The microstructures and grain boundary morphologies of a novel Co-9A1-9W-2Ta-0.02B alloy doped with yttrium (Y) (0.01,0.05,0.10,and 0.20;at％) were investigated as functions of aging temperatures (900 and 1000 ℃) and time (50 and 150 h).The aged alloys all exhibit a γ/γ’-Co3(Al,W) coherent microstructure in grain interiors,whereas an intermetallic κ-Co3(W) phase precipitates at grain boundaries.Y is found to fully segregate at grain boundaries and changes grain boundary precipitate morphologies.For 0.01Y alloy,bright κ-Co3(W) stripes precipitate along grain boundaries,where a needlelike κ-Co3(W) phase grows from grain boundaries or κ-Co3(W) stripes toward grain interior.As the nominal Y content increases,the stripe and needlelike κ-Co3(W) precipitates at grain boundaries are strongly restrained and disappear in 0.20Y alloy,leaving fine κ-Co3(W) particles scattered at grain boundaries.It is noted that more Y segregation may increase the number of low-angle grain boundaries (LABs,with misorientations of ＜15°),whereas it eliminates O impurities from grain boundaries.Finally,the effect of Y segregation on tensile behavior of Co-Al-W-Ta-B alloy was discussed from the viewpoints of grain boundary precipitate morphologies,grain boundary character distribution (GBCD),and impurity segregation.