Образование интерметаллидных фаз в функциональных покрытиях, полученных методом холодного газодинамического напыления

Chapter 12 - Study of Two-Dimensional Phases on Electrodes

ABSTRACTTwenty years of research have now been devoted to investigating reaction products obtained by annealing metal-layer/silicon structures. A wide variety of cases have been analyzsed and a considerable amount of data has been produced. Despite the vast amount of information available, several aspects concerning phase formation and kinetic processes are not yet well established. The purpose of this paper is to investigate the mechanisms of phase formation and to show the importance of kinetic factors in the appearance of various compounds. Results will be shown for a single metal layer deposited on silicon, for bilayers. and for alloys. Depending upon the starting structure, metal-rich or silicon-rich silicides can be formed. Moreover, by modifying the boundary conditions, it is possible to change the growth kinetics of the silicide phase that forms.

We systematically investigate the internal friction properties of a Fe–(43 at.%)Al powder mixture compact during the heating process with the expectation to understand the phase formation and transition process. Three internal friction peaks are successively observed during the heating process from room temperature to 750°C, but almost completely disappear in the subsequent cooling process. Three internal friction peaks exhibit obvious measuring frequency dependence, which increases with decreasing the frequency. The first internal friction peak originates from the micro-sliding of weak bonding interface between Al particles corresponding to a recrystallization process of deformed Al particles. The second internal friction peak is attributed to a phase formation process associated with the formation of the intermediate phase Fe2Al5. The third internal friction peak is considered to result from the formation of the FeAl intermetallic compound owing to the reaction of Fe2Al5 and residual Fe initiated by a dramatic thermal explosion reaction.

Unveiling the role of Cs ion in perovskite phase formation during solid–vapor reactions

In the present paper, some further results of application of the generalized Gibbs' approach to describing new-phase formation processes are outlined. The path of cluster evolution in size and composition space is determined taking into account both thermodynamic and kinetic factors. The basic features of these paths of evolution are discussed in detail for a simple model of a binary mixture. According to this analysis, size and composition of the clusters of the newly evolving phase change in an unexpected way which is qualitatively different as compared to the classical picture of nucleation-growth processes. As shown, nucleation (i.e., the first stage of cluster formation starting from metastable initial states) exhibits properties resembling spinodal decomposition (the size remains nearly constant while the composition changes) although the presence of an activation barrier distinguishes the nucleation process from true spinodal decomposition. In addition, it is shown that phase formation both in metastable and unstable initial states near the classical spinodal may proceed via a passage of a ridge of the thermodynamic potential with a finite work of the activation barrier even though (for unstable initial states) the value of the work of critical cluster formation (corresponding to the saddle point of the thermodynamic potential) is zero. This way, it turns out that nucleation concepts-in a modified form as compared with the classical picture-may govern also phase formation processes starting from unstable initial states. In contrast to the classical Gibbs' approach, the generalized Gibbs' method provides a description of phase changes both in binodal and spinodal regions of the phase diagram and confirms the point of view assuming a continuity of the basic features of the phase transformation kinetics in the vicinity of the classical spinodal curve.

The processes of structure and phase formation during the formation of combined physical deposition (PVD)/chemical deposition (CVD) coatings on a hard-alloy tool are investigated. It is shown that the formation of the Cr PVD barrier layer prevents the formation of the η phase with the subsequent deposition of the TiC-Ti(C,N)-TiN CVD coating. Treatment of the CVD coating with the ion-plasma flow and deposition of the finishing TiN PVD layer lead to the formation of the texture {111} in the coating and the diminishment of its subgrains to ∼30 nm. Upon going from articles with CVD coatings to combined PVD-CVD-PVD coatings, the strength and hardness of hard-alloy blades increase. This is explained by the effect that the corresponding phase and structural transformations that occur in the coating and at the phase interfaces during their formation have on these characteristics. The results of certification tests of cutter tools for continuous and intermittent cutting indicate the efficiency of these coatings both for turning work and, especially, for mill operation, when high toughness, along with hardness, is the most important characteristic.

