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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Smart or intelligent materials embedded in a metalic matrix o ff er improved performance or even additional features Mortensen and Llorca (2010) such as in situ sensing Yanaseko et al. (2019); Lemkova et al. (2023), actuation Li et al. (2017); Bournias-Varotsis et al. (2019) and health monitoring or self-healing Moghadam et al. (2014); Srivastava and Gupta (2019); Lemkova et al. (2023). Furthermore, the applicability of HPT with ceramics has been extensively reviewed, highlighting its role in phase transformation, defect engineering, and the synthesis of high-entropy and black oxides for energy and environmental applications Edalati et al. (2025). Standard manufacturing routes carry the challenge to maintain the chemical or physical properties of the active embedded component. Unlike resin-based embedment, powder metallurgy as a main processing route for MMCs often involves compression in addition to di ff usion bonding. This leads to high process temperatures, similar to most additive manufacturing and casting processes Hahnlen and Dapino (2010). Hence, the properties of interest tend to degrade Ramanathan et al. (2021); Li et al. (2017). In contrast to most processing routes such as powder sintering, additive manufacturing and stir casting, mechanical alloying results in composites with a very high strength Ye et al. (2005) and large ductility due to its intrinsic grain refinement by the large conversion during consolidation. Following a bottom up route, ultra-fine grained or nanocrystalline (nc) materials can also be synthesized atom by atom. The limitation of these deposition techniques of material synthesis originates in the up scaling of samples to a feasible and industrial use. Common strategies are chemical vapor deposition (CVD), physical vapor deposition (PVD) and pulsed electrodeposition (PED) Qian et al. (2014). The incorporation of particles is limited to very small particle sizes and therefore su ff ers from particle coagulation. A contrary top down approach allows to produce bulk materials by mechanical consolidation, conversion and / or mechanical alloying e.g. by ball milling. Murty et al. (1992) even for large ceramic particle sizes. Severe plastic deformation (SPD) techniques such as high-pressure torsion (HPT) allows besides the bulk route to combine a wide range of base materials in the form of micro powders and to design structural and chemical properties from easy to handle green bodies for mechanical alloying. Because of the large volume fraction of grain boundaries Chadwick and Savin (2006), nc materials tend to prema ture grain growth resulting in low thermal and mechanical microstructure stability Wegner et al. (2009); Kapp et al. (2017). The small grain size in the range of less than 100 nm can principally be stabilized by the following approaches: • In the case of SPD, the energy of the unrelaxed grain boundaries is comparatively high in the as-produced state. A subsequent annealing further increases the hardness and stabilizes the nc microstructure Alfreider et al. (2020). • The microstructural stability is increased if a powder is consolidated instead of starting from bulk material during HPT by oxide segregation at the grain boundaries (solute drag e ff ect) Bachmaier and Pippan (2011). • While the material performance can also su ff er from dispersoids and ceramic particles acting as stress concen tration or delamination sites, the microstructural stability could further benefit from an additional Zener Pinning Smith (1948). Although, embedding of ceramics enhance the sti ff ness, strength and wear resistance of MMCs, these dispersoids can severely a ff ect the fracture toughness and the ductility of the material Romanova et al. (2009). Finite element (FE) simulations Qin et al. (1999) showed in agreement with tensile testing results Spowart and Miracle (2003) that ceramic reinforcements not only cause a higher yield stress by an enhanced load carrying, but also trigger premature internal damage Mortensen and Llorca (2010). The mechanical heterogeneity between reinforcements and matrix realizes a complex stress strain state in the vicinity of the ceramic / metal interfaces. If the ceramic reinforcements have a higher elastic modulus, as this is usually the case, this complex stress state is dominated by tension and promotes failure Romanova et al. (2009). With regard to geometry, a high angularity of the particles, even for slight roughness or imperfections Romanova et al. (2009), increases the local stress concentration at the metal / ceramic interface at edges. This not only leads to accelerated plastic damage accumulation and pore formation in the matrix nearby the interface, but also to particle fracture or to interfacial failure. The constraints that sti ff er ceramics exert on the matrix metal result in larger strain accumulation and higher stress peaks when the particles are more sharp-edged Llorca et al. (1991). This mechanical contrast between particles and matrix causes high strain gradients in the matrix near the interface, which causes by