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. 7. Isothermal heat treatment at 500 ◦ C .The grain size distribution after (a) 15 min and (b) 120 min reveals stability.

We observed the same for our particles. They seem to be fractured probably by the large hydrostatic pressure of 7 . 6 GPa . During the advancing rotation these broken particle clusters were eroded by the large shearing and have shrunken. If the mechanical contrast is high one can expect that the particles drift and float in a surrounding ductile matrix. The suppression of plastic flow from the hydrostatic pressure is triggered due to a lack of high shear stresses, too. This ongoing erosion is directly visible by the rounding of the broken particles or rather particle clusters. The ongoing e ff ect decelerates because the excess stresses from edges vanishes due to rounding. The stress concentration at the sharp edges of the particles were verified by the 2D FE simulations. The shear stress component of the plain strain stress tensor is shown in Fig. 8. Pronounced excess von Mises stress correlates with convex particle region. The higher the aspect ratio of an edge the larger this excess stress. It is not surprising that this causes edges to break o ff . The higher the mechanical contrast the higher the excess stress level. It makes no di ff erence whether unbroken or broken particles are considered, due to the high overlayed hydrostatic pressure during HPT. Hence, edges or protruding cluster particles are eroded. The TKD measurements underline the results of the FE simulations. In concave areas, no excess shearing (Fig. 8o) ccurs in the simulation. Therefore, the stress on the matrix is reduced during HPT deformation. The partially enclosed matrix is shielded through the particle during torsion. This results in a locally reduced level of deformation and therefore in a reduced grain refinement thus larger grains as shown in Fig. 5. A good bonding between metal matrix and ceramic interface is achievable by HPT as revealed by micro-beam bending. To ensure that a particle pre-cracking during the HPT is avoided, a good pre-dispersion and a smaller initial particle size should be intended. Dispersing ceramics in a nanocrystalline metal matrix improves the thermal microstructure stability. An additional Zener pinning enhances the stability by 0.1 of the homologous temperature in addition to a solute drag e ff ect. HPTo ff ers a free choice of materials but the mechanical contrast is crucial for • the processability because crack initiation and cracking in the matrix must be avoided. • the local microstructure evolution near particles in order to avoid weak points under service conditions later. • the particle dispersion itself. 5. Conclusion