TiAl Phase Research Articles

Microstructures and Creep Behavior of As-Cast TiAl-Nb Alloy

By means of creep properties measurement and microstructure observation, an investigation was carried out to study the creep behaviors of the as-cast TiAl-Nb alloy at temperature near 910 oC. Results showed that the microstructure of as-cast TiAl-Nb alloy consisted of γ/α2 phases with the lamellar feature, and the boundaries with irregular serrated configuration consisted of single γ phase, and located in between the lamellar γ/α2 phases with different orientations. The as-cast TiAl-Nb alloy displayed a better creep resistance at high temperature and longer creep life. Compared to α2 -Ti3Al phase, the γ phase possessed a weaker strength, therefore, the crack was easily initiated along the boundaries at about 45° angles relative to the stress axis, and propagated along the boundaries parallel to the orientation of lamellar γ/α2 phases up to the occurrence of creep fracture. Thereinto, the tearing edges formed on the surface of the fracture being inclined with the lamellar γ/α2 phases are attributed to the α2-Ti3Al phase with better strength hindering the crack propagation during creep.

Vacuum brazing TiAl to Ti3Al using two Ti-based filler metals

Vacuum brazing of TiAl and Ti3Al was conducted with Ti–Zr–Cu–Ni and Ti–Zr–Fe filler metals. Microstructure and shear strength of TiAl/Ti3Al joints were investigated. The results indicated that the joint mainly consisted of Ti-rich, Ti2Al, and Ti3Al phases and residual filler metal. A columnar structure in all of the brazed seams adjacent to TiAl substrate was found, and it was resulted from the interdiffusion between filler metals and solid-state TiAl substrates. The fracture surface analysis demonstrated that the existence of excessive residual filler metal and the columnar structure was detrimental to joint strength. The joint brazed at 1050 °C for 5 min using Ti–Zr–Fe filler metal presented the maximum shear strength of 196.3 MPa, and the fracture mechanism was characterized by a mixture of cleavage and quasi-cleavage fracture.

Influence of thermodynamic activities of different masteralloys in pack powder mixtures to produce low activity aluminide coatings on TiAl alloys

Titanium aluminides are interesting high temperature materials, but show insufficient oxidation resistance as well as embrittlement at higher temperatures (>750 °C). Al-enriched coatings can be manufactured by pack cementation on many high temperature alloys to promote the formation of a protective alumina layer at high temperatures, which not only protects the alloy from oxidation but is also expected to impede embrittlement of TiAl at high temperatures. One drawback of such coatings is that Al-rich phases are very brittle. Therefore the major intermetallic aluminide phase in the coating plays a critical role for the protection behavior. Based on thermodynamic calculations different masteralloys were chosen to control the pack cementation process. Particular attention is given to the gradient between the aluminum activity of the different masteralloy powders and the aluminum activity of the substrate surface (alloy TNM®-B1) in order to control the deposited phase at the surface. It is revealed that powder pack with Al as masteralloy provides a high Al activity and produces thick multi-layered coatings consisting of brittle TiAl3 and TiAl2 phase and aluminum-rich TiAl. By using different chromium aluminides as masteralloys, thinner, low-activity coatings could be produced, consisting of a bi-layer of brittle TiAl2 phase and aluminum-rich TiAl or just the targeted pure aluminum-rich TiAl, which is known to have much better mechanical properties.

Thermal stability of Ni/Ti/Al ohmic contacts to p-type 4H-SiC

Low resistivity Ni/Ti/Al ohmic contacts on p-type 4H-SiC epilayer were developed, and their thermal stabilities were also experimentally investigated through high temperature storage at 600 °C for 100 h. The contact resistance of the Al/Ti/Ni/SiC contacts degraded in different degrees, and the contact morphology deteriorated with the increases of the average surface roughness and interface voids. X-ray spectra showed that Ni2Si and Ti3SiC2, which were formed during ohmic contact annealing and contributed to low contact resistivity, were stable under high temperature storage. The existence of the TiAl3 and NiAl3 intermetallic phases was helpful to prevent Al agglomeration on the interface and make the contacts thermally stable. Auger electron spectroscopy indicated that the incorporation of oxygen at the surface and interface led to the oxidation of Al or Ti resulting in increased contact resistance. Also, the formation of these oxides roughened the surface and interface. The temperature-dependence of the specific contact resistance indicated that a thermionic field emission mechanism dominates the current transport for contacts before and after the thermal treatment. It suggests that the Ni/Ti/Al composite ohmic contacts are promising for SiC devices to be used in high temperature applications.

