PSI - Issue 68

Mirko Teschke et al. / Procedia Structural Integrity 68 (2025) 936–941 M. Teschke and F. Walther / Structural Integrity Procedia 00 (2025) 000–000

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3

a)

b)

Fig. 2: Fatigue testing setup: a) Tests at RT; b) Tests at 800 °C. [5]

In addition to the investigation of the microstructure, the SEM Tescan Mira 3 XMU was used to investigate the fracture surface after the fatigue tests to find the characteristics (size, location, and shape) of the fracture-inducing defect (“killer defect”). To measure these values the software ImageJ was used. In order to determine the quantitative distribution of the elements of the different material conditions, the Electron Beam Microprobes Analyzer JEOL JXA-iHP200F was used applying wavelength-dispersive X-ray spectroscopy (WDS). 3. Results and discussion Fig. 3 shows the microstructure of the three observed material conditions. Besides the PBF-EB/M manufactured AB state, the additional HIP condition as well as the HIP DED-LB condition were investigated. In general, the same phases occurred in all three conditions, which were the dark γ-phase, the lamellar α 2 /γ-phase, and the light β 0 -phase. However, due to the high temperatures and holding times, as well as the low cooling rates during the HIP, a significant grain coarsening can be overserved. The lamellar phase is finer in the DED-LB state compared to the PBF-EB/M HIP state. The quantitative distribution of the chemical elements in the initial state is shown in Table 1.

Table 1: Chemical analysis (WDX) of the specimen material in the initial state of the TiAl-alloy TNM-B1.

Process

Ti

Al

Nb

Mo

PBF-EB DED-LB

At.-% At.-%

Bal. Bal.

44.54 43.60

4.07 3.87

1.01 1.01

Fig. 4 shows the three types of fracture-inducing defects. Besides lack-of-fusion (LOF) defects, failure was mainly induced by gas pores (GP) or microstructure (MS) features like specific lamellar with an orientation orthogonal to the force direction.