Phase Formation In Ti Research Articles

Omega Phase Formation in Ti-3wt.%Nb Alloy Induced by High-Pressure Torsion.

It is well known that severe plastic deformation not only leads to strong grain refinement and material strengthening but also can drive phase transformations. A study of the fundamentals of α → ω phase transformations induced by high-pressure torsion (HPT) in Ti–Nb-based alloys is presented in the current work. Before HPT, a Ti–3wt.%Nb alloy was annealed at two different temperatures in order to obtain the α-phase state with different amounts of niobium. X-ray diffraction analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied for the characterisation of phase transitions and evolution of the microstructure. A small amount of the β-phase was found in the initial states, which completely transformed into the ω-phase during the HPT process. During HPT, strong grain refinement in the α-phase took place, as did partial transformation of the α- into the ω-phase. Therefore, two kinds of ω-phase, each with different chemical composition, were obtained after HPT. The first one was formed from the β-phase, enriched in Nb, and the second one from the α-phase. It was also found that the transformation of the α-phase into the ω-phase depended on the Nb concentration in the α-Ti phase. The less Nb there was in the α-phase, the more of the α-phase was transformed into the ω-phase.

Phase Formation in Ti–Ni Binary System during Solid-State Synthesis

Much effort has been made to synthesize porous NiTi alloys using powder metallurgy techniques. However, the sintered products from elemental Ti and Ni powders often contain Ti2Ni and Ni3Ti in addition to the desired NiTi phase, and the thermal and mechanical properties of the sintered products are inferior comparing to cast or wrought NiTi alloys. This study investigated the solid-state diffusion reactions between elemental Ti and Ni powders during sintering to delineate the origin of the formation of these undesired intermetallic phases and proposed a physical model to explain the phase formation processes. The intermediate diffusion reaction products include Ti(Ni) and Ni(Ti) solid solutions, Ti2Ni, Ni3Ti and NiTi. The Ti2Ni and Ni3Ti intermetallic phases are the primary reaction products between Ti and Ni, and NiTi phase is formed only as a secondary reaction product from Ti2Ni and Ni3Ti. In addition, two NiTi phases of different Ni contents are formed from either Ti2Ni or Ni3Ti. These findings clarify some uncertainties and common misunderstanding on NiTi formation through elemental powder sintering and provide a guide for the design of powder metallurgy of Ti and Ni.

Metalorganic chemical vapor deposition of Ti–O–C–N thin films using TBOT as a promising precursor

Ti–O, Ti–O–C and Ti–O–C–N thin films have been synthesized successfully via metalorganic chemical vapor deposition (MOCVD) technique. Tetrabutyl orthotitanate (TBOT) is used as a precursor in presence of Ar, H 2, and N 2 as process gases. By controlling deposition temperature and type of process gases, it was possible to control the composition of the deposited films. The deposited films are composed mainly of Ti and O when H 2 is used as a process gas in the temperature range 350–500 °C. As the temperature increased up to 600 °C, thin films containing anatase (TiO 2) and titanium carbide (TiC) phases are deposited and confirmed by XRD and EDX analyses. As the temperature increased to 750 °C, a transformation from anatase to rutile phase (TiO 2) is started and clearly observed from XRD patterns. Titanium nitride (Ti 2N and TiN) phase in addition to TiO 2 and TiC phases are formed at 600–1000 °C in presence of nitrogen as a process gas. SEM images for all investigated film samples showed that the films are deposited mainly in the form of spherical particles ranged from few nano- to micrometer in size with some additional special features regardless the type of the process gas. Films containing carbon and nitrogen show higher hardness than that containing only oxygen. The obtained results may help in better understanding and controlling film composition and its phase formation in Ti–O–C–N system by MOCVD technique.

Mechanism and kinetics of the intermediary phase formation in Ti–Al and Ti–Al–Si systems during reactive sintering

Abstract In this work, chemical reactions and phase transformations during reactive sintering of Ti – Al and Ti – Al – Si materials were investigated by differential thermal analysis. Kinetics and mechanism of the formation of intermediary phases are described on an experimental model system consisting of a bulk titanium sample exposed to molten aluminium or AlSi50 alloy. The results show that the reactive sintering is a reaction-controlled process in both systems. The reaction rate increases significantly with addition of silicon. According to the kinetics study, reactive sintering of TiAl36 and TiAl15Si15 alloys was carried out. TiAl15Si15 alloy was successfully sintered, while TiAl36 material showed extremely high porosity and presence of unreacted components.

Metastable phase formation in Ti–Al–Nb undercooled melts

Metastable phase formation processes in ternary Ti–Al–Nb alloys were studied by containerless electromagnetic levitation for melt undercooling up to 300 K below the liquidus temperature. Dendrite growth velocities of 15–25 m s −1 for highly undercooled Ti–Al–Nb melts were consistent with primary β-phase formation, which is promoted by Nb addition. From double-recalescence events in Ti 40Al 50Nb 10 and Ti 45Al 50Nb 5 melts beyond a critical undercooling a subsequent β to α phase transformation in the semi-solid state was inferred. A second recalescence near 1300 °C, which was attributed to an α to γ solid state transformation, was observed in the pyrometer trace for the Ti 45Al 50Nb 5 and Ti 40Al 50Nb 10 alloys. The γ phase formation was suppressed in favour of a homogeneous α 2 phase in undercooled Ti 45Al 45Nb 10 samples quenched onto a chill substrate, whereas in Ti 40Al 50Nb 10 high undercooling enabled a direct γ phase solidification.

