Absence Of Phase Transformation Research Articles

Contrasting irradiation behavior of dual phases in Ti-6Al-4V alloy at low-temperature due to ω-phase precursors in β-phase matrix

Aiming to simulate the radiation damage effect on a dual α+β phase Ti-6Al-4V alloy utilized as high-intensity accelerator beam window material, a series of irradiation experiments were conducted with a 2.8 MeV-Fe2+ ion beam in several dpa regions at room temperature. The nano-indentation hardness increased steeply at 1 dpa and unchanged up to 10 dpa, due to the saturation of defect clusters and tangled dislocations in the dominant α-phase matrix with a size of 2–3 nm and a density of about 1×1023 m−3. In contrast in the intergranular β-phase, larger loops of 20–30 nm diameter were observed with much less density of about 5×1020 m−3. The diffraction pattern showed rectilinear diffuse streaks between the β-phase reflections, corresponding to the ω-phase precursor, without dose dependency in its intensity. FFT/I-FFT analysis of the HREM revealed a sub-nanometer-sized lattice disorder with local fluctuations, not discrete but continuous, and homogeneously distributed within the matrix β-phase stably against the irradiation. The significantly low dislocation density and the absence of phase transformation in the β-phase matrix could be attributed either to the strong sink effect expected for this distinctive sub-nanometer-sized homogeneous lattice disorder or to the anomalous point defect recombination induced by the high mobility of vacancies, both of which are originated from the metastable ω-phase precursors specifically formed in the β(BCC) phase of group-4 transition metals.

Calculation of the Heat Capacity Baseline in a Model of a Two-Phase Region in the Absence of Phase Transformations and Other Transitions

Calculation of the Heat Capacity Baseline in a Model of a Two-Phase Region in the Absence of Phase Transformations and Other Transitions

Novel Ni-Doped Bismuth-Magnesium Tantalate Pyrochlores: Structural and Electrical Properties, Thermal Expansion, X-ray Photoelectron Spectroscopy, and Near-Edge X-ray Absorption Fine Structure Spectra.

The samples of Ni-doped bismuth magnesium tantalate pyrochlores with the general formula Bi1.4(Mg1–xNix)0.7Ta1.4O6.3 (x = 0.3, 0.5, 0.7) were obtained by solid-phase synthesis. The crystal structure of the pyrochlore type (sp. gr. Fd3̅m:2) was clarified by the Rietveld method on the basis of X-ray powder diffraction data. The unit cell parameters increase with the decreasing nickel content in the range from 10.5319(1) to 10.5391(1) Å. The electronic state of atoms is established by the XPS method. According to XPS analysis, bismuth atoms have an effective charge of +3, nickel atoms +(2 + δ), and tantalum ions +(5 – δ). The coefficient of thermal expansion of the lattice of the samples was calculated from high-temperature X-ray structural measurements in the range of −180 to 1050 °C. The average values of linear TECs α in the temperature ranges of 30–570 and 600–1050 °C are 5.1 × 10–6 and 8.1 × 10–6 °C–1, respectively. The monotonicity of the change in the thermal expansion coefficient in the temperature range from −100 to 1050 °C indicates the absence of phase transformations. All samples are dielectric and exhibit high activation energies ∼2.0 eV, moderately high dielectric constants ∼24–28, and tangent dielectric losses ∼0.002 at 1 MHz and 21 °C. The electrical properties of the samples are described by a simple parallel equivalent scheme. The chemical composition of the materials has little effect on the polarizability of the medium or on the value of the activation energy of the conductivity. Ionic processes in investigated materials at frequencies 200–106 Hz and at temperatures 100–450 °C were not detected.

Synthesis, spectroscopic analysis and DFT studies of N-(2-methyl-5-nitro-phenyl)benzamide organic single crystal

N-(2-methyl-5-nitro-phenyl)benzamide (N2M5NPBA) compound was synthesized by benzoylation of 2-methyl-5-nitroaniline. N2M5NPBA single crystal was grown by slow evaporation technique using DMSO as solvent. Single crystal X-ray diffraction study of N2M5NPBA crystal have monoclinic system under space group P21/C with the lattice parameters a = 4.9490 Å, b = 10.1916 Å, c = 24.969 Å, α = γ = 90°, β = 91° and V = 1259.1 Å3. Nitro and amide functional groups presence in N2M5NPBA crystal was identified in FT-IR spectral characterization. The molecular geometry of title molecule was optimized and theoretical analysis was performed by density functional theory (DFT) using B3LYP/6-311++G (d, p) basis set. UV-Visible spectrum analysis of N2M5NPBA predict the cut-off wavelength was 278 nm (bandgap energy ~4.19 eV). 1H & 13C NMR spectrum were recorded the grown N2M5NPBA crystal to confirm the chemical structure. The thermal intransigence of the grown N2M5NPBA crystal was determined by the TGA (thermal stability up to 528 K) and accurate melting point was confirmed from DSC. The absence of phase transformation was confirmed by TG-DTA analysis. The second harmonic generation (SHG) efficiency of N2M5NPBA single crystal was evaluated by Kurtz-Perry powder test and the outcome of N2M5NPBA crystal is 2.2 times higher than that of KDP crystal.

