Alloy design concepts for refined gamma titanium aluminide based alloys

The influence of the Al content and the addition of further alloying elements on the cast microstructure of γ(TiAl) + α₂(Ti₃Al) alloys has been examined. The results show that particularly fine and homogeneous microstructures without strong segregation can be obtained for certain alloy compositions solidifying through the β phase. This behavior can be attributed to the avoidance of peritectic solidification and to the alloying influence on the kinetics of the β ⇒ α transformation following solidification. The experimental findings were used to propose a design concept for γ-TiAl + α₂-Ti₃Al alloys. This concept aims at the production of high-quality castings as well as at ingot material for wrought processing routes because the chemically homogeneous and fine-grained microstructures would be a good precondition for improved workability.     

Creep feed grinding of gamma titanium aluminide and burn resistant titanium alloys using SiC abrasive

Following a brief introduction to titanium alloys and their machinability, the cutting performance of a gamma titanium aluminide intermetallic (γ-TiAl) alloy: Ti–45Al–8Nb–0.2C wt% and a burn resistant titanium (BuRTi) alloy: Ti–25V–15Cr–2Al–0.2C wt%, is compared with creep feed grinding using SiC abrasive. The work utilised 2 separate L9 Taguchi fractional factorial arrays. Typically G-ratios were a factor of 10× greater for γ-TiAl than BuRTi, with on average 10% lower maximum power and 25% lower maximum specific energy for the γ-TiAl alloy. A combination of a moderately high wheel speed: 35 m/s, low depth of cut: 1.25 mm and low feed rate: 150 mm/min, produced the lowest average workpiece surface roughness (Ra1.4 μm). Workpiece surface integrity evaluation indicated that with lower operating parameter levels, in particular a wheel speed of 15 m/s, surfaces free of burn and cracks could be produced, while at higher wheel speeds: 35 m/s, extensive workpiece surface burn was evident, with the γ-TiAl alloy suffering extensive cracking. Microhardness measurements showed in some instances slightly increased workpiece surface hardness of around 50–60HK_0.025 for the BuRTi alloy and 200HK_0.025 for the γ-TiAl material over respective bulk hardness values of 375HK_0.025 and 400HK_0.025.     

Hydrogen behavior in titanium aluminide alloys

This is a synthetical report about hydrogen behavior in titanium aluminide alloys in our group. There are two kinds of hydrogen solubility in titanium aluminides, one is the overall solubility at high temperature in the matrix without hydride and the other is the terminal solubility at low temperature in the matrix in equilibrium with the hydride. The former decreases but the later increases with increasing temperature. Hydrogen as a temporary β stabilizer clearly decreases the size of the α2 phase, and increases greatly the amount of β phase, and then increases evidently the mechanical properties of Ti3Al＋Nb. The cathodic corrosion of TiAl during charging is due to hydride on the surface. The decrease of the strength, the strain to fracture and fracture toughness for hydrogenated samples is due to hydride. The enrichment of atomic hydrogen at the crack tip during charging under sustained load can enhance localized plastic deformation and cause hydrogen-induced delayed cracking.     

Hot rolling of gamma titanium aluminide foil

Metal flow and microstructure evolution during the thermomechanical processing of thin-gage foil of a near-gamma titanium aluminide alloy, Ti–45.5Al–2Cr–2Nb, with an equiaxed-gamma microstructure was investigated experimentally and theoretically. Foils of thickness of 200–250 μm were fabricated via hot rolling of sheet in a can of proprietary design. The variation in gage of the rolled foils was ±15 μm except in very sporadic (local) areas, with variations of approximately 60 μm relative to the mean. Metallography revealed that the larger thickness variations were associated with large remnant colonies lying in a hard orientation for deformation. To rationalize these observations, a self-consistent model was used to estimate the strain partitioning between the softer (equiaxed-gamma) matrix and the remnant colonies. Furthermore, the efficacy of pre- or post-rolling heat treatment in eliminating remnant colonies was demonstrated and quantified using a static-spheroidization model. The elimination of remnant colonies via spheroidization prior to foil rolling gave rise to improved gage control.     

Force control grinding of gamma titanium aluminide

This paper addresses the grinding of ordered intermetallic compounds and their brittleness at ambient temperature. The depth of plastic deformation is supposed as the measure of surface integrity. The current paper expands the work of a previously reported indentation model that correlated the depth of plastic deformation and the normal component of the grinding force. This paper studies the indentation model using force control grinding of gamma titanium aluminide (TiAl-γ). Reciprocating surface grinding is carried out for a range of normal force 15–90 N, a cutting depth of 20–40 μm and removal rate of 1–9 mm³/sec using diamond, cubic boron nitride (CBN) and aluminum oxide (Al₂O₃) abrasives. The measured depths of plastic deformation are in the range of 150–300 μm. The deviations from the indentation model are explained by changes in the ductility during the grinding process. Furthermore, a force-based model for specific energy is developed and evaluated. The measured specific energies are in the range of 40 J/mm³ (diamond) to 400 J/mm³ (CBN).     

