Microstructure,Phase Composition,and Thermal Stability of Two Zirconium Alloys Subjected to High‐Pressure Torsion at Different Temperatures

The structure, phase composition, and thermal stability of the industrial zirconium alloys, namely, E110 (Zr–1% Nb) and E635 (Zr–1% Nb–0.3% Fe–1.2% Sn), which are subjected to high‐pressure torsion (HPT) at room temperature (RT), 200, and 400 °С have been studied. HPT of Zr‐alloys at RT (10 revolutions) leads to the formation of grain–subgrain nano‐sized structure and to increase the microhardness by 2.1…2.8 times. The increase in the HPT temperature to 200–400 °С leads to the increase in the structural‐element average size. The structural‐element size in the complexly alloyed E635 alloy in all cases is lower compared with the E110 alloy. The hardening of the alloys after HPT at RT and 200 °С is close, and at 400 °С is much less. HPT initiates the α‐Zr → (ω‐Zr + β‐Zr) transformation, which is the main factor for alloys hardening. The α‐Zr → (ω‐Zr + β‐Zr) transformation in the E635 alloy occurs less quickly. The maximum amount (ω‐Zr + β‐Zr) phase in the structure of the alloys is observed after HPT at RT and 200 °C, and the minimum ? at 400 °C. During heating, the alloys undergo the reverse (ω‐Zr + β‐Zr) → α transformation which depends on both the alloy composition and HPT temperature.

     

Producing High‐Strength Metals by I‐ECAP

Incremental equal channel angular pressing (I‐ECAP) is used in this work to produce ultrafine‐grained (UFG) pure iron, aluminum alloy 5083, commercial purity titanium (grade 4), and magnesium alloy AZ31B. Pure iron is processed at room temperature, aluminum alloy at 200 °C, titanium at 320 °C, and magnesium alloy at 150 °C. Strength improvement, attributed to the grain refinement below 1 μm, is reported for all processed materials. The yield strength increase is the most apparent in pure iron, reaching almost 500 MPa after one pass of I‐ECAP, comparing to 180 MPa in the as‐forged conditions. UFG titanium, aluminum, and magnesium alloys obtained in this study reached yield stress of 800, 350, and 300 MPa, respectively, in each case exhibiting the yield strength increase by at least 30%, comparing to the alloys processed by conventional metal forming operations such as forging and rolling.     

Densification Behavior and Wear Response of Spark Plasma Sintered Iron‐Based Bulk Amorphous Alloys

In the present study, spark plasma sintering (SPS) process is used to sinter Fe‐based bulk amorphous alloys from starting amorphous powder. The sintering was performed in supercooled liquid region (at 630 °C with uniaxial pressure of 70 MPa) where plastic flow of the amorphous powder results in better densification without crystallization. To study the crystallization behavior and its influence on mechanical behavior, the sintered discs were de‐vitrified by annealing at 700 and 800 °C. Detailed characterization of phase development, microhardness at different loads, and wear behavior of the as‐sintered and annealed alloys is presented.     

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.

     

High‐performance Cu/Al laminated composites fabricated by horizontal twin‐roll casting

Horizontal twin‐roll casting technology was successfully introduced to produce high‐performance copper/aluminum (Cu/Al) laminated composites. The interface morphology, electrical properties and peeling strength after different annealing and cold rolling processes were investigated and contrasted with Cu/Al clad plates fabricated by conventional methods. The results show that sound metallurgical bonding between the copper and aluminum matrix can be attained after the horizontal twin‐roll casting processes and Al₂Cu is the only intermetallics at the interfacial region, the thickness of interfacial interlayer is about 0.7 μm. The peeling strength is 31.4 N/mm and can be further increased to 37.1 N/mm after annealing at 250 °C. However, higher temperature like 400 °C will cause the excessive growth of intermetallics so that peeling strength sharply decreases to 9.2 N/mm. Electrical conductivity of the clad plate is 51 MS/m. At the same electrical current intensity, the temperature‐rise of the composite plate is between the pure copper plate and the aluminum plate, and closer to the copper plate. All of the properties are outstanding than that of Cu/Al clad plate fabricated by conventional methods.     

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.     

