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.     

Correlation Between the Pitting Potential Evolution and $$\varvec\sigma$$ Phase Precipitation Kinetics in the 2205 Duplex Stainless Steel

The aim of this work is to correlate the pitting potential (E_pit) evolution with the kinetics of σ phase precipitation in the 2205 duplex stainless steel aged at 850 °C after solution treatment at 1150 °C. The potentiodynamic polarization curves indicate a reduction of the pitting corrosion resistance with the aging time, which is revealed by a decrease in the E_pit values from 0.65 to 0.40 V_SCE. Thus, E_pit values are used to determine the kinetics parameters of the σ phase precipitation. The experimental transformed fraction agrees well with the one calculated by using the modified Kolmogorov–Johnson–Mehl–Avrami equation with an impingement parameter c?=?0.6.     

Superior Mechanical Properties of AlCoCrFeNiTi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High-Entropy Alloys upon Dynamic Loading

High-entropy alloys with composition of AlCoCrFeNiTi _x (x: molar ratio; x = 0, 0.2, 0.4) under quasi-static and dynamic compression exhibit excellent mechanical properties. A positive strain-rate sensitivity of yield strength and the strong work-hardening behavior during plastic flows dominate upon dynamic loading in the present alloy system. The constitutive relationships are extracted to model flow behaviors by employing the Johnson-Cook constitutive model. Upon dynamic loading, the ultimate strength and fracture strain of AlCoCrFeNiTi _x alloys are superior to most of bulk metallic glasses and in situ metallic glass matrix composites.     

Influence of Zinc on Coarsening of δ-Ni<Subscript>2</Subscript>Si Particles,Aging Behavior and Hardness in a Cu-Ni-Si Alloy

In the present paper, the aging precipitation and coarsening of disk-like δ-Ni₂Si particles in Cu and Cu-10Zn alloys aged at 450 °C have been investigated by hardness, electric resistivity measurement and transmission electron microscopy observation. The coarsening dynamics of the average diameter of the δ-Ni₂Si particles coincides with the t ^1/3 time law for both alloys. The coarsening of the diminution of supersaturation related to aging time t coincides with the t ^?1/3 time rule. Adding Zn to the Cu-Ni-Si alloy increases the growth and coarsening rate of the particles mainly because of the increased diffusivity D of the δ-Ni₂Si particles in the matrix. The value of D of the δ-Ni₂Si particles in the Cu-xZn (x = 0, 10 wt.%) matrix and the Cu/δ-Ni₂Si interfacial energy γ are independently calculated by using the Lifshitz–Slyozov–Wagner theory which was extended to include disk-like particles by Boyd and Nicholson. The values of D and γ increase from 0.77 × 10^?19 to 2.21 × 10^?19 m²/s and 0.19 to 0.63 J/m², respectively, when Zn is added to the Cu-Ni-Si alloy. These calculations and the analysis show that the properties of Cu-Ni-Si-Zn alloy can significantly be enhanced by reducing the aging temperature.     

Development of Texture in Interstitial-Free Steel Processed by Equal-Channel Angular Pressing

Ti + Nb-stabilised interstitial-free steel is deformed by equal-channel angular pressing (ECAP) adopting a route B_C up to an equivalent strain of 24. Upon ECAP the grain size decreases to ultrafine level and it becomes strongly textured. At ε_vm = 0.6-6, components of both {110} fiber, \( J_{\uptheta } \), \( \bar{J}_{\uptheta } \) and of 〈111〉 fiber, D _1θ, D _2θ with common components of E _θ, \( \bar{E}_{\uptheta } \) are existing but after ε_vm ≥9, only 〈111〉 fiber components are observed. At large strain, ε_vm = 9-24, 〈111〉 fiber texture is recorded with monoclinic symmetry. At ε_vm = 0.6, coarse grains get split into deformation bands. Fragmentation of bands (at ε_vm = 3) suppress \( \bar{J}_{\uptheta } , \) \( J_{\uptheta } \) components. At ε_vm = 6, formation of lamellar structures increases intensity of mainly D _1θ, D _2θ. At ε_vm = 9, oriented ribbon grains result in strong D _1θ, D _2θ components with 〈111〉 fiber. At ε_vm = 15-24, conversion of ribbon grains to near-equiaxed shaped grains maintains 〈111〉 fiber texture with enhanced intensity of D _1θ and D _2θ components.     

