Studies On High-temperature Chemical Stabilities Of Cu-Co-Cr Alloys Prepared By Mechanically Alloying

Advanced high-temperature alloys or coatings often contain the ternary or more components in order to obtain the necessary combination of good mechanical properties and good corrosion resistance under working conditions. Mechanical alloying is a good method for decreasing the critical Cr contents needed to form the continuous Cr2O3 scales over the alloy surface and promotes to form the continuous Cr2O3 scales under much lower Cr content in ternary or more alloys. These studies are very important for improving high-temperature oxidation theories and developing some novel materials against high-temperature oxidation.The oxidation behavior of Cu-20Co-20Cr alloy prepared by mechanically alloying (MA) and powder metallurgy (PM) was studied at 600-800℃in 0.1 MPa pure O2. The two alloys are composed of two phases, the oxidation kinetics deviates from the parabolic rate law. The two alloys formed the multi-layer oxide scale structures. The external layer is composed of CuO, while internal layer is mainly composed of alloys and oxides. Moreover, the contents of Cr in scales increase gradually from the external to internal layer, but the alloy is unable to form the continuous and thin Cr2O3 scales on the alloy surface finally. The oxidation rates of MACu-20Co-20Cr alloys are significantly higher than those of PMCu-20Co-20Cr alloys. This shows Cu-20Co-20Cr alloys are unable to form the selective external oxidation of reactive component Cr after the grain size is refined.The oxidation behavior of PMCu-20Co-20Cr and MACu-20Co-20Cr alloys was studied at 700-900℃in 0.1 MPa pure O2. The two alloys are composed of three phases. PMCu-20Co-20Cr alloys formed the multi-layer oxide scale structures. The external layer is composed of CuO, while internal layer is mainly composed of mixture of Cu, Co and Cr oxides. The black layer containing high content of Cr close to the alloy substrate is composed of Co and Cr oxides. Moreover, the contents of Cr in scales increase gradually from the external to internal layer, but the alloy is unable to form the continuous and thin Cr2O3 oxide scales on the alloy surface finally. The porous oxide scale has an irregular shape, and extends to the alloy along a phase. MACu-20Co-30Cr alloys formed the dense and continuous Cr2O3 scales at 700-900℃. The bright Cr depleted layer under the oxide scale becomes wide with the increasing temperature.The oxidation behavior of PMCu-50Co-30Cr and MACu-50Co-30Cr alloys at 700-900℃in 0.1MPa pure O2 was studied. Each alloy is composed of three phases, and the morphologies of the oxide scales on the PMCu-50Co-30Cr and MACu-50Co-30Cr alloy surface are similar to those of PMCu-20Co-30Cr and MACu-20Co-30Cr alloys. PMCu-50Co-30Cr alloy also formed the multi-layer structures, and the more close to the alloy, the higher content of Cr the oxides have. But the alloys are unable to form the continuous and compact Cr2O3 scales. However, MACu-50Co-30Cr alloys formed the dense and continuous Cr2O3 scales at 700-900℃, which avoid the alloy from further oxidation.