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Bulk three-phase nanocrystalline (NC) Ag-20Cu-30Cr alloy was obtained by hot-pressing mechanically alloyed powders, and oxidation tests were completed in 0.1 MPa pure O 2  at 700 and 800 °C. The oxidation behaviour of the alloy and the effect of the grain size were also studied in comparison with the previous coarse-grained (CG) Ag-20Cu-30Cr alloy prepared by a powder metallurgy route. At the two temperatures the oxidation kinetic curves of the NC Ag-20Cu-30Cr alloy are composed of three parabolic stages, and their parabolic rate constants become smaller as the oxidation time increases. The oxidation rate at 700 °C is higher before 5 h, but lower after 5 h than that at 800 °C. Furthermore, the oxidation rate of the NC Ag-20Cu-30Cr alloy is lower than that of the CG Ag-20Cu-30Cr alloy at the same temperature. Moreover, the NC Ag-20Cu-30Cr alloy forms an outer oxide layer composed of the Cu oxides and double oxides of the metallic Ag, Cu and Cr as well as an inner regular, continuous and protective chromia layer. Thus, the Ag-20Cu-30Cr alloy can complete the transition from an internal to external oxidation of the reactive component Cr, and in the end form a regular, continuous and protective chromia layer after nanocrystallization.

Effect of grain size on oxidation behaviour of Ag-20Cu-30Cr alloys in 0.1 MPa pure O 2 at 700 and 800 °C