Liquid Ti Al Alloys Research Articles

Effect of the third element on the wetting of alumina by TiAl-X (X = Fe, Ni) alloys at 1500 °C

In this work we show how the addition of a third element, namely Fe or Ni, influences the interfacial behaviour of liquid TiAl alloys on polycrystalline alumina. Wetting of these alloys was studied by the sessile drop method at 1500 °C and the interfacial microstructures were characterised to determine the relation between the contact angle and the interfacial reaction with the aid of the relevant phase diagrams. The equiatomic TiAl intermetallic wetted the alumina substrate forming a contact angle of 60°. On the other hand, increasing concentrations of Fe and Ni were found to improve the wetting behaviour of ternary TiAl-X alloys: for third-element atomic concentrations of 5 at. %, contact angles decreased to 32° and 29° by adding Fe and Ni, respectively. Neither reactive-formed interlayers nor newly formed phases were observed for any binary or ternary composition as confirmed by thermodynamic calculations. A slight dissolution of the ceramic occurred during liquid/solid contact and became more evident with the increase of concentration of the third element. The introduction of oxygen into the melt provided by the dissolution of alumina improved the wetting behaviour.

Thermodynamic Evaluation of the Surface Tension and Viscosity of Liquid Quaternary Alloys: The Ti-Al-Cr-Nb System

Surface tension and viscosity of complex Ti-based industrial alloys are important for simulation of liquid assisted industrial processes such as casting, joining, crystal growth and infiltration. Modelling of the interface and mass transport during liquid-solid phase transition requires reliable surface tension and viscosity data. Therefore, to obtain accurate predictions of microstructural evolution during solidification related processes, only reliable input data are necessary. In the case of liquid Ti-Al alloys, the experimental difficulties related to high temperature measurements and reactivity of these alloys with supporting materials or containers as well as inevitable presence of oxygen may lead to data gaps including a complete lack of property data. An alternative for container-based methods are containerless processing techniques that offer a significant accuracy improvement and / or make possible to measure temperature and composition dependent thermophysical properties of metallic melts, as in the case of the Ti-Al-Cr-Nb system. Advanced mathematical models and computer simulations, developed in several theoretical frameworks, can be used to compensate the missing data; on the other side, for the validation of theoretical models, the experimental data are used. In the present work, an evaluation of the surface tension and viscosity of liquid Ti-Al-Cr-Nb alloys by means of the predictive models and a comparison to the available experimental data were done. The proposed methodology is a tool to assess the reliability of thermophysical properties data of multicomponent alloy systems.

Accurate interatomic potential for the nucleation in liquid Ti-Al binary alloy developed by deep neural network learning method

For the traditional empirical potentials, the accuracy of chemical bonding in a wide temperature range is regrettable, and thereby they are not an appropriate choice to explore the nucleation process of crystals. As a developed superalloy in aerospace, the liquid structure and nucleation mechanism of Ti-Al alloy desire to be investigated to comprehend the solidification process deeply. This work develops an accurate deep neural network (DNN) potential for Ti-Al binary system by employing a machine-learning method. Then, the local structure and the nucleation process of liquid Ti-48 %Al and Ti-52 %Al (atomic percentage) alloys from the normal to the undercooled state are performed by molecular dynamics simulation with the DNN potential. It is found that the formation of the locally ordered structures depends on the Ti-Al atom pair during the solidification. The Honeycutt-Andersen indices of 1431,1541 and 1551 are dominant, and the liquid structure is mainly characterized by perfect and distorted icosahedral clusters. By Voronoi Polyhedron (VP) analysis, Ti-centered VPs favor the formation of body-centered-cubic (BCC) structure, while Al-centered VPs have the main contribution to the icosahedral structure. Besides, liquid Ti-52 %Al alloy prefers to form face-centered-cube (FCC) nuclei than BCC nuclei in a high undercooling, which is closely related to the ability to capture Ti-Al bonds. The favor of the nucleation of the γ phase causes the nuclei with non-equilibrium composition, comprised of Ti-rich BCC and Al-rich FCC coordinated atoms, respectively. Novelty, the FCC nuclei in liquid Ti-Al alloys are independently generated without the precursor of BCC coordinated atoms. High undercooling can transform the nucleation mechanism from two steps to one step.

