High-Pressure Torsion for Pure Chromium and Niobium

Two kinds of body centered cubic (bcc) structure refractory metals, pure Cr and Nb, were subjected to severe plastic deformation through high-pressure torsion (HPT) under applied pressures of 2 and 6 GPa for 2, 3, 4 and 5 revolutions at room temperature. Vickers microhardness is plotted as a function of the distance from the disk center and equivalent strain. It is shown that all hardness values fall on a single curve when they are plotted against equivalent strain for both metals. Vickers microhardness increases with increasing equivalent strain at an early stage of straining and then reaches steady state with the grain size of 200–250 nm in Cr and 240–270 nm in Nb irrespective of the applied pressures. In the steady state, there is no changing in hardness even in applying further straining. Tensile and bending tests show that brittle fracture occurs in Cr but in Nb, the strength significantly increases with some ductility after HPT processing

Microstructures and Mechanical Properties of Pure V and Mo Processed by High-Pressure Torsion

Two body centered cubic (bcc) metals, V and Mo, were processed by high pressure torsion (HPT) at ambient temperature. Hardness variation as well as microstructural evolution was examined with strain under a pressure of 2 to 6 GPa. It was shown that the hardness increases with straining and saturates to a constant level with the grain size of 330–400 nm in V irrespective of the applied pressures. Although the hardness variation with strain is the same for Mo with the grain size of ∼350 nm at the saturation level when the applied pressure is 6 GPa, the hardness level lowers below the saturation level and the grain size becomes coarser as the pressure is lowered. Tensile tests show that the strength significantly increases with some ductility for V after processing under any pressure and for Mo under lower pressures, but brittle fracture occurs in the Mo specimen processed at 6 GPa. The slower evolution of microstructure as well as the lower hardness levels observed in Mo is due to the applied pressure which is lower than the yield stress and thus due to the insufficient generation of dislocations for grain refinement

Grain Refinement of AZ31 and AZ61 Mg Alloys through Room Temperature Processing by UP-Scaled High-Pressure Torsion

This study presents application of an up-scaled high-pressure torsion (HPT) process to AZ31 and AZ61 Mg alloys for ultrafine grain refinement. Disks with 30 mm diameter were processed at room temperature under 6 to 7 GPa using the up-scaled HPT facility with a maximum capacity of 5 MN (500 ton). Microstructural evolution was evaluated by hardness measurement and microscopy observations including tensile testing. The grain size was well refined to ∼150 nm and ∼100 nm at the saturated state for the AZ31 and AZ61 alloys, respectively. Superplastic elongations of ∼520% and ∼550% were then attained in the corresponding alloys when tested in tension at elevated temperatures because of the grain boundary sliding controlled by grain boundary diffusion. Upsizing of the disk sample makes for a chance to extract the tensile specimens at different radial distance within the same disk and therefore the effect of the equivalent strain on the superplastic elongations was effectively evaluated

Softening and Microstructural Coarsening without Twin Formation in FCC Metals with Low Stacking Fault Energy after Processing by High-Pressure Torsion

Gold (Au), silver (Ag) and copper (Cu) were severely deformed through the process of high pressure torsion (HPT). The grain sizes were reduced to the submicrometer range and the hardness increased significantly by the HPT process. However, after holding at room temperature, Au and Ag exhibited grain growth and thus softening without heating. The softening and microstrcutural coarsening occurred rather quickly in Ag and moderately in Au but nothing changed in Cu even after keeping for prolonged time. No twins were formed along with the grain growth in the HPT-processed Au and Ag. High density lattice defects and enhanced diffusivity in the metals are responsible for such an unusual softening behavior

Achieving highly strengthened Al-Cu-Mg alloy by grain refinement and grain boundary segregation

An age-hardenable Al-Cu-Mg alloy (A2024) was processed by high-pressure torsion (HPT) for producing an ultrafine-grained structure. The alloy was further aged for extra strengthening. The tensile strength then reached a value as high as ~1 GPa. The microstructures were analyzed by transmission electron microscopy and atom probe tomography. The mechanism for the high strength was clarified in terms of solid-solution hardening, cluster hardening, work hardening, dispersion hardening and grain boundary hardening. It is shown that the segregation of solute atoms at grain boundaries including subgrain boundaries plays a significant role for the enhancement of the tensile strength