Alkaline corrosion properties of laser-clad aluminum/titanium coatings

Purpose– The purpose of this paper is to study the use of titanium as a protecting element for aluminum in alkaline conditions.Design/methodology/approach– Aluminum coatings containing up to 20 weight per cent Ti6Al4V were produced using laser cladding and were investigated using light optical microscope, scanning electron microscope – energy-dispersive X-ray spectroscopy and X-Ray Diffraction, together with alkaline exposure tests and potentiodynamic measurements at pH 13.5.Findings– Cladding resulted in a heterogeneous solidification microstructure containing an aluminum matrix with supersaturated titanium (<1 weight per cent), Al3Ti intermetallics and large partially undissolved Ti6Al4V particles. Heat treatment lowered the titanium concentration in the aluminum matrix, changed the shape of the Al3Ti precipitates and increased the degree of dissolution of the Ti6Al4V particles. Corrosion testing showed significant localized dissolution of the aluminum matrix.Research limitations/implications– Increased titanium concentration and heat treatment gave improved alkaline corrosion properties. At pH 13.5, the Al3Ti phases were protected, while the aluminum matrix corroded.Practical implications– For alkaline corrosion-protection of aluminum in the automobile industry, titanium might be useful at pH values below 13.5 or by using other coating techniques.Originality/value– This is the first study testing the use of titanium as a protective element of aluminum in stringent alkaline conditions.

Phase Formation and Interfacial Phenomena of the <i>In-Situ</i> Combustion Reaction of Al-Ti-C in TiC/Mg Composites

Phase evolution and interfacial characteristics through the combustion reaction of Al-Ti-C system were investigated, in terms of the reaction mechanism. The phase formation in the reaction system at 650, 660, and 670°C by in-situ high temperature X-ray diffraction (HT-XRD) showed the formation of the solid Al3Ti phase, along with melting of the Al at 660°C. Microstructural analysis of the Al-Ti-C system after holding at 670°C was carried out to identify the reaction mechanisms, which were the formation and growth of the Al3Ti phase by dissolving Ti in molten Al. This phase occurs with further contact with C, and would initiate the combustion reaction to produce a more thermodynamically stable TiC phase. Mg-infiltrated 3.04mm in the Al-TiC substrate was compared to 5.42mm in the Al-Ti-C system, at the same time and temperature from the infiltration test. The apparent activation energy obtained for Al-TiC system was 350.84kJ/mol, which was higher than that of the Al-Ti-C system (307.31kJ/mol). The formation of the Al3Ti phase in the Al-Ti-C system was also observed from the crystal structural analysis on the infiltrated area; therefore, the in-situ combustion reaction in Al-Ti-C system promoted the wetting of Mg. [doi:10.2320/matertrans.M2014323]

Effects of Titanium and Boron Additions with Mechanical Stirring on Mechanical Properties in Al-Si Alloys

Al-Si based alloys are widely used to automobile parts for weight reduction. The mechanical stirring method was performed to achieve the homogeneously dispersed strengthening phases such as AlB2, Al3Ti and TiB2 phases since the specific gravity of strengthening phases is different from the Al matrix. 500 rpm of mechanical stirring results in well dispersion of strengthening phases compared with lower speed (100 rpm) and higher speed (1,000) stirring rate. The higher elastic modulus, hardness and good wear resistance is obtained in the Al12mass%Si-2.3mass%Ti-1mass%B alloy due to the formation of strengthening phases such as Al3Ti and boride containing (AlTi)B2 compared with Al-12mass%Si, Al-12mass%Si-2.3mass%Ti and Al-12mass%Si-1mass%B alloy. Relationship between the microstructure and mechanical properties is discussed based on the solidification phenomena. [doi:10.2320/matertrans.L-M2014848]

