Rapidly solidified titanium alloys for high-temperature applications

The application of rapid solidification technology (RST) to titanium alloy systems is relatively new and became the subject of active research since it was demonstrated that novel titanium alloys of higher temperature capability can be synthesized through new alloy design based on rapid solidification processing. The effects of rapid solidification on the occurrence of metastable phases, microstructures and mechanical properties in binary and ternary titanium alloys are reviewed. In particular, earlier results from RS-Ti alloy research have shown that many different novel dispersoids, some of which are coarsening-resistant at elevated temperatures (600 to 800° C), can be created in the matrix through RST. The alloys containing novel dispersoids also exhibit good creep resistance at elevated temperatures. Further studies on/- and-Ti alloys through RST, in conjunction with the development of various processing technologies for bulk alloy manufacturing, are clearly desirable.     

Simulation of processes of high-temperature gas corrosion of titanium alloys in vacuum

A physicomathematical model of the process of high-temperature interaction of titanium alloys of the system Ti-Al-Mn in a rarefied medium has been considered that takes into account gas saturation of the metal and sublimation of the alloying element. The example of the alloy OT4-1 illustrates the fact that the model representations and the experimental data are in satisfactory agreement. The model representations can be apparently extended also to other types of alloys. The model can be used for expert estimation of the effect of a vacuum in the heat treatment or the utilization of titanium alloys.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 6, pp. 29–34, November–December, 1990.     

Incoherent radiative processing of titanium silicides

Titanium silicide thin films were formed after short-time processing of thin films of metallic titanium over single-crystal silicon and polycrystalline silicon. Radiation from high intensity lamps provided a directional driving force for the reaction, which was carried out both in the presence of oxygen in the reactor and under vacuum. The effect of oxygen on the reaction was monitored using sheet resistance, X-ray diffraction and Auger electron spectroscopy (AES) measurements. The film quality was found to be strongly influenced by an oxygen partial pressure in the reactor. The effect of the processing time was also assessed and the optimum time and power input interval were determined. High quality, low resistivity films with TiSi₂ as the major phase were obtained after 10 s under a roughing vacuum. AES studies indicated that most of the oxygen and other contaminants remained in a narrow surface layer after processing.     

Strengthening of threaded connections of titanium and high-temperature nickel-chromium alloys by thermovibration treatment

The article presents results of a study of the strengthening of threaded connections M6 and M8 of alloys ÉI698, VT16, and VTZ-1 by thermovibration treatment to raise their creep resistance. Treatment regimes and empirical formulas for determining the index of strengthening (for the ÉI698 alloy) are discussed. Data on long-term strength of threaded connections in initial and post-treatment states, and on the effect of aging on retention of the strengthening effect, are presented along with some mechanical and fatigue characteristics.Translated from Problemy Prochnosti, No. 9, pp. 98–103, September, 1990.     

Characterization of diamond films deposited on titanium and its alloys

Titanium and its alloys have important applications for example in aerospace or as bioimplants. Some of these applications would be improved by diamond coatings. However the large thermal expansion mismatch between diamond and titanium or its alloys creates high residual stresses, up to about 7 GPa at 800 °C, which represent an important drawback. In this study, polycrystalline diamond films were deposited on pure titanium and Ti-6Al-4V in a classical tubular microwave plasma reactor from C-H(-O)-containing gas mixtures, at a temperature in the range 600–900 °C. Raman spectroscopy provided information about the diamond grain stress, which is obviously related to the deposition temperature. X-ray diffraction indicates the presence of titanium carbide or oxycarbide. Some other characterizations by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) are reported. It is shown that XPS coupled to argon ionic etching allows us to study the first steps of the deposition process. The structure and the chemical composition at the interface of a thicker deposit are obtained by TEM and EELS.     

X-ray diffraction study of mechanically alloyed amorphous-crystalline titanium silicides

The structure of mechanically alloyed (MA) Ti-Si powders has been investigated by means of X-ray diffraction radial distribution functions analysis and computer-generated quasi-crystalline models. It was established that the investigated samples with compositions Ti₃₃Si₆₇ and Ti₄₂Si₅₈ consist of an amorphous matrix, with chemical short-range order (SRO) similar to that of the TiSi phase, in which crystallites of the Ti₅Si₃ and Ti₅Si₄ phases are embedded. For the composition Ti₄₄Si₅₆, the SRO resembles the structural arrangement in the Ti₅Si₃ phase. An attempt has been made to explain these results using the formation enthalpies of the amorphous and the crystalline phases formed in earlier stages of MA. The Ti₅Si₄ and Ti₅Si₃ phases have a much lower formation enthalpy than the other Ti-Si phases. That is why the amount of mechanical energy imparted during MA is not sufficient completely to drive the amorphization in these two phases.     

