Stability of the ultrafine-grained microstructure in silver processed by ECAP and HPT

The high-temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking fault energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures were achieved by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) at room temperature (RT). The defect structure in the as-processed samples was examined by electron microscopy and X-ray line profile analysis. The stored energy calculated from the defect densities was compared to the heat released during DSC. The sum of the energies stored in grain boundaries and dislocations in the ECAP-processed samples agreed with the heat released experimentally within the experimental error. The temperature of the DSC peak maximum decreased while the released heat increased with increasing numbers of ECAP passes. The released heat for the specimen processed by one revolution of HPT was much smaller than after 4–8 passes of ECAP despite the 2 times larger dislocation density measured by X-ray line profile analysis. This dichotomy was caused by the heterogeneous sandwich-like microstructure of the HPT-processed disk: about 175 μm wide surface layers on both sides of the disk exhibited a UFG microstructure while the internal part was recrystallized, thereby yielding a relatively small released heat.     

Hardness and microstructure of interstitial free steels in the early stage of high-pressure torsion

The hardness and microstructure distributions in interstitial free (IF) steel disks processed via the high-pressure torsion (HPT) process with an early stage (up to 1 turn) are investigated using experimental and simulation approaches. The results indicate that the deformation in the HPT-processed IF steel disk is inhomogeneous, providing almost linearly increasing hardness from the center to the edge regions. In particular, near the surface of the disk is a soft region that shrinks with increasing numbers of revolutions. Compared with the compression-only disks by the HPT die, there is a hardness hill in the center region of the HPT-processed disk. The hardness distributions in the HPT disks indicate that the deformation proceeds gradually from the edge to the center with the degree of revolutions. In addition, as the degree of revolutions increases, the strain in the center region increases and the plastic deformation becomes uniform along the radial direction. The finite element analyses strongly support the conclusions of the experimental results.     

High-pressure torsion for fabrication of high-strength and high-electrical conductivity Al micro-wires

Iron (Fe) is commonly found in aluminum (Al), but its contents are usually kept as low as possible, because the formation of intermetallic phases may induce fracture. In this study, high-pressure torsion (HPT) was used to control the microstructure in an Al-2 %Fe alloy in conjunction with wire drawing and an aging treatment, in order to improve not only their mechanical properties but also the electrical conductivity. It is shown that HPT processing of ring-shaped samples produced ultrafine grains with a size of ~150 nm in the matrix, while intermetallic phases were fragmented to nanosizes with some Fe fraction dissolved in the matrix. Semi-rings were extracted from the HPT-processed samples and swaged to a round section with 0.4-mm diameter. The HPT-processed sample was successfully drawn to a final diameter of 0.08 mm (25:1 ratio, 96 % reduction in area), whereas the sample without HPT processing failed after drawing to 0.117-mm diameter (12:1 ratio, 91 % reduction in area). The electrical conductivity increased to ~65 IACS % in the HPT-processed rings and to ~54 IACS % in the wires by aging for 1 h after the drawing.     

Fabrication of nanograined silicon by high-pressure torsion

This paper describes fabrication of Si nanograins through allotropic phase transformation by concurrent application of high pressure and intense straining using high-pressure torsion (HPT). Single-crystalline Si(100) wafers were processed by HPT under a pressure of 24 GPa at room temperature. X-ray diffraction and Raman analysis revealed that the HPT-processed samples were composed of metastable Si-III and Si-XII phases and amorphous phases in addition to the original diamond-cubic Si-I phase. It was found that nanograins formed because the Si-I diamond phase had transformed to high-pressure phases (Si-II, Si-XI, and Si-V) having metallic nature, and it then became easier to generate a high density of dislocations to form grain boundaries. The high-pressure phases were further transformed to the Si-XII and Si-III phases via the Si-II phase upon unloading and they existed as metastable phases at ambient pressure. Subsequent annealing at 873 K gave rise to reverse transformation to Si-I but with nanograin sizes. Although no appreciable photoluminescence (PL) peak was observed from the HPT-processed sample, a broad PL peak centered around 600 nm was detected from the annealed sample due to quantum confinement in the Si-I nanograins.     

Nanoindentation analysis for local properties of ultrafine grained copper processed by high pressure torsion

Severe plastic deformation (SPD) techniques have recently been developed for producing bulk ultrafine grained metallic materials. High pressure torsion (HPT) produces finer microstructures than those achieved by other SPD processes because of the higher imposed strain and hydrostatic pressure. It is known that HPT-processed metals show a highly heterogeneous microstructure not only along the radius due to the nature of torsional deformation but also through the thickness. Since the sample size for HPT is small, the local properties of HPT-processed specimens have not been investigated yet. In this paper, we propose a method to obtain stress–strain curves from nanoindenting curves by combining the finite element method and the recursion method. The nanoindentation technique was employed to elucidate the local mechanical properties, especially the stress–strain behavior. The method to extract the stress–strain curves from the load–displacement curves obtained by nanoindentation tests was applied to the edge region of the HPT-processed sample. The extracted properties correlated well with experimental results qualitatively.     

