Recovery kinetics of nanostructured aluminum: Model and experiment

A model is suggested which, for a deformed and partially annealed material, can factor out the recovery component of the flow stress for conditions where recrystallization occurs simultaneously with recovery processes. The model is applied in an analysis of the isothermal rates of recovery of hardness upon annealing of commercial purity aluminum at temperatures between 140 and 220 °C of samples cold rolled to strains 2 and 4. This analysis is successfully validated by a microstructural characterization. The recovery rates are described by a first-order chemical reaction rate equation with an activation energy that varies from 89 kJ mol⁻¹ at the onset of recovery to 125 kJ mol⁻¹ at its conclusion. The recovery rate increases significantly with an increase in strain, thereby markedly reducing the thermal stability of the material deformed to the largest strain.     

Recovery and recrystallization in commercial purity aluminum cold rolled to an ultrahigh strain

Recovery and recrystallization were studied in commercial purity aluminum cold rolled to an ultrahigh strain (ε_vM = 6.4) and isothermally annealed at 300 °C. The deformed material consists of three layers with similar fractions of high-angle boundaries (HABs) and similar lamellar boundary spacings, but with different textures and different spatial arrangements of the rolling texture components. Annealing leads initially to a coarsening of the lamellar microstructure, accompanied by a reduction in the HAB fraction. Ex-situ experiments using very short annealing times indicate that such microstructural changes are consistent with a process of coarsening via triple junction motion. The recovery proceeds similarly in the center and subsurface layers, but because of the different initial spatial arrangement of the texture components in these layers, the loss of HABs is significantly greater in the subsurface compared with the center layer. Further annealing leads to discontinuous recrystallization, which occurs differently in the center and subsurface layers. In the center layer, recrystallization proceeds more rapidly and with a larger frequency of nuclei, resulting in a smaller recrystallized grain size. In contrast, pronounced recrystallization in the subsurface layers is delayed, and the recrystallized grain size is larger than in the center. It is concluded that the changes taking place during recovery are very significant in determining the subsequent recrystallization behavior in terms of the final grain size and texture.     

Constitutive modeling and processing map for elevated temperature flow behaviors of a powder metallurgy titanium aluminide alloy

The flow behaviors of PM titanium aluminide alloy were studied by isothermal compression simulation test. The apparent activation energy of deformation was calculated to be 313.53 kJ mol^?1 and a constitutive equation had been established to describe the flow behavior. Processing map was developed at a strain of 0.7. With an increase of strain, two domains can be found: dynamic recrystallization and superplastic deformation, which are further confirmed by microstructural observations. The dynamic recrystallization occurs extensively at 1000 °C and 10^?3 s^?1, with a peak efficiency of 50%, and the superplastic deformation occurs at 1100 °C and 10^?3 s^?1, with a peak efficiency of 60%. At a strain rate higher than 10^?1 s^?1, the alloy exhibits flow instability.     

Phase separation kinetics in a Fe–Cr–Al alloy

The α–α′ phase separation kinetics in a commercial Fe–20 wt.% Cr–6 wt.% Al oxide dispersion-strengthened PM 2000? steel have been characterized with the complementary techniques atom probe tomography and thermoelectric power measurements during isothermal aging at 673, 708, and 748 K for times up to 3600 h. A progressive decrease in the Al content of the Cr-rich α′ phase was observed at 708 and 748 K with increasing time, but no partitioning was observed at 673 K. The variation in the volume fraction of the α′ phase well inside the coarsening regime, along with the Avrami exponent 1.2 and activation energy 264 kJ mol^?1, obtained after fitting the experimental results to an Austin–Rickett type equation, indicates that phase separation in PM 2000? is a transient coarsening process with overlapping nucleation, growth, and coarsening stages.     

