High-temperature Plastic Deformation Research Articles

Characteristics of oxide-dispersion strengthened alloys produced by high-temperature severe deformation

This study is to explore an economically attractive and technically feasible processing method for oxide-nanoparticle strengthened alloys for fusion reactor application. Despite many scientific merits of the advanced oxide-dispersion strengthened (ODS) alloys, such as the nanostructured ferritic alloy (NFA) 14YWT, the only viable production path for a high-quality NFA is the high-power mechanical alloying process. This process is often a multi-day high-speed ball milling of alloy powder with a small quantity of yttria (Y2O3) powder, followed by the milled-powder consolidation using extrusion or other methods and additional thermomechanical processing (TMP) for property control. This complex production path, including the low-temperature mechanical alloying in particular, has limited technical advancement toward the cost-effective and scale-up production of ODS alloy components. To overcome such a practical limitation, we proposed to explore alternative low-cost processing routes using traditional thermomechanical processing (TMP) method only. A series of continuous TMP cycles, which were designed to impose high-temperature severe plastic deformation (HT-SPD) conditions to the consolidated powder mixtures, were applied to achieve the effective distribution of oxide particles in nanograin structure and thus desirable mechanical properties. Since the reduced-activation ferritic-martensitic (RAFM) alloy powders (Fe-10Cr and Fe-14Cr alloys with various Y contents) are available in our inventory, we focused to utilize the new solid-state synthesis approach for controlling oxide (oxygen source) dissolution and nanoscale clustering in nanograin structure in those alloys. A combination of powder consolidation at 900 °C and continuous thermomechanical activation at 600 °C yielded two essential ODS alloy microstructure contents–nanograin structure and nanoparticle distribution–and thus demonstrated a good combination of strength and ductility.

АНАЛИЗ ВЛИЯНИЯ ПРЕДВАРИТЕЛЬНОЙ И ЦИКЛИЧЕСКОЙ ПЛАСТИЧЕСКОЙ ДЕФОРМАЦИИ НА ХИМИКО-ТЕРМИЧЕСКУЮ ОБРАБОТКУ КОНСТРУКЦИОННЫХ СТАЛЕЙ

The subject of the study was thermodiffusion nitrided and silicified layers on steels obtained after plastic deformation. The influence of preliminary plastic deformation on the low-temperature nitriding process and cyclic plastic deformation on the high-temperature silicification process is studied. The results obtained are of interest for accelerating the production processes of chemical-thermal treatment of steels. It is established that preliminary surface plastic deformation contributes to the intensification of the process of ion-plasma nitriding of structural steel. The presence of high-temperature cyclic plastic deformation ensures the intensification of diffusion processes during the resorption of the diffusion silicified layer on structural steel.

Optimization of the Mechanical and Corrosion Resistance of Alloy 625 through Aging Treatments

In the as-annealed condition, the nickel-based Alloy 625 has excellent mechanical and corrosion properties compared to those of common stainless steels. This peculiarity enables its exploitation in several industrial fields at cryogenic and high temperatures and in the presence of severely corrosive atmospheres. However, in this alloy, when high-temperature plastic deformation processes and heat treatments are not carefully optimized, the occurrence of excessive grain coarsening can irremediably deteriorate the mechanical strength, possibly leading to incompatibility with the standard requirements. Therefore, this research work investigated the possibility of adopting single- and double-aging treatments aimed at improving such strength loss. Their optimization involved identifying the best compromise between the hardening effect and the loss in corrosion resistance induced by the simultaneous formation of intergranular chromium-rich carbides during aging. The investigation of the aging treatments was performed using hardness, tensile and intergranular corrosion tests considering different time–temperature combinations in a range from 621 °C to 732 °C. Double aging resulted in a considerable acceleration in the hardening response compared to single aging. However, even after its optimization in terms of both temperature and time, the intergranular corrosion resistance remained a critical aspect. Among all the tested conditions, only single aging at 621 °C for 72 h was acceptable in terms of both mechanical and corrosion properties. The influence of longer exposures will be investigated in a future study.

