The paper concerns the deposition and high temperature oxidation behavior of simple aluminide and SiAl coatings on the TiAl-based alloy TNB-V5. The coatings were produced using pack cementation method with varying content of Si and Al in the pack. The samples were thermally cycled at 850 °C in 23 h cycles up to 131 cycles (total of 3013 hours) in order to obtain mass change curves. The results of cyclic oxidation tests were related to the microstructure of the as deposited coatings. Special effort has been done in order to study the growth of protective oxide scales as well as the evolution of the metal-scale interfaces in detail using analytical high resolution Scanning Transmission Electron Microscopy (STEM).

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The contour scan strategies in laser powder bed fusion (LPBF) of Ti-6Al-4V were studied at the coupon level. These scan strategies determined the surface qualities and subsurface residual stresses. The correlations to these properties were identified for an optimization of the LPBF processing. The surface roughness and the residual stresses in build direction were linked: combining high laser power and high scan velocities with at least two contour lines substantially reduced the surface roughness, expressed by the arithmetic mean height, from values as high as 30 µm to 13 µm, while the residual stresses rose from ~340 to about 800 MPa. At this stress level, manufactured rocket fuel injector components evidenced macroscopic cracking. A scan strategy completing the contour region at 100 W and 1050 mm/s is recommended as a compromise between residual stresses (625 MPa) and surface quality (14.2 µm). The LPBF builds were monitored with an in-line twin-photodiode-based melt pool monitoring (MPM) system, which revealed a correlation between the intensity quotient I2/I1, the surface roughness, and the residual stresses. Thus, this MPM system can provide a predictive estimate of the surface quality of the samples and resulting residual stresses in the material generated during LPBF.

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Intermetallic Al-Si-based coatings can greatly increase the oxidation resistance of γ‑TiAl alloys. However, the effects of the Si addition are not fully understood. Therefore, it is difficult to determine the Si content that is optimal for oxidation resistance. Therefore, pure Al and several Al-Si coatings with varying Si contents between 1 and 81 at.% were studied. The coatings were produced using a combinatorial magnetron sputtering process. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used for structure and chemical analysis. The phases were identified by X‑ray diffraction. Cyclic oxidation tests at 900 °C were conducted up to 5000 cycles of 1 h each and subsequently evaluated by thermogravimetric analysis. Si addition in the range of 1 to 12 at.% did not deteriorate the oxidation resistance compared to a pure Al coating up for 1000 cycles (1 h) of oxidation at 900 °C, while higher Si contents led to a high mass gain. For oxidation times up to 5000 cycles (1 h), a sufficient thickness of the coatings is crucial for good oxidation resistance. The main effect of Si addition is to enhance the transformation speed of the deposited Al and Si to the high temperature stable Ti(Al,Si)3 phase during the heat treatment. Si additions of up to 12 at.% led to increased initial mass gains and a decrease in the oxidation rates during subsequent exposures compared to pure Al coatings.

The paper presents the effect of pre-oxidation on the high temperature oxidation behavior of γ-TiAl alloy. The pre-oxidation treatments were performed at 900 °C for 2 hours under various O and Ar mixtures which resulted in the formation of very thin (200-300 nm) oxide scales and oxygen enriched subsurface layers, containing the α2-Ti3Al(O) and the Ti5Al3O2 Zphase. It is revealed that the pre-oxidation treatment can lower the oxidation rate of the alloy up to 300 hours. Detailed high resolution STEM investigations were performed to characterize the oxide scales and metal-scale interfaces.

MAX‐phases are of increasing interest as coating material for high temperature applications due to their unique metallic as well as ceramic properties. Herein, the deposition of Cr2AlC and Ti3AlC2 or Ti2AlC MAX‐phase forming coatings by magnetron sputtering is demonstrated. Using pure elemental targets, the manufacturing with a coating thickness of above 7 μm is established. The MAX‐phase forming coatings are characterized by high‐temperature X‐ray diffraction (HT‐XRD) measurements and provide a good oxidation behavior due to the development of protective thermally grown oxide layers. The performance of the MAX‐phases is strongly depended on the substrate material and the accompanying interdiffusion processes. Therefore, the Ti–Al–C coating is favored for TiAl alloys due to the thermodynamic stability of the Ti2AlC MAX phase in particular in the presence of the γ‐TiAl phase. An excellent oxidation behavior is confirmed up to 300 h at 850 °C due to the development of an alumina layer above a homogenous Ti2AlC phase coating. The Cr2AlC MAX‐phase coating degrades after 100 h at 800 °C due to interdiffusion processes between coating and substrate and the accompanying development of carbides and nitride phases. Nevertheless, the oxidation resistance of the Cr–Al–C‐coated TiAl alloy is given by the formation of the Ti2AlC MAX‐phase.