Deformation behavior of isothermally forged Ti–5Al–2Sn–2Zr–4Mo–4Cr powder compact

The isothermal deformation behavior of hot isostatic pressed (HIPed) Ti–5Al–2Sn–2Zr–4Mo–4Cr(Ti-17) powder compact was investigated by compression testing in the temperature range of 810–920 °C and constant strain rate range of 0.001–1 s^?1. The true stress–true strain curves of the powder compact exhibit flow oscillation and flow softening phenomenon in both beta field and beta + alpha field. The flow softening behavior is related to the globularization of the primary acicular microstructure and deformation heating. The apparent activation energy for deformation in beta field is estimated to be 149 kJ mol^?1, indicating that the deformation is controlled by diffusion. The high apparent activation energy of 537 kJ mol^?1 for deformation in beta + alpha field may be related to the dynamic recrystallization of the primary acicular microstructure. Constitutive equations with the form of Arrhenius-type hyperbolic-sine relationship are proposed to delineate the peak flow stress as a function of the strain rate and the temperature for isothermal forging HIPed Ti-17 powder compact.     

Flow softening and microstructural evolution of TC11 titanium alloy during hot deformation

Hot compression tests on samples of the TC11 (Ti–6.5Al–3.5Mo–1.5Zr–0.3Si) titanium alloy have been done within the temperatures of 750–950 °C and strain rate ranges of 0.1–10 s^?1 to 40–60% height reduction. The experimental results show that the flow stress behavior can be described by an exponential law for the deformation conditions. The hot deformation activation energy (Q) derived from the experimental data is 538 kJ mol^?1 with a strain rate sensitivity exponent (m) of 0.107. Optical microstructure evidence shows that dynamic recrystallization (DRX) takes place during the deformation process. Moreover, only α DRX grains are founded in the titanium alloys. The influences of hot working parameters on the flow stress behavior and microstructural features of TC11 alloy, especially on the type of phase present, the morphologies of the α phase, grain size and DRX are analyzed. The optimum parameters for hot working of TC11 alloy are developed.     

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix

The coarsening behavior of Ni–Al solid–solution precipitates in an Ni₃Al matrix was investigated in alloys containing 22.0–22.8 at.% Al aged at 650–800 °C for times exceeding 1800 h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1 ± 16.6 kJ mol⁻¹, agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate–matrix interface, controls the kinetics.     

Microstructure and texture evolution during annealing a cryogenic-SPD processed Al-alloy with a nanoscale lamellar HAGB grain structure

The grain structure and texture evolution during annealing a Al–0.13% Mg submicron-grained alloy, deformed by plane-strain compression (PSC) at cryogenic temperatures, has been investigated by transmission electron microscopy and electron backscatter diffraction. After deformation the alloy contained a lamellar grain structure with a high-angle grain boundary (HAGB) spacing of 190 nm and an area fraction of ～80%. On annealing the grain structure coarsened and transformed from lamellar to equiaxed. Remarkably, the fraction of low-angle grain boundaries (LAGBs) progressively increased during annealing, to ～50% above 300 °C, leading to instability and discontinuous recrystallization at higher temperatures. This resulted in a “bimodal grain structure” comprised of bands of coarser grains and fine subgrains, arising as a result of the increase in proportion of lower-mobility LAGBs. The surprisingly large increase in LAGB fraction on annealing is shown to be related to orientation impingement, originating from the strong texture present after PSC in liquid nitrogen.     

Kinetics and microstructure of laser chemical vapor deposition of titanium nitride

Titanium nitride (TiN) films were deposited onto Ti–6Al–4V substrates by laser chemical vapor deposition using a cw CO₂ laser and TiCl₄, N₂ and H₂ reactant gases. Laser-induced fluorescence (LIF) and pyrometry determined relative titanium gas phase atomic number density and deposition temperature, respectively. Auger electron spectroscopy found substoichiometric films, caused by diffusion of nitrogen through TiN grain boundaries to the titanium alloy substrate. The morphology is a polyhedral structure with crystallite sizes ranging from 10 to 1000 nm. The activation energy was calculated to be 122 ± 9 kJ mol⁻¹ using growth rates measured by film height and 117 ± 23 kJ mol⁻¹ using growth rates measured by LIF signals. Above N₂ and H₂ levels of 1.25% and below TiCl₄ input of 4.5%, the growth rate has a half-order dependence on nitrogen and a linear dependence on hydrogen. The rate-determining steps of TiN growth are discussed.     