Кераміка ZrB2 з добавками MoSi2, SiC і B4C: кінетика ущільнення, фазоутворення та опір повзучості

A study was carried out of the processes of compaction, structure formation and mechanical properties of ceramics based on zirconium boride with sintering-activating additives of boron, silicon and chromium carbides, as well as molybdenum silicide, obtained under hot pressing conditions in a CO atmosphere. In ZrB2—18% (vol.) B4C ceramics, the use of the B4C additive reduces the optimal hot pressing temperature to 1940 °C and accelerates the compaction process of the ceramics. The influence of the sample preparation background on high-temperature creep has been established, as a result of which either plastic flow of the material occurs over a wide temperature range, or a high temperature threshold for yield and brittle fracture. In ZrB2—SiC ceramics, during high-temperature plastic deformation both during sintering and in creep tests, a bidisperse structure with a submicrograined component is formed, which is responsible for high creep rates. In ZrB2—B4C ceramics there is no submicrograin component, which provides high creep resistance up to 2000 °C. The phase composition of ZrB2—MoSi2 ceramics changes dramatically during hot pressing; it is represented by a composition of a ZrB2 solid solution with the second phases of SiC and B4C, and in terms of creep resistance it occupies an intermediate position between two other ceramics. Keywords: ultra-high temperature ceramics, zirconium diboride, silicon, boron and chromium carbides, hot pressing in a CO atmosphere, compaction kinetics, structure, creep resistance.

Underlying mechanisms of enhanced plasticity in Ti/Al laminates at elevated temperatures: A molecular dynamics study

The Ti/Al laminated metal components have attracted considerable attention due to their impressive strength and lightweight properties, positioning them as promising materials for aerospace and automotive applications. Although experimental evidence has highlighted their exceptional elongation and formability at elevated temperatures, there is still limited understanding of the underlying atomic-level mechanisms governing high-temperature plastic deformation, particularly with regards to the titanium aluminide (TiAl3) layer. This study utilized molecular dynamics (MD) simulations to investigate the tensile deformation of Ti/Al laminates, emphasizing the effects of temperature and TiAl3 layer thickness on atomic structural evolutions and mechanical properties. The MD simulations confirmed the decreases in the elastic modulus and peak stress at elevated temperatures, consistent with the experimental findings. The total dislocation length displayed an increasing trend with temperature, reaching a maximum at 600 K, before decreasing at 900 K due to amorphization. Moreover, the coordinated deformation among the Ti, Al and TiAl3 layers at elevated temperatures contributed to enhanced plasticity of the Ti/Al laminates. Furthermore, the TiAl3 layer thickness positively influenced the total dislocation length. However, increasing the layer thickness also leads to more high-strain bands in the TiAl3 layer, resulting in crack nucleation and decreased ductility of the Ti/Al laminate. This study provides valuable insights into the plastic deformation of Ti/Al laminates at elevated temperatures from the molecular dynamics perspective, benefitting further improvement in their plastic forming technique and engineering applications.

Dynamic Recrystallization Behavior of Q370qE Bridge Steel

Bridge steel has been widely used in recent years for its excellent performance. Understanding the high-temperature Dynamic Recrystallization (DRX) behavior of high-performance bridge steel plays an important role in guiding the thermomechanical processing process. In the present study, the hot deformation behavior of Q370qE bridge steel was investigated by hot compression tests conducted on a Gleeble 3800-GTC thermal-mechanical physical simulation system at temperatures ranging from 900 ℃ to 1100 ℃ and strain rates ranging from 0.01 s−1 to 10 s−1. The obtained results were used to plot the true stress-strain and work-hardening rate curves of the experimental steel, with the latter curves used to determine the critical strains for the initiation of DRX. The Zener-Hollomon equation was subsequently applied to establish the correspondence between temperature and strain rate during the high-temperature plastic deformation of bridge steel. In terms of the DRX volume fraction solution, a new method for establishing DRX volume fraction was proposed based on two theoretical models. The good weathering and corrosion resistance of bridge steel lead to difficulties in microstructure etching. To solve this, the MTEX technology was used to further develop EBSD data to characterize the original microstructure of Q370qE bridge steel. This paper lays the theoretical foundation for studying the DRX behavior of Q370qE bridge steel.

Analysis of the High Temperature Plastic Deformation Characteristics of 18CrNi4A Steel and Establishment of a Modified Johnson–Cook Constitutive Model

To investigate the plastic deformation behavior of 18CrNi4A steel at high temperatures, an isothermal hot compression test was conducted on a Thermecmastor-Z series test machine at deformation temperatures from 1273 K to 1423 K and strain rates ranging from 0.01 s−1 to 10 s−1. The effects of these two factors on the flow stress were analyzed. Based on the true stress–strain experimental curves, the original Johnson–Cook constitutive model was applied to determine the flow stress data under different deformation conditions. The prediction results of the model were compared with the experimental data. The main reason for the large deviation observed between them was that the coupling relationship between the deformation temperature and the strain rate was not considered, so the original Johnson–Cook model was modified. The correlation coefficient and average absolute relative error of the original Johnson–Cook model were 0.962 and 16.36%. The prediction accuracy of the modified Johnson–Cook model was improved to 0.991 and 5.58%, respectively. The results show that the modified Johnson–Cook model exhibits higher prediction precision, which is beneficial for the broader application of 18CrNi4A steel in the industry.

