Phase Decomposition Mechanisms Research Articles

Investigation of phase separation mechanism in AlCoCrFeMo0.05Ni2 high-entropy alloy by first-principles calculations

AlCoCrFeMo0.05Ni2 high-entropy alloy (HEA) has great potential for high-temperature applications. However, its FCC matrix will decompose during annealing, which seriously limits its high-temperature applications. Here, the lattice distortion, Gibbs free energy change, phonon spectra, density of states, and bond length distribution of the FCC matrix were calculated by first-principles calculations. The findings indicate that the primary barrier to the formation of a single homogeneous solid solution in the alloy is more closely associated with Gibbs free energy rather than the degree of lattice distortion. The vibration frequency of Al is not in sync with the overall structure, making it prone to desolvation at elevated temperatures. Furthermore, an analysis of bond lengths reveals that the Al–Ni bond length is significantly shorter than the theoretical value, facilitating the formation of a (Ni, Al)-rich phase. In contrast, the longer bond lengths of Al–Cr and Al–Mo facilitate the detachment of Cr and Mo from the (Ni, Al)-rich phase, resulting in the formation of (Cr, Mo)-rich phase. This in-depth investigation sheds light on the phase decomposition mechanism of high-entropy alloys, offering valuable insights for future research in this field.

Morphological, compositional and phase evolutions in sulfur-rich W-S targets during magnetron sputtering

A sulfur-rich WS2.7 target was manufactured via ball milling of sulfur and tungsten-disulfide powders, followed by sintering. The target morphology, composition, and phase changes induced by sputtering processes were investigated by a number of analytical methods and compared to a stoichiometric WS2 target used as a control. Radio frequency magnetron sputtering of the WS2.7 target was conducted with progressively increasing time intervals, from a few minutes to several hours, where the target was intermittently removed for characterization. Selective sputtering of sulfur within both the control WS2 and the sulfur-rich WS2.7 targets yielded sulfur depletion over the extended sputtering time. Depressions on the target surface formed from the selective sputtering of sulfur, where at the base of these regions developed growth of metallic tungsten dendrites and additional unbonded sulfur segregation over sputtering time. A catalytically enhanced WS2 phase decomposition mechanism was proposed based on detailed analysis of phases and oxidation states from site specific spectroscopic and x-ray diffraction analysis. The observed phenomenological process exemplifies the need for monitoring and maintenance of sulfur-rich sputtered targets in their use for thin film growth.

Strong Interfacial Coupling of Tunable Ni-NiO Nanocomposite Thin Films Formed by Self-Decomposition.

The next-generation spintronic devices including memristors, tunneling devices, or stochastic switching exert surging demands on magnetic nanostructures with novel coupling schemes. Taking advantage of a phase decomposition mechanism, a unique Ni-NiO nanocomposite has been demonstrated using a conventional pulsed laser deposition technique. Ni nanodomains are segregated from NiO and exhibit as faceted "emerald-cut" morphologies with tunable dimensions affected by the growth temperature. The sharp interfacial transition between ferromagnetic (002) Ni and antiferromagnetic (002) NiO, as characterized by high-resolution transmission electron microscopy, introduces a strong exchange bias effect and magneto-optical coupling at room temperature. In situ heating-cooling X-ray diffraction (XRD) study confirms an irreversible phase transformation between Ni and NiO under ambient atmosphere. Synthesizing highly functional two-phase nanocomposites with a simple bottom-up self-assembly via such a phase decomposition mechanism presents advantages in terms of epitaxial quality, surface coverage, interfacial coupling, and tunable nanomagnetism, which are valuable for new spintronic device implementation.

Experimental and theoretical investigations on the phase stability and mechanical properties of Cr7Mn25Co9Ni23Cu36 high-entropy alloy

Understanding the mechanisms of phase formation and their influence on the mechanical behavior is crucial for materials used in structural applications. Here, the phase decomposition under heat treatment in the Cr7Mn25Co9Ni23Cu36 (atomic percentage) high-entropy alloy and how secondary phases formed affect its tensile mechanical response are reported. The microstructural analysis shows that heat treatment at 800 °C /2 h and 600 °C /8 h led to the formation of sigma phase, but the sigma phase was not observed for 2 h heat treatment at 600 °C and below. The experimentally observed thermal stability and phases are compared to the calculated phase diagram and rationalized by recourse to thermodynamics and kinetics. The mechanism of phase decomposition is discussed based on ab initio calculations, indicating that decomposition into two solid solution phases is energetically preferred over a single solid solution phase with nominal composition.

