Effect of <Emphasis Type="Italic">r</Emphasis>-value and texture on plastic deformation and necking behavior in interstitial-free steel sheets

Three initial tensile specimens having different textures and, in consequence, different r-values were cut from a sheet of an interstitial-free steel. Using these specimens, the effect of r-value and texture on plastic deformation and the necking behavior were studied by tackling the strain state and texture during tensile tests. A reduced decrease in work hardening rate of tensile specimens with higher r-values led to a slower onset of diffuse necking which offers an increased uniform elongation. A slower reduction in thickness of specimens with a higher r-value provided a favorable resistance against onset of failure by localized necking.     

Dynamic recrystallization behavior and constitutive modeling of as-cast 30Cr2Ni4MoV steel based on flow curves

The compression deformation of 30Cr2Ni4MoV steel at different temperatures and strain rates is carried out on Gleeble 1500 thermal mechanical simulation tester. Based on the experimental flow curves, the strain hardening rate curves (θ = dσ/dε versus σ) are derived, from which the characteristic stresses and strains are identified. Meanwhile, the dependences of the characteristic stresses and strains on Zener-Hollomon parameter are determined and the results show that the value of the critical stress of dynamic recrystallization is close to the value of the steady stress. With the aid of the experimental flow curves, the Avrami equation is employed to describe the kinetics of dynamic recrystallization. The time exponent (n) is expressed as a power law function of Zener-Hollomon parameter and the Avrami constant (k) is determined as a function of half of the time for the complete dynamic recrystallization (t ₅₀). Furthermore, a constitutive model is presented based on the rule of mixtures when the dynamic recrystallization occurs. Validation of the constitutive model is implemented and the simulated results agree well with the experimental results.     

Determination of Johnson–Cook Plasticity Model Parameters for Inconel718

In order to simulate foreign object damage (FOD) phenomenon in aircraft high-pressure compressor blades made of a nickel-based super-alloy, Johnson–Cook (J–C) plasticity model was used. For prediction of material’s plastic behavior at temperature of 400 °C (working temperature of the blades) in the range of strain rates associated with the FOD phenomenon (in order of 10⁶ s^?1), material parameters of A, B, C, n and m for the J–C plasticity model had to be determined experimentally. Parameters of A, B and n with values of 1108, 699 MPa and 0.5189, respectively, were obtained from quasi-static tensile tests. Moreover, m was determined to be 1.2861, also through quasi-static tensile tests with a strain rate of 1 s^?1 at three temperatures of 475, 550 and 625 °C. However, in order to determine C, firstly a steel ball was impacted on the surface of a flat specimen made of a precipitation-hardening alloy, and then, the impact site was 3D scanned to obtain the induced crater profile. Finally, the impact test (ballistic) was simulated using Abaqus, and a C value of 0.0085 was determined by comparing the actual crater profile with the one obtained from the simulation through a trial-and-error approach.     

Temperature-Dependent Flow Behavior and Microstructural Evolution During Compression of As-Cast Mg-7.7Al-0.4Zn

The microstructure and mechanical properties improve substantially by hot working. This aspect in as-cast Mg-7.7Al-0.4Zn (AZ80) alloy is investigated by compression tests over temperature range of 30-439°C and at strain rates of 5 × 10^?2, 10^?2, 5 × 10^?4 and 10^?4 s^?1. The stress exponent (n) and activation energy (Q) were evaluated and analyzed for high-temperature deformation along with the microstructures. Upon deformation to a true strain of 0.80, which corresponds to the pseudo-steady-state condition, n and Q were found to be 5 and 151 kJ/mol, respectively. This suggests the dislocation climb-controlled mechanism for deformation. Prior to attaining the pseudo-steady-state condition, the stress-strain curves of AZ80 Mg alloy exhibit flow hardening followed by flow softening depending on the test temperature and strain rate. The microstructures obtained upon deformation revealed dissolution of Mg₁₇Al₁₂ particles with concurrent grain growth of α-matrix. The parameters like strain rate sensitivity and activation energy were analyzed for describing the microstructure evolution also as a function of strain rate and temperature. This exhibited similar trend as seen for deformation per se. Thus, the mechanisms for deformation and microstructure evolution are suggested to be interdependent.     

