In the present work, a combined process of laser powder bed fusion (LPBF) and hot working in terms of microstructure refinement was investigated for Fe-25Al-1.5Ta alloy samples. Uniaxial compression tests were carried out parallel and perpendicular to the building direction (BD) at 1000 °C, where BCC A2-phase was stable, at a strain rate of 0.0013 s⁻¹. The true stress–true strain curves indicated a broad flow stress peak followed by a slight decrease, which is typical for dynamic recrystallization (DRX) of conventional BCC metals such as ferritic iron. A negligible dependence in the flow stress behavior on the compression direction was observed. DRX initiated at a stress of 18.7 MPa for the sample compressed parallel to the BD, corresponding to a true strain of 0.011, and at 18.1 MPa for the samples compressed normal to the BD, which corresponded to a true strain of 0.010. The microstructural investigations by electron backscatter diffraction (EBSD) showed that the relatively coarse and elongated grains of the as-LPBF builds were significantly refined after hot working. The microstructure of the compressed samples mainly consisted deformed grains. These were fragmented by sub-grains bounded by low-angle boundaries independent of the compression axis, indicating the occurrence of dynamic recovery (DRV) during hot working. In addition, a few equiaxed, small grains were observed in the pre-existing grain boundaries, which formed due to DRX. Most pores in the as-LPBF builds were closed after hot compression, particularly in the central region of the deformed specimens where the compressive stress state is dominant. In summary, hot compression reveals a practical thermomechanical post-processing treatment for Fe-Al-Ta iron aluminides built by LPBF. The hot working refines the epitaxially elongated microstructure of the as-LPBF builds by DRV/DRX and reduces the porosity.

Fe-Al-Ta alloys are expected to replace high-alloyed steels in steam turbine blades. However, the mechanical properties of the forged blades are still not optimal due to limited grain refinement during hot forging and the coarse-grained microstructure inherited from the as-cast precursor. It is, therefore, essential to investigate the hot deformation behavior of the alloy to identify the optimum range for the deformation parameters leading to good hot workability with significant grain refinement. The hot deformation behavior and hot workability of an Fe-25Al-1.5Ta (at.%) alloy were investigated in the present work using constitutive modeling and the concept of processing maps. Uniaxial compression tests were conducted in a strain rate range from 0.0013 s⁻¹ to 1 s⁻¹ and in a temperature range from 900 °C to 1100 °C, where a disordered A2 α-(Fe, Al) matrix phase along with a C14-(Fe, Al)₂Ta Laves phase were confirmed by X-ray diffraction. The flow stress–strain curves showed a broad maximum followed by a slight drop in stress until a steady state was reached. The optimum processing window for the studied alloy was located at 910–1060 °C/0.0013–0.005 s⁻¹, where the efficiency of the power dissipation (η) and strain rate sensitivity (m) reached 50% and 0.33, respectively. The material underwent a combination of dynamic recovery and dynamic recrystallization over the whole tested deformation range. No flow instabilities were predicted based on Prasad’s flow instability criterion when deformation was performed up to a true strain of 0.5 and 0.8, indicating a high degree of hot workability of the studied alloy over the entire deformation range tested. The current study reveals a well-suited parameter range for the safe and efficient deformation of Fe-Al-Ta alloys, which may contribute to the optimization of the thermomechanical processing of this alloy.

We report a systematic study on the structural and magnetic properties of off-stoichiometric polycrystalline bulk spinels ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_4$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_4$ using neutron and x-ray diffraction, ferromagnetic resonance, and magnetic measurements. Both compounds show a weak tetragonal distortion with $c/a<1$, where the crystal structure could be refined in the tetragonal space group $I{4}_1/amd$. Both ${\mathrm{Co}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ ions are located at the tetrahedral $A$ site, and ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Co}}^{3+}$ at the octahedral $B$ site. Ferrimagnetic (FI) ordering of ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_4$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_4$ sets in below 184 and 164 K, respectively. Magnetic structure analysis revealed that the ferrimagnetically coupled $A^{2+}$- and $B^{3+}$-site moments are aligned parallel to the tetragonal $c$ axis. Additionally, a noncollinear antiferromagnetic order appears in the $ab$ plane, where the moments point along [110] and $\left[1\overline{1}0\right]$. The net magnetic moment [$2{\mu }_{\mathrm{FI}}({\mathrm{Mn}}_B$/${\mathrm{Co}}_B)-{\mu }_{\mathrm{FI}}({\mathrm{Co}}_A$)] of ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_4$ obtained from neutron data varies between 0.88–1.08 ${\mu }_B$ which is in good agreement with $M=0.89\text{–}1.13$ ${\mu }_B$ as determined from magnetization measurements. However, for the Cu-containing compound a larger discrepancy in the magnetic moment was observed between the neutron data (1.89–1.92 ${\mu }_B$) and low-temperature ($T=1.9$ K) field-dependent ($H=90$ kOe) magnetization data ($0.97\text{–}1.21$ ${\mu }_B$). From the three-sublattice model we obtained canting angles ${28}^{\circ }$ and ${25}^{\circ }$ for ${\mathrm{Mn}}_{1.15}{\mathrm{Co}}_{1.85}{\mathrm{O}}_4$ and ${\mathrm{Mn}}_{1.17}{\mathrm{Co}}_{1.60}{\mathrm{Cu}}_{0.23}{\mathrm{O}}_4$, respectively. Both the bulk systems exhibit high magnetocrystalline anisotropy ($K_{\text{U}}\sim 9\times {10}^5$ and $7.5\times {10}^5$ erg/${\mathrm{cm}}^3$) and a field-induced transition ($H_{\text{D}}$) across 4.0 kOe due to the domain reorientation. Temperature ($1.9\text{–}350$ K) and field ($\pm 90$ kOe) dependence of magnetization data confirms the high-spin ($S=\frac{3}{2}$ and $S=2$) ground-state configuration for both the divalent Co and trivalent Mn.