Unstable Microstructure Research Articles

Superior high-temperature strength of a carbide-reinforced high-entropy alloy with ultrafine eutectoid structure

Refractory alloys with high-temperature softening resistance are crucial for extreme high-temperature components in aerospace and weapon equipment. Traditional alloys and single-phase refractory high-entropy alloys suffer from unstable microstructures and loss of strength at high temperatures. Here we report a strategy to obtain a superior strong high-entropy alloy by introducing carbides to form micro-nano scale eutectic and eutectoid structures. These metal-carbide interfaces remain stable under high-temperature deformation and exhibit strong dislocation blocking effects. The ultrafine eutectoid structure provides a primary strengthening effect due to its numerous enhanced phase interfaces. The resulting alloy achieves a high temperature yield strength of 1.17 GPa at 1473 K and 0.92 GPa at 1673 K. This work provides valuable insights for optimizing the high-temperature performance and microstructure design of high-temperature composites to further extend their potential applications in high-temperature areas.

Microstructural stability and mechanical properties of the as-cast and heat-treated newly developed TiNbCrTa refractory complex concentrated alloy

In this study, a TiNbCrTa refractory complex concentrated alloy (RCCA) was prepared using vacuum arc remelting. The microstructural evolution and mechanical properties of both as-cast and heat-treated RCCA samples were analyzed. Heat treatment (HT) was performed at 800–1200 °C for 1 h in a vacuum-sealed environment. These samples exhibited a formation of Cr2Nb and Cr2Ti Laves phases. A variation in elemental distribution was observed, with interdendritic (ID) regions showing higher fractions of Ti and Cr, while the dendritic regions had a greater concentration of Ta and Nb. Micro-segregation at the IDs was confirmed through energy dispersive x-ray spectroscopy mapping, which inferred the formation of Cr- and Ti-rich phases during HT at 800–1200 °C. High-temperature HT at 1200 °C for 1 h led to the evolution of the hcp omega phase. Prolonged HT at 1200 °C for 96 h resulted in the evolution of a Cr-rich Laves phase (Cr2Ta), which was homogeneously distributed within the microstructure, indicating an unstable microstructure. Furthermore, despite prolonged HT, a variation in the elemental distribution persisted due to the presence of dendritic and ID regions. Electron backscattered diffraction analysis revealed the presence of bcc and hcp phases in the dendritic and ID regions, respectively, of the as-cast and HTed samples. The as-cast samples demonstrated a high compressive strength of approximately 2 GPa. Micro-hardness values increased with the HT temperature up to 1000 °C. Further increases under HT conditions did not significantly reduce the microhardness value, whereas prolonged HT at 1200 °C led to an increase in the microhardness value. Overall, the newly developed TiNbCrTa RCCA exhibited high-strength behavior even after the phase transformation.

Synergistically interface networked organic–inorganic photocatalytic membrane for highly stable Cr(VI) reduction and hydrogen production

Artificial conversion technology based on solar energy is one of most strategic artificial technologies for addressing environmental hazards and energy crisis. However, micro/nano-sized particulate catalysts mostly have the drawbacks of unstable microstructure and difficult recovery, which inevitably limits the practical application of photocatalysis technology. In order to overcome the defects of most inorganic particulate photocatalysts, a novel organic–inorganic photocatalytic membrane based on highly active Bi4Ti3O12/CdS composite and polyvinylidene fluoride (PVDF) was processed for simulated sunlight (SSL)-driven Cr(VI)-to-Cr(III) (CTC) reduction and H2O-to-H2 (HTH) conversion. Due to the maximum light harvesting capacity and synergistically electronic interaction of this synergistically interface networked organic–inorganic membrane, it possesses accelerated dynamics for separating/transferring photoexcitons and restraining photoexciton recombination, thereby outstanding CTC reduction rate (1.25 × 10-2 min−1) and HTH conversion rate (7.45 mmol∙m−2∙h−1) were achieved under SSL-irradiation, and highly stable photoactivity was maintained even being recycled for dozens of times owing to the convenient separation/recovery performance and stable organic–inorganic matrix. Subsequently, a panel reactor for wastewater purification was constructed to achieve 6.5 × 10-3 min−1 of CTC reduction rate under SSL-irradiation. This study provides a prospective insight for application of photocatalysis technology based on the membrane materials.

Microstructure and Mechanical Properties of the Joints from Coarse- and Ultrafine-Grained Al-Mg-Si Alloy Obtained via Friction Stir Welding.