Strain-rate sensitivity of powder metallurgy superalloys associated with steady-state DRX during hot compression process

Strain-rate sensitivity (SRS) is an important parameter to describe the thermodynamic behavior in plastic deformation process. In this research, the variation of SRS associated with steady-state DRX in P/M superalloys was quantitatively investigated. Based on the theoretical analysis and microstructural observation of the alloy after deformation, the SRS coefficient was employed to identify the deformation mechanism of the alloy. Meanwhile, the corresponding relationship between SRS coefficient m, stress exponent n and deformation mechanism was revealed. The stress exponent n in the Arrhenius constitutive model of P/M superalloys was calculated. In addition, it is found there is a relatively stable stress exponent range (n = 4-6), indicating that dislocation evolution played as the major hot deformation mechanism for P/M FGH4096 superalloy. Furthermore, the Bergstrom model and Senkov model were used and combined together to estimate the SRS coefficient in the steady-state DRX and the m value maintains at 0.2-0.22, which are consistent with the microstructural evolution during hot deformation process. The SRS coefficient distribution map and power dissipation efficiency distribution map were finally constructed associated with the microstructural evolution during hot deformation, which can be used to optimize the processing parameters of the superalloys.     

Preparation and high-frequency soft magnetic property of FeCo-based thin films

A series of FeCo-based thin films were prepared by magnetron sputtering without applying an induced magnetic field.The microstructure,electrical properties,magnetic properties and thermal stability of FeCo,FeCoSiN monolayer thin film and[FeCoSiN/SiN_x]_n multilayer thin film were investigated systematically.When FeCo thin film was doped with Si and N,the resistivity and soft magnetic properties of the obtained FeCoSiN thin film can be improved effectively.The coercivity(H_c),resistivity(ρ) and ferromagnetic resonance frequency(f_r) can be further optimized for the[FeCoSiN/SiN_x]_n multilayer thin film.When the thickness of FeCoSiN layer and SiN_x layer is maintained at 7 and 2 nm,the H_c,p and f_r for[FeCoSiN/SiN_x]_n multilayer thin film are 225 A·m~(-1)392 μΩ·cm~(-1) and 4.29 GHz,respectively.In addition,the low coercivity of easy axis(H_(ce) ≈ 506 A·m~(-1)) of[FeCoSiN/SiN_x]_n multilayer thin film can be maintained after annealing at 300 ℃ in air for 2 h.     

Analysis of Plastic Flow Instability During Superplastic Deformation of the Zn-Al Eutectoid Alloy Modified with 2 wt.% Cu

The aim of this work is to analyze the plastic flow instability in Zn-21Al-2Cu alloy deformed under 10^?3 s^?1 and 513 K, which are optimum conditions for inducing superplastic behavior in this alloy. An evaluation using the Hart and Wilkinson–Caceres criteria showed that the limited stability of plastic flow observed in this alloy is related to low values of the strain-rate sensitivity index (m) and the strain-hardening coefficient (γ), combined with the tendency of these parameters to decrease depending on true strain (ε). The reduction in m and γ values could be associated with the early onset of plastic instability and with microstructural changes observed as function of the strain. Grain growth induced by deformation seems to be important during the first stage of deformation of this alloy. However, when ε > 0.4 this growth is accompanied by other microstructural rearrangements. These results suggest that in this alloy, a grain boundary sliding mechanism acts to allow a steady superplastic flow only for ε < 0.4. For ε values between 0.4 and 0.7, observed occurrences of microstructural changes and severe neck formation lead to the supposition that there is a transition in the deformation mechanism. These changes are more evident when ε > 0.7 as another mechanism is thought to take over.     

Deformation mechanisms in Cr20Ni80 alloy at elevated temperatures

The mechanisms of plastic deformation of Cr20Ni80 nichrome with an initial grain size of 80 μm were studied in the temperature range 600–950°C and the strain-rate range 1.5 × 10^?6?5 × 10^?2s^?1. Nichrome is shown to exhibit anomalously high values of stress exponent n and a high deformation activation energy Q. These unusual properties were found to be caused by “threshold” stresses below which deformation does not occur. An analysis of the deformation behavior with allowance for threshold stresses reveals the regions of hot, warm, and cold deformation in nichrome. At normalized strain rates \(\dot \varepsilon \) kT/D _{1 Gb} < 10^?8, the true values of n and Q are ～4 and 285 ± 30 kJ/mol, respectively. In the normalized-strain range 10^?8?10^?4 n ～ 6 and the deformation activation energy decreases to 175 ± 30 kJ/mol. This change in the deformation-behavior characteristics is explained by the transition from high-temperature dislocation climb, which is controlled by lattice self-diffusion, to low-temperature dislocation climb, which is controlled by pipe diffusion, as the temperature decreases. At \(\dot \varepsilon \) kT/D _{1 Gb} = 10^?4, a power law break-down takes place and an exponential law (which describes the deformation behavior in the range of cold deformation) becomes operative.     

