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Duplex stainless steels (DSSs) possess desirable combinations of mechanical properties and excellent corrosion resistance due to their composition and equilibrium microstructure of roughly equivalent fractions of ferrite and austenite. They are used in harsh environments such as marine infrastructures, oil & gas, and paper & pulp industries. Components with complex geometries are often required for these applications. Additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) can be harnessed to fabricate components with greatest complexity. However, AM fabrication is well-known to promote non-equilibrium microstructures with high dislocation densities and Cr2N precipitates, resulting in inferior ductility. This is generally regarded as a challenge, however, short heat treatments of such as-built microstructures have been shown to attain refined duplex equilibrium microstructures. Recently, annealed LPBF DSS 2205 has been reported to possess strength higher than wrought counterparts and ductility properties better than the as-built state. However, the microstructural phenomena and deformation mechanisms behind these attractive properties remain poorly understood. Through multi-scale microstructural characterization, we show that the improved strength results not only from the hard ferrite phase, but also fine austenite grain size and nanoscale oxide dispersion strengthening. The enhanced ductility may be attributed to a combination of deformation mechanisms including dislocation slip, stacking fault formation, deformation twinning, and a deformation-induced martensitic transformation. We discuss how the level of microstructural complexity and solid-state phase transformations during LPBF and annealing can unlock multiple strengthening mechanisms during tensile deformation. Such fundamental understanding is crucial for designing AM parts with reproducible and optimised mechanical properties.

Additive manufacturing (AM) of alloys has attracted much attention in recent years for making geometrically complex engineering parts owing to its unique benefits, such as high flexibility and low waste. The in-service performance of AM parts is dependent on the microstructures and grain boundary networks formed during AM, which are often significantly different from their wrought counterparts. Characteristics such as grain size and morphology, texture, and the detailed grain boundary network are known to control various mechanical and corrosion properties. Advanced understanding on how AM parameters affect the formation of these microstructural characteristics is hence critical for optimising processing parameters to unlock superior properties. In this study, the difficult-to-weld nickel-based superalloy Inconel 738 was fabricated via electron-beam powder bed fusion (EPBF) following linear and random scanning strategies. Random scanning resulted in finer, less elongated, and crystallographically more random grains compared to the linear strategy. However, both scanning strategies achieve unique high grain structure stability up to 1250 ℃ due to the presence of carbides pinning the grain boundaries. Despite significant difference in texture and morphology, majority of grains terminated on {100} habit planes in both linear and random built samples. The results show potential for controlling grain boundary networks during EPBF by tuning scan strategies.

17-4 precipitation hardening (PH) stainless steel is a multi-purpose engineering alloy offering an excellent trade-off between strength, toughness, and corrosion properties. It is commonly employed in additive manufacturing via laser powder bed fusion owing to its good weldability. However, there are remaining gaps in the processing-structure-property relationships for AM 17-4 PH that need to be addressed. For instance, discrepancies in literature regarding the as-built microstructure, subsequent development of the matrix phase upon heat treatment, as well as the as-built residual stress should be addressed to enable reproducible printing of 17-4 builds with superior properties. As such, this work applies a comprehensive characterisation and testing approach to 17-4 PH builds fabricated with different processing parameters, both in the as-built state and after standard heat treatments. Tensile properties in as-built samples both along and normal to the build direction were benchmarked against standard wrought samples in the solution annealed and quenched condition (CA). When testing along the build direction, higher ductility was observed for samples produced with a higher laser power (energy density) due to the promotion of interlayer cohesion and, hence, reduction of interlayer defects. Following the CA heat treatment, the austenite volume fraction increased to ∼35 %, resulting in a lower yield stress and greater work hardening capacity than the as-built specimens due to the transformation induced plasticity effect. Neutron diffraction revealed a slight reduction in the magnitude of residual stress with laser power. A concentric scanning strategy led to a higher magnitude of residual stress than a bidirectional raster pattern.

The crystallography and chemistry of interfaces between austenite and ferrite in duplex steels control many important materials properties but remain poorly understood. In this study, we experimentally show that in an additively manufactured and heat treated duplex stainless steel, the majority of austenite-ferrite interfaces terminate on {111}A/{110}F planes, and this behaviour is more pronounced for rational interfaces with the Kurdjomov-Sachs orientation relationship. Interface segregation was found to be controlled by not only the interface crystallography but also the bonding properties of solute atoms. Solute elements showed higher interfacial excess at irrational interfaces. Furthermore, a heterogeneous distribution of selected solute elements in austenite-ferrite interfaces planes was observed. Our findings reinforce the importance and, in fact, necessity to consider five independent crystallographic parameters and chemical architecture of interphase boundaries for advanced control of mechanical and other critical properties in duplex materials.

