Microstructural evolution of mechanically alloyed Ni-based alloys under high temperature oxidation

Abstract

To further improve the cost-benefit of MCrAlX (X is a reactive element like Y, Hf, Ta, etc.) alloys, the microhardness, microstructure and oxidation behavior of two Ni-based alloys, Ni60Co30Al10 and Ni60Co10Cr20Al10 without reactive elements were studied by scanning electron microscopy, X-ray diffraction and microhardness Vickers. These alloys were synthesized by mechanical alloying in a high energy ball mill and conventional sintering at 1100°C under inert atmosphere. Milling time and Cr additions have a significant effect on microhardness of as-milled and sintered samples. Bulk samples were oxidized at 1000°C for different times to analyze the possible growth of protective oxide scale, as well as the effect of Cr addition on the chemical composition of the oxide scale. After 3h of oxidation the formation of Al2O3 oxide scale in the Ni60Co30Al10 alloy is evident. By reducing the Co content and incorporating 20wt.% of Cr, the formation of an Al2O3 oxide scale with the presence of Cr2O3 was observed. Even the formation of homogeneous, continuous and protective oxide products in the surface of samples is evident, the microstructural changes after oxidation treatments have a significant effect on the decrease in microhardness of both alloys.