Influence of Tension-Compression Asymmetry on the Mechanical Behavior of AZ31B Magnesium Alloy Sheets in Bending

Magnesium alloys are promising materials for lightweight design in the automotive industry due to their high strength-to-mass ratio. This study aims to study the influence of tension-compression asymmetry on the radius of curvature and energy absorption capacity of AZ31B-O magnesium alloy sheets in bending. The mechanical properties were characterized using tension, compression, and three-point bending tests. The material exhibits significant tension-compression asymmetry in terms of strength and strain hardening rate due to extension twinning in compression. The compressive yield strength is much lower than the tensile yield strength, while the strain hardening rate is much higher in compression. Furthermore, the tension-compression asymmetry in terms of r value (Lankford value) was also observed. The r value in tension is much higher than that in compression. The bending results indicate that the AZ31B-O sheet can outperform steel and aluminum sheets in terms of specific energy absorption in bending mainly due to its low density. In addition, the AZ31B-O sheet was deformed with a larger radius of curvature than the steel and aluminum sheets, which brings a benefit to energy absorption capacity. Finally, finite element simulation for three-point bending was performed using LS-DYNA and the results confirmed that the larger radius of curvature of a magnesium specimen is mainly attributed to the high strain hardening rate in compression.     

The Effect of Synthesis Conditions on the Phase Composition and Structure of EuBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + δ</Subscript> Samples

The composition and the structure of ceramic EuBa₂Cu₃O_{6 + δ} (Eu-123) oxide samples annealed in steps with varying processing conditions (in air or oxygen and argon atmosphere at a temperature of 940–960°С for 1–70 h with or without homogenization) were studied by the X-ray phase and chemical analysis, electron diffraction pattern analysis, elemental analysis, and high-resolution transmission electron microscopy. Regardless of the processing conditions, Eu-123 nanostructured oxide with a tetragonal or orthorhombic structure and domains 1–20 nm in size was obtained as a result of annealing. Nanostructuring of the samples, which was revealed by high-resolution electron microscopy, is attributed to their chemical nature: the presence of identical structural elements in members of the homologous Eu _n Ba _m Cu_{m + n}O _y series of oxides allows them to intergrow coherently and create an illusion of a single crystal. Just like any other member of the Eu _n Ba _m Cu_{m + n}O _y series, oxide Eu-123 is disproportionate depending on the annealing conditions to form other members of this series located on either side of the dominant oxide. Temperature T_c of the superconducting transition of each member of the series depends on the average oxidation state of copper \(\overline {Cu} \). At \(\overline {Cu} \) < 2, all members of the series have a tetragonal structure and do not exhibit superconducting properties. At \(\overline {Cu} \) = 2.28, five members of the Eu _n Ba _m Cu_{m + n}O _y series with matrices (Ba : Cu) 5 : 8, 3 : 5, 2 : 3, 5 : 7, and 3 : 4 exhibit superconducting properties with T_c = 82–90 K.     

On the Characteristic Potentials of Pitting Corrosion of Compact and Powder Steel

The influence of chloride concentration and current density on the characteristic potentials of pitting corrosion found during potentiodynamic E _po ^pd and galvanostatic E _po ^cr polarization and E_rp repassivation of the PC process of 10Х17Н13М2Т and 12Х18Н10Т cast steels and 316L and 316L + 3% Сu 304HLD powder steels with addition of 10 and 15% Cu in a neutral chloride solution was studied. The porosity of the powder steel significantly decreases the pitting resistance. Addition of 3% Сu increases the pitting resistance, but does not allow achieving the pitting resistance level of compact molybdenum steel; neither does addition of 10 and 15% copper do so for 12Х18Н10Т steel.     