Improved High-Temperature Microstructural Stability and Creep Property of Novel Co-Base Single-Crystal Alloys Containing Ta and Ti

The influence of Ta and Ti additions on microstructural stability and creep behavior in novel Co-Al-W base single-crystal alloys has been investigated. Compared to the ternary alloy, the γ′ solvus temperature and γ′ volume fraction were raised by individual additions of Ta and Ti, and increased further in the quinary alloy containing both alloying additions. In contrast to ternary and quaternary alloys, an improved microstructural stability with the stable γ–γ′ two-phase microstructure and more than 60% γ′ volume fraction existed in the quinary alloy after prolonged aging treatment at 1050°C for 1000 h. The creep behavior at 900°C revealed lower creep rates and longer rupture lives in the quaternary alloys compared to the ternary alloy, whereas the quinary alloy exhibited even better creep resistance. When the creep temperature was elevated to about 1000°C, the creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys and first-generation Ni-base single-crystal superalloys. The improved creep resistance at approximately 1000°C was considered to be associated with high γ′ volume fraction, γ′ directional coarsening, and dislocation substructure, which included γ–γ′ interfacial dislocation networks and the sheared γ′ precipitates containing stacking faults and anti-phase boundaries.     

Phase Equilibria of the Ternary Al-Cu-Zn Alloys on Al-Zn Rich Side

Phase equilibria of the Al-Cu-Zn system on Al-Zn rich side was experimentally determined with 16 alloys annealed at 360 °C. The annealed alloys were examined by means of x-ray diffraction, electron probe microanalysis and differential scanning calorimetry. Five single-phase regions and seven two-phase regions as well as three three-phase regions, i.e. α-(Al)?+?θ-Al₂Cu?+?τ′-Al₄Cu₃Zn, α-(Al)?+?τ′-Al₄Cu₃Zn?+?ε-CuZn₄ and α-(Al)?+?ε-CuZn₄?+?(Zn), were determined. The partial isothermal section of the Al-Cu-Zn system on Al-Zn rich side at 360 °C was constructed based on the obtained experimental data in this work. It was observed that the solid solution phase α-(Al) would easily decompose into ε-CuZn₄, (Zn) and α′-(Al) at the ambient temperature in the early stages. The ternary phase τ′-Al₄Cu₃Zn would form and ε-CuZn₄ would disappear gradually along with the extension of aging time.     

The efficiency of bridges in a crack

Bridges are cross connections in a crack that contract its faces behind the crack tip. They appear because of the inhomogeneity of the material or differences in the kinematics of fracture. We obtained a general solution to the nonlinear singular integral equation that relates the crack opening h(x) to the stress caused by bridges p(x) and determined the stress-intensity factor F(B) (degree of unloading of the crack tip due to the presence of bridges) depending on the dimensionless stiffness of bridges B. As the measure of the efficiency of bridges, we suggest the ratio of the stress for crack start in a matrix without bridges σ_m to that in the presence of bridges σ_∞. When bridges with a strength σ_u occupy a fraction f of the fracture area, their efficiency Y(ζ, B) depends on the degree of reinforcement ζ = fσ_u/σ_m and on their stiffness B. The Y(ζ, B) dependences have been obtained in an explicit form for the limiting cases of brittle and fully ductile bridges.     

The Microstructure and Gamma Prime Distributions in Inertia Friction Welded Joint of P/M Superalloy FGH96

A gamma prime (γ′) precipitation (~35% in volume)-hardened powder metallurgy (P/M) superalloy FGH96 was welded using inertia friction welding (IFW). The microstructure and γ′ distributions in the joints in two conditions, hot isostatic pressed state and solution-treated and aged state, were characterized. The recrystallization of grains, the dissolution and re-precipitation of γ′ in the joints were discussed in terms of the temperature evolutions which were calculated by finite element model analysis. Regardless of the initial states, fully recrystallized fine grain structure formed at welded zone. Meanwhile, very fine γ′ precipitations were re-precipitated at the welded zone. These recrystallized grain structure and fine re-precipitated γ′ resulted in increasing hardness of IFW joint while making the hardness dependent on the microstructure and γ′ precipitation.     

