactions. CoCrMo alloy surfaces undergo microstructural changes during wear, including the formation of ε-martensite and, occasionally, a nanocrystalline surface layer. It is not clear whether these changes are beneficial or detrimental to the performance of the component. Thus, high-pressure torsion (HPT) was employed to produce different grain sizes and crystallographic structures in a CoCrMo alloy and the corrosion and tribocorrosion behaviour were critically investigated as a function of grain size. The results reveal a degradation of the corrosion resistance for the HTP processed samples. The contributions of mechanical and corrosion material loss in tribocorrosion is also examined