In the present study, the welding of coarse- (CG) and ultrafine-grained (UFG) Al-Mg-Si alloy using friction stir welding (FSW) was attempted. The purpose of welding the UFG material was to check the possibility of applying FSW to materials with a thermally unstable microstructure, which is achieved by severe plastic deformation. This group of materials has significant potential due to the enhanced mechanical properties as a result of the elevated number of structural defects. The CG sample was also examined in order to assess whether there is an influence of the base material microstructure on the weld microstructure and properties. To refine the microstructure, incremental equal channel angular pressing was used. Plastic deformation resulted in grain refinement from 23 µm to 1.5 µm. It caused an increase in the microhardness from 105 HV0.1 to 125 HV0.1 and the tensile strength from 320 MPa to 394 MPa. Similar welds obtained using an FSW method exhibited good quality and grain size in a stir zone of 5 µm. For both welds, a decrease in the microhardness occurred in the stir zone. However, for the weld of UFG Al-Mg-Si, the microhardness distribution was homogeneous, while for the weld of the CG, it was inhomogeneous, which was caused by different characteristics of the second-phase precipitates. The tensile strength of the welds was lowered and equaled 269 MPa and 220 MPa for the CG and UFG welds, respectively.

Z-scheme electron transfer mechanism of MoS2/CoP heterostructure for simulated solar light induced hydrogen production

Photocatalytic hydrogen production is a potential strategy to convert solar energy into chemical energy. Molybdenum disulfide (MoS2) semiconductor has been broadly used to synthesize high-efficient photocatalysts because of its superior light response and easy photoexcitation. However, its unstable microstructure and low-crystallinity generally lead to rapid recombination of photo-generated carriers, which severely restrict its performance. Herein, cobalt phosphide (CoP) was selected as cocatalyst to build MoS2/CoP heterostructure by a hydrothermal method. Research demonstrated the formation of Z-scheme electron transfer mechanism in the heterointerface, which effectively suppressed the recombination of photoinduced carriers due to the synergetic effect of internal electric field. Thereby, the optimum enhanced simulated solar light (SSL)-induced HER activity was achieved by the MoS2/CoP composite (76.45 μmol·g−1·h−1), which is ∼3.8 fold-greater of MoS2 (∼20.14 μmol·g−1·h−1) and ∼4.5 fold-greater of CoP NPs (∼16.92 μmol·g−1·h−1). Meanwhile, about 10.08% and 3.36% of apparent quantum yields were achieved at the wavelengths of 350 nm and 500 nm, which also reveals the high-efficiency of this catalyst for hydrogen production. This research provides a simple strategy to construct stable and high-efficient MoS2-based photocatalysts for hydrogen production.

Interface propagation and energy dissipation in Shape Memory Alloys

Recent experiments showed that the macroscopic interface propagation of the martensitic phase transformation in Shape Memory Alloys (SMAs) includes unstable microstructure evolution in the macroscopic diffuse interfacial zone (with nucleation, branching, merging and/or annihilation of numerous small domains), releasing the stored energy of the diffuse interface. In this paper, with a one-dimensional Cahn-Hilliard model, a quantitative relation between the energy dissipation and the interfacial properties (interface energy and interface thickness) is revealed: the energy dissipation is governed by the energy barrier caused by the diffuse interface. The relation is verifiable with existing experiments. It can also help understand the interfacial effect on the material's fatigue failure and provide a hint to search for low-hysteresis SMAs: weak first-order phase transformation implies weak energy dissipation (low hysteresis).

Supersaturated Ag-Cu nanoalloy film for high reliability power electronic packaging

Nano-Ag sintering can realize superior thermal, electrical, and mechanical properties for power electronic packaging, while it suffers from unstable microstructure and electrochemical migration. In this work, a supersaturated Ag2.8 wt%Cu nanoalloy film has been developed using pulsed laser deposition, which can be sintered at 250 °C in air for electronic packaging without sacrificing bondability. It was surprisingly found that both thermal stability and electrochemical migration resistance were significantly improved when alloying the Ag with 2.8 wt% Cu. The in situ formed Cu2O/CuO from Ag-Cu nanoalloy existed on the pore surface and along the grain boundaries of Ag/Ag-Cu nanoalloy, which stabilized the microstructure of bondline from 400 h to 1400 h at 300 °C. The synergistic effect of Ag-Cu nanoalloy and Cu oxides realized an obvious protective effect in the process of anodic dissolution, exhibiting a short-circuit time 2.4 times of the pure Ag electrode. The die attach process is compatible with most commercial equipment and Si/SiC dies with improved performance, enabling the supersaturated Ag-Cu nanoalloy to be a promising material for high reliability power electronic packaging.