Experimental measurement and thermodynamic optimization on the solidus miscibility gap of the Mo-V binary and the Mo-Nb-V ternary systems

The solidus miscibility gap and the phase relations of the Mo-V binary system at the low temperature region below 851 °C have been determined by experimental approaches. The homogenized alloy samples were measured by differential scanning calorimetry (DSC), and the obtained results indicate that below 851 °C there exists an immiscible area. The observations of both the alloy samples and the diffusion couples isothermally treated at 600 and 750 °C by electron probe microanalysis (EPMA) reveal the separations of the solid solution phase (BCC_A2) into two phases, the Mo-rich Bcc_A2(Mo,V)1 (βMo) and the V-rich Bcc_A2(Mo,V)2 (βV) phases, which have the same crystal structure but different compositions. Two typical phase decomposition mechanisms, the nucleation-growth and the spinodal decomposition, are confirmed by means of the transmission electron microscopy (TEM). By consideration of the present experimental data for the solidus miscibility gap and the related literature reports for the liquid-solid phase equilibria, the phase diagram of the Mo-V binary system was constructed. Then a set of self-consistent thermodynamic parameters for describing the Gibbs energies of the constituent phases was assessed. The thermodynamic calculations agree well with the experimental data of the solidus miscibility gap and the spinodal decomposition range of the Mo-V binary system. Furthermore, on the basis of the studies on the Mo-V and the Nb-V binary systems, the isothermal sections of the Mo-Nb-V ternary system at 600 and 750 °C were experimentally measured and thermodynamically optimized. The experimental isothermal sections compare well with the result of the thermodynamic calculations.

Thermal decomposition of Zn[(C6H5)2PSSe]2 single-source precursor for the chemical vapour deposition of binary and ternary zinc chalcogenides: a theoretical study.

The mechanistic pathways for the formation of zinc selenide, zinc sulphide and ternary zinc selenide sulphide in the gas phase decomposition of Zn[(C6H5)2PSSe]2 single-source precursor has been explored on the singlet and doublet potential energy surfaces using density functional theory method in order to understand the thermodynamic and kinetic properties. The optimized geometries and the predicted energies were obtained using the M06 functional and a combination of the basis sets LACVP* for Zn and 6-31(d) for light atoms. The rate constants of each elementary reaction have been calculated using the transition state theory. The results indicate that the steps that lead to ternary ZnSexS1−x formation on both the singlet and doublet potential energy surfaces is favoured kinetically and thermodynamically over those that lead to ZnSe and ZnS formation. Density functional theory calculations of the gas phase decomposition of the complex indicate that the deposition of ternary ZnSexS1−x in chemical vapour deposition may involve more than one step but the steps that lead to its formation are consistent with a dominant role for both thermodynamic and kinetic factors. The theoretical study showed important insights as a general tool to anticipate the gas phase decomposition mechanism of a novel precursor when direct experimental measurements are not available.

A study of phase separation processes in presence of dislocations in binary systems subjected to irradiation

Dislocation-assisted phase separation processes in binary systems subjected to irradiation effect are studied analytically and numerically. Irradiation is described by athermal atomic mixing in the form of ballistic flux with spatially correlated stochastic contribution. While studying the dynamics of domain size growth we have shown that the dislocation mechanism of phase decomposition delays the ordering processes. It is found that spatial correlations of the ballistic flux noise cause segregation of dislocation cores in the vicinity of interfaces effectively decreasing the interface width. A competition between regular and stochastic components of the ballistic flux is discussed.

Exploration for the potential precursors for zirconium carbide atomic layer deposition via comprehensive computational mechanistic study of the gas phase decomposition of neopentyl zirconium derivatives

We previously reported the probable gas phase decomposition mechanism of tetraneopentyl zirconium (ZrNp4) under typical MOCVD (metalorganic chemical vapor deposition) conditions (400 to 800 °C) using computational thermochemistry. By the same approach, we performed a mechanistic study of the gas phase decomposition of trineopentyl zirconium monochloride (ZrNp3Cl) to evaluate its possibility as a CVD precursor for ZrC film growth. It was demonstrated that strong Zr-Cl bonding would require much higher growth temperatures to drive the gas phase decomposition of ZrNp3Cl, compared to ZrNp4. The higher temperature growth would pose the problem of accelerated gas phase parasitic reactions, which potentially hamper ZrC deposition on the surface. However, strong Zr-Cl bonding offers the possibility of ZrC ALD (atomic layer deposition) using dineopentyl zirconium dichloride (ZrNp2Cl2), and a postulated scheme is presented based on the results from the gas phase decomposition study of ZrNp4 and/or ZrNp3Cl.

