Volume 79, Issue 6

Using laser powder bed fusion (L-PBF), nickel-free austenitic stainless steel was manufactured from mixing AISI 420S martensitic stainless-steel powder with austenite-stabilizing components. Chromium nitride (Cr 2 N), chromium carbide (Cr 3 C 2 ), chromium (Cr) and graphite (C) powder were admixed in different quantities. The resulting microstructures were investigated using light- and electron microscopy, X-ray diffraction, and hardness indentations. Nitrogen, carbon, and chromium from the admixed powders were dissolved in solid solution; no remnants of nitrides/carbides were identified. The as-built specimens had a lower nitrogen content than the mixed powders. Insufficient additions of austenite-stabilizing elements resulted in a dual-phase microstructure of austenite and martensite, which experienced in-situ tempering of martensite during fusion of consecutive layer(s) in the L-PBF process. Relatively high contents of austenite-stabilizing elements resulted in a fully austenitic microstructure with a hardness of 380–500 HV 5 , depending on Cr and interstitial content. The tendency for forming hot cracks was found to correlate with the solidification interval as calculated using a modified version of the Scheil-Gulliver model.

The spray-forming process is a promising processing route for producing high-performance high-carbon steels. In this study, as-received and hardened/tempered spray-formed (SF) AISI 440C-Mod steel is compared to its vacuum cast (VaC) counterpart. For materials microstructure characterization scanning electron microscopy, X-ray diffraction analysis, energy dispersive spectroscopy, and electron backscatter diffraction were utilized. For the characterization of the mechanical properties and performance hardness, tensile, and compression testing were performed. The results show that the SF steel has a microstructure that is very different from the VaC condition. The differences in the microstructure of the steels after the different processing routes can be explained consistently by differences in the solidification rate and the rate of subsequent cooling of the solidified structure. In particular, the identity, morphology, and size of the primary carbides depend strongly on the combination of solidification and cooling rates. The differences in microstructures have a decisive influence on the mechanical properties of as-manufactured and hardened/tempered steels. The present findings demonstrate the potential of spray forming to produce high-performance AISI 440C-Mod martensitic stainless steel with improved mechanical properties compared to VaC steel.

We investigated hydrogen permeation in titanium aluminium nitride (TiAlN) using ab initio density functional theory (DFT) for cubic and hexagonal crystal structures. Despite the significance of hydrogen barriers, the potential of TiAlN has not been fully explored. We analyzed site specificity, temperature-dependent insertion, and atomic hydrogen migration path energies. Our research highlights the decisive role of crystallographic structure over chemical composition in designing materials resistant to hydrogen absorption. However, once absorbed, hydrogen diffusion is governed by the local chemical environment. Specifically, hydrogen migration through an Al-N plane requires more energy than through Ti-N, which affects the overall diffusion process. We found hydrogen absorption is highly endothermic, with insertion energies from 50 to 320 kJ/mol of hydrogen atoms, indicating low uptake probability at ambient conditions. Higher temperatures further increase the energy required, making absorption less favourable. We also identified substantial energy barriers in the hexagonal structure, with peaks up to 276 kJ/mol, indicating a very low probability of migration for hydrogen. These findings underscore TiAlN’s exceptional resistance to hydrogen permeation, making it suitable for hydrogen storage applications.