High‐Modulus Low‐Cost Carbon Fibers from Polyethylene Enabled by Boron Catalyzed Graphitization

Currently, carbon fibers (CFs) from the solution spinning, air oxidation, and carbonization of polyacrylonitrile impose a lower price limit of ≈$10 per lb, limiting the growth in industrial and automotive markets. Polyethylene is a promising precursor to enable a high‐volume industrial grade CF as it is low cost, melt spinnable and has high carbon content. However, sulfonated polyethylene (SPE)‐derived CFs have thus far fallen short of the 200 GPa tensile modulus threshold for industrial applicability. Here, a graphitization process is presented catalyzed by the addition of boron that produces carbon fiber with >400 GPa tensile modulus at 2400 °C. Wide angle X‐ray diffraction collected during carbonization reveals that the presence of boron reduces the onset of graphitization by nearly 400 °C, beginning around 1200 °C. The B‐doped SPE‐CFs herein attain 200 GPa tensile modulus and 2.4 GPa tensile strength at the practical carbonization temperature of 1800 °C.     

High‐Throughput Near‐Field Optical Nanoprocessing of Solution‐Deposited Nanoparticles

The application of nanoscale electrical and biological devices will benefit from the development of nanomanufacturing technologies that are high‐throughput, low‐cost, and flexible. Utilizing nanomaterials as building blocks and organizing them in a rational way constitutes an attractive approach towards this goal and has been pursued for the past few years. The optical near‐field nanoprocessing of nanoparticles for high‐throughput nanomanufacturing is reported. The method utilizes fluidically assembled microspheres as a near‐field optical confinement structure array for laser‐assisted nanosintering and nanoablation of nanoparticles. By taking advantage of the low processing temperature and reduced thermal diffusion in the nanoparticle film, a minimum feature size down to ≈100 nm is realized. In addition, smaller features (50 nm) are obtained by furnace annealing of laser‐sintered nanodots at 400 °C. The electrical conductivity of sintered nanolines is also studied. Using nanoline electrodes separated by a submicrometer gap, organic field‐effect transistors are subsequently fabricated with oxygen‐stable semiconducting polymer.     

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.     

Stress Evolution With Temperature in Titanium‐Rich NiTi Shape Memory Alloy Films

Abstract: The effect of deposition temperature on residual stress evolution with temperature in Ti‐rich NiTi films deposited on silicon substrates was studied. Ti‐rich NiTi films were deposited on 3″ Si (100) substrates by DC magnetron sputtering at three deposition temperatures (300, 350 and 400 °C) with subsequent annealing in vacuum at their respective deposition temperatures for 4 h. The initial value of residual stress was found to be the highest for the film deposited and annealed at 400 °C and the lowest for the film deposited and annealed at 300 °C. All the three films were found to be amorphous in the as‐deposited and annealed conditions. The nature of the stress response with temperature on heating in the first cycle (room temperature to 450 °C) was similar for all three films although the spike in tensile stress, which occurs at ～330 °C, was significantly higher in the film deposited and annealed at 300 °C. All the films were also found to undergo partial crystallisation on heating up to 450 °C and this resulted in decrease in the stress values around 55–60 °C in the cooling cycle. The stress response with temperature in the second thermal cycle (room temperature to 450 °C and back), which is reflective of the intrinsic film behaviour, was found to be similar in all cases and the elastic modulus determined from the stress response was also more or less identical. The three deposition temperatures were also not found to have a significant effect on the transformation characteristics of these films such as transformation start and finish temperatures, recovery stress and hysteresis.     

Microstructural Parameters and Their Effect on the Indentation Hardness of Electrodeposited and Annealed Nickel–Iron Micro‐Specimens

Using the direct‐LIGA technology, nickel–iron micro‐specimens are serially produced by a micro‐gear drive manufacturer and subsequently annealed within the temperature range between 180 and 800 °C. The microstructure (grain size, lattice strain, and texture) is characterized using XRD measurements. Following electrodeposition, nano‐crystalline microstructures result with grain size of approximately 10 nm. The transmission electron microscope images confirm the XRD results. The lattice strain decreases in the temperature range from 200 to 300 °C and grain growth results for an annealing temperature from approximately 260 °C. The annealing treatment produced no essential changes in the material's texture. Analysis of the indentation hardness and indentation modulus demonstrates considerable changes above 200 °C.     

High‐Temperature Corrosion of NiTi‐Shape‐Memory Alloys

Semi‐finished products and components made of NiTi‐shape‐memory alloys (NiTi‐SMA) are often subjected to heat treatment after their fabrication. During this heat treatment, oxide layers begin to form which contain a high amount of titanium. In this investigation special attention was drawn to the selective oxidation of Ti because a TiO_X‐layer can represent a Ni‐barrier and may therefore be of special use for medical applications. A comparison of the following three samples was carried out: A sample oxidised at room temperature, another that was heat‐treated in ambient air (600 °C/1min) and a third sample that was subjected to a heat treatment (600 °C/1min) in an atmosphere that oxidises titanium but reduces NiO in order to achieve a selective oxidation of the titanium. The analysis of the oxide layers was carried out by means of x‐ray photoelectron spectroscopy (XPS). It was shown that the ratio of titanium to nickel in the oxide layer can be substantially increased when performing the annealing treatments in a partial reducing atmosphere. Furthermore, a thermo‐gravimetric investigation of the material was carried out at 600 °C in dry air in order to estimate the growth of the oxide layers.     