Surface Tension of Liquid Ti-Al Alloys

The surface tension of liquid Ti-Al alloys at 1758 K was determined by an improved sessile drop method. The surface tension of liquid Ti-Al alloys was calculated using the Butler's model and several modified ideal solution models. Results show that the surface tension of liquid Ti-Al alloy decreases with increased Al concentration. Experimental results agree well with both the Butler's model and the modified ideal solution model. The segregation of Al atoms to the surface occurs at all bulk concentrations of Ti-Al alloys. Al with lower surface tension prefers to segregate on the surface of liquid Ti-Al alloy, whereas Ti with higher surface tension prefers to segregate inside the molten alloy. The effect of surface active solute on the surface tension of liquid Ti-Al alloy was also discussed. Sulfur is found to have a larger effect on the surface tension of alloy. The surface tension of liquid Ti-Al alloy decreases with increased sulfur concentration. Sulfur concentration on the surface is higher than that in the bulk. Thus, the adsorption of sulfur for Ti-Al alloys is a positive adsorption.

Molecular dynamic simulation of nanocrystal formation and tensile deformation of TiAl alloy

The formation of BCC structure in the melt was the key in TiAl crystalline nucleation, and liquid TiAl alloy completely crystallized at the quenching rate of 0.02 K ps−1.

Substantial undercooling and rapid dendrite growth of liquid Ti-Al alloy

It is highly desirable to undercool titanium based alloy melts and modulate their dendritic solidification process due to the relevant applications in aerospace engineering. But the serious chemical reactivities of this category of alloys result in potent heterogeneous nucleation and suppress remarkable undercoolings in the course of normal material processing. This paper shows that such a challenge can be solved by containerless processing approach. Liquid Ti-25 wt.%Al alloy is highly undercooled and rapidly solidified under containerless state by both electromagnetic levitation and drop tube techniques. Its metastable undecoolability, crystal nucleation mechanism and dendrite growth process are examined experimentally and analyzed theoretically. Those heterogeneous nuclei with wetting angles above 60 are found to be quite difficult to eliminate even during levitation processing, thus reducing the undercoolability of this alloy. The maximum undercooling of bulk alloy melt reaches 210 K (0.11 TL). The thermodynamic driving force to initiate the nucleation of -Ti phase increases almost linearly with the enhancement of undercooling. The phase dendrite displays a growth velocity up to 11.2 m/s, indicating that the rapid solidification is realized at the relatively slow cooling rate of levitated alloy melt. With the increase of undercooling, phase dendrite experiences a kinetic transition from solute diffusion controlled to thermal diffusion controlled growth. Once undercooling exceeds 100 K, the nonequilibrium solute trapping effect brings about the practically desirable segregationless solidification. Nevertheless, the single condition of substantial undercooling is insufficient to suppress the solid state transformation of phase. It is decomposed into 2-Ti3Al phase plus a small amount of -TiAl compound after containerless solidification at levitated state. A more efficient approach to controlling and modulating the solidification microstructures is to utilize the coupled effects of high undercooling and rapid quenching, which proves to be feasible through the rapid solidification of alloy droplets inside drop tube. For those alloy droplets with diameters ranging from 77 to 1048 m, their cooling rates attain a maximum of 1.05105 K/s, and the predicted maximum undercooling is 227-778 K. In this case, phase dendrites are well refined and kept in a metastable state until ambient temperature. The heat transfer calculations indicate that the thermal radiation is the dominant cooling mechanism for the large alloy droplets above 690 m, whereas thermal convection becomes the major cooling mechanism for the small alloy droplets below 690 m. The microgravity condition during free falling does not show apparent effect on the microstructural formation of these alloy droplets.

Interfacial interactions between liquid Ti–Al alloys and TiB2 ceramic

High-temperature interactions of liquid Ti and Ti74.3Al25.7, Ti49.3Al50.7, Ti26.5Al73.5 and Ti3.5Al96.5 (at.%) alloys with TiB2 ceramic are studied using the sessile drop technique. Molten Ti and Ti–Al alloys show a very good wetting of TiB2. A homogeneous TiB layer with a thickness of about 20 μm forms at the Ti/TiB2 interface. An approximately 200-μm-thick layer of densely packed TiB whiskers is observed at the interface of a Ti74.3Al25.7/TiB2 couple. The Ti49.3Al50.7 liquid penetrates the grain boundaries in the TiB2 ceramic and disconnects the grains in the near-interface region. The Ti26.5Al73.5 melt does not disconnect the TiB2 grains, but penetrates the grain boundaries over a distance of about 300 μm from the interface. Ti3.5Al96.5 just fills the pores in the ceramic up to a distance of about 120 μm from the interface.