The Study on Microstructure and Wear Resistance of In-situ Synthesis of TiAl(CN)/Fe Cermet Composite Coating

With Ti powder, Al powder and graphite powder as the coating material, and with industrial N2 as the shielding gas and reactive gas, the TiAl(CN)/Fe Cermet Composite Coating were prepared on the surface of Q235 steel base on Reactive Nitrogen Arc cladding by the TXII500 argon arc welding machine. The microstructure, phases and micro-hardness were investigated by means of scanning electron microscope (SEM), X-ray diffraction (XRD) and MH-6 micro-hardness instrument. The wear loss of TiAl(CN)/Fe coating and 65Mn steel were analyzed and compared by HSR-2M friction and wear testing machine under room temperature dry sliding wear. The experiments show that the coating and the substrate forms metallurgical bond without stomata andcrack; the coating are composed of TiAl(C0.51N0.12) phase, AlFe3 phase,FeC phase; the micro-hardness of TiAl(CN)/Fecoating is 1110HV0.5 high, about 4 times of base metal; comparing with the 65Mn,the wear weight loss of TiAl(CN)/Fe coating is about 1/2 times, the wear resistance of TiAl(CN)/Fe coating is excellent. KeywordsIn-Situ Synthesis; Reactive Nitrogen Arc Cladding; Tial(CN) Cermet; Microstructure; Wear Resistance

Microstructure and mechanical properties of Ti2AlC-reinforced TiAl composites

Abstract Ti2AlC/TiAl composites were successfully in-situ synthesized by reaction hot pressing using Ti, Al, TiC and CNTs as starting materials. The results indicate that the as-sintered composites mainly consist of Ti2AlC and TiAl phases. The mechanical properties of the 23 wt.% Ti2AlC/TiAl composite exhibit the highest values, with a flexural strength and fracture toughness of 652.5 ± 76.8 MPa and 6.6 ± 0.5 MPa m1/2, respectively. The complex structure coupled with dispersed Ti2AlC micro-particulates is responsible for the enhancement in the strength. Toughening of the composite is mainly attributed to crack deflection, crack bridging, crack branching and pull-out of the Ti2AlC particles, as well as transgranular cracking.

The influence of carbon content on Al–Ti–C master alloy prepared by the self-propagating high-temperature synthesis in melt method and its refining effect on AZ31 alloy

The influence of carbon content on Al–Ti–C master alloy prepared by self-propagating high-temperature synthesis (SHS) in melt method was investigated. The results indicate that when the carbon content in the preform is insufficient, the master alloy mainly contains TiAl3 and TiC phases. With the increasing of carbon content, the TiAl3 phase transforms into TiC particles gradually. Further increase the carbon amount results in the formation of Al4C3 phase. The master alloy with sufficient carbon, which contains Al4C3 and TiC phases, is the most excellent refiner and exhibits satisfactory performance in refining AZ31 alloy. With 1.5% master alloy addition, the grain size reduced from 280μm to 109μm. The tensile properties were dramatically enhanced with the refinement of grain structure. The ultimate tensile strength increased from 105MPa to 156MPa while the elongation increased from 8.4% to 13.6%.

Effect of Ultrasound on Microstructure and Tensile Properties of <i>In Situ</i> TiB<sub>2</sub><sub>p</sub>/7055 Composite Synthesized by Melt Direct Reaction

TiB2/7055 composite was synthesized by a new melt direct reaction from 7055-(Al-3B)-Ti system with ultrasound. The phase composition and the microstructures of the as-prepared samples were investigated by XRD, SEM and EDS. The result shows that no Al3Ti phase is in existence. With the assistance of ultrasound, grains are significant refined. Moreover, the wettability between matrix and particles is improved. The shapes of TiB2 particles are mainly hexagonal or near-spherical, and the size is about 500nm. TiB2 particles are with uniform distribution. Compared with 7055Al matrix alloy, the tensile strength of the composite reaches 303MPa, increased by 10.2%. However, the elongation is 7.3%, decreased by 4.8%.

