Microstructural evolution 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. It is shown that there are generally smaller grains at the edges of the disks by comparison with the disk centers. The hardness results in these two alloys are mutually consistent, but they are different from those generally reported for conventional single-phase materials. The significance of this difference is examined.     

Microstructure and microtexture in pure copper processed by high-pressure torsion

The evolution of microstructure and microtexture in high purity copper was examined after processing by high-pressure torsion (HPT). Copper disks were annealed for 1 h at 800 °C and later processed monotonously in HPT at ambient temperature for 1/4, 1/2, 1, and 5 turns under a pressure of 6.0 GPa. Electron backscattered diffraction (EBSD) measurements were taken for each disk at three positions: center, mid-radius, and near-edge. Results from EBSD for samples processed between 1/4 and 1 turn indicate the formation of Σ3 twin boundaries by recrystallization before complete microstructural refinement. The results show a gradual increase in the homogeneity of the microstructure with increasing numbers of turns, reaching a stabilized ultrafine-grained structure at 5 turns with a bimodal distribution of fine and coarse grains of 0.15 and 0.5 μm in diameter, respectively. The occurrence of recrystallization in the early straining stages was further supported by examining microtexture development with increasing numbers of turns, where this shows a gradual transition from a shear texture to a mixture of shear and recrystallization and later to a shear texture at high HPT strains. The promotion of recrystallization during HPT is probably related to the high purity of the copper.     

Microstructural evolution in ultrafine-grained titanium processed by high-pressure torsion under different pressures

A grade 2 commercially pure (CP) titanium was processed by high-pressure torsion (HPT) at pressures of 3.0 and 6.0 GPa in order to achieve improved strengths. The microhardness values for these Ti samples were plotted against the imposed strain, and the plots show that a higher saturation microhardness of 320 Hv is achieved for the sample processed at 6.0 GPa compared to a microhardness of 305 Hv when using a pressure of 3.0 GPa. The omega ω-phase has been reported in some earlier HPT investigations of pure titanium, but it was not detected in this investigation even after processing at 6.0 GPa. The absence of the ω-phase is attributed to the relatively high level of oxygen (0.25 wt%) in these CP titanium samples. The higher saturation hardness for the 6.0 GPa sample is consistent with the smaller average grain size of ~105 ± 12 nm compared with the measured grain size of ~130 ± 18 nm after processing with an imposed pressure of 3.0 GPa.     

Tribology testing of ultrafine-grained Ti processed by high-pressure torsion with subsequent coating

A grade 2 pure Ti was processed by high-pressure torsion (HPT) under 3.0 GPa for 10 revolutions to achieve an improved strength. Wear tests revealed that HPT only slightly improved the wear resistance of pure Ti. Subsequently, a TiN coating with a thickness of 2.5 μm was deposited on different Ti substrates to improve the wear resistance. Both indentation and scratch testing demonstrated a much improved load-bearing capacity when ultrafine-grained Ti was chosen as the substrate compared with coarse-grained Ti. All results indicate that pure Ti processed by HPT, when combined with a subsequent coating, represents a good candidate material for bio-implant applications.     

Microstructural evolution and the mechanical properties of an aluminum alloy processed by high-pressure torsion

Processing by high-pressure torsion (HPT) was performed on disks of an Al-7075 alloy at room temperature. The alloy was initially annealed at 753 K and then processed by HPT under a pressure of 6.0 GPa up to a maximum of ten turns. Measurements of the Vickers microhardness showed lower values at the centers of the disks after small numbers of turns but higher numbers of turns led to a reasonable hardness homogeneity across each disk. After five turns, the grain size at the edge of the disk was ~250 nm. It is demonstrated that results from mechanical testing are consistent with the hardness and microstructural data.     

Review: achieving superplasticity in metals processed by high-pressure torsion

It is now well established that processing by equal-channel angular pressing (ECAP) leads to grain refinement and produces materials having the potential for exhibiting extensive superplastic flow at elevated temperatures. High-pressure torsion (HPT) is also an effective procedure for refining the grain sizes of polycrystalline metals to the submicrometer or even the nanometer level, and recent results show that this processing method also gives materials that exhibit excellent superplastic characteristics. This report examines the various publications describing superplasticity in metallic alloys processed by HPT. A comprehensive tabulation is presented listing all of the results to date showing true superplastic elongations of at least 400 % after processing by HPT. Examples of superplastic elongations are described for tensile tests conducted using specimens cut from either disk or ring samples. An analysis shows that the flow behavior of various Al and Mg alloys is in good agreement with the predicted flow behavior for conventional superplastic materials.     

