TiSi2 Phase Research Articles

Wetting and spreading behavior of Sn–Ti alloys on SiC

The isothermal wetting behavior of Sn–Ti alloys in contact with SiC were studied by the sessile drop method in the 900 °C–1000 °C temperature range, and the interfacial microstructures were characterized. Ti additions led the system to evolve from a non-wetting (θ = 151°) to a wetting state with the formation of interfacial layers (TiC and Ti5Si3). As the Ti concentration increased, more Ti5Si3 phases grew on the TiC layer and lower final contact angles were obtained. The lowest value θ = 20° was obtained at 1000 °C for the Sn-2.0Ti/SiC system. The Sn-5.0Ti melt kept the contact angle of 20° as well, whereas the formation of solid Sn–Ti intermetallics inhibited the kinetics of spreading. Lower temperatures had no influence on the final contact angles, only slowed down the spreading rate. The spreading of Sn–Ti melts on SiC consisted of three stages: (i) reaction-limited stage, (ii) transition stage and (iii) diffusion-limited stage. The activation energy of the first stage was calculated to be 210.3 kJ/mol. These results offer important information to optimize SiC brazing and for the preparing SiC-based composites.

Competing growth of titanium nitrides and silicides in Ti thin films processed in expanding microwave plasma: Morphology and microstructural properties

The diffusion of nitrogen into Ti silicide films allows the performance of complementary metal oxide semiconductor (CMOS) components to be improved. In this work, the thermochemical treatment is carried out in an expanding microwave plasma reactor using (Ar-33%N2-1%H2) gas mixtures. This process promotes the chemical reactions on the surface of metals. The diffusion of nitrogen into the film is improved by the reducing effect of NHx and/or H species towards passive layers such as oxides which form a barrier of diffusion in the surface layers during the process. The simultaneous formation of Ti nitrides and silicides at the surface and at the film-substrate interface, respectively gives rise to two competing processes which result in the growth of the Ti nitride phase at the expense of the Ti silicide phase at a critical temperature of 800°C. This paper reports on a comprehensive analysis of the evolution of TiSi2 and TiN phases and microstructural properties of films by means of X-ray diffraction, Raman spectroscopy, transmission electron microscopy and selected area electron diffraction investigations. Square shaped crystals of TiN are identified on the top of round shaped crystals of TiSi2. The growth of the TiN phase at the expense of TiSi2 induces a catastrophic decrease of the intensity of the (040) diffraction line of TiSi2 and a huge increase of the (220) reflection line of TiN. The microstructural properties changes during the process such as the formation of TiN crystals of nanometric size in the bulk of the TiSi2 phase as well as the migration of free Si which epitaxially grows at the film-substrate interface have been evidenced by very detailed investigations for the first time. The results are related to the mechanism of formation of TiN from the reaction between TiSi2 and nitrogen.

Precipitation location of secondary phase and microstructural evolution during static recrystallization of as-cast Ti-25V-15Cr-0.3Si titanium alloy

Abstract The microstructural evolution and precipitation location of the secondary phase of an as-cast Ti-25V-15Cr-0.3Si titanium alloy were investigated via isothermal compression experiments and heat treatment. The average aspect (length-to-width) ratio, average area and size of the grains at different heat treatment temperatures and holding time were analyzed and the effects of deformation and annealing time on the grain area and size were considered. It was found that the grain size was strongly influenced by the height reduction and holding time. Grain growth was significant when annealing time increased from 10 min to 2 h at 950 °C and height reduction of 30%; however, grain growth was minimal at annealing time between 2 and 4 h. Many dispersion particles were observed to form in continuous chains; the precipitation location was confirmed to be along initial grain boundaries, and the dispersion particles were identified to be Ti5Si3 phase by TEM.