The Microstructural Evolution and Special Flow Behavior of Ti-5Al-2Sn-2Zr-4Mo-4Cr During Isothermal Compression at a Low Strain Rate

The microstructural evolution and special flow behavior of Ti-5Al-2Sn-2Zr-4Mo-4Cr during isothermal compression at a strain rate of 0.0001 s^?1 were investigated. The dislocation climbs in elongated α grains resulted in the formation of low-angle boundaries that transform into high-angle boundaries with greater deformation, and the elongated α grains subsequently separated into homogenous globular α grains with the penetration of the β phase. The simultaneous occurrence of discontinuous dynamic recrystallization and continuous dynamic recrystallization in the primary β grains resulted in a trimode grain distribution. The β grains surrounded by dislocations presented an equilateral-hexagonal morphology, which suggests that grain boundary sliding through dislocation climbs was the main deformation mechanism. The true stress–strain curves for 1073 and 1113 K abnormally intersect at a strain of ~0.35, related to the α → β phase transformation and distinct growth of the β grain size.     

Warm Temperature Deformation Behavior and Processing Maps of 5182 and 7075 Aluminum Alloy Sheets with Fine Grains

The tensile deformation behavior and processing maps of commercial 5182 and 7075 aluminum alloy sheets with similarly fine grain sizes (about 8 μm) were examined and compared over the temperature range of 423–723 K. The 5182 aluminum alloy with equiaxed grains exhibited larger strain rate sensitivity exponent (m) values than the 7075 aluminum alloy with elongated grains under most of the testing conditions. The fracture strain behaviors of the two alloys as a function of strain rate and temperature followed the trend in their m values. In the processing maps, the power dissipation parameter values of the 5182 aluminum alloy were larger than those of the 7075 aluminum alloy and the instability domains of the 5182 aluminum alloy were smaller compared to that of the 7075 aluminum alloy, implying that the 5182 aluminum alloy had a better hot workability than the 7075 aluminum alloy.     

Kinetics and fracture behavior under cycle loading of an Al–Cu–Mg–Ag alloy

The behavior of aluminum alloy AA2139 subjected to T6 treatment, including solution treatment and artificial aging, has been studied using cyclic loading with a constant total strain amplitude. Upon low-cyclic fatigue in the range of total strain amplitudes ε_ac of 0.4–1.0%, the cyclic behavior of the AA2139-T6 alloy is determined by the processes that occur under the conditions of predominance of the elastic deformation over plastic deformation. The AA2139 alloy exhibits stability to cyclic loading without significant softening. The stress-strained state of the alloy upon cyclic loading can be described by the Hollomon equation with the cyclic strength coefficient K' and the cyclic strain-hardening exponent n' equal to 641 MPa and 0.066, respectively. The dependence of the number of cycles to fracture on the loading amplitude and its components (amplitudes of the plastic and elastic deformation) is described by a Basquin–Manson–Coffin equation with the parameters σ′/E = 0.014, b =–0.123, ε′_f= 178.65, and c =–1.677.     

Study on the Hot Workability of SiCp/Al Composites Based on a Critical Strain Map

We used the isothermal compression test (conducted in a Gleeble-3500 system) to study the hot deformation behaviors of SiCp/Al composites over a wide range of temperatures (623-773 K) and strain rates (0.001-10 s^?1). A 3D hot-processing map was constructed based on the Malas stability criteria and experimental data. An artificial neural network model of four hot work quality characteristic parameters (strain rate sensitivity m, its derivative m′, temperature sensitivity s, and its derivative s′) were established. A new hot-processing map, known as a hot-processing critical strain map, has been proposed based on the smallest strain prior to instability. Two optimized processing regions at 623-660 K, 0.05-0.075 s^?1 and 720-773 K, 0.04-0.18 s^?1 were determined based on this map.     

Constitutive modeling and understanding of the hot compressive deformation of Mg–9.5Zn–2.0Y magnesium alloy with reduced number of strain-dependent constitutive parameters

The hot compressive flow behavior of the cast Mg–9.5Zn–2.0Y alloy as a function of strain was analyzed, and the degree of dependence of the parameters (A: material constant, n ₂: stress exponent, Q _c: activation energy for plastic flow and α: stress multiplier) of the constitutive equation (\(\dot \varepsilon = A{\left[ {\sinh \left( {\alpha \sigma } \right)} \right]^{{n_2}}}\exp \left( {\frac{{ - {Q_c}}}{{RT}}} \right)\)) upon the strain was examined in a systematic manner. This is to explore the possibility of representing the hot compressive deformation behavior of metallic alloys in a simple way by using a reduced number of strain-dependent constitutive parameters. The analysis results for several different cases can be interpreted as follows: (1) Q _c can be treated as being strain-independent, which is physically sensible; (2) while only the microstructure changes as a function of strain at low flow stresses, as the flow stress increases, the power-law creep deformation and power-law breakdown mechanisms change; (3) the regime where only A is strain dependent expanded to higher strain rates and lower temperatures as the strain increased, suggesting that the number of the strain-dependent parameters decreases as the initial microstructure is refined by dynamic recrystallization, and the microstructure approaches a steady state.     

