To understand mechanical behavior of metastable metal-matrix composites, Cu@Ag core–shell powders of two compositions, 51:49 and 80:20 (Cu:Ag in wt%), were compacted by spark plasma sintering. The microstructures of these metastable metal-matrix composites are characterized by single spherical particulates of pure Cu in the matrix of pure Ag in the former case while by aggregates of Cu particulates in Ag matrix in the latter one. The plastic deformation showed enhancement of the ultimate tensile strength compared to both pure component metals by a factor of ca. 3. It is shown that a part of plastic deformation can be correlated by a logarithmic dependence but a possibility to apply a polynomial (quadratic) correlation is suggested. As expected, the shape of the particulate changes during plastic deformation and depends on the level and type of the plastic deformation. A model is proposed showing that this dependence is of a hyperbolic character.

The present work describes the effect of long-term (8 weeks) high-temperature oxidation (500 °C) on the formation of an oxide layer as well as on the microstructure and mechanical properties of the 3D-printed 18Ni-300 maraging steel. For this purpose, samples produced by additive manufacturing in the as-built and the as-built + solution annealed and aging treated states were used. The as-built + solution annealed and aging treated material was found to be more prone to oxide layer formation due to a homogeneously distributed Ni₃Mo intermetallic phase in the material matrix compared to the as-built material. The 8 weeks long exposure to a temperature of 500 °C has caused the formation of a thick oxide layer that exhibited a very bad adhesion with the metal matrix/oxide. The X-ray diffraction analysis confirmed the formation of a layer with a complex phase composition: martensite, austenite, Fe₂O_3, and Fe₃O₄. Moreover, the presence of CoFe₂O₄ was determined on the thin outer oxide layer using X-ray photoelectron spectroscopy. The phenomenon of over-aging was found to be the most significant after the first week of high-temperature oxidation. Then, a negligible change in the microhardness was observed throughout the entire experiment. X-ray diffraction analysis and energy dispersive spectroscopy confirmed the phase composition of the alloy corresponding to 75% of martensite + 25% of austenite as well as the change of Ni₃Mo precipitate to Ni₃(Mo, Ti) type after the long-term oxidation.

Metal matrix composites represent an interesting class of materials with an exclusive combination of properties. In this study, a unique Ag–W metastable metal matrix composite was produced from W@Ag core–shell powders using a spark plasma sintering technique at a temperature of 700 °C and a pressure of 80 MPa. The microstructures of a default powder and as-produced composite were observed by scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. As expected, the composite is characterized by a dual microstructure: a soft matrix of pure Ag with a submicrometer grain size reinforced by W particulates with a grain size of up to 30 μm. In addition, tensile and compression tests were performed with a deformation rate of 10^–3 s⁻¹ at ambient temperature. The value of the compression yield stress of Ag–W MMC is higher than the compression yield stress of pure Ag by approximately 467%. Observation of the microstructure of the deformed composite material revealed that the interface between the matrix and the reinforced particles is the weakest place, which is a key factor influencing the performance and properties of the composite material. Graphical Abstract

The present work focuses on the characterization of the ultra-high-strength 1.2709 maraging steel produced by the Direct Energy Deposition (DED) technique, either in its as-built or as-built + heat-treated state. Scanning electron microscope micrographs and X-ray diffraction patterns showed that the heat treatment (namely, solution annealing and aging) had minimal impact on the microstructure changes of the maraging steel. The material is characterized by fine cellular or dendritic microstructure containing several percent of the ductile γ-austenite phase in both as-built and as-built + heat-treated states. A small amount of the Ni₃Mo_0.5Ti_0.5 intermetallic phase was observed even in the as-built state of the material. The heat treatment caused a substantial improvement of the mechanical properties through the homogeneous precipitation of nano-sized needle-shaped Ni₃Mo_0.5Ti_0.5 intermetallic phase. Tensile yield strength increased from 753 to 1957 MPa, ultimate tensile strength—from 991 to 2024 MPa, and microhardness—from 350 to 700 HV0.1. The present results are also compared with those obtained for the same material produced by the more commonly used Laser Powder Bed Fusion (L-PBF) technique. Despite having a coarser microstructure with a presence of γ-phase than the LPBF-printed material, the DED-printed maraging steel exhibited greater precipitation hardening while maintaining 5% ductility after heat treatment.