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In this study, the ferritic stainless steel SS441 was produced with excellent mechanical properties using laser powder bed fusion (L-PBF) compared to samples produced by conventional casting and hot-rolling. In addition, thermodynamic calculations were utilized to study the phase stability at elevated temperatures and to understand the solidification behavior. The hot-rolled sample showed a grain size up to several hundred mu m with additional precipitates of TiN and Nb(C,N). In contrast, the as-built L-PBF samples displayed a grain size in the mu m range. Spherical precipitates with a size of around 50 nm could be observed and were attributed to a corundum phase from the thermodynamic calculations. The printed material shows superior mechanical properties, with more than 30 times higher impact energy compared to the hot-rolled alloy (217 +/- 5 J vs. 7 +/- 0.5 J). Furthermore, the properties are anisotropic for the L-PBF produced alloy, with the highest tensile strength vertical to the build direction. The superior mechanical properties of the L-PBF produced sample can be attributed to a smaller grain size, giving a higher strength according to the Hall-Petch relationship. The anisotropy of the material can be eliminated by heat treatments at 900 degrees C followed by water quenching, but the absolute strength decreases slightly due to formation of intermetallic phases such as Nb(C,N) and the Fe2Nb Laves phase. The results clearly illustrates that L-PBF provides a promising manufacturing mute for enhanced strength of ferritic stainless steels.

In this study, the crystal structure and phase stability of gas atomized equiatomic AlCoCrFeNi powder was investigated. This alloy is usually described as a high entropy alloy forming a solid solution phase stabilized by a high mixing entropy. However, thermodynamic calculations show that the high entropy phase is stable only at very high temperatures close to the melting point and that a mixture of several phases are the most stable state at lower temperatures. This suggest that kinetic effects may influence the phase composition of atomized powder. The unique features of X-ray diffraction, neutron diffraction as well as transmission electron microscopy were used to study the atomic structure of the atomized powder in detail. The results show that the powder crystallises in an ordered B2 (CsCl-type) structure with a preferred site occupation of Al and Fe on the (½ ½ ½) position and Co and Ni on the (0 0 0) position. During heat-treatment of the powder, the B2 phase decomposes into fcc and σ phases and the final phase composition is highly dependent on the heating rate. The effect of heat-treatment on the atomized powder was also investigated and revealed a significant phase transformation with e.g. the formation of σ phase preferably at the surface of the powder particles. The phase content was also dependent on the size fraction of the powder particles. Sintering of green bodies made with different heat cycles showed that the phase composition of the starting material had a significant impact on the final phase composition and microstructure of the sintered components. The results illustrate the importance of well-defined powder materials for powder consolidation, especially additive manufacturing (binder jetting) of high entropy alloys.

Additive manufacturing of a high-entropy alloy, AlCoCrFeNi, was studied with selective laser melting from gas atomized powder. A wide process parameter window in the SLM process was investigated but it was impossible to produce crack-free samples, attributed to stresses that originate during the building processes. The microstructure and elemental segregation in the SLM samples were compared with induction-melted AlCoCrFeNi. The induction-melted sample crystallizes in randomly oriented large grains (several hundred microns). Dendritic and inter-dendritic areas with slightly different chemical composition can be observed. Within these areas a spinodal decomposition occurs with a separation into FeCr- and NiAl-rich domains. Further spinodal decomposition within the FeCr-rich regions into Cr- and Fe-rich domains was observed by atom probe tomography.

In contrast, the SLM-samples crystallizes in much smaller grains (less than 20 μm) with a dendrite-like substructure. These dendrite-like features exhibit distinct chemical fluctuations on the nm-scale. During annealing more pronounced chemical fluctuations and the formation of Cr-rich and Cr-poor regions can be observed. The difference in microstructure and spinodal decomposition between the induction-melted and SLM samples is attributed to the significantly higher cooling rate for SLM. This study shows that, by using different synthesis pathways, it is possible to modify the microstructure and segregation of element within alloys. This can be used to tune the materials properties, if the cracking behavior is handled e.g. by change of alloy composition to minimize phase transformations or use of a heating stage.

High density components of an AlCoCrFeNi alloy, often described as a high-entropy alloy, were manufactured by binder jetting followed by sintering. Thermodynamic calculations using the CALPHAD approach show that the high-entropy alloy is only stable as a single phase in a narrow temperature range below the melting point. At all other temperatures, the alloy will form a mixture of phases, including a sigma phase, which can strongly influence the mechanical properties. The phase stabilities in built AlCoCrFeNi components were investigated by comparing the as-sintered samples with the post-sintering annealed samples at temperatures between 900 degrees C and 1300 degrees C. The as-sintered material shows a dominant B2/bcc structure with additional fcc phase in the grain boundaries and sigma phase precipitating in the grain interior. Annealing experiments between 1000 degrees C and 1100 degrees C inhibit the sigma phase and only a B2/bcc phase with a fcc phase is observed. Increasing the temperature further suppresses the fcc phase in favor for the B2/bcc phases. The mechanical properties are, as expected, dependent on the annealing temperature, with the higher annealing temperature giving an increase in yield strength from 1203 MPa to 1461 MPa and fracture strength from 1996 MPa to 2272 MPa. This can be explained by a hierarchical microstructure with nano-sized precipitates at higher annealing temperatures. The results enlighten the importance of microstructure control, which can be utilized in order to tune the mechanical properties of these alloys. Furthermore, an excellent oxidation resistance was observed with oxide layers with a thickness of less than 5 mu m after 20 h annealing at 1200 degrees C, which would be of great importance for industrial applications.