Flexible sensor based on Hair-like microstructured ionic hydrogel with high sensitivity for pulse wave detection

Flexible sensors will be widely used in wearable pulse wave detection. However, there are few reports that can be used as pulse wave detection both without external pressure and under large pressure. Because it is difficult for flexible pressure sensor with centrosymmetric microstructures to further improve the sensitivity and broaden the pressure range. Based on the modified lithography processes, the PVA-H3PO4 ionic hydrogel film with hair-like tunable microstructures can be synthesized. The flexible sensor with hair-like unstable microstructures achieve high sensitivity of 2296 kPa−1 at 0∼4 kPa, 1167 kPa−1 at 4∼40 kPa and 511 kPa−1 at 40∼100 kPa, respectively. Moreover, the sensor shows low detect limit (∼3.9 Pa), rapid response time (37 ms), long loading/unloading times (2900 cycles) and accurate dynamic response to follow the pressure change. The flexible sensors can monitor the pulse waves both without external pressure and under cuff compression. Not only different arterial pulse waves and heart rate can be detected with no pressure, but also pulse waves can be monitored under different static pressure, which enables the blood pressure measuring. In addition, ionic hydrogels can be collected and reproduced for environmental protection and resources utilization.

Effect of solution temperature on microstructure and mechanical properties of selective laser melted Fe–22Cr–5Ni-0.26N duplex stainless steel

Selective Laser Melting (SLM) technology is effective for fabricating complex shape parts with superior strength and ductility. However, the high-temperature gradient and rapid solidification rate in the SLM process led to unstable material microstructure, which indicated an excellent strength and poor elongation of the SLM specimen. In order to achieve outstanding strength-plasticity matching, the influences of different solution temperatures (1000 °C, 1100 °C, and 1200 °C) on the evolution of phase and microstructure of Fe–22Cr–5Ni-0.26 N duplex stainless steel (DSS) fabricated by SLM were investigated. The results were then used to clarify the corresponding variations in mechanical properties of the specimens under tensile loading. The results revealed that the AS-SLM specimen showed high yield strength and poor elongation which was 1162.52 MPa and 4.97%. Solution treatment at 1000 °C can increase the elongation from 4.97% to 29.21% due to the two-phase structures recovered, grains refined, as well as eliminate dislocation and residual stress through recrystallization. As the temperature rose, the yield strength declined, while elongation remained almost unchanged. Except for elongation, the tensile strength, yield strength, and hardness were all higher than those of forged specimens. Therefore, solution treatment is an efficient strategy to improve the mechanical properties of the SLMFe-22Cr–5Ni-0.26 N DSS with excellent yield strength and plasticity.

Evolution of dislocation and stacking-fault densities for a Cu-0.7Cr-0.07Zr alloy during cryogenic tensile test: An in-situ synchrotron X-ray diffraction analysis

Processing at cryogenic temperatures is one of the ways of improving mechanical properties of copper alloys. However, such processing routes may result in unstable microstructures, which make it difficult to understand the deformation mechanisms controlling microstructure evolution during cryogenic deformation. In this study, the microstructure evolution of a Cu-0.7Cr-0.07Zr alloy was analyzed as a function of the flow behavior during tensile tests performed at 123 K and 298 K, through in-situ X-ray diffraction (XRD) experiments in a synchrotron source. The tensile behavior was analyzed in terms of the dynamic recovery rate using the Kocks-Mecking model, while the evolution of stacking-fault density, dislocation density and crystallite size during the tests was estimated by the modified Williamson-Hall method. Solution-treated and peak-aged conditions were tested to assess the influence of particles distribution. At 298 K, the strain-hardening rate was controlled by dislocation glide and the flow stress was affected mainly by particle strengthening. At 123 K, a simultaneous increase in strength and ductility was attributed to the twinning induced plasticity (TWIP) effect, promoted by precipitate interfaces acting as nuclei for stacking-faults, thus delaying necking and increasing the plasticity. Based on the evolution of dislocation and stacking-fault densities with increasing plastic strain, it was concluded that, at 123 K, mechanical twinning predominated at strain levels below 0.15, mainly for aged conditions, while dislocation glide between twin boundaries, and its accumulation, started after TWIP mechanism reached its saturation. • In-situ synchrotron X-ray diffraction of Cu-0.7–0.07Zr has been performed. • Solutionized and peak-aged conditions were evaluated at 298 K and 123 K. • Both temperature and precipitates affected the plastic strain regime. • At 123 K, mechanical twinning predominated at strains below 0.15. • Twinning induced plasticity (TWIP) effect occurred during plastic flow at 123 K.

