Microstructure of bulk nanocrystalline Ag50Ni50 alloy prepared by hot pressing of mechanically alloyed powders

A bulk nanocrystailine Ag50Ni50 alloy has been prepared by hot pressing of mechanically alloyed powder at 620 °C under a pressure of 58 MPa. The microstructural characteristics of the alloy were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show. that after mechanical alloying for 200 h, a solid solubility of Ag in Ni reached 4.96% ± 0.21% (mole fraction) and Ni in Ag reached 0.84% ± 0.30% (mole fraction). After hot pressing, the Ag- and Ni-rich phases in the alloy were still in a state with certain degree of supersaturation, such as a solid solubility of Ag in Ni being 0.45% ± 0.11% (mole fraction). The results also reveal that the solubility of Ag in Ni is (0.21-0.24) % ± 0.11% (mole fraction) and that of Ni in Ag is less than 0.1% (mole fraction) at 600-700 °C. The grain size of the mechanically alloyed powders was about 6 nm. After hot pressing, it grew to 40-60 nm and then after annealing treatment it grew further to 100-110 nm. Furthermore, the influence of the variation of the grain size and the internal stress on the broadening of the X-ray diffraction peaks was evaluated in detail and the role of the nanocrystalline structure in the fast densification process of the powders was also discussed.