PSI - Issue 77

Valeria Lemkova et al. / Procedia Structural Integrity 77 (2026) 279–291 Valeria Lemkova and Florian Schaefer / Structural Integrity Procedia 00 (2026) 000–000

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Fig. 4. Dispersion and erosion of the Si 3 N 4 particles in the Ni matrix after (a) 5 rotations at room temperature (5R) and (b) 50 rotations at 300 ◦ C followed by 5 rotations at room temperature (50T + 5R). The rotations at room temperature caused a severe increase in hardness and resulted in cracks emitting from the particles. The mechanical incompatibility stresses because of the mechanical contrast between matrix and particles are likely to be the reason for these cracks

4. Discussion

4.1. Thermal Stability

As seen in Fig. 1 and already mentioned in subsection 3.1 the samples with an additional treatment at 300 ◦ C gained a better thermal stability and a further increased in hardness. This is attributed to a better dispersion of particles by additional pre-rotations at elevated temperature. The just additional 5 rotations at room temperature are su ffi cient to obtain a grain size distribution comparable to the saturation state at room temperature after 10 rotations. This is attributed to a smaller grain size after 50 T rotations compared to the inital state. The temperature of the hardness declination is increased from 0.4 to about 0.5 of the homologous temperature T H for Ni and to even 0.6 for the Al matrix material. The dispersed particles contribute to an additional Zener pinning of the grain boundaries besides the solute drag e ff ect of grain boundary segregation from oxides during the powder route. The shift of the overall curve to a higher hardness level and a further increased thermal stability after 50T rotations followed by 5R rotations supports the evidence for a Zener pinning. Evidently, the hardness correlates directly to the grain size. The hardness decrease can be attributed to a grain growth identified by the planimetric analysis as shown in Fig. 2. The large scattering of the measured hardeness values is due to the inhomogeneous material properties between particles and matrix possibly also under the hardness indent below the sample surface. By comparing the in situ images of the grains during heat treatment an improved thermal stability can be indicated directly, too. Fig. 3 shows that the grain size after the embedment of particles with SiO 2 in a Ni matrix at 500 ◦ C has just negligibly changed, whereas the EBSD scan for the Ni reference sample, also manufactured via the powder route, however without ceramic embedments, revealed an already increased grain size. This further gain in thermal stability can be directly linked to a Zener pinning by the dispersoids additional to the solute drag e ff ect from the grain boundary oxides. The grain size at elevated temperatures seems to stay constant during isothermal holding as shown in Fig. 7. This plateau is also evidence for an additional Zener pinning by which grain boundaries have to overcome an activation energy to pass the non coherent ceramic particles. This pinning vanishes at latest at 650 ◦ C . At this temperature all material combinations tested had undergone a severe increase of the average grain size measured by EBSD. The dispersion process is crucial for the interface region. As shown in Fig. 4 the mechanical contrast between the ceramic reinforcements and the metal matrix can lead to crack initiation at the interface and cracking of the 4.2. Mechanical Stability