Ti Si Alloys Research Articles

Hot corrosion behavior of silicide coating on an Nb–Ti–Si based ultrahigh temperature alloy

Hot corrosion behavior of an Nb–Ti–Si alloy and its silicide coating in Na2SO4–25wt.% NaCl molten salts at 900°C was investigated. For the bare alloy, a thick and porous corroded scale (Na-containing oxides) was formed after hot corrosion. The silicide coating showed a good oxidation resistance at 900°C, while the presence of the molten salts aggravated the damage process of the silicide coating by forming a thick corroded scale which mainly consisted of a Na2TiSi2O7 outer layer and a porous and cracked NaNbO3+Na2TiSi2O7 inner layer. The possible hot corrosion mechanism of the silicide coating has been explored.

Designing primary phase-embedded Cu-based bulk metallic glass composites: a computational thermodynamic approach

In this paper, a computational thermodynamic approach was used to design the compositions of bulk metallic glass composites. This approach indicates that both the liquidus temperature and the solidification temperature range of the primary phase of a Cu–Zr–Ti–Si alloy system increase with the addition of silicon. With the aid of the calculation, the crystal-embedded Cu-based bulk metallic glass composites have been fabricated successfully. The as-cast composites were characterized by X-ray diffraction and energy dispersive spectroscopy. The characterizations demonstrate that the increase of in the liquidus temperature and the extension of the primary phase region facilitate the formation of primary phase-embedded composite structures. This work suggests that the computational thermodynamics approach can be serve as a potential tool to construct bulk metallic glass composites, especially to fabricate the primary phases-embedded bulk metallic glass composites. The findings of this work can be utilized to understand the formation of composite structures of bulk metallic glass composites.

Ab initio-based prediction and TEM study of silicide precipitation in titanium

The literature contains many contradictory data about the most thermodynamically favorable structure of precipitates in α-Ti matrix of the hypoeutectoid Ti–Si alloys. In this work we applied our recently developed thermodynamic model to predict the structure of Ti–Si precipitates in α-Ti matrix of the Ti–Si alloy with total Si concentration of 0.7wt.%. We considered all prominent Ti–Si phases such as Ti3Si,Ti5Si3,Ti5Si4,TiSi and two TiSi2 phases and discovered that formation of the Ti5Si3 phase is more favorable than that of the Ti3Si in contradiction with the known phase diagrams. Theoretical result was confirmed by experimental investigation of microstructure and phase composition of the model Ti–0.7Si alloy annealed at 873K for 10h. Indeed, the only observed phase has hexagonal Ti5Si3 structure. To ensure the completeness of the results we calculated ab initio elastic constants for all considered Ti–Si phases.

Compositions and morphologies of TiAlSi intermetallics in different diffusion couples

Two kinds of diffusion couples were designed to investigate the formation of ternary TiAlSi phases in Al–Si–Ti alloys. It was found that different diffusion processes result in various compositions and morphologies of TiAlSi intermetallics. The melted Al, Si and Ti atoms in the diffusion couple leads to the formation of flake-like TiAlSi phase through liquid–liquid reaction. Besides, unidirectional diffusion of Al and Si atoms into blocky TiAl3 particles or Ti powders via a liquid–solid diffusion process also results in the formation of TiAlSi, while keeping the block-like morphology. This kind of diffusion is a gradual process, driven by the concentration gradient. The reactions in the diffusion couples are helpful to understand the compositional and morphological evolutions of TiAlSi as reported in previous work.

Top-seeded solution growth of three-inch-diameter 4H-SiC using convection control technique

The top-seeded solution growth of 4H-SiC at three inches in diameter has been investigated using Si–Ti alloy as a solvent. A perforated graphite disk called “immersion guide” (IG) was positioned in the solution in order to control solution flow. The morphological instability was improved and growth rate was significantly increased using the IG compared with conventional growth without the IG. Numerical fluid flow analysis with coupled heat and mass transportation was performed as well to understand convection and growth behavior. The origins of these improvements in the growth performance are discussed based on the numerical results. By controlling solution flow, we could successfully grow a three-inch-diameter 4H-SiC with 4-mm thickness.