Microstructure and properties of FeCrMnNiCx compositionally complex bulk alloys

FeCrMnNiCx (x = 0.1, 0.3, 0.5 and 1) bulk alloys were synthesised by Vacuum Arc Smelting. The alloy with 2 at. % of carbon (x = 0.1) is a single-phase and uniform material with no element or phase segregation. The alloys with higher carbon content were two phase materials. The equiatomic FeCrMnNiC alloy with two phases had the best corrosion resistance in 3.5% NaCl solution at room temperature under atmospheric pressure. Ion irradiation of FeCrMnNiC0.1 with FCC structure by 600 KeV Xe+2 to 50 displacements per atom (dpa) at 623 K revealed the alloy high radiation hardness by the absence of phase transformation and elemental segregation. The single-phase FeCrMnNiC0.1 alloy has a hardness of 7.5 ± 0.84 GPa and reduced Young's modulus of 250 ± 28.1 GPa.

Microstructures in a Nb-Cr-V-W-Ta high entropy alloy during annealing

A Nb-Cr-V-W-Ta high entropy alloy has been subjected to annealing treatment at 600, 700 and 800 °C in air for 6 hours. Five different phases have been identified based on their composition in the as received condition. The annealing treatment does not alter the phase analysis and compositions except for the W-rich phase where minor changes in the composition have been observed. This is considered as an experimental evidence of sluggish diffusivity associated with HEAs. Absence of phase transformation during the heating and the minimal changes in observed compositions of the phases is an indication of the resistance to diffusion offered by the elements of the high entropy alloy. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and color X-ray elemental mapping have been used to conduct this research.

Physicochemical and DFT studies on new organic Bis-(2-amino-6-methylpyridinium) succinate monohydrate good quality single crystal for nonlinear optical applications

X-ray single-crystal study on the slow evaporation technique grown 2-amino-6-methylpyridinium succinate (2A6MS) exposes its crystal system which belongs to triclinic with centrosymmetric space groupP1¯. A crystal having a dimension of 20 × 8 × 4 mm3 was achieved. 1H NMR spectrum was recorded to investigate the chemical surroundings of various hydrogens in the grown 2A6MS complex and to compliment the X-ray single crystal analysis. FTIR and Raman spectral analyses supported the presence of identified functional groups in the chemical structure. The optical studies by UV–Vis and PL have shown the cut-off wavelength at 348 nm and an occurrence of a high optical emission band at 380 nm under an excitation with 342 nm. The thermal/chemical stability up to 104.3 °C and the absence of phase transformation were confirmed by the TG-DSC study. Vicker’s micro hardness test at room temperature and Chemical etching perceived the mechanical property and the grown-in features of the 2A6MS crystal. SHG study confirmed that the grown 2A6MS crystal has SHG which is very well compared with the standard KDP crystal. Density functional theory (DFT) computation was used on B3LYP/6–311++G (d, P) basis set to optimize molecular geometry, HOMO-LUMO energies, Mulliken population analyses, molecular electrostatic potential, NBO, and first order hyperpolarizability.

Roentgenometry of the Zr–2.5Nb alloy under cyclic loads

Studies were carried out in this work to establish the influence of thermal cyclic and mechanical cyclic loads that simulated the factors of operational impact on structural stability and crystal lattice dilatation of zirconium alloys. It is shown that during cold cyclic deformation at the first moment of loading the Zr–2.5Nb alloy is characterized by significant instantaneous deformation. The effect of anomalous thermal deformations of the zirconium alloy crystal lattices, which are typical for phase transitions of type I and type II, was detected in the operating temperature range (250–350 °C). The absence of phase transformation in this temperature range allows us to suggest the existence of a causal relationship between the anomalous thermal expansion effects of zirconium crystal lattices with orientation changes and phase transformations in grain boundaries. The performed calculations showed the presence of an anomalous change in the crystallite size distribution at a given temperature. The coincidence that temperatures were correlated with size effects and phenomena such as blistering and flecking has been established. The hypothesis of grain-boundary transformations is indirectly confirmed by the results of the second thermal cycle, during which a second jump of the crystal lattice thermal deformations, not related to structural phase transitions, was detected in the range of 550–650 °C.