Nonnegligible Temperature Drop Induced by Thermocouple on Samples in Gleeble Tests

High-temperature plastic deformation is one of the main methods for the fabrication of titanium-based alloys. Accurate determination of the mechanical constitutive relation is pivotal for the design, optimization, as well as the prediction of the mechanical behavior of materials. In this study, finite element simulations were carried out to simulate the Gleeble thermal compression experiment of titanium alloy samples, focusing on different thermocouple design parameters, including thermocouple material and wire diameter, etc. The results show that the heat dissipation of the thermocouple distorted the local temperature field of the contact point between the sample and the thermocouple, resulting in a deviation of the measured temperature. Through finite element method (FEM) simulation and analysis of the changes caused by various factors and comparison with the metallographic morphology of titanium alloy samples from relevant heat treatment experiments, it was shown that the material and wire diameter of the thermocouple, the thermal conductivity coefficient of sample, and the testing temperature of the sample all affected the measurement results. The thermal conductivity of sample had the largest influence on the measurement accuracy. Based on the finite element simulation and experimental comparison, the corresponding correction method and correction formula are proposed.

A discrete dislocation dynamics model of creep in polycrystals

The dislocation creep represents high temperature plastic deformation of metallic materials. However, the subject is usually approached with constitutive and phenomenological formulations. The notable exceptions are the recent use of numerical modeling. For example, single-crystal discrete dislocation dynamics (DDD) has been adopted to involve dislocation glide and climb together with diffusion of vacancies. This naturally provides a physics-based framework for dislocation creep. However, the existing model(s) are limited to single crystals and hence are limited in scope. This study extended a single crystal dislocation creep model into polycrystalline ensemble. This was then used to simulate steady state creep rate (ε˙) with respect to creep stress (σ), temperature (T) and grain size (d) in polycrystalline aluminum. Our virtual experiments showed an Arrhenius relationship between ε˙ and T, and a power law scaling with σ. Further, our simulations also revealed a power law increase in ε˙ with d. This is striking, as available experimental data indicate both grain size dependence as well as independence. The answer to this apparent contradiction also emerged from our numerical simulations. It was shown that ε˙ is controlled by interactions of dislocations with static obstacles and grain boundaries. Dominance of static obstacles, in particular, significantly diminished the grain size effect. The role of dislocation mean free path on the pinning and dislocation creep behavior of polycrystalline metallic material was thus mechanistically established.

Intrinsic-Strain Engineering by Dislocation Imprint in Bulk Ferroelectrics.

We report an intrinsic strain engineering, akin to thin filmlike approaches, via irreversible high-temperature plastic deformation of a tetragonal ferroelectric single-crystal BaTiO_{3}. Dislocations well-aligned along the [001] axis and associated strain fields in plane defined by the [110]/[1[over ¯]10] plane are introduced into the volume, thus nucleating only in-plane domain variants. By combining direct experimental observations and theoretical analyses, we reveal that domain instability and extrinsic degradation processes can both be mitigated during the aging and fatigue processes, and demonstrate that this requires careful strain tuning of the ratio of in-plane and out-of-plane domain variants. Our findings advance the understanding of structural defects that drive domain nucleation and instabilities in ferroic materials and are essential for mitigating device degradation.

High-Temperature Plastic Deformation of MgO Products

High-Temperature Plastic Deformation of MgO Products

Effect of short-range order on the high-temperature plastic deformation behavior of Ni–Cr–W, Ni–W, and Ni–Re single crystals