Very rapid growth of aligned carbon nanotubes on metallic substrates

Aligned carbon nanotubes were grown on metallic substrates using a microwave plasma-enhanced chemical vapor deposition system. The substrates were Ni and Cu, and the catalyst was an Fe–Si alloy thin film. The effects of substrate and catalyst characteristics and growth temperature were studied. We show, via the use of a microwave shield, and with optimized catalyst thickness and growth temperature, that carbon nanotubes (CNTs) with a length of up to 2.15 mm and an unprecedently high growth rate of 177 μm min^?1 can obtained. Non-isothermal growth was performed to investigate the growth kinetics and therefore to obtain the activation energies of CNTs grown on Ni and Cu. Very similar activation energies for the growth of CNTs on Ni and Cu substrates were determined to be 101.5 (1.05 eV) kJ mol^?1 and 102.3 (1.06 eV) kJ mol^?1, respectively.     

Densification and grain growth during early-stage sintering of Ce0.9Gd0.1O1.95−δ in a reducing atmosphere

The present work investigates the processes of densification and grain growth of Ce_0.9Gd_0.1O_1.95?δ (CGO10) during sintering under reduced oxygen partial pressure. Sintering variables were experimentally characterized and analyzed using defect chemistry and sintering constitutive laws. Based on the results achieved, the grain size–relative density relationship, the densification rate and the grain-growth rate were determined. The activation energies for densification and grain growth were evaluated, and the dominant densification mechanism was indicated. For comparison, the densification behavior of CGO10 sintered in air was also studied. Accelerated densification was observed in early-stage sintering of CGO10 in a reducing atmosphere. This might be attributed to the oxygen vacancies generated by the reduction of Ce⁴⁺ to Ce³⁺ in the reducing atmosphere, which facilitate the diffusion of ions through the lattice. The densification activation energy of CGO10 in the reducing atmosphere was evaluated to be 290 ± 20 kJ mol^?1 in the relative density range of 0.64–0.82, which was much smaller than that of CGO10 sintered in air (770 ± 40 kJ mol^?1). The grain-growth activation energy of CGO10 sintered in the reducing atmosphere was evaluated to be 280 ± 20 kJ mol^?1 in the grain size range of 0.34–0.70 μm. The present work describes a systematic investigation of sintering behavior of CGO10 under reduced oxygen partial pressure, which contributes to the first known determination of the fundamental parameters associated with densification and grain growth during early-stage sintering of CGO10 in a reducing atmosphere.     

Atomic-scale investigation of ε and θ precipitates in bainite in 100Cr6 bearing steel by atom probe tomography and ab initio calculations

Carbide precipitation during upper and lower bainite formation in high-carbon bearing steel 100Cr6 is characterized using transmission electron microscopy and atom probe tomography. The results reveal that both ε and θ carbides precipitate in lower bainite isothermally held at 260 °C and only θ precipitates form in upper bainite isothermally held at 500 °C. ε and θ precipitate under paraequilibrium condition at 260 °C in lower bainite and θ precipitates under negligible partitioning local equilibrium condition in upper bainite at 500 °C. In order to theoretically study ε and θ precipitation and the ε → θ transition in bainite, thermodynamic calculations have been carried out using ab initio techniques. We find that ε and θ carbides in ferrite have almost identical thermodynamic stability, and hence have similar formation probability. In austenite, however, cementite formation is clearly preferred: it is favored by 5 kJ mol^?1 at room temperature and still by 4 kJ mol^?1 at 500 °C. Hence, the thermodynamic predictions agree well with the atom probe tomography results.     

Thermal behavior of Ni (99.967% and 99.5% purity) deformed to an ultra-high strain by high pressure torsion

