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

Microstructure and texture characterization of Mg–Al and Mg–Gd binary alloys processed by simple shear extrusion

Microstructural and textural evolutions of pure Mg, Mg–2 wt% Al, and Mg–2 wt% Gd were investigated after extrusion and simple shear extrusion (SSE). Microstructural studies revealed that the grain size of all extruded samples decreased after 4 passes of SSE at 553 K (280 °C). In the fine-grained Mg–2Gd alloy, however, a duplex structure consisting of fine recrystallized grains and coarse unrecrystallized patches was formed. Although the 2.5 μm size of the recrystallized grains did not experience a significant change, the volume fraction of the coarse 15 μm-wide unrecrystallized patches decreased, and the overall microstructural homogeneity was improved after 4 passes of SSE. In Mg–2Gd, fragmentation of large grains and unrecrystallized patches encouraged the formation of HAGBs, while Mg–2Al did not experience significant changes in the fraction of HAGBs. Contrary to pure Mg and Mg–2Al alloy, Mg–2Gd alloy developed a new “rare earth texture component” with the 〈1 1 2¯ 1〉 direction parallel to the extrusion direction in the extruded condition. However, all three materials developed the conventional “extrusion texture” after SSE. Keywords: Severe plastic deformation, Simple shear extrusion, Electron backscattered diffraction, Textur