Effect of annealing on wear resistance and electroconductivity of copper processed by high-pressure torsion

The influences of annealing temperature on the wear properties and electrical conductivity of Cu were studied after processing by high-pressure torsion (HPT). The annealing of Cu specimens processed by HPT leads to an increase in electroconductivity and a decrease in the wear rate. It is apparent that a nanotribolayer at the surface induced during wear sliding plays a more significant role than the ultrafine-grained structure. A slight increase was observed in the microhardness of HPT copper specimens upon annealing at a relatively low temperature (100 °C), and this is most likely due to a change in texture. The annealing leads to an increase in the Taylor factor by ~5 %, which is in good agreement with the increase in the microhardness level which is also by ~5 %. It is apparent that low-temperature annealing of HPT copper may produce optimal properties of the specimens including high strength and electroconductivity with a lower wear rate.     

高压扭转对GW83K镁合金耐腐蚀性能的影响

采用高压扭转(HPT)工艺对Mg-8Gd-3Y-0.4Zr(GW83K)合金进行剧烈塑性变形加工,通过显微组织观察、物相分析、显微硬度测定、开路电位测试、动极化曲线测试、交流阻抗谱及扫描开尔文探针分析,对比研究了HPT前后GW83K合金的显微组织和耐蚀性。结果表明:经5圈高压扭转后,GW83K合金的晶粒细化不显著,物相结构亦无变化,晶粒中出现大量孪晶和位错,试样中心区域和边缘区域的晶粒尺寸无明显差异,但边缘区域硬度远高于中心区域。HPT后镁合金在空气中伏打电位较低,在3.5%(质量分数)NaCl溶液中浸泡时开路电位迅速稳定(约1h)但稳定值较负,腐蚀电流密度较大,电化学阻抗较小且随浸泡时间延长而减少,其耐蚀性不及未HPT加工的初始态合金。在0.1mol/L NaOH溶液中,浸泡初期初始态和HPT态合金开路电位较接近,交流阻抗相差不大,浸泡6h后初始态的电化学阻抗容抗弧直径则明显大于HPT态样品。极化曲线显示HPT态合金在碱性溶液中的维钝区间减小,维钝电流密度增大,初次击穿电压负移。HPT加工虽有助于镁合金表面快速形成钝化膜(不能有效保护基体),却使镁基体更活泼(溶解加速),GW83K镁合金耐蚀性下降。     

Grain-boundary diffusion and precipitate trapping of hydrogen in ultrafine-grained austenitic stainless steels processed by high-pressure torsion

This study was conducted to clarify the effects of grain boundaries and precipitates on room-temperature hydrogen transport in two types of austenitic stainless steels with ultrafine-grained structures produced by high-pressure torsion (HPT) and subsequent annealing. The grains in the Fe-25Ni-15Cr (in mass%) alloy containing Ti and the Fe-25Cr-20Ni alloy were refined by the HPT-processing to ∼150 and ∼85 nm, respectively. The high-temperature annealing after the HPT processing led to the precipitation of η-Ni₃Ti for the former and σ-FeCr for the latter. In the HPT-processed specimens, hydrogen diffusivity was enhanced through short-circuit diffusion because of the increased population of grain boundaries in comparison with the increased opportunity of hydrogen trapping on dislocations. As for the post-HPT-annealed specimens having the precipitates, the hydrogen diffusion was hindered by the hydrogen trapping on η-Ni₃Ti precipitates, but was not affected by σ-FeCr precipitation. This depends on the affinity between hydrogen and constituting elements.     

The corrosion behaviour of commercial purity titanium processed by high-pressure torsion

Commercial purity (CP) titanium was processed by high-pressure torsion (HPT) under an applied pressure of 6.0 GPa for different numbers of torsional revolutions and then exposed to a 3.5 % NaCl solution for open-circuit potential measurements followed by electrochemical impedance spectroscopy and potentiodynamic polarization tests. The electrochemical results exhibit a complicated relationship between the corrosion resistance and grain refinement. Thus, microhardness measurements reveal an improvement in hardness for CP titanium after processing by HPT but the corrosion resistance is lower in the NaCl solution than for the annealed coarse-grained Ti. It is shown that the corrosion susceptibility of the HPT-processed samples decreases with increasing torsional strain. The effect of grain size and microstructure on the corrosion properties of ultrafine-grained CP Ti is also examined.     

