Junhee Han

The grain boundary character distribution (GBCD) of an equiatomic CoCrFeMnNi high-entropy alloy (HEA) was statistically determined using electron backscatter diffraction. While the overall GBCD of this face-centered cubic (fcc) HEA is comparable to that of elemental metals with the same structure (Au, Cu, and Ni), it exhibits an exceptionally high fraction of Σ3 coherent twin boundaries. These boundaries constitute 94% of the HEA’s total Σ3 grain boundary (GB) population, significantly higher than in Cu (51%) and Ni (61%). This is due to a consistent inverse relationship between GB population and energy, which aligns with a Boltzmann-like distribution, but with a smaller GB energy anisotropy in the HEA. The high-entropy effects, associated with configurational entropy, lattice distortion, and varying atomic radii, reduce the GB energies, most notably the (1 1 1) twist boundary, leading to distinctly higher populations of this GB type at multiple misorientations (i.e., Σ7, Σ13b, and Σ21a).

This study investigated the effect of the secondary phases on multi-step phase transitions and the magnetocaloric properties depending on the Ge content in the MnFeCoPSiGe alloys. Two-step phase transitions were observed by the variations of the Fe₂P-type hexagonal structure (first-order) and secondary phases (second-order). The Curie temperature alters with non-linear behavior consistent with change of the lattice parameters. In addition, the magnetic entropy change decreased with the increase of the Ge content and, subsequently, fractions of the secondary phases. However, the morphological variation of microstructure, distributed as a circular-type shape of the Fe₂P-type hexagonal structure in the Ge-rich matrix, increased the magnetic entropy change. Therefore, the addition of Ge enables the control of the Curie temperature to be applicable for high temperature operating devices. The control of the secondary phases and morphology of the microstructure are crucial to improve the phase transition and magnetic entropy change.

This study investigated the improvement in magnetocaloric properties with magnetocrystalline anisotropy by Ge doping in MnFeHfPSi alloys. The maximum magnetic entropy change gets larger in alloys with higher magnetocrystalline anisotropy. This result is induced by the fractions of easy and hard magnetization planes. In addition, the magnetocaloric properties indicate large differences based on the direction between the easy magnetization plane and the applied magnetic field. Especially, the difference in magnetic entropy change under a low magnetic field is higher than that under a strong magnetic field. Besides, when magnetic transition occurs from a ferromagnetic to a paramagnetic state, the fraction of the easy magnetization plane decreases. Therefore, orientations with an easy magnetization plane play an important role in the enhancement of magnetocaloric properties.

Single-phase solid-solution refractory high-entropy alloys (RHEAs) have been receiving significant attention due to their excellent mechanical properties and phase stability at elevated temperatures. Recently, many studies have been reported regarding the precipitation-enhanced alloy design strategy to further improve the mechanical properties of RHEAs at elevated temperatures. In this study, we attempted to develop precipitation-hardened light-weight RHEAs via addition of Ni or Co into Al_0.8NbTiV HEA. The added elements were selected due to their smaller atomic radius and larger mixing enthalpy, which is known to stimulate the formation of precipitates. The addition of the Ni or Co leads to the formation of the sigma precipitates with homogeneous distribution. The formation and homogeneous distribution of sigma particles plays a critical role in improvement of yield strength. Furthermore, the Al_0.8NbTiVM_0.2 (M = Co, Ni) HEAs show excellent specific yield strength compared to single-phase AlNbTiV and NbTiVZr RHEA alloys and conventional Ni-based superalloy (Inconel 718) at elevated temperatures.