Microstructural properties of bulk nanocrystalline Ag-Ni alloy prepared by hot pressing of mechanically pre-alloyed powders

A bulk nanocrystallined Ag50Ni alloy has been prepared by hot-pressing the mechanically pre-alloyed powders at 620degreesC under a normal pressure of 58 MPa in vacuum. The microstructural characteristics of the alloy were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results of the precise determination of the lattice parameters of the phases in the powders and in the alloy by XRD show that, after mechanical alloying for 200 h, the solid solubility of Ag in Ni reaches 4.85 0.21 at %, while that of Ni in Ag reaches 0.84 +/- 0.30 at %. After hot pressing, the Ag- and Ni-rich phases in the alloy still show a certain degree of supersaturation, with a solid solubility of 0.45 +/- 0.11 at % of Ag in Ni. After further annealing of the alloy at 700 degreesC for 24 h, the solubility decreases to a value of 0.21 0.11 at % for Ag in Ni and to less than 0.1 at % for Ni in Ag. The grain size of the mechanically alloyed powders was of ca 6 nm. After hot pressing, the grain size of the alloy increased to 40-60 nm and then grew further to 100-110 nm after annealing. The influence of the variation of the grain size and the internal stress on the line breadth of the X-ray diffraction peaks has been evaluated in detail. Finally, the role of the nanocrystalline structure in the fast densification process of the powders is also discussed.