Abstract
The paper works at γ/β duplex-phase MCrAlY(Re) alloys, and its main part is the improvement of the high-temperature anti-oxidation properties of these alloys.The present work studies the microstructural features, the oxidation behaviors and mechanisms of the tested alloys, and subsequently predicted the in-service lifetime of those alloys.
With the help of back-scattered electron microscopy and energy-dispersive analysis, the present work is to study the influences of heat-treatment and alloying elements on the high-temperature oxidation behaviors of γ/β duplex-phase MCrAlY(Re) alloys. Heat-treatment homogenizes the distribution of Al-rich phase,which is beneficial to prompt the formation of the α-Al2O3 protective layer, lower the oxidation rate and improve the anti-oxidation properties of alloys.In order to clarify the influence mechanisms of alloying elements on the tested alloys during the oxidation process, the paper conducted systematic studies on the partition and precipition of alloying elements, the oxidation kinetics, the compostion and the phase transformation of oxides, and the element-distribution characteristics at the scale/alloy interface. Compared with the individual addition of Al or Re, the combined addition of Al and Re could increase the amount of β-phase; strengthen the precipitation behaviors of α-(Cr,Re) underneath the scale/alloy interface and lower oxidation rate. Additon of 3~5wt.% Pt could form a Pt-rich β-phase. By contrast with the original alloys, the Pt-modified alloys showed the accelerated phase transformation, lowered oxidation rate and consumption rate of β-phase. Pt could suppress the formation of the internal oxidation and spinels. Alloys with or without adding small amount of Dy showed the parellel microstructural features. The effects of Dy on the tested alloys were sensitive to the temperature and the existence of Re.For MCrAlY alloy, addition of Dy could bring the accelerated phase transformation,the refined oxide grains and the significantly lowered oxidation rate. Under cyclic oxidation at 1100℃ ,Dy addition formed a sole and protective α-Al2O3 scale, which elminated the cracks and spallations of the oxide scale and internal nitrides. For Re-containing alloy, Dy had not obvious effects on the refinement of the oxides and inhibited the phase transformation. At 800 ℃, Dy had no influence on the anti-oxidation properties. However, Dy could greatly lower the oxidation rate;suppress the formation of spinels and improve the anti-oxidation resistance at higher temperature.
Based on the mechanisms of the consumption of β-phase, a model was proposed to predict the in-service lifetime of the tested alloys. The prediction results revealed the doubled lifetime of the Dy-modified alloys compared with the original alloys.
Key words: MCrAlY(Re) alloys; high-temperature oxidation behaviors;heat treatment; alloying elements; in-service lifetime
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