Improvement of strength and conductivity in Cu-alloys with the application of high pressure torsion and subsequent heat-treatments

Quenched and slowly cooled (annealed) Cu–0.7 %Cr, Cu–0.9 %Hf, and Cu–0.7 %Cr–0.9 %Hf alloys were processed by high pressure torsion (HPT). The microstructures of the alloys were studied immediately after HPT and subsequent annealing. It has been shown that the microhardness and the thermal stability of the severely deformed microstructure increase, while the average grain size decreases in the order of Cu–0.7 %Cr, Cu–0.9 %Hf, and Cu–0.7 %Cr–0.9 %Hf alloys. The microhardness in all alloys is higher after quenching and HPT, than after annealing and HPT. The largest dislocation density is achieved by quenching and HPT in Hf-containing samples. Cu₅Hf phase precipitations in Hf-containing alloys are more effective in retarding grain growth in comparison with Cr particles and lead to additional hardening during aging. It has been demonstrated that HPT-processing with subsequent heat-treatment might yield the combination of large hardness and high electrical conductivity in Cu alloys.     

Controllable bimodal structures in hypo-eutectoid Cu–Al alloy for both high strength and tensile ductility

To effectively demonstrate the dependence of ductility improvement on the scheme of introducing bimodal structure into nanostructured materials, a three-step processing was adopted in hypo-eutectoid Cu–Al alloys to obtain controllable bimodal structure of micrometer-grained pre-eutectoid phase embedded on ultrafine-grained (UFG) matrix with eutectoid composition: (1) pre-deformation heat-treatment was proposed to achieve controlled distribution of pre-eutectoid phase in the matrix with eutectoid composition, (2) both pre-eutectoid phase and eutectoid matrix were refined to submicrometer level by usage of high-pressure torsion (HPT), (3) annealed HPT-processed samples at selected temperature. All samples subjected to this novel processing route imparted a high strength, meanwhile obvious uniform plastic elongation in tensile deformation was also observed at those with bimodal structure.     

Using Post‐Deformation Annealing to Optimize the Properties of a ZK60 Magnesium Alloy Processed by High‐Pressure Torsion

A ZK60 magnesium alloy with an initial grain size of ≈10 µm is processed by high‐pressure torsion (HPT) through 5 revolutions under a constant compressive pressure of 2.0 GPa with a rotation speed of 1 rpm. An average grain size of ≈700 nm is achieved after HPT with a high fraction of high‐angle grain boundaries. Tensile experiments at room temperature show poor ductility. However, a combination of reasonable ductility and good strength is achieved with post‐HPT annealing by subjecting samples to high temperatures in the range of 473–548 K for 10 or 20 min. The grain size and texture changes are also examined by electron back scattered diffraction (EBSD) and the results compared to long‐term annealing for 2500 min at 450 K. The results of this study suggest that a post‐HPT annealing for a short period of time may be effective in achieving a reasonable combination of strength and ductility.

     

Effect of microstructural stability on fatigue crack growth behaviour of nanostructured Cu

Constant amplitude fatigue crack growth tests were carried out on commercial and high purity nanostructured copper processed by High Pressure Torsion (HPT). Due to strong grain refinement the HPT processed materials show higher tensile strength but also faster crack growth rates when compared to coarse grained material. Crack growth curves of nanostructured copper determined at different stress levels, however, showed that the occurrence of grain coarsening at low stress amplitudes leads to a retardation of crack growth in commercial and high purity HPT Cu. This effect was not observed for high purity HPT Cu with a bimodal microstructure. Crack propagation rates depend significantly on the coarsening phenomenon which on the other hand depends on the applied stress amplitude. A comparison of these results with cyclic deformation tests in the high cycle fatigue regime suggests that grain coarsening during crack growth depends more on the stored energy of the materials while a similar coarsening during cyclic deformation depends more on the activation enthalpy for annealing of defects.     

Effect of Initial Annealing Temperature on Microstructural Development and Microhardness in High‐Purity Copper Processed by High‐Pressure Torsion

The effect of the initial annealing temperature on the evolution of microstructure and microhardness in high purity OFHC Cu is investigated after processing by HPT. Disks of Cu are annealed for 1 h at two different annealing temperatures, 400 and 800 °C, and then processed by HPT at room temperature under a pressure of 6.0 GPa for 1/4, 1/2, 1, 5, and 10 turns. Samples are stored for 6 months after HPT processing to examine the self‐annealing effects. Electron backscattered diffraction (EBSD) measurements are recorded for each disk at three positions: center, mid‐radius, and near edge. Microhardness measurements are also recorded along the diameters of each disk. Both alloys show rapid hardening and then strain softening in the very early stages of straining due to self‐annealing with a clear delay in the onset of softening in the alloy initially annealed at 800 °C. This delay is due to the relatively larger initial grain size compared to the alloy initially annealed at 400 °C. The final microstructures consist of homogeneous fine grains having average sizes of ≈0.28 and ≈0.34 µm for the alloys initially annealed at 400 and 800 °C, respectively. A new model is proposed to describe the behavior of the hardness evolution by HPT in high purity OFHC Cu.     