Microstructure and mechanical properties of hypereutectic aluminium‐silicon alloy annealing at liquid‐solid region

The microstructure and mechanical properties of hypereutectic Al?Si alloys containing 16 wt.%, 21 wt.% and 30 wt.% Si (AlSi16, AlSi21 and AlSi30) have been investigated in “as‐cast” and “annealed at the liquid‐solid region”. The results show that the morphologies of primary Silicon in AlSi30 changed hardly after being annealed at 587 °C for different time. The morphology of eutectic silicon in all Al?Si alloys changed from flake‐like to granular after being annealed at 587 °C for 5 min to 15 min. With the increase of silicon content, the average size of the granular eutectic silicon decreased. The spheroidized effect for the eutectic silicon in AlSi21 was the best. After being annealed for 15 min, the tensile strength of AlSi16, AlSi21 and AlSi30 alloys decreased by 10.4 %, 11.7 % and 11.5 %, respectively. Their elongation increased 61.1 %, 50.0 % and 33.3 %, respectively. The comprehensive mechanical properties of AlSi16 and AlSi21 alloys were improved markedly after being annealed at 587 °C for 15 min.     

The influence of defect and temperature on the fatigue behaviours of Al‐Si‐Cu‐Mg‐Ni alloy

High‐cycle fatigue (HCF) properties of two Al‐Si‐Cu‐Mg‐Ni alloys with different defect sizes named as alloys A (smaller ones) and B (bigger ones) were investigated at 350°C and 425°C, respectively. The results indicate that fatigue strengths of both alloys decrease as the temperature increases. Fatigue cracks originated from pores and oxide films at both temperatures. They propagated preferentially through cracked matrix at 350°C and debonded interface and grain boundary at 425°C. Alloy A exhibits higher fatigue life and fatigue strength than alloy B at 350°C due to its smaller pore sizes. However, it has slightly worse fatigue properties than alloy B at 425°C because the fatigue crack initiation is controlled by oxide film at this temperature and is not affected by its size. This indicates that there is a transition of predominant initiation site from pores to oxide films when the temperature increases. The fatigue strength estimated through defect size is consistent with the experimental results at 350°C, while unsuitable at 425°C.     

Switching Vertical to Horizontal Graphene Growth Using Faraday Cage‐Assisted PECVD Approach for High‐Performance Transparent Heating Device

Plasma‐enhanced chemical vapor deposition (PECVD) is an applicable route to achieve low‐temperature growth of graphene, typically shaped like vertical nanowalls. However, for transparent electronic applications, the rich exposed edges and high specific surface area of vertical graphene (VG) nanowalls can enhance the carrier scattering and light absorption, resulting in high sheet resistance and low transmittance. Thus, the synthesis of laid‐down graphene (LG) is imperative. Here, a Faraday cage is designed to switch graphene growth in PECVD from the vertical to the horizontal direction by weakening ion bombardment and shielding electric field. Consequently, laid‐down graphene is synthesized on low‐softening‐point soda‐lime glass (6 cm × 10 cm) at ≈580 °C. This is hardly realized through the conventional PECVD or the thermal chemical vapor deposition methods with the necessity of high growth temperature (1000 °C–1600 °C). Laid‐down graphene glass has higher transparency, lower sheet resistance, and much improved macroscopic uniformity when compare to its vertical graphene counterpart and it performs better in transparent heating devices. This will inspire the next‐generation applications in low‐cost transparent electronics.     

Ultrafast Flame Annealing of TiO2 Paste for Fabricating Dye‐Sensitized and Perovskite Solar Cells with Enhanced Efficiency