Interaction potential function for the deformation analysis of potassium dihydrogen phosphate using molecular dynamics simulation

Potassium dihydrogen phosphate (KDP) is an important nonlinear optical material which plays a core role in electro-optic switches and laser spectroscopy. However, KDP is also one of the most difficult-to-handle materials due to its fragility, unstable microstructure and complex mechanical behaviour. Molecular dynamics (MD) simulation is an appropriate method to explore the deformation mechanisms of the material at the atomic scale. However, the challenge is that there is not a suitable potential function for describing the mechanical behaviour of KDP by using MD simulation. This paper successfully developed a potential function, which enables such insightful investigations. It was found that the established potential function can reliably predict the mechanical properties of KDP including its modulus in different crystal directions and structural changes under various loading conditions.

From diluted solid solutions to high entropy alloys: Saturation grain size and mechanical properties after high pressure torsion

Effects of solutes on saturation grain size and mechanical properties are investigated for the Cantor alloy and Ni-enriched variations ((CrMnFeCo)xNi1-x) with x=0.8, 0.4, 0.08 and 0. Indentation on coarse-grained and severely deformed states shows increasing hardness with increasing alloying content due to higher solid solution strengthening and Hall-Petch contributions. Nanoindentation strain rate jump tests reveal similar rate sensitivities of the deformed states without pronounced transient regimes. All compositions exhibit a history dependent softening indicating an unstable microstructure. The saturation grain size ds after HPT deformation inversely correlates with the solid solution strengthening contribution, i.e. the higher Δτ the lower ds.

Microstructure evolution and mechanical properties improvement of selective laser melted Co-Cr biomedical alloys during subsequent heat treatments

Abstract Selective laser melting (SLM) technology has garnered considerable attention as an advanced manufacturing method that produces biomedical Co-Cr devices specifically developed for dental restoration applications. However, SLM technology inevitably causes unstable microstructures and excessive residual stress due to rapid heating and cooling, which negatively affects their mechanical properties. Also, information on the phase transformation and precipitation behavior during subsequent heat treatments is limited. In this work, SLM-manufactured Co-Cr specimens were initially subjected to stress-relieving annealing (SRA) at various temperatures (850, 950, 1050, and 1150 °C) and then porcelain firing (PF) treatments, and the microstructure evolution and mechanical properties improvement during subsequent heat treatments was investigated in detail. Scanning electron microscopy (SEM) combined with energy-dispersive x-ray spectroscopy (EDS), x-ray diffractometer, transmission electron microscopy, and Image Pro Plus 6.0 (IPP) software were used to analyze alloy microstructures, whereas tensile tests were performed to evaluate corresponding performance. Results showed that after SRA and/or PF treatments, a large number of nanoscale precipitates were observed, and acicular or lamellar e phase appeared. For SRA-treated specimens, as annealing temperatures increased, the volume fraction of γ phase decreased, especially after annealing at 950 °C, the γ phase was completely transformed into the e phase. In contrast, for (SRA + PF)-treated specimens, the volume fraction of γ phase first decreased and then increased with the increase of annealing temperatures. IPP analysis found that the amount, size, and morphology of the precipitates varied with the change in annealing temperature. During annealing, isothermal martensite transformation (γ →e) dominated due to the long duration between 600 and 900 °C, while the amount of athermal martensite transformation (γ →e) gradually increased when porcelain firing. Changes in the above-mentioned microstructures affected corresponding mechanical properties. From 850 °C to 1050 °C, tensile strength and elongation continue to improve. At 1150 °C, both tensile strength and 0.2% yield strength decreased, but the elongation was further improved. The variation in hardness (HV10) with temperature was the same as that of the volume fraction of e phase, that is, the higher the volume fraction of e phase, the higher the hardness. SEM/EDX analysis of tensile fracture surfaces revealed a quasi-cleavage fracture. In conclusion, 1150°C-SRA and PF treatments are an effective method for homogenizing microstructure and improving mechanical properties.

