Frank Peter Schimansky

The Characterisation of a Powder Metallurgically Manufactured TNM™ Titanium Aluminide Alloy Using Complimentary Quantitative Methods

Abstract In order to be able to use intermetallic titanium aluminide in industrial applications, a quick and affordable method of quantitatively analysing their microstructures is required. In the presented work it was able to demonstrate on a powder metallurgical manufactured TNM™ alloy of nominal composition Ti-43.5Al-4Nb-1Mo-0.1B (at.%), that by electrolytic-polishing and colour etching a quick and cost effective quantitative microstructural analysis may be carried out via light-optical microscopic images. In doing so, the phase fractions and microstructural constituents of the various types of microstructures present are determined using complementary analysing techniques. Both light-optical and scanning electron microscopic images were captured from each of three different types of microstructures. These were then quantitatively evaluated using an image analysis program. The results were compared with those obtained from X-ray diffraction experiments. The possibilities and limits of the quantitative phase evaluation of light-optical microscopic images of colour etched microstructures are also explained and their relationship to the choice of parameters used for the colour etching and electro-polishing operations discussed.

Metal Injection Moulding of Titanium and Titanium-Aluminides

Metal injection moulding (MIM) attracts growing interest as an economic net-shape manufacturing technique for the processing of titanium and titanium alloys. Even for titanium-aluminides, intended for high-temperature applications, MIM is seen as a reasonable technique to overcome processing problems with conventional methods. In this paper, basic requirements in terms of raw materials, facilities and processing in order to produce high performance components are presented. Main focus is laid on the well-known Ti-6Al-4V alloy. It is shown that the tensile properties of specimens after MIM processing can exceed the requirements given by ASTM standards even without performing an additional HIP process. For an oxygen content ranging from 0.15 to 0.33 wt% plastic elongation yields excellent 14%. Fatigue measurements performed by means of 4-point-bending tests show that grain size is more important than residual porosity in order to achieve a high endurance limit. This is shown by addition of boron powder which refines the microstructure dramatically. The modified alloy Ti-6Al-4V-0.5B yields an endurance limit of 640 MPa compared to 450 MPa of MIM parts made from standard alloy powder. Sintered components from Ti-45Al-5Nb-0.2B-0.2C (at%) powder made by inert gas atomising (EIGA technique) and processed by MIM exhibit a residual porosity of only 0.2% and tensile properties comparable to cast material.

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.

Texture Formation during Hot-Deformation of High-Nb Containing γ-TiAl Based Alloys

In this study texture and microstructure formation in high-Nb containing TiAl alloys during lab-scale compression experiments and “near conventional” forging on an industrial scale are investigated. The deformation temperatures range from 700 °C up to temperatures close to the α transus temperature (Tα = 1295 °C). Depending on the deformation conditions, the texture of the tetragonal γ-TiAl phase is formed by pure deformation components, components related to dynamic recrystallization, or transformation components. This changing corresponds with microstructural observations. The hexagonal phases α2-Ti3Al and α-Ti(Al) show a similar texture as it is known for Ti and Ti-base alloys after compressive deformation at elevated temperatures. In contrast to the γ texture, no significant change of the α/α2 texture was observed in the investigated temperature range. In the alloy with a composition of Ti-45Al-10Nb (in at.%) even deformation textures of ternary intermetallic phases, as the hexagonal ωo-Ti4Al3Nb and the cubic βo-TiAl(Nb) phase, respectively, were measured and analyzed.

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.

Microstructure of Ti-45Al-5Nb and Ti-45Al-10Nb Powders

Gas-atomised spherical powders of Ti-45Al-5Nb and Ti-45Al-10Nb alloys were produced using the plasma melting induction guided gas atomisation (PIGA) technique. The phase composition was determined by X-ray diffraction at the synchrotron beamline HEMS at PETRA III (DESY), as well as by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), two dimensional and focused ion beam (FIB) based three dimensional electron back scatter diffraction (EBSD) measurements. Due to the high cooling rates the alloy composition of both alloys consists of hexagonal-close-packed α- and body-centred-cubic β-phase. The α-phase is dominant in the larger powder size fractions. Considerable amounts of the β phase were only found in the powder particle size fractions smaller than 32 μm for the Ti-45Al-5Nballoy and smaller than 45 μm for the Ti-45Al-10Nb. A pronounced dendritic cauliflower-like structure was observed in bigger powder particles of the Ti-45Al-10Nb alloy. This gives proof that diffusion took place during the initial β-grain formation, even though there is no orientation relation between the final grain and the dendrite structure in the powder particles. The presence of dendritic structures showed that the cooling rate during powder atomization was still too low to reach the critical growth rate for a planar solidification. The absence of preferred misorientation angles between α-grains indicates that α-grains are not formed out of already solidified β-grains by a solid state phase transformation.

Novel Processing Techniques for γ-TiAl Alloys

The processing of large or near-net shaped parts from g-TiAl alloys is extremely challenging. The forging of large-scale TiAl-parts is hampered by the unavailability of high-quality, chemical homogenous pre-material. However, using an innovative combination of forging and joining large discs of TiAl with excellent mechanical properties can be produced. Metal Injection Moulding (MIM) facilitates the production of near-net shaped parts of high-strength TiAl-alloys with strength comparable to the cast condition.

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.