Structure and mechanical properties of as-cast Ti–Si alloys

In the present work, the microstructure and mechanical properties of as-cast Ti–Si alloys with a Si content ranging from 1 to 12.5 wt% prepared using a dental cast machine were investigated and compared with commercially pure titanium (c.p. Ti). X-ray diffraction (XRD) for phase analysis was conducted using a diffractometer. Three-point bending tests were performed to evaluate the mechanical properties of all specimens and their microstructure and fractured surfaces were observed using scanning electron microscopy (SEM). Experimental results indicated that the diffraction peaks of the Ti–Si alloys matched those of α-Ti and Ti5Si3. All the Ti–Si alloys had higher bending strengths and bending moduli than those of c.p. Ti. For example, the bending strength of Ti–5Si was about 2.6 times that of c.p. Ti, and both Ti–10Si and Ti–12.5Si had the highest bending moduli, which were about 1.8 times higher than that of c.p. Ti. Additionally, Ti–1Si exhibited ductile properties and Ti–3Si and Ti–5Si had a combination of brittleness and ductility. When the Si content was 7.5 wt% or greater, the alloys showed brittle properties. Judging from the results of the mechanical properties and deformation behavior, Ti–1Si, Ti–3Si, and Ti–5Si can be considered highly feasible alloys for prosthetic dental applications if other properties necessary for dental casting are obtained.

Anodized Ti-Si Alloy as Gate Oxide of Electrochemically-Fabricated Organic Field-Effect Transistors

Anodized Ti-Si Alloy as Gate Oxide of Electrochemically-Fabricated Organic Field-Effect Transistors

Porous Ti-Si Alloys for Implants

Porous alloys are very perspective materials for medical implants, particularly for surgical and dental applications. The reason - besides their biocompatibility - is their density. This is why the implants and bone replacements are lighter and more similar to a human bone in its structure and mechanical properties. Another advantage is good osseointegration, i.e. tissue growing through pores in the material, this makes the body accept the implant better and there is also no risk of rejection. New Ti-Si biomaterials were prepared by powder metallurgy using reactive sintering, during which the desired porous structure of the material is formed. In this experiment the observed subject was the microstructure of Ti-Si alloys, properties determined were porosity and yield strength in compression.

The role of Nd on the microstructural evolution and compressive behavior of Ti–Si alloys

The rare earth (RE) added Ti–Si alloys are potential candidates for lightweight structural materials, especially in high-temperature fields, but up to now related fundamental information is still rare. In this work, samples of Ti–8Si–5Nd, Ti–13Si–5Nd, and Ti–20Si–5Nd are designed to investigate the effect of RE element Nd on the microstructural evolution and compressive properties of the Ti–Si alloys. Experimental results indicate that all the samples are composed of α-Ti, β-Ti, Nd and Ti5Si3 phases according to the XRD patterns. The Ti–8Si–5Nd alloy shows a typical hypoeutectic structure with primarily precipitated bulky Ti5Si3 and Nd. Ti–13Si–5Nd alloy has lamellar eutectic structure and the Ti–20Si–5Nd alloy exhibits a coarsened homogeneous primary Nd, Ti5Si3 precipitation and microcrystalline eutectic structure. The added Nd and primarily-precipitated Nd5Si3 particles work as inoculants and refine the grain size of Ti5Si3, because of their similar hexagonal structure, atomic bond and atomic distribution of the outer crystal surface. The refined grain size and homogeneous distribution of Ti5Si3 phases in the Nd added Ti–Si alloys (especially the hypoeutectic alloy) are effective to improve the mechanical properties. The Ti–8Si–5Nd alloy exhibits the best plasticity (14.545%) and compressive strength (1191MPa) among the three samples and shows good potential for structural materials.