Microstructures of the TiAl joints brazed with Ti-Zr-based filler metals

TiAl is brazed with Ti-Zr-Fe and Ti-Zr-Cu-Ni-Fe brazing foils at 1,323 and 1,373 K for 5 min. Ti-rich, Ti3Al, and white blocky Ti-Zr-Al-Fe phase are found in the joint brazed with Ti-Zr-Fe filler metal at 1,323 K. Ti2Al phase is formed with the improved brazing temperature. When using Ti-Zr-Cu-Ni-Fe filler metal, the joints are mainly comprised of Ti-rich, Ti3Al, and Ti2Al and white blocky Ti-Zr-Al-(Cu, Ni, Fe) phase. The joint widths and the size and amount of white blocky Ti-Zr-based phases in the center of the joints are increased when the brazing temperature is increased. And under the same brazing condition, the joint width, the size of Ti3Al and Ti-Zr-Al-(Cu, Ni, Fe) phase in the joint brazed with Ti-Zr-Cu-Ni-Fe filler metal, are larger than that of Ti-Zr-Fe filler metal. More TiAl base metal dissolved into the joint induces the wider joint and the higher concentration of element Al in the joint, which lead to form Ti2Al phase. Too large size compounds such as Ti-Zr-based phases and Ti3Al would deteriorate the joint property. A lot of these phases also might have a negative effect on the joint property.

Study on Preparation and Refining Efficiency of Al-5Ti-1C Master Alloy Refiner

Al-5Ti-1C master alloy refiner was prepared by melt reaction method, and its refining efficiency was examined by purity aluminum in the paper. The microstructures under different process conditions were studied by OM, SEM and XRD. The results shown that the grains were finer, TiAl3 and TiC phases were uniformly distributed at 900¡æ, held time for 15min. The refining efficiency was best at addition of 0.4% (wt.%) refiner and held time for 10min.

The Effects of Rare Earth Element on the Al-Ti-B Master Alloy Second Phase

Al-Ti-B-RE master alloy is an efficient long-term new refiner. Al-Ti-B-RE refiner was prepared by Al-Ti-B master alloy doped with LaF3, La-rich RE carbonate and Ce respectively. This paper studies the effect of different type and mass fraction of RE on the Second Phase Particles distributions and shape in Master Alloys. The addition of RE have a certain effect of refining TiAl3 and TiB2 intermediate phase. Moreover, it prevents aggregation of TiB2 intermediate phase. As the different type and addition of RE has an effect on the microstructure of master alloy, then the reasonable addition of RE is put forward.

Phase evolution and alloying mechanism of titanium aluminide nanoparticles

The evolution of phase composition of titanium aluminide nanoparticles synthesized by the flow-levitation method was systematically investigated by adjustment of the evaporating temperature of the mixed metallic droplet and the X-ray diffraction spectrum. Their alloying mechanism was analyzed according to the variation of phase composition. ε(h, l)-TiAl3, γ-TiAl and α2-Ti3Al phases are gradually formed in TiAl alloy nanoparticles with the increasing of evaporating temperature of the mixed droplet. The alloying reaction is possible to perform between the small clusters of Ti and Al during the cooling process with high cooling rate. And the alloying mechanism can be explained based on the Gibbs free energy of alloying reaction of Ti and Al small clusters.

Anodisation of sputter deposited aluminium–titanium coatings: Effect of microstructure on optical characteristics

Magnetron sputtered coatings of aluminium containing up to 18 wt.% titanium were deposited on aluminium substrates to study the effect of microstructure on the optical appearance of the anodised layer. The microstructure and morphology were studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), and glow discharge optical emission spectroscopy (GDOES), while the optical appearance was investigated using photospectrometry. The microstructure of the coatings was varied by heat treatment, resulting in the precipitation of Al3Ti phases. The reflectance of the anodised surfaces decreased with titanium content in the as-deposited, and heat-treated states, and after anodisation of the as-deposited coatings. Specimens turned grey or black when anodising after heat treatment. Partially anodised Al3Ti phases were found in the anodised layer, and the interface between substrate and anodised layer was rough, causing light trapping. Progressive darkening of the anodised layer is assumed to be due to the synergetic effect of morphology of incorporated intermetallics and substrate interface.

