Microstructure and grain boundary morphology evolution of a novel Co-9Al-9W-2Ta-0.02B alloy doped with yttrium

The microstructures and grain boundary morphologies of a novel Co-9 Al-9 W-2 Ta-0.02 B alloy doped with yttrium(Y)(0.01,0.05,0.10,and 0.20; at%) were investigated as functions of aging temperatures(900 and1000 ℃) and time(50 and 150 h). The aged alloys all exhibit a γ/γ’-Co(Al,W) coherent microstructure in grain interiors, whereas an intermetallic κ-Co(W) phase precipitates at grain boundaries. Y is found to fully segregate at grain boundaries and changes grain boundary precipitate morphologies. For 0.01 Y alloy, bright κ-Co(W) stripes precipitate along grain boundaries, where a needlelike κ-Co(W) phase grows from grain boundaries or κ-Co(W) stripes toward grain interior. As the nominal Y content increases, the stripe and needlelike κ-Co(W) precipitates at grain boundaries are strongly restrained and disappear in 0.20 Y alloy, leaving fine κ-Co(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-AlW-Ta-B alloy was discussed from the viewpoints of grain boundary precipitate morphologies, grain boundary character distribution(GBCD), and impurity segregation.

The microstructures and grain boundary morphologies of a novel Co-9 Al-9 W-2 Ta-0.02 B alloy doped with yttrium(Y)(0.01,0.05,0.10,and 0.20; at%) were investigated as functions of aging temperatures(900 and1000 ℃) and time(50 and 150 h). The aged alloys all exhibit a γ/γ'-Co_3(Al,W) coherent microstructure in grain interiors, whereas an intermetallic κ-Co_3(W) phase precipitates at grain boundaries. Y is found to fully segregate at grain boundaries and changes grain boundary precipitate morphologies. For 0.01 Y alloy, bright κ-Co_3(W) stripes precipitate along grain boundaries, where a needlelike κ-Co_3(W) phase grows from grain boundaries or κ-Co_3(W) stripes toward grain interior. As the nominal Y content increases, the stripe and needlelike κ-Co_3(W) precipitates at grain boundaries are strongly restrained and disappear in 0.20 Y alloy, leaving fine κ-Co_3(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-AlW-Ta-B alloy was discussed from the viewpoints of grain boundary precipitate morphologies, grain boundary character distribution(GBCD), and impurity segregation.