In situ lift-out dedicated techniques using FIB–SEM system for TEM specimen preparation

The recent emergence of the focused ion-beam (FIB) microscope as a dedicated specimen preparation tool for transmission electron microscopy (TEM) has extended the reach of TEM to a wider variety of problems in materials science. This paper highlights three examples of using FIB–SEM lift-out techniques for preparing site-specific and crystallographic orientation-specific thin-foil specimens. An in situ lift-out technique used to extract thin foils from across a local grain boundary in bulk Al alloy and from individual fine Al atomised powder particles (down to 20μm in diameter) was performed with real-time secondary electron imaging within the chamber of a FIB–SEM system. In conjunction with electron backscatter diffraction (EBSD), the FIB is used for extracting TEM foil with a specific crystallographic orientation aligned normal to the broad plane of the foil. The above technique has been demonstrated using a dual-phase Ti–Si alloy for the exploration of orientation relationship between constituent phases. Furthermore, it is suggested that FIB is more applicable for preparing thin foils from hydrogen-sensitive metals (such as titanium alloys) than conventional thinning techniques, which tend to induce ambiguous artifacts in these foils.

Liquid–solid phase equilibria in Nb-rich corner of the Nb–Ti–Si–B system

Nb–Ti–Si based alloys are considered as candidates of next-generation high temperature materials (i.e., working temperature >1200°C). Boron has a beneficial effect in enhancing the oxidation resistance and reducing the anisotropy ratio ac/aa of the T2 phase in the Nb–Si alloys. The liquid–solid multiphase equilibria in the Nb–Ti–Si alloys with B additions have been investigated via an approach of integrating thermodynamic modeling with designed experiments. The present study suggests that Ti and Si additions increase the stability of Nb3B2, and the primary region of Nb3B2 will appear in the Nb–Ti–Si–B quaternary, while there is no primary solidification region of Nb3B2 in the constitute binaries and ternaries. The proposed liquidus surface of the Nb-rich Nb–Ti–Si–B system is associated with eight primary solidification regions of Nbss, T2, T1, D88, (Nb,Ti)3Si, NbB, TiB and Nb3B2. The direct eutectic solidification occurs between Nbss and T2 in the Nb-rich region in the Nb–Ti–Si–B liquidus projection, which could provide new opportunities for alloy design based on the Nb–Ti–Si–B system.

Investigation of crack patterns and cyclic performance of Ti–Si nanocomposite thin film anodes for lithium ion batteries

Three different Ti–Si nanocomposite thin films are prepared by the co-sputtering method with Si target and Ti target. The film thickness ranges between 250 and 480 nm. X-ray diffraction (XRD), high resolution transmission electron miscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) reveal significant charge transfer between Ti and Si and uniform dispersion of Ti–Si alloy nanograins in the amorphous Si thin films. It is found that decreasing the grain size of the Ti x Si y alloy below 2 nm, can improve the cyclic performance over pure Si thin film electrodes and those containing larger Ti–Si grains. This is mainly related to the improved mechanical properties that result from dispersing small grains of Ti–Si. The throughout thin film cracks formed after 10 electrochemical cycles are finer and more curved compared to the other thin films with the same film thickness. Furthermore, the width of the cracks, as well as, the area and junction angles of the remained fractured particle size of all electrode films considered, is compared and analyzed, after 10 cycles.

Thermodynamic aspects of grain growth restriction in multicomponent alloy solidification

The growth restriction factor Q is the key quantity in current descriptions of the solutal effect on grain growth and grain refinement during solidification of alloys. A rigorous treatment for the evaluation of Q in multicomponent alloys based on consistent thermodynamic descriptions of the alloy phase equilibria is presented. On closer inspection the conventional approach to calculate Q in multicomponent alloys from liquidus gradient m i and partition coefficient k i must fail for a wide range of common alloys exhibiting minute amounts of primary crystallizing intermetallic phase, exemplified for Mg–Al–Mn and Al–Si–Ti alloys. The rigorous approach provides an extension of the applicability range of the concept of Q. The qualitative similarity of inoculant particles and primary intermetallic phases is verified by calculations for Al–Si–Ti–B alloys.

