W-based amorphous phase stable to high temperatures

A quaternary W-Co-Fe-C amorphous alloy synthesized by mechanical alloying at room temperature is found to have unusual thermal stability and a complex evolution of phases as it decomposes and crystallizes. The structure of this alloy under continuous heating to 1200 °C was characterized using a combination of various techniques, including X-ray diffraction, calorimetry and electron microscopy coupled with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. Prior to crystallization, we observe phase separation of the alloy into two distinct amorphous compositions. Subsequently, alloy carbides (M6C and M12C), metallic crystalline solutions (Co-rich body-centered cubic/face-centered cubic) and intermetallic phases (W6Co7) emerge, but even to an extremely high temperature of 1200 °C we find that about half of the alloy remained amorphous, with a local amorphous phase composition of W35.0Co36.2Fe14.0C14.8. The unusual sequence of phases that evolve and the stability of this amorphous composition are analyzed on the basis of thermodynamic calculations, and in light of the asymmetric diffusivity of carbon vis-à-vis the metallic elements in the system.