The subject of the research is features of synthesis of alumina cements on the basis of compositions of system CaO – CoO – Al2O3 – MoO3 with use of cobalt‐molybdenum‐containing waste for reception of refractory materials with a complex of the set properties. The processes of phase formation in raw mixtures have been investigated. X‐ray phase studies of clinkers fired at different temperatures and holding times proved that as a result of the interaction of the raw materials of the mixture in the material synthesizes a mixture of hydraulically active calcium mono‐ and di‐aluminate and refractory cobalt spinel, which provides the resulting complex high strength, accelerated curing time, fire resistance. The absence of phases corresponding to molybdenum compounds is explained by the fact that they are part of hydraulically active aluminates as limited solid solutions, deforming the crystal lattice and increasing hydraulic activity. Thus, it was found that the high strength of cement is due to the presence of calcium hydroaluminates such as C2AH8, aluminum hydroxide, as well as unhydrated grains of calcium aluminates, which will contribute to further long‐term strength. It is this coexistence of phases in both the crystalline and colloidal states provide high strength cement stone.

The thermodynamic and ab initio molecular dynamic approaches have been applied to analyze the phase formation processes in amorphous alloys of Fe–Zr system. The thermodynamic calculations of the kinetics of the phase formation process of binary amorphous alloys of the Fe–Zr system with continuous slow heating and pulsed laser processing are carried out. It was shown that the processes of phase formation in the binary alloys of the Fe–Zr system take place in two stages: initially, there are processes of phase stratification on two amorphous phases according to the type of spinodal decomposition, then crystallization of each amorphous phase. The calculations carried out within the framework of the modified theory of homogeneous nucleation for binary systems have shown that the process of crystallization of binary alloys of the Fe–Zr system with continuous slow heating is two-step: first, the solid solution Zr in α-Fe crystallizes, then the chemical compound Fe3Zr, with complete crystallization occurs in the temperature interval of 90 K. It was shown that the region of amorphization of binary alloys of the Fe–Zr system is within (3–15)% at. zirconium, which is well in agreement with experimental data. The theoretical confirmation of the possibility of a course of explosive crystallization during pulsed laser annealing in binary amorphous alloys of the Fe–Zr system at a temperature lower than 60 K for the temperature of the beginning of intensive crystallization at slow heating. First-principles molecular dynamics simulations of amorphization and crystallization process in the Fe–Zr system have been presented. The atomic positions in the Fe29Zr3 supercell were modeled by simulating annealing by the density functional theory in the generalized gradient approximation. Changes in the density of electronic states of the Fe29Zr3 supercell under liquid-amorphous-crystalline phase transitions are discussed. The most marked difference between the electronic spectrum of liquid and amorphous phases is a pseudogap at the Fermi level which is consistent with the Nagel–Tauc electronic criterion of the amorphous metallic alloy thermal stability. Further simulating annealing in the isothermal/isenthalpic ensemble under the higher temperatures leads to the drastical changes of the electronic spectrum and rearrangement of atoms, which we assign to the first stage of the amorphous alloy crystallization.KeywordsAmorphous alloyRelative integral free gibbs’ energyVolume part of crystalline phaseCrystallization processExplosive crystallizationPulsed laser annealingElectronic structureAb initio molecule dynamic

Corrosion Resistance of Materials Obtained By Electron-Beam Cladding of Fe – Cr – Ni – Ti Powder Mixtures

Mechanical alloying of nanocrystalline intermetallic compound TiFe doped by aluminum and chromium

Due to their outstanding dielectric and ferroelectric properties, barium titanate (BaTiO(3))-based ceramics have found many applications in electronic devices. To optimise the final quality of such ceramics, a detailed knowledge of the complex processes involved in the formation of BaTiO(3) is required. The phase formation process in ordered structures of the BaCO(3)/TiO(2) system was analysed by X-ray diffraction and by Raman spectral imaging (RSI) as a function of the annealing temperature. RSI was used for the first time as a locally resolving method for phase analysis, and proved to be a useful tool in examining the formation process of BaTiO(3) starting from spherical, core-shell structured precursors of the type TiO(2) core/BaCO(3) shell. The Raman spectra of different BaO-TiO(2) phases appearing as intermediate phases during the formation of BaTiO(3) were recorded for separately-prepared pure substances. Using these spectra as fingerprints, and choosing phase filters by setting wave number windows, "phase landscape pictures" of the samples at different temperatures during the genesis of BaTiO(3) could be created with a lateral resolution of up to 200 nm. These pictures confirm shell-like formation of the different barium titanate phases according to the diffusion of barium and oxygen ions from the Ba-rich shell into the TiO(2) core. At an intermediate state of the phase formation process, the phase sequence Ba(2)TiO(4), BaTiO(3), BaTi(2)O(5), BaTi(4)O(9) and BaTi(5)O(11) to TiO(2) was detected from the outer to the inner parts of the core-shell structures.