Influence of Si and Ti contents on the microstructure, microhardness and performance of TiAlSi intermetallics in Al–Si–Ti alloys

In this study, the influence of Si and Ti contents on the microstructure, microhardness and morphology of TiAlSi intermetallics in ternary Al–Si–Ti alloys was investigated. The increase of Si addition in Al– xSi–2Ti alloys leads to an increase of Si content in TiAlSi phase as well as an increase of microhardness. A phase evolution from Ti(Al 1− x Si x ) 3 to Ti 7Al 5Si 12 at about 14 wt.% Si was detected, while all the TiAlSi intermetallics exhibit flake-like. The increase of Ti content promotes the morphological transformation of TiAlSi from flake-like to block-like and primary Si particles are replaced by blocky TiAlSi particles in hypereutectic Al–Si alloy. The high-temperature strengthening effect of TiAlSi particles in a piston alloy was investigated and the strength is increased by 11.9% in this article, while the elongation and yield strength are increased by 17.6% and 10.1%, respectively.

Electrochemical extraction of Fe–Ti–Si alloys direct from Ti bearing compound ores

This study demonstrated the successful direct electrochemical reduction of Ti bearing compound ores to Fe–Ti–Si alloys by using the solid oxide membrane (SOM) process. This electrochemical process was carried out in molten CaCl2 at 1100°C and 3·5–4·0 V, the pressed cylindrical pellet of Ti bearing compound ores sintered in air at 1150°C for 2 h acted as a cathode, and the carbon saturated liquid metal contained in an yttria stabilised zirconia tube served as an anode. The electrolytic characteristics were investigated and the products obtained from different conditions were characterised by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. It is found that Ti bearing compound ores can be directly electrochemically reduced to Fe–Ti–Si alloys, the titanomagnetite mixed in the compound ores is first deoxidised to the metal Fe, and the pure Fe extends into the centre of the pellet through the metal/oxide/electrolyte three-phase interlines, which may promote the subsequent electrochemical reactions effectively, and lead to the electrodeoxidation process proceed fast. Importantly, it is observed that some metallic impurities in the Ti bearing compound ores, such as the magnesium, calcium and aluminium, are partly or completely removed during the electrolysis.

Microstructure and tribological properties of in-situ Y2O3/Ti-5Si alloy composites

Abstract The Y2O3/Ti-5Si alloy composites were prepared by nonconsumable vacuum arc melting through the synthesis reaction from Ti, Si, Y and SiO2. The thermodynamics of the insitu synthesis reaction were analyzed. Microstructural analysis showed that the Ti5Si3 and Y2O3 reinforcements were in-situ synthesized in the Ti matrix. Continuous and quasicontinuous network-shape Ti5Si3 + Ti eutectic cells are abundant, and Y2O3 grows from near-equiaxed shape to dendritic shape with increasing Y content in the composite. The introduction of in-situ Y2O3 particles results in a strong interfacial bonding between the reinforcement and matrix, which can significantly improve the wear resistance and frictional stability of Ti–Si alloy, due to the load-bearing function of the Y2O3 particles and the Ti5Si3 phase. Homogeneous distribution of the in-situ reinforcements helps to resist the propagation of subsurface microcracks and then improve the wear properties of in-situ composites.

