The effect of Al and Ti on the microstructure, mechanical properties and oxidation resistance of γ'-Ni3(Al, Ti) strengthened austenitic stainless steels

Both Al and Ti are the key elements for the formation of γ′-Ni3(Al, Ti) phase and protective oxide scale in the γ′-Ni3(Al, Ti) precipitation strengthened austenitic stainless (AS) steels. To obtain good mechanical properties, especially at elevated temperatures, high volume fraction of γ′ phase is required, which is strongly dependent on the total amounts of Al and Ti. For a given concentration of (Al + Ti), changing Al or Ti addition also has significant effects on the microstructure, mechanical properties and oxidation resistance of alloys. In this study, three alloys are designed by moderately adjusting Al and Ti addition while keeping (Al + Ti) concentration constant (8.7 at.%). It is found that increasing Ti and decreasing Al addition lead to fine Laves precipitates and close spacing between them, which in turn refine grain size and enhance strength at temperature lower than 750 °C. But at about 800 °C, the strength of there alloys is almost the same. Increasing Al and decreasing Ti addition have no apparent influence on the γ′ phase but promote the formation of B2–NiAl phase, resulting in reduced ductility at elevated temperature. Besides, high Al addition facilitates the formation of continuous protective oxide layer (mainly alumina) and is beneficial for oxidation resistance at 750 °C for 500 h. As Ti replaces Al, the integrity of protective alumina layer is destroyed and the discontinuous Al-rich oxides are interlaced with Cr-rich and Ti-rich oxides.