Computational study on the decomposition of tetraneopentyl zirconium for the chemical vapor deposition of zirconium carbide

The overall gas phase decomposition mechanism of tetraenopentyl zirconium precursor (Zr[CH2C(CH3)3]4) for the chemical vapor deposition of zirconium carbide thin films was investigated by using computational thermochemistry. Density functional theory (DFT) and harmonic vibrational frequency calculation were used to generate thermodynamic properties at each reaction step, based on which thermodynamic or kinetic preference of a reaction pathway was evaluated. While the preference of γ-hydrogen abstraction of neopentane over α-hydrogen abstraction was confirmed in the initial stage of ZrNp4 decomposition, they turned out to be competing instead of the dominant preference of γ-hydrogen abstraction. Methane formation at three subsequent reaction steps was explained by β-methyl migration, and the following α-hydrogen abstraction of methane based on the suggestion that α- and γ-hydrogen abstractions of neopentane are competing kinetically in previous reaction steps. Computational thermochemistry showed a possibility as a general tool to anticipate the gas phase decomposition mechanism of a precursor in chemical vapor deposition.

HRTEM Observation of Alpha-Phase in Al Added 60/40 Cu-Zn Alloy

The effect of Al-addition on the mechanism of phase decomposition from β1to α-phase in Cu-40at.%Zn alloy (base alloy) has been investigated using hardness measurement and HRTEM observation. The peak hardness during annealing and the number density of α-phase in Al-bearing alloy were the same as those in the base alloy. Many striations which have been considered as the periodicity of 9R structure were observed in the α-phase of the Al-bearing alloy. There were a lot of striations in the α-phase at the early annealing time, and the number of these striations decreased with increasing annealing time. The rate of decreasing of this striation in the α-phase of the Al-bearing alloy was the slower than that of the base alloy. According to the analysis of the selected area electron diffraction (SAED) patterns obtained for these α-phase in those alloys, transition time from the 9R structure to fcc structure t (9R-fcc) of has been detected by the changing of angles in SAED patterns. The t (9R-fcc) of α-phase in the Al-bearing alloy showed the slower than the base alloy.

Research on the phase decomposition of Lu xY 1−xAlO 3:Ce crystals at high temperatures

Cerium-doped lutetium ortho-aluminate LuAlO 3:Ce crystal is a good scintillator with excellent properties, while the single crystal is very difficult to grow and the phase decomposition mechanism remains unknown. In this paper, based on the analyses of high-temperature DTA and X-ray diffraction (XRD), it is supposed that LuAlO 3:Ce melts at 1905 °C, it tends to decompose when the temperature goes above 1200 °C, and the decomposition products are Lu 3Al 5O 12:Ce and Lu 4Al 2O 9:Ce. This decomposition is an irreversible endothermic reaction. This phenomenon is suggested to be caused by the unfitness of the radius of Lu 3+ ion for the perovskite-type structure of LuAlO 3, and this is the intrinsic cause for the instability of LuAlO 3:Ce. Introducing ions with larger radius (such as Y 3+) into LuAlO 3:Ce crystal may increase its phase stability.

Experimental studies of the phase separation mechanism in Ti-15at%A1

Abstract The phase decomposition mechanism operating in a binary Ti-A1 alloy in the two-phase α+α2 region has been investigated. A Ti-15at%A1 alloy was aged in the range 550–750°C for various times to produce material at different stages of phase separation. The resulting microstructures have been examined experimentally using three-dimensional atom probe field ion microscopy and neutron diffraction. Results indicate that significant ordering occurs in the system prior to the onset of chemical phase separation which occurs via a spinodal reaction. The mechanism is believed to lie in the class of ‘conditional spinodal’ mechanisms in which one stage of the phase separation process is thermodynamically contingent upon another.

Short range order and phase separation in Ti–Al alloys

The phase decomposition mechanism operating in binary Ti–Al alloys in the two phase α+α2 region has been investigated. A Ti–15at.%Al alloy was aged in the range 450–750°C for various time periods. The resulting microstructures were studied by transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM). Computer simulations of phase separation in this alloy were also carried out using a dynamic Ising model. Results from the computer simulations suggest decomposition occurs via a conditional spinodal route, whereby compositional phase separation is contingent upon prior ordering of the matrix. Experimental studies to date are fully consistent with this mechanism.