Size dependence of the melting temperature of metallic nanoclusters from the viewpoint of the thermodynamic theory of similarity

The generalized Thomson formula T_m = T_m^(∞)(1-δ)R for the melting point of small objects T_m has been analyzed from the viewpoint of the thermodynamic theory of similarity, where R is the radius of the particle and T_m^(∞) is the melting point of the corresponding large crystal. According to this formula, the parameter δ corresponds to the value of the radius of the T_m(R^-1) particle obtained by the linear extrapolation of the dependence to the melting point of the particle equal to 0 K. It has been shown that δ = αδ₀, where α is the factor of the asphericity of the particle (shape factor). In turn, the redefined characteristic length δ₀ is expressed through the interphase tension σ_sl at the boundary of the crystal with its own melt, the specific volume of the solid phase v_s and the macroscopic value of the heat of fusion λ_∞:δ₀ = 2σ_slv_s/λ_∞. If we go from the reduced radius of the particle R/δ to the redefined reduced radius R/r₁ or R/d, where r₁ is the radius of the first coordination shell and d ≈r₁ is the effective atomic diameter, then the simplex δ/r₁ or δ/d will play the role of the characteristic criterion of thermodynamic similarity. At a given value of α, this role will be played by the simplex Estimates of the parameters δ₀ and δ₀/d have been carried out for ten metals with different lattice types. It has been shown that the values of the characteristic length δ₀ are close to 1 nm and that the simplex δ₀/d is close to unity. In turn, the calculated values of the parameter δ agree on the order of magnitude with existing experimental data.     

Effect of Al and Mg Contents on Wettability and Reactivity of Molten Zn–Al–Mg Alloys on Steel Sheets Covered with MnO and SiO<Subscript>2</Subscript> Layers

The reactive wetting behaviors of molten Zn–Al–Mg alloys on MnO- and amorphous (a-) SiO₂-covered steel sheets were investigated by the sessile drop method, as a function of the Al and Mg contents in the alloys. The sessile drop tests were carried out at 460 °C and the variation in the contact angles (θ_c) of alloys containing 0.2–2.5 wt% Al and 0–3.0 wt% Mg was monitored for 20 s. For all the alloys, the MnO-covered steel substrate exhibited reactive wetting whereas the a-SiO₂-covered steel exhibited nonreactive, nonwetting (θ_c?>?90°) behavior. The MnO layer was rapidly removed by Al and Mg contained in the alloys. The wetting of the MnO-covered steel sheet significantly improved upon increasing the Mg content but decreased upon increasing the Al content, indicating that the surface tension of the alloy droplet is the main factor controlling its wettability. Although the reactions of Al and Mg in molten alloys with the a-SiO₂ layer were found to be sluggish, the wettability of Zn–Al–Mg alloys on the a-SiO₂ layer improved upon increasing the Al and Mg contents. These results suggest that the wetting of advanced high-strength steel sheets, the surface oxide layer of which consists of a mixture of MnO and SiO₂, with Zn–Al–Mg alloys could be most effectively improved by increasing the Mg content of the alloys.     

Integrated constitutive model for flow behavior of pure Titanium considering interstitial solute concentration

The aim of this work is to develop a constitutive model that can predict the flow behavior of pure Ti with different interstitial concentrations and grain sizes. To build a database required for identifying material constants, three different grades of Ti were subjected to tensile tests at temperatures of 223, 300, 473, 673 or 773 K and at a fixed strain rate of 10^?3 s^?1. In the modeling procedure, the mechanical threshold stress model was further modified to capture both the hardening effects attributed to the changes in equivalent oxygen concentration (O _eq ) and the softening effect caused by deformation heating at high strain rates. The developed model can reasonably predict the flow behavior of pure Ti having different O _eq (0.14–0.32 wt%), and grain size (14.5–90 μm) over a temperature range of 135 to 673 K, and a strain rate range of 2×10^?4 to 1400 s^?1.