Effects of Ta Addition on the Microstructure, Compression Properties and Oxidation Resistance of NiAl-Cr(Mo) Alloy

The effects of Ta addition on the microstructure, compression properties and oxidation resistance of NiAl-32Cr-6Mo alloy were investigated. The microstructure of NiAl-32Cr-6Mo hypereutectic alloy comprises Cr(Mo) primary dendrites and NiAl + Cr(Mo) eutectic. After Ta addition, Cr₂Ta phase, NiAl + Cr(Mo) two-phase eutectic and NiAl + Cr(Mo) + Cr₂Ta three-phase eutectic are observed. Increasing Ta content changes the primary phase from Cr(Mo) to Cr₂Ta, disrupts the formation of eutectic lamellae and degrades the eutectic cell. Ta addition leads to a decreasing room-temperature compression strength but a significantly increasing high-temperature compression strength. At 1000 °C, NiAl-Cr(Mo)-6Ta alloy has a compression strength of 475.1 MPa, which is about nine times than that of binary NiAl alloy and two times than that of NiAl-Cr(Mo) alloy. Room-temperature fractures present brittle pseudo-cleavage with tearing edges and cleavage steps, while high-temperature fractures indicate typically plastic deformation. Cracks in alloys containing Ta no longer propagate along the interfaces between eutectic cells or between the primary phase and eutectic. Improved high-temperature strength of Ta-added alloys results from precipitation strengthening of Cr₂Ta, solid solution strengthening of Ta and strong interface. Furthermore, Ta addition obviously enhances the oxidation resistance of NiAl-Cr(Mo) alloy, especially at higher temperature.