Large-strain softening of aluminum in shear at elevated temperature

Pure aluminum deformed in torsion (shear) at elevated temperatures reaches a broad "peak" stress and then undergoes about a 17% decrease in flow stress with deformation to roughly 1-2 equivalent uniaxial strain. Beyond this strain the flow stress is approximately constant. The sources for this softening are unclear. The suggested basis includes texture softening, microstructural softening, and enhanced dynamic recovery. Experiments were performed where specimens were deformed in torsion to various strains within the softening regime followed by compression tests at ambient and elevated temperature. Analysis of the compressive yield strengths indicate that the softening is at least substantially explained by a decrease in the average Taylor factor due to the development of texture

Mechanical property evaluation of an Al-2024 alloy subjected to HPT processing

An aluminum-copper alloy (Al-2024) was successfully subjected to high-pressure torsion (HPT) up to five turns at room temperature under an applied pressure of 6.0 GPa. The Al-2024 alloy is used as a fuselage structural material in the aerospace sector. Mechanical properties of the HPT-processed Al-2024 alloy were evaluated using the automated ball indentation technique. This test is based on multiple cycles of loading and unloading where a spherical indenter is used. After two and five turns of HPT, the Al-2024 alloy exhibited a UTS value of ~1014 MPa and ~1160 MPa respectively, at the edge of the samples. The microhardness was measured from edges to centers for all HPT samples. These results clearly demonstrate that processing by HPT gives a very significant increase in tensile properties and the microhardness values increase symmetrically from the centers to the edges. Following HPT, TEM examination of the five-turn HPT sample revealed the formation of high-angle grain boundaries and a large dislocation density with a reduced average grain size of ~80 nm. These results also demonstrate that high-pressure torsion is a processing tool for developing nanostructures in the Al-2024 alloy with enhanced mechanical propertie

The evolution of homogeneity during processing of commercial purity aluminium by ECAP

Billets of a commercial purity aluminium Al-1050 alloy were processed by equal-channel angular pressing (ECAP) for up to a maximum of 6 passes. Following processing, the billets were sectioned and hardness measurements were recorded on both longitudinal and transverse sections. These measurements showed the hardness increases significantly in the first pass and continues to increase by small amounts in subsequent passes. Initially, there are regions of lower hardness running in bands near the top and bottom surface of each billet. The region of lower hardness near the upper surface disappears with increasing numbers of passes but near the bottom surface the lower hardness remains even after 6 passes. The results show that, neglecting the small region near the bottom of the billet, there is an excellent potential for achieving microstructural homogeneity within the Al-1050 alloy after pressing through a sufficient number of passes in ECAP

Processing of aluminium and titanium alloys by severe plastic deformation

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Microstructure and microhardness of OFHC copper processed by high-pressure torsion

An ultra-high purity oxygen free high conductivity (OFHC) Cu was investigated to determine the evolution of microstructure and microhardness during processing by high-pressure torsion (HPT). Disks were processed at ambient temperature, the microstructures were observed at the center, mid-radius and near-edge positions and the Vickers microhardness was recorded along radial directions. At low strains, ?3 twin boundaries are formed due to dynamic recrystallization before microstructural refinement and ultimately a stabilized ultrafine grain structure is formed in the near-edge position with an average grain size of ~280 nm after 10 turns. Measurements show the microhardness initially increases to ~150 Hv at an equivalent strain of ~2, then falls to about ~80 Hv during dynamic recrystallization up to a strain of ~8 and thereafter increases again to a saturated value of ~150 Hv at strains above ~22. The delay in microstructure and microhardness homogeneity by dynamic recrystallization is attributed to the high purity of Cu that enhances dislocation mobility and causes dynamic softening during the early stages of straining

Hardness homogeneity on longitudinal and transverse sections of an aluminum alloy processed by ECAP

Billets of a commercial-purity aluminum Al-1050 alloy were processed by equal-channel angular pressing (ECAP) at room temperature for up to six passes and microhardness measurements were recorded on the longitudinal and cross-sectional planes of each billet. Large numbers of datum points were recorded in order to minimize the errors in the results. The measurements show the hardness increases significantly after the first pass and then increases by very small amounts in subsequent passes. There are regions of lower hardness running in bands near the top and bottom surfaces of each billet. With increasing numbers of passes, the lower hardness region near the top surface disappears and the region near the lower surface remains in place but becomes less extensive. Neglecting the very small region of lower hardness near the bottom surface, the results show there is a potential for achieving excellent three-dimensional homogeneity after six passes of ECAP.<br/

The evolution of homogeneity during processing aluminium alloys by HPT

Disks of a commercial purity aluminium Al-1050 alloy and Al-1%Mg alloy were processed by high-pressure torsion (HPT) at room temperature for up to a maximum of 5 turns under a pressure of 6 GPa. Following processing, hardness measurements were recorded across the surfaces of the disks. These measurements showed low values of hardness at the center and high values near the edges of the disks and the hardness increased in both alloys with increasing numbers of turns. The evolution of homogeneity in hardness was rapid in Al-1050 compared to the Al-1%Mg alloy. After 5 turns of HPT under a pressure of 6 GPa, the hardness was fully homogeneous across the total surface of the Al-1050 disk whereas there was a region of lower hardness around the center of the Al-1%Mg disk. The results reveal the significant difference between both alloys where the higher rate of recovery in the Al-1050 alloy leads to a rapid evolution of the hardness homogeneity

The development of hardness homogeneity in pure aluminum and aluminum alloy disks processed by high-pressure torsion

Processing by high-pressure torsion was conducted on four different materials: high-purity (99.99%) aluminum, commercial purity (99.5%) aluminum, an Al-1% Mg solid solution alloy and a commercial aluminum Al-6061 alloy. Disks of each material were processed through 1/4, 1 and 5 turns and detailed microhardness measurements were recorded to permit the construction of color-coded hardness maps and three-dimensional representations of the hardness distributions. There are significant differences between these four materials. Whereas the hardness is initially high in the centers of the HPT disks of high-purity aluminum, the hardness is initially low in the centers of the disks for the other three materials. The hardness achieves saturation values after 5 turns in high-purity Al and the commercial purity Al but more torsional strain is required to achieve homogeneity in the other two alloys. There is evidence that it is difficult to achieve a well-defined saturation hardness in the Al-6061 allo