Elevated Temperature Compression Deformation Behavior of Commercial Pure Zr Combined with the Constitutive Equation and Processing Map

The compression deformation behavior of commercial pure zirconium at elevated temperature is systematically investigated at different conditions. The corresponding deformation mechanism of pure Zr is clarified for analyzed flow stress, work hardening rate, activation energy, and microstructure. A modified Arrhenius‐type equation is proposed for future numerical simulation, which shows a good relativity between the theoretical and experimental data. The apparent deformation activation energy is 209 kJ mol−1, indicating that a cross‐slip of dominant dislocation mechanism controls the deformation process. Established based on the dynamic material model (DDM), processing maps indicate that the optimum processing conditions are 700–900 °C at 0.1–0.14 s−1 and 905–950 °C at 3–20 s−1 with the high efficiency of power dissipation. Microstructure examination reveals that the main restoration mechanism in the safe domain is related to dynamic recrystallization and the unstable regional microstructure results in the flow localization and cracking behavior.

Fabrication of IPL-Sintered Ag-MWCNT composite circuits and their flexibility characteristics

Printed electronics have been used because of various advantages such as low cost, productivity, and eco-friendly processing. However, for fabricating printed electronics, the conventional sintering process conducted at high temperatures has problems such as thermal damage and long processing time. To solve these problems, low-temperature processes such as intensive pulsed light (IPL) have been widely studied. Research of composite materials has become important in supporting these low-temperature processes. We fabricated Ag-decorated, multi-walled carbon nanotubes (Ag-MWCNTs) in composite pastes and sintered printed circuits using IPL energy. We investigated the electrical resistivity, microstructure, and flexibility of the Ag-MWCNT composite circuit with various durations of IPL energy irradiation and Ag-MWCNT content (0, 1, 2, and 3 wt%). The electrical resistivity of the circuit sintered with IPL energy decreased with irradiation time, but it had an unstable microstructure from the drastic sintering process. The Ag-MWCNTs could solve this problem using the net structure of the MWCNTs. The Ag-MWCNT composite nanoparticles could be a solution to overcome the problems in the photonic sintering process using IPL energy.

Superplastic-Like Elongation by Transition of Deformation Mechanism from Grain Boundary Sliding to Solute Drag Creep in Fine-Grained Al-Mg Solid Solution Alloy

It is widely accepted that the dominant deformation mechanism of fine-grained superplasticity is through grain boundary sliding (GBS) that occurs in fine-grained materials. However, it has been reported that in “Class I” solid solution alloys, superplastic-like behavior controlled by trans-granular deformation occurs by solute drag creep. In this study, we have investigated superplastic behavior in a fine-grained aluminum solid solution alloy with a thermally unstable microstructure. To obtain fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum (Al−Mg) alloy. An equiaxial fine-grained microstructure with a grain size of 7.4 μm was obtained after FSP; however, this microstructure was unstable at high temperatures. Generally, for fine-grained superplasticity or GBS to occur or continue, the fine-grained microstructure must be smaller than 10 μm during high-temperature deformation. However, a large elongation of over 200% was observed at high temperatures despite the occurrence of grain growth. From microstructural observations, it was determined that a fine-grained microstructure is maintained in the early stage of deformation, but at strain levels greater than 100%, trans-granular deformation occurs. The microstructural feature of this trans-granular deformation is similar to the deformation microstructure of solute drag creep observed in “Class I” solid solution alloys. This indicates that a change in the deformation mechanism from GBS to solute drag creep takes place during high-temperature deformation. Here, based on our observations on our model system, which is a thermally unstable aluminum solid solution alloy, we discuss the possibility of a superplastic elongation occurring by means of a transition of the deformation mechanism.

Study on hot deformation and process parameters optimization of Ti-10.2Mo-4.9Zr-5.5Sn alloy

The isothermal constant strain rate compression test of Ti-10.2Mo-4.9Zr-5.5Sn alloy was carried out by Gleeble-3500 thermomechanical simulator. The hot deformation behavior at temperature of 670–820 °C and strain rate of 0.001–10s−1 was studied, and the activation energy of hot deformation was calculated. The results show that the flow stress of the alloy is more sensitive to the deformation temperature and strain rate, and is reduced by the increase of the deformation temperature and the decrease of the strain rate; the activation energy of the hot deformation of the alloy is higher than the self-diffusion activation energy of pure α titanium and pure β titanium alloy. According to the error calculation, the correlation coefficient and the average relative error of the constitutive model of Ti-10.2Mo-4.9Zr-5.5Sn alloy established by strain compensation are 0.9716 and 6.24%, and they own good precision. Based on the hot processing map of the Ti-10.2Mo-4.9Zr-5.5Sn alloy set up by the dynamic material model, instability zone's unstable form is mainly local flow and stable zone's deformation mechanism is mainly dynamic recovery and dynamic recrystallization through analyzing stable and unstable zones' microstructure; and it is found that the suitable deformation parameters of Ti-10.2Mo-4.9Zr-5.5Sn alloy are as follows: deformation temperature 670–700 °C, strain rate 0.001–0.003s−1, deformation temperature 790–820 °C, and strain rate 0.001–0.1s−1 through the processing map and microstructure observation.