The development of a Ni-based superalloy that consists of only the γ-solid-solution phase is highly desired for some applications such as high-temperature gas-cooled reactors in power plants. In this study, the temperature dependence of the plastic deformation behavior of Ni–22Cr–8W (at%) alloy was examined for the first time by using a single crystal, focusing on the influence of the short-range order (SRO), especially upon high-temperature deformation behavior. The critical resolved shear stress (CRSS) for (111)[101¯] slip exhibited a rapid decrease as the temperature increased at low temperatures but an almost constant value of ∼45 MPa between 500 and 900 °C. In addition, a slight yield stress anomaly (YSA) was observed, reaching a maximum value at 1000 °C.To clarify the influence of the SRO on the YSA behavior, the mechanical properties of Ni–8W and Ni–5Re single crystals were also examined. Similar temperature dependence for the CRSS, including the occurrence of slight YSA, was observed for Ni–8W, in which SRO developed. In the Ni–5Re single crystal in which SRO did not significantly develop, however, the CRSS monotonically decreased as the temperature increased. The results suggest that the presence of SRO in a Ni–Cr–W alloy contributes to the strengthening of the alloy not only at low temperatures but also at high temperatures up to ∼1000 °C. The results strongly suggested that the pseudo-PLC effect is related to the origin of YSA appearing in Ni-based γ-solid-solution alloys at high temperatures.

High-temperature plastic deformation of MgO-products

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In situ observation of deformation and failure behavior in the burn-through instability zone of in-service welding

Based on the in-situ high-temperature SEM and laser scanning confocal microscope (LSCM), the high-temperature plastic deformation and failure behavior of X65 pipeline steel at 1000℃ and 1200℃ were studied in this paper, and the strain evolution behavior in this process was analyzed by DIC technology. In this study, the precipitation and dissolution of carbide at the austenite grain boundary were observed during the in situ heating process. The external load showed a great effect on the growth of the grains, and the grains grew rapidly by swallowing each other. The grain size, material performance, and misorientation played an important role during the deformation and crack evolution. At high temperatures, cracks were mainly initiated and propagated in the position with a large plastic strain. The strain concentration showed a significant effect on crack initiation.

Enhanced strength and plasticity in a Nb2MoWC0.5 alloy via eutectic dilute carbide

Refractory niobium alloys are widely used in aero-engine combustion chambers and rocket propulsion systems. Conventional Nb-based alloys often suffer from insufficient high-temperature strengths or poor room-temperature plasticity. Here we report a strategy to develop a novel Nb2MoWC0.5 niobium alloy with a good strength-plasticity synergy by introducing eutectic NbC carbide. Microstructure characterization reveals that the as-prepared alloy exhibits a hypoeutectic microstructure, in which low-energy eutectic interface remains stable under high-temperature compression up to 1473 K. The stable low-energy metal-carbide interface enables not only a superior high-temperature strength (>1 GPa at 1473 K and 0.61 GPa at 1673 K) but also possesses good room-temperature plasticity (a true plastic strain of 16%) via microcrack tip blunting. The dilute NbC carbide with alloying elements has low stacking fault energy and improved high-temperature plastic deformation. This strategy provides theoretical guidance for designing next-generation high-performance high-temperature alloys to bypass the limitation of alloying elements in conventional refractory alloys.

A Study of the Metallurgical and Mechanical Properties of Friction-Stir-Welded Pure Titanium

Commercially pure titanium (CP-Ti) plates were friction-stir welded (FSWed) using a welding tool with a tungsten carbide (WC) pin. The bead-on-plate technique was applied to reduce the effects of welding defects, such as incomplete penetration. An X-ray inspection and fractography showed that the FSWed material was free of defects and of WC particles, which may have originated from the welding tool. The appearance of refined equiaxed grains in the thermo-mechanically affected zone (TMAZ) may have been related to dynamic recrystallization (DRX) occurring during the FSW due to the high temperature and intensive plastic deformation involved in the process. Grain refinement, mechanical twinning, and increased dislocation density were detected within the TMAZ, and these microstructural changes were considered to be responsible for the improved mechanical properties of the TMAZ. The TEM study reported in the current paper revealed the presence of nano-sized grains in the FSWed material due to dynamic recrystallization (DRX) occurring during the processing stage. The microstructure obtained during FSW of Ti has been reported in a several publications, yet many discrepancies can be found in these publications. Among these discrepancies are the size and the shape of the grains at the various zones, as well as the presence or non-presence of various zones at the vicinity of the weld. The current study contradicts the argument for correlations between the conditions prevailing at different points across the TMAZ and microstructural changes, which were previously proposed by several researchers.

