Effect of Ni on Microstructure and Mechanical Properties of CrMnFeCoNi High Entropy Alloy

The effect of Ni content on microstructure and mechanical properties of the CrMnFeCoNi high entropy alloy (HEA) has been studied. The Ni content varied from 0 to 20 at% in the composition (CrMnFeMn)_100?xNi_x, where x?=?0, 2.5, 5, 10, 15, and 20 at%. The alloys were synthesized by vacuum arc melting and the microstructure as well as hardness of the as-cast alloys were studied. Alloys with low Ni content (x?≤?2.5%) consists of a two-phase microstructure of dendritic and inter-dendritic regions with fcc (matrix) and tetragonal (sigma) crystal structure, respectively. When the Ni content is 5 at%, two-phase structure with fcc (matrix) and bcc (secondary phase) is observed, with the addition of Mn-rich inclusions that are present in the entire matrix. Alloys with higher Ni content (x?≥?10, at%) exhibit a single phase of fcc structure. Hardness of the HEAs decreases from 320 to 120 Hv with increase in Ni content, and the high hardness of these alloys with low Ni content is due to the mixture of both fcc and hard tetragonal (sigma) phases.

     

Structural anisotropy of amorphous alloys with creep-induced magnetic anisotropy

Amorphous ribbons of different composition were annealed under tensile stress. This yielded a creep-induced magnetic anisotropy with an easy magnetic plane perpendicular to, or an easy axis parallel to, the ribbon direction, depending on the alloy composition. X-ray diffraction experiments and simple thermal expansion measurements show that the stress-annealed samples reveal a structural anisotropy which is released by post-annealing as a residual strain. This strain increases with the annealing stress and is therefore correlated with the induced magnetic anisotropy. The origin of this frozen-in strain is discussed in terms of structural heterogeneity in the strength of local atomic bonds. It is suggested that the induced magnetic anisotropy is related to the local magneto-elastic coupling in regions with strong bonding forces.     

Microstructural changes upon annealing in ODS-strengthened ultrafine grained ferritic steel

In this study, the stability of grain size and oxide nanoparticles in the ODS steel upon annealing at high temperature (650–1350 °C) has been evaluated. The ODS Fe–Cr–W–Ti–Y₂O₃ steel has been manufactured by powder metallurgy, consolidated by hot isostatic pressing and processed by hydrostatic extrusion. Such a processing brings about ultrafine grain structure reinforced with oxide nanoparticles (few nm in diameter) and results in superior mechanical properties. The stability of nano-oxides has been analyzed by small angle X-ray scattering together with transmission electron microscopy. The results obtained revealed excellent thermal stability of ultrafine grained ODS ferritic steel, which was attributed to the resistance of oxides against coarsening.