Cyclic oxidation of high-temperature alloys

Abstract

Many applications of high-temperature alloys involve high-temperature oxidation under thermal cycling conditions. The oxide scales formed have lower coefficients of thermal expansion than the metallic alloys and significant thermal stresses can arise during temperature variations. These thermal stresses add to the residual growth stresses which accompany oxide formation. Under certain conditions, stresses in oxide scales may be partly relieved by plastic deformation of scale or alloy. However, when these mechanisms cannot be operative, scale buckling or cracking occur depending on interfacial and oxide fracture strengths. Eventual oxide spallation causes rapid degradation since depleted regions of alloy are in contact with the oxidizing atmosphere. Incorporation of ‘active elements’ such as yttrium, in reducing the residual growth stresses significantly improves the cyclic oxidation resistance of high-temperature nickel-, cobalt-, and iron-base alloys. The present paper attempts to review briefly the mechanisms involved in these phenomena and the tentative cyclic oxidation models. MST/443     

Backscattering analysis of the successive layer structures of titanium silicides

Solid state reactions of titanium thin films with silicon and with SiO₂ were studied using a backscattering technique. For the Ti-Si system layers of TiSi₂, TiSi and Ti₅Si₃ were detected in the temperature range 500–600°C. For the Ti-SiO₂ system layers of TiO_x and Ti₅Si₃ were formed in the temperature range 700–900°C. At temperatures above 1000°C the oxygen in the film disappeared and silicon was found to reach the film surface. A surface structure of concentric circular rings was observed.     

Mechanism of carburization of high-temperature alloys

An investigation of the mechanism of gaseous carburization in a reducing environment was conducted for selected Fe- and Ni-base alloys. Carburization kinetics were measured as functions of temperature in the range 870–980 °C. Scanning electron microscopy, analytical electron microscopy and X-ray diffractometry were employed for microstructural characterization and microchemical analysis. Changes in mechanical strength produced by carburization were determined from microhardness and tensile property measurements. Kinetic studies indicated that the carburization reaction followed a parabolic rate law. Depending upon the nature of surface scale formed in the presence of a carburizing environment, the rate-determining step of the reaction varied from C diffusion into the alloy in the presence of a carbide scale to that in the presence of an oxide scale. Under reducing carburizing conditions, alloys inherently protected by Cr₂O₃-base scale were found to develop a surface carbide scale which allowed C to penetrate into the alloy with relative ease and, thus, the carburization kinetics was accelerated. In contrast, an alloy capable of forming Al₂O₃ developed and maintained a protective surface oxide scale which acted as an effective barrier to C diffusion into the alloy. Degradation of mechanical strength due to precipitation of carbides in the alloy was correlated with the rate of attack and consequently the nature of the surface scale.     

Self-propagating high-temperature synthesis of titanium borides

The application of self-propagating high-temperature synthesis (SHS) to prepare a few borides of titanium was investigated. Using the plane wave propagation mode, the synthesis of titanium borides in the cold-pressed cylindrical specimens of the component powder mixtures was effected and was studied as a function of boron content in the initial mix and the specimen size. SHS reaction in compacts having diam. of 6 mm or less and high bulk density could not be initiated and/or sustained and was considered to be a result of rapid heat dissipation.     

Microstructural design for thermal creep and radiation damage resistance of titanium aluminide alloys for high-temperature nuclear structural applications

Microstructure plays an important role in strengthening of metallic materials. Various microstructures can be developed in titanium aluminide (TiAl) alloys, which can enable different combinations of properties for various extreme environments in advanced nuclear systems. In the present paper the mechanisms for microstructural strengthening and the effects of various microstructural features on thermal creep and radiation damage resistance of TiAl alloys are reviewed and compared. On the basis of the results, the evidence-based optimum microstructure for the best combination of thermal creep and radiation damage resistance of TiAl alloys is proposed. The heat treatment processes for manufacturing the optimal microstructure are also discussed.     

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M₇C₃. carbides in a eutectic mixture of austenite and more M₇C₃. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration ( 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.     

A practical guide to high-temperature alloys

The requirement for materials which can operate at ever-higher service temperatures continues. Corrosion problems are a limiting factor and have been countered by improvements in material compositions, coating procedures and fabrication techniques. Existing high-temperature alloys are reviewed, leading to a discussion of their performance in a variety of corrosion environments — oxidation, sulfidation, hot corrosion, fuel ash corrosion, carburisation and halogen attack are considered. Options are given for each environment but it is emphasised that material selection cannot be separated from consideration of other factors of the high-temperature system, including process control and better overall design specifications.     

Diffusion wear in milling titanium alloys

Abstract

In this paper, diffusion wear during milling of titanium alloys is reported. In high speed milling, tool wear is mainly caused by diffusion. The wear pattern is characterised by the combined extension of crater wear on the rake face and glacier wear on the flank. Evidence of diffusion of cobalt and carbon at the interface between the milling cutter and the workpiece has been obtained. It was demonstrated for the first time that, as diffusion wear occurred, a carbon rich layer was formed at the tool/workpiece interface while the tool material below the flank wearland was depleted in carbon. Wear occurred as a result of embrittlement and weakening of the tool surface resulting from the diffusion process.

MST/674     

Morphological features of the crystalline or quasicrystalline silicides observed in the quaternary Al-Fe-Mo-Si alloys

Rapidly solidified Al-Fe-Mo-Si powders were produced by centrifugal atomization and then consolidated by hot extrusion. Five compositions were chosen in order to obtain a theoretical constant volume fraction of (silicide + Al₁₂Mo) of 20%, with different relative fractions of [silicide]/[-matrix] and different ratios of [Fe]/[Mo] within the silicide. This paper presents the microstructures observed in both as-atomized and consolidated states. The only phase detected by X-Ray Diffraction, the non equilibrium ternary Al_12-13(Fe,Mo)₃Si, exhibits different sizes and morphologies and is inhomogeneously distributed within the matrix. Accurate electron microscopy observations indicate that this silicide may grow from a quasicrystalline precursor. Grains within the polycrystalline silicide often exhibit orientation relationships (micro twinning) giving rise to quasicrystalline-like patterns.