Formation Of Titanium Aluminides Research Articles

Selective Al-Ti reactivity in laser-processed Al/Ti multilayers

ABSTRACTMultilayers consisting of five (Al/Ti) bilayers were deposited on (100) silicon wafers. On top was deposited the Ti layer, aimed at preventing Al from diffusing to the surface upon laser treatment. The total thickness of the thin-film structure was 200 nm. Laser irradiations with Nd:YAG picoseconds laser pulses in the defocused regime were performed in air. Laser beam energy was 4 mJ and laser spot diameter on the sample surface was 3 mm (fluence 0.057 J cm−2). The samples were treated with different numbers of laser pulses. Structural characterizations were performed by different analytical methods and nano-hardness was also measured. Laser processing induced layer intermixing, formation of titanium aluminides, oxidation of the surface titanium layer and enhanced surface roughness. Aluminum appears at the sample surface only for the highest density of laser irradiation. Laser processing induces increment of nano-hardness by approximately 20% and decrease of residual Young’s modulus for a few percentages from the starting value of the untreated samples. These results can be interesting toward achieving structures with a selective extent of Al-Ti reactivity in this multilayered system, within the development of biocompatible materials.

Implementation of DSC analysis in reaction kinetics during heating of Ti–50 at.%Al powder mixture

In this study, the reactions in Ti–50 at.%Al powder mixture upon heating at constant heating rates were studied using DSC analysis. Heating up the mixture resulted in melting of aluminum at temperatures close to its melting temperature, which was manifested as an endothermic peak in DSC curves. The melting of aluminum was the onset of an intense exothermic reaction, so-called combustion synthesis reaction which results in the formation of titanium aluminide. The shift in exothermic peak temperature in various linear heating rates was used to calculate the apparent activation energy according to Kissinger-type model-free methods. Heating rates of 10, 20, 30, 40, and 50 K min−1 were used to estimate the activation energy based on DSC data. In order to study the effect of ball-milling on reaction behavior, the starting powder mixture was ball-milled for different times, and a comparison was made between non-ball-milled and ball-milled DSC curves. The results showed that ball-milling tends to diminish the endothermic peak of melting of aluminum and shift the exothermic reaction temperature to lower temperatures, apparently altering the mechanism from a solid–liquid to an almost solid–solid one. The activation energy of the process in the non-ball-milled state was found to be close to the activation energy of diffusion of aluminum in TiAl (220 kJ mol−1). However, the application of same model-free methods to the ball-milled samples showed unexpectedly increased values for activation energy. It would be appropriate to check other methods as well as for calculation of activation energy in the ball-milled systems.

Oxidation protection behaviour of titanium aluminide coatings developed by TIG technique

Titanium alloys are attractive in aerospace applications for low density and good mechanical properties, but they have poor oxidation and wear resistance. A coating layer of titanium aluminide can mitigate these problems to some extent and make the alloys suitable for hot structure applications. This paper discusses the formation of titanium aluminide coatings on commercial purity titanium (CPTi) surfaces by melting a preplaced aluminium and titanium powder mixture, using a tungsten inert gas welding torch. Depending on powder composition and energy input, the resolidified melt layer produced a single-phase α2–Ti3Al or a dual-phase α2 and γ–TiAl microstructures of lamellar or columnar dendritic types. The microhardness varied from 400 to 600 Hv based on the microstructure variation within the coating. Testing the resistance to oxidation, by heating and cooling through 9 cycles at 750 °C for a total of 100 h in air, gave a weight gain of 1.00 mg cm−2 for the α2–Ti3Al coating compared to 2.60 mg cm−2 for the CPTi specimen. The dual-phase coating showed much improved oxidation resistance with a weight gain of 0.35 mg cm−2 after exposure under similar conditions.