Microstructural Characteristics and Mechanical Properties of a Nb/Nb5Si3 based Composite with and without Directional Solidification

The Nb/Nb5Si3 based composites were fabricated by conventional casting (CC) and directional solidification (DS) methods. Micro structural characteristics, compressive properties and fracture toughness of the CC and DS composites were investigated by SEM, XRD, TEM, bending and compression tests. The results demonstrate that in the CC Nb/Nb5Si3 based composite, the intergrowth of fine (Nb,Ti)ss and α-(Nb,Ti)5Si3 phases leads to the formation of eutectic structure and the coarse α-(Nb,Ti)5Si3 dendritic phase prefers to grow along eutectic cell boundary. The (Nb,Ti)3Si, (Ti,Nb)5Si3 and Dy2O3 phases mainly segregate along the eutectic cell boundary and moreover there is an orientation relationship between the (Nb,Ti)3Si and (Nb,Ti)ss phases: [001] (Nb,Ti)3Si//[112](Nb,Ti)ss and (110)(Nb,Ti)3Si//(110)(Nb,Ti)ss. The DS processing promotes the formation of coarse primary α-(Nb,Ti)5Si3 phase, (Ti,Nb)5Si3/(Nb,Ti)ss eutectic and α-(Nb,Ti)5Si3/(Nb,Ti)ss eutectic in the DS Nb/Nb5Si3 based composite. Moreover, the (Nb,Ti)ss and α-(Nb,Ti)5Si3 phases are aligned paralleling to the DS direction and exhibits strong crystal orientation preference. In addition, an orientation relationship between the (Nb,Ti) ss and α-(Nb,Ti)5Si3 phases is observed: [310]α(Nb,Ti)5Si3//[110](Nb,Ti)ss. Compared with the CC Nb/Nb5Si3 based composite, the DS Nb/Nb5Si3 based composite possesses the higher yield strength and fracture toughness, which should be ascribed to the microstructure optimization.

Effects of Si and Cu contents on grain size of Al–Si–Cu alloys

ABSTRACTThe influence of the silicon and copper contents on the grain size of high-purity Al–Si, Al–Cu, and Al–Si–Cu alloys was investigated. In the Al–Si alloys, a poisoning effect was observed and a poor correlation between the grain size and growth restriction factor was obtained. A possible cause of the poisoning effect in these alloys is the formation of a TiSi2 monolayer on the particles acting as nucleation sites or another poisoning mechanism not associated with TiSi2 phase formation. In the Al–Cu alloys, a good correlation between the grain size and growth restriction factor was found, whereas in the Al–Si–Cu alloys, the correlation between these two parameters was inferior.

NaBH4 reduction of Ti[sbnd]Si[sbnd]O nanotubes photoanode for high-efficiency photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting based on doping-modified one-dimensional (1D) TiO2 is a promising approach to sustainable hydrogen evolution. Here we successfully fabricated high-efficiency reduced TiSiO nanotubes photoanode through anodizing two-phase TiSi alloy and one step NaBH4 reduction, and investigated its PEC water splitting properties. It was found that Si-doped TiO2 nanotubes mainly grew at the α-Ti region of Ti5Si alloy substrate, and they easily dissolved at the Ti5Si3 phase region of the alloy. Both Si element and Ti3+/oxygen vacancy (VO) were successfully doped into the oxide nanotubes, resulting in enhanced optical absorption and facilitated separation of photogenerated electron-hole pairs. The reduced TiSiO nanotubes photoanode with co-doped Si element and Ti3+/VO could have a high photocurrent density of 1.39 mA/cm2 at 0 V vs. Ag/AgCl. The solar to hydrogen efficiency could reach 0.88% and it was 4.89 times that of undoped TiO2. It demonstrated that co-doping of Si and Ti3+/VO shows great potential in improving the PEC water splitting properties of undoped TiO2.

Metal matrix to ceramic matrix transition via feedstock processing of SPS titanium composites alloyed with high silicone content

