Rainer Gerling

Diffusion bonding of γ-TiAl sheets

Abstract Using advanced γ-TiAl based alloys and various ingot and powder metallurgical routes, several aerospace engine components are now being developed. Further application of this material in the aerospace industry can only be implemented provided that successful joining and cost effective fabrication methods for this material are developed. In this context, diffusion bonding and superplastic forming of this material is considered to be of particular interest. The possibility of producing sound bonds in γ-TiAl sheets by industrially available diffusion bonding equipment designed for conventional Ti-alloys was investigated. Defect-free bonds were achieved at 1000°C with pressure levels of 5, 10 and 20 MPa, the holding time varying between 5 and 8 h. The bond qualities were assessed by shear testing at room temperature. Reasonable shear strength levels were obtained by bonding at 1000°C. The bonds were also post bond heat treated at 1430°C for 30 min, which improved the bond quality in all cases.

Experimental studies and thermodynamic simulation of phase transformations in high Nb containing γ-TiAl based alloys

Abstract Solid-state phase transformations and phase transition temperatures in Ti-45 at.% Al and Ti-45 at.% Al-(5, 7.5, 10) at.% Nb alloys were analyzed experimentally and compared to thermodynamic calculations. Results from scanning electron microscopy, high-energy and conventional X-ray diffraction as well as differential scanning calorimetry were used for the characterization of the prevailing phases and phase transformations. For the prediction of phase stabilities and phase transition temperatures, thermodynamic calculations using the CALPHAD method were conducted. In order to achieve better agreement between calculated and experimental results, a commercially available database was modified using our own results from thermo-physical measurements and annealing treatments.

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

Abstract 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.

Spray forming of Ti 48.9Al (at.%) and subsequent hot isostatic pressing and forging

Abstract Binary Ti 48.9Al (at.%) has been processed via two different processing routes, both comprising the spray forming technology: (i) spray forming followed by hot isostatic pressing (HIP) and subsequent thermal treatments and (ii) spray forming and subsequent high temperature forging. After each process step, the material is characterized by porosity measurements, microstructural investigations and tensile tests. The as sprayed state is characterized by a porosity of 1.0%. Both, HIP and forging yield a further reduction in porosity to 0.06 and 0.04%, respectively. The as sprayed microstructure is of high homogeneity, which is maintained during all subsequent treatments. High strength (UTS=470 MPa) and a plastic elongation of 0.9% have been measured for the nearly lamellar as sprayed microstructure. The duplex microstructure, which has been established by a specific heat treatment subsequently to HIP provides the highest plastic elongation of 2.7%. The fine near gamma microstructure after forging shows at 2%-plastic elongation the highest strength (UTS=520 MPa) among the various microstructures.

Properties of two-phase intermetallic (Ti, Nb)3 (Al, Si) + (Ti, Nb)5 (Si, Al)3 P/M bulk and sheet material

Abstract Intermetallic Ti-based alloy powder with nominal composition Ti 23.5Al 5.5Si 10Nb (at.%) was produced by argon gas atomization. This dual phase alloy is composed of the α 2 -(Ti, Nb) 3 (Al, Si)-matrix phase and a eutectic mixture of α 2 -(Ti, Nb) 3 (Al, Si) + ξ-(Ti, Nb) 5 (Si, Al) 3 . The powder was compacted by hot isostatic pressing, subsequently hot rolled into sheets, and heat treated. The microstructures of the HIPed compacts and of the sheet materials were metallographically characterized and correlated with tensile properties at temperatures between 20 and 1000°C. Due to networks of the brittle ξ-silicide phase, the HIPed state showed no ductility at 20°C. Break-down of the silicide network during rolling led to an improved ductility at 20°C. After post rolling annealing, the yield strengths at 20 and 700°C were between 750-490 MPa and 700-420 MPa for specimens taken parallel and perpendicular to the rolling direction, respectively. These values are considerably higher than those reported for γ-TiAl sheets. At 1000°C the sheet material could be superplastically deformed.