Reactive synthesis and characterization of titanium aluminides produced from elemental powder mixtures

The formation of titanium aluminides in Ti–Al elemental powder mixtures containing 25, 50 and 75 at.% Al, has been studied using differential scanning calorimetry (DSC). Phase evolution in the mixture was followed by heating the compacted samples up to 1273 K at 7.5 and 15 K min−1. The cooled samples were characterized using X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy. The results showed that the primary combustion product in all the samples was TiAl3, and the combustion reaction occurred below the melting point of aluminum only in Ti-rich samples. In Al-rich samples (75 at.% Al), TiAl3 was obtained as a porous, single-phase product after combustion. In samples containing 25 and 50 at.% Al, the combustion reaction was incomplete and the unreacted titanium particles were covered by a layer of TiAl3. In these samples, other intermetallic compounds such as TiAl2, TiAl and Ti3Al were observed to form upon heating beyond the combustion peak and are attributed to the solid-state reaction between unreacted titanium and TiAl3. Heating the samples with 25 at.% Al to 1273 K for an hour led to the formation of a homogenous Ti3Al product, while a multiphase product with a dominant TiAl phase was observed in samples containing 50 at.% Al. Calculations based on DSC data show that the formation of TiAl3 through the reaction between solid titanium and molten aluminum is associated with an apparent activation energy of 195 ± 20 kJ mol−1 and an enthalpy of −114 ± 5 kJ mol−1.

Nucleation and growth of titanium aluminide in an explosion-welded laminate composite

Processes of nucleation and growth of titanium aluminide in a 23-layer aluminum-titanium composite produced by explosion welding have been studied. In the vortex zones of seven upper welds, microvolumes of melted metal whose microhardness is ∼5500 MPa have been revealed, which corresponds to the microhardness of the intermetallic compound Al3Ti. No formation of titanium aluminide in welded junctions that were not subjected to additional heat treatment has been revealed by X-ray diffraction. The holding of the composites at 630°C is accompanied by the formation of interlayers of intermetallic compounds of the Al3Ti type. Intermetallic compounds of two morphological types are formed in the welds. In the regions of vortex zones, compact precipitates of Al3Ti are formed; in the other regions of the welds, intermetallic compounds in the form of a film are precipitated. The intermetallic compounds of the first type grow more rapidly and in final account absorb the precipitates of the film type. The activation of diffusion in the upper junctions that occurs upon heating of the welded composites is favored by the nonequilibrium state of the material caused by the strain hardening of the initial samples. In the welds located deeper than the 13th layer, no signs of the formation of compact intermetallic compounds have been revealed upon the annealing for 5 h and less.

Synthesis mechanism of an Al-Ti-C grain refiner master alloy prepared by a new method

The mechanisms of in-situ synthesis of an Al-Ti-C grain-refiner master alloy, prepared by adding a powder mixture of potassium titanium fluoride and carbon into an aluminum melt, have been systematically studied. It was found that vigorous reactions occurred at the initial stage of reaction and then slowed down. After about 20 minutes, the reactions, which led the formation of blocky titanium aluminides and submicron titanium carbides in the aluminum matrix, appeared to reach completion. Potassium titanium fluoride reacted with aluminum and carbon at 724 °C and 736 °C, respectively, resulting in the formation of titanium aluminides and titanium carbides in the aluminum matrix as well as in the formation of a low-melting-point slag of binary potassium aluminofluorides. The reaction between potassium titanium fluoride and carbon is believed to be the predominant mechanism in the synthesis of TiC by this method.

Reaction synthesis of titanium aluminides

The formation of titanium aluminides from the elemental powders has been investigated. A traditional powder metallurgy route of compaction (by cold isostatic pressing, hot pressing or hot extrusion) followed by heat treatment was compared with the novel technique of hot extrusion reaction synthesis (HERS). The products from these different production methods were characterised by x-ray diffraction and microscopy (light and scanning electron). The intermetallic compound formed under most processing conditions wasTiAl3. Only when there was a rapid increase in temperature to high temperatures, as found in induction heating of compacts or in HERS, were the compounds Ti3Al and TiAl formed.