Titanium silicides are promising candidates for use as a reinforcement in advanced light-weight composites due to their excellent mechanical properties and oxidation resistance at high temperatures, sufficient wear resistance, and high chemical stability in various corrosion environments. Direct in-situ synthesis of such composites from titanium-silicon (Ti-Si) powder feedstock by spark plasma sintering (SPS) was used in this study with a particular attention on the effect of the powder processing parameters (blending, co-milling, milling + blending) on the microstructure formation and mechanical properties of the sintered composites. As opposed to the previous silicide-reinforced Ti studies, this was done for high silicone content (20 wt%). It was found that, despite the powders initial identical composition, the microstructure and phase content of the compacts varied significantly with the used powder fabrication route. Taking advantage of this, composites ranging from relatively soft metal-matrix (52 vol% metallic Ti; using non-milled Ti and coarse or fine-milled Si) to hard ceramic-matrix (11 vol% metallic Ti, using fine-dispersed joint-milled mixture of Ti and Si) were obtained. Due to in-situ formation of various TiSi2, TiSi, Ti5Si4 and Ti5Si3 silicide reinforcement phases contents with high hardness and stiffness, all the sintered composites showed superior hardness and wear resistance (an increase as much as 44×) in comparison to pure Ti. Importantly, hardness and elastic modulus of intermediate compounds TiSi2, TiSi, Ti5Si4 and Ti5Si3 were measured using instrumented indentation technique for the first time and are presented in the paper.

Early oxidation behavior of Si-coated titanium

Oxidation of pure Ti coated with a thin Si layer was investigated to clarify the role of titanium silicide phases in improving oxidation resistance of titanium alloys. The system initially reacted to form a two-layer structure with Ti5Si3 and SiO2. Oxidation proceeded by upward diffusion of Ti to form TiO2 wedge-like crystals on the surface and inward oxidation of Ti5Si3 into a mixed nanocrystalline TiO2 and SiO2 internal scale that was locally layered. The Ti5Si3 phase inhibited oxygen inward diffusion, thereby limiting oxygen ingress into Ti and retarding inward Ti oxidation until the silicide layer was completely oxidized.

Microstructure evolution and mechanical properties of TiAl binary alloys added with SiC fibers

Binary TiAl (Ti29.8Al and Ti33.3Al, weight percent, hereafter in wt.%) alloys with different SiC fibers contents were prepared by arc melting. Microstructure, phase transformation, mechanical properties and related mechanism were elaborately researched. For the microstructure of Ti29.8Al alloy, the matrix consisted of fully α2/γ lamellar colonies, the grain morphology changed from columnar to equiaxed when the SiC content reached 0.9%. For Ti33.3Al alloy, the matrix consisted of α2/γ lamellar colonies and γ phases, the columnar grains changed to equiaxed grains when the SiC content was more than 0.3%, and the grain size decreased from 150.9 to 69.3 μm with increasing SiC content. Ti5Si3 phases formed at the grain boundaries of both SiC-containing binary alloys. Ti2AlC particles would appear in the lamellar colonies when the SiC content was more than 0.9% in Ti29.8Al alloy and 0.3% in Ti33.3Al alloy, respectively. The formation of Ti5Si3 and Ti2AlC phases resulted from the reaction of SiC fibers and TiAl alloy. The solid solution of C element in α2 phase is higher than that in γ phase, Ti2AlC will be more easily formed in Ti33.3Al alloy due to less α2 phases and more γ phases comparing with Ti29.8Al alloy. Compressive testing results showed that the strength and the strain of SiC-containing Ti29.8Al alloy were almost unchanged until the SiC content was 0.9%, which were improved to 1383 MPa and 6.5%, respectively. A good combination of strength and ductility of SiC-containing Ti33.3Al alloy was obtained, the highest values of them were 2196 MPa and 27.5% when the SiC content was 0.5%. Improvement of mechanical properties is caused by the grain refinement, grain boundary strengthening of Ti5Si3 particles, load transfer strengthening of Ti2AlC particles and solid solution strengthening of C and Si atoms.

Direct laser cladding of the silicide dispersed titanium aluminide (Ti45Al5Nb0.5Si) composites

The present study concerns development of titanium silicide dispersed titanium aluminide composite with addition of 5 wt.% and 10 wt.% Si in Ti45Al5Nb0.5Si base alloy by direct laser cladding under varied process parameters. Direct laser cladding has been conducted using a high power (3 kW) fiber optic delivered Nd:YAG laser (with a beam diameter of 2 mm) using a 3-axis handling system in a layer by layer fashion to develop a coupon with a dimension of 10 mm × 10 mm × 4 mm. The effect of process parameters (applied power, scan speed and powder feed rate) on the depth and integrity of the clad zone, microstructures and microhardness have been studied in details to optimize process parameters. Addition of Si leads to formation of micro-cracks in the microstructures under varied parameters. There are formations of different silicides (Ti5Si3, Ti7Al5Si12, TiSi2 and TiSi phases) in the microstructure of the base alloy with the presence α2 + γ phase. The microhardness of the clad layer increases with addition of Si. However, optimization of process parameters needs to be undertaken for the formation of defect free microstructure with improved hardness.

