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Phase equilibria and stabilities of intermetallic phases appearing in the Co-rich portion of the Co-W-Ga ternary alloys were investigated and isothermal section diagrams at 1200, 1100, 1000, and 900 °C were determined. A fine cuboidal phase with an L12 structure was observed at 900 and 800 °C, where the composition of the new ternary compound obtained by aging at 900 °C for the Co-7.4W-12.0Ga alloy was Co-11.2W-11.4Ga (at.%). It was confirmed that this compound is metastable at 900 °C but is more stable at 800 °C. These results mean that the thermodynamic stability of the metastable Co3W L12 phase, especially in the low temperature region, increases by the addition of Ga. [ABSTRACT FROM AUTHOR]