DEVELOPMENT OF CAST NI45CO15CR11.75FE12AL9.75TI6.5 HIGH ENTROPY SUPERALLOY FOR AEROSPACE APPLICATIONS

Abstract

Conventionally, designing alloy is confined to one element (major) to which other minor elements are added to enhance their structural/functional properties. This concept of alloy design restricts the degree of freedom of alloy composition and also the development of desired properties and special microstructure. In context with this, a new concept of high entropy alloys (HEAs) has been introduced in the recent past (in 1996). The HEAs with body-centered cubic (BCC) microstructure show very superior strength but very limited ductility. On the other hand, the FCC microstructure of HEAs is highly ductile and shows limited mechanical strength. Hence, the concept of developing FCC plus one or more second phases with other crystal structure(s) is getting more attention for structural applications in the moderate range of temperature (for example 100-600°C). This prompted researchers to look into new high-temperature alloys to replace Ni-based superalloys. Cast high entropy superalloys (HESAs) have a microstructure of γ and γ´ similar to that of conventional superalloys with high entropy concepts, so the name was given as high entropy superalloys. The present reports focus on designing a new Ni45Co15Cr11.75Fe12Al9.75Ti6.5 alloy having face-centered cubic (γ) and γ΄ (L12) microstructure in the as-cast condition. The comparative analysis has been performed for all three types of specimens, i.e., as-cast, homogenized, and aged alloy samples in terms of microstructural characterization, hardness, tensile, and wear testing. The hardness was found to be a maximum of 408 ± 5.50 HV in the as cast state as compared to homogenized (317 ± 5.50 HV) and aged alloy (317 ± 5.50 HV) samples. Also, for the as-cast alloy, yield strength and elongation to failure are obtained as 864 MPa and 2.6% whereas for aged alloy samples, 677 MPa and 3.11% are obtained. In the aging alloy sample, the average size γ΄ was found to be 0.196 µm. Precipitation strengthening contributes significantly to yield strength. The wear performance of the as-cast alloy sample was found to be superior at 300°C due to the formation of the protective oxidized layer in comparison to wear performed at 20°C and 500°C. Finally, a structure-property correlation has been made to understand various mechanisms of strengthening.