Microstructural and Mechanical properties of Ni-based high entropy superalloys prepared by mechanical alloying and spark plasma sintering

Abstract

Recently, high entropy superalloys (HESAs) have attracted a lot of research attention for possible medium to high temperature applications such as various parts of jet turbine, aerospace, energy industries and gas turbine due to its high thermal stability and mechanical properties. In this study, nickel-based HESAs were designed based on three different compositions, i.e., Ni45Fe12Co15Cr11.75Al9.75Ti6.5 (HESA-1), Ni43Fe12Co15Cr11.75Al9.75Ti8.5 (HESA-2) and Ni41Fe12Co15Cr11.75Al11.75Ti8.5 (HESA-3). All three compositions were developed by mechanical alloying (MA) technique using a high energy ball-mill (HEBM) followed by spark plasma sintering (SPS). Change in crystallite size with respect to milling time was evaluated by analyzing x-ray diffraction (XRD) profiles of the milled samples, and milling time was optimized to achieve super-saturated alloys. Formation of FCC phase was confirmed after 25h of milling producing a uniform and homogeneous structure. The 25h milled samples were compacted and consolidated using SPS at 1000 °C for a duration of 5 min at an applied pressure of 60MPa. The SPS was performed using 10mm diameter graphite die-punch. Relative density of all three-samples was evaluated using Archimedes method. All three sintered samples were subjected to XRD for phase analysis. Electron backscatter diffraction (EBSD) technique and transmission electron microscopy (TEM) were used to study micrographical features such as grain size, and its homogeneity for all three SPSed alloy specimens. Mechanical properties of the SPSed samples were evaluated using Vickers microhardness and uniaxial compression tests. Various second phases such as γ′ (Ni3Al), γ′ (Ni3Ti), Cr2Ti are found to evolve after the SPS, which are crucial for precipitation strengthening of metal-matrix. Stress-strain plot generated through compression tests were analyzed to assess and equivalent ultimate compression strength (UCS) yield strength (YS). The HESA-3 alloy specimen resulted with better mechanical properties as compared to other two compositions. This is ascribed to higher Al content in the HESA-3, which participated to evolve more amount of γ′ (Ni3Al) precipitates within the matrix. Various strengthening mechanisms were analyzed to correlate mechanical properties of the alloys, especially, the YS.