Effects of abnormal grain growth on the mechanical properties and creep behavior of Ni45Co17Al13Cr11Ti5W5Mo2Nb1Ta1 high entropy superalloy

A new Ni 45 Co 17 Al 13 Cr 11 Ti 5 W 5 Mo 2 Nb 1 Ta 1 (Ni45Co17) high entropy superalloy (HESA) was designed to have a larger volume fraction and higher thermal stability of γ′ precipitates embedded in a face-centered cubic (FCC) matrix. Using a powder metallurgical (PM) technique, a newly developed HESA was prepared. The resulting alloy had a density of 8.14 g/cm 3 , and its microstructure was composed primarily of an L12 precipitate and an FCC matrix, with minor amounts of oxides and carbides. To overcome the limitations of a powder metallurgy-processed HESA, the grain size was controlled with abnormal grain growth (AGG) via cold-rolling and annealing processes, thereby enhancing its mechanical properties at elevated temperatures. The AGG Ni45Co17 exhibited tensile yield strengths of 1270 MPa at room temperature, 1060 MPa and 756 MPa at 600 °C and 800 °C, respectively. Furthermore, the AGG Ni45Co17 showed significantly enhanced creep resistance at 800 °C, showing 45 % and 25 % reduction in creep rates when compared to those of Inconel 718 and Ni45Co17 without AGG, respectively. The enhancement of the elevated-temperature properties in AGG Ni45Co17 is attributed to the suppression of grain boundary sliding and diffusion kinetics. This study provides a method to overcome limitations of alloy fabricated via the powder metallurgy route, as well as describes a new HESA with the potential for high temperature structural applications.