Creep correlations of metals at elevated temperatures

Creep data for pure metals at temperatures above those at which rapid recovery occurs (above about 0.45 the melting temperature) are correctable by means of the equations ? = f (te^?ΔH/RT, σ) and σ = f (?&#x0307 _s e^ΔH/RT). These correlations were applied successfully to data for aluminum, iron, nickel, copper, zinc, platinum, gold, and lead as well as for simple alloys. For a given metal, ΔH is a constant about equal to the activation energy for self-diffusion.     

Temperature Dependence of the Surface Energy of the Low Index Planes of UO<Subscript>2</Subscript> and ThO<Subscript>2</Subscript>

The temperature dependence of the surface energy γ of the low index (111), (110), and (100) planes of UO₂ and ThO₂ was calculated for the first time, using a simple method based on physical and thermodynamic quantities and considerations. The extrapolated to 0 K surface energy values agreed well with the available theoretical data reported in literature for the (111) and (110) surfaces, whereas they were significantly lower for the (100) surface. The γ ₁₀₀/γ ₁₁₁ ratio for UO₂ and ThO₂ revealed the formation of crystals with truncated octahedron structure in thermodynamic equilibrium. This structure shifts to octahedral with increasing temperature. Comparison with experimentally determined surface energies of the polycrystalline oxides showed that their values are in the range between γ ₁₁₁ and γ ₁₀₀.     

Effect of tensile stresses on the Δ<Emphasis Type="Italic">E</Emphasis> effect in ferromagnetic Fe<Subscript>64</Subscript>Co<Subscript>21</Subscript>B<Subscript>15</Subscript> ribbons

Effect of elastic tensile stresses on the Δ E -effect magnitude in ferromagnetic Fe₆₄Co₂₁B₁₅ amorphous ribbons subjected to magnetic annealing and dc treatment was studied. In the case of relatively low tensile stresses, the maximum magnitude of the negative Δ E effect in the ribbons under study is shown to increase whatever their treatment. The subsequent increase in elastic tensile stresses leads to a decrease in the maximum value of the negative ΔE effect. The maximum sensitivity of the ΔE effect to the applied elastic tensile stresses is characteristic of samples subjected to magnetic annealing. The results obtained are explained using a conception of the effect of magnetic domain structure and mechanisms of its transformation induced by magnetic field and elastic tensile stresses on the ΔE effect in amorphous ferromagnetic ribbons.     

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.     

Phase transformations during deformation of Fe-Ni and Fe-Mn alloys produced by mechanical alloying

Compositions of Fe_{(100 ? x)}Mn _x (x = 10 and 12 at. %) and Fe_{(100 ? y)}Ni _y (y = 18 and 20 at. %) were produced by combined mechanical alloying of pure-metal powders and annealed in the austenitic field. After annealing and cooling to room temperature, the alloys had a single-phase austenitic structure. During deformation, the γ phase partially transforms into the α ₂ phase (and/or ? phase in Fe-Mn alloys). The phase composition of the alloys after deformation depends on the amount of alloying elements and the predeformation annealing regime. The amount of martensite in the structure of a bulk alloy obtained by powder compacting grows proportionally to the degree of deformation of the sample.     

On the Reaction Mechanism of MCrAlY Alloys with Oxide–Sulfate Deposits at 1100 °C

The corrosion of γ-(Ni,Co) + β-(Ni,Co)Al NiCoCrAlY alloys exposed to CaO-rich, Na₂SO₄-containing deposits in air and CO₂–H₂O–O₂ at 1100 °C was studied with a focus on the mechanisms governing the accelerated attack of γ-rich compositions. Internal oxidation of Al resulted from the breakdown of an initially formed Al₂O₃ scale, due to its reaction with the deposit. Compared to deposit-free conditions, this scale exhibited: (i) a delayed Al₂O₃ structure transition from metastable θ to stable α; (ii) finer grains; (iii) a reduced adherence to the metal, to the extent that a gap developed at the metal/oxide interface; and (iv) significant permeability to S, as well as N during air exposures. The occurrence of sulfidation in S-free gases reflected the establishment of an elevated S activity under the molten Na₂SO₄, due to the locally reduced \(p_{{{\text{O}}_{ 2} }}\). The factors affecting the Al₂O₃-scale microstructure were examined, as well as the associated consequences on Al consumption and on the transition to internal oxidation. The roles of alloy and gas compositions in the identified mechanisms were also discussed.     

