TiAl Melt Research Articles

Formation mechanism of Ti3AlC2 in TiO2–Al–C/TiC systems at high temperatures

Ti 3 AlC 2 /Al 2 O 3 composites were synthesized by aluminothermic reduction of TiO 2 /Al/C and TiO 2 /Al/TiC. The effects of carbon sources (carbon black and TiC), steps heating-up profile, and Al-excess of raw materials composition on the Ti 3 AlC 2 content in final products were investigated. The results showed that in contrast with carbon black, TiC used as a carbon source is beneficial to the formation of Ti 3 AlC 2 at lower temperatures. However, Ti 3 AlC 2 content in the product with carbon black was improved by the steps heating-up profile and Al-excess, promoting intermediate phases TiAl 3 and TiC. Furthermore, the formation mechanisms of Ti 3 AlC 2 with different carbon sources in aluminothermic reaction systems were also elucidated. For the TiO 2 /Al/C system, Ti 3 AlC 2 is precipitated from the Ti–Al melt at high temperatures via the dissolution-precipitation mechanism, which has slower nucleation but faster growth. In contrast, in the TiO 2 /Al/TiC system, Ti 3 AlC 2 is mainly growing from the intact TiC grains through the epitaxial nucleation mechanism, which has faster nucleation but lower growth.

Microstructure and mechanical properties of hybrid in-situ Ti2AlCw/ Mo2B5p reinforced TiAl alloy

The Ti–48Al–6Nb alloy is reinforced by the addition of the AlMo/B4C inoculant powders after high-energy ball milling. The in-situ reactions in TiAl melt lead to the formation of Ti2AlC whiskers and Mo2B5 nanoparticles. Good interface matching between the in-situ phases and the TiAl alloy is observed, which is the key point leading to the uniform distribution of the in-situ phases in the TiAl matrix. On this basis, the grain size and lamellar width of the alloy are significantly refined. The compression and three-point bending tests show that both the strength and toughness are improved. Casting twinning between γ-TiAl lamellas is detected in this study, which could be attributed to the internal stress of the melt TiAl as it solidifies. Multiple strengthening and toughening mechanisms induced by the dual-scaled Ti2AlCw and Mo2B5p are the main reason for the improvement on the mechanical properties.

Growth kinetics analysis of Nb–Al intermetallic compounds interfacial layers based on Nb–Al phase diagram

A model for studying the growth kinetics of intermetallic compounds by means of binary phase diagrams was proposed with some assumptions. The parabolic growth relationship of intermetallic compound is deduced mathematically using analytical solution of diffusion equations and the mass balance law. By comparing the solubility of the adjacent phase in intermetallic compound phase, the predominant parameter controlling the growth kinetics is the solute diffusion coefficient. The diffusion coefficient of Al atoms in intermetallic layers is quantitatively described and the smallest diffusion coefficient is 0.373 × 10−21 m2/s in Nb3Al phase. It is demonstrated intermetallic layers formed at interface indeed inhibit the Nb container being corroded further by TiAl melt, and Nb3Al layer is a dominant factor.

Interaction of Carbon Fiber with a Ti–Al Melt during Self-Propagating High-Temperature Synthesis

Interaction of Carbon Fiber with a Ti–Al Melt during Self-Propagating High-Temperature Synthesis

The high temperature wetting and corrosion mechanism analysis of Nb by TiAl alloy melt

High temperature corrosion behavior of Nb containing a TiAl melt is studied connecting with grain boundary wetting (GBW). Three intermetallic compounds (IMC) layers are formed and corroded step by step via the TiAl melt. The interfacial morphology and wetting angle evolution are investigated theoretically and experimentally. The morphology of the grooves controlled by interfacial tension, while nonzero wetting angles θd controlled by surface diffusion are established at the roots. By quantitative assessment, when θd is more than 140°, the grooves terminating on a thin sheet, which is responsible for the effective inhibition of further wetting and corrosion.