Formation of intermetallic nanocomposites in the Ti–Al–Si system by mechanical alloying and subsequent heat treatment

Formation of nanocrystalline Al3Ti, TiAl, Ti3Al, and Ti5Si3 composites by mechanically alloying (MA) in the Ti–Al–Si system and subsequent annealing treatment are investigated. Microstructure development was monitored by x-ray diffraction, differential thermal analysis, and transmission electron microscopy. An amorphous phase could be generated through milling for 100 h. The results of annealing at different temperatures on this amorphous phase show that the formation of titanium aluminides (Al3Ti, TiAl, and Ti3Al, according to the initial relative amount of Ti and Al) and Ti5Si3 (the only silicide produced by the crystallization reaction) take place. Annealing produces nanocrystalline composites of Al3Ti, TiAl, Ti3Al, and Ti5Si3 with a grain size less than 20 nm. With increasing annealing temperature, the crystalline sizes of the phases increased.

Titanium aluminide formation in Ti implanted aluminium alloy

Ion implantation has been extensively used for modifying the surfaces of engineering materials. In the present report, the implantation of type 6061 aluminium alloy with Ti ions has been investigated as a function of dose and substrate temperature. Retained doses in the range 0.8−6 × 10 17 ions cm −2 were studied in order to examine the effect of Ti concentration on intermetallic compound stoichiometry. The formation of titanium aluminides following implantation was monitored by means of glancing angle X-ray diffrection (GAXRD) while the Ti depth profiles were obtained from Rutherford backscattering (RBS) analysis. Peak concentrations of the implanted Ti were found to vary with substrate temperature. At the lower end of the range studied, high peak concentrations of Ti were observed and GAXRD analysis indicated the presence of AlTi 3. In contrast, substrate temperatures of ∼ 400°C yielded broader implant distributions, extending to greater depths and with lower peak concentrations of implant. These were attributed to temperature induced diffusional effects which favoured the formation of an aluminium rich intermetallic compound Al 3Ti.

TiAl composites formed from elemental powders by hot-isostatic pressing processing

Recently several studies have produced intermetallics and intermetallic composites from elemental powders using self-propagating, high-temperature synthesis (SHS) [1-3]. Of particular interest has been the formation of titanium aluminides [4]. It has been observed that by preheating a Ti and A1 powder mixture, by increasing the sintering time or by applying pressure, or both, the density and form of the resultant intermetallics can be controlled [1]. Previous studies by our group have shown that by hot-pressing Ti and A1 powders, high-density, nearnet-shape titanium aluminide intermetallics are produced [5, 6]. The thermodynamics and kinetics of the Ti-A1 SHS reaction, as well as the resultant microstructure, were explained as functions of the processing parameters. The mechanical properties of the SHS hot-pressed intermetallics were found to be strongly dependent on the material density. It has also been shown that the selective addition of a third component will produce dual SHS reactions, yielding in situ formation of intermetallic composites [6]. In this study the technique of forming intermetallics by hot-pressing was extended to processing the elemental powders in a hot isostatic press (HIP) facility using "QUICK HIP"*. The aim of this study was to utilize the higher pressures that can be obtained in an HIP furnace to produce fully dense, near-net-shape intermetallics in shorter times at lower temperatures. A series of Ti*AI-X? powders (approximately 40-50 g each) were placed in a stainless steel can 2.54 cm in diameter and approximately 20 cm high. The can included powder mixtures of Ti*A1 with 5 mol % addition of B, C, Si, Fe or Ni powder, or A1203-coated SiC whiskers. B, C and Si additions formed second-phase titanium intermetallics resulting in intermetallic-intermetallic composites. Fe and Ni additions reacted with the A1, forming aluminides, resulting in intermetallic-intermetallic composites. The coated SiC whiskers were added to form an intermetallic-ceramic composite. A previ-