The potential use of germanide thin films as self‐aligned metallization in Ge‐based Complementary Metal‐Oxide Semiconductor (CMOS) technology has drawn interest in the solid‐state reactions of Ge and metal films [1‐3]. In order to integrate these germanide thin films as a contact material, a complete analysis of the solid‐state reactions of amorphous Ge (a‐Ge) and metals is required. The solid‐state reactions in the Co‐Ge thin films systems previously examined by in situ x‐ray diffracting annealing experiments only probed the crystal structures present in the film and did not identify the morphological variations of the process [1]. Furthermore, to the authors knowledge previous works Co‐Ge thin film systems have only analyzed systems with Ge in excess of Co. In this study, the phase formation and crystallization behavior of a‐Ge and Co thin film layers are investigated by ex situ transmission electron microscopy (TEM) coupled with in situ x‐ray diffraction (XRD) annealing experiments. Four different specimens were prepared, alternating the stacking order and film thickness (200 nm and 30 nm), in order to explore the influence of the free surface on the crystallization and the phase reactions with Co in excess of a‐Ge. Thin film layers of a‐Ge and Co were deposited via electron beam evaporation onto (100) silicon wafers with the native oxide film present on the surface (~1–2 nm). The thin film specimens were characterized by TEM before and after annealing during the phase formation process. Bright field TEM images of the 200 nm Co on top of 30 nm a‐Ge stack are shown in Figure 1: (a) as‐deposited, (b) annealed at 280°C and (c) 400°C. Upon heating to 280°C, the a‐Ge layer reacts with the Co layer to form CoGe, which then undergoes an additional phase transformation to CoGe 2 with continued heating to 400°C. The influence of the free surface ( fs ) on the crystallization process was examined on the 200 nm a‐Ge and 30 nm Co film stack. The scanning electron microscopy images of the specimens after annealing to 400°C are shown in Figure 2. (a) The specimen with a‐Ge at the free surface (a‐Ge fs ) shows two distinct contrast indicating the presence of two different crystalline phases near the surface, while (b) the specimen with Co at the free surface (Co fs ) shows a more homogeneous contrast throughout the surface. Both specimens have voids visible at the surface, which formed during annealing. By viewing the specimens in cross‐section, the presence of large voids is visible in (c) the specimen a‐Ge fs , but the voids are not present in (d) the specimen with Co fs . The resulting formation of voids in only one of two specimens indicates the free surface has a direct influence of the phase formation process.

Fire hazard of titanium powder layers mixed with inert nano TiO2 powder

The phase formation processes in the zone of directional laser irradiation of compacted Al2O3–TiO2–Y2O3 mixtures have been investigated. It has been established that the formed ceramic material (track) has a complex structure. Its surface layer contains α-Al2O3 as the main phase and Y2Ti2O7 inter layers. With increase in the distance from the surface of the track, the content of α-Al2O3 decreases, and the content of Y2Ti2O7 increases. The volume layers of the track contain Y2Ti2O7 as the main phase and inclusions of Y3Al5O12. The absence of Al2TiO5 or its small content is explained by the decomposition of aluminum titanate into Al2O3 and TiO2 under the conditions of rapid cooling of eutectic melt. Released titanium oxide takes part in the formation of the Y2Ti2O7 phase.

Previously formulated physical principles are used to develop a mathematical model of thermally induced processes of diffusion, phase formation, and stabilization of a spatially inhomogeneous structural-phase state in layered systems containing three isotopes of two elements. The program realization of the model allows quantitative characterization of the kinetics of these processes under arbitrary conditions of thermal annealing. The results of theoretical calculations and the experimental data obtained by Mossbauer spectroscopy and X-ray diffractometry of the layered structure consisting of 57Fe (0.07 μm)-Ti (1 μm)-Fe0.96Ti0.04 (13 μm)-57Fe(0.07 μm) are found to be in good agreement.