Enhancement of high-temperature strength of Ni-based films by addition of nano-multilayers and incorporation of W

Nanocrystalline Ni thin film exhibits poor thermo-mechanical properties due to its unstable microstructure at elevated temperature. Here, the paper endows a new approach to solve above issue via addition of nano-multilayers and incorporation of W for nanocrystalline Ni-based films, to provide novel Ni/Ni3Al-W nano-composite multilayered structure with high hardness and good thermal stability. The thermal evolution of microstructure and mechanical properties was investigated to reveal nanocrystalline stability and strengthening mechanisms for co-sputtered Ni/Ni3Al-W multilayers with varied W concentrations and annealing temperature. The lamellar structure and nonequilibrium phases are well maintained in 600 °C annealed multilayers, while nano-grains are further refined with increasing W addition. Annealing at 800 °C results in the appearance of elemental redistribution and phase separation in multilayers, leading to the layered structure dissolved and globular W-related particles precipitated. Annealing hardening is founded in most of annealed Ni/Ni3Al-W multilayers. Based upon microstructure observation, grain boundary relaxation and W-related phase precipitation are mainly responsible for the hardness enhancement of multilayers at 600 °C and 800 °C, respectively. Notably, the best hardness is achieved at the value of 15.6 GPa for 800 °C annealed 12.5 at% W doped Ni/Ni3Al-W multilayer, which shows the residual layer interfaces with larger precipitations in microstructure. This hardness increment for annealed Ni-based multilayers can be attributed that the high degree of strengthening is provided by a combination of hardening precipitation and survived lamellar structure via the Orowan mechanisms, offering a feasible insight to develop nano-metallic coatings for further increasing thermo-mechanical properties.

Directional solidification and growth characteristics of Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramic by laser floating zone melting

Directionally solidified Al2O3/Er3Al5O12/ZrO2 ternary eutectic ceramic in situ composite rods with length of 110 mm have been fabricated by laser floating zone melting. The microstructural characteristics of steady growth zone, initial growth zone and solid/liquid interface are investigated under high temperature gradient. In the steady growth zone, the eutectic spacing (λ) is rapidly decreased as increasing the growth rate (V), and the corresponding relationship between growth rate and eutectic spacing is determined to be λ = 11.14 × V −1/2. The temperature gradient has been measured to be about 5.3 × 103 K/cm. In the initial growth zone, the melting process and temperature distribution are recorded by infrared thermal imager, and several unstable complex microstructures are observed. In the quenched zone, the regular eutectics with minimum eutectic spacing of 200 nm are obtained. Moreover, the solid/liquid interface during solidification shows convex interface morphology and the interface height is gradually decreased as increasing the growth rate. The eutectic growth behaviors at the center and edge of the as-grown rod are compared and discussed.

Superplastic Elongation through Deformation Mechanism Transition during High-Temperature Deformation in Thermally Unstable Fine-Grained Aluminum Solid Solution Alloy

In this study, the superplastic behavior on a fine-grained aluminum solid solution alloy consisting of thermally unstable microstructures was investigated. In order to obtain the fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum alloy. An equiaxial fine-grained microstructure of 7.8 mm was obtained after FSP, but this microstructure was thermally unstable at high temperatures. Commonly, for fine-grained superplasticity to occur (or to continue grain boundary sliding (GBS)), it is necessary to keep the fine-grained microstructure to less than 10 mm during the high-temperature deformation. However, in this study, a large elongation of over 200% was observed at high temperatures in spite of the occurrence of grain growth. From the microstructural observations, it was determined that the fine-grained microstructure was maintained until the early stage of deformation, but the transgranular deformation was observed at a strain of over 100%. The microstructural feature of the abovementioned transgranular deformation is similar to the deformation microstructure of the solute drag creep occurring in "Class I"-type solid solution alloys. This indicates that the deformation mechanism transition from GBS to the solute drag creep occurred during high-temperature deformation. Here, the possibility of occurrence of the superplastic elongation through deformation mechanism transition is discussed as a model of the thermally unstable aluminum solid solution alloy.