2相ステンレス鋳鋼におけるフェライトの相分離過程のメスバウアー分光測定

Duplex stainless steels become brittle at medium temperatures, owing to the phase decomposition of a ferrite into a Cr-rich α′ phase and an Fe-rich α phase. A cast CF3M duplex stainless steel containing 25% volume fraction of ferrite was aged at 723 K for time periods up to 10000 h to investigate the phase decomposition mechanism and to evaluate the composition of the decomposition products using Mössbauer spectroscopy at room temperature. The half width of the internal magnetic field distribution curve estimated from the experimental Mössbauer spectra increases in the initial stage of aging, suggesting compositional fluctuation due to spinodal decomposition. The ferrite phase finally decomposes into the Fe-rich α and Cr-rich α′ phases after prolonged aging. The composition of the Fe-rich α phase is estimated to be Fe-11 at%Cr-5 at%Ni by numerically analyzing the internal magnetic field distribution based upon the local environmental effect of solute atoms on the central Fe nucleus. The result is consistent with the one previously obtained by atom-probe analysis. The isomer shift of 57Fe in the α′ phase is estimated to be −0.114±0.002 mm/s at room temperature by decomposing an overlapped paramagnetic absorption peak into component peaks. The Cr content of the phase is estimated to be about 85 at% by plotting the isomer shift on an experimental isomer shift versus Cr content diagram.

Synthesis, Characterization and Evaluation of Zinc-Amides as Potential Dopant Sources for ZnSe OMVPE

ABSTRACTCompounds of the general forms Zn[N(R)2]2, Zn(N(R)(R')2] and Zn{[N(R)2][N(R')2]} have been prepared, these new compositions have been characterized by multinuclear NMR, GC/MS, FTIR, elemental analysis and single crystal x-ray diffraction, and they have been evaluated for their potentialto serve as “designer dopants” in the epitaxial growth of p-type ZnSe. Retention of the Zn-N bond during deposition should insure selective location of the nitrogen atom on the native selenium lattice site. Precursor vapor pressures, vapor phase decomposition mechanisms, and thin film properties are presented. Results from materials characterization by XRD, SIMS, PL, Raman and SEM are presented in the context of evaluating dopantlevel.

In situ mass spectroscopy and thermogravimetric studies of GaAs MOCVD gas phase and surface reactions

Two special MOCVD reactors systems designed for in situ monitoring of gas phase and surface reactions are presented. The gas phase decomposition mechanisms of Ga(CH 3) 3, Ga(C 2H 5) 3 and As(CH 3) 3 in H 2, He and D 2 are investigated by molecular beam sampled mass spectroscopy. The data show that the initial thermal pyrolysis reaction of Ga(CH 3) 3 and As(CH 3) 3 is a unimolecular reaction leading to the loss of methyl groups. This is supported by the observations of similar decomposition temperatures in H 2 and He, an increased C 2H 6 formation in He, and the production of CH 3D in D 2 carrier gas. The decomposition of Ga(C 2H 5) 3 proceeds either via the loss of ethyl radicals or β-elimination of ethene, and the latter mechanism dominates above 650 K. This model is supported by variation in the product distribution of C 2H 4, C 2H 5·, C 2H 6 and C 4H 10 with pyrolysis temperature. A new thermogravimetric MOCVD system is used to measure surface reaction rates of Ga(CH 3) 3 and As(CH 3) 3 at low pressures, where mass transfer effects are absent. Data for varying total pressures and constant partial pressures of Ga(CH 3) 3 and As(CH 3) 3 indicate that the growth rate at reduced pressures is influenced by gas phase reactions. At low pressures surface reactions appear to dominate and at high temperatures the growth rate goes to zero and etching takes place.

Phase decomposition phenomena in polystyrene/(polyvinylmethylether)

AbstractThis paper reports studies on polystyrene/poly‐(vinylmethylether) blends to elucidate phase decomposition phenomena. Small angle neutron scattering (SANS) and temperature jump light scattering (TJLS) techniques were used to study both static and kinetic properties. The SANS methods were used to characterize the blends with respect to phase diagram, the Flory‐Huggins interaction parameter, and the critical fluctuation length as a function of temperature. From the TJLS experiments, we have demonstrated the possibility of two kinds of phase decomposition mechanisms which should lead to distinct morphology and material properties.