Phase formation in Ti after high fluence/high temperature nitrogen implantation

Nitrogen ion implantation in titanium using plasma immersion ion implantation (PIII) is investigated at a pulse voltage of 15keV and elevated temperatures between 300 and 750°C. Starting the implantation at room temperature leads to the parallel formation of TiN and Ti2N, as observed by X-ray diffraction (XRD). In contrast, additionally external heating with a start of the ion implantation at 350°C results in the dominance of Ti2N. The nitrogen surface concentration is in all cases at 30–35 at.%. Different activation energies and kinetics for the processes (1) Ti →ε-Ti2N, (2) Ti →δ-TiN and (3) ε-Ti2N→δ-TiN, with the activation energy increasing from (1) to (2) and (3), are postulated. Control experiments with plasma nitriding can be interpreted as an indication of additional radiation effects.

Mechanico-Chemical Synthesis, Structure and Properties of Alloys Based on the System Ti ― Al

The kinetics of structure and phase formation in Ti ― Al and Ti ― Al ― Sc alloy powders in the process of mechano-chemical synthesis from elemental powders was investigated. It was established that the formation of microdimensional x-ray amorphous composite particles with nanodimensional structural components occurs. It was shown that in consolidation of the mechanically alloyed powders by hot isostatic pressing structural heredity is realized, and that the materials produced have unusually high microhardness compared to that of similar cast alloys. This is characteristic of nanostructured materials. In addition to high hardness the materials are characterized by high work of plastic deformation at room temperature. Microalloying the alloy powders with scandium has a complex positive effect on the strength properties of the hot pressed materials, due to the formation of a dispersion hardened microstructure with purified nanodimensional grains.

Prediction of ω phase formation in Ti–Al–X alloys

The ω phase can form in TiAl based alloys from a precursor high temperature bcc type phase of A2 or B2 structure. The different structures of ω and the stability of the ω phase are discussed with reference to the role of Al and transition metal alloying additions to Ti. Experimental data on ω phase stability for TiAl–X based alloys follows the same trend as for Ti–TM alloys in a general plot of the crystallographic parameter, Δ , versus the free electron per atom per volume of unit cell, φ, of the beta phase of the A2 or B2 structure. It is unlikely that the formation of 3D ω phase could be avoided in TiAl–X alloys when φ<0.135.

Phase Formation in Ti (Ta)-Ni and Co-Ti Films Deposited on (001)Si in N2 Atmospheres.

ABSTRACTA series of electron microscopy investigations on the microstructure produced by surface-diffusion reactions between Si substrates heated to 700–800°C and Ti-Co, Ti-Ni or Ta-Ni alloys deposited in a nitrogen ambient with and without barrier layers are described. The TEM data show clear phase separation into TaSi2 and NiSi2 for the Ta-Ni film deposited in a high N2 pressure ambient. Deposition at lower N2 pressure led to the formation of a mixed Ni-Ta-Si layer. The phase separation effect was also absent for Ni-Ti films even at high N2 pressure. The presence of barrier layers strongly affected the surface diffusion reactions in the Co-Ti-Si system. Formation of Ti-(O) or CoSix amorphous layers at the Si surface prevented the interdiffusion of Si and Co, such that even pure Co or Co2Si layers could be formed.

Phase Formation during the Surface-Diffusion Growth of Ti–Co–Si–N and Ti–Co–N Thin Films on Si and SiO2

The kinetics of phase formation in Ti–Co–Si–N and Ti–Co–N thin films on Si and SiO2is investigated experimentally. With the deposition on Si, rapid thermal annealing (T∼ 900°C) is shown to cause phase separation that ends in a TiN/CoSi2/Si structure. If SiO2is used, the alloy reacts with the substrate to produce compounds that are difficult to remove with selective etchants. This limits the potential uses of this process in the fabrication of contact systems for CMOS devices. It is shown that structure- and phase-dissimilar films can be formed on Si and SiO2by means of the surface-diffusion reactions between a Ti–Co–Si–N or Ti–Co–N alloy and the substrate at 650–700°C. The effect of a TiN, Ti, or CoSi2thin layer at the alloy–substrate interface on the phase separation is investigated.

Thermodynamic conditions of phase formation in Ti3Al based alloys

In the titanium enriched part of Ti-Al diagram there were determined the temperatures of the beginning and of the end of phase transformation with the participation of liquid and solid β, α and α2 -phases. Nonmonotonous character of fusible diagram and of the curves of ordering α→α2 was established. The position of maximum at fusible diagram is determined, the most optimum for single crystal production there are the alloys, in which the composition corresponds to the maximums at the curves of L[math]β and α[math]α2 phase transformations.