Effect of Grain Size on Oxidation Behavior of Fe-40Ni-15Cr Alloys

Fe-40Ni-15Cr alloys with different grain size were prepared by powder metallurgy (PM) and mechanical alloying (MA) through hot pressing technique. Their oxidation behavior was examined at 1073-1173 K in 1.01 x 10(2) kPa pure oxygen. Results show that the oxidation kinetics of the coarse grained PMFe-40Ni-15Cr and nanocrystalline MAFe-40Ni-15Cr alloys deviates from the parabolic law and their instantaneous parabolic rate constants change with time irregularly. At 1073 K, mass gain of MAFe-40Ni-15Cr alloy is lower than that of PMFe-40Ni-15Cr alloy. At 1173 K, mass gain of MAFe-40Ni-15Cr alloy is higher than that of PMFe-40Ni-15Cr alloy for initial 4 h and afterwards becomes lower than that of PMFe-40Ni-15Cr alloy up to 24 h. PMFe-40Ni-15Cr alloy formed iron oxide scales, while MAFe-40Ni-15Cr alloy formed the continuous and protective scales of chromia on the alloy surface at 1073-1173 K. The reason may be that grain refinement caused the increment of the grain boundary which leads to the faster diffusion of the reactive component Cr from the alloy to alloy/scale interface. Finally, MAFe-40Ni-15Cr alloy formed the continuous and protective scales of chromia.