A Study on the Characterization of MASHS Processed Ti-Si-Al System

Many promising titanium aluminides and silicides could be synthesized in Ti-Si-Al system via mechanically activated self-propagating high-temperature synthesis (MASHS). The influence of mechanical activation and initial composition on phase formation, microstructure and synthesis behavior of mechanically activated Ti-rich mixtures in Ti-Si-Al system is investigated. Results indicate that Al can participate in the reaction and forms Ti3Al and TiAl3 phases beside Ti5Si3 phase by using mechanically activated reactants. Si and Al could be substituted each other in titanium aluminide and silicide to form solid solution of Ti3(Si,Al) and Ti5(Si,Al)3. In Ti-Si-Al system, combustion temperature is increased by increasing Si amount in the system while, ignition temperature is decreased profoundly from 1100 to 800 ∘C by applying mechanical activation prior to reaction and increasing Al in the system. In addition, Mechanical activation prior to synthesis gives rise to narrow grain size distribution and more uniform microstructure in comparison with SHS products.

Improving the Strength of the ZrC-SiC and TC4 Brazed Joint Through Fabricating Graded Double-Layered Composite Structure on TC4 Surface

In order to improve the ZrC-SiC ceramic and TC4 brazed joint property, graded double-layered SiC particles (SiCp)-reinforced TC4-based composite structure (named as GLS for convenience) was designed to relieve the residual stress in the joint. The GLS was successfully fabricated on TC4 substrate by double-layered laser deposition technology before the brazing process. The investigation of the GLS shows that the volume fraction of SiCp in the two composite layers was graded (20 and 39 vol pct, respectively). Ti5Si3 and TiC phases formed in the GLS due to the reaction of SiCp and TC4. The laser power-II (the laser power for the second deposition layer) affected the microstructure of the GLS significantly. Increasing the laser power-II would promote the reaction between the SiCp and TC4. But the high laser power-II made the layer I remelt completely and the two layers became homogeneous rather than graded structure. In the ZrC-SiC and TC4 brazed joint, the CTE (coefficient of thermal expansion) was graded from the TC4 to the ZrC-SiC due to the GLS, and the strength of the joint with the GLS (91 MPa) was higher than that without the GLS (43 MPa).

Effect of adding of SiC particulate on the microstructure and shear strength of SiC ceramic joint brazed with Si-24Ti alloy

In order to refine the grain size of TiSi2 silicide and reduce the formation of micro-defects in the joint, and thereby increasing the joint strength of SiC ceramic brazed with Si-24Ti (wt.%), a small amount of SiC particulates were added in the brazing alloy. The microstructure and mechanical strength of SiC joints was investigated by using field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction spectrometer, and shear strength test. The results indicated that SiC particulates enhanced the nucleation and grain refinement of the TiSi2 and Si phase. The adding of appropriate content of SiC (<1 wt.%) could effectively refine the size of TiSi2 phase and increase the fraction of Si-TiSi2 eutectic zone. However, excess addition of 1.5 wt.% SiC caused the coarsening of TiSi2 phase due to the clustering of added fine SiC particulates. With the increasing of SiC particulate content, the shear strength of the joints increased at first and then decreased. The maximum shear strength of 106.3 MPa of SiC joint was obtained for the joint brazed with 1 wt.% SiC addition, which was ~19% higher than that of the joint brazed without SiC particulates.