Severe Plastic Deformation under High Pressure: Upsizing Sample Dimensions

It is well known that severe plastic deformation (SPD) produces ultrafine-grained structures in bulk metallic materials. The SPD process becomes more versatile when it is performed under high pressure as high-pressure torsion (HPT) and high-pressure sliding (HPS). Not only the grain size is more refined but also the process is applicable to hard-to-deform materials such as intermetallics, semiconductors and ceramics, leading to enhancement of functional properties as well as structural properties. The major drawback is that the sample size is small so that the applicability is limited to a laboratory scale and it is an important subject to increase the sample dimensions. This paper presents an overview describing efforts devoted thus far to deal with this upscaling issue

Contactless measurement of electrical conductivity for bulk nanostructured silver prepared by high-pressure torsion: A study of the dissipation process of giant strain

We measured the electrical conductivity of bulk nanostructured silver prepared by high-pressure torsion (HPT) in a contactless manner by observing the AC magnetic susceptibility resulting from the eddy current, so that we could quantitatively analyze the dissipation process of the residual strain with sufficient time resolution as a function of temperature T and initial shear strain γ. The HPT process was performed at room temperature under a pressure of 6 GPa for revolutions N = 0–5, and we targeted a wide range of residual shear strains. The contactless measurement without electrode preparation enabled us to investigate both the fast and slow dissipation processes of the residual strain with sufficient time resolution, so that a systematic study of these processes became possible. The changes in the electrical conductivity as a function of N at room temperature were indeed consistent with changes in the Vickers microhardness; furthermore, they were also related to changes in structural parameters such as the preferred orientation, the interplanar distance, and the crystallite size. The dissipation process at N = 1, corresponding to γ ≈ 30, was the largest and the fastest. For N = 5, corresponding to γ ≈ 140, we considered the effects of grain boundaries, as well as those of dislocations. The strain dissipation was quite slow below T = 290 K. According to the analytical results, it became successful to conduct the quantitative evaluation of the strain dissipation at arbitrary temperatures: For instance, the relaxation times at T = 280 and 260 K were estimated to be 3.6 and 37 days, respectively

Fatigue Property and Cytocompatibility of a Biomedical Co–Cr–Mo Alloy Subjected to a High Pressure Torsion and a Subsequent Short Time Annealing

In the present study, we evaluated the effects of high pressure torsion (HPT) and subsequent short time annealing processing on fatigue properties and cytocompatibility of the biomedical Co–Cr–Mo alloy (CCM). Before processing, CCM was solution treated (CCMST) to achieve a microstructure composed of coarse single γ-phase equiaxed grains with no internal strain. Through HPT processing, an inhomogeneous microstructure containing both micro- and nano-scaled grains is obtained in CCM specimens, which were named as CCMHPT, accompanied by high internal strain and extensive ε martensite. Following a subsequent short time annealing, a uniform single γ-phase ultrafine-grained microstructure with small local strain fields dispersed forms in CCM specimens, which were named as CCMHPTA. This microstructure change improves fatigue strength in CCMHPT, and further in CCMHPTA, because of the enhanced crack initiation and/or propagation resistance. For cytocompatibility evaluation, the cells cultured on CCMST show an immobilization tendency, while those cultured on CCMHPT exhibit a locomotion tendency. The cells cultured on CCMHPTA have an intermediate pattern. Compared with CCMST, much larger numbers of cells are proliferated in both CCMHPT and CCMHPTA. All these results demonstrate that the CCMHPTA offers an improved fatigue property and a good cytocompatibility. Therefore, it is promising for use in biomedical applications

Aging behavior of Al-Li-(Cu, Mg) alloys processed by different deformation methods

Structural features and aging behavior of Alsingle bondLi, Al-Li-Cu and Al-Li-Mg alloys under different equivalent strains (ε ) were investigated. Following solid-solution treatment, high-pressure torsion (HPT), asymmetric rolling (ASR) and cold rolling (CR) were adopted to introduce high, middle and low amount of strains to Al-Li-(Cu, Mg) alloys. After deformation, for the HPT processed alloys under high equivalent strains, the highest as-deformed hardness was obtained. Transmission electron microscopy (TEM) revealed that the grain size was refined to 210 nm, 120 nm and 150 nm, respectively. single bond Under severe plastic deformation condition (ε > 30), the Alsingle bondLi alloy lost age-hardenability, however, the aging of the asymmetric rolled Alsingle bondLi alloys increased the hardness further and the highest hardness was obtained in this alloy. For the Al-Li-Cu and Al-Li-Mg alloys, a further increase in hardness was achieved by aging the as-deformed alloys, regardless of the equivalent strains. Meanwhile, the peak hardness increases with increasing the equivalent strains. During aging treatment, the behavior of the precipitates was discussed in the present work