Microstructure evolution and mechanical properties of Ti–46Al–4Nb alloy modified by in-situ Si3N4-graphene core-shell nanoparticles

This paper reports a new casting strategy of titanium aluminum alloy: Si3N4-graphene core-shell nanoparticle (SGP) inoculant prepared by high-energy ball milling was added to Ti–46Al–4Nb alloy. The graphene and Si3N4 in the inoculant has an in-situ reaction with TiAl melt to form SiC and Ti2AlN particles. TiC@SiC core-shell structure can be observed in TiAl matrix, which indicates Si3N4 particles could be wrapped by graphene after milling treatment. Compared with matrix alloy, the microstructure of composite consists of full lamellar (FL) equiaxed grains. The grain size and lamellar spacing were significantly refined with the increase of inoculant content. Compression experiments were carried out at room temperature and high temperature respectively, and the results show that the specimens exhibit a compressive strength of 2.53 GPa with a strain about 16.0% in 273 K and 711 MPa in 1273 K. The increase of compressive strength may be related to the fine Si5C3 particles at the grain boundary. After that, the refining effect and the micromechanics model were discussed in detail.

In situ Al3Ti/Al composites fabricated by high shear technology: microstructure and mechanical properties

ABSTRACTIn situ Al3Ti/Al composites were produced by the reaction between Ti powder and Al melt with high shear technology. As Ti powder is starting material, it is important to investigate its size effect on the microstructure and mechanical properties of Al3Ti/Al composites. The results indicate that the necessary reaction time increases with increasing Ti particle diameter, which results in the Al3Ti phase coarsening. Particle sedimentation occurs at a short mixing time applying large Ti powder. Due to the presence of stiff Al3Ti particles, Young's modulus increases with particle volume fraction and fit well with the predicted values from the Halpin–Tsai equation. Moreover, together with grain refinement, these Al3Ti particles with a homogeneous distribution also lead to an increase in tensile strength.

Interaction of Graphite with a Ti–Al Melt during Self-Propagating High-Temperature Synthesis

We have studied in detail the structuring of combustion products in the Ti–Al–C system at low carbon concentrations (96 wt % (Ti + Al) + 4 wt % graphite) prepared by self-propagating high-temperature synthesis and compared the results to those for carbon-free samples prepared by the same method. A thin (~500 nm) TiC carbide layer has been shown to be formed on the surface of unreacted graphite particles, followed by the growth of the Ti2AlC MAX phase, which has a laminate structure. The presence of the Ti2AlC phase in the synthesized samples has been confirmed by X-ray diffraction and energy dispersive analysis. TiC has not been detected by X-ray diffraction, presumably because it is present in a small amount.

Correlation of Oxygen and Aluminum Contents of Molten Titanium-Aluminum Alloys in Alumina and Calcia Crucibles

As a key part of investigating a new proposed Ti-alloy production process, Ti-Al alloys were produced by aluminothermic reduction experiments at 1973 K in Al2O3 crucibles. Besides, pure Ti and Al were melted inside CaO crucibles at 1973 K to investigate the possibility of melt refining in equilibrium with CaO. The experiments were carried out in a vacuum induction furnace under argon gas at atmospheric pressure. The experimental results were proved by thermodynamic evaluations including also literature data. The correlation of oxygen and aluminum in Ti melt with 11 to 23 wt pct Al in contact with Al2O3 was assessed as $$ 3\ln X_{\text{O}} = -\,2\ln X_{\text{Al}} - 9.1 \pm 0.4. $$ This correlation in Ti melt with 3 to 10 wt pct Al in contact with CaO was $$ 3\ln X_{\text{O}} = - \,2\ln X_{\text{Al}} - 14.9 \pm 0.3. $$ Partial molar excess Gibbs free energies of mixing and combinations thereof in Ti-rich Ti-Al-O-Ca melt were assessed as $$ RT \ln (\gamma_{\text{Al}}^{2} \cdot \gamma_{\text{O}}^{3} ) = - 894 \pm 6\,({\text{kJ}}/{\text{mol}}) $$ , $$ RT \ln \left( {\gamma_{\text{Ca}} \cdot \gamma_{\text{O}} } \right) = - 195 \pm 4\,({\text{kJ}}/{\text{mol}}) $$ , $$ RT \ln \gamma_{\text{O}} = - 258 \pm 6\,({\text{kJ}}/{\text{mol}}) $$ , $$ RT \ln \gamma_{\text{Ca}} = 62 \pm 7\,({\text{kJ}}/{\text{mol}}). $$ The activity of Al2O3 in CaO-saturated slag was assessed as 0.003 ± 0.001. The changes of Gibbs free energy for the dissolution of 1 wt pct oxygen and 1 wt pct calcium in Ti-Al melt were estimated as −630 ± 6 and −11 ± 7 (kJ/mol), respectively. A model was developed and applied to calculate the basic data for refining Ti-Al alloy.

