Strain weakening and superplasticity in a Bi-Sn eutectic alloy processed by high-pressure torsion

High-pressure torsion (HPT) was conducted on disks of a Bi-Sn eutectic alloy under a pressure of 6.0 GPa. The microstructural evolution was studied by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Measurements of Vickers microhardness showed decreasing strength caused by strain weakening after HPT processing. Tensile testing was performed under initial strain rates from 10?4 to 10?2 s?1 at room temperature. The results demonstrate a much improved elongation to failure for the Bi-Sn alloy after HPT- processing. The Bi-Sn alloy processed through 10 turns gave an elongation to failure of more than 1200% at an initial strain rate of 10?4 s?1 at room temperature which is significantly larger than the elongation to failure of ~110% in the as-cast Bi-Sn alloy under the same tensile condition

Shape memory effect of NiTi alloy processed by equal-channel angular pressing followed by post deformation annealing

Processing by Equal-Channel Angular Pressing (ECAP) is generally considered superior to most other SPD techniques because it uses relatively large bulk samples. However, due to their low deformability it has proven almost impossible to successfully process NiTi alloys by ECAP at room temperature and therefore the processing is conducted at elevated temperatures. Recently, a new billet design was introduced and it was used to achieve the successful processing of NiTi shape memory alloys by ECAP. In this procedure, a NiTi alloy was inserted as a core within an Fe sheath to give a core-sheath billet. In this research, a NiTi was processed by one pass ECAP with this new billet design at room temperature. The structural evolution during annealing was investigated by X-ray diffraction (XRD) and microhardness measurements. Post deformation annealing (PDA) was carried out at 400°C for 5 to 300 min and the results indicate that the shape memory effect improves by PDA after ECA

The significance of self-annealing in two-phase alloys processed by high-pressure torsion

The Zn-22% Al eutectoid alloy and the Pb-62% Sn eutectic alloy were processed by high-pressure torsion (HPT) over a range of experimental conditions. Both alloys exhibit similar characteristics with significant grain refinement after processing by HPT but with a reduction in the hardness values by comparison with the initial unprocessed conditions. After storage at room temperature for a period of time, it is shown that the microhardness of both alloys gradually recovers to close to the initial unprocessed values. Electron backscatter diffraction (EBSD) measurements on the Pb-Sn alloy suggest that the self-recovery behaviour is correlated with the fraction of high-angle grain boundaries (HAGBs) after HPT processing. Thus, high fractions of HAGBs occur immediately after processing and this favours grain boundary migration and sliding which is important in the self-annealing and recovery process. Conversely, the relatively lower fractions of HAGBs occurring after annealing at room temperature are not so conducive to easy migration and slidin

Microstructures and mechanical properties of pure tantalum processed by high-pressure torsion

A body-centred cubic (BCC) structure metal, tantalum, was processed by high- pressure torsion (HPT) at room temperature with different numbers of rotations. The microstructural evolution was studied by electron backscatter diffraction (EBSD). The grain sizes were significantly refined at the disk edge area in the early stages of deformation (N = 0.5) but tended to attain saturation after the numbers of rotations was increased to N = 5. As the deformation continued, some coarse grains appeared in the disk edge areas and it appeared that there was structural recovery at the expense of grain boundary migration in the tantalum during HPT processing. Microhardness measurements showed the hardness gradually evolved towards a more homogenized level across the disk surfaces as the numbers of rotations increased. The hardness level after N = 10 turns was slightly lower than after N = 5 turns, thereby indicating the occurrence of a recovery process after 5 turn

Developing ultrafine-grained materials with high strength and good ductility for micro-forming applications

Materials with ultrafine grain sizes are attractive for use in micro-forming operations. High-pressure torsion (HPT) is an effective technique that allows disc materials subjected to torsional deformation to generate microstructures with significant grain refinement. Generally, ultrafine-grained materials exhibit high strength but their ductility is limited because they have both a low rate of strain hardening and a low strain rate sensitivity. A special processing route by utilizing HPT and short term annealing at different temperatures was developed to obtain balanced high strength and good ductility in an ultrafine-grained Al-1% Mg alloy which will increase the production rate and quality in micro-forming

Investigating anvil alignment and anvil roughness on flow pattern development in high-pressure torsion

High-pressure torsion (HPT) is a processing technique in which samples are subjected to a high pressure and torsional straining. Anvil alignment and anvil roughness are two important factors related to the successful application of the HPT processing technique. Using a two-phase duplex stainless steel as a model material, experiments were conducted by placing the anvils in different amounts of initial misalignment. Experiments show that the flow patterns (the development of double-swirl patterns) in HPT are dependent upon the alignment of the anvils within the HPT facility. Through carefully designed experiments, it is shown that the presence of a double-swirl is a feature of HPT processing when the initial positions of the anvils have a small lateral misalignment. The effect of the double-swirl patterns on the hardness evolution was also evaluated quantitatively. By comparing the flow patterns developed on the disc upper surface using both rough and smooth anvils with a fixed anvil misalignment, it was demonstrated that there are some differences in the flow patterns which are dependent upon the anvil surface roughness

The mechanical properties of ultrafine-grained metals at elevated temperatures

Ultrafine-grained materials, having grain sizes in the submicrometer or nanometer range, may be readily produced by processing bulk solids through the application of severe plastic deformation (SPD) and this leads to the possibility of revealing different flow mechanisms when these materials are tested at elevated temperatures. Experiments show the two-phase Zn-22% Al alloy and various magnesium alloys exhibit excellent superplastic properties after processing by SPD whereas it is not possible to reveal different creep mechanisms in high-purity aluminum because the ultrafine grains are unstable at high temperature

Structural impact on the hall-Petch relationship in an Al-5Mg alloy processed by high-pressure torsion

The evolution of microstructure and microhardness was studied in a commercial 5483 Al-5 Mg alloy processed by high pressure torsion (HPT) under a pressure of 6.0 GPa up to 10 turns. Significant grain size refinement was observed even after 1/4 turn and additional processing led to a further grain size reduction and a shift in the distribution of grain boundary misorientation angles towards higher values. An essentially fully homogeneous microstructure was reached after 10 turns with a final grain size of ~70 nm, a saturation Vickers microhardness of Hv?240 which was attained at and above equivalent strains of ~150, a relatively narrow grain size distribution and a fraction of ~80% of high-angle grain boundaries. Analysis shows the Hall-Petch plot deviates from the conventional linear relationship for samples processed through small numbers of turns but after 3 or more turns there is a direct correlation between the results obtained in HPT processing and coarse-grained sample

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

Evolution of microstructure and hardness in an AZ80 magnesium alloy processed by high-pressure torsion

An AZ80 magnesium alloy with an initial grain size of ?25 ?m and a hardness of Hv ? 63 was processed by high-pressure torsion (HPT) at room temperature for up to 10 turns under an imposed pressure of 6.0 GPa. After processing, the specimens were examined by optical microscopy and transmission electron microscopy and measurements were taken of the Vickers microhardness along diameters of the HPT discs. The results show that the grains are refined to ?200 nm after 5 and 10 turns of HPT and the hardness increases to Hv ? 120 at an equivalent strain of ?30. There is a saturation condition and no further hardening at additional equivalent strains up to >200