Polycrystalline Ni of two purities (99.967% (4N) and 99.5% (2N)) was deformed to an ultra-high strain of ε_vM = 100 (ε_vM, von Mises strain) by high pressure torsion at room temperature. The 4N and 2N samples at this strain are nanostructured with an average boundary spacing of ～100 nm, a high density of dislocations and a large fraction of high angle boundaries (>15°) of 0.68–0.74, as determined by transmission electron microscopy, and 0.8–0.83, as determined by electron backscattering diffraction. The deformed samples were annealed isochronally for 1 h at temperatures from 100 to 600 °C, and the evolution of the structural parameters (boundary spacing, average boundary misorientation angle and the fraction of high angle boundaries), crystallographic texture and hardness were determined. Based on microstructural parameters the energy stored in the deformed state was estimated to be 14 MPa and 24 MPa for 4N Ni and 2N Ni, respectively. The isochronal annealing leads to a drop in hardness in three stages: a relatively small decrease at low temperatures (recovery), followed by a rapid decrease at intermediate temperatures (recrystallization) and a slow decrease at high temperature (grain growth). Both recovery and recrystallization of the 2N Ni are strongly retarded by the presence of impurities reducing the mobility of boundaries. In the recrystallization stage, changes in hardness, microstructure and texture show that the 4N Ni recrystallizes discontinuously, in spite of a large fraction of high angle boundaries in the deformed state. This finding contradicts previous experiments and theory, which suggest that recrystallization is continuous when the fraction of high angle boundaries is high. In the 2N Ni, the observations suggest that some structural coarsening (continuous recrystallization) may take place simultaneously with discontinuous recrystallization. The findings emphasize the importance of alloying, which can delay the process of recovery and recrystallization and thereby enable tailoring of the microstructure and properties through an optimized annealing treatment.     

Hydrogen diffusion and trapping in a precipitation-hardened nickel–copper–aluminum alloy Monel K-500 (UNS N05500)

Hydrogen uptake, diffusivity and trap binding energy were determined for the nickel–copper–aluminum alloy Monel K-500 (UNS N05500) in several conditions. The total atomic hydrogen (H) concentration increased from 0 to 132 wppm as the hydrogen overpotential decreased to ?0.5 V in alkaline 3.5% NaCl electrolyte at 23 °C. The room-temperature H diffusion coefficient ranged from 0.9 to 3.9 × 10^?14 m² s^?1 for single-phase solid solution, aged, and cold worked then aged microstructures. Diffusivity was independent of lattice H concentration but depended weakly on metallurgical condition, with slower H diffusion after aging. The apparent activation energy for H diffusion was in the range of 29–41 ± 1.5 kJ mol^?1 at the 95% confidence level. The lower value approached nearly perfect lattice transport, while the high value was strongly influenced by traps of low-to-intermediate strength. Atomic hydrogen trapping at metallurgical sites, strongly suggested to be spherical-coherent γ′ (Ni₃Al) precipitates, was evident in the aged compared to the solution heat treated + water-quenched condition. Both thermal desorption and classical Oriani trap state analyses confirmed that the apparent hydrogen trap binding energy interpreted as Ni₃Al (10.2 ± 4.6 kJ mol^?1) interfaces was significantly less than the activation energy for perfect lattice diffusion (25.6 ± 0.5 kJ mol^?1) in this nickel-based alloy system.     

Evolution of strength and microstructure during annealing of heavily cold-drawn 6.3 GPa hypereutectoid pearlitic steel wire

Hypereutectoid steel wires with 6.35 GPa tensile strength after a cold-drawing true strain of 6.02 were annealed between 300 and 723 K. The ultrahigh strength remained upon annealing for 30 min up to a temperature of 423 K but dramatically decreased with further increasing temperature. The reduction of tensile strength mainly occurred within the first 2–3 min of annealing. Atom probe tomography and transmission electron microscopy reveal that the lamellar structure remains up to 523 K. After annealing at 673 K for 30 min, coarse hexagonal ferrite (sub)grains with spheroidized cementite, preferentially located at triple junctions, were observed in transverse cross-sections. C and Si segregated at the (sub)grain boundaries, while Mn and Cr enriched at the ferrite/cementite phase boundaries due to their low mobility in cementite. No evidence of recrystallization was found even after annealing at 723 K for 30 min. The stability of the tensile strength for low-temperature annealing (<473 K) and its dramatic drop upon high-temperature annealing (>473 K) are discussed based on the nanostructural observations.     

