Titanium-based alloys are susceptible to hydrogen embrittlement (HE), a phenomenon that deteriorates fatigue properties. Ti-6Al-4V is the most widely used titanium alloy and the effect of hydrogen embrittlement on fatigue crack growth (FCG) was investigated by carrying out crack propagation tests in air and high-pressure H₂ environment. The FCG test in hydrogen environment resulted in a drastic increase in crack growth rate at a certain $\Delta$ K, with crack propagation rates up to 13 times higher than those observed in air. Possible reasons for such behavior were discussed in this paper. The relationship between FCG results in high-pressure H₂ environment and microstructure was investigated by comparison with already published results of cast and forged Ti-6Al-4V. Coarser microstructure was found to be more sensitive to HE. Moreover, the electron beam melting (EBM) materials experienced a crack growth acceleration in-between that of cast and wrought Ti-6Al-4V.

Electron beam melting is a powder bed fusion (PBF) additive manufacturing (AM) method for metals offering opportunities for the reduction of material waste and freedom of design, but unfortunately also suffering from material defects from production. The stochastic nature of defect formation leads to a scatter in the fatigue performance of the material, preventing wider use of this production method for fatigue critical components. In this work, fatigue test data from electron beam melted Ti-6Al-4V specimens machined from as-built material are compared to deterministic fatigue crack growth calculations and probabilistically modeled fatigue life. X-ray computed tomography (XCT) data evaluated using extreme value statistics are used as the model input. Results show that the probabilistic model is able to provide a good conservative life estimate, as well as accurate predictive scatter bands. It is also shown that the use of XCT-data as the model input is feasible, requiring little investigated material volume for model calibration.

Laser powder bed fusion (L-PBF) has attracted great interest in the aerospace and medical sectors because it can produce complex and lightweight parts with high accuracy. Austenitic stainless steel alloy 316 L is widely used in many applications due to its good mechanical properties and high corrosion resistance over a wide temperature range. In this study, L-PBF-processed 316 L was investigated for its suitability in aerospace applications at cryogenic service temperatures and the behavior at cryogenic temperature was compared with room temperature to understand the properties and microstructural changes within this temperature range. Tensile tests were performed at room temperature and at −196 °C to study the mechanical performance and phase changes. The microstructure and fracture surfaces were characterized using scanning electron microscopy, and the phases were analyzed by X-ray diffraction. The results showed a significant increase in the strength of 316 L at −196 °C, while its ductility remained at an acceptable level. The results indicated the formation of ε and α martensite during cryogenic testing, which explained the increase in strength. Nanoindentation revealed different hardness values, indicating the different mechanical properties of austenite (γ), strained austenite, body-centered cubic martensite (α), and hexagonal close-packed martensite (ε) formed during the tensile tests due to mechanical deformation.

In this work, friction stir spot welding of 5754 aluminum alloy to dual phase steel was investigated using two different ratios of martensite and ferrite (0.38 and 0.61) for steel sheet initial microstructure and varying tool rotation speed (800, 1200 and 2000 rpm). The effect of these parameters on the joint formation was evaluated by studying the plunging force response during the process and the main characteristics of the joint at (i) macrolevel, i.e., hook morphology and bond width, and (ii) microlevel, i.e., steel hook and sheet microstructure and intermetallic compounds. The plunging force was reduced by increased tool rotation speed while there was no significant effect from the initial steel microstructure ratio of martensite and ferrite on the plunging force. The macrostructural characterization of the joints showed that the hook morphology and bond width were affected by the steel sheet initial microstructures as well as by the tool rotation speed and by the material flow driver; tool pin or shoulder. At microstructural level, a progressive variation in the ratio of martensite and ferrite was observed for the steel hook and sheet microstructure. The zones closer to the tool presented a fully martensitic microstructure while the zones away from the tool showed a gradual increase in the ferrite amount until reaching the ratio of ferrite and martensite of the steel sheet initial microstructure. Different types of FexAly intermetallic compounds were found in three zones of the joint; the hook tips, in the hooks close to the exit hole and in the corner of the exit hole. These compounds were characterized by a brittle behavior with hardness values varying from 456 to 937 HV01.

This study explores the use of Fe-Mn-Cr-Si-C master alloy as a sustainable alternative to nickel in press and sinter powder metallurgy, with the goal to improve hardenability and mechanical performance. Two master alloy compositions were optimized for liquid phase sintering using thermodynamic software and then produced through gas atomization. The melting behavior of the master alloys was characterized and compared with thermodynamic predictions. Sintering experiments were performed on compacts from mixes with and without master alloy additions. Continuous cooling transformation (CCT) diagrams were generated to assess hardenability. The results demonstrated that the master alloys improve the hardenability, even at lower cooling rates compared to alloying with Ni. Mechanical testing showed notable improvements in yield strength and apparent hardness comparable to those achieved with Ni additions. These findings support the use of Fe-Mn-Cr-Si-C master alloy as a viable, more sustainable alternative to nickel in powder metallurgy steels.

In this study, ultrasonic nanocrystal surface modification (UNSM) was employed to improve the performance of the top surface layer of a ferritic-pearlitic steel sheet, which is most susceptible to degradation during service. The microstructural changes induced by UNSM treatment and their underlying mechanisms were examined.Significant grain refinement within a surface layer with a maximum depth of 80 μm, and surface compressive residual stress were found for all treatment conditions. The combination of a refined microstructure and accumulated plastic deformation led to a 25% to 68% increase in hardness. Based on the evaluation of surface hardness, compressive residual stresses and surface roughness, the optimal processing parameters were found to be a single pass with 80 N of static load and 50 μm of vibration amplitude.The present study demonstrates the potential of UNSM as a sustainable, room-temperature surface modification technique for enhancing the durability and performance of the surface layer of pipeline steel and provides guidance for the future selection of processing parameters.