Effect of Al on the Wetting Behavior Between TiC x and Molten Ti-Al Alloys

The wetting behavior and the interfacial reactions between TiC x substrate and molten Ti-Al alloys with different Al contents were studied using the Sessile Drop method at 1758 K (1485 °C) in argon atmosphere. It is found that the wettability and interface reaction products depend on Al content in the molten alloy. The initial contact angles between the molten Ti-Al alloy and TiC0.78 surface reduces from 110 to 80 deg when Al content in the alloy changes from 40 to 80 wt pct. The reduction in the initial contact angle is due to the decrease of surface tension of the molten Ti-Al alloys with increasing Al contents. The segregation of Al atoms to the surface occurred at all bulk concentrations of Ti-Al alloys. Al with lower surface tension tends to segregate on the surface of liquid Ti-Al alloy. In the spreading stage, the interfacial reaction led to the decrease in the contact angle. The adhesion in Ti-Al/TiC x system can be interpreted in terms of strong chemical interactions, which is greatly affected by the diffusion of C. The equilibrium contact angle was measured less than 10 deg. Finally, the reaction sequence at the Ti-Al melt and TiC x substrate interface is proposed.

Glass formation and icosahedral medium-range order in liquid Ti–Al alloys

In this paper, molecular dynamics simulations are applied to explain the glass formation processes of Ti–Al alloys in structural perspective and to reveal the microstructural evolution of Ti–Al alloys during rapid solidifications in icosahedral medium-range order. The icosahedral medium-range order structural evolutions are described in detail by icosahedral connectivity parameter and coordination number during the rapid solidifications. The results reveal that the icosahedral medium-range order is an appropriate structural unit, which aptly describes glass formation processes of liquid and amorphous Ti–Al alloys in structural aspect. Furthermore, hexagonal connected icosahedral clusters, which link via volume-sharing, have good structural stability and configural continuity, and act as the junction closely connects nearby extended icosahedral clusters. At the same time, the number of the hexagonal formation after the rapid solidification could properly reflect the glass forming abilities of the Ti–Al alloys with different compositions.

Growth of icosahedral medium-range order in liquid TiAl alloy during rapid solidification

The formation and evolution of icosahedral medium-range order structures of TiAl alloy during the rapid solidification are investigated based on molecular dynamics simulations. The icosahedral medium-range order structural evolutions are described in detail by coordination number and icosahedral connectivity parameters. Simultaneously, the evolution of the medium-range order icosahedral clusters is analyzed by nano-clusters tracing method. The results reveal that the full growths of icosahedral medium-range order structures improve the glass forming ability and stability of metallic glass. Meanwhile, the hexagonal connected icosahedral clusters which link via volume-sharing have excellent structural stability, and the dendritic icosahedral clusters based on the hexagonal formation keep continuous growth below the Tg. The junctions which consist of the hexagonal formations enhance the compactness of icosahedral medium-range order structure. Moreover, the icosahedral medium-range order is an appropriate structural characterization which accurately defines the characteristics of amorphous TiAl alloy during the rapid solidification.

Evolution of icosahedral clusters during the rapid solidification of liquid TiAl alloy

The evolution characteristics of icosahedral clusters during the rapid solidification of TiAl alloy under different cooling rates are investigated based on molecular dynamics simulations. The short-range order structural properties of liquid and amorphous TiAl alloy are analyzed by several structural characterization methods. It is found that the cooling rate plays a key role during the evolution of icosahedral clusters and has significant effect on the glass transition temperature. Simultaneously, the medium-range order structural evolutions are described in detail by quantitative method and visualization technology during the rapid solidification. The results reveal that the medium-range order icosahedral clusters have good structural stability and configural continuity during the rapid cooling process. Furthermore, the icosahedral structures have significant improvements with decreasing cooling rate. The structures block the crystal nucleation and improve the glass forming ability of supercooled liquid.

Factors Affecting the Separation of Ti-Al Alloy in the TiPro Process

Abstract This paper presents and discusses the factors influencing the yield of Ti-Al alloy in the TiPro process which is a process developed at the University of Waikato for producing titanium alloy powders by mechanically activating Al/TiO2 powder mixtures and subsequently preheating the resultant composite powder in order to ignite a combustion synthesis reaction and separate the liquid Ti-Al alloy by extrusion. In this study, TiO2/Al composite powders with different powder particle microstructures have been produced and used to study the effects of starting composite powder particle microstructure on the solid/liquid separation of TiAl from solid Al2O3 by extrusion. Results obtained so far indicate that maximizing the time the Ti-Al alloy phase is maintained in the liquid state after the reaction between TiO2 and Al is one of the critical factors to increase the yield of Ti-Al alloy produced through the separation of liquid Ti-Al from the solid Al2O3 phase by extruding the mixture of liquid Ti-Al and Al2O3 formed through reactions and heating.

