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

Comparisons of self-annealing behaviour of HPT-processed high purity Cu and a Pb–Sn alloy.

Early published results have demonstrated that high purity Cu and a Pb–62% Sn alloy exhibit very different behaviour during high-pressure torsion (HPT) processing at room temperature and subsequent room temperature storage. High purity Cu showed strain hardening behaviour with a refined grain structure during HPT processing whereas a Pb–62% Sn alloy displayed a strain weakening behaviour because the hardness values after HPT processing were significantly lower than in the initial as-cast condition even though the grain size was reduced. During room temperature storage after HPT processing, high purity Cu with lower numbers of rotations softened with the time of storage due to local recrystallization and abnormal grain growth whereas the Pb–62% Sn alloy hardened with the time of storage accompanied by grain growth. Through comparisons and analysis, it is shown that the low absolute melting point and the high homologous temperature at room temperature in the Pb–62% Sn alloy contribute to the increase in hardness with coarsening grain size during room temperature storage

Ultrafine grained plates of Al-Mg-Si alloy obtained by Incremental Equal Channel Angular Pressing : microstructure and mechanical properties

In this study, an Al-Mg-Si alloy was processed using via Incremental Equal Channel Angular Pressing (I-ECAP) in order to obtain homogenous, ultrafine grained plates with low anisotropy of the mechanical properties. This was the first attempt to process an Al-Mg-Si alloy using this technique. Samples in the form of 3 mm-thick square plates were subjected to I-ECAP with the 90˚ rotation around the axis normal to the surface of the plate between passes. Samples were investigated first in their initial state, then after a single pass of I-ECAP and finally after four such passes. Analyses of the microstructure and mechanical properties demonstrated that the I-ECAP method can be successfully applied in Al-Mg-Si alloys. The average grain size decreased from 15 - 19 µm in the initial state to below 1 µm after four I-ECAP passes. The fraction of high angle grain boundaries in the sample subjected to four I-ECAP passes lay within 53-57 % depending on the examined plane. The mechanism of grain refinement in Al-Mg-Si alloy was found to be distinctly different from that in pure aluminium with the grain rotation being more prominent than the grain subdivision, which was attributed to lower stacking fault energy and the reduced mobility of dislocations in the alloy. The ultimate tensile strength increased more than twice, whereas the yield strength - more than threefold. Additionally, the plates processed by I-ECAP exhibited low anisotropy of mechanical properties (in plane and across the thickness) in comparison to other SPD processing methods, which makes them attractive for further processing and applications

Application of high-pressure torsion to Al-Si alloys with and without scandium additions

Al-2 wt. % Si alloys with and without 0.25 wt. % scandium additions were processed by high-pressure torsion up to five turns at room temperature under a pressure of 6.0 GPa. Microstructural examination of the as-cast Al-2Si-0.25Sc alloy revealed the presence of Al3Sc precipitates which refined the Al grain structure, whereas no major changes were observed in the morphology of the Si particles. Processing by HPT of both experimental alloys revealed submicrometer grains with uniformly distributed Si particles. The mechanical properties were obtained using hardness measurements and the ball-indentation technique. The results show the hardness increased in the first turn of HPT and further increased with increasing numbers of turns. In addition, the hardness values were lower at the centers and continuously increased towards the edges of the disks. The difference in hardness values between the centre and the edge decreased with increasing turns, thereby suggesting an increasing homogeneity with increasing processing. The scandium addition and HPT processing of the Al-2Si alloy strongly influences the grain refinement and mechanical properties. The grain size reduction in the Al-2Si alloy was similar to Al whereas the presence of Sc in Al-2Si during HPT processing was responsible for large precipitation networks and a submicrometer grain formation

Influence of scandium on an Al-2% Si alloy processed by high-pressure torsion

High-pressure torsion (HPT) was used to process Al–2% Si and Al–2% Si–0.25% Sc alloys for up to five turns and the mechanical properties of the processed materials were evaluated using the ball indentation technique (BIT). The results show that the presence of Al3Sc precipitates is effective in producing higher strength levels and greater grain refinement in the Al–2% Si–0.25% Sc alloy. The introduction of scandium reduces the grain size of the Al–2% Si alloy from not, vert, similar0.38 to not, vert, similar0.15 ?m after 5 turns of HPT and the corresponding maximum tensile strength is increased from not, vert, similar325 to not, vert, similar375 MPa. The grain and substructure formation in the Al–2% Si alloy is similar to aluminum with dislocation cell formation and a reasonably recovered microstructure whereas in the Al–2% Si–0.25% Sc alloy it is non-homogeneous with arrays of non-equilibrium boundaries and dislocation tangles within the grain

A visualization of shear strain in processing by high-pressure torsion

Optical microscopy was used to examine the shear strain imposed in duplex stainless steel disks during processing by high-pressure torsion (HPT). The results show a double-swirl pattern emerges in the early stages of HPT and the two centres of the swirl move towards the centre of the disk with increasing revolutions. Local shear vortices also develop with increasing numbers of revolutions. At 20 revolutions, there is a uniform shear strain pattern throughout the disk and no local shear vortice

Elemental redistribution in a nanocrystalline Ni–Fe alloy induced by high-pressure torsion

An electrochemically deposited nanocrystalline supersaturated face-centred-cubic Ni–21 at.% Fe alloy with an initial average grain size of ?21 nm was processed using high-pressure torsion (HPT) that resulted in grain growth via grain rotation and coalescence to an average grain size of ?53 nm. Atom probe tomography investigations revealed that the supersaturated Ni–Fe solid solution was stable under HPT and that C and S atoms, which are the major impurities in the material and segregated to the grain boundaries (GBs) of the as-deposited material, migrated from disappearing GBs to the remaining GBs during HPT. We propose that the elemental redistribution was facilitated by GB diffusion and the motion of a large volume of HPT-induced defects at the GB regions during the grain growth process. This elemental redistribution process is different from other HPT-induced elemental redistribution processes reported in the literatur

Strain hardening and softening in a nanocrystalline Ni–Fe alloy induced by severe plastic deformation

The strain response of an electrochemically deposited nanocrystalline Ni–20 wt.% Fe alloy processed by high-pressure torsion (HPT) was investigated by monitoring changes in hardness. Strain hardening was observed in the very early stage of HPT, followed by strain softening before the onset of a second strain hardening stage. Structural investigations revealed that the two hardening stages were associated with an increase in dislocation density, whereas the strain softening stage was accompanied by a reduction in the dislocation and twin densities, thereby demonstrating the main dependence of hardness on the dislocation density in this material. Grain growth occurred during HPT and its role in the hardness evolution is also discusse