Tailoring supersaturation from amorphous solid dispersions

The maximum achievable concentration of a drug in solution is dictated by the chemical potential of the solid form. Because an amorphous solid has a higher chemical potential than the corresponding crystal form, in the absence of phase transformations, a higher transient solubility is expected. However, the chemical potential of an amorphous drug can be reduced by mixing with another component. Therefore, upon mixing with a polymer to form an amorphous solid dispersion (ASD), the maximum solution concentration achieved can be potentially altered, in particular if the polymer is poorly soluble in the dissolution medium. Such changes in the chemical potential of the drug may be a critical factor in determining the maximum achievable solution concentration, and could alter the crystallization driving force of the drug. Therefore, the aim of this study was to gain insights into the impact of poorly soluble polymers on the “amorphous solubility” of drugs formulated as amorphous solid dispersions. Lopinavir was selected as a model drug with a low crystallization tendency, enabling determination of the amorphous solubility as a function of ASD composition. Model polymers included cellulose acetate (CA), CA phthalate (CAP), ethylcellulose (EC), Eudragit® RL PO (EUD), hydroxypropylmethylcellulose (HPMC), HPMC acetate succinate (HPMCAS), and HPMC phthalate (HPMCP). The “amorphous solubility” of the drug alone was determined and then the changes in maximum achievable concentration were measured as a function of drug loading. Drug-polymer interactions were characterized using infrared spectroscopy (IR), differential scanning calorimetry (DSC) and moisture sorption analysis. The results showed that the maximum achievable concentration (“amorphous solubility”) of lopinavir varied with the extent of drug-polymer interactions, as well as the drug weight fraction in the ASD. This information is of great value when evaluating the maximum achievable concentration of amorphous systems formulated with pH responsive polymers, and should contribute to a broader understanding of drug phase behavior in the context of ASDs.

Effect of thermal cycling process parameters on recrystallization kinetics for processing of fine-grained pure copper

ABSTRACTThe present work developed a two-step thermal cycling technique for processing of fine-grained pure copper for improved properties. This included initial annealing of specimens followed by heavy cold rolling and a series of heating and cooling cycles. The study investigated the important microstructural changes occurring in the cold deformed grains in the absence of phase transformations. A major interest of the present research was to closely describe the effect of thermal cycling parameters on the recrystallization and grain growth kinetics for processing of fine-grained structure. The study determined the optimum values of process parameters for the developed thermal cycling route including the extent of cold deformation, annealing temperatures, holding periods, and the number of thermal cycles. The thermal cycling process produced closely distributed fine grains with bi-modal microstructure leading to increased hardness and strength without any significant loss in electrical conductivity.

Strain localization during discontinuous plastic flow at extremely low temperatures

• The paper deals with an extended, physically based, multiaxial constitutive model of discontinuous plastic flow, including new feature of strain localization at near 0 K temperatures in polycrystalline materials. • Another new feature defines the drop of stress being entirely determined by dislocations density before and after the serration. • Regular way of nucleation and free unconstrained glide of slip bands is observed within the sample, in the absence of phase transformation. • Contrary to former views, strong coupling of discontinuous plastic flow with fcc-bcc phase transformation in metastable materials is observed. The phenomenon of strain localization in the course of discontinuous plastic flow (DPF) at extremely low temperatures is investigated. DPF is observed mainly in fcc metals and alloys strained in cryogenic conditions, practically down to absolute zero. These materials undergo at low temperatures a process similar to dynamic strain ageing, manifested by the so called serrated yielding (DPF). DPF is attributed to the mechanism of local catastrophic failure of lattice barriers (including Lomer–Cottrell locks), under the stress fields related to the accumulating edge dislocations. Failure of LC locks leads to massive motion of released dislocations, accompanied by step-wise increase of the strain rate (macroscopic slip) and drastic drop of stress. Recent experiments indicate strong strain localization in the form of shear bands propagating along the sample. The plastic power dissipated in the shear band is partially converted to heat, which results in a local drastic increase of temperature promoted by the so-called thermodynamic instability (nearly adiabatic process). The Dirac-like temperature function is measured by two thermometers located in the gage length of the sample. Spatio-temporal correlation indicates smooth shear band propagation, as long as the process of phase transformation remains on hold. A physically based multiaxial constitutive model presented in the paper describes both DPF and strain localization, accompanied by temperature distribution represented by Green-like solution of heat diffusion equation. The model accounts for the thermodynamic background, including phonon mechanism of heat transport, accompanied by specific heat vanishing with the temperature approaching absolute zero. Experimental identification of parameters of the constitutive model is carried out. A projection of the model to the range where the phase transformation takes place is discussed.