Determining thermophysical properties of undercooled liquid Ti–Al alloy by electromagnetic levitation

Owing to the high liquidus temperature and reactivity, it is usually difficult to undercool liquid Ti–Al alloys and measure their thermophysical properties. Here, the surface tension of liquid Ti80Al20 alloy is determined in the temperature range of 1706–2273K by using oscillating drop method under electromagnetic levitation condition, and a maximum undercooling of 257K (0.13 TL) is achieved. The measured surface tension of Ti80Al20 alloy exhibits a linear dependence on temperature. Furthermore, based on the measured surface tension, the viscosity is also derived.

Specific heat measurement of stable and metastable liquid Ti–Al alloys

The specific heats of liquid Ti-20at.%Al and Ti- 51at.%Al alloys are determined to be 33.01 ± 2.75 and 31.27 ± 2.91 J mol −1 K −1 in the stable superheated and metastable undercooled states by using an electromagnetic levitation drop calorimeter. The experimental temperature ranges are 1733-2133 K and 1511-1948 K, and maximum undercoolings of 230 (0.12 TL) and 242 K (0.14 TL) are achieved, respectively. On the basis of the experimental re- sults, the specific heat dependence on the composition is ob- tained for binary Ti-Al alloys.

Surface tension of substantially undercooled liquid Ti–Al alloy

It is usually difficult to undercool Ti–Al alloys on account of their high reactivity in the liquid state. This results in a serious scarcity of information on their thermophysical properties in the metastable state. Here, we report on the surface tension of a liquid Ti–Al alloy under high undercooling condition. By using the electromagnetic levitation technique, a maximum undercooling of 324 K (0.19 T L) was achieved for liquid Ti-51 at.% Al alloy. The surface tension of this alloy, which was determined over a broad temperature range 1429–2040 K, increases linearly with the enhancement of undercooling. The experimental value of the surface tension at the liquidus temperature of 1753 K is 1.094 N m−1 and its temperature coefficient is −1.422 × 10−4 N m−1 K−1. The viscosity, solute diffusion coefficient and Marangoni number of this liquid Ti–Al alloy are also derived from the measured surface tension.

A Molecular Dynamics Study for the Thermophysical Properties of Liquid Ti–Al Alloys

Molecular dynamics simulations (MDS) employing an embedded-atom-method (EAM) are applied to calculate the density and specific heat of liquid Ti–Al alloys at temperatures above and below the melting temperature in a wide composition range. Both the temperature and composition dependences of these two properties are investigated. The excess volume of Ti–Al alloys is calculated from the predicted density, and shows a negative value. The specific heat of liquid Ti–Al alloys increases linearly with a decrease of temperature. Unlike the monotonic change of density with the addition of aluminum, the specific heat reaches its maximum value at the composition of Ti–50at%Al alloy. Thus, both the density and specific heat show highly nonideal behaviors, indicating that Neumann–Kopp’s rule does not apply.

Thermodynamic properties of highly undercooled liquid TiAl alloy

The specific heat of an undercooled liquid TiAl alloy was measured by an electromagnetic levitation drop calorimeter and the related thermodynamic properties, difference in Gibbs free energy and change in entropy are calculated based on the experimental results and some well known models. Under the containerless processing condition, the liquid alloy was undercooled by up to 219 K (0.122TL). The change in enthalpy shows a linear relationship with the temperature and a nearly constant specific heat was determined even though this alloy was undercooled far below its liquidus temperature. The calculated results of the thermodynamic properties reveal that even though semiquantitative models fit with the experimental curve of the difference in Gibbs free energy well, they lead to different degrees of deviation in the entropy difference calculation.

Production and characterization of Ti:sapphire thin films grown by reactive laser ablation with elemental precursors

Crystalline Ti:sapphire (Ti:Al(2)O(3)) thin films were grown at low temperatures upon Al(2)O(3) (0001) substrates by reactive crossed-beam laser ablation at 248 nm by use of a liquid Ti-Al alloy target and O(2) . The films were investigated ex situ by x-ray diffraction, x-ray photoelectron spectroscopy, and Rutherford backscattering spectrometry. Low-temperature luminescence was identical to that for Ti(3+) ions in bulk samples of Al(2)O(3) .