Key Engineering Materials Vol. 1043

Abstract: Titanium scaffolds produced by additive manufacturing were studied using the scanning electron microscopy (SEM) and the confocal optical microscopy (COM). The previous research has shown that the titanium scaffolds with porous filaments (14 % porosity) exhibited markedly better fatigue resistance than those with compact filaments (6 % porosity). This article is devoted to macroscopic and microscopic images of fracture surfaces of both types of scaffolds after cyclic compression (CC) tests and after cyclic three-point bending tests (CTPB). A high density of cracks and broken filaments was indicated particularly in scaffolds with porous filaments. The fatigue crack growth was highly affected by the microporosity. Fracture facets were smaller and rougher for the porous filaments compared to the compact ones. Values of roughness parameters S_a and S_v for porous filaments were significantly higher than those for compound fibres. Both SEM and COM studies confirmed an important role of crack-pore interactions especially in the porous filaments.

Abstract: The paper summarizes the results of the analysis made on the ruptured tube of the reactor made of austenitic stainless steel in which the conversion of aniline to diphenylamine occurs at elevated temperature and pressure and in the presence of a catalyst. The tube wall perforation occurred in the piping used to measure the pressure in the reactor and a fire occurred after an aniline, ammonia and catalyst vapour leak. The material analyses carried out clearly showed that the thinning and subsequent perforation of the tube wall was due to a specific corrosion attack, so-called metal dusting, which occurs at elevated temperatures and in the presence of a carburizing atmosphere.

Abstract: Titanium alloy Ti6Al4V was oxidation annealed in the beta-phase region (1050 °C/3 hours + WQ) in a furnace under a non-protective atmosphere. The above treatment caused the formation of an alpha-case layer on the surface. The above layer, because of its high hardness and strength, has a significant effect on the surface properties of the alloy. However, undesirable effects include the formation of cracks in this layer and a change in the mechanism of initiation and propagation of fatigue cracks. Based on the above findings, it is also very complicated to predict the fatigue life of Ti alloys processed in this way because of the presence of cracks in the alpha-case layer and the varying thickness of this layer. From the fractography and microstructural point of view, the initiation of fatigue cracks in the heat-treated alloy is realized by transcrystalline cleavage and the formation of pronounced fracture facets as a consequence of surface cracks in the alpha-case layer. Fatigue crack propagation (in the alpha-case layer region) is realized along the interface of the alpha-phase needles and the beta-phase primer grain without the significant presence of so-called fatigue striations.

Abstract: With search for clean and sustainable energy sources European union is determined to fully substitute natural gas with hydrogen in 2050. With these requirements it is really important to study materials used in existing gas infrastructure and evaluate their resistance to hydrogen embrittlement, which is common phenomena in material degradation. In this article, a comparative study was conducted using slow strain rate testing (SSRT) in situ at 10 MPa hydrogen pressure and tensile testing in air on X-52, X-60, and X-70 pipeline steels commonly used in Czech gas infrastructure. The results revealed reduction in elongation and a significant reduction in contraction in SSRT samples exposed to hydrogen compared to those tested in air, while yield and tensile strengths remained nearly unchanged. Furthermore, fracture morphology transitioned from ductile dimple to cleavage/quasi-cleavage. These findings suggest that hydrogen primarily affects the plastic properties of the materials, leading to a shift towards a lower energy fracture mechanism.

Abstract: Fatigue damage is one of the key degradation mechanisms affecting the service life and reliability of aluminum alloys in a wide range of technical applications. The present study focuses on the fracture mechanisms of aluminum alloys under cyclic loading, with a view to the initiation and analysis of fatigue crack propagation in the context of the microstructural characteristics of the material. Special attention was paid to the influence of grain morphology, distribution and type of intermetallic phases, as well as the presence of casting defects on the initiation and development of cracks. Fatigue experiments were performed on a selected Al-Mg alloy of the EN AC 51200 type for the use of three-point bending loading. The results show that the key factors affecting the fatigue behavior are the size and distribution of precipitates, the nature of the interfaces between the phases and the occurrence of microcracks initiated mainly in areas of stress concentration. The knowledge gained contributes to a deeper understanding of fatigue mechanisms in aluminum alloys and provides a basis for their optimization in terms of composition and technological processing in order to increase their resistance to fatigue failures.