Effect of silicon on corrosion resistance of Ti–Si alloys

Abstract The corrosion resistance of Ti–Si alloys has been studied in acid solutions and the alloys exhibit a high resistance to corrosion. SEM examinations combined with EDAX allowed to conclude that the passive films on Ti–Si alloys are mainly composed of TiO 2 /SiO 2 oxides. XPS analysis indicated the formation of Si–O and Si–O–Ti bonds in the passive film, respectively corresponding to SiO 2 and Si-doping TiO 2 . The effect of silicon on the corrosion was correlated to the formation of a stable SiO 2 film, Si-doping on TiO 2 and the extended lattice imperfections formed along TiO 2 /SiO 2 grain boundaries and phase-boundaries. The calculated donor densities based on the point defect model were shown to depend exponentially on pH and linearly on potential.

Study on Si–Ti alloy dispersed in a glassy matrix as an anode material for lithium-ion batteries

A nanosized silicon based composite consisting of Si, Si–Ti alloy phases dispersed in a glassy matrix is prepared by high energy mechanical milling (HEMM), followed by thermal treatment. X-ray diffraction (XRD), field emission scanning electron microscope (SEM) and transmission electron microscope (TEM) are used to determine the phases obtained and to observe the microstructure and distribution of the components. The galvanostatic discharge/charge test is carried out to characterize the electrochemical properties of the composite. The composite electrode delivered a reversible capacity of 738.6 mAh g −1 after 50 cycles and the coulombic efficiency remains above 95% from the 3rd cycle. When the discharge capacity is limited to 700 mAh g −1, the reversible capacity is maintained at 675.8 mAh g −1 for 60 cycles without overcharge.

Structure and microhardness of nanocrystalline composite alloys on the basis of Al and Ti

A nanotechnology for production of layered composite nanocrystalline Al-Si, Al(1Hf + 0.2Nb + 0.2Sn, wt %)-Si, Al(0.5Ce + 0.5Re + 0.1Zr, wt %)-Si, and Ti-Si alloys has been suggested. The structure of the nanocrystalline composites obtained has been studied by the methods of optical microscopy and scanning and transmission electron microscopy. Experimental data on the microhardness of layered composite nanocrystalline alloys on the basis of aluminum and titanium are given. The microhardness of the nanocrystalline two-layered Al-Si composite in comparison with submicrocrystalline aluminum increased by a factor of three. In the nanocrystalline two-layered Ti-Si composite (compacted from powders), the microhardness also increased by a factor of almost six in comparison with nanocrystalline titanium. In the nanocrystalline two-layered composite alloys of aluminum, the increase in microhardness in the case of the Al(Hf,Nb,Sn)-Si alloy was up to 45% as compared to the composite on the basis of the metallic foil and by a factor of two as compared to the powder-compact composite; and in the case of the Al(Ce, Re,Zr)-Si alloy, by a factor of 1.6 and 2.7, respectively. An increase in the number of layers in the composites had no significant effect on the level of microhardness.

Effect of cyclic heat treatment on the microstructures and mechanical properties of Ti–Si alloys

Abstract Samples of Ti-8Si, Ti-13.67Si and Ti-23Si (at.%) alloys treated using different cyclic heat treatment processes have been investigated using X-ray diffraction, optical microscopy and scanning electron microscopy. The microstructures, Vickers hardnesses and compressive properties of the as-cast and annealed samples are compared. For the Ti-8Si and Ti-23Si alloys, only the eutectic microstructure in the as-cast alloy is obviously altered after annealed at 900 °C (where β-Ti is stable) for 20 min and then cooled in air with six cyclic treatments, or firstly treated at 900 °C (where β-Ti is stable) for 20 min followed by quenching in air, and then treated at 700 °C (where α-Ti is stable) for 40 min followed by cooling in furnace, with six cycles) cyclic heat treatment. The microstructure of Ti-13.67Si alloy changes from the features of clubbed and larger sized Ti5Si3 grains to finer grain size during different cyclic heat treatment. Vickers hardness of all alloys is significantly increased in the cyclic processing. For the Ti-23Si alloys, particles in the eutectic structure of as-cast alloy were spheroidized after cyclic heat treatment, resulting in increasing of both Vickers hardness and compressive properties.