Phase Formation in the Ti–Al–C System during SHS

The phase formation of Ti–Al–C powder mixtures with compositions close to MAX phases during self-propagating high-temperature synthesis (SHS) is investigated using time-resolved X-ray diffraction. It is found that the formation of the material during combustion in air under low heat-removal rates is a staged process. At the first stage, the dominant reaction is titanium carbide formation, providing major heat release and combustion front propagation. This reaction leads to the formation of TiC crystals surrounded by the Ti–Al melt. Titanium carbide is dissolved in the surrounding melt behind the combustion front with the subsequent crystallization of the Ti2AlC ternary compound. No TiC formation is observed during synthesis in helium, which provides rapid heat removal. The first phase appearing in the diffraction field is Ti2AlC. The TiC life cycle of 5–10 s for the mixtures synthesized in air substantially decreases when performing the process in helium and does not exceed 1 s. SHS leads to the formation of the composite material based on Ti2AlC phase containing less than 20 wt % TiAl and 2 wt % TiC. The material structure is characterized by lamellar Ti2AlC grains surrounded by the TiAl matrix. The microhardness of synthesized materials is 4.0–4.5 GPa and corresponds to the microhardness of the Ti2AlC phase. The dispersity of Ti2AlC grains during the synthesis in helium is lower than during the synthesis in air. Lamellar MAX-phase grains grow to 8–15 μm in length and 2–5 μm in width during slow cooling in air. The dispersity of Ti2AlC grains grown in helium is lower, being no larger than 8 and 1 μm, respectively.

Microstructures and Mechanical Properties of Al3Ti/Al Composites Produced In Situ by High Shearing Technology

Due to its low density, high strength, and stiffness the intermetallic phase Al3Ti is a good candidate as reinforcement for Al alloys. In this work, in situ Al3Ti particle reinforced Al composites are fabricated from Ti particles and Al melt via melt stirring with a high shearing mixer. Microstructure and mechanical properties are investigated. The results indicate that, owing to the high shearing effect and intensive macroscopic flow of the melt, reinforced particles are distributed homogeneously on the microscopic and macroscopic scale. Furthermore, Al3Ti particles are proved to be effective nuclei for heterogeneous nucleation of α‐Al, thus the grain size of the Al matrix is significantly decreased. As a result of the fine grains and the uniform distribution of Al3Ti particles, E‐modulus, yield, and tensile strength of the composites are enhanced.

Evaluation of Ca-doped BaZrO3 as the crucible refractory for melting TiAl alloys

In this study, a new Ca-doped BaZrO3 refractory was designed by using thermodynamics approaches and tested for its applicability for vacuum induction melting (VIM) of TiAl alloys. The influence of CaO on the BaZrO3 phase constitution and microstructure, as well as the key features of the TiAl melt interaction with the Ca-doped BaZrO3 crucibles were investigated by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). Results revealed that the Ca-doped BaZrO3 refractory consisted of Ba1-xCaxZrO3 and CaO phases. An obvious interaction occurred during the melting of the TiAl alloy in the Ca-doped BaZrO3 crucible along with the generation of BaAl2O4 as a reaction product, with formation of a reaction layer up to 5 µm thick. Dissolution of Ca-doped BaZrO3 refractory in the TiAl melt was the main reason for the alloy-crucible reaction. Moreover, the Ca-doped BaZrO3 crucible was found to substantially reduce the contamination of the TiAl alloy, with lower oxygen concentration as compared with other conventional oxide crucibles. Overall results confirmed that vacuum induction melting using the Ca-doped BaZrO3 refractory can be considered as an appropriate method for the fabrication of TiAl alloys.