In-situ neutron diffraction study of thermal phase stability in a γ-TiAl based alloy doped with Mo and/or C

In-situ heating/cooling neutron diffraction experiments were performed on a γ-TiAl based alloy doped with Mo and/or C. The structural analysis was complemented by differential scanning calorimetry and electron microscopy. Description of high temperature α phase with new model of partially disordered α2′-Ti3Al phase instead of a fully disordered α-Ti is proposed. The atomic redistribution in the γ phase is related to a wide α-transus signal on the DSC curves. Sections of temperature phase stability diagrams constructed from diffraction data for heating and cooling are presented.

Role of Ti/Al Ratio of Ti-Al-X (X=Cr, Nb, Ta and W) Intermetallics on High Temperature Tensile Properties

The mechanical properties of g-TiAl at elevated temperatures have been investigated extensively over the last 30 years. Designed alloys have been proposed from the first generation alloy (Ti-48Al-2Cr-2Nb) to the second, the third and the fourth generations. However, a decisive chemical composition of g-TiAl has not been agreed among researchers yet. The main reasons for this situation are difficulties in compositional control of Ti-Al-X-Y. In this paper, the high temperature tensile properties of g-TiAl alloy with lots of different composition have been examined from the room temperature to 1200C and the tensile strength data of those specimens have been summarized. It is clear that Ti/Al atomic ratio plays an important role on the behaviors of the high temperature strength since the Ti/Al atomic ratio is strongly related to the phase stabilities between g and a2 phases in the binary Ti-Al phase diagram. A very narrow confine of a/a2 atomic ratio exists in the specimens having the comparatively high tensile strength at the elevated temperatures. Moreover, additions of the third elements such as Cr, Nb, Ta and W to g-TiAl contribute on the increase of the tensile strength and the shift of the phase stability among a2, b and g phases. In order to utilize g-TiAl alloys in the various machine components at high temperatures, the severe process controls of melting, casting, thermo-mechanical treatments and heat treatments are indispensable.

Microstructure and high temperature tensile properties of Ti22Al25Nb alloy prepared by reactive sintering with element powders

The Ti22Al25Nb alloy was prepared by reactive sintering with element powders. The reactive sintering process was conducted by step reactive sintering in order to prevent loss of Al; thus, the process was performed at 903K, holding 60min and 20MPa, and then increased to 1523K for 120min and 30MPa, followed by furnace cooling. The O (orthorhombic Ti2AlNb) phase and B2 phase were obtained in the sintered Ti22Al25Nb alloy, and a few Ti3Al phases. Subsequently, the microstructures and phases of the sintered alloy were analyzed at two sintering stages. Moreover, the forming mechanism of O (orthorhombic Ti2AlNb) phase and reactive mechanism were analyzed. In view of examining the tensile property, the tensile tests were carried out from 1233K to1313K at an interval of 20K. The tensile datum indicated that the elongations increased with the increasing temperature and decreasing strain rate; the tensile strength decreased with increasing temperature and decreasing strain rate. From the experimental results, the maximum elongation of the sintered alloys was up to 146% at 1313K at1.04×10−4s−1, and tensile strength was above 85MPa.

Space tribological properties of metal matrix space lubricant coating prepared on titanium surface

In order to improve space tribological properties of pure Ti substrate, Ti3Al/Ag metal matrix lubricant coating has been synthesized successfully on pure Ti substrate by laser cladding using Al and Ag powders as the precursor. The prepared Ti3Al/Ag metal matrix coating is mainly comprised of Ti3Al and Ag phases. Space tribology properties of the prepared Ti3Al/Ag metal matrix coating and pure Ti substrate were systematically evaluated in a simulated space environment such as high vacuum (VC), atomic oxygen (AO) and ultraviolet (UV) irradiation in comparison with those in an air environment through the space tribological test system. It has been found that in the simulated space environment Ti3Al/Ag metal matrix coating has good wear resistance and lower friction coefficient than the pure Ti substrate with relative lubricity.