Creep behavior and its correlation with defect chemistry of La0.58Sr0.4Co0.2Fe0.8O3−δ

The creep behavior of La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) perovskite was studied in the temperature range 750–950 °C in air and vacuum (P_O2 ≈ 4 mbar). A transition in the apparent activation energy was found between 800 and 850 °C for both oxygen partial pressures. The apparent activation energy is ～250 kJ mol^?1 for the temperature range 700–800 °C under vacuum (P_O2 ≈ 4 mbar) and ～480 kJ mol^?1 for 850–950 °C in both atmospheres. Above 850 °C, the creep rate of LSCF is higher in vacuum than in air although the same cubic structure exists. The stress exponent of the creep law is in the range 1.9–2.5 for all temperatures, which excludes a transition of creep mechanism. It is suggested that, below 800 °C, cation vacancies originate from the necessary balance with the substituted cations in LSCF, and the determined activation energy reflects the energy barrier for cation migration via these vacancies. Above 850 °C, additional vacancies appear to be formed intrinsically, and the activation energy represents the sum of the thermally activated formation energy of cation vacancies and migration energy of cations.     

A phenomenological study of the incomplete grain evolution in a deformed vanadium alloy

The dynamic recrystallization (DRX) behavior of a V-5Cr-5Ti (wt%) alloy was studied with a view to optimizing its hot working behavior. Uniaxial compression tests were performed over the temperature range 1373 to 1673 K and strain rate range 0.001 to 1.0 s^− 1 and the microstructural changes were examined by EBSD. Discontinuous dynamic recrystallization (d-DRX) was observed to take place in addition to continuous dynamic recrystallization (c-DRX) despite the bcc nature of this alloy. The new grains nucleated at triple junctions and along grain boundaries to form a necklace structure. Some DRX grains formed within shear bands and deformation bands as well as in the matrix when the Zener-Hollomon (Z) parameter and the strain were increased. The critical stresses and strains increased with Z while the DRX grain size decreased with Z.     

Pressure effects and grain growth kinetics in the consolidation of nanostructured fully stabilized zirconia by pulsed electric current sintering

The effect of applied uniaxial pressure on the densification and grain size of nanocrystalline cubic zirconia (c-YSZ) was investigated during sintering by the pulsed electric current sintering (PECS) method. The role of the pressure depended on temperature, being highly significant at lower temperatures and of little significance at higher temperatures. The kinetics of grain growth were determined under PECS conditions. Analysis of the results indicated a grain growth process that is retarded, probably due to the effect of the current on grain boundary energy or dopant segregation. The activation energy for grain growth of c-YSZ was determined as 252 ± 34 kJ mol^?1, a value that is slightly smaller than reported values for microcrystalline samples.     

Synthesis of monodispersed La2Ce2O7 nanocrystals via hydrothermal method: A study of crystal growth and sintering behavior

Monodispersed and uniformly distributed La₂Ce₂O₇ nanocrystals were synthesized via the hydrothermal method using polyethyleneglycol (PEG) as surfactant. X-ray diffraction (XRD), Thermogravimetric analysis/Differential scanning calorimeter (TG/DSC), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and High resolution transmission electron microscopy (HRTEM) were utilized to characterize the thermal decomposition, phase structure and morphology of the products. Qualitative analysis indicates that the products are comprised of well-dispersed square particles with cubic fluorite structure. The specific surface area and the average crystallite size of the as-prepared products are 195.59 m² g^− 1 and 10–15 nm, respectively. The low effective activation energy (15.27 ± 0.03 kJ mol^− 1) for crystal growth was obtained in the calcination temperature range of 700–1300 °C. The sintering behavior of the compacted body was also investigated, revealing that La₂Ce₂O₇ has a low relative density and open channel morphology.     

Orientations of recrystallization nuclei developed in columnar-grained Ni at triple junctions and a high-angle grain boundary

A directionally solidified, high-purity nickel sample with a strong 〈0 0 1〉 fiber texture was cold rolled to 40% reduction. Electron backscattered diffraction (EBSD) was used to identify triple junctions (grain edges) before annealing the sample at 430 °C, after which further EBSD was performed to identify 17 recrystallization nuclei at the initially characterized triple junctions on the rolling plane. In addition, a longitudinal section was cut on which a further seven recrystallization nuclei were found and characterized. This characterization of all nuclei revealed that the majority of nuclei could be broadly associated with a 40° 〈1 1 1〉 misorientation to the matrix in which they formed, while a much lower percentage were found to have orientations as those of the local deformation substructure. However, a substantial fraction (around one-third) could not be associated with either of these types. Mechanisms for the formation of nuclei with new orientations are discussed, as is evidence of reorientation of material ahead of the recrystallization front.