Understanding conversion mechanism of NiO anodic materials for Li-ion battery using in situ X-ray absorption near edge structure spectroscopy

Nano-scaled NiO particles (nano-NiO) are prepared by a ligand stabilization method and compared with micron-sized NiO particles (micro-NiO) as anodic material of Li-ion battery. The structural and physical properties are characterized by N2 physisorption, transmission electron microscopy, and X-ray diffraction. The nano-NiO shows uniform spheres with an average particle size of 9 nm with high and stable discharge capacity of 637 mAh g−1, while the micro-NiO forms irregularly shaped particles with an average particle size of 750 nm with low capacity of 431 mAh g−1 at 0.5C. In situ X-ray absorption near edge structure (XANES) analysis reveals that the capacity and reversibility of the NiO anode is highly affected by the particle size of the NiO. The micro-NiO exhibits a low capacity with absence of phase transformation upon the discharge/charge cycles. In contrast, the nano-NiO exhibits a high capacity with reversible phase transformation between NiO and Ni metal upon the cycle test.

High Cycling Stability and Extreme Rate Performance in Nanoscaled LiMn2O4 Thin Films.

Ultrathin LiMn2O4 electrode layers with average crystal size of ∼15 nm were fabricated by means of radio frequency sputtering. Cycling behavior and rate performance was evaluated by galvanostatic charge and discharge measurements. The thinnest films show the highest volumetric capacity and best cycling stability, retaining the initial capacity over 70 (dis)charging cycles when manganese dissolution is prevented. The increased stability for film thicknesses below 50 nm allows cycling in both the 4 and 3 V potential regions, resulting in a high volumetric capacity of 1.2 Ah/cm3. It is shown that the thinnest films can be charged to 75% of their full capacity within 18 s (200 C), the best rate performance reported for LiMn2O4. This is explained by the short diffusion lengths inherent to thin films and the absence of phase transformation.

Molten salt corrosion of high density graphite and partially stabilized zirconia coated high density graphite in molten LiCl–KCl salt

Corrosion studies were performed on uncoated high density graphite and plasma sprayed partially stabilized zirconia (PSZ) coated high density graphite with NiCrAlY bond coat in molten LiCl–KCl eutectic salt at 600°C for periods of 250h, 1000h and 2000h under inert argon atmosphere. High density graphite showed weight loss while PSZ coated high density graphite showed weight gain. There is no significant attack and degradation of top PSZ coating in molten salt, however microcracks were observed at the bond coat-substrate interface after 2000h of exposure. PSZ coated high density graphite exhibited excellent corrosion resistance in molten LiCl–KCl salt due to chemical stability and absence of phase transformation as confirmed from scanning electron microscopy, X-ray diffraction and laser Raman studies, however adhesion of the coating has to be improved.

Elastic Modulus Evolution and Behavior of Si/Mullite/BSAS-Based Environmental Barrier Coatings Exposed to High Temperature in Water Vapor Environment

Si-based ceramics (e.g., SiC and Si3N4) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the surface recession that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H2O vapor), appropriate refractory oxides are engineered to serve as environmental barrier coatings (EBCs). The current state-of-the-art EBCs multilayer system comprises a silicon (Si) bond coat, mullite (3Al2O3·2SiO2) interlayer and (1 − x)BaO·xSrO·Al2O3·2SiO2, 0 ≤ x ≤ 1 (BSAS) top coat. In this article, the role of high-temperature exposure (1300 °C) performed in H2O vapor environment (for time intervals up to 500 h) on the elastic moduli of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates was investigated via depth-sensing indentation. Laser-ultrasonics was employed to evaluate the E values of as-sprayed BSAS coatings as an attempt to validate the indentation results. Fully crystalline, crack-free, and near-crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The absence of phase transformation and stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers after harsh environmental exposure provides a plausible explanation for the almost crack-free coatings observed. The relationships between the measured elastic moduli of the EBCs and their microstructural behavior during the high-temperature exposure are discussed.