Novel design concepts for gamma-base titanium aluminide alloys

Two-phase titanium aluminide alloys are being considered as light-weight materials to replace nickel-base superalloys for some high temperature applications in energy conversion systems. Thus, their mechanical properties have to be assessed against the high standard set by the superalloys currently in use. In this respect most titanium aluminides are particularly inferior in high temperature strength and creep resistance even if these properties are related to density. In an attempt to overcome these problems several studies have been performed on titanium aluminides which have been subjected to solid solution and precipitation hardening. The intention of the present study is to examine more closely these strengthening processes in order to assess their potential for extending the service range of the titanium aluminides towards higher temperatures. There is growing evidence that two-phase titanium aluminides, microalloyed with carbon or niobium, can provide the necessary performance. Particular emphasis will be placed on processing routes acceptable for these materials.     

Mechanistic understanding of creep in gamma-base titanium aluminide alloys

The paper describes an experimental study of creep processes in two-phase titanium aluminide alloys. The investigations involve long-term creep test at relatively low stresses and temperatures. These test conditions lead to very low strain rates, which are characteristic of the intended service conditions. The creep tests were coupled with detailed electron microscope observations involving high-resolution imaging techniques and in situ heating studies. Long-term creep leads to spheroidization and coarsening of the lamellar morphology, which involve phase transformations and recrystallization. Climb velocities were analyzed in terms of the critical vacancy supersaturation necessary for the operation of diffusion assisted dislocation climb sources. The mechanisms are closely related to the atomic structure of the interfaces and are probably driven by a non-equilibrium phase composition.     

Recycling of gamma titanium aluminide scrap from investment casting operations

In investment casting processes for TiAl alloys, a substantial amount of the original raw material does not end-up as a final product but is instead solidified in runners and feeders of the casting system or as a skull in the crucible. Because of the high prices for the virgin alloys, there is a strong interest in remelting this scrap. This is a challenging task due to the tremendous oxygen affinity of titanium alloys. The contact of the melt with shell mould and crucible material leads to oxygen pickup during casting. Up to now no technique exists to avoid this effect, thus any recycling process for TiAl casting scrap potentially needs a deoxidation technique. Within the IMPRESS project, experimental proof for the feasibility of a novel treatment has been obtained in a series of tests and results are presented in this paper. Casting scrap from pilot-trials has been consolidated via classic vacuum induction melting (VIM) in special ceramic crucibles and then successfully deoxidized by pressure electroslag remelting (PESR) under reactive slags. Further refining by vacuum arc remelting (VAR) finally leads to removal of dissolved reducing agent and impurities like non-metallic-inclusions.     

Aspects of material removal mechanism in plain waterjet milling on gamma titanium aluminide

Due to providing reduced mechanical and thermal damages to workpiece surfaces, waterjet machining that is one of the most promising non-conventional processing methods found its niche application in cutting/shaping of materials with low machinability indexes. It can be even a more attracting technology if plain waterjet (PWJ) milling is employed due to reduced running costs (absence of abrasives) and the elimination of surface contaminations (grit embedment). The paper reports for the first time PWJ milling of a notoriously difficult-to-cut material, gamma titanium aluminide (γ-TiAl). Trials of different jet paths with varied milling parameters (e.g. water-hammer pressures, stepovers, number of passes) were conducted for understanding the removal mechanism of γ-TiAl in plain waterjet milling. The findings showed that the threshold water-hammer pressure for eroding the target material and for achieving uniform erosion were in the vicinity of 800 MPa and greater than 1 GPa, respectively. In addition, different fracture modes were observed on γ-TiAl when PWJ milling: (i) plastic deformation and crack initiation; (ii) stress wave propagation; (iii) micropits due to joint of crack lines; (iv) intergranular cracking/fracture, triple split and interlamellar/translamellar fracture. The stages (i)–(iii) occurred at lower water-hammer pressures and number of passes while the subsequent stage (iv) was only observed at higher water-hammer pressures or number of passes. The knowledge accumulated when studying the material removal mechanism and surface morphology enabled successful generation of 3D PWJ milled features (e.g. shallow pocket). To evaluate the capability of PWJ milling process, the geometrical accuracy and surface quality of the pocket has been examined. Finally, the advantages and drawbacks of the PWJ milling process are discussed to allow the definition where the technology is economically viable.     

Process and properties of hydride formed on gamma titanium aluminide during cathodic charging

A cathodic charging procedure was used to study the process of the formation of the hydride layer on gamma titanium aluminides. This electrolytic process was carried out at constant current densities of 1 and 2 A/m² for 24 h of charging in a 1N sulfuric acid solution. The hydride layer formed as a result of the charging process was observed using scanning electron microscopy. Nanohardness and microhardness of this hydride layer were also measured. Results show that the hydride forms initially as isolated islands and becomes continuous with increased charging time. The hydride layer is quite brittle and its degree of porosity increases from the metal surface outward. The thickness of the hydride layer also increases with charging current density. This is confirmed by the hardness measurements. EDS signals show the presence of the constitutive elements of gamma titanium aluminide in the hydride. ICPS analysis of the electrolyte indicates increasing metal content with increasing time of exposure probably as a result of the brittle hydride flaking off and falling into the electrolyte during the charging process.     