Solid-state amorphization of Cu + Zr multi-stacks by ARB and HPT techniques

A series of CuZr binary alloys with wide composition range were fabricated through ARB and HPT techniques using pure Cu and Zr metals as the starting materials. Bulk alloy sheets with thickness of about 0.8 mm after ARB process and alloy disks with 0.30 mm in thickness and 10 mm in diameter after HPT process can be obtained, respectively. The structures of all the alloys were found to be gradually refined with the increase of ARB cycles or HPT rotations. As a result, nanoscale multiple-layered structure was formed for the 10 cycled ARBed specimens, which could partially transform into amorphous phase during subsequent low temperature annealing. While for the as-HPTed sample, the alloy was completely amorphized after 20 rotations without any heat treatment. The thermal stabilities of the amorphous alloys were studied. The deformation behavior and the amorphization mechanism during the ARB and HPT process were put forward and discussed.     

Using high-pressure torsion for the cold-consolidation of copper chips produced by machining

High-pressure torsion (HPT) was used to consolidate chips machined from coarse-grained copper and from copper processed by ECAP. The results are compared with discs prepared by HPT and with discs processed by a combination of equal-channel angular pressing (ECAP) and HPT. It is demonstrated that the consolidated discs have exceptionally fine microstructures and high hardness. The results suggest the possibility of a saturation in grain refinement which may be related to a release in heat during high-pressure straining.     

Achieving homogeneity in a two-phase Cu–Ag composite during high-pressure torsion

A Cu–8 wt%Ag alloy was processed at room temperature either by high-pressure torsion (HPT) or using a two-step deformation mode of equal-channel angular pressing (ECAP) and HPT. After HPT deformation, different flow patterns were observed on the disk surface without any post-deformation treatment thereby indicating an inhomogeneous shearing deformation. The microhardness distributions throughout the disks were compared after the two different processing routes. It is shown that the microhardness remains very low near the center of the disk although saturated in the outer regions after 10 revolutions. By contrast, an intrinsically homogeneous microhardness may be attained throughout the disk after the two-step deformation of ECAP and HPT. This study suggests a convenient procedure for achieving full homogeneity in high-strength materials during HPT processing.     

溶胶-凝胶法制备c轴取向生长ZnO薄膜

采用sol-gel法,在普通载玻片上使用旋转涂覆技术生长了具有c轴择优取向生长的ZnO薄膜.采用热分析、XRD、SEM等手段对薄膜样品进行了表征.热分析结果表明,二水醋酸锌-乙醇胺-乙二醇甲醚体系sol-gel的热分解过程与纯二水合乙酸锌的分解过程大相径庭.ZnO薄膜的sol-gel分解趋于在较窄的温度范围内一步完成.样品在XRD图谱中表现出明显的c轴择优取向.此外,SEM照片也表明:ZnO薄膜样品中间区域和边缘区域的表面形貌相差甚远.探讨了预热处理温度、退火温度等工艺条件对ZnO薄膜的结构性能的影响,最佳的预热处理温度被认为在ZnO相完全生成温度附近.     

Solid-state transformation in ball milled nickel powder

A solid-state tetragonal transformation of ball milled and cold pressed unmilled nickel powder is proposed. The phase evolution was characterized using the X-ray diffraction (XRD) technique, while the phase transition was analysed by the differential scanning calorimetry (DSC). However, the lattice parameters obtained on the surface of the cold pressed sample are smaller than the lattice parameters of the 2 h ball milled powder. Crystallization of two exothermic peaks was observed during DSC analysis of the milled powder, which was not the case in cold pressed powder.     

High temperature thermal stability of ultrafine-grained silver processed by equal-channel angular pressing

The high temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking-fault-energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures in two samples having purity levels of 99.995 and 99.99 at.% were achieved by four passes of equal-channel angular pressing at room temperature. The defect structure was studied by electron microscopy, X-ray line profile analysis, and positron annihilation spectroscopy before and after the exothermic DSC peak related to recovery and recrystallization. The heat released in the DSC peak was correlated to the change of defect structure during annealing. It was found for both compositions that a considerable fraction of stored energy (~15–20 %) was retained in the samples even after the DSC peak due to the remaining UFG regions and a large density of small dislocation loops in the recrystallized volumes. The larger impurity level in Ag yielded a higher temperature of recrystallization and a lower released heat. The latter observation is explained by the much lower vacancy concentration before the DSC peak which is attributed to the segregation of dopants at grain boundaries resulting in a smaller free volume in the interfaces.