A FeCoNi(B_0.5Si_0.5) high-entropy alloy with the face-centered cubic (FCC) crystal structure was synthesized by mechanical alloying and spark plasma sintering (SPS). Phase evolution, microstructure, morphology and annealing behaviors were investigated. It was found that a single FCC solid solution appears after 50 h of milling. The grain size was 10 nm after 150 h of milling. Microstructure parameters were calculated by the Rietveld fitting of the X-ray Diffraction patterns. Magnetic characterizations of milled and annealed powders at 650 °C for 1 h were investigated. The heat treatment improves the magnetic properties of the milled powders by enhancing the saturation magnetization value from 94.31 to 127.30 emu/g and decreasing the coercivity from 49.07 to 29.57 Oe. The cohabitation of the FCC phase with the equilibrium crystalline phases observed after annealing is responsible of this magnetic softening. The as-milled powder was also consolidated by spark plasma sintering at 750 and 1000 °C. The obtained specimen consolidated at 750 °C improved the coercivity to 25.06 Oe and exhibited a compressive strength of 1062 Mpa and Vickers hardness of 518 ± 14 HV, with a load of 2 kN. The nanoindentation technique with the Berkovich indentor gave hardness and indentation elastic modulus of 6.3 ± 0.3 Gpa (~640 HV) and 111 ± 4 Gpa for samples consolidated by SPS at 750 °C.

Soft magnetic Fe₆₅Ni₂₈Mn₇ (at. %) alloy was successfully synthesized by mechanical alloying and spark plasma sintering (SPS) and, in parallel, the same composition was prepared by arc melting (AM) for comparison. Several SPS conditions were tested. X-ray diffraction and scanning electron microscopy were used to investigate the structure, phase composition, and morphology of the samples. It was found that mechanical alloying produced BCC and FCC supersaturated solid solution after 130 h of milling, with a fine microstructure (i.e., crystallite size of 10 nm). Spark plasma sintering performed at 750 °C and 1000 °C under two pressures of 50 MPa and 75 MPa revealed stable FCC phases. A single FCC phase was observed after the arc melting synthesis. The magnetic properties of milled powders and solids obtained by AM and SPS were investigated. The specimen consolidated by SPS at 1000 °C under the pressure of 50 MPa exhibits soft magnetic behavior (coercivity 0.07 Oe), whereas the mechanically alloyed sample revealed hard magnetic behavior. The specimen consolidated at 750 °C under a pressure of 75 MPa showed a higher compressive strength of 1700 MPa and a Vickers hardness of 425 ± 18 HV. As a result, sintering at 750 °C/75 MPa can be utilized to enhance the mechanical properties, while those sintered at 1000 °C/50 MPa increase magnetic softness.

When a current is involved, as in spark plasma sintering, metallic powders are heated by the Joule effect through both tool and specimen. Other mechanisms might occur, but it is difficult to separate the role of the temperature from the role of the current inside the sample as, in most cases, the two parameters are not controlled independently. In this paper, the consolidation and the densification of a pure copper powder were studied in three configurations for obtaining different electric current paths: (i) current flowing through both the powder and the die, (ii) current forced into the powder and (iii) no current allowed in the powder. Electrical conductivity measurements showed that even low-density samples displayed higher conductivities than graphite by several orders of magnitude. FEM simulations confirmed that these copper specimens were mainly heated by the graphite punches. No modification of the microstructure by the flow of current could be observed. However, the absence of current in the specimen led to a decrease in densification. No significant temperature difference was modeled between the configurations, suggesting that differences are not linked to a thermal cause but rather to a current effect.