PSI - Issue 74

Karel Slámečka et al. / Procedia Structural Integrity 74 (2025) 85 – 90 Karel Slámečka / Structural Integrity Procedia 00 (202 5 ) 000 – 000

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Coffigniez et al. (2021), and Kachit et al. (2022)). When the microporosity exceeds ~8 vol.%, the pores form an interconnected, three- dimensional network that remains accessible to the external environment, as described by German (2020) (Fig. 1). Such interconnected micropores are particularly attractive in biomedical applications, where they promote tissue ingrowth, as well as in surface-intensive processes requiring high specific surface area, such as catalysis (du Plessis et al. (2022)). A recent study on DIW titanium (Ti) lattices by Slámečka et al. (2023) has shown that these interconnected micropores can deflect and bifurcate fatigue cracks , thereby contributing to improved fatigue resistance. To enhance surface hardness, wear resistance, and potentially further improve fatigue performance, Ti may be functionalized by gas nitriding. This thermochemical treatment results in the formation of an interstitial nitrogen solid solution within the Ti matrix, inducing beneficial residual compressive stresses in the diffusion zone (DZ), and in the development of a surface compound layer (CL) composed of hard titanium nitrides (Fig. 1b). While gas nitriding is well established for dense materials, its effects on open-porous, sintered Ti remain largely unexplored and require further investigation. Our recent work (Slámečka et al. (2025)) demonstrated that low-temperature (LT) gas nitriding of open-porous Ti leads to volumetric nitriding, in which both external and internal surfaces accessible via the open pore network become coated. Nitriding at 500 °C and 600 °C for 10 h produced thin, heterogeneous coatings that increased flexural modulus of single nitrided DIW filaments by ~25% without compromising their strength. In contrast, nitriding at 700 °C resulted in a significant reduction in flexural strength, attributed to brittle fracture of over-nitrided, isolated sintering necks (localized bridges between adjacent powder particles), which act as the weakest load -bearing microstructural features (Fig. 1c).

Fig. 1. (a) Surface and cross-sectional appearance of a DIW Ti filament with ~20% open microporosity; (b) Cross-section of a filament nitrided at 600 °C for 10 h; the inset shows a detail of the nitride coating; (c) Fracture surface of a filament nitrided at 700 °C for 10 h, with the detail highlighting brittle fracture of a sintering neck in the porous zone. In the present study, we develop an initial version of a simple finite-element model of a single nitrided sintering neck , aiming to better understand the mechanistic origins of the observed strength degradation. The model incorporates experimentally measured thicknesses of the DZ and CL, as well as graded material properties in the DZ, scaled using a complementary error function based on diffusion theory. Simulations of cooling from the stress-free state at nitriding temperature, followed by an in silico tensile test, are used to evaluate the residual thermal stresses and the effective composite behaviour. These results provide a foundation for future fracture- mechanics modelling of crack initiation and growth in nitrided, microporous Ti systems. Note that this neck -scale formulation complements our earlier multiscale finite- element analysis of Ti lattices reported by Slámečka et al. (2024), together forming a coherent framework for predicting the mechanical behaviour of gas-nitrided Ti lattices.