Facile synthesis of micro-sized Ni–Al alloy powders through low-temperature chemical alloying

Micro-sized Ni–Al alloy powders were synthesized using different quantities of Ni and Al powders as well as AlCl₃ as an activator in a quartz batch reactor at ≤773 K. This method, named low-temperature chemical alloying (LTCA), which allows Al atoms to diffuse into micro-sized nickel particles mediated by aluminum chloride (chemical promotor), is distinct from conventional processes, such as cast-and-crush, and gas atomization. As-synthesized Ni–Al alloys were characterized using different analytical techniques that included X-ray diffraction (XRD), particle size analysis, and field emission scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy (FESEM-EDS), to confirm the formation of alloy phases, such as Ni₃Al, NiAl, Ni₂Al₃, and NiAl₃. The analytical results showed that the crystalline phase compositions of the Ni–Al alloys were highly dependent upon the initial amounts of Ni and Al powders employed at the given alloying conditions (alloying temperature, 773 K; alloying time, 20 h; amount of AlCl₃, 1.2 wt%). As a result of the thermal treatment of Ni–Al powder mixtures with the Al contents of (5, 15, 30, and 50) wt.% under continuous powder mixing by rotation, each powder was found to have (i) Ni solid solution + Ni₃Al, (ii) Ni₃Al, (iii) NiAl, and (iv) Ni₂Al₃ + NiAl₃ phases, respectively, corresponding to the equilibrium states of the Ni–Al phase diagram. The cross-sectional analyses showed that the alloy structures of the heat-treated powders exist in a single-phase or core-shell form, depending on the number of crystalline phase compositions predicted from the phase diagram. In particular, the Ni–50 wt% Al powder has a unique Ni₂Al₃@NiAl₃ core-shell structure. We further evaluate the performance of the as-developed Ni–Al alloy powders as oxidation-resistant materials and template materials for high-surface area (∼60 m²/g_cat) nickel catalysts.