INVERSE MELTING OF METASTABLE NB-CR SOLID-SOLUTIONS

Metastable body-centered-cubic (bcc) Nb-Cr solid solutions were produc ed by mechanical alloying over a wide concentration range, and investi gated by x-ray diffraction, differential thermal analysis, and transmi ssion electron microscopy. for comparison, metastable bcc and amorphou s Nb-Cr alloys were fabricated by sputtering. Upon annealing, the soli d solutions prepared by mechanical alloying undergo a transformation t o an amorphous phase. The vitrification is polymorphous for a composit ion Nb45Cr55 whereas two-phase mixtures of amorphous and bcc phases ar e formed for other concentrations, indicating that metastable equilibr ia between these two phases develop upon heat treatment. These results , combined with a detailed analysis of the thermodynamic functions of the system, reveal that the amorphous phase has a lower enthalpy and e ntropy than the bcc phase near equiatomic composition. In consequence, the Gibbs-energy curves of bcc and liquid/amorphous Nb45Cr55 phases v ersus temperature exhibit two points of intersection, i.,e., two melti ng points, with the amorphous phase being thermodynamically more stabl e than the bcc phase at low temperatures. Inverse melting is thus ther modynamically possible in the Nb-Cr system, so that the metastable cry stalline phase would melt upon cooling. (C) 1996 American Institute of Physics.