Effect of Ti Content on the Microstructure and High-Temperature Creep Property of Cast Fe-Ni-Based Alloys with High-Al Content

The cast Fe-Ni-based austenitic heat-resistant alloys with 4.5 wt% Al and varying Ti content were developed for high-temperature application. With increase in Ti content, strength of model alloys increased gradually at 700 °C and 750 °C. At 750 °C, alloys with 35Ni-(2~4)Ti composition showed a significant increase in creep rupture life compared to 30Ni-1Ti alloy, attributed to the increase in γ'-Ni (Al,Ti) precipitates due to higher Ni and Ti content. Among the 35Ni-(2~4)Ti alloys, increasing Ti content from 2 to 4 wt% gradually increased the creep rupture life in the as-cast condition. The creep rupture life was improved after solution annealing treatment, however, the beneficial effect of higher Ti content was not evident for 35Ni-(2~4)Ti alloys. After solution annealing, interdendritic phases were partially dissolved, but coarse B2-NiAl phases were formed. The size and amount of coarse B2-NiAl phases increased with Ti content. In the creep-tested specimens, creep void nucleation and crack propagation were observed along the coarse B2-NiAl phases, especially for high-Ti alloys. Therefore, the beneficial effect of the increase in γ'-Ni (Al,Ti) precipitates for high-Ti alloys on creep property was limited due to the detrimental effect of the presence of coarse B2-NiAl phases.