In-situ fabrication of laminated SiC/TiSi2 and SiC/Ti3SiC2 ceramics by liquid silicon infiltration

The laminated silicon carbide/titanium silicon (SiC/TiSi2) and silicon carbide/titanium silicon carbide (SiC/Ti3SiC2) ceramics were successfully designed and fabricated by liquid silicon (Si) infiltration. When the thickness of TiC layer was 150 and 450 µm, the TiSi2 and Ti3SiC2 phases were the main products in the TiC layer, respectively. The as-fabricated structural unit of laminated SiC/Ti3SiC2 ceramics consisted of five layers of functionally graded materials, which has multiscale layered structure containing macro-layered structure and nano layered structure. The generation of hierarchical structure was attributed to the diffusion of Ti elements and in-situ formation of TiSi2 and Ti3SiC2. The growth direction of Ti3SiC2 was anisotropic, thus providing more paths for the crack propagation via deflection, branching, and delamination during fracture process. However, the crack propagation inside the Ti3SiC2 phase included the pull out, bridging, lamination, deflection, and fracture of the single layer, which are the energy absorption and damage tolerance mechanisms of the Ti3SiC2 phase.

Microscopic characteristics and properties of titaniferous compound reinforced nickel-based wear-resisting layer via in situ precipitation of plasma spray welding

TiC, TiB2 particles reinforced nickel-based composite wear-resistant layer can be generated by NiCrBSi, Ti and Si composite powder via plasma spray welding. Research has shown that composite powder spray welding layer mainly consisted of γ-Ni, FeNi3 and β1-Ni3Si matrix phases, as well as TiC, TiB2, CrB, M7C3, M23C6, SiC, TiSi2 and other hard phases inlaid in the matrix. Fine TiB2, TiC, M7C3, M23C6 and all SiC, CrB, TiSi2 are dispersed in the matrix, which can refine the sprayed-layer structure effectively. The grown TiB2 (6–50 µm) can be observed on the NiCrBSi+Ti+Si mixed powder spray-welding layer, TiB2 shows stepped growth through screw dislocation. The microhardness of NiCrBSi+Ti and NiCrBSi+Si spray-welding layer was lower than that of NiCrBSi+Ti+Si mixed powder spray-welding layer, and their abrasive resistance was 19 times and 26 times of that of H13 substrate respectively. When the content of added Ti and Si is 15%, the best properties of the NiCrBSi+Ti+Si mixed powder spray-welding layer are obtained with the microhardness reaching up to 980 HV0.5, which is approximately 2 times greater than that of the pure NiCrBSi spray-welding layer. Furthermore, its wearing capacity is merely 6.5 mg, so that the wear-resistance property is more than 5 times that of the pure NiCrBSi spray-welding layer, and even 50 times that of the H13 base material. However, the toughness of the spray-welding layers will be reduced, and even cracks that run through the entire spray-welding layer will be generated.

Experimental Investigation of Phase Equilibria in the Ho-Ti-Si Ternary System at 973 K (700 °C)

Phase equilibrium relations of the Ho-Ti-Si ternary system at 973 K (700 °C) were experimentally researched by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectrometer (EDS). The isothermal section of the system at 973 K (700 °C) consists of 14 three-phase regions, 27 two-phase regions, and 14 single-phase regions. The phases Ti5Si3, Ti5Si4, TiSi, TiSi2, Ho5Si3, Ho5Si4, HoSi, αHoSi2-b, and βHoSi2-a are proved to exist at 973 K (700 °C). Previously reported HoTiSi and Ho2Ti3Si4 ternary compounds were confirmed to exist at this temperature. The solubility of Ho in the intermediate phases (i.e., TiSi2, TiSi, Ti5Si4, and Ti5Si3) at the Ti-Si side is extremely small. The maximum solubilities of Ti in HoSi2-b, Ho5Si4, and Ho5Si3 are confirmed to be 8.0, 7.2, and 6.0 at. pct, respectively.

