Effect Of Grain Size On High-Temperature Chemical Stabilities Of Fe-Ni-Cr And Fe-Cr-Al Alloys

Chemical stabilities of high-temperature alloys depend on the continuous, good adhesion and slow growth oxide scales formed on the alloy surface in high temperature oxidation processes. Ternary multiphase alloys of coarse-grained prepared by conventional arc-melting can not avoid from the microstructure with bigger grain size and the non-uniformity of phase distribution, which requires a higher critical concentration of the reactive component to form selective external oxidation. To decrease the critical concentration needed to form selective external oxide scales, microstructure refinement by mechanical alloying and hot pressing is one of the better ways. In this paper, the nanocrystalline Fe-Ni-Cr and Fe-Cr-Al alloys were prepared by mechanical alloying and afterwards hot pressing, while corresponding coarse-grained alloys were prepared by powders metallurgy. Moreover, the oxidation behavior of these alloys at 700-900℃under 0.1MPa oxygen pressures and the effect of grain size on their oxidation behavior were also studied as compared with the corresponding coarse-grained alloys.The oxidation kinetics of PMFe-40Ni-20/15Cr, and MAFe-40Ni-20/15Cr alloys all deviate from the parabolic law. At the same temperature, the oxidation rates of PMFe-40Ni-20/15Cr are higher than those of MAFe-40Ni-20/15Cr alloy. In the high temperature oxidation process, protective Cr2O3 scales are unable to form on the surface of PMFe-40Ni-20/15Cr alloy but form on the surface of MAFe-40Ni-20/15Cr alloy. At the same temperature, the oxidation rate of PMFe-40Ni-20Cr alloy is higher than that of PMFe-40Ni-15Cr alloy,and the oxidation rate of MAFe-40Ni-20Cr alloy is lower than that of MAFe-40Ni-15Cr alloy.The oxidation kinetics of PMFe-60Ni-15Cr and MAFe-60Ni-15Cr alloys all deviate from the parabolic law. The oxidation rates of MAFe-60Ni-15Cr alloy is lower than those of PMFe-60Ni-15Cr alloy at the same temperature. In the high temperature oxidation process, protective Cr2O3 scales are unable to form on the surface of PMFe-60Ni-15Cr alloy but formed on the surface of MAFe-60Ni-15Cr alloy.The oxidation kinetics of PMFe-10Cr-2.5Al alloy and MAFe-10Cr-2.5Al alloys all deviate from the parabolic law. The oxidation rates of MA Fe-10Cr-2.5Al alloy are significantly smaller than those of PMFe-10Cr-2.5Al alloy at the same temperature. In high temperature oxidation process, protective Cr2O3 scales are unable to form on the surface of PMFe-10Cr-2.5Al alloy but formed on the surface of MA Fe-10Cr-2.5Al alloy.Mechanical alloying decreases the grain size and produces a large number of grain boundaries, and increases effective diffusion coefficients of components. Thus, the reactive components in the alloy are able to diffuse outward faster, which are benefit to decrease the critical concentrations needed to form Cr oxide Cr2O3 scales and form the continuous scales of Cr2O3 scales as compared with the corresponding coarse-grained alloys alloy under the same conditions.