Temperature induced porosity in hot isostatically pressed gamma titanium aluminide alloy powders

In this study the occurrence of temperature induced porosity (TIP) in hot isostatically pressed (HIP) compacts of different gamma Titanium aluminide alloys was investigated. Two gamma Titanium aluminide alloys Ti-48.9at.%Al and an advanced Niobium containing alloy Ti-46at.%Al-9at.%Nb have been atomized by gas atomization and by centrifugal atomization in an inert gas atmosphere. The alloy powders were studied regarding porosity and the content of inert gas entrapped in the powder particles. Selected powder batches were hot isostatically pressed at 1280 °C and were investigated with respect to TIP evolution after a high temperature exposure to 1390 °C for short and long time periods. It was found that gas atomized Titanium aluminide alloy powders contain a certain amount of atomization gas, the concentration of which increases with the powder particle size. The amount of inert gas entrapped in centrifugally atomized powders is higher as compared to powders produced by gas atomization. The occurrence of TIP after high temperature annealing of the HIP’ ed compacts depends on the grain size, the processing medium (Argon or Helium), the amount of entrapped inert gas and the annealing time. Guidelines are presented for minimizing or prevention of TIP in γ-TiAl alloys processed by powder metallurgy.     

The effect of postweld heat treatment on gas tungsten arc welded gamma titanium aluminide

Postweld heat treated (postweld heat treatment, PWHT) structures of gamma titanium aluminides were analyzed by optical microscopy and X-ray diffraction techniques. It was found that PWHT promotes the formation of the γ phase. This microstructural change resulted in the reduction of the fusion zone microhardness mainly due to the reduction of the hard, brittle α₂ phase.     

Processing titanium aluminide foils

In an effort to extend the available manufacturing technologies for Ti₃Albase aluminides, the potential success of a new hot-rolling process for the production of high-quality foils has been investigated. Processing of Super-α₂ aluminide foils approximately 0.15 mm thick takes full advantage of the combined benefits of controlled rolling and a unique pack rolling process. Based on the results of a study of phase transformation kinetics, microstructure and hardness of continuously-cooled and isothermally transformed specimens, the processing parameters are optimized to provide enhanced surface quality and superior ductilities in the as-rolled condition.     

The effect of machining on the fatigue strength of a gamma titanium aluminide intertmetallic alloy

Experimental data is presented which compares the effect of grinding and high speed milling (HSM) on the fatigue strength of a gamma titanium aluminide intermetallic alloy. Results showed that HSM significantly increased fatigue strength by as much as 200 MPa over polished samples. Measurement and analysis of workpiece subsurface microhardness and microstructure indicated that the high run-out values correlated to high hardness and plastic deformation of the near surface lamellae.     

Intermetallic protective coatings on titanium

In the presented work, the powder siliconizing and liquid phase alloying were used for a surface hardening of titanium and to protect titanium against high-temperature oxidation. The powder siliconizing was carried out in a pure Si powder at 900–1100 °C/3–48 h and the liquid phase alloying was realized in an Al–20 wt.% Si melt at 800 °C/5–40 min. It was shown that the coating methods produced hard multi-phase surface layers composed of various kinds of silicides (Ti₅Si₃, Ti₃Si and TiSi) and ternary Ti(Al_XSi_1−X)₂ (τ₂) phase. The binary silicide layers grew in accordance with the parabolic law while the ternary layer grew very rapidly. It was shown that the powder siliconizing at 900–1100 °C/3 h produced sufficiently thick and compact protective layers. The liquid phase alloying at 800 °C/10 min was efficient for preparation of protective layers. The oxidation experiments were conducted at 850 °C in air. Both the powder siliconized and liquid phase alloyed coatings were shown to provide a good protection against high-temperature oxidation.     

Effects of dislocation dynamics and microstructure on crack growth mechanisms in two-phase titanium aluminide alloys

The fracture behaviour of two-phase titanium aluminide alloys was characterized by fracture toughness tests performed in a wide temperature range on chevronnotched three point bending bars. Temperature and rate dependent deformation processes were characterized by temperature and strain rate cycling tests. The alloy investigated had compositions and microstructures which are currently being considered for engineering applications. The paper considers the effects of microstructure and crack tip plasticity on the crack growth resistance. The temperature dependence of the fracture toughness was rationalized in terms of micro-processes which determine the glide resistance of the dislocations in the plastic zone of crack tips. The implications of such observations for the engineering application of the materials are addressed briefly.     

Continuous fiber-reinforced titanium aluminide composites

Recent work on a lightweight, elevated-temperature intermetallic-matrix composite (SCS-6 SiC/α₂ Ti-14Al-21Nb), including investigations of fabrication techniques, microstructural characteristics and mechanical behavior, indicates that the material appears promising for a number of demanding applications. If successfully implemented, this material, or a derivative, will provide substantial weight savings in aerospace systems. To realize this potential, major challenges must be conquered—low-temperature ductility and environmental resistance must be improved, and the cost must be brought to competitive levels.