Mesoporous TiO₂ nanoparticle (NP) films are broadly used as electrodes in photoelectrochemical cells, dye‐sensitized solar cells (DSSCs), and perovskite solar cells (PSCs). State‐of‐the‐art mesoporous TiO₂ NP films for these solar cells are fabricated by annealing TiO₂ paste‐coated fluorine‐doped tin oxide glass in a box furnace at 500 °C for ≈30 min. Here, the use of a nontraditional reactor, i.e., flame, is reported for the high throughput and ultrafast annealing of TiO₂ paste (≈1 min). This flame‐annealing method, compared to conventional furnace annealing, exhibits three distinct benefits. First, flame removes polymeric binders in the initial TiO₂ paste more completely because of its high temperature (≈1000 °C). Second, flame induces strong interconnections between TiO₂ nanoparticles without affecting the underlying transparent conducting oxide substrate. Third, the flame‐induced carbothermic reduction on the TiO₂ surface facilitates charge injection from the dye/perovskite to TiO₂. Consequently, when the flame‐annealed mesoporous TiO₂ film is used to fabricate DSSCs and PSCs, both exhibit enhanced charge transport and higher power conversion efficiencies than those fabricated using furnace‐annealed TiO₂ films. Finally, when the ultrafast flame‐annealing method is combined with a fast dye‐coating method to fabricate DSSC devices, its total fabrication time is reduced from over 3 h to ≈10 min.     

Creep Behavior of High‐Nb TiAl Alloy at 800–900 °C by Directional Solidification

Cold crucible directional solidification Ti44Al6Nb1.0Cr alloy is crept at 800–900 °C. Experimental results show that creep lifetime significantly decreases with the increasing creep temperature. When creeping at 900 °C under 130 MPa, the T_Q twinning is activated in lamellar structures. The T_Q twinning shows a strong dependency on temperature during creep under low creep‐stress and it can overcome α₂ lamellae and transfer into adjacent γ lamellae. The hardening by mechanical twinning and the softening by α₂ lamellar dissolution take place at different zones in lamellar structures and the strain incompatibility between hardening zone and softening zone promotes the microcracks to form in lamellar structures. The deformation characteristic of hard and soft lamellae is studied. Moreover, recrystallization γ phase formed in lamellar structures near colony boundary during creep at 900 °C accelerates the creep failure.

     

Mikrostrukturelle Veränderungen von Magnesium‐Druckgusslegierungen nach langzeitiger thermischer Beanspruchung

Microstructural Changes of Pressure Die Cast Magnesium Alloys after Long‐Term Thermic Loading The expansion of the application of pressure die cast magnesium alloys for automobiles requires the development of new alloys and the comprehensive assessment of available alloys on aggravated conditions, too. Such conditions are also given at higher temperatures, which can cause the creep of the material and lead to the component failure. Because the microstructural stability decisively depends on the thermic loading, this paper deals with the change of the microstructure and the hardness of the alloys AZ91, AM50 and AE42 after a long‐term annealing at 150 °C and 200 °C in comparison to the pressure die as‐cast condition. The results reveal clear differences of the microstructural stability of the alloys AZ91 and AM50 on the one hand and the alloy AE42 on the other hand. Due to the long‐term annealing at 150 °C the alloys AZ91 and AM50 show chiefly an intense precipitation of Mg₁₇Al₁₂ from the Al‐rich eutectic α‐phase. Furthermore at 200 °C, it is observed the coagulation and coarsening of these precipitates, too. The last appearances are connected with a weakening of the material. Regarding the alloy AE42, the changes of the precipitation state are not so intensely and do yet not lead to a microstructural weakening.     

Coercivity and domain structure of nanograined Fe–C alloys after high-pressure torsion

The microstructure and magnetic properties of binary hypo- and hyper-eutectoid Fe–C alloys were studied. The investigations have been carried out on the samples in the as-cast state, after a long annealing at 725 °C and on the specimens after the high-pressure torsion (HPT). The deformation was carried out at the ambient temperature and the pressure of 5 GPa. The grain size after HPT is in the nanometer range. Long annealing leads to a drastic decrease of the coercivity in comparison with the as-cast alloys. In all alloys the coercivity H _c increases with increasing carbon content. The distance L between pinning points for domain walls decreases with increasing carbon content. Increase of the coercivity and decrease of L are more pronounced below the eutectoid concentration. The coercivity of the nanostructured samples is higher than that of the as-cast alloys. Due to the pinning of domain walls by the cementite particles, the hysteresis loop in the coarse-grained alloys both in as-cast and annealed states has a narrowing near the zero magnetization.     

High‐temperature low‐cycle fatigue behaviour of a cast Al–12Si–CuNiMg alloy

The low‐cycle fatigue behaviour of a cast Al–12Si–CuNiMg alloy, with a high content of Si, is investigated at 200, 350 and 400 °C. The fatigue test results show that the alloy exhibits symmetrical hysteresis loops, moderate cyclic softening and higher fatigue resistance at higher temperature. The fracture surface analysis reveals that more tear ridges are formed at higher temperature, which strongly affect the fatigue resistance. Furthermore, evaluation of the material fatigue resistance using an energy‐based Halford–Marrow model indicates that the material's ability to absorb and dissipate plastic strain energy is enhanced as temperature increases.