Recent investigations have indicated that compositional heterogeneity, which is compositional undulation of alloying elements in single-phase solid solutions, can enhance the strength without sacrificing ductility, similar to cluster strengthening, of single-crystal nano-pillars and bulk nanocrystalline metallic materials. However, the impact of compositional heterogeneity on the mechanical properties of coarse-grained metallic materials is not yet fully understood. In this study, we examine the influence of compositional heterogeneity on the tensile mechanical properties of a Cu-9Al (at.%) alloy with a coarse-grained structure fabricated using wire arc additive manufacturing and with an ultrafine-grained structure after further processing. Our results demonstrate that the effects of compositional heterogeneity depend significantly on the grain size. Specifically, in ultrafine-grained samples, there is a remarkable improvement in strength with only a slight reduction in ductility. However, as grain size increases, the strengthening effect of compositional heterogeneity diminishes, resulting in softening with a significant loss of ductility. These phenomena can be attributed to the behaviour of dislocations in the materials. Overall, this study highlights the importance of understanding the relationship between compositional heterogeneity, grain size, and mechanical properties in metallic materials, which can inform the design of new materials with improved strength and ductility.

Thermal gyrations inherent to metal additive manufacturing (AM) result in complex microstructural heterogeneities that are linked to the process parameters, geometry, and alloy composition. AM 17–4 precipitation hardened (PH) stainless steel displays heterogeneities due to its multi-phase microstructure where different as-built phases have been reported. These phases include austenite, martensite, and ferrite, as well as varying degrees of AM induced Cu clustering and/or precipitation. To enable a systematic study of the evolution of these strengthening phases, we utilize a non-standard concentric scan strategy to achieve variations in thermal conditions across a build. Using atom probe microscopy and thermal modeling, we show that the response of Cu varies across the build from no clustering at the outer regions to an increased tendency for Cu precipitation towards the center. Our results will enable the tuning of processing parameters to achieve homogenous Cu precipitation, and thus improve mechanical properties of AM 17–4 PH.

The complex microstructure evolution and heterogeneities in metal additive manufacturing (AM) continue to delay the adoption of AM parts by additional industries. Achieving uniform and superior properties in AM parts requires better fundamental understanding of the microstructural evolution. A suitable pathway to gain such understanding is via in situ techniques such as high-speed X-ray imaging, high-resolution infrared cameras, or via synchrotron and neutron diffraction. However, these methods are complex and resource intensive. Modeling may be a more economical avenue, yet, to make these models more robust and reliable, data from in situ techniques are often required. We believe that in some cases, physical simulation methods originally developed for research on conventional processing such as forging, rolling, and welding may provide similar insights. This viewpoint article discusses existing experimental methods for tracking the microstructure evolution during AM in lab-scale settings, focusing on Ni-based superalloys as a case study. The proposed physical simulation methods include the Gleeble thermo-mechanical simulator, dilatometry, and the arc-melting heat treatment technique. These methods can also be integrated into various X-ray, synchrotron, and neutron diffraction set-ups. We discuss how insights derived from thermo-kinetic modeling can underpin the experimental observations from physical simulations. Last, in situ transmission electron microscopy is evaluated as a powerful method with unparalleled resolution for observing the microstructure evolution directly during simulated AM processes. We believe that these methods can be extended to other alloy systems, enhancing scientific understanding, and streamlining the efficient development of AM parts with superior and more uniform properties, promoting the more widespread adoption of AM.

Gallium‐based liquid metal alloys exhibit unconventional and intriguing properties as metallic solvents, demonstrating an exceptional potential to dissolve and reconfigure a vast array of elements within the liquid metal matrix. Leveraging on these distinctive characteristics of gallium‐based alloys, the synthesis of high‐entropy liquid metal alloys (HELMAs) in low dimensions is reported. The nanoscale HELMAs offer advantages including the solvation of multiple metallic elements at room temperature, while promoting their atomic dispersion at elevated concentrations. Entropy estimations for HELMAs surpass those of high‐temperature molten metals, leading to the realization of high‐entropy liquid metal systems at room temperature. Through a proof‐of‐concept hydrogen evolution reaction comparison, the potential of these HELMAs in enhancing the activities of nanocatalysts is demonstrated. In this case, atomic dispersion of Pt is shown in senary GaIn‐AuCuPtPd HELMA, contrasting with lower entropy systems in which Pt forms discernible clusters. These presented features can lead to catalytic systems with enhanced and tailored activities.