Synthesis of Ti–Si–Al coatings on the surface of Ti–6Al–4V alloy via hot dip siliconizing route

Hot dip siliconizing method was proposed as a fast, simple and easy-echo route for synthesis of Ti–Si–Al coatings on the surface of Ti–6Al–4V alloy. Rectangular blocks were dipped into the Al-Si melt at the different melt temperatures and immersion intervals. Results showed that coatings with three sequential layers have been formed on the surface of the bare alloy after coating process; the first layer attached to the substrate composed mainly of TiAl, the second one consisted of TiSi2 and Ti5Si3 and TiAl0.3Si1.7 phases and the third layer containing Al2Ti, Al11Ti5 and TiSi2 phases. Moreover, the thickness of the Ti-Si intermediate layer increased with increasing process temperature and soaking time. The growth kinetic of the silicide layer follows the Arrhenius' relationship with the activation energy of 100 kJ/mol. Furthermore; the temperature of 850 °C was introduced as the optimum process temperature to obtain a titanium silicide layer with a proper thickness at a desirable immersion time. Finally, it was indicated that the coating process improved the high-temperature oxidation resistance of the Ti–6Al–4V alloy.

Effect of laser scanning speed on surface properties of Ti-Si laser clad intermetallic coatings fabricated on Ti-6Al-4V alloy

A binary Ti-Si (1 : 1) alloy coating was fabricated on Ti-6Al-4V alloy using laser cladding technique where Neodymium yttrium aluminium garnet laser was utilised. Micro-atomised powders of Ti and Si were used for deposition of multi-track clad coatings utilising power of 1.5 kW and a varied speed of 1.2 m/min, 1.6 m/min and 2.0 m/min. Scanning electron microscope coupled with energy dispersive spectrometer was used for microstructural characterisation and elemental analysis while a digital light microscope was used for optical imaging and coating thickness measurements. Phase identification was conducted using X-ray diffraction analyser and transverse microhardness evaluated at 100 gf for 10 s dwell time was achieved with the use of a Vickers hardness machine. The laser fabricated coatings were found to exhibit a decrease in grain size with an increase in the laser scanning speed. The microstructure of the coatings ranged from irregular refined grains, faceted grains and lamellar eutectic network of TiSS + Ti5Si3. The hardness of the coatings exhibited Ti, Si, TiS2 and Ti5Si3 phases averaged a 247.4% increase with maximum hardness recorded at 1.2 m/min scanning speed.

Micro-structural evolution during diffusion bonding of C-SiC/C-SiC composite using Ti interlayer

Diffusion bonding of C-SiC composite was achieved with Ti interlayer at 1500°C for 3h with the applied pressure of 20MPa. The joined C-SiC composite showed the apparent shear strength of 19MPa and the sample fractured away from the joint. The microstructure of joint consists of Ti3SiC2 MAX phase, TiSi2 and TiC in which Ti3SiC2 was the major phase. The distribution of phases in the microstructure of joint was categorized into three different zones based on electron backscattered diffraction (EBSD) results. The first zone consists of a thin continuous TiSi2 phase, which was followed by a thick zone of Ti3SiC2 MAX phase. In the middle of the sample, a mix phase zone was found. The reaction mechanism was established for the evolution above microstructure of joined C-SiC composite.

Eutectic evolution of directionally solidified Nb-Si based ultrahigh temperature alloys

The purpose of this work is to reveal the eutectic evolution in near-eutectic Nb-Si based alloys. The Nb–xTi–ySi–4Cr–2Al–2Hf (22Ti-15Si; 24Ti-15Si; 22Ti-16.5Si, at.%) alloys were processed by directional solidification with the withdrawal rates of 400 and 500μm/s. Three typical eutectics were observed: quasi-regular eutectics I, quasi-regular eutectics II and eutectic dendrites colonies. The decrease in the content of Ti and the increase in the withdrawal rate increased the growth rate, and led to the transformation from quasi-regular eutectics I to quasi-regular eutectics II. The increase in the content of Si and the decrease in the withdrawal rate reduced the growth rate, led to larger degree of segregation, and eventually promoted the generation of eutectic dendrites colonies. The sizes of quasi-regular eutectics I and II decreased with the increase in the withdrawal rate and increased with the higher contents of Ti and Si. For the quasi-regular eutectics I and II, the volume fractions of γ-(Nb, Ti)5Si3 phases increased with increasing the content of Ti. Compared with the quasi-regular eutectics I, more γ-(Nb, Ti)5Si3 phases existed in the quasi-regular eutectics II, due to the growth pattern transition of the γ-(Nb, Ti)5Si3 phases.