Strain rate response and rheological characteristics of ZrCuNiAl bulk metallic glass in supercooled liquid region

In this study, the plastic deformation behavior of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass (BMG) in the supercooled liquid region (SLR) was studied by high temperature compression experiments at various strain rates. The results show that the high temperature plastic deformation behavior of the BMG in SLR is highly sensitive to the strain rate. With the decrease in strain rate, the steady-state flow stress and peak stress decreases, and the viscosity increases. Meanwhile the stress overshoot gradually disappears, the deformation of the BMG specimen changes from non-Newtonian flow to near-Newtonian flow, and there is a critical strain rate of 5 × 10−4 s−1. The surface deformation profile changes from an "S" shaped truncated cone to a true truncated cone. The plastic deformation of BMGs at room temperature is completed by the initiation and expansion of the shear bands, while the high temperature plastic deformation is caused by the generation and annihilation of free volume.

Structural evolutionary behavior of Zr-based bulk metallic glasses under thermoplastic deformation

Zr55·7Cu22·4Al14.7Ni7.2 alloy rods with a diameter of 3 mm were prepared by copper mold suction casting. The microstructure, thermomechanical properties and high temperature plastic deformation behavior of the alloy specimens before and after uniaxial compression in supercooled liquid region (SLR) were investigated by X-ray diffractometer, scanning electron microscope, differential scanning calorimeter and microcomputer-controlled electronic universal testing machine. And the effects of different deformation conditions on the thermomechanical properties and microhardness of the specimens after high temperature deformation were analyzed. The results show that during plastic deformation in SLR, this alloy exhibits stress overshoot in the low temperature region - high strain rate conditions, exhibiting non-Newtonian fluid properties, while alloy softening occurs in the high temperature region - medium to high strain rate, showing the deformation behavior of a Newtonian fluid. The alloy crystallizes after deformation at high temperature and low strain rate (750 K, 2 × 10−4 s−1 and 760 K, 2 × 10−4 s−1∼5 × 10−4 s−1), generating dendritic τ3 phase (Zr51Cu28Al21), while still maintaining an amorphous structure in the low and medium temperature region, but this structure has part of the amorphous matrix that has been transformed into ordered clusters of atoms, and the degree of transformation increases sharply with decreasing strain rate, which in turn causes a sharp reduction in the crystallization incubation period and the exothermic enthalpy of crystallization. And the effect of high temperature deformation on the Vickers microhardness of metallic glass was analyzed.

The Wear and Electrical Performance of Cu-CNTs Composites with Network Structure CNTs Fabricated by Multi-Directional Forging

Cu-[Formula: see text] CNTs composites ([Formula: see text], 1, 2[Formula: see text]vol.%) were successfully prepared through a combination of pre-treatment, powder metallurgy and multi-directional forging (MDF) processes. It provided a solution for the industrial production of structural composites with reticulated CNTs. The effect of CNTs contents on wear and electrical performance of the composites was investigated. The addition of CNTs results in a significant increase in the wear resistance of the composites. During the wear process, reticulated CNTs will carry the load barrier to high temperature softening and plastic deformation of the Cu-matrix, which protects the matrix eventually. In general, the addition of CNTs will reduce the conductivity of the composites. Under MDF process, the CNTs were organically linked together and formed a network structure as the volume fraction increased from 1% to 2%, resulting in a significant promotion of conductivity by providing paths for electron transfer. The contact resistance increased slightly when 2[Formula: see text]vol.% CNTs were added, with an average value of 5.87[Formula: see text]m[Formula: see text]. The contact resistance volatility of the Cu-2[Formula: see text]vol.% CNTs was the most stable (variance of 1.485). Reticulated CNTs can effectively attenuate the erosive effect of molten pools and welding bridges on the anode material, and are ideal reinforcers for Cu-based electrical contact materials.

Research on high temperature deformation behavior and constitutive relation of Ti2AlNb alloy

A high-temperature plastic deformation is a failure form of Ti2AlNb alloy that deserves attention in the service process of aero-turbine engines. In this study, the deformation behavior of Ti2AlNb alloy has been studied through compression at 900 ~1100 °C and strain rates of 0.1 s−1, 0.01 s−1, and 0.001s−1. Then, the constitutive model of Ti2AlNb alloy has been established based on the true stress-strain curve. The results show that Ti2AlNb alloy has a high sensitivity to strain rate and temperature at the experimental condition. Ti2AlNb alloy still maintains good resistance to deformation at 900 °C, and the microstructure at the core region is basically unchanged after compression. In the boundary region, the microstructure changes significantly under the influence of stress. However, the yield strength of Ti2AlNb alloy decreases to about 5 MPa at 1100 °C, 0.001s−1, and the microstructure of the alloy has significant transformation which the O-phase structure decomposes and forms the α2 strip structure.