A Study of Recrystallization and Phase Transitions in Intermetallic Titanium Aluminides by In-Situ High-Energy X-Ray Diffraction

High-energy synchrotron X-ray diffraction is a novel and powerful tool for bulk studies of materials. In this study, it is applied for the investigation of an intermetallic γ-TiAl based alloy. Not only the diffraction angles, but also the morphology of reflections on the Debye-Scherrer rings are evaluated in order to approach lattice parameters and grain sizes as well as crystallographic relationships. An in-situ heating cycle from room temperature to 1362 °C has been conducted starting from massively transformed γ-TiAl which exhibits high internal stresses. With increasing temperature the occurrence of strain relaxation, chemical and phase separation, domain orientations, phase transitions, recrystallization processes, and subsequent grain growth can be observed. The data obtained by high-energy synchrotron X-ray diffraction, extremely rich in information, are interpreted step by step.

Metal injection moulding using intermetallic γ-TiAl alloy powder

Argon gas atomized γ-TiAl alloy powder of high purity has been used for metal injection moulding. In order to keep the pick-up of impurities as low as possible most of the process steps were done under inert gas or high vacuum. The binder system used had been especially developed for titanium alloys. In the sintered structures the nitrogen and carbon levels are low, oxygen scatters over a wide range. The porosity of ∼4 % after sintering could be reduced to ∼0.4 % by additional hot isostatic pressing. Tensile tests at room temperature indicate promising properties of σ 0.2 = 409 MPa, UTS = 433 MPa and e pl = 0.6 %.

Gas Atomization of High Melting Reactive Metals by a Crucible- and Ceramic-Free Technique

Electrode Induction Melting Gas Atomization (EIGA) is a crucible free technique for powder manufacturing by gas atomization. It is especially suited for reactive and high melting metals/alloys. On application of this technique a metal rod dips into an induction coil. The rod is inductively heated up and the melt drops into a gas nozzle were it is atomized. The process can be conducted ceramic-free. One aim of the present developments was an increase in melt flow rate for an improved process efficiency. For titanium rods, 60 mm in diameter, the melt flow rate has been successively increased from initially 26 kg/h to 50 kg/h. Ar has been used as atomization gas and identical atomization conditions were applied for different melt flow rates. The powder particle size distribution was found to be independent from the melt flow rates. In order to test the potential of EIGA with respect to very high melting metals, atomization experiments using niobium (melting temperature approx. 2470 °C) have been performed. Nb has been successfully atomized. Although the processing conditions are not yet fully optimized, the fine powder yield is quite high, 15 wt% < 45 μm. Powder particles of all size fractions are spherical to a high degree and the tap density of 69 % is very high.

Internal Friction of a High-Nb Gamma-TiAl-Based Alloy with Different Microstructures

An intermetallic Ti-46Al-9Nb (at%) alloy with different microstructures (near gamma, duplex, and fully lamellar) was studied by internal friction measurements at 300 K to 1280 K using different frequency ranges: (I) 0.01 Hz to 10 Hz and (II) around 2 kHz. The loss spectra in range I show (i) a loss peak of Debye type at T ≈ 1000 K which is only present in duplex and fully lamellar samples; (ii) a high-temperature damping background above ≈ 1100 K. The activation enthalpies determined from the frequency shift are H = 2.9 eV for the loss peak and H = 4.1–4.3 eV for the high-temperature damping background. The activation enthalpies for the visco-elastic high-temperature damping background agree well with values obtained from creep experiments and are in the range of those determined for self-diffusion of Al in TiAl. These results indicate that both properties (high-temperature damping background and creep) are controlled by volume diffusion-assisted climb of dislocations. The loss peak is assigned to diffusion-controlled local glide of dislocation segments which, as indicated by transmission electron microscopy observations, are pinned at lamella interfaces.