Effects of Alloying Elements on the Formation of Core-Shell-Structured Reinforcing Particles during Heating of Al-Ti Powder Compacts.

To prepare core-shell-structured Ti@compound particle (Ti@compoundp) reinforced Al matrix composite via powder thixoforming, the effects of alloying elements, such as Si, Cu, Mg, and Zn, on the reaction between Ti powders and Al melt, and the microstructure of the resulting reinforcements were investigated during heating of powder compacts at 993 K (720 °C). Simultaneously, the situations of the reinforcing particles in the corresponding semisolid compacts were also studied. Both thermodynamic analysis and experiment results all indicate that Si participated in the reaction and promoted the formation of Al–Ti–Si ternary compounds, while Cu, Mg, and Zn did not take part in the reaction and facilitated Al3Ti phase to form to different degrees. The first-formed Al–Ti–Si ternary compound was τ1 phase, and then it gradually transformed into (Al,Si)3Ti phase. The proportion and existing time of τ1 phase all increased as the Si content increased. In contrast, Mg had the largest, Cu had the least, and Si and Zn had an equivalent middle effect on accelerating the reaction. The thicker the reaction shell was, the larger the stress generated in the shell was, and thus the looser the shell microstructure was. The stress generated in (Al,Si)3Ti phase was larger than that in τ1 phase, but smaller than that in Al3Ti phase. So, the shells in the Al–Ti–Si system were more compact than those in the other systems, and Si element was beneficial to obtain thick and compact compound shells. Most of the above results were consistent to those in the semisolid state ones except the product phase constituents in the Al–Ti–Mg system and the reaction rate in the Al–Ti–Zn system. More importantly, the desirable core-shell structured Ti@compoundp was only achieved in the semisolid Al–Ti–Si system.

Characterization of Gd-rich precipitates in a fully lamellar TiAl alloy

We report microstructural characterization of a Gd-modified TiAl alloy with a fully lamellar structure by utilizing Cs-corrected transmission electron microscopy. Two types of Gd-contained precipitates, Gd2O3 and Al2Gd, with sizes ranging from several nanometers to micrometers were detected at grain boundaries and lamellar interfaces. Atomic-scale images reveal a well-defined crystallographic relationship between the nano-scale Al2Gd precipitates and γ-TiAl while Gd2O3, formed from TiAl melt during solidification and precipitated from α phase, is incoherent with TiAl matrix. The microstructural characterization provides direct evidence that the microstructure refinement of the Gd-modified TiAl is mainly from both oxide and intermetallic precipitates.

Interface reactions between TiAl alloys and diacetatozirconic acid–yttria molds differentiated by ammonium metatungstate addition

Two types of diacetatozirconic acid–yttria molds which were labeled as ZYW mold (diacetatozirconic acid–yttria mold with traces of ammonium metatungstate in the slurry) and ZY mold (diacetatozirconic acid–yttria mold without ammonium metatungstate) were prepared by traditional investment casting. The ZYW and ZY molds were sintered in a carbon-reducing atmosphere and in air, respectively, at 1550 °C for 4 h. Interactions between Ti–47Al–2Cr–2Nb (at%) alloys and the molds at 1650 °C for 30 min were studied. The microstructures of the metal–mold interfaces and the presence of inclusions and oxygen contamination in the TiAl bulk alloy were analyzed, and the influences of ammonium metatungstate were discussed. The results indicate that WO3 releases O into the TiAl melt, introducing tungsten metal into the alloy matrix. The TiAl alloy ingots cast in the ZY molds had the fewest inclusions and the lowest oxygen contamination.

