Nonequilibrium Phases in Melt-Quenched Co-W-C Alloys and Their Powder-Forming Tendency by Comminution

Nonequilibrium phases of amorphous, fcc β-Co and orthorhombic M_3C have been found in Co-W-C ternary alloys quenched rapidly from the melts. The formation of their phases is limited to the range of 12 to 17 at% W and 13 to 18 at% C for the totally amorphous phase, 8 to 20 at% W and 10 to 20 at% C for the partially amorphous phase containing β-Co or WC particle, 0 to 20 at% W and 0 to 10 at% C for the β-Co solid solution, and 0 to 7 at% W and above 20 at% C for the M_3C carbide. Whilst the totally amorphous phase and the β-Co phase exhibit a good bending ductility, the partially amorphous phase possesses a highly brittle nature combined with hardness as high as about 700-1100 DPN. The β-Co phase is a supersaturated solid solution containing large amounts of tungsten and carbon and the lattice parameter and the hardness increase almost linearly from 0.3543 to 0.3641 nm and from 280 to 645 DPN, respectively, with increasing tungsten and carbon contents. The partially amorphous ribbon exhibiting white luster was found to be readily comminuted into irregularly polygonal powder of desirable size ranges by a hammer milling treatment as a consequence of the extreme brittleness of the nonequilibrium duplex phases. The resulting Co-W-C powder dissolving tungsten and carbon as much as about 40 at% (≃53 wt%) is highly attractive as raw material which is consolidated into bulk form by conventional consolidation techniques of powder metallurgy owing to the advantage that the amorphous phase decomposes into an aggregate of stable hcp α-Co phase including a uniform dispersion of ultrafine WC carbide