Effect of nitrogen on sigma formation in Cr-Ni steels at 1200°F (650°C)

The addition of nitrogen (0.10 to 0.20 pct) to Fe-Cr-Ni alloys of simulated commercial purity results in a real displacement of the σ phase boundaries to higher chromium contents. The effect is small for the (γ + σ)/γ boundary, but is pronounced for the (γ + α + σ)/(γ + α) boundary. Although there is an indication of an exceptionally large shift of the σ boundaries to higher chromium contents, especially in steels with nitrogen over 0.2 pct, the major portion of this apparent shift results from the fact that carbide and nitride precipitation cause “chromium impoverishment” of the matrices. The effect of combined additions of nitrogen and silicon to the Fe-Cr-Ni phase diagram is demonstrated also. Nitrogen can nullify the effect of about 1 pct Si in shifting the (γ + σ)/γ phase boundary to lower values of chromium at all nickel levels from 8 to 20 pct. Nitrogen can nullify this σ-forming effect of about 2 pct Si at the 8 pct Ni level, but not at the 20 pct Ni level. The alloys studied were in both the cast and the wrought conditions. There are indications that the σ phase forms more slowly in the cast alloys than in the wrought alloys if both are in the completely austenitic state. The presence of δ ferrite in the cast alloys accelerates the formation of σ. Cold working increases the rate of σ formation in both cast and wrought alloys.     

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.     

Electrical Conductivity and Dielectric Permittivity of γ-Irradiated Nanocomposites Based on Ultrahigh-Molecular-Weight Polyethylene Filled with α-SiO<Subscript>2</Subscript>

UHMWPE/SiO₂ composites were obtained from a homogeneous mixture of ultrahigh-molecular- weight polyethylene (UHMWPE) and silicon dioxide (α-SiO₂) by hot pressing. The temperature dependence (20–170°C) of their conductivities σdс prior to and after γ irradiation (D = 200 kGy), the effect of absorbed dose on the σ_dс value (dose dependence), the behavior of the function logσ_dc = f(T) under heating–cooling conditions, and the frequency dependences (25–106 Hz) of the real (ε') and imaginary (ε'') parts of the complex dielectric constant were studied. The logσ_dc = f(T) dependence was shown to have in both cases a complex pattern: “breaks” due to phase transitions were observed. With an increase in the concentration of α-SiO₂, the ε' and ε'' (tanδ) values in the matrix increased and the frequency dependences of these values corresponded to an exponential rule.     

Phase Equilibria in the Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>–InSb System

The phase diagram of the (Sb₂Te₃)_100?x –InSb _x system was determined based on x-ray diffraction (XRD) analysis, differential thermal analysis (DTA), and microhardness and density measurements. An intermediate compound with composition Sb₂Te₃·2InSb was formed as a result of syntectic reaction, melting incongruently at 553 °C. This compound has tetragonal lattice with unit cell parameters of a = 4.3937 Å, b = 4.2035 Å, c = 3.5433 Å, α = 93.354°, and β = γ = 90°. Sb₂Te₃·(2 + δ)InSb (?1 ≤ δ ≤ +1) and (Sb₂Te₃)_100?x (InSb) _x (90 ≤ x ≤ 100) solid solutions exist in the investigated system, based on the intermediate compound Sb₂Te₃·2InSb and on InSb, respectively. Also, two invariant equilibria exist in the system, with eutectic point coordinates at compositions of x = 60 and x ≈ 85 mol% InSb and eutectic temperatures of T _E = 541 and T _E ≈ 501 °C, respectively.     

Investigation of Fe content in Cu–Al–Ni Shape Memory Alloys

Polycrystalline Cu–Al–Ni–Fe-based shape memory alloys with different chemical composition were produced in an arc-melting furnace under an argon atmosphere. Homogenized and aged specimens were prepared for multiple analyses. The temperatures of reversible martensitic transformations, namely A_s, A_f, M_s, M_f, A_max and ΔH enthalpy values were determined by a DSC device. The phase transition analysis from the room temperature to 850°C was undertaken by DTA. To characterize the lattice structure, an XRD analysis was conducted, the results of which were confirmed by microstructure images obtained from optical microscope observations.