Powdered nickel and SiCNP (7.5 vol.%) were milled under an argon atmosphere using a QM-3SP4 planetary ball mill, with a ball-to-powder (BPR) weight ratio of 10:1, in an agate jar (1 L) and spherically shaped agate balls (Φ 6 mm and Φ 10 mm)[32] and [33]. Prior to use, Ni-powder was firstly milled for 48 h to form a protective layer on the surface of the balls and milling jar, so as to minimize contamination from the milling media[34]. Stearic acid (C18H36O2, 1 wt%) was used as a process control agent (PCA) in all milling experimental tests[35] and [36]. Using this technique, four different powders, milled individually for 8, 24, 36 and 48 h, were prepared; with the rotation speed of the disc set at 200 r/min, and the rotation speed of the jars set at 400 r/min. The milled powders were then die-pressed into cylindrical shapes with a pressure of 20 MPa and sintered under vacuum (1 Pa) at 50 MPa to 1150 °C for 30 min in a spark plasma sintering (SPS) furnace (KCE-FCT-HP D 25/4-SD) to avoid excessive metal oxidation. These Ni–SiCNP composite samples had a diameter of 52 mm and height of 20 mm, with densities, calculated using Archimedes principle, reaching more than 98.5% of the theoretical densities. An unreinforced pure nickel sample was prepared by the same route (SPS without a ball mill) for comparative purposes. After sintering, the samples were thermally treated for 50 min (5 min/mm) at 1100 °C in atmosphere followed by water quenching in order to weaken the stress concentration and mitigate the formation of large defect sizes caused by the SPS process.
The bulk samples were cut into 0.5 mm thick sheets and subsequently thinned to 140 μm by mechanical polishing. Standard transmission electron microscopy (TEM) disc specimens (Φ = 3 mm) were punched out from these sheets and further thinned to a thickness of 80 μm using silicon carbide paper. The samples were then double-jet polished in a solution of 5% perchloric acid and 95% alcohol for 3 min at a temperature of −31 °C, followed by ion milling with 4 keV Ar+ ions at a beam angle of 4° for 30 min in order to minimize the adverse effects of possible contaminant ions on grain boundaries. The microstructures were characterized using a Tecnai G2 F20 transmission electron microscope with an accelerating voltage of 200 kV. In addition, grain sizes of pure Ni and Ni–SiCNP composite were measured using electron backscatter diffraction (EBSD) methods within the scanning electron microscope.
Each of the Ni–SiCNP composite samples was cut into dogbone-style tensile specimens using a wire-electrode cutting machine in order to investigate their tensile properties, as shown in Fig. 2. All tensile specimens were polished and tested using a Zwick Amsler 100 HFT 5100 tensile testing machine with a constant strain rate of 0.0007 s−1 in order to obtain the yield strength (σ0.2), tensile strength (σb) and elongation percentage. σ0.2 and σb were determined, respectively, at a 0.2% offset and the maximum stress, while elongation percentage was defined as the maximum elongated gage length divided by the original gage length. In addition, the tensile fracture morphology was analyzed on both the pure Ni and the Ni–SiCNP composites by scanning electron microscopy (SEM), which enabled us to clarify their fracture behavior.