PHASE FORMATION IN TI–AL–C SYSTEM DURING SHS

The phase formation of Ti–Al–C powder mixtures with compositions close to the composition of the MAX phases in self-propagating high-temperature synthesis (SHS) was investigated using time resolved X-ray diffraction. It is found that material formation during combustion in air under low heat removal rates is a staged process. At the first stage, the dominant is the reaction of titanium carbide formation providing major heat release and combustion front propagation. As a result, TiC crystals surrounded by the Ti–Al melt are formed. Behind the combustion front titanium carbide dissolves in the surrounding melt and then the Ti 2 AlC ternary compound is crystallized. TiC formation is not observed with the synthesis in helium providing high heat removal rates. The first phase emerging on the diffraction field is Ti 2 AlC. The TiC life cycle of 5–10 s for air-synthesized mixtures is significantly reduced for helium processes and does not exceed 1 s. SHS reaction in helium yielded a Ti 2 AlC-based composite containing less than 20 wt.% of TiAl, and 2 wt.% of TiC. The material structure is characterized by laminated Ti 2 AlC grains surrounded by the TiAl matrix. The microhardness of synthesized materials was 4,0–4,5 GPa that corresponds to that of the Ti2AlC phase. Ti 2 AlC grains synthesized in helium are smaller than in air. Laminated MAX-phase grain sizes grow up to 8–15 μm in length and 2–5 μm in width at slow air cooling. The Ti 2 AlC grain size in helium is lower – up to 8 μm in length and 1 μm in width.

Effect of carbon reactant on microstructures and mechanical properties of TiAl/Ti2AlC composites

TiAl/Ti2AlC composites were successfully synthesized by vacuum arc melting with TiC, graphite powder, and carbon nanotubes (CNTs) as carbon sources. As the amount of Ti2AlC ceramic increased, the microhardness and compressive strength of the TiAl/Ti2AlC composites improved linearly. Importantly, the TiAl/Ti2AlC prepared with TiC as the carbon source exhibited higher microhardness and compressive strength than that composites prepared with graphite powder or CNTs. During vacuum arc melting, three reactions occur successively: Ti+Al→TiAl, Ti+C→TiC, and TiAl+TiC→Ti2AlC. Ti2AlC was formed by peritectic reaction between the TiAl melt and TiC particles. A large amount of TiC was left in the TiAl matrix, which acted to improve the hardness and strength of the composites. The fracture behavior of the composite was mainly transgranular fracture. The pinning, pulling out, and the effect of crack deflecting of TiC particles, together with the interlayer tearing, plastic shearing, folding, and crimping of Ti2AlC played a key role in the improvement of the strength and plasticity of the composites.

Effect of reaction temperature on the microstructures and mechanical properties of high-intensity ultrasonic assisted in-situ Al3Ti/2024 Al composites

Al3Ti/2024 Al composites were fabricated by high-intensity ultrasonic assisted in-situ casting method. The effect of reaction temperature on microstructures and mechanical properties of the in-situ composites was studied. The results show that the reaction between Ti particles and Al melt was incomplete at low temperatures (e.g. 735 °C and 785 °C) after ultrasonic vibration for 15 min. When the temperature was increased to 835 °C, all Ti additions reacted with the Al melt to form fine Al3Ti particles which are evenly distributed in the α-Al matrix. As the temperature increasing, the average grain size of α-Al matrix decreased, while the yield strength was improved significantly. The ultimate tensile strength and yield strength of the composite which was fabricated at 835 °C were 446 MPa and 357 MPa, respectively, which are increased by 30% compared with those of the composite without ultrasonic treatment. Moreover, the effect of ultrasonic vibration on formation of Al3Ti particles was studied. The effect of in-situ formed Al3Ti particles on grain refinement of the α-Al matrix was presented. The strengthening mechanism of the in-situ Al3Ti/Al composites was also discussed.

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.

Flow field and its effect on microstructure in cold crucible directional solidification of Nb containing TiAl alloy

A 3D finite element model was established to investigate the flow field in TiAl melt under different process parameters that include the position of the solidification interface, the meniscus height, the heating power and the current frequency. A square cold crucible with an inner cross section of 36mm×36mm was employed to directionally solidify Ti–46Al–6Nb–B (at.%) ingots. The flow pattern in the melt is highly parameter-related. Generally, there are two flow swirls with opposite directions in the melt. The flow near the solidification interface can be minimized with an optimal meniscus height. The flow velocity is decreased with a decrease of heating power and an increase of current frequency. A weaker lateral flow near the solidification interface is beneficial for the continuous growth of columnar grains. The microsegregation of a directionally-solidified ingot can be reduced by controlling the flow in the melt, which is one of the advantages of cold crucible directional solidification.