Experimental Simulation of the Solidification of Steel at Higher Cooling Rates

Today, continuous casting is the common technology for casting commercial steel grades. Conventional casting processes, like slab casting, are characterised by a moderate heat withdrawal and a rather low solidification velocity and cooling rate. Linking the casting and the rolling process demands a higher casting velocity, an increased heat withdrawal and thus, a higher local cooling rate. The absence of phase transformations during cooling and reheating before the rolling process makes the solidification microstructure more important for the behaviour of the steel during rolling and for the final product properties. Over several years the Christian Doppler Laboratory for "Metallurgical Fundamentals of Continuous Casting Processes" has developed an experimental setup for the simulation of solidification at higher cooling rates. The experiment is based on the principle of a dipping test under inert gas atmosphere inside a vacuum induction furnace. Recently, this apparatus has been equipped with a pyrometer in order to measure the temperature of the solidified sample during the subsequent cooling phase and also with a furnace in order to simulate different cooling and heat treatment strategies. Thus, it is possible to reproduce solidification and subsequent cooling of the cast material in casting-rolling processes and to characterise the microstructure and the mechanical properties of the solidified samples. The present work will give an overview on heat transfer in conventional casting processes, present a laboratory scale simulation of solidification a higher cooling rate, touch some aspects like the numerical simulation of the experiment and conclude with some results and an outlook on further planned work.

Absence of phase transformation at low temperature in Co-doped LiMn2O4 samples

The phase transition in spinel LiMn2O4 was investigated using powder synchrotron radiation diffraction. A transition from cubic (Fd3m) to orthorhombic (Fddd) was observed around T = 220 K. Increasing the average valence of Mn by doping Co into the [Mn2O4] framework, LiMn1.9Co0.1O4, reduced the concentration of Mn3+ ions, suppressed the Jahn–Teller distortion and the cubic phase was retained at low temperature. p

Neutron Diffraction Study of La3Ni2O7: Structural Relationships Among n=1, 2, and 3 Phases Lan+1NinO3n+1

Structural relationships among the n=1, 2, and 3 members of the Ruddlesden–Popper series Lan+1NinO3n+1 are used to predict a lowering of symmetry in the n=2 phase, subsequently observed and refined against neutron powder diffraction data. Resistivity and magnetic susceptibility measurements are presented for n=2 and 3 that suggest the possibility of a further symmetry lowering at low temperature, such as occurs for n=1; however, no evidence for this is found in low-temperature neutron data. Re-refinement of the n=3 phase reveals significant strain within the perovskite-type layers that appears to increase with n. This strain is used to explain the absence of low temperature phase transformations for n>1. In the absence of a phase transformation, possible origins for the resistivity and susceptibility anomalies are considered.

Pseudobinary intermetallic compounds in Hf2M′–Hf2M″ (M′, M″=Mn, Fe, Ni, Cu) systems and their interaction with hydrogen at high pressures

Abstract Ternary alloys in the Hf 2 Ni–Hf 2 Mn, Hf 2 Ni–Hf 2 Fe, Hf 2 Ni–Hf 2 Cu and Hf 2 Fe–Hf 2 Cu systems have been prepared and characterised by X-ray diffraction and pressure–composition isotherm measurements. Single phase compounds have been obtained for all Ni-containing alloys with a general composition of Hf 2 Ni 0.5 M 0.5 . These compounds retained the structure type of parent Hf 2 M intermetallics: cubic Ti 2 Ni type for Hf 2 Ni 0.5 Fe 0.5 and Hf 2 Ni 0.5 Mn 0.5 and tetragonal MoSi 2 type for Hf 2 Ni 0.5 Cu 0.5 . The new hydrides obtained from ternary intermetallics had the composition of Hf 2 Ni 0.5 M 0.5 H x where x =3.1 (M=Cu) and 4.9–5.0 (M=Mn, Fe). Formation of hydride phases from cubic Mn- or Fe-containing compounds occurred without any change in structure type but was accompanied by unit cell volume expansion of 23–24%. In the case of Cu, there was a tetragonal to orthorhombic transformation. An absence of phase transformations in high pressure range of 100 to 2000 atm for new compounds unlike the earlier studied Hf 2 M–H 2 systems has to be noted.

Buckling of a nonlinearly elastic slab lying on the surface of a liquid with allowance for phase transformation

The equations of the three-dimensional nonlinear theory of elasticity [1] are used to formulate equilibrium conditions with finite strains for an arbitrary thermoelastic body undergoing a phase transformation. These conditions are then used to study the equilibrium of a circular uniform slab lying on the surface of a melt in a gravitational field. We use the model of a non-Hookian material as the governing relation for the material of the slab, this model being one possible generalization of the model of an incompressible linearly elastic body to the case of finite strains. The method of superimposing a small strain on a finite strain [1] is used to study local loss of stability of the slab due to its compression in the radial direction. The critical strains are determined numerically. A similar approach is used to study buckling of the slab in the absence of phase transformation.