Interpretation of hardness evolution in metals processed by high-pressure torsion

The processing of metals through the application of high-pressure torsion (HPT) provides the potential for achieving exceptional grain refinement in bulk disks. Numerous reports are now available describing the application of HPT to a range of pure metals and simple alloys. Excellent grain refinement was achieved using this processing technique with the average grain size often reduced to the nanoscale range. By contrast, the development of microstructure and local hardness is different depending upon the material properties. In order to make HPT processing more practical, it is indispensable to investigate the nature of the sample characteristics immediately after conventional HPT processing. Accordingly, this report demonstrates the different models of hardness evolution using representative materials of AZ31 magnesium alloy, high-purity aluminum, and Zn–22 % Al eutectoid alloy processed by HPT. Separate models are described for the evolution of hardness with equivalent strain, and the correlation between these models is suggested by the homologous temperature of HPT processing. A special emphasis is placed on examining the numerical expression of the level of strain hardening or softening of these metals with increasing equivalent strain.     

Microstructure evolution of CuZr polycrystals processed by high-pressure torsion

The microstructure evolution of extruded Cu–0.18 wt% Zr polycrystals processed by high-pressure torsion (HPT) at room temperature at the pressure of 4 GPa and the different number of the HPT revolutions (i.e. different strain) was investigated using the combination of the electron back-scatter diffraction, microhardness measurements and the X-ray diffraction. A significant transition from the inhomogeneous microstructure after few HPT revolutions into the homogeneous equiaxed microstructure with increasing number of HPT rotations was observed. HPT straining leads to the grain size refinement by a factor more than 100 after the 25 HPT revolutions. Moreover, the EBSD revealed an increase in the fraction of high-angle grain boundaries (HAGBs) with increasing HPT straining reaching the value of 70% after 25 revolutions. Additionally, a slight increase of the twin-related CSL Σ3 grain boundaries occurred during the microstructure refinement. The microhardness measurements confirmed the billet radial inhomogeneity at early stages of the HPT straining, whereas with increasing number of the HPT rotations, causing the specimen fragmentation and homogenization, the microhardness values increased. The average crystallite size and the average dislocation density in individual specimens determined by the XRD diffraction were in the range of approximately 100–200 nm and 2 × 10¹⁵ m⁻², respectively. Moreover, XRD measurements confirmed the absence of residual stresses in all specimens.     

Allotropic phase transformation of pure zirconium by high-pressure torsion

Pure Zr is processed by high-pressure torsion (HPT) at pressures in the range of 1–40 GPa. A phase transformation occurs from α to ω phase during HPT at pressures above 4 GPa while the total fraction of ω phase increases with straining and saturates to a constant level at higher strain. This phase transformation leads to microstructural refinement, hardness and strength enhancement and ductility reduction. Lattice parameter measurements confirm that c for α phase is expanded about 0.6% by the presence of ω phase. The temperature for reverse transformation from ω to α phase increases with straining and thus, straining under high pressure increases thermal stability of ω phase. The ω phase obtained by HPT is stable for more than 400 days at room temperature.     

Long-term self-annealing of copper and aluminium processed by high-pressure torsion

High-pressure torsion (HPT) is recognized as the most effective method for producing ultrafine-grained and even nanocrystalline structures in metallic systems. Although there are many reports on microstructural refinement of pure metals and metallic alloys, several important problems remain unresolved. For example, more information is needed on the homogeneity of the processed microstructure and on the heat release and temperature rise during HPT processing. Recently, there were reports of hardening in HPT-processed pure metals under conditions of self-annealing. This report presents new experimental data on the relaxation processes in HPT copper and aluminium processed at room temperature under loads of 6.0 and 1.0 GPa for zero and one turn. The experimental results were obtained using X-ray diffraction and by measuring the average Vickers microhardness after ageing.     

Unusual macroscopic shearing patterns observed in metals processed by high-pressure torsion

A duplex stainless steel was processed by high-pressure torsion (HPT) and then examined by optical microscopy. The results reveal unusual flow patterns including double-swirl strains, shear vortices, and the presence of significant local turbulence. Similar flow behavior was also visible in disks of high-purity aluminum and a Zn–22%Al eutectoid alloy. These complex flow patterns and the presence of double-swirls are consistent with the presence of a Kelvin–Helmholtz instability during HPT processing where this may arise if there are local shear velocity gradients between adjacent positions within the HPT disks.     

Tribocorrosion behaviour of nanostructured titanium substrates processed by high-pressure torsion

Aseptic loosening induced by wear particles from artificial bearing materials is one of the main causes of malfunctioning in total hip replacements. With the increase in young and active patients, complications in revision surgeries and immense health care costs, there is considerable interest in wear-resistant materials that can endure longer in the harsh and corrosive body environment. Here, the tribological behaviour of nanostructured titanium substrates processed by high-pressure torsion (HPT) is investigated and compared with the coarse-grained samples. The high resolution transmission electron microscopy reveals that a nanostructured sample has a grain size of 5-10?nm compared to that of ～ 10?μm and ～ 50?μm for untreated and annealed substrates, respectively. Dry and wet wear tests were performed using a linear reciprocating ball-on-flat tribometer. Nanostructured samples show the best dry wear resistance and the lowest wear rate in the electrolyte. There was significantly lower plastic deformation and no change in preferred orientation of nanostructured samples attributable to the wear process. Electrochemical impedance spectroscopy (EIS) shows lower corrosion resistance for nanostructured samples. However, under the action of both wear and corrosion the nanostructured samples show superior performance and that makes them an attractive candidate for applications in which wear and corrosion act simultaneously.     

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.     

Different models of hardness evolution in ultrafine-grained materials processed by high-pressure torsion

High-pressure torsion (HPT) is an attractive processing method in severe plastic deformation techniques involving the application of high compressive pressure with concurrent torsional straining. Excellent grain refinement is anticipated when using this technique to average grain sizes of the submicrometer or even nanometer ranges. Because of the significant microstructural changes during processing, there are numerous reports showing evolution in local hardness toward homogeneity throughout a disk diameter with increasing numbers of revolutions. The achieved hardness after HPT is mostly much higher than that in the as-received condition because of exceptional grain refinement although there are a limited number of metals and alloys showing softening or weakening after HPT processing. This paper was initiated to review recent discoveries in the experimental results on hardness evolution toward homogeneity during HPT processing and discuss the different models of hardness developments with respect to imposed equivalent strain by HPT processing for a range of metals and alloys. Moreover, recent results of hardness homogeneity and heterogeneity through thicknesses of the processed disks are discussed toward a complete understanding of hardness evolution in the UFG metals processed by HPT.     

关键参数对铜粉锥形件HPT成形的影响

高压扭转工艺具有极强的晶粒细化和粉末固结效果,而且非常适用于回转体零件的生产,为了制备出性能优异的药型罩,将高压扭转工艺应用于铜粉锥形件成形.采用单因素实验法,利用HPT专用液压机、倒置金相显微镜和金相软件系统,以及排水法测相对密度等方法,研究了温度、压力、扭转角速度和扭转圈数对致密度和晶粒尺寸的影响.结果表明：随着温...     

Evolution of microstructure and hardness in NiTi shape memory alloys processed by high-pressure torsion

Experiments were conducted on Ni-50.2 at.% Ti and Ni-50 at.% Ti alloys in order to examine the evolution of hardness and microstructure after processing by high-pressure torsion at room temperature. Disks were pressed through different numbers of revolutions up to a maximum of 40 using an applied pressure of 2.0 GPa. It is shown that there is a gradual evolution in both the hardness and the microstructure with increasing numbers of turns but even after 40 turns there is not full homogeneity. There is evidence that after 10 turns the edges of the disks achieve a well-defined saturation hardness and by further processing to 40 turns the hardness in the centers of the disks increases. The results show that a martensite-to-austenite transformation occurs during processing. The austenitic transformation around the edge of the disks achieves saturation after 5 and 10 turns in the Ni-50 at.% Ti and Ni-50.2 at.% Ti alloys, respectively.     

Mechanical properties and microstructure evolution in an aluminum 6082 alloy processed by high-pressure torsion

A commercial aluminum 6082 alloy was used to investigate the effect of the initial condition on subsequent processing by high-pressure torsion (HPT). The alloy was prepared in two different initial conditions: (i) in a T651 annealed condition and (ii) after a solution treatment followed by over-aging and subsequent processing by equal-channel angular pressing (ECAP). All samples were processed by HPT through 1/2, 1, 2, 5, and 10 turns and then the microstructures were examined using electron backscattered diffraction (EBSD). Significant grain refinement was achieved after processing by HPT through 5 turns with measured grain sizes of ~0.5 μm in both types of alloy. Microhardness measurements were conducted to evaluate the evolution of hardness after HPT for the two initial conditions. It is demonstrated